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Thursday, January 30, 2014

Who Carries the GPS/GNSS Gold Standard Now?

As reported by GPS WorldChina’s BeiDou system claimed a user range error (URE) of 2.5 meters zero age of data (ZAOD) 95% recently.  The parallel GPS specifications commit to 6 meters 95% ZAOD and 7.8 meters 95% all AODs.  Does this mean that BeiDou is more accurate than GPS? Not so fast.  

In late December, director Ran Chengqi of China’s Satellite Navigation System Management Office announced the BeiDou Navigation Satellite System (BDS) Public (or Open) Service Performance Standard. The document details the public service performance parameters of the BeiDou system, including service area, accuracy, integrity, continuity, and availability. It is a basic commitment to customers from BDS providers, but also an important basis for customers to choose, use, and evaluate the system performance.

A few important qualifications of BeiDou’s performance standard first:
According to the foreword of the document, “This document specifies the BDS open service performance standard at the current stage.” This is as it should be.
A paragraph on service volume, however, highlights the fact that BeiDou is as yet a regional service.
“4.4 BDS OS Service Volume
The BDS OS service volume is defined as the OS SIS coverage of the BDS satellites where both the BDS OS horizontal and vertical position accuracy are better than 10 meters (probability of 95%). At the current stage, the BDS regional service capability has been achieved, which can provide continuous OS to the area as shown in Figure 2 & Figure 3, including the most part of the region from 55°S to 55°N, 70°E to150°E.”
The BDS Service Area.
The BDS Service Area.
This means that BeiDou commits to 2.5 meter accuracy in China, as well as neighboring countries — and importantly, trading partners — in Southeast Asia plus Australia.
Does this mean that once BeiDou attains global status, it will provide 2.5 meter accuracy everywhere, on its basic single frequency, open service?  Hard to tell.  Much of its strength, its core strength, one might say, comes from 5 geostationary Earth orbit (GEO) satellites and 5 Inclined Geosynchronous Satellite Orbit (IGSO) satellites. The GEOs  hover over the Equator more or less permanently, south of but in the general longitude of  China’s sovereign national territory. The IGSOs move back and forth from the northern to the southern hemispheres in the same area.
When BeiDou achieves its planned global reach, an event scheduled for 2020, the constellation will consist of 35 satellites: 5 GEOs, stationed at longitudes so their footprints cover China,  27 medium Earth orbit (MEO) satellites encircling the globe in continuous paths as do those of GPS, and 3 IGSOs over the East and Southeast Asian regions.
Will globally available accuracy at that point match what is achievable in China?  It takes a better geometric mind than mine to fathom this.
Even disregarding the geographic limit of the 2.5-meter claim, and ignoring for the moment the mathematical conundrum outlined above, there are reasons to scrutinize the BeiDou Performance Standard more closely, as John Lavrakas of Advanced Research Corporation has done.  His notes, and an illuminating table, follow below after a bit more introduction and background on the general topic.
The publishing of the Public Service Performance Standard, a common practice among GNSS operators, is also a prerequisite for BeiDou system involvement in international civil aviation, international maritime, 3rd Generation Mobile [phone] System, and other international standard-setting organization activities.
The document has Chinese and English versions. Because document download from the BDS government website can be difficult, Richard Langley has made them available at the University of New Brunswick website:
John Lavrakas of Advanced Research Corporation posted the following comment to the an earlier online article announcing the Performance Standard document.
“I took a quick look at comparing the BeiDou Open Service Performance Standard with the GPS Standard Positioning Service Performance Standard and obtained mixed results.”
Table 1. Coded to show green for the GNSS service committing to a more stringent standard over the other. Courtesy of Advanced Research Corporation.
Table 1. Coded to show green for the GNSS service committing to a more stringent standard over the other. Courtesy of Advanced Research Corporation.
“In some cases, the commitments from BeiDou were stronger (URE accuracy, vertical position), and in other cases the commitments from GPS were stronger (continuity of service, advance notice of outages).
“The good news is that GNSS systems are documenting the service levels that users can expect. What we will need next is monitoring to verify these service levels are being met.
“Here is a link to my quick look:
Thank you, John.
A final note.  As the GPS stewards from the U.S. Air Force carefully and proudly remind us at each GNSS conference where they deliver a briefing, actual GPS performance has almost always bettered its specs over the last decade or two — often by a considerable margin.
And with that, I think we may all return to our various pursuits, secure in the knowledge that while the gold standard may — repeat, may — at times pass in limited special circumstances or under particular conditions, from system to system, overall GNSS Things Are Getting Better All the Time.