Survey and Mapping

Precise Point Positioning and Its Challenges, Aided-GNSS and Signal Tracking

Q: What is precise point positioning (PPP), and what are its requirements, advantages and challenges?

A: Precise point positioning (PPP) is a method that performs precise position determination using a single GPS receiver.

This positioning approach arose from the advent of widely available precise GPS orbit and clock data products with centimeter accuracy. These data can be applied to substantially reduce the errors in GPS satellite orbits and clocks, two of the most significant error sources in GPS positioning.

Q: What is precise point positioning (PPP), and what are its requirements, advantages and challenges?

A: Precise point positioning (PPP) is a method that performs precise position determination using a single GPS receiver.

This positioning approach arose from the advent of widely available precise GPS orbit and clock data products with centimeter accuracy. These data can be applied to substantially reduce the errors in GPS satellite orbits and clocks, two of the most significant error sources in GPS positioning.

Combining precise satellite positions and clocks with a dual-frequency GPS receiver (to remove the first order effect of the ionosphere), PPP is able to provide position solutions at centimeter to decimeter level, which is appealing to many applications such as airborne mapping. PPP is different from double-difference RTK (real-time kinematic) positioning that requires access to observations from one or more base stations with known coordinates. The word “precise” is also used to distinguish it from the conventional point positioning techniques that use only code or phase-smoothed code as the principal observable for position determination.

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

Q: Does Aided-GNSS improve signal acquisition, tracking, or both?

A: A-GPS (Aided/Assisted-GPS) and more recently its extensions, A-GNSS, have been introduced to substitute for missing satellite broadcast data when access is intermittent, difficult, or impossible due to signal obstruction. It has expanded the capabilities of the traditional receiver in reducing the time to first fix (TTFF), enabling “high sensitivity” modes, improving the performance in urban canyons and indoors, and incidentally, boosting the receiver’s efficient use of power.

Multiple ways have been developed to deploy an A-GPS server, and to distribute the process flow between the server and the mobile. The mobile station–based method places the position determination in the receiver, while the network-based method relegates it back to the server.

Other notable factors influence the architecture. The assistance can be one-way, where information accessible at the server flows down to the receiver, or in closed loop, where the information is uploaded to the server, processed remotely applying far larger computational resources and extra knowledge not available to the receiver, and then pushed back to the receiver in its final form.

We will now introduce two simple rules that will illustrate the rest of the explanations:

Rule 1: For A-GPS to be practical, the assistance information should not be stale when ready to be used at the mobile. In more technical terms, the assistance information persistence needs to be longer than the sum of network latency plus server and mobile processing times,

Rule 2: In a closed loop architecture, the information collected at the mobile, and processed at the server should be returned fast enough before the internal state of the mobile changes too much. We can reformulate it as the sum of the round trip network delay plus server and mobile processing times have to be shorter than the process time constant to control the mobile.

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

A New Version of the RTCM SC-106 Standard, the Probability of Solving Integer Ambiguities

Q: The RTCM has announced a new version of its widely used differential GPS (DGPS) standard. Why did the group decide a new standard — Version 3 — was needed, and what are the benefits compared to Version 2?

Q: The RTCM has announced a new version of its widely used differential GPS (DGPS) standard. Why did the group decide a new standard — Version 3 — was needed, and what are the benefits compared to Version 2?

A: Initially, the RTCM (Radio Technical Commission for Maritime Services) Subcommittee (SC104) Version 2 standard was developed with marine DGPS as the target application. One of the design goals for the RTCM SC 104 standard was to have correctional information readily available for user equipment. The outlines of the messages were specifically tailored for low bit rate data links.

Not until the beginning of the 1990s did RTK (real-time kinematic) surveying applications come into focus for RTCM. The committee drafted new RTK messages based on the proven DGPS messages. Although the DGPS messages only contain corrections, the RTK messages also allow transmission of raw observables from the satellite signals. RTCM tentatively published redundant means of disseminating precise RTK information with the aim of gaining practical experience during their implementation.

The first implementations by different manufacturers had diverse interoperability issues. For instance, various manufacturers have different sign conventions for representing the carrier phase observations, which resulted in incompatibilities when mixing receivers of different manufacturers.

(For the rest of Dr. Hans-Jürgen Euler’s answer to this question, please download the complete article using the PDF link above.)

Q: What is the probability of correctly resolving integer ambiguities and how can it be evaluated?

A: Resolving, or “fixing”, carrier phase ambiguities to integer values is ultimately based on statistical assumptions and testing. As such, a probability is associated with resolving any particular ambiguity correctly. Evaluating the probability of correct fix (PCF), that is, the probability that the ambiguities are fixed to the correct integer values, is particularly important for safety-critical applications where an incorrect ambiguity fix would produce hazardously misleading information (HMI).

In fixed-ambiguity carrier phase processing, the usual procedure is to begin by estimating the carrier phase ambiguities as real-valued (“float”) parameters and then to determine their integer values. A difficulty with this method is that, although the least-squares adjustment or Kalman filter used to estimate the real-valued ambiguities provides an estimate of their quality (a covariance matrix), it is not obvious how to obtain an estimate of the quality of the integer ambiguities.

In most carrier phase ambiguity estimation methods, integer quality is validated using some sort of statistical test. These generally involve testing the least-squares sum-squared residuals of the best fitting integer solution against the second best fitting solution. The test statistic is then compared against a threshold value, the idea being that if the best solution is sufficiently better than the second-best solution, then it must be correct.

(For the rest of Kyle O’Keefe and Mark Petovello’s answer to this question, please download the complete article using the PDF link above.)

GPS + GLONASS for Precision

The SC Geodetic Survey (SCGS) has combined the technologies of the GPS, GLONASS, cellular communications and high-speed server networks to provide centimeter-level accuracy in real-time for surveying, mapping, and engineering applications.

The SC Geodetic Survey (SCGS) has combined the technologies of the GPS, GLONASS, cellular communications and high-speed server networks to provide centimeter-level accuracy in real-time for surveying, mapping, and engineering applications.

Named the SC Virtual Reference Station (VRS*) Network, the system is composed of 45 global navigation satellite system (GNSS) receivers installed statewide and connected by high-speed Internet to servers in the state capital, Columbia. Users connect in the field via cellular digital data communications to access the servers and obtain near real-time custom corrections to position objects or automate vehicle operations.

The South Carolina Department of Transportation has partnered with the SCGS with the intention of using the VRS for machine control to automate highway construction. South Carolina is the only state in the nation to use this technology to include the Russian GLONASS satellites as well as GPS satellites for a more robust solution.

Important to the implementation of the VRS is the provision of a common and consistent connection to the North American Datum NAD83 (2007) via the South Carolina State Plane Coordinate System. All coordinates produced through the use of VRS can be directly tied to NAD83 (2007). Surveyors and engineers will no longer need to be concerned about datum issues and coordinate conversions.

This article will describe how SCGS, which operates within the state Budget & Control Board’s Office of Research and Statistics, designed, implemented, tested, and operates the GNSS VRS network today.

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

Ruth Neilan: The Global Grid Master

Ruth Neilan outside the history library at the GeoForschungsZentrum in Potsdam, Germany.

When Ruth Neilan was named director of what is now known as the Central Bureau of the International GNSS Service (IGS), she had an immense undertaking before her.

A voluntary civilian federation, the IGS compiles and analyzes GPS (and more recently, GLONASS) satellite data. From these, the IGS creates highly accurate products —such as precise satellite orbit and clock files — and makes them freely available to engineers, scientists, and researchers all over the world.

When Ruth Neilan was named director of what is now known as the Central Bureau of the International GNSS Service (IGS), she had an immense undertaking before her.

A voluntary civilian federation, the IGS compiles and analyzes GPS (and more recently, GLONASS) satellite data. From these, the IGS creates highly accurate products —such as precise satellite orbit and clock files — and makes them freely available to engineers, scientists, and researchers all over the world.

The latter folks use the IGS data to improve the accuracy of their own GNSS positioning and timing results based on observations from the same set of satellites, using IGS products in place of the broadcast data.

Originally known as the International GPS Service for Geodyanmics, the standardized global tracking network was initiated by NASA and NOAA in the late 1980s. Today, the IGS Central Bureau is managed by NASA’s Jet Propulsion Laboratory (JPL) at Caltech in Pasadena, California, where Neilan has worked for nearly 25 years. IGS has 200 participating organizations —mostly public, government, and research agencies — with upwards of 400 permanent ground stations and data and analysis centers in more than 80 countries.

But in the early 1990s, all of that was far in the future. Neilan and the IGS had to create the building blocks themselves: setting standards, agreeing upon specific formats for data collection and processing, deciding how much to log to guarantee the precise results they needed. They succeeded in great part because of Neilan’s passion and optimism that GNSS technologies could — and do — bridge geopolitical boundaries.

“Through IGS, developing countries can join an international effort. People are very enthusiastic about contributing,” she said. “Off-the-shelf products have developed to such a point that they can leapfrog into the highest technology that’s available. The difficulty we have is getting enough resources to put their efforts on solid ground and ensure sustainability.”

Never Say “Never”

Neilan’s internationalist bent was established early in life.

She recalls crawling beneath the drafting tables in her father’s engineering firm in Somerset, a small Appalachian town in southwest Pennsylvania, and losing herself in books. She especially like the one with fascinating photographs of Asia and, in fact, went on to study Mandarin Chinese for five years.

As a child, she was convinced that she would “never” be an engineer. But blessed with a surefire sense of direction that she calls “Zen navigation,” Neilan loved reading maps and making precise measurements.

At college, she gravitated to The Pennsylvania State University’s engineering technology program, earned an associate’s degree, and became a surveyor. But she still wasn’t convinced that engineering should be her life’s work, so she took a detour.

A two-year globe-spanning tour started her on the path that combined her passion — Asia and the world — with what turned out to be her calling: engineering and the development of GNSS.

On her sojourn Neilan crossed Turkey and Afghanistan, worked as English editor for a Taipei magazine, climbed to the Mount Everest base camp, and attained an altitude of 19,200 feet— without oxygen — while crossing into the Rowalling Valley along the Tibetan border. Along the way, Neilan realized she was good at engineering.

She returned to the United States to earn a bachelor’s degree in civil and environmental engineering plus a minor in Asian studies, graduating with distinction from the University of Wisconsin at Madison in 1983.

Although the buzz about GPS began perking in the early 1980s, nothing was taught at the university level. After graduating, Neilan visited a friend at the Jet Propulsion Laboratory at California Institute of Technology in Pasadena, California. Hoping for nothing more than ideas for a thesis topic, Neilan arrived wearing flip-flops and shorts. She met with several people working on GPS and, by the end of the day, she had a job.

She put herself on what she laughingly refers to as the first in a series of “five-year plans,” working for JPL while pursuing her master’s degree. She finished her thesis, “An Experimental Investigation of the Effect of GPS Satellite Multipath,” in 1986.

Growing the Global Grid

Neilan’s first major project out of graduate school put her at the hub of the emerging GPS infrastructure. JPL assigned her to get the Deep Space Network’s first GPS receivers and meteorological instrumentation up and running. As that project unfolded, she also managed seminal projects measuring crustal deformation, tectonic motion, and earthquake fault monitoring using GPS techniques.

Starting in 1990, a planning group of five leaders — including Neilan and her mentor at JPL, Bill Melbourne — began meeting to plan the way forward for a global network. Neilan led implementation and operation of ground data systems for the GPS Ground Tracking Network. At about the same time, she also took on the separate task of coordinating the sub-network of six GPS tracking stations required for mission support of the GPS precise orbit determination experiment flown on the satellite TOPEX/Poseidon.

In 1992, she became GPS Operations Manager for the NASA/JPL global network and for scientific support of regional experiments, overseeing project management and technical direction of field engineering for NASA scientists and geodetic tasks at the University NAVSTAR Consortium (UNAVCO) in Boulder, Colorado.

One year later, she was named director of IGS.

The Global Grid and Beyond

Neilan’s serves on the advisory board of the U.S. Positioning, Navigation and Timing Executive Committee, which addresses such issues as policy, planning, management, services, capabilities, and funding.

“This board provides an additional assurance that there is a voice for users,” Neilan says. “The board includes international people, which emphasizes the global nature of GPS.” It also underscores the value that the executive committee places on the international community as part of the process, she adds

Since 2005 she also has served as vice chair of the Global Geodetic Observing System, which provides continuous, precise observations of the three fundamental geodetic observables and their variations: the Earth’s shape, gravity field, and rotational motion. She says a stable, sustainable global reference frame is crucial for all Earth observation and for practical applications ranging from agriculture to the dynamics of atmosphere and the oceans.

The system is beginning to help scientists get their arms around the complexities of seismic activity and climate variation. “Natural hazard detection and mitigation is a really important use of GPS and continuous networking,” Neilan says. “Less than a day after the earthquake that triggered the tsunamis in the Indian Ocean in 2004, we could see that our IGS station in Singapore had moved almost an inch. Our IGS stations in India also had moved.”

Neilan’s zest for travel makes her an ideal fit for her job, which requires frequent trips to developing countries. She lights up when talking about bringing Africa’s 50-plus nations into the grid. This effort, known as AFREF (which stands for unification of African Reference Frames) includes vertical data as well as gravity observations. However, she emphasizes that planning on a continental scale does not have to be “top down or all at once.” Instead, she focuses on assisting newcomers put in GPS systems that meet standards and follow conventions used by the rest of the world.

If Neilan had just one magic GNSS wish, it would that everyone understood the importance of tying into the global grid, known officially as the International Terrestrial Reference Frame. “It’s easy to remotely sense an area, but often contractors or consultants set up their own little local reference system,” she explains. “Then, when they try to link up or extend their project, it has no relationship to the country’s grid, much less the international grid. It’s been very hard to get this across to the mapping and GIS people.”

NASA’s return to space exploration opens up the possibility of extending GPS-like constellations to the Moon and to Mars. “We need to have a way of commonly and seamlessly referencing all the vehicles,” Neilan says. “An extended coordinate timing system would reduce errors.”

Neilan’s daily contacts with colleagues around the globe reinforce her optimism that GNSS technologies can bridge geopolitical boundaries.

“Open availability [of data] is seeding so much innovation and fostering better understanding of our world,” she says. “IGS is the global sandbox. Everybody can have fun and play.”

Neilan’s coordinates:
34° 12′ 5.7" N
118° 10′ 27.47" W
h = 372 meters

Ruth Neilan’s Many GNSS Hats

• Vice Chair, Global Geodetic Observing System (GGOS) since 2005
• Director, Central Bureau of the International GNSS Service (IGS), since 1993
• Advisory board, US Positioning, Navigation and Timing (PNT) Executive Committee
• Ad Hoc Strategic Committee on Information and Data (SCID), International Council for Science (ICSU)
• International Committee on GNSS (ICG), representing the International Association of Geodesy (IAG)
• Executive Committee, International Association of Geodesy
• IGS website http://igscb.jpl.nasa.gov

COMPASS POINTS

Engineering Specialties
Surveyor, geodetic surveying, and civil and environmental engineering.

GNSS Mentor
Bill Melbourne at the Jet Propulsion Laboratory, a visionary who’s many accomplishments included leading GPS technology developments at JPL. “His support in shaping the GPS global network and working with our international partners laid the foundation for the IGS.”

Favorite Equation
Geoid Separation H=h-N

Heights that are given from GPS (h) are relative to the GPS ellipsoid WGS84. “To get the vertical position of a point, the separation between the geoid and the ellipsoid (N) must be known — with care. You can think of the geoid as the surface of the earth approximated by the mean sea level.”

Her Compass Points
Neilan credits her family first – her “brilliant” jazz pianist husband, “terrific” kids, and parents who “are always there, even for GPS observations in American Samoa, Easter Island, and remote areas of Mexico.” Her other compass points are “the wonderful IGS/GNSS community of colleagues and friends,” and “The great game — soccer!”

Fell in love with GPS when . . .
. . . she realized that this revolutionary technology could provide precise position and navigation aids for anyone, anywhere, anytime. “(This) levels the playing field to an extent — especially in the developing countries.”

Knew GNSS had arrived when. . .
. . . CASA UNO ’88 deployed the first civilian global tracking network for orbit improvements to monitor crustal deformation at many stations in Central and South America. “This was the first large-scale study of crustal deformation. Now it is done continuously for hundreds of stations around the globe.”

Influences of Engineering on her Private Life
“Time motion studies in the kitchen!”

Popular Notions about GNSS that Most Annoy
“That GPS will operate according to specifications when actually it is far, far better than that. We need to develop the notion of performance-based capabilities and delivery of services.”

What’s next?
For Neilan and IGS, this includes integrating the upcoming signals from GALILEO, COMPASS and other new GNSSes into the mix. They are aiming for seamless incorporation to take advantage of the signals “so that we can use this technology over several decades in order to better understand our changing world.” And, as always, IGS’s on-going effort to promote dialog and a forum for the international use of GNSS.

Human Engineering is a regular feature that highlights some of the personalities behind the technologies, products, and programs of the GNSS community. We welcome readers’ recommendations for future profiles. Contact Glen Gibbons, glen@insidegnss.com.