Our esteemed editor-in-chief and publisher at GPS World, Alan Cameron, penned an editorial in January concerning claims made by the People’s Republic of China regarding the Gold Standard for PNT (position, navigation and timing). The Chinese recently claimed that its BeiDou system averaged a user range error (URE) of 2.5 meters using zero age of data (ZAOD), 95% of the time.
Alan correctly made the point that today BeiDou is strictly a regional system, and that while the published and arcane (30-year-old) standard for GPS is 6 meters under the same conditions, this is merely a standard, a never-to-exceed boundary, and not an actual URE measurement. GPS has always provided significantly better than 6 meters accuracy, with a reasonable age of data, while the GPS numbers for URE have significantly improved on a consistent basis since 1978 and today are the best in the world for any global PNT system.
Dr. Bradford Parkinson, the father of GPS, after reviewing the Chinese data pointed out that, “ If a GNNS has full view and an immediate update (such as Compass [BeiDou]) they can drive the AOD down, effectively becoming a WAAS system. This result would not represent a global capability. Plus, there are other errors for a single-frequency receiver in addition to the ionosphere (that is calibrated by WAAS and EGNOS), including troposphere modeling errors, and multipath that drive the ranging error up for a civil user depending on the situation.”
The civilian version of the GPS statistics and accuracy parameters for a single-frequency GPS civilian receiver can be found at http://www.nstb.tc.faa.gov/DisplayGPSReportCardArchive.htm.
Civil Report Card on GPS Performance – Accuracy Parameters
RMS Single Frequency User Range Error
CY 2012 2.00 meters
Sept 2013 1.89 meters
Oct 2013 2.31 meters
CY 2012 2.38 meters
Sept 2013 2.26 meters
Oct 2013 3.30 meters
This data is very useful for GDOP (Geometric Dilution of Precision) statistics, which are quite surprising – and come about because of the 30+ GPS satellites in view and the resulting excellent geometry available.
The public data clearly shows that the GPS system is every bit as accurate, and indeed comparatively nominally much more accurate, than BeiDou, and GPS covers the entire globe, not just an area over China and portions of Australia.
It All Starts Here — GPS SIS URE
The GPS accuracy equation begins with the signal in space (SIS). Since 1978, the SIS figures for GPS satellites have continuously improved, as I said primarily due to more accurate orbit determination and more stable atomic reference systems, while the GPS URE numbers have continued to decline. Which is a good thing – smaller URE numbers are better.
Indeed, this clearly explains, in my opinion, why SVN49, which is a perfectly healthy GPS satellite, has never been set to healthy status. While the SVN49 GPS signals are all well within the published 6 meter URE – a never exceed threshold – they are significantly greater than 2 meters. Accuracy matters with GPS, so until corrections can be made, the satellite will remain in test status. Today, it serves as a very useful orbiting GPS test bed but does not enter into the SIS or URE equation.
GPS SIS URE is best explained as the pseudo-range inaccuracy due to ephemeris (orbit) and clock (atomic reference system) errors, which are common to all modern space PNT systems. The SIS root-mean-square (RMS) URE for GPS has been steadily declining over time (smaller numbers are better) and, consequently, so have the user range errors for users on the Earth. However, for my technical readers and space physics buffs, SIS errors are not determined by simple equations and therefore are sometimes difficult to describe accurately because they are neither purely stochastic nor deterministic. Indeed, Ph.D.-level subject matter experts such as Liang Heng, Grace Xingxin Gao, Todd Walter, Sherman Lo and Per Enge, from Stanford University, have clearly shown that SIS errors do not necessarily have a normal distribution Also, the traditional statistics such as sample mean and sample standard deviation may be affected by extreme excursions or outliers. However, these deviations do not significantly affect URE for most users, as they are effectively smoothed by long-term trend analyses and an active Kalman filter.
Certainly, better atomic reference systems with frequency stabilities on the order of 1×10-E13 or better are partially responsible for these improvements, since one nanometer of clock stability typically equals one foot of position accuracy on the Earth’s surface. That number is important because I clearly remember the day in 1990 at the 1CACS (1st Command and Control Squadron) in Cheyenne Mountain (the 1 CACS is now located at Vandenberg AFB in California), when it was explained that the nominal ephemeris tracking error via optical systems for GPS satellites was on the order of two kilometers. The 1 CACS was responsible for providing collision-avoidance support during NASA shuttle missions and is still responsible for maintaining an extensive space satellite and space object catalog. Today, that error, using different tracking methods — including a global network of dual-frequency GPS tracking and monitoring sites — for GPS SVs approaches two centimeters or better. Consequently, better (more stable) clocks and more precise orbit determinations have greatly reduced the signal-in-space errors and significantly improved the position accuracy for all GPS users on a global scale. And for me that is the crux of the issue for GPS versus any other space-borne PNT system in existence today, or for any system in the near future.
A Global System
GPS is and has always been a global system, since its inception (1973) 41 years ago this year and since President Reagan decreed on September 16, 1983, just 15 days after Korean Air Flight 007 was tragically shot down by fighter aircraft from the Soviet Union (there were four other similar tragedies involving the Soviet Union on record) for being off course, that the Global Positioning System would be a gift from the United States to the world for precise navigation, so that this type of disaster need never happen again. Since that time GPS has been a truly global system for all users, friend or foe, without distinction. Of course longevity and dependability are merely two of the important factors that makes GPS the PNT Gold Standard.
GPS Stands Alone
I do not intend nor do I need to defend GPS as the global Gold Standard for PNT, the figures speak for themselves, however I do feel that the words Gold Standard, as I and many other subject matter experts, interpret them, may need a bit of an explanation.
One of my professional colleagues and a dear friend, for more years than I care to count, and I have long disagreed on this terminology. He feels the term Gold Standard is easily misinterpreted and should not be applied to GPS simply because it is not always well understood; instead he prefers the term system of first choice. However, that just does not have the same ring or historical significance as the Gold Standard.
What is a Gold Standard?
Leaving aside the monetary or financial implications for our PNT purposes, a Gold Standard is defined as the best one or the very best example of its kind — with synonyms such as: a system benchmark, a yardstick, a touchstone, the criterion, a paradigm and the epitome. Add to these descriptors the sense of longevity, endurance, dependability, and quality the GPS engenders among users — and you may be approaching the true sense of the phrase “Global PNT Gold Standard.” I can say unequivocally that the GPS is the only space-based PNT system in existence today that meets all these exacting and more fluid definitions simultaneously.
The Global Positioning System has had a continuous on-orbit presence since the second NRL Test and Development satellite was launched in 1977. GPS achieved IOC or Initial Operating Capability with 24 SVs (satellite vehicles) on December 8, 1993 (2SOPS celebrated the 20th anniversary of GPS IOC in December 2013). GPS FOC or Full Operational Capability was achieved on April 27, 1995, just 16 months later. Since that date, the GPS has never been less than fully operational, providing both the military Precise Positioning Service (PPS) and the civil Standard Positioning Service (SPS) to global users. As the staff writers at GPS Daily stated in a recent anniversary article:
Amazingly, though many Navstar satellites have been launched and been decommissioned over the past 20 years, four of the original Block IIA satellites which made up the IOC constellation (SVN-23,SVN-26, SVN-34, and SVN-39) are still operating and providing reliable PNT services as of this 20th Anniversary of IOC.
GPS has grown to become a vital worldwide utility serving billions of users around the globe. GPS multi-use PNT services are integral to the United States global security, economy, and transportation safety, and are a critical part of our national infrastructure. GPS contributes vital capabilities to our nation’s military operations, emergency response, agriculture, aviation, maritime, roads and highways, surveying and mapping, and telecommunications industries, as well as recreational activities.
It is not an overstatement to say GPS is fundamental to today’s technical infrastructure and culture. GPS provides the ‘winning edge’ to our warfighters and allies by delivering premier space-based PNT services to the nation and the world.
This indeed supports the definition, as I see it, of a Gold Standard for global PNT. A system that is long-lived, dependable, and just keeps improving every day. A ubiquitous utility that has changed the world we live in and the way we live our lives for the better, a system that now defines not only the critical national infrastructure of the United States but of many nations around the globe.
As for GLONASS, Galileo and BeiDou, we can have this discussion again in 20 years or so when they have been IOC and FOC for a credible period of time and have proven their accuracy, longevity and utility. For now, there is only one Gold Standard and that is the Global Positioning System.
What Is Don Reading?
This month, my reading preferences centered around mythical and real life figures in the CIA or Central Intelligence Agency. And frankly, sometimes it was hard to tell the difference.
I devoured this 740-page tome in one weekend and was looking for more when I finally finished. This is one of those books you don’t want to end. It describes the life of the young Jack Ryan as a CIA operative during the Cold War, and of his son, Jack Jr., today. The authors manage the timeline to and fro adroitly so that it is never an issue. As usual, the action spans the globe and as far as I can determine the historical facts are accurate and the scenarios are riveting but believable.
Tom Clancy passed way just about two months before this final book was published. He managed to write 28 books in 30 years, a prodigious feat considering most of them were on the order of 700 pages or more (Threat Vector runs 840 pages). But to my mind, they were all too short, and Tom managed to exit, as any writer would desire, leaving his avid readers yearning for more.
Until next time, happy navigating, and think about what a difference GPS, the PNT Gold Standard, has made in your life. You might be surprised. And then grab a good Tom Clancy book. You have 28 excellent volumes from which to choose.