Chris Bartone, Ohio University

GNSS receivers seem to get all the attention. Go to any technical GNSS conference and the lion’s share of presentations are about receiver design and techniques: better algorithms, signal processing, integration with other sensors, spoofing detection, and on and on.

GNSS receivers seem to get all the attention. Go to any technical GNSS conference and the lion’s share of presentations are about receiver design and techniques: better algorithms, signal processing, integration with other sensors, spoofing detection, and on and on.

But here’s a fundamental fact of radio science: without antennas, GNSS receivers are essentially useless. Antennas are the component that picks up the GNSS signals out of the RF noise and channels them to the receiver proper — the better the antenna, the better the signals that receivers have to process.

Moreover, largely constrained by the laws of physics, the physical aspects of antennas play a substantial role in the size, weight, and power parameters within which receiver designers must work. And, by extension, antennas are a key variable in the cost factors associated with receiver manufacturing.

With these factors in mind, we turned to Dr. Inder “Jiti” Gupta for insights into the current state of GNSS antennas and their role in GNSS positioning, navigation, and timing. Currently a research professor with the Department of Electrical and Computer Engineering of The Ohio State University, Gupta has focused on GNSS antennas and antenna electronics for the past 17 years. An Edmond S. Gillespie Fellow of the Antenna Measurement Techniques Association (AMTA) the recipient of the 2007 AMTA Distinguished Achievement Award, he has authored more than 150 journal and conference papers.

IGM: What changes are taking place in the GNSS operational environment that pose increasing challenges for successful PNT applications?

GUPTA: Many changes are taking place, including increased GNSS operation in dense urban environments, inside buildings, on platforms that change rapidly with time, e.g., rotorcrafts. The major challenge, however, is posed by spectrum crowding and radio frequency interference (RFI) that could be intentional or unintentional. Spectrum crowding will lead to high-energy signals next to GNSS frequency bands and will require filters with very narrow passband and very high rejection ratio outside the pass band.

One will be looking at brick wall type of filters that are not only costly but can distort the signals of interest (satellite signals). RFI is within the GNSS signals frequency band and cannot be filtered in the frequency domain without affecting the satellite signals. Other approaches need to be applied for successful operation of GNSS receivers under strong RFI environments.

IGM: Can improved receiver antenna design help in these operational environments?

GUPTA: Yes. Currently, fixed-reception-pattern antennas (usually a single element and single feed) are used with GNSS receivers. As the name indicates, the response of these antennas does not change with the RF environment. If we replace these antennas with multiple element antennas whose weights can be controlled (adapted) in real time, then we can easily obtain spatial and polarization discrimination. For example, the signals received by various antenna elements can be combined to increase the gain along selected GNSS satellites.

One can also adapt the element weights to steer antenna nulls along the sources of RFI. Note that controlled reception pattern antennas (CRPA) used with many military GNSS receivers carry out null steering. One can combine beamforming with null steering to increase the antenna gain along the satellite direction while suppressing the RFI simultaneously. One can form ring nulls to suppress multipath or RFI originating around the horizon.

For applications where it is not possible to install antennas with multiple elements, one can use multiple feeds with a single aperture (microstrip patch antennas) and use the output of these feeds to carry out null steering and/or polarization discrimination.

IGM: What innovations in receiver/antenna software seem most promising?

GUPTA: I do not know if we can call it innovation or not, but array signal processing is one area that has not been exploited by GNSS receiver designers. Only recently has the navigation community started using this powerful technology to enhance the receiver performance in strong multipath and RFI environments and to geolocate the sources of interfering signals. With the advancements in field programmable gate arrays (FPGAs), we need to incorporate array signal processing in GNSS receivers.

IGM: Some application developers and handset manufacturer have expressed interest in implementing multi-GNSS capability in consumer products. What should be the considerations for antenna design & development to support the implementation of such capability?

GUPTA: A GNSS antenna is supposed to have omnidirectional (for handheld receivers) or upper hemispherical coverage (for mounted receivers). Thus, these antennas should be low directivity antennas. Also, a GNSS antenna should be an efficient antenna.

Two main factors dictate the antenna efficiency. First, how well is the antenna matched to the receiver? This is also called the return loss of the antenna (S₁₁ parameter). A good number to shoot for is better than 10 decibels over all the frequency bands. The second factor is the radiation efficiency of the antenna which tells us how much of the incident RF energy antenna passes to the receiver. A good number to shoot for is better than 75 percent radiation efficiency over all frequency bands.

Another parameter to consider during the design and development is the antenna polarization. GNSS signals have right hand circular (RHC) polarization. For the best performance, GNSS antennas should have RHC polarization over all the frequency bands and field of view, which is upper hemisphere for mounted receivers and whole sphere for handheld receivers.

IGM: What challenges does one face in designing antennas for handheld multi-GNSS receivers?

GUPTA: For handheld GNSS receivers, major challenges are size and weight. The current commercial handheld GNSS receivers use GPS L1 C/A coded signals or maybe an L1 band GLONASS signal. The bandwidth of these signals is approximately two megahertz at 1575.42 MHz. The percentage bandwidth, thus, is very small.

It is easy to design a lightweight antenna with small volume for small percentage bandwidth. As the bandwidth increases, it becomes more and more difficult to make the antenna small without losing its efficiency. Either one has to use multiple antennas to cover all the frequency bands or use frequency independent antennas (spiral type antennas). In both cases, one will need more real estate, and that is a challenge for handheld GNSS receivers. In this case, one may want to consider wearable antennas for multi-GNSS receivers.

IGM: What are the relative strengths and weaknesses of anechoic chamber measurements versus real-world trials for testing of GNSS antennas?

GUPTA: Let us start with the weakness. The major weakness of anechoic chamber measurements is that it is difficult to simulate the real-world physical environment. Let us say that we are interested in measuring a GNSS receiver antenna mounted on a large SUV. I do not know many anechoic chambers that are large enough to measure antennas mounted on large SUV at GNSS frequency bands. Also, it is hard to duplicate the surroundings.

On the other hand, anechoic chamber provides a very controlled RF environment. One can choose what signals to be simulated and the relative strengths of those signals. For example, one can transmit very strong (more than 20-decibel signal-to-noise ratio) signals in GNSS frequency bands to measure the antenna response (gain and phase) at those frequencies. Thus, one can obtain very accurate antenna response without very long integration. One can also sweep the frequency to cover the whole frequency band of interest. With current technology, frequency sweep is trivial and extremely fast. One can cover the whole L-band in a few seconds. Also, using two independent motion controls, one can control the attitude of the antenna under test to measure its response over the whole field of view. Thus, anechoic chambers are well suited to verify the antenna design by measurements.

The post GNSS Antennas with Dr. Inder Gupta appeared first on Inside GNSS - Global Navigation Satellite Systems Engineering, Policy, and Design.

A global navigation satellite system seems like such solid thing, like the pyramids, perhaps, or a mountain. Permanent, fixed, immutable.

Nor is this surprising. After all, GNSS distinguishes itself from many other technologies of the moment by its grounding in a large and widespread infrastructure: a master control station, launch facilities, far-flung monitoring stations, the space segment with dozens of massive satellites that can operate 20 years or more as did a recently retired GPS Block IIA spacecraft.

A global navigation satellite system seems like such solid thing, like the pyramids, perhaps, or a mountain. Permanent, fixed, immutable.

Nor is this surprising. After all, GNSS distinguishes itself from many other technologies of the moment by its grounding in a large and widespread infrastructure: a master control station, launch facilities, far-flung monitoring stations, the space segment with dozens of massive satellites that can operate 20 years or more as did a recently retired GPS Block IIA spacecraft.

From the user perspective, even those unaware of the tons of metal and electronics behind the signals their devices receive, the persistent and robust availability of the resource historically only adds to an expectation of continuity.

But that sense of unchanging permanence is deceptive. Indeed, a GNSS program is in constant motion, reaching for the future even as it secures its present with the labor and investments of the past.

Every GNSS program in existence today, including regional systems, is undergoing constant change — modernizing, evolving, expanding: the Global Positioning System with its GPS Block III program and next-generation operational control system, BeiDou moving from Phase II to Phase III as the system progresses toward a global presence, GLONASS working to bring new signals online with new satellites while completing a System of Differential Correction and Monitoring.

Even Europe, with only about a third of its initial Galileo space segment in place, began looking beyond its projected completion in 2020 with a formal GNSS “evolution” initiative launched in 2007. To gather more insight into how one GNSS program got out ahead of its inevitable modernization needs, we called on Prof. Dr. Günter Hein.

Hein was the head of the EGNOS and GNSS Evolution Department at the European Space Agency (ESA) from December 2008 until the end of 2014. He continues to organize the ESA/JRC International Summer School on GNSS, was appointed 2015 by the University FAF Munich as Emeritus of Excellence, and is now a member of the Executive Board of Munich Aerospace.

IGM: How far into the future is the current Galileo and EGNOS Evolution looking?

HEIN: Three years ago, the European Space Agency had already started within the GNSS Evolution Program a draft of the Galileo Second Generation (G2G) mission and system-level requirements, system architecture, and deployment scenarios (Phase A studies), as well as technology pre-developments. Considering that the first Galileo in-orbit validation (IOV) satellite launch took place in 2011 and taking into account a satellite reliability of 0.88 after 12 years in space, replenishment of the constellation has to be started after 2023.

Counting backwards and allocating sufficient time for the production of the satellites, the file supporting the decision for Galileo Evolution needs to be prepared over the next two to three years. This shows that there is not much margin left.

On the EGNOS side, the objective is to follow GPS signal modernization, and in particular the fact that the P/Y code tracking of GPS L2 will no longer be guaranteed in the long term. ESA will develop and qualify EGNOS V2.4.2 by the end of 2018, solving present hardware obsolescence and further improving LPV200 performance for civil aviation with respect to coverage and continuity.

The EGNOS second generation, called EGNOS V3, will augment GPS and Galileo on both L1/E1 and L5/E5a. New services will be introduced progressively in the first part of the 2020s. EGNOS V3 System Definition studies with two consortia have just been finished and V3 system test beds are established.

IGM: What are the key metrics/goals for improving Galileo and EGNOS performance through the Evolution program?

HEIN: The main goals for G2G are the resolution of shortcomings in the present system through “lessons learnt,” the implementation of potential, new functions or missions stemming from the evolution of the international GNSS environment in order to be competitive, emerging needs and/or opportunities offered by evolving technologies, and optimization of the exploitation efficiency of the system both in terms of cost and operability.

For that purpose, several options are under study, including optimization of G1G, an advanced payload on middle-Earth-orbit (MEO) satellites, a regional constellation incorporating inclined geo-synchronous orbit (IGSO) satellites with the MEO constellation, and inter-satellite links between MEO and/or IGSO satellites.

On the EGNOS V3 side the goal is to build up a dual-frequency, dual-constellation (GPS + Galileo) augmentation system with improved performance and new services (not only for aviation).

IGM: What possibilities are being considered for changes in the Galileo signal design and spectrum plan?

HEIN: For the Open Service L-band signals, in particular on E1, an enhancement of existing capabilities is being studied: faster time to first fix, enhanced robustness at the signal and message level, enhanced data delivery for challenging environments, and authentication. In addition, an improved accuracy is also planned taking advantage of a better clock and orbit modeling.

IGM: The Evolution program is investigating the possibility of inter-satellite links (ISL) for ranging and/or communications? What would the objectives be for introducing ISL and what factors appear to be most important in deciding whether to do so?

HEIN: Both objectives are being studied. One goal for inter-satellite links is coming from communication: investigating faster distribution of information in the Galileo system. Another goal would be the capability to improve the ranging among satellites and therefore produce improved orbit determination. One of the factors for a decision is the benefit versus the cost versus the number of satellites that would need to be equipped in order to benefit from ISL.

IGM: What non-navigation payload improvements are anticipated for the space vehicles’ physical design and capabilities?

HEIN: Most likely the G2G satellites will take advantage of a larger platform in order to increase their capabilities. At the same time, we want — as a minimum — to maintain the same launch cost efficiency. Therefore, the use of electrical propulsion is being studied not only for orbit changes — for example to change from one orbital plane to another — but also for the transfer from launch to the final orbit.

IGM: What are the prospects for integration of EGNOS into Galileo to create a unified constellation under one system operation?

HEIN: EGNOS like WAAS and other augmentations are guaranteed to be available for aviation until 2035. Regulations of most civil aviation authorities (CAA) require the control of augmentation signals and augmentations. This cannot be guaranteed by a single GNSS that is often under military control. Therefore, in the foreseeable future we will see that augmentation systems will increase and cover most of the Earth, say about 70 percent.

A change may come with Advanced Receiver Autonomous Integrity Monitoring (ARAIM) — still to be finally developed — but for the vertical requirements of aviation, this most likely will not occur before 2035. Thus, no real plans for the integration of EGNOS into Galileo exist besides the possibility of G2G to transmit EGNOS signals from IGSOs (inclined geosynchronous orbits) — a very low probability, however.

IGM: Is it possible that any of the evolutionary improvements might be included on the third-block purchase of FOC satellites to complete and sustain a 30-SV Galileo constellation?

HEIN: This has to be decided by the program manager, the European Commission, but the completion of the Galileo constellation as soon as possible is the clear program objective. 

The post Galileo & EGNOS Evolution appeared first on Inside GNSS - Global Navigation Satellite Systems Engineering, Policy, and Design.

At one time, GPS was expected to supplant a wide range of navigation technologies in the world’s positioning, navigation, and timing (PNT) portfolio. But an unexpected thing happened along the way.

As GPS — and more recently, GNSS — moved from concept, to development, to reality, its vulnerabilities became more apparent, along with its remarkable qualities of accessibility, accuracy, and affordability. Interference to low-power GNSS signals, fear of spoofing attacks and intentional jamming, diminished performance in some operational environments — these and other factors have led policy makers and some user communities to reconsider their expectation of GPS universality and, instead, to seek alternative PNT (APNT) resources. In the United States, a 2004 presidential directive mandated creation of a backup system for GPS to ensure the uninterrupted provision of PNT services.

We called on Sherman Lo help us understand what is at stake in the search for APNT. Lo is a senior research engineer in the GPS Laboratory at Stanford University, where he earned his Ph.D. in aeronautics and astronautics. For the last several years, he has served as an investigator for the Federal Aviation Administration’s evaluation of APNT alternatives.

IGM: What are the leading functions/values/features being looked for in alternative PNT systems?

LO: This topic is a source of much discussion in the APNT community as there is not general agreement in several areas. I think the features needed from APNT are generally agreed upon: robustness, integrity/authenticity, and accuracy (timing or positioning).

However, a debate exists about what those things actually entail. For example, while it is clear that APNT must handle a GNSS outage, no consensus has emerged regarding the length of time and extent in area for which an outage must be managed. I think the differences of opinion come from several factors. First, there have been few major incidents of GNSS denial or spoofing. Second, the uses of and threats to PNT are evolving.

I think that the various stakeholders have different time horizons in mind for APNT. The PNT targets and threats in 2035 will be different than those today or 2025. The challenge with PNT (APNT or otherwise) infrastructure is that it takes time to build out, equip, and modify. It is not like consumer devices or software where we can count on rapid turnover or updates. We must think and plan for the future, sometimes far into the future, and get consensus on what is needed.

For example, FAA APNT currently needs to only provide about one-nautical-mile position accuracy. Hence, distance measuring equipment (DME) — either DME/DME or DME/DME with inertial reference unit (IRU) avionics — is sufficient. However, in the future airspace (2025 and later) as envisioned under the FAA Next Generation Air Transportation System (NextGen), reliance on GNSS and GNSS level performance will be greater and some APNT capabilities will need to improve — to perhaps 0.3 to 0.5 nautical mile accuracy along with better low-altitude coverage.

I suspect that similar issues face other industries such as telecommunications, where one-microsecond timing is sufficient for today but we may be talking about 100 nanoseconds or better in the near future.

IGM: The Federal Aviation Administration (FAA) has emerged as a leader in the search for APNT. Are other regulated carriers —rail, maritime, commercial transport, etc. — substantially engaged in this activity and, if so, how?

LO: The FAA has shown great leadership in seeking robust and redundant navigation capability. As for other regulated carriers, the work with which I am most familiar is that of the General Lighthouse Authorities of the United Kingdom and Ireland (GLA) with maritime navigation. The GLA has been at the forefront developing eLoran for maritime APNT and are leading international standardization efforts on eLoran for maritime applications.

However, I feel that other civil agencies are becoming increasingly cognizant of the need for APNT. The Department of Homeland Security (DHS) has engaged in numerous activities to scope out the need for APNT and examining alternatives. It has held interference exercises to help users understand these threats and develop mitigations awareness and supported development of enhanced Loran (eLoran) through its cooperative agreements.

IGM: What are the leading candidate technologies for providing APNT service?

LO: Right now, it seems as if each agency has its own leading candidates. For FAA, DME [distance measuring equipment] will be a basis for APNT in the near term. DHS is looking into eLoran, DARPA is developing chip scale inertials, and so on.

This diversity of solutions is due to many reasons — each group has systems to meet specific special needs of their mission. On one hand, heterogeneous solutions are a good thing, as they make it more challenging for attackers. But on the other hand, from a cost and a security perspective, I feel that it is not good to have too many different solutions. Having fewer solutions will allow us to focus on the security of each solution rather than just counting on security through diversity.

I believe that, as different APNT technologies mature, there will be a convergence of solutions with a handful of trusted systems. We track the development of robust alternatives by other groups, as they may offer significant benefits to our FAA APNT efforts. eLoran, should it further develop in the United States, may provide a source of robust time synchronization for APNT. The availability of low-cost, high-accuracy inertial would be very useful for APNT and reduce the requirements on ground infrastructure.

IGM: What is the state of play for APNT in other nations and what kinds of relations/interactions are there between their efforts and those in the United States?

LO: Several other nations and groups have expressed interest in US FAA APNT activities. Eurocontrol has talked to us about their APNT efforts and plans to investigated it within the next set of Single European Sky ATM Research (SESAR) activities. SESAR is the European airspace modernization plan similar to the US NextGen. We have and continue to work with the German Aerospace Center (DLR) and National Cheng Kung University in Taiwan on various APNT research within their airspaces. We interact regularly with these groups. DLR is a participant with US and European APNT activities. I think FAA leadership on APNT has led many other groups to look more closely at this issue.

Beyond aviation, I think the other major area of APNT is the ongoing eLoran effort in South Korea, the United Kingdom, and the United States. Korea is planning to modernize their existing stations to eLoran and build three new stations. This eLoran is based on the know-how developed in US/UK efforts.

IGM: Is APNT a significant concern for consumers — the people with smartphones, PNDs, in-vehicle navigation systems, etc. — and, if so, how should those concerns be addressed?

LO: I absolutely think that APNT will be a significant concern for consumers for all those items that you mentioned. Positioning is cheap and I think in the future devices will integrate GNSS chips for even minor benefits (for example, GNSS in cameras and laptops).

As for how to address the need for APNT, it depends on the product category and it characteristics. For me, two characteristics come to the forefront. The first is the likelihood and effect on safety of losing PNT. In this dimension, I also think about the likelihood of loss — are there any incentives for jamming or spoofing the GNSS signal in this application. The second characteristic is how quickly would we add new PNT systems should the need arise.

Smartphones seem to be on one end of the spectrum in terms of these two characteristics. First, for these devices the GNSS degradation (interference, spoofing, or otherwise) is generally a nuisance but not a safety event. Second, and most important, smartphones are technology items for which people refresh the technology very quickly, and so new technology can quickly be fielded to address PNT problems if needed.

Other consumer devices (unmanned aerial vehicles, robot lawn mowers) may be more challenging.

The post Alternative PNT appeared first on Inside GNSS - Global Navigation Satellite Systems Engineering, Policy, and Design.

Now that we have had GNSS-driven precision in the fields for nearly 20 years, with widespread and growing acceptance by farm vehicle manufacturers and farmers, what lies ahead for precision agriculture?

Now that we have had GNSS-driven precision in the fields for nearly 20 years, with widespread and growing acceptance by farm vehicle manufacturers and farmers, what lies ahead for precision agriculture?

The unobstructed views of the sky, which eased the task of ensuring robust signal availability for use with commodity crops such as corn and wheat, is narrowing as growers turn their attention to high-value orchard fruit and wine grapes. Moreover, the scarcity of skilled workers in some sectors is undercutting traditional reliance on manual labor for such tasks as pruning, chemical applications, and harvesting.

These forces have encouraged farmers to look toward increasing automation of equipment to ensure continued efficiencies on the farm. To help us sort out these issues, we turned to Francisco Rovira-Más, director of the Agricultural Robotics Laboratory (ARL) at Polytechnic University of Valencia. Dr. Rovira-Más obtained a Ph.D. in agricultural engineering from the University of Illinois at Urbana-Champaign in the United States.

Among the ARL’s activities is participation in the VineRobot project, an EU-funded effort to integrate machine vision, infrared, GNSS, and other technologies to optimize vineyard management, decision-making, and improve grape quality.

IGM: What are some of the more promising sensors and technologies being incorporated with GNSS to augment and enhance the precision guidance, navigation, and control (GNC) of automated farm equipment?

ROVIRA-MÁS: The major complement to global positioning is local perception. Agricultural environments are open and unpredictable; therefore automation and guidance can never be achieved safely unless vehicle surroundings are reliably sensed. In addition, the tight spacing between crop rows often results in tolerances of a few inches, where real-time fine adjustments from neighboring features are instrumental to avoid collisions.

Machine vision provides a rich source of information that can be analyzed with efficient processing techniques at high rates. However, working outdoors poses serious challenges for long-term stability due to the continuous changing in the relative orientation between the sun and the farm vehicle and the common presence of shadows and reflections. Stereoscopic vision, on the other hand, is more robust to changes in ambient light because intensity variations affect the left- and right-hand images similarly, allowing their pixel-wise correlation as long as a minimum level of texture exists, which is usually granted in off-road environments. The fact that stereo perception provides a 3D representation of a vehicle’s vicinity is key to detect obstacles and estimate how far away they are.

The main disadvantage of 3D stereo has traditionally been its computational cost limiting real time capabilities, although current processors perform excellently with images of moderate resolution. An alternative to computer vision for finding guidance cues is represented by laser rangefinders known as lidar sensors. These provide a faster response and can typically detect obstacles at greater distances, but they usually scan in one plane and, as a result, are more prone to noise, especially in the dusty atmosphere of field terrains.

IGM: Automated precision guidance of agricultural equipment has most commonly been associated with large-scale production of commodity crops in open spaces. However, GNSS and integrated positioning technologies are also being used in specialty crops, orchards, and vineyards. Could you comment on some of these applications?

ROVIRA-MÁS: The added value associated with specialty crops makes their growers prone to adapt new technologies such as precision farming, robotics, and information technologies. The high competitiveness of global markets and the lack of young farmers in industrialized countries practically leave the incorporation of automated or semi-automated technologies in the field as the only alternative.

The apple industry in Washington State has been demanding automated solutions for a long time, the citrus sector in Spain cannot cope with labor costs, and the winegrowers in France’s Burgundy region have trouble finding skilled workers for vine pruning in the winter. The requirement is basically the same: cost-efficient precise machines capable of delivering specialized work at a near-human pace.

The case of the wine industry is especially attractive for technology-based solutions, as wine can be considered a luxury rather than a basic product, and investment in high-performance equipment is easier to justify.

Premier wine requires the identification of grapes with homogeneous characteristics and here is where GNSS becomes irreplaceable, as mixing grapes of varying quality is the recipe for a mediocre wine. Global positioning allows the mapping of vineyards according to quality and harvest readiness, which in turn is the gateway to differential harvesting, the longstanding dream of many viticulturists.

Most medium-size orchard farmers, however, are not willing to pay a subscription fee for a higher quality differential signal; so, such commercial applications must offer safe solutions in light of this constraint.

Although differential corrections can remove important atmospheric errors, multipath and signal blockage often occur when a vehicle traverses an orchard or gets close to farm buildings. The main challenge is, therefore, finding the right balance between cost-efficiency and data robustness.

IGM: What issues, technical or other, still need resolution to realize the full potential of GNSS and related technologies in automated precision farming?

ROVIRA-MÁS: The most important issue by far is the long-term availability, reliability, and consistency of data. The rugged terrains and weather conditions of farm land require solutions comparable to those achieved by army vehicles but limited to much smaller budgets for equipment acquisition. All agricultural vehicles — intelligent or manned — have to be cost-efficient. The size and power of farm equipment ranges from gigantic harvesters of several tons and hundreds of horsepower to small scouting robots operating on electric batteries that are beginning to appear on the market.

IGM: What are the trends for operator involvement with integrated equipment? For instance, do they operate field vehicles remotely or on board?

ROVIRA-MÁS: So far, precision farming applications usually require operators to interact with a monitor on board the vehicle, but the current trend is to access and transfer data via mobile platforms such as cell phones and tablets. A crucial challenge for system integrators is the design of the user interface, as the complexity of operating these systems, regardless of their underlying intricacy, must be comparable to that of cell phone browsing or even less difficult. Intelligent off-road vehicles will generally be operated by farmers and field managers, who are not IT experts and have no time for tutorials.

IGM: Autonomous and semi-autonomous navigation raises issues of safety for agricultural workers as well as preventing damage to crops and the machines. What kinds of safeguards are being incorporated into GNC systems of automated field equipment?

ROVIRA-MÁS: All auto-steered agricultural vehicles require the presence of the operator inside the cabin. In automatic mode, when GPS signal reception is weak or if the driver stands up, removing weight from the seat, the vehicle stops. The only exception is California where a law allows automated machines to operate without a driver as long as there is a safety remote switch to control throttle, clutch, and brakes, and speed is below three km/h.

Many potential solutions never reached the commercial stage due to liability issues, especially with traditional equipment of considerable dimensions. New designs tend to reduce vehicle size to decrease the risk of accidents. For these cases, obstacle detection sensors — imaging or lidar — and a reliable GNSS fault-detector would suffice for performing specific farm tasks. Nevertheless, safety requirements remain a big barrier to widespread farm vehicle automation.

The post Farm Vehicle Automation appeared first on Inside GNSS - Global Navigation Satellite Systems Engineering, Policy, and Design.

Nowhere has the fact that GNSS can guide things besides military weapons and transport manifested itself more profoundly than in agriculture.

While Google and automotive manufacturers struggle to figure out how to put autonomous vehicles on the highway, farmers have been using GNSS for well over a decade to guide equipment through their fields — along with a host of other ag-related, site-specific applications.

Indeed, GNSS — along with an array of other high-tech resources — is transforming agriculture at an accelerating rate.

Nowhere has the fact that GNSS can guide things besides military weapons and transport manifested itself more profoundly than in agriculture.

While Google and automotive manufacturers struggle to figure out how to put autonomous vehicles on the highway, farmers have been using GNSS for well over a decade to guide equipment through their fields — along with a host of other ag-related, site-specific applications.

Indeed, GNSS — along with an array of other high-tech resources — is transforming agriculture at an accelerating rate.

To help us get our arms around the state of the arts and sciences in precision farming, we turned to Dr. John Fulton, associate professor in the Department of Food, Agricultural, and Biological Engineering at Ohio State University. Previously the assistant manager of a 2,000-acre, family owned fruit and vegetable farm in Ohio for 24 years, Fulton has conducted research and written extensively on the use of advanced technologies on U.S. farms of which he estimates around 70 percent are using some level of GPS/GNSS.

IGM: What level of positioning accuracy is most widely sought for these applications? Has there been a trend toward obtaining greater GNSS precision/accuracy in the agricultural community?

FULTON: The level of positioning accuracy depends on the field operation and technology being used. Typical application equipment such as sprayers and fertilizer applicators will use Wide Area Augmentation System (WAAS) or sub-meter correction since high accuracy is not required. These operations occur generally at higher ground speeds versus other field operations in the range of 10 to 20 mph. Therefore, adjacent passes occur in less than 15 minutes; so, precision of the positioning system is sufficient for both guidance and rate control technology on these machines.

However, the trend during the past 10 years in agriculture has been towards decimeter- to centimeter-level accuracy due primarily to reduced pricing but also as a result of the availability of higher accuracy correction services, in particular state-provided continuously operating reference stations (CORS) and private network solutions. These have enabled the number-one precision ag application using GNSS technology: guidance systems, including both lightbar guidance and autoguidance systems for agricultural equipment. Field operations such planting, tillage, and harvesting are using GNSS technologies. Real-time kinematic (RTK) adoption continues to increase in agriculture and has become one of the leading correction services used by farmers.

IGM: What are the leading farming applications of high-precision (i.e., centimeter-level) GNSS technology?

FULTON: High-precision or real-time kinematic (RTK) has become well adopted into autoguidance technology for tractors and harvesters to improve machine control and to accurately maintain spacing for adjacent passes. More specifically, RTK provides the necessary accuracy where machine and/or implement alignment is critical for subsequent field operations.

A typical example would be conducting a strip-till operation, then planting to ensure seeds are placed at the center of the tilled strip. Operation near the edge or even off of the tilled strip can negatively impact crop yield. Therefore, RTK level accuracy is required to maintain accurate alignment of passes over time. This requirement also exists for situations where the harvester needs to be aligned accurately with crop rows to minimize harvesting loss. We are also seeing RTK used to more accurately place fertilizers in relation to a row of plants to maximize uptake and reduce environmental risks of off-site movement of nutrients.

IGM: Do farmers seeking access to high-precision (i.e., centimeter-level) GNSS resources tend toward local, ground-based RTK systems or satellite-based commercial services, including virtual reference station techniques?

FULTON: For RTK application in agriculture, farmers use a range of options. In general, single-baseline solutions are the high percentage versus a true network solution.

However, the use of network solutions, whether virtual (VRS) or not, has grown significantly in recent years. A few agricultural companies still only offer local, ground-based RTK solutions that customers must purchase. The biggest shift for most farmers has been away from purchasing their own base station and being responsible for managing it. Some still use this option, but most new users will have a local base station network or network solution (VRS, CORS, etc.) available in which they can purchase an annual subscription.

IGM: The scalability of precision farming technologies, including GPS & GNSS, has long been a subject of discussion. Recognizing that varying factors, such as crop type, acreage, and applications, will affect decisions to adopt these technologies, at what size of farm operation do we see precision farming methods used?

FULTON: Today, precision agriculture is being used by small and large farms. The price of technology has greatly reduced over the past 20 years and, in particular, over the last 6, allowing any size of farm to take advantage of precision ag technologies. Not only have costs been reduced, the functionality of technology has greatly grown. Farmers do not just purchase a guidance system today; instead they purchase technology that has integrated capabilities for guiding and controlling the application on agricultural machines. So, at a relatively low cost, a farmer can purchase technology having a range of capabilities.

Further, manufacturers have integrated precision agriculture technology into the farm equipment when it’s built. An unlock fee or GNSS receiver purchase may be necessary, but over time costs have gone much lower — between 50 and 80 percent compared to 10-15 years ago. There is also exponential growth in app development within agriculture, allowing farmers to leverage consumer products for capturing spatial data and taking advantage of integrated GPS or GNSS technology within smartphones, iPads, tablets, or similar products. You may not see small farmers using RTK but do not be surprised if they have autoguidance.

IGM: Precision farming technologies traditionally have been adopted for large-scale operations producing commodity crops such as corn, wheat, and soybeans. Have GNSS technologies made inroads into higher-value crop production such as orchards, vineyards, cane berries, etc.?

FULTON: Yes. Today, GNSS is used to conduct scouting of crops and spatially mark data collection sites in both specialty and row crops. Remotely sensed imagery is being used to document in-season crop health or identify issues for a variety of crops. There is continual growth of geographic information system (GIS) and GNSS technologies for improving management of vegetables and tree crops. Automation of machinery and visioning technology improve placement of pesticides and nutrients while enhancing harvesting.

IGM: Is there a significant use of multi-GNSS system technology (e.g., GPS + GLONASS) in precision farming today? As other GNSS systems are completed (e.g., BeiDou, Galileo) what are the prospects and drivers for multi-GNSS applications?

FULTON: The existence of tree lines around fields along with rolling or steep terrain in many regions of the United States limited GPS-only solutions, especially for RTK. Many times, the availability of GPS satellites to derive positions dropped near or below the required number due to shadowing of a portion of the sky during field operations. Therefore, the technology was unable to function properly when working along tree lines or other obstacles, and the operator was back to manually driving the machine or manually turning on and off implements. So, GPS+GLONASS overcame these issues , allowing the technology to operate uninterrupted across the whole field. The addition of new satellite navigation systems will only improve uptime or reliability of the GNSS positioning sensor around the globe.

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The availability of carrier phase tracking — counting the cycles of GNSS signals between satellites and a receiver — has long enabled high-precision users to achieve greater accuracy than using the navigation messages or pseudoranges. Improvements in high-end receivers and techniques such as real-time kinematic (RTK) and precise point positioning (PPP) have made once inconceivably accurate results almost routinely accessible.

The availability of carrier phase tracking — counting the cycles of GNSS signals between satellites and a receiver — has long enabled high-precision users to achieve greater accuracy than using the navigation messages or pseudoranges. Improvements in high-end receivers and techniques such as real-time kinematic (RTK) and precise point positioning (PPP) have made once inconceivably accurate results almost routinely accessible.

The presence of ever more GNSS satellites and signals holds the promise of improving the situation even more. However, this has not eliminated the need to solve some fundamental challenges, especially the effects of the ionosphere on signals passing through it and determining the exact number of cycles by integer ambiguity resolution (IAR).

To help us understand the current state of the art and the implications of having multiple GNSS systems to draw on for high-precision positioning, we called on Dennis Odijk, a research fellow in the Department of Spatial Sciences at Curtin University’s Western Australian School of Mines in Perth. Odijk obtained his doctor of engineering degree from Delft University of Technology in the Netherlands, where he also spent seven years as a GNSS researcher focusing on signal-processing for high-precision applications.

In order to have acceptable convergence times to a solution — say, less than 10 minutes — both PPP and PPP-RTK (a technique in which PPP provides rapid convergence to a reliable centimeter-level positioning accuracy based on an RTK reference network) rely on precise ionospheric corrections. This also holds for RTK, if one wants to extend the baseline to a distance beyond which the differential ionospheric biases cannot be neglected.

The availability of multiple GNSS constellations offers opportunities for more precise ionospheric modeling, Odijk points out, as the ionosphere will be intersected at many more “piercing points” than when using data from a single constellation. In addition to this, the availability of triple-frequency observations enables researchers like Odijk to estimate and model second-order ionospheric effects, possibly resulting in better ionospheric models.

However, having more multi-GNSS, multi-frequency carrier-phase data means more cycle slips are likely to occur, he says; so, cycle slip correction techniques must be optimized to address this situation.

One of the research challenges associated with IAR is the increased dimension of the ambiguity vector in a multi-GNSS case. For example, Odijk offers the example of a PPP-RTK user who is tracking data from three fully operational constellations on three frequencies per constellation. Assuming eight satellites per constellation are being tracked, the dimension of the ambiguity vector reads 3*3*(8-1) = 63 (versus 14 for dual-frequency GPS). This high dimension may slow down ambiguity resolution and, therefore, hamper real-time applications, Odijk says. However, with so many ambiguities it may not be necessary to resolve the full vector — partial IAR techniques may be able to resolve an optimal subset of ambiguities.

IGM: What is the current state of the art in performance of RTK and PPP techniques – in terms of such variables as real-time or postprocessed, single vs. dual-frequency, static vs. dynamic?

ODIJK: With both RTK and PPP techniques, centimeter-level positioning accuracy is feasible, although with (standard) PPP this may take several hours before the solution has converged to such high accuracy. This is typically only reached in (static) post-processing mode, taking into account the most precise orbit and clock products, as well as a priori corrections.

Decimeter-level PPP accuracy can be reached much quicker (tens of minutes; static receiver). For PPP based on a single-frequency receiver, it is hereby essential that corrections for the ionosphere are available.

With RTK, centimeter-level accuracy is achievable very quickly — even instantaneously when dual-frequency receivers are used. RTK based on single-frequency receivers typically needs more time (as it requires a sufficient number of satellites).

IGM: What signal processing challenges are common to RTK positioning and PPP? What signal-processing techniques are common to both?

ODIJK: With RTK the carrier-phase ambiguities are estimable as double-differenced parameters and therefore “automatically” integers. It is well known that IAR is the key to fast high-precision positioning. With PPP the ambiguities are, however, not estimable as integers, as the information to restore their “integerness” is lacking in the standard correction products.

In order to resolve PPP integer ambiguities additional information is needed about the satellite hardware phase biases. If this information is provided to PPP users, their estimable ambiguity parameters are very similar to those of RTK, namely double-differenced, but now relative to one of the receivers of the reference network from which the satellite phase bias corrections are generated. This is the principle of PPP with ambiguity resolution (PPP-AR) or PPP-RTK: while the method is conceptually equivalent to PPP, it provides the potential high accuracy of RTK. The standard PPP solution is a special case of the PPP-RTK solution: it corresponds to the PPP-RTK solution in which the ambiguities “float.”

IGM: Multi-GNSS precise positioning has to address the issue of intersystem differences. What challenges do such factors add to achieving multi-GNSS precise positioning?

ODIJK: The largest benefit of multi-GNSS is that the positioning model becomes much stronger with more satellites and more frequencies. For example, we have demonstrated using real data collected at the Curtin University campus that RTK based on single-frequency GPS L1 + BeiDou (BDS) B1 is feasible with an instantaneous success rate close to 100 percent. Such performance is not possible based on single-constellation, single-frequency GPS data.

This performance improvement is conditioned on a proper handling of the biases between the different constellations. Although each GNSS transmits its satellite positions in its own coordinate frame, the differences between these frames are expected to be small, as they are realizations of the International Terrestrial Reference System (ITRS) — at least for GPS, Galileo, and BDS. For (short-baseline) RTK these differences, therefore, will cancel out. This will not be the case for PPP (-RTK), however, and one has to take them into account. Time offsets between systems also must be accounted for, either by correcting the observations or by estimating them in the processing. The calibration and correction of inter-system biases is essential to align the observations of different constellations to one constellation.

IGM: How can precise positioning methods be made more robust/reliable when operating under adverse conditions, e.g., urban canyons or under foliage?

ODIJK: The availability of new signals with higher power and better tracking performance in themselves will improve positioning in adverse environments. Moreover, precise positioning based on multi-GNSS will be more robust than that based on a single constellation, with more satellites available and consequently a stronger geometry. This means that when operating under adverse conditions, such as (low-elevation) multipath or in an urban canyon, we can apply a higher cut-off elevation than with only one constellation.

For example, we have demonstrated that RTK based on single-frequency data of GPS+BDS+Galileo+QZSS still results in an instantaneous ambiguity success rate of almost 100 percent in these conditions based on a high 35-degree cut-off elevation, whereas when using only GPS the success rate was 8 percent. (In the latter case we could not always compute a solution using this cut-off because of insufficient satellites). Despite this high cut-off elevation, the fixed positioning accuracy based on the four-constellation data was at the centimeter level.

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In August, a group of scientists at the Scripps Institution of Oceanography reported that the severe drought gripping the western United States in recent years is causing a “uplift” in the western United States.

About the same time, governmental agencies were reporting widespread cases of land subsidence in California’s central San Joaquin Valley caused by overpumping of water from wells there.

In August, a group of scientists at the Scripps Institution of Oceanography reported that the severe drought gripping the western United States in recent years is causing a “uplift” in the western United States.

About the same time, governmental agencies were reporting widespread cases of land subsidence in California’s central San Joaquin Valley caused by overpumping of water from wells there.

According to the Scripps researchers, the largest uplift of the tectonic plate — that piece of the Earth’s crust on which the western states rest — amounted to only 15 millimeters (about half an inch) in California’s mountains and an average of four millimeters (0.15 of an inch) across the West.

How did they do it? How did they detect so precisely such a small movement over thousands of square miles of terrain?

From analysis of massive sets of data accumulated over the past 11 years from high-precision GNSS monitoring stations scattered across the region. Indeed, GNSS technology has created a figurative common ground where space science meets Earth science, which in its larger scope encompasses the field of geodesy: the measuring and monitoring of the size and shape of our planet.

Finding out how that has come about led us to Gerald Mader, chief of the Geosciences Research Division of the National Geodetic Survey (NGS), an office of the U.S. National Oceanic and Atmospheric Administration (NOAA). Mader joined the NGS in 1983 and has seen GNSS grow from an obscure military program into, among other things, a crucial tool for modern geodesy.

Mader is a coauthor of the original RINEX format, a cofounder of the International GNSS Service, and the developer of NGS’ antenna calibration program. He has also written and supervised NGS’s GPS software for precise static and kinematic positioning.

NGS is the oldest scientific agency in the nation, descended from the Survey of the Coast, founded in 1807 by President Jefferson. Through both research and development, NGS focuses on defining and delivering to the public the National Spatial Reference System (NSRS). A good example of its activities is OPUS, NGS’s Online Positioning User Service, now referred to as OPUS-S. Mader promoted development of this web-based service that provides users submitting GPS static positioning data with NSRS coordinates accurate to a few centimeters with which to improve their positions.

A few years ago, he launched the Kinematic GPS Challenge, an effort to attract volunteers to help NGS facilitate development of its GPS processing method for GRAV-D (Gravity for the Redefinition of the American Vertical Datum). Interested researchers and companies from around the world were invited to compute and submit position solutions from samples of actual GRAV-D data.

High-precision GNSS accuracy has made possible, or greatly enhanced, a variety of remote sensing applications, including photogrammetry, LIDAR mapping, synthetic aperture radar, and airborne gravity. Road grading, precision agriculture, and monitoring the stability of buildings, bridges, and dams are part of a very long list of applications facilitated by GNSS.

“Just about any type of platform that moves, whether on the surface of the earth, in air or in orbit, has had GNSS attached to it to precisely measure its motion,” says Mader.

IGM: What do you believe has been the single most significant effect that GNSS has had on the field of geodesy?

MADER: The overwhelming benefit of GNSS to geodesy is the accuracy and the time required to achieve that accuracy. The older optical techniques required line-of-sight and traversing, sometimes over long distances, over mountains, across rivers, etc., to position a new mark with respect to a known mark. Now, geodetic control is transferred from a global network of marks (the International GNSS Service network) through the GNSS satellites to any other mark on the Earth that needs to be positioned.

IGM: How has GNSS technology affected, if not transformed, the work of geodetic survey agencies?

MADER: The impact of GNSS on surveying is revolutionary and comparable only to the impact of the computer itself. Labor-intensive surveys requiring days or weeks can now be done in hours. State-wide, real-time networks provide centimeter-level accuracies in seconds to minutes. While GNSS is not appropriate for every survey, just about every survey will use GNSS to connect to local geodetic control. Survey agencies are defining and maintaining the infrastructure needed to support this GNSS surveying and provide some continuity to the legacy surveys they have archived.

IGM: How has GNSS contributed to the definition of the International Terrestrial Reference Frame (ITRF)?

MADER: Given the number of GNSS tracking stations compared to Very Long Baseline Interferometry (VLBI), Satellite Laser Ranging (SLR) and Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS), GNSS has certainly become the dominant contributor to the ITRF. While the techniques contributing to the ITRF offer comparable accuracies, there are small biases which are still being studied.

Perhaps ironically, the traditional optical techniques are needed and being used to precisely find the vectors connecting these techniques where combinations of these techniques are co-located at the same site. There are multiple IGS GNSS tracking stations on every continent and just about any remote island that may provide additional coverage. These precisely determined reference stations provide the means to conveniently and accurately connect local and national datums to this global reference frame.

IGM: How much have GNSS techniques improved the accuracy of horizontal and vertical datums? Have they produced a truly unified three-dimensional capability?

MADER: The improvement to the accuracy of datums has been enormous as you might imagine given the high accuracy and efficiency of GNSS techniques. The continuously operating reference stations (CORS) may typically show horizontal repeatability of 3-5 millimeters and vertical repeatability of 6-9 millimeters. Of course, this vertical is, strictly speaking, distance from the geocenter (since GNSS uses Cartesian coordinates) and is usually expressed as a height with respect to an ellipsoid that approximates the oblate shape of the Earth in order to get more reasonable numbers.

This ellipsoid height, however, is not the height required for mapping and engineering. That is the orthometric height which follows an equipotential surface and is often colloquially, but incorrectly, referred to as mean sea level.

The two height systems are related by the geoid, which can be determined from measurements of gravity. NGS is currently working on a project (GRAV-D) using terrestrial, airborne, and satellite gravity data to improve the geoid so that more accurate orthometric heights can be derived from GNSS determined ellipsoid heights.

IGM: Has GNSS enabled modern geodesy to expand from the realm of scientific endeavor to a more service-oriented discipline, such as can be found, for example, in the NGS’s Online Positioning User Service (OPUS)?

MADER: GNSS data processing software is now capable of sub-centimeter accuracies between stations thousands of kilometers apart and estimating satellite positions to within a few centimeters. Yet these highly complex programs can be made accessible to the public through a simple web-based interface as NGS has done with OPUS.

This allows long term tracking of stability and motion by users to be based on uniform and consistent physical models and parameterization. For example, subsidence from ground fluid extraction can be monitored with the best possible accuracy by periodically submitting RINEX files to OPUS. There are now thousands of geodetic quality GNSS receivers in use every day in the United States. Providing the capability to easily and accurately process a portion of these data and share results is an important and increasingly popular service.

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For at least two decades, GPS experts, geodesists, and public agencies have been working together to develop high-accuracy, large-scale continuously operating GPS reference stations that provide them the capability to monitor and model crustal deformation, tectonic plate movement, and the effects of geohazards such as earthquakes and volcanic eruptions.

Now, GNSS-augmented advance warning systems are going into place that can give us a crucial margin of safety in the event of an earthquake.

And none too soon.

For at least two decades, GPS experts, geodesists, and public agencies have been working together to develop high-accuracy, large-scale continuously operating GPS reference stations that provide them the capability to monitor and model crustal deformation, tectonic plate movement, and the effects of geohazards such as earthquakes and volcanic eruptions.

Now, GNSS-augmented advance warning systems are going into place that can give us a crucial margin of safety in the event of an earthquake.

And none too soon.

The latest Updated National Seismic Hazard Maps recently released by the U.S. Geological Survey (USGS) indicate a higher level of earthquake risk for the West Coast and some areas of the Midwest and East Coast then previously thought. (See the related news article in this issue on page 18.) In the next 30 years, the USGS says, California has a 99.7 percent chance of a magnitude 6.7 or larger earthquake, and the Pacific Northwest has a 10 percent chance of a magnitude 8 to 9 megathrust earthquake on the Cascadia subduction zone.

The Federal Emergency Management Agency (FEMA) has estimated the average annualized loss from earthquakes nationwide to be $5.3 billion. According to FEMA, 77 percent of that figure ($4.1 billion) comes from California, Washington, and Oregon, with 66 percent ($3.5 billion) from California alone.

So, an ongoing effort by the USGS and partner agencies and institutions to establish a West Coast Earthquake Early Warning (WC-EEW) system as the prototype for an eventual nationwide “ShakeAlert” system seems especially timely.
The early warning system exploits physical characteristics of earthquakes, which generate two main types of waves: rapidly moving primary or P-waves and the slower secondary (S) and surface waves that cause more intense and damaging ground shaking. (See accompanying figure.)

By detecting and analyzing the location and magnitude of an earthquake reflected in the P-wave energy, expected ground-shaking levels across a region can be estimated and warnings sent to local populations before more damaging shaking arrives with or after the S-wave. The advanced warning can range from seconds up to more than a minute, depending on the distance an affected area is from the earthquake’s origin.

Ken Hudnut, a geophysicist at the USGS Earthquake Science Center in Pasadena, California, and chair of the GNSS Working Group for the WC-EEW, has a long history in working in the area of geohazards. Dr. Hudnut received an A.B. degree in Earth sciences from Dartmouth College and a Ph.D. in geology from Columbia University. Before joining the USGS in 1992, he was a post-doctoral fellow at the California Institute of Technology Seismological Laboratory and currently is a visiting associate in geophysics on the faculty of the California Institute of Technology.

We called on Dr. Hudnut to discuss the state of the art in seismic science and the role of GNSS in that research and in the design and operation of earthquake early warning systems.

IGM: The USGS has a long history of developing instrumentation for the study of earthquakes and other types of Earth movement. What does GNSS positioning bring to the task that seismic sensors do not provide and, more specifically, how do GPS/GNSS data benefit EEW systems?

HUDNUT: GNSS positioning is especially good at rapidly giving us the change in a station’s position. Seismic sensors measure vibrations very well, but GNSS is better at measuring permanent displacement.
In a big earthquake, a station might move by several meters in several seconds, and not just in a simple straight line. The shaking may include erratic oscillatory displacements that are several times larger than the permanent displacement. Even though GNSS was never intended to measure such large, sudden, and jerky movements, we find that it works very well and provides a great augmentation to the seismic sensors that are currently in use for earthquake early warning.

IGM: How is GNSS data different from that obtained from these seismic sensors and how is it merged in an EEW system?

HUDNUT: GNSS data add to system robustness because they are an independent measurement. The seismic sensors are basically a mass on a spring, whereas GNSS is measuring position variation using changes in ranges to a constellation of satellites, so it’s a totally different kind of observation. Having both types of data makes the system stronger because we can immediately rule out glitches coming from one sensor type or the other. The diversity of observations gives us more strength.

As for merging the data, there is an abundance of literature and we are testing everything from uncoupled to loosely coupled and tightly coupled, and using a variety of methods. There are trade-offs in terms of simplicity, speed, and smoothing that we’re evaluating on an ongoing basis. We are creating a hair-triggered system that is also very robust even during large dynamic displacements, which is not a slam dunk. We’ve learned a lot by studying the methods of combining being done for strap-down avionics, that is, navigation and positioning systems, and looking at both commercial off-the-shelf solutions and open-source options.

IGM: What GPS/GNSS signals, satellite observables, and signal components (e.g., code vs carrier phase) are used in the EEW system?

HUDNUT: Right now, we are mostly reliant on GPS alone, but we have upgraded to GNSS receivers at our stations over the past several years. Of course we’re doing phase-differential, dual-frequency processing to get the few-centimeter accuracy in real-time; so, we do widelaning and narrowlaning, but code is relatively unimportant to us — we rely heavily on the carrier phase. We’re using precise point positioning with ambiguity resolution, which is possible for GPS these days and in the future may be possible for GLONASS as well. Our limited telemetry bandwidth doesn’t allow us to bring back all of the GLONASS and other GNSS data just yet.

IGM: What practical benefits are provided by an impending seismic movement alert on the order of tens of seconds?

HUDNUT: Applications envisioned are getting school children to safety under their desks that much sooner, and operating automatic shut-off valves, putting computer systems into a safer state, or switching other automated systems to try to prevent loss of life or damage to property. If you were having surgery performed at that time, wouldn’t you want the surgeon to remove the scalpel to safety right before the shaking started?

We want to make it possible for people to invent their own applications, and we expect this to happen here as it has in Japan, Mexico, and other countries that have already had EEW for many years and even decades. In Japan, EEW protects the Shinkansen (bullet train) system. In California, BART is testing use of EEW and figures it could help prevent or lessen the severity of future derailments.

IGM: In recent years, a number of demonstration campaigns have been conducted, involving public agencies and citizen participants in sending and receiving test notifications of an earthquake. What have been some the most important lessons learned from those campaigns?

HUDNUT: ShakeOut is our annual public drill to encourage “Drop, Cover, and Hold On” by everybody. We started this in 2008 in California and it has grown worldwide. We use that as an earthquake hazard awareness opportunity for publicity for EEW. In general, ShakeOut encourages a personal action that could be done even quicker if one had an operational public EEW.

With the ShakeAlert EEW system, what we have been doing for the past couple of years is a slow roll-out through selected “beta-users.” We don’t want to roll this out to the public before it’s ready because of the “cry wolf” gotcha. Most county-level and large cities’ emergency operation centers, plus Caltrans and BART for example, have the ShakeAlert UserDisplay installed so that they could potentially relay an alert through dispatch communications systems.

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In the “gee-whiz” awesomeness of proliferating GNSS apps, it’s sometimes hard to remember that Global Positioning System originated as a military system designed to meet strategic and tactical needs on the battlefield.

And, with the U.S. Air Force continuing its 40-year mission as the executive agent for sustaining GPS, that undiminished military role plays no small part in ensuring the availability and reliability of the U.S. contribution to the GNSS system of systems.

In the “gee-whiz” awesomeness of proliferating GNSS apps, it’s sometimes hard to remember that Global Positioning System originated as a military system designed to meet strategic and tactical needs on the battlefield.

And, with the U.S. Air Force continuing its 40-year mission as the executive agent for sustaining GPS, that undiminished military role plays no small part in ensuring the availability and reliability of the U.S. contribution to the GNSS system of systems.

The long road traveled by the Department of Defense (DoD) in bringing GPS to its prominent, even dominating, presence in the world of positioning, navigation, and timing (PNT) has encountered a growing number of challenges in recent years: budgetary constraints, pressures to modernize in a technologically evolving world, the recognition of GNSS vulnerabilities and physical constraints, and the need to accommodate civil, commercial, and scientific user communities along with the military services.

In the early years of the GPS program, for instance, some people thought that the new satellite-based technology would replace many legacy navigation systems, including those operated by the DoD. More recently, with growing awareness of the limitations of GPS access in some operational environments, as well as vulnerability to interference and jamming, defense officials seem to have re-set their expectations for GPS as they look for robust and ubiquitous PNT capabilities.

Another challenge: Even as the new military M-code signal becomes implemented on a growing number of GPS satellites, the GPS Directorate continues to search for the best way forward in getting M-code–capable receivers onto platforms and, especially, into the hands of military personnel in the field.

To address these and other questions regarding the role of GNSS in military affairs and the role of the military in GNSS affairs), we turned to Doug Taggart, president of Overlook Systems Technologies, Inc. Based in Vienna, Virginia, Overlook has a deep grounding in GPS, supporting over the years the Office of the Secretary of Defense (C3I, NII, AT&L and CIO), the Joint Staff, Secretary of the Air Force Acquisition Office, Departments of the Army and Navy, Air Force Space Command Headquarters, 50th Space Wing, and Space and Missile Systems Center (SMC) Global Positioning Systems (GPS) Directorate.

A former radionavigation program manager for the U.S. Coast Guard, Taggart earned a BSEE degree from the U.S. Coast Guard Academy and an M.S. degree in electrical and electronic engineering from Purdue University.

IGM: How would you characterize the role/ status of GPS within overall PNT-related military programs and policies in the medium and long term?

TAGGART: No one reading this magazine needs me to recount the virtues of GPS — it is clearly a critical enabling technology that is threaded throughout almost every facet of DoD’s infrastructure. It has undeniably redefined how the DoD conducts operations. DoD dependence on the enabling capabilities of GPS has awakened an awareness of the value of precise positioning, navigation, and timing (PNT) and illuminated the necessity of ensuring the warfighter has a robust, resilient, and ubiquitous PNT information source.

DoD leadership is aware of the current dependence on GPS to provide the required level of very precise PNT. Fortunately, in parallel with awareness of this dependency, there is now a growing understanding that PNT information must come from more than one source. With that said, I fully expect GPS to remain the cornerstone of DoD’s global PNT information source well into the foreseeable future. However, the rest of the PNT structure needed to fill the gaps and provide for ubiquitous PNT across the full spectrum of military requirements still needs to be developed and implemented.

IGM: What are some of the leading issues associated with the idea of combining GPS with other military or security-oriented GNSS signals and services, such as the Galileo Public Regulated Service?

TAGGART: Untangling the challenges of addressing this question clearly involves technical, operational, policy, and cost issues. It also requires a well-vetted and documented military requirement. I believe that the first significant challenge would be to convince those participating in the DoD’s requirements process that the addition of other GNSS signals would provide distinctive benefits to enhance military PNT capabilities, particularly since those signals share similar vulnerabilities when compared to GPS.

As to the specific question of the forecast Galileo Public Regulated Service (PRS), beyond the issue of having vulnerabilities similar to GPS, it is my sense that technical details of how PRS will be fielded, controlled, and/or made operational are not yet known by the DoD. Until those details are made available by those designing Galileo, it is difficult for me to even begin to speculate how any technical, operational, or policy issues might be resolved.

IGM: What is your assessment of the current status and way forward for M-code user equipment?

TAGGART: Fielding military user equipment has always been a challenge for GPS. It is exacerbated by the fast pace of today’s technology (Moore’s Law), constrained by the rigors of the DoD’s requirements process, and burdened by the economic realities imposed by a military user base that includes new and legacy GPS-enabled systems.

In my view, the Air Force’s plan for fielding M-code user equipment beginning with increment one (platform focused) and following with increment two (personnel and specialized mission areas such as munitions) was driven by the DoD’s requirements process. The plan is complicated by the challenges of designing and managing the M-code security architecture and maintaining a balance between the simplistic “one size fits all” approach and the specialized but unaffordable “every solution is unique” approach.

Although this may be somewhat beyond the scope of the question, an architecture that allows for graduated PNT capabilities needed to meet specific system or mission requirements might allow for signals of opportunity and other PNT information sources to be integrated into military user equipment. This offers a broader collection of capabilities ranging from uniquely civil/commercial sources at one end of the spectrum all the way to sole reliance on military generated and derived (trusted) PNT solutions on the other. But even in this graduated approach to obtaining PNT, the challenges of dealing with how M-code user equipment will be made available are still complicated and could ultimately be more costly based on a decreased overall military market/user base.

IGM: How would you assess the state of civil/ military cooperation in GPS affairs?

TAGGART: My immediate response would be to suggest that cooperation could be better. But characterizing what is meant by saying “could be better” requires that I establish a reference. In the mid 1990s when GPS was declared fully operational (July 17, 1995) and Presidential Decision Directive/National Science and Technology Council (PDD/NSTC- 6) (March 1996) was issued, there was interagency coordination regarding the national goals being pursued. The primary goal was to advance the role of GPS internationally and make it the global standard for U.S. military alliances and civil, commercial, and scientific enterprises. That goal was achieved and represents the high-water mark for civil/ military cooperation.

In comparison, civil/military GPS cooperation today is no longer focused on coordinated national goals but has been fragmented by competing equities among agencies interested in advancing PNT applications based on GPS and other systems. In my opinion, this situation has been created by a national policy that reflects the criticality of PNT to the nation but fails to stimulate agency commitment toward implementing solutions to sustain the level and of civil/ military cooperation that made GPS the success it is today.

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The world’s GNSS systems are entering a phase of transformation — modernization of existing systems (the U.S. Global Positioning System and Russia’s GLONASS) and development of new systems (China’s BeiDou and Europe’s Galileo) that benefit from the lessons learned from the original GNSSs.

Notable among the modernization initiatives is an interest in implementing new satellite signal designs. These include the GPS L5, L2C, and L1C signals as well as those signals designed for Galileo and BeiDou. GLONASS designers are also working on modernized signals.

The world’s GNSS systems are entering a phase of transformation — modernization of existing systems (the U.S. Global Positioning System and Russia’s GLONASS) and development of new systems (China’s BeiDou and Europe’s Galileo) that benefit from the lessons learned from the original GNSSs.

Notable among the modernization initiatives is an interest in implementing new satellite signal designs. These include the GPS L5, L2C, and L1C signals as well as those signals designed for Galileo and BeiDou. GLONASS designers are also working on modernized signals.

In many cases, these new signals adopt or build on features designed and being introduced as part of the modernization of GPS, the first and still most widely used GNSS. Among others, these include such innovations as higher transmit power to improve reception under challenging conditions, longer codes for a better cross-correlation between satellites signals, data-less pilot channels that facilitate long integrations and improve the sensitivity threshold, and secondary codes — short pseudorandom noise (PRN) codes to simplify the data synchronization.

A unique cooperative agreement signed in 2004 between the United States and the European Union calls for common use of a binary offset carrier (BOC) modulation at 1575.42 MHz. Under this agreement, Galileo and GPS system operators (the European Space Agency and the U.S. Air Force, respectively) are implementing two different versions of a multiplexed BOC(6,1,1/11) signal.

Although currently operating as a regional system, the Phase III plan for the BeiDou B1 civil signal also calls for shifting to the L1 frequency centered at 1575.42 MHz and transmitting a multiplex binary offset carrier (MBOC 6,1,1/11) modulation similar to the modernized GPS civil signal (L1C) and the Galileo L1 Open Service signal.

GLONASS says it will introduce CDMA signals at 1575.42 MHz, which has emerged as the common frequency for current and future civil signals, in place of the frequency division multiple access signals currently transmitted at higher frequencies. But the new signals’ specifications, including such parameters as data rate and signal structure, are still under development.

We asked A. J. Van Dierendonck, one of the pioneers in GPS system development with 40 years in the satellite navigation field, to comment on some of the innovations seen in these new signals. Dr. Van Dierendonck is a codeveloper of the L5 signal structure that will be carried by the GPS spacecraft beginning with the Block IIFs now being launched. He also participates in the US/ EU bilateral discussions that take place under the auspices of the 2004 agreement.

IGM: More recent GNSS signals have adopted a variety of features not used in earlier signal designs — such as longer codes, higher data rates, message error detection and control methods, use of pilot channels, multiplexing, and so forth. What do you think have been the most important improvements and what benefits have they brought?

VAN DIERENDONCK: Some of these improvements were introduced first in the L5 signal design, although L2C was the first modernized signal to be implemented. The longer codes reduce the amount of self-code interference and cross-correlation, and thus, increase tracking margin. Acquisition margin is also increased, but with the penalty of the time it takes to search the longer code. Modern receivers will overcome that with the implementation of more correlators or channels.

Higher data rates were not introduced on L2C or L5. In fact, the data rate on L2C is cut in half in order to make up for implementing the pilot channel. (More on that later.) Galileo has increased the data rate on their E1 and E5b channels, but only to provide integrity data (which, so far hasn’t been implemented, and may never be). Higher data rates reduce tracking margin; so, if the higher rates are not needed, it is better that they are not implemented.

GPS has always had message error detection (called message parity), although it is not very strong. Aviation has made up for it by requiring that the message be collected twice (and be in agreement) before it can be used. The error detection (forward error correction or FEC) on the L5 and L2C signals is strong enough without the extra message collection. This error correction only improves the data collection margin, but does not improve tracking margin. FEC was implemented on satellite-based augmentation system (SBAS) signals before being added to GPS.

A pilot channel is a common practice in communication systems to improve signal tracking, but still retain a good data capability. It was introduced into the L5 signal design by Tom Stansell and Charlie Cahn as a quadrature channel. However, pilot channels have limitations. The extra channel steals power from the data channel. In the case of L5, we had the possibility to define signal power requirements, although these were not fully implemented of the Block IIF satellites (–154.9 dBW min instead of –154 dBW min). The desired power will be implemented on the Block III satellites.

The pilot channel does not have the same PRN code as the data channel. Thus, tracking can be completely independent. However, normally pilot-channel tracking is more robust (because of the possible narrower pre-detection bandwidth); so, it can be used to aid data channel tracking.

In the case of L2C, the PRN code on the data channel is a much shorter code to provide faster acquisition. On L1C, the power split is 3/4 on the Pilot and 1/4 on the data channel, for more robust tracking in challenged environments. These challenge environments usually get the data messages via another means, such as via the internet or Bluetooth, and thus, the motivation for more power on the pilot channel. Obviously, this is not possible in aviation applications and via SBAS.

The multiplexed BOC(6,1) signal added at the Galileo and GPS L1 frequency also steals power from those signals. Its purpose was to provide some signal in an additional GPS military Mcode null. GPS L1C does the same thing, providing the same spectrum but implemented differently. For the Galileo design, the (6,1) signal is added to the (BOC1,1) signal and can only be deleted by filtering. In the GPS design, higher rate code bits are multiplexed into the BOC(1,1) code to provide a TDMA version (TMBOC). These extra bits can be blanked with less loss.

Receivers can be designed to ignore the multiplexed signal, but in the case of Galileo, the power in that part of the signal spectrum is also lost. So, I don’t think that the benefit justifies the loss of power and the extra complexity.

IGM: What features in new GNSS signals have helped improve receivers’ performance in the presence of multipath? Of RF interference?

VAN DIERENDONCK: Higher chipping rates are usually associated with longer codes. However, that doesn’t necessarily increase performance in the presence of multipath unless there is also an increase in transmitted (and received) bandwidth. The big advantage of higher chipping rates is that the receivers can be implemented with narrower correlators (in terms of seconds, not chips).

The first receiver implemented that way used the C/A code. That worked so well because the signal transmit bandwidth was 20 megahertz or more, at least 20 times the C/A code chipping rate, allowing a correlator spacing as narrow as 0.05 chips. But that is only half a P-code chip or an L5 chip, Thus, the multipath performance of the P code or the L5 code tracking would be just as good using 1/2 chip spacing.

However, code-tracking performance would be better by the square-root of 2 with the same spacing in terms of seconds. For example, in the presence of noise or interference, code-tracking accuracy would be better using the higher chipping rate, but multipath performance would be about the same given the same receiver bandwidth. This is because multipath performance is based upon chip edge sharpness, whereas noise performance is based more on the square root of chip width.

In other words, not much in the new signals has or will improve multipath performance.

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