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September 1, 2007

The MBOC Modulation

As emphasized in the European Commission (EC) “white paper” on European transport policy for 2010, the European Union (EU) needs an independent satellite navigation system. Galileo is Europe’s contribution to the global navigation satellite system of systems (GNSS) and has committed itself from the very beginning to developing a signal plan that would provide sufficient independence from GPS, while also being compatible and interoperable with it.

As emphasized in the European Commission (EC) “white paper” on European transport policy for 2010, the European Union (EU) needs an independent satellite navigation system. Galileo is Europe’s contribution to the global navigation satellite system of systems (GNSS) and has committed itself from the very beginning to developing a signal plan that would provide sufficient independence from GPS, while also being compatible and interoperable with it.

The historic Agreement on the Promotion, Provision, and Use of Galileo and GPS Satellite-Based Navigation Systems and Related Applications between the United States and the European Commission (EC) signed in 2004, wherein both parties agreed to work together, affected the originally planned Galileo signals but has intensified the cooperation on interoperability and compatibility issues between Galileo and GPS for the maximum benefit of GNSS users worldwide.

The final touch to the Galileo signal plan was achieved in 2006 when the Working Group on GPS and Galileo compatibility and interoperability, under the auspices of the 2004 agreement, finally settled on a new modulation for the common signal in the E1/L1 frequency, namely the multiplex binary offset carrier, or MBOC for short. This decision was pursuant to efforts mainly driven by the European side and fully recognized by the U.S. representatives.

The journey to the signals Galileo has today for its baseline has been tedious and long, but from the outset the journey has followed a consistent logic. At the very beginning, one of the main challenges that Galileo set for itself was to offer three wideband signals, satisfying at the same time the requirements of the mass market and pushing the potential performance of the navigation signals to their natural limits.

This article will try to shed some light on the long process that has led to the signal baseline we have today. Special care will be placed on describing all the modulations of the final Galileo Signal Plan.

(For the rest of this story, please download the complete article using the link above.)

By Inside GNSS

January 14, 2007

GPS: The Way Ahead

On December 4, 2004, George W. Bush signed a National Security Presidential Directive (NSPD) that established the national policy for U.S. Space-Based Positioning, Navigation, and Timing (PNT). The directive updated the 1996 National Policy on the Global Positioning System (GPS). It also provided guidance for the procurement, management, and protection of GPS and its U.S. government operated augmentations.

On December 4, 2004, George W. Bush signed a National Security Presidential Directive (NSPD) that established the national policy for U.S. Space-Based Positioning, Navigation, and Timing (PNT). The directive updated the 1996 National Policy on the Global Positioning System (GPS). It also provided guidance for the procurement, management, and protection of GPS and its U.S. government operated augmentations.

The policy created the National Space-Based Positioning, Navigation and Timing Executive Committee (EXCOM), which was charged with advisory and coordinative roles among federal agencies regarding policies, issues, and initiatives involving GPS and related space-based PNT systems. The article will briefly describe the EXCOM’s role and activities as well as that of the staff organization created to support it.

EXCOM: Members and Staff
The space-based PNT EXCOM is co-chaired by the Hon. Gordon England, the deputy secretary of the Department of Defense (DoD), and his counterpart at the Department of Transportation (DoT), the Hon. Maria Cino. Committee members include representatives at the equivalent level from the departments of state, commerce, and homeland security; the Joint Chiefs of Staff, NASA, and from other departments and agencies as required. The EXCOM met four times in 2006, addressing key issues related to policy implementation, GPS program modernization, international competition, and cooperation.

The National Coordination Office for Space-Based Positioning, Navigation and Timing was established on November 1, 2005, to serve as the administrative secretariat for the National Positioning, Navigation and Timing Executive Committee. The coordination office is staffed by subject matter experts from the member EXCOM organizations.

The Coordination Office monitors the status of policy implementation actions, coordinates outreach initiatives, and manages key EXCOM-directed activities and studies. It has been involved in a wide-range of international, national, and local issues during the past year. Interagency efforts also focused on ensuring the GPS modernization program proceeds on a schedule to meet growing public demands for space-based positioning, navigation and timing services.

External Activities
During the past year, U.S. government departments and agencies initiated a variety of mechanisms to promote international global navigation satellite systems both in a bilateral way and through international multilateral bodies.

Working groups established under the U.S./European Union GPS-Galileo Agreement convened during 2006 to review items of mutual interest and strive to ensure the future availability of easily accessible, accurate, and reliable PNT systems and services for users throughout the world. In March 2006, a bilateral expert group on radio frequency compatibility and interoperability (Working Group A) successfully produced a jointly optimized common signal after 21 months of cooperative effort. Working Group B on trade and civil applications just held its first meeting March 16–17 in Washington, D.C. A joint statement from that discussion is available on the PNT Web site discussed later in this article.

Working Group C, which generally will deal with design and development of next-generation satellite navigation and timing, is expected to hold its first meeting this spring with a particular focus on interoperable safety-of-life services between GPS and Galileo. The fourth working group, which deals with security issues, held its first meeting last year.
In addition, working in cooperation with the United Nations Office of Outer Space Affairs, the United States welcomed the creation of the U.N. International Committee on Global Navigation Satellite Systems (ICG). The ICG will provide a significant opportunity to reach out to developing countries to discuss and interact on technical issues involved in space-based PNT services.
Those activities are anticipated to include providing assistance to developing countries in identifying and eliminating sources of electromagnetic interference with GPS as well as organizing seminars on a variety of GNSS topics such as applications and spectrum protection. The United States also worked with the ICG to establish a Providers’ Forum to promote multilateral engagement with other service provider nations, including Russia (GLONASS); the European Union (Galileo and the European Geostationary Navigation Overlay Service or EGNOS); Japan (the MTSAT Space-based Augmentation System, MSAS); and India (the Indian Radionavigation Satellite System, IRNSS, and the GPS and GEO Augmented Navigation system, GAGAN).

The forum will provide a vehicle to promote multilateral engagement with foreign space-based positioning, navigation, and timing service providers. This forum will provide a means to promote discussion on key issues and principles such as compatibility, interoperability, trade, and business practices.

The U.S. government will maintain an active role in the ICG and the Providers’ Forum in order to develop important alliances with developing countries regarding space-based PNT services.

Public Outreach
A key U.S. policy goal is to promote U.S. technological leadership in applications involving space-based PNT services, particularly the Global Positioning System. To support this goal, the Coordination Office developed and produced brochures for national and international users, decision-makers and the general public. These brochures emphasize the economic, safety, security, scientific, and personal benefits of using GPS.

Brochures and online products produced in English, Spanish, and French provide factual, authoritative information in easy-to-understand formats. The Coordination Office also plans to translate this material into Arabic and other languages, as required. Brochures are available in hardcopy for conferences, meetings and symposia through the Coordination Office.

A newly created Web site provides information about the EXCOM and Coordination Office, PNT policy, the U.S. Space-Based PNT Advisory Board, and frequently asked questions. It also contains public domain presentations for information and field use. Since its inception, more than 70,000 visitors have explored the site’s resources, and the site currently gets about 250 visitors per day. For PNT info, visit <http://www.pnt.gov>.

A second Web site was launched last September to cover GPS applications and serve as a repository for news media and general public information, including the brochures mentioned earlier. The site, which has had 11,500 visitors since it opened, also has links to related online material. To learn more about GPS, visit: <http://www.gps.gov>.

National PNT Advisory Board
The EXCOM established a National Space-Based Positioning, Navigation, and Timing Advisory Board in early 2006. NASA will host and provide administrative support for this group, which will operate solely in an advisory capacity as directed by the National PNT Policy and in accordance with the Federal Advisory Committee Act. The board will advise on U.S. space-based PNT policy, planning, program management, funding, and other pertinent issues.

The board charter was approved by the EXCOM in early 2006 and signed by the NASA Administrator on April 18, 2006. The Coordination Office managed EXCOM member input for study topics and will present them at the board’s first meeting in March 2007.

PNT Coordination Office
The EXCOM has tasked the Coordination Office to work with the DoD National Security Space Office and the DoT Research and Innovative Technology Administration to develop a U.S. space-based PNT architecture. This architecture will help guide future PNT systems’ investment and implementation decisions. The objective is to provide effective and efficient PNT capabilities in the post-2025 timeframe and an evolutionary path for PNT systems and services.

The Coordination Office provides the EXCOM with requested information and assessments of U.S. government space-based PNT programs, policies, budgets, and activities, with a focus on national interests and interdependencies. Agency and department planning includes the development, acquisition, deployment, operation, sustainment, and modernization of U.S. space-based PNT systems, including GPS and its U.S. augmentation.

The Coordination Office gathers data to provide the EXCOM with the required information to develop inclusive national requirements for space-based PNT and to identify interagency program dependencies, such as between the GPS program and the Federal Aviation Administration’s Wide Area Augmentation System.

Looking to the future: 2007 and beyond
In 2007 and beyond, the Coordination Office will continue to evolve in support of the Executive Committee. The Coordination Office will work closely with the departments and agencies that comprise the EXCOM, as they continue to implement U.S. government PNT policies and strategies within their individual areas of responsibility.

During the coming years, the Coordination Office will remain active in PNT symposiums and conferences delivering the U.S. message to international, national, and local audiences. The Coordination Office will report on GPS-related issues and identify opportunities to promote U.S. policy goals through our Web sites, individual presentations, media outlets, and conferences.
Our goal is to focus efforts on making sustained progress toward achieving the goals set by the 2004 National Positioning, Navigation and Timing Policy and maintaining free, uninterrupted public access to GPS and similar space-based PNT services throughout the world.

By

September 2, 2006

Player Pianos, Sex Appeal, and Patent #2,292,387

. . . It is simply too good to be true.

But by now we know that Hedy Lamarr and George Antheil were awarded U.S. Patent No. 2, 292,387 on August 11, 1942 for a "Secret Communication System."

And that, indeed, the two artists invented the "frequency hopping" method of radio signal transmission, later to be known as "spread spectrum."
As they explained in their patent application:

. . . It is simply too good to be true.

But by now we know that Hedy Lamarr and George Antheil were awarded U.S. Patent No. 2, 292,387 on August 11, 1942 for a "Secret Communication System."

And that, indeed, the two artists invented the "frequency hopping" method of radio signal transmission, later to be known as "spread spectrum."
As they explained in their patent application:

“This invention relates broadly to secret communication systems involving the lie of carrier waves of different frequencies and is especially useful in the remote control of dirigible craft, such as torpedoes. . . Briefly, our system as adapted for radio control of a remote craft employs a pair of synchronous records, one at the transmitting station and one at the receiving station, which change the tuning of the transmitting and receiving apparatus from time to time . . . we contemplate employing records of the type used for many years in player pianos, and which consist of long rolls of paper having perforations variously positioned in a plurality of longitudinal rows along the records. In a conventional Player Piano record there may be 88 rows of perforations. And in our system such a record would permit the use of 88 different carrier frequencies, from one to another of which both the transmitting and receiving station would be changed at intervals. . ."

How did it happen?

Hedy Lamarr, born Hedwig Kiesler in 1913 or 1914 to a Vienna banking family, did not study electrical engineering. She studied at Max Reinhardt’s famous Berlin acting school and was the first actress to appear (tastefully) nude in a major motion picture, Ekstase, in 1933.

Hitler’s Germany banned the film because Kiesler was Jewish. The United States banned the film because it was erotic.

That same year, the young actress married a husband 14 years her senior. Austrian “Cartridge King” Fritz Mandl was director general of weapons manufacturer Hirtenberger Patronenfabrik.

The year of his marriage, Mandl was embroiled in a notorious illegal arms transport scandal, the “Hirtenberger Waffenaffaire,” in which his company smuggled German and Austrian weapons out of the country under Swiss labels.

The company made shells and grenades as well as aircraft from the mid- 1930s on and supplied arms for Mussolini’s 1935 invasion of Africa. Mandl was also interested in control systems. He frequently entertained key buyers and sellers of arms — as well as Hitler and Mussolini themselves — during the runup to World War II.

The bright Madame Mandl’s role as hostess provided her link to discussions of weapons technology and, most likely, radio-controlled torpedoes and the need for an anti-jamming device.

Within four years of her unhappy marriage, Madame Mandl had escaped her domineering husband and sailed to the United States, where she eventually became a citizen in 1953.

She continued her acting career for film factory Metro Goldwyn Mayer, whose legendary boss, Louis B. Mayer, renamed her “Lamarr.” His publicity department marketed her as “the most beautiful girl in the world.”

Meanwhile, George Antheil had been composing movie scores in Hollywood since his return from Europe in 1933.

Antheil was born in Trenton, New Jersey in 1900. In the 1920s, he joined the Lost Generation in Paris, where he lived above Sylvia Beach’s famous Left Bank bookstore, Shakespeare & Company. He made a name for himself as a daring composer and concert pianist in Paris and Berlin.

He said of his most famous piece, “Ballet Mechanique is the first piece of music that has been composed OUT OF and FOR machines, ON EARTH.”

The Paris Tribune announced the first performance in 1924, saying, “Paris will hear the strident screech and crash of giant machines evocative of modern industrial America very shortly . . . Ballet Mechanique. . . will be played on four player pianos simultaneously, with electricity as the motive power and a further volume of sound supplied by four electric bells, and two electric motors driving a steel propeller and a wooden rattle. . .

In 1940, the renamed Hedy Lamarr met Antheil at a Hollywood party. The two made a perfect pair — technologically if not romantically.

Lamarr had an idea for an anti-jamming device for radio controlled torpedoes. Antheil’s artistic use of machines foreshadowed the electronic age. A 1990 Forbes magazine article on the pair said, “Antheil understood instantly that synchronizing a series of split-second hops between radio frequencies would be no more difficult — than synchronizing player pianos.”

Lamarr talked about quitting MGM and working for the National Inventor’s Council (NIC), a government agency formed during WWII as a technology transfer link between citizen inventors and the military. She submitted her concept, and the NIC encouraged Lamarr and Antheil to develop it into a patentable idea.

Hedy Lamar filed the patent as Hedwig Kiesler Markey, her name in private life during her marriage to screenwriter Gene Markey. For years, no one made the connection between the patent holder and the movie star.

The War Office immediately classified the new invention, and the patent lapsed 17 years later. Lamarr and Antheil apparently considered it their contribution to the war effort and neither made money from their brilliant idea until 1998, when wireless technology developer Wi-LAN, Inc. acquired a 49 percent claim to the patent from Lamarr for an undisclosed amount of stock.

Antheil continued to compose a number of Hollywood movie scores, among them The Plainsman (1936) and In a Lonely Place (1950). He died in 1959.

Lamarr made many films, most notably Samson and Delilah (1949). In 1997, three years before her death, she was honored by the Electronic Frontier Foundation for her contributions to wireless technology. The EFF is a civil liberties advocacy organization that investigates, among other things, patent abuses.

Today, No. 2,292,387 is considered the foundational patent for spread spectrum technologies.

And the story behind it is every bit as good as “Casablanca.”

This article appeared in the September 2006 issue of Inside GNSS as a sidebar to the Working Papers column on spread spectrum: The Galileo Code and Others

Copyright 2006 Gibbons Media & Research LLC

By

April 1, 2006

Adaptive Antenna Arrays, Multi-GNSS Tropospheric Monitoring, and High-Dynamic Receivers

Q: What is adaptive nulling vs. adaptive beamforming? What are the advantages and disadvantages?

A: Adaptive arrays are perhaps the single most powerful antijamming tool in the GNSS systems engineer’s toolkit. They can provide anywhere from 15 to 90 dB of jamming rejection depending on the specific architecture used. Their main disadvantage is that they require an array of antenna elements, each spaced about four inches apart (center to center), and thus are physically large.

Q: What is adaptive nulling vs. adaptive beamforming? What are the advantages and disadvantages?

A: Adaptive arrays are perhaps the single most powerful antijamming tool in the GNSS systems engineer’s toolkit. They can provide anywhere from 15 to 90 dB of jamming rejection depending on the specific architecture used. Their main disadvantage is that they require an array of antenna elements, each spaced about four inches apart (center to center), and thus are physically large.

Two general types of adaptive array antenna are used with GNSS receivers: single-output nulling antennas and multiple-output beamsteering antennas. Most deployed systems are single-output adaptive nulling antennas that operate as an antijamming appliqué. In this way, a GPS receiver need not know to what type of antenna it is connected, be it a fixed or controlled reception pattern antenna (FRPA or CRPA). New development systems tend to emphasize multiple-output beamsteering antennas because of their better performance. However, in order to handle the multiple output channels, a new receiver is required, too. The trend is to integrate the array processing with the GNSS receiver in a single unit.

(For the rest of Logan Scott’s answer to this question, please download the complete article using the PDF link above.)

Q: How will Galileo benefit the troposphere monitoring community?

A: L-band RF signals experience propagation delays dependent on pressure, temperature, and humidity in the neutral atmosphere. We can measure this effect using GNSS receivers and extract information about atmospheric properties. Of particular interest are atmospheric moisture measurements, because water vapor is an important greenhouse gas and a major factor in weather systems.

Over the past decade, meteorologists have exploited GNSS as an atmospheric remote sensing tool, with applications in weather forecasting and climate change. The availability of Galileo signals, when combined with those from GPS (and/or GLONASS), will enable more accurate estimates of water vapor using ground-based receivers, with higher temporal and spatial resolution. By deploying Galileo receivers onboard low-Earth orbiters, vertical profiles of atmospheric temperature and humidity may be derived with improved accuracy over current GPS-based methods.

(For the rest of Susan Skone’s answer to this question, please download the complete article using the PDF link above.)

Q: Are special GNSS receivers required for high-dynamic applications such as on fighter aircraft?

A: Yes — high dynamic applications do often require special types of GNSS receivers. When designing a GNSS receiver, many tradeoffs need to be made, and certain design decisions are necessary in order to be able to track during high dynamics.

Several different factors can affect the ability of a receiver to maintain lock on the GNSS signal for a given level of dynamics, including the loop filter characteristics, oscillator phase noise, and oscillator vibration sensitivity. Each of these will be explained separately.

(For the rest of John Raquet’s answer to this question, please download the complete article using the PDF link above.)

By Inside GNSS

On Target: Offshore Scoring of Precision Guided Munitions

The development and testing of precision ballistic and guided weapons require the occasional discharge of those weapons. For newer weapons with greater operational envelopes, this often requires testing over very large areas. To meet this growing requirement, weapon testing and training has migrated from land based firing ranges to offshore ranges, such as the U.S. Air Force (USAF) Eglin Joint Offshore Test and Training Area in the Gulf of Mexico near the northern coast of Florida.

The development and testing of precision ballistic and guided weapons require the occasional discharge of those weapons. For newer weapons with greater operational envelopes, this often requires testing over very large areas. To meet this growing requirement, weapon testing and training has migrated from land based firing ranges to offshore ranges, such as the U.S. Air Force (USAF) Eglin Joint Offshore Test and Training Area in the Gulf of Mexico near the northern coast of Florida.

Offshore ranges pose a challenge for determining the location of impact. On land an impact can be sighted and measured after the event, while at sea an impact can only be observed at the moment of occurrence. This observation must also account for absolute position, which is difficult in a marine surface environment without any permanent physical landmarks.

Several methods have been used to determine the location of an offshore impact event. All these methods rely on knowing the location of the observation point and the direction or distance to the observed event. Use of the Global Positioning System (GPS) provides the primary method for determining the location of the observation sensors. Direction can be measured by sighted instruments such as optics or radar, while distances can be measured by active radar and passive underwater acoustic arrival time.

The Tactical Acoustic Realtime Geolocation and Training (TARGT) system, developed by Trident Research LLC, is a distributed floating array that provides an accurate and low cost underwater acoustic method for locating offshore weapon impacts in near-realtime, using a time difference of arrival algorithm. This article describes the engineering development of the TARGT system and the results of deployment of the system during weapon trials on the USAF Eglin Offshore Test and Training Area test range.

Concept of Surface Scoring
The use of underwater acoustics provides several advantages compared to above-water scoring methods. Among these advantages is the ability to operate during the day or at night, in poor environmental conditions including fog and heavy precipitation, and in broad ocean areas regardless of sea state or depth of water. Surface impact scoring relies on detecting the acoustical signal created by the impact. Underwater transducers, or hydrophones, can detect such signals from several kilometers away. An audio circuit then converts the hydrophone output to a digital record that can be further processed.

At a standoff distance of 0.5 kilometer, the impact of a 500-pound weapon generates an acoustical signal that is easily measured by a TARGT Acoustic Geolocation Sensor (AGS). The time of arrival of the signal can be precisely identified and correlated with a time code generated from a GPS receiver.

As an acoustic wave propagates through an array of AGS sensors, each sensor detects the wave, computes a signal arrival time, and relays the time and its position to the shipboard command and control system (CCS). Once the CCS has acquired at least four event-timing messages, it computes time difference of arrival (TDOA) values that are then evaluated in a trilateration algorithm to determine the location of the weapon impact relative to the array of AGS sensors.

Variations in depth, temperature, and salinity may cause fluctuations in the aquatic velocity of sound. However, TARGT is unaffected by these variations as the physical properties of the ocean can be considered homogeneous in the vicinity of the test area. Any minor variations that are present will appear as a consistent bias across all the sensor records and will not affect the least squares fit in the trilateration algorithm. Other factors such as ambient noise, geometric attenuation, and absorption of the signal cannot be neglected.

Experience has shown that by limiting the array size to roughly 2×2 kilometers, an ample acoustic signal budget exists to recognize the weapon impact from the background noise. The TARGT system has also demonstrated the ability to detect the impact of the weapon on the bottom of the sea floor. This can provide an additional benefit for organizations that wish to recover the deployed weapon from the sea floor for diagnostic inspection.

Early Acoustic Systems
The U.S. Navy developed some of the early acoustic-based weapon surface-scoring capabilities in the mid- to late-1970s. These systems, such as the Sonobuoy Missile Impact Location System (SMILS) and the Barking Sands Tactical Underwater Range (BARSTUR), commonly used an array of transducers mounted and surveyed on the sea floor.

These fixed transducers provided a geodetic reference frame for triangulating the location of weapon impacts or of objects equipped with cooperative transducers or “pingers.” By using known sound velocity profiles, these methods produced consistently reliable results with impact position accuracies approaching 10
meters.

With the advent of GPS, significant advances have occurred in acoustic-based scoring, principally in improving accuracy and portability. Two prominent systems developed by the Navy in the mid to late 1990s include an aircraft-deployed broad ocean area scoring system and a ship-deployed version that utilized self-propelled autonomous surface vehicles.

Although both of these systems significantly improved the performance and reduced the cost of weapons scoring, both remain principally data collection systems, providing weapon impact score only after post-mission data processing. The most recent Navy effort has been the Integrated Maritime Portable Acoustic Scoring and Simulator (IMPASS) system, which can provide near-realtime scoring, but cannot meet weapon testing accuracies.

Development of TARGT
The TARGT design team played a major role in the development of two Navy underwater acoustic scoring systems fielded in 1994 and 2000. From this experience, motivation and ideas arose for a more effective system with commercial marketability. By applying state-of-the-art consumer-grade technologies, the team set out to design a system that would exceed the performance of these earlier systems while reducing acquisition, operation, and costs.

Some of the initial design goals for the system included a sensor capable of:

•    realtime scoring at relative GPS positioning accuracies of one-three meters,
•    post-mission scoring with accuracies below one meter,
•    a form factor that allowed for single-person deployment and recovery from small seagoing vessels,
•    an operational duration of 24 hours, and
•    an architecture built upon readily available commercial off the shelf (COTS) components to minimize cost while maximizing flexibility to support multiple applications.

Initially, TARGT began as a cylindrical prototype built from PVC pipe and available COTS components. Circuit designs were developed, built, and tested to precisely time the detection of a munitions acoustic signal, incorporate the timing and positioning data from a GPS receiver, and coordinate the communications and realtime triangulation algorithm for the command and control system (CCS).

By far the greatest challenge in the development of TARGT was precise acoustic timing. For underwater acoustic triangulation, the ability to precisely time the arriving signal is directly proportional to the accuracy to which a score can be produced. With an approximate speed of sound in water of 1,500 meters per second, timing delays of as little as 1 millisecond lead to ranging errors of 1.5 meters.

To meet the accuracy goal of sub-meter accurate weapons scoring, Trident expended significant effort in designing and validating an acoustic detection and processing capability to achieve a roughly 15-microsecond timing accuracy.

Parallel to the sensor development was the development of a realtime CCS built upon a Linux operating system. The CCS provides a means for command and control of each sensor, monitors the AGS array status, and upon receipt of sufficient acoustic event messages, computes the realtime impact position using a trilateration algorithm.

Developmental testing commenced on a component-by-component basis, with the acoustics, GPS, communications, and command and control system software all designed and tested independently in the laboratory and again following system integration. Full system acoustic detection, RF data communications, and impact event localization processing was carried out on the bench using known acoustic source signals, electronics testing equipment, and simulation and modeling software.

Making an Operational System
When system testing was ready to progress to an operational environment, we incorporated batteries, electronics circuitry, and a hydrophone into the prototype sensor housing. The first series of floating tests, conducted in swimming pools and in three nearby lakes, provided performance data on timing precision and accuracy, and on GPS positioning accuracy in an aquatic environment. These tests also served to provide a basis for design upgrades to the sensor hull, buoyancy, metacentric height, and operator deployment and recovery handling.

Having served its purpose, the PVC tube was replaced by a machined aluminum hull tube, with ends fitted for O-rings. Various mechanical configurations for sealing the ends of the tubes were tested. The sensor hull design needed to not only provide water-tight integrity, but also provide penetrations for hydrophone and radio cables, a structural support for buoyancy ballast and an RF antenna mast, and lateral stability to ensure robust GPS tracking and RF communications. Three-dimensional CAD tools were used to model and draft the housing in the latter part of the system development with the final AGS design shown in the accompanying photo.

We then conducted open water tests with the aluminum sensor hulls to verify AGS-sensor RF command and control, acoustic detection, GPS timing and positioning, data transmission reliability, and CCS processing. The open water tests also provided an opportunity to test buoyancy, weight distribution, and flotation collar schemes.

At the end of first-run production testing, we identified and implemented design changes in the areas of acoustic circuitry, RF radio configuration, mechanical hull sealing, and onboard data storage. We integrated programmable gain circuitry into the acoustic electronics board. This new feature allowed us to remotely adjust the acoustic gain in response to local environmental conditions on the day of the test.

Regarding the RF radio, we identified and isolated an internal electronic noise source and optimized radio-modem settings to improve the reliability of data communications. The hull sealing clamps were modified so that they could be mechanically screwed into the base hull to provide greater strength at the upper seal. We also welded the bottom plate to increase hull strength and prevent water intrusion.

We also designed and implemented an on-board data storage capability using microcontrollers, flash memory chips, and an SPI communication routine. This new internal recording capability provides four hours of GPS and acoustic event data recording; a sufficient amount to perform postmission data processing, if needed.

The AGS sensor firmware was modified to provide more functionality and situational awareness at the command and control system (CCS). These modifications included: built-in self test of basic functions, permanent unit identification, transmission time synchronization to mitigate data collision over the air, preset onboard data record timing, battery lifetime checking and report, and memory state (on/off) and remaining data recording availability.

During this refinement effort, the opportunity arose to test the TARGT system during several weapons tests at the Eglin Air Force Base (AFB) Joint Offshore Test and Training Area (OTTA) against the Eglin AFB Instrumented Target System (ITS). The deployment and testing of TARGT during actual weapons tests provided not only a realistic operational environment for validating the system, but also benefited the Air Force by providing an additional independent means of weapon scoring.

Field Deployment of the ITS
Flight test preparations for the OTTA missions include preparation and deployment of multiple sensor systems, including the USAF 46th Test Wing’s Instrument Barge (IB) and ITS. The IB is a 40×140-foot, four-point moored platform for supporting multiple high-speed digital video and data cameras, high accuracy GPS receivers, and RF communications modems. The IB also hosts the command and control and communications networks essential for remote ITS management after the range is cleared of personnel.

The ITS is a smaller moored target platform equipped with multiple GPS antennas for determining the precise attitude and location of the platform relative to the Eglin AFB GPS reference tower on Santa Rosa Island, Florida. The ITS is also equipped with replaceable deck witness panels, which provide a definitive physical impact point on the platform.

In determining the exact truth coordinates, the x-y distance of the weapon impact point on the witness panel are manually measured relative to a fixed GPS antenna location on the ITS platform. We account for the platform orientation by applying a three-dimensional orientation correction using GPS-derived attitude information. The ITS GPS antennas’ positions are then determined through carrier phase–based differential GPS (DGPS) processing relative to the ground reference tower on Santa Rosa Island.

Dual-frequency, 24-channel all-in-view GPS receivers, and survey grade choke-ring antennas, provide for sub-meter accurate positioning of the ITS over the 21 mile (34 km) baseline. DGPS postprocessing of the mission data set produced forward and backward solutions that we combined in a weighted solution to provide the best estimate of position. Postprocessed, absolute accuracy is estimated to be 20 centimeters (1 sigma).

Field Deployment of TARGT
In preparation for field operations, each TARGT AGS sensor is assembled on the deck of the support vessel prior to deployment. Assembly and checkout procedures include installation of the RF antenna mast, preparation of the mooring assembly, and a communications, GPS tracking, and acoustic check of each sensor from the shipboard CCS to ensure that all sensors are operating nominally. The relatively small size and weight of the AGS sensor allows for deployment by a single operator from almost any size ocean vessel.

For OTTA missions to date, the deployment vessel has been a 30–40 foot chartered fishing or sight-seeing boat. Deployment of six to eight AGS sensors requires approximately 40 minutes, regardless of sea state condition. Sensors are typically placed around the impact site at a range of 200 to 500 meters from the intended impact point to ensure good geometry for scoring. Good geometry is defined as sensor placement providing a horizontal dilution of precision (HDOP) of less than one, and providing direct-path acoustic signal reception. Placement of sensors in a specific location is not required.

During operation each AGS sensor periodically reports its status and position information, allowing the CCS-equipped support vessel to monitor the status of the AGS array during the mission. The AGS information is also used to compute range and bearing to each sensor to facilitate recovery. Upon detection of an acoustic event greater than observed ambient noise conditions, each sensor immediately reports the event time to the CCS.

Once four or more event messages are received, the CCS computes a realtime two-dimensional geodetic position and estimated error ellipse for each event. Events spaced more than 0.5 seconds apart can be individually determined in realtime by the system. Following weapon release and impact, the support vessel returns to the test area and commences recovery of the deployed AGS sensors. Sensor recovery is typically performed by a single operator in approximately 45 minutes; however, additional time has been required under severe conditions (greater than sea state 4).

We perform postmission DGPS processing that combines dual-frequency receiver data with the AGS sensors’ 12-channel single-frequency receivers’ GPS data to produce a precise relative position vector for each AGS sensor at the time of impact. These improved sensor positions are used to reproduce a refined impact score for the weapon strike. This process results in an absolute WGS84 geodetic impact score relative to the Eglin AFB Santa Rosa Island reference tower.

Scoring Results
On March 12 and May 25, 2004, three precision weapons were deployed in OTTA missions approximately 22 miles south of Destin, Florida. The ITS truth and TARGT systems were successfully deployed for both operations. In each mission the TARGT system successfully produced realtime impact scores within 5 seconds after the weapons struck the water. The accuracies of the realtime scores were within 5 meters (2D RMS, 1 sigma) of ITS-determined truth for all three weapons.

The post-mission score for each weapon was produced within 24 hours of system recovery. Following report of the post-mission TARGT scores to the 46th Test Wing, the final scores produced by the ITS truth system were provided to Trident Research. The post-mission scores produced by the TARGT system indicated accuracies within 3.7 meters of truth for all three weapons, with a mean accuracy of 2.47 meters (2D RMS, 1 sigma).

On November 15, 2005, the TARGT system was deployed in support of a three-weapon mission at the OTTA. One of the principal goals for this mission was to determine if the system could detect the impact of a weapon on the sea floor. The ability to time the sea floor impact would provide a position and estimate of the velocity of the weapon upon sea floor impact.

These two quantities provide an ability to determine the likelihood that the weapon survived the impact, and a location to assist divers in possible recovery of the weapon for diagnostic examination or for environmental remediation. In addition, if both surface and sea floor impact strikes are accurately positioned, the entry angle of a weapon through the water may be approximated. Residual errors would include unknown variables such as surface deflection and variations in subsurface trajectory caused by asymmetrical body damage at surface impact.

In post-mission processing of the digitized acoustic files, compelling evidence indicated that the weapon strikes on the sea floor were detected with sufficient fidelity to time the accuracy to within five milliseconds. Sea floor impact times ranging from 135 to 165 milliseconds after surface strikes were observed from multiple sensor positions by multiple depth hydrophones. Additional testing is planned to refine the processing and analysis methods for a ‘turn key’ sea floor impact scoring capability for the TARGT system.

A second objective of the November 15 mission was to determine how accurately TARGT sensors could be positioned using differential carrier phase–based processing. The predicted TARGT error budget indicates that sensor positioning errors as high as 60 centimeters (2D RMS, 1 sigma) would result in an impact score with an estimated error of less than 1 meter. To achieve this objective, TARGT sensors were equipped with versions of the single-frequency 12-channel GPS receivers capable of carrier-phase tracking.

Postmission processing of the sensor GPS data demonstrated positioning accuracies ranging from 30 to 40 centimeters (1 sigma) during the mission window. This result successfully validated the GPS portion of the error budget for sub-meter TARGT scoring. The next planned OTTA mission will be the final validation test for TARGT and will demonstrate the ability to attain a level of accuracy never before achieved in underwater acoustic-based weapon scoring.

Conclusion
The TARGT system has met or exceeded the initial design objectives. The acquisition cost is sufficiently low that each sensor can be considered an expendable item when retrieval is deemed too expensive or dangerous. The size of a TARGT sensor supports deployment and retrieval by one person. Internally, the electronics of the system are modular and designed with an open architecture approach; so, improvements and modifications can be rapidly integrated. Three separate military weapons tests have proven the success of the design and have provided useful data as to the systems reliability and performance.

Future plans for the TARGT system will depend on mission requirements, but a minimum objective is to validate weapon surface impact scoring with submeter accuracy and sea floor impact scoring. Additional modifications are being reviewed to evaluate the possibility for more advanced acoustic processing for such applications as mammal detection for range clearance and swimmer detection for harbor security.

Trident Research has also integrated robust satellite communications and autonomous stationkeeping into other marine instrumentation and is reviewing the adaptation of these capabilities into the TARGT system so that larger, relatively fixed networks of sensors can be operated and controlled from anywhere on the globe. In addition, advances in other remote sensing technologies, such as chemical monitoring, short range radar, and video imaging are being investigated with the intent of expanding the utility of TARGT.

The design team at Trident Research is well versed in these technologies and is looking forward to aiding weapons system designers and developers as they continue the migration of precision munition testing to the sea.

For figures, graphs, and images, please download the PDF of the article, above.

By Inside GNSS