GPS World: GPS Embarrassment of Riches
Sam Wormley
http://sidt.gpsworld.com/gpssidt/content/printContentPopup.jsp?id=506615
Apr 1, 2008
By: Alan Cameron
GPS World
GPS Embarrassment of Riches
The Constellation by the Numbers
As this magazine went to press, the GPS Operations Center announced that SVN48/PRN07 launched on March 15 was set usable on
March 24 at 20:08 UTC. Launch to operational status in only nine days (9.58 days to be exact) is believed to be a GPS record.
At the present time, all 31 active GPS satellites and all 16 active GLONASS satellites are set healthy, for a total combined
constellation of 47 satellites.
SVN48/PRN07 is the sixth modernized Block IIR-M satellite to join the constellation. It has assumed the plane A, slot 4
position, replacing SVN27, the U.S. Air Force said. Two further IIR-M satellites are scheduled to be launched this year, in June
and September, respectively.
The IIR-M satellites carry an upgraded antenna panel that provides increased signal power to both military and civilian
receivers on the ground, two new military signals for improved accuracy, enhanced encryption and anti-jamming capabilities for
the military, and a second civil signal, L2C.
The IIR-M to launch in June will broadcast the first L5 signal by a GPS satellite (WAAS satellites have been transmitting L5).
GPS World plans to publish observations and analyses of these first L5 signals at the earliest opportunity.
Musical Chairs. The earlier setting of Space Vehicle Number (SVN) 32, broadcasting on channel Pseudo Random Noise (PRN) 01, to
unusable March 17 was unrelated to SVN23/PRN32 being set to healthy, the U.S. Air Force GPS Wing clarified on March 24. The SV
number is fixed per satellite; the PR number, the satellite’s unique PRN code, can be changed among satellites at the discretion
of the GPS Wing and the 2nd Space Operational Squadron (2SOPS).
GPS administrators had issued a Notice Advisory to NAVSTAR Users (NANU) on March 13, advising that satellite SVN32, transmitting
L-band code as PRN01, would soon be set unusable no earlier than March 17. Some observers suspected the change in SVN32’s status
may have been related to the GPS control segment and another satellite, SVN23, recently set to healthy and broadcasting as
PRN32, which had caused problems for some older receivers.
The Air Force stated that SVN32’s status was unrelated to SVN23’s status, however. In response to a story published March 14 in
the Navigate! daily e-newsletter (the confusion in similar numbers led to the error in the earlier Navigate! story,
necessitating the correction), the GPS Wing e-mailed the following:
“Space Vehicle Number (SVN32/PRN01 was unusable beginning 17 Mar 08 at 1953 Zulu and removed from the Broadcast Almanac at 2200
Zulu. With SVN32/PR-01 in this configuration the performance of the clock can still be monitored by Architecture Evolution Plan
(AEP). How it behaves may help determine whether SVN32 is a good candidate for long-term storage or not. The aforementioned
SVN32/PRN01 actions are unrelated to SVN23/PRN32 which was set healthy on 26 Feb 08.”
The GPS Wing did not elaborate as to exactly why it is evaluating SVN32 for long-term storage. Some have suggested that the GPS
control segment may have difficulty addressing more than 31 healthy satellites.
Civilian observers report that prior to it being set unusable, SVN32 was behaving nominally; as of the time this story was
published — as the GPS Wing indicated — the satellite’s L-band transmitters were still on and it was transmitting a navigation
message with orbit and clock data, even though it had been removed from the broadcast almanac.
Backstory. According to unconfirmed reports, the atomic clock on PRN01/SVN32 (a Cesium clock, the last operational clock
on-board) has been in use since 1996 and is finally failing. 2SOPS had been keeping the satellite’s user range error (URE) to a
minimum by uploading clock updates several times a day, averaging six uploads in a 24-hour period, and this task became a major
burden for the 2SOPS crews since they have 32 satellites to keep updated. The plan is to set SVN32/PRN01 to test mode once
removed from the broadcast almanac. In test mode, the performance of the atomic clock will still be monitored by the AEP. If GPS
Wing determines that SVN32 is a good candidate for long-term storage, it could become a standby asset in space, to be called
upon in the future if necessary.
How Much Is Too Many? Reports during the preceding month have indicated, as suspected, that there is a problem for some GPS
receivers when there are 32 PRNs in the almanac and broadcasting. Some of it is due to older equipment that was never designed
for 32 PRNs and some is because the military signal specifications were not followed correctly or completely. There are also
some military systems that have problems with 32 PRNs for various reasons and that has caused consternation in the military GPS
receiver world. A GPS World editor has spoken several times with representatives of Rockwell Collins, who maintain that none of
their systems (PLGRs and DAGRs) have problems with 32 PRNs being broadcast. So it is equipment built by other manufacturers, of
which there are several.
Technically some of the problems are because many GPS receiver manufacturers used scientific notation when counting PRNs; they
started with zero and went through 31. However, the military started with one and went through 32. The result is the same number
of satellites received, but obviously problems with the almanac and internal nomenclature.
Currently neither the GPS Wing nor the 2SOPS will release information about the systems affected. It is not classified but
considered for official use only (FOUO). They say they are working to fix the problem.
“In my opinion,” stated one anonymous source, “we will not see 32 PRNs until the problem is resolved or the military decides to
live with it and replace the affected systems.”
Single-String Mode. In further complications, by October 2009 the GPS constellation could lose as many as nine satellites that
are now in single-string failure mode (one of the critical systems is running on back-up and there is no further back-up when
that system fails). Such problems may or may not have anything to do with the GPS payload; they can be batteries, solar cells,
reaction wheels, receivers, and so on. “In my opinion, this circumstance allows the GPS Wing and the 2SOPS time to determine a
fix for the 32 PRN problem,” stated the source. “Both organizations have assured me that the problem is not with AEP, although
it is true that the older OCS could not handle more than 29 PRNs.”
Galileo, Compass on Collision Course
As Galileo looks forward to its second satellite launch (see Expert Advice), program officials fret over possible technical and
security-related threats posed by the nascent Compass system touted by China. Talks during the week of April 21 between the
European Space Agency (ESA) and European Commission (EC) and the Chinese vice minister for science and technology may or may not
resolve the impasse.
The EC says it must commit to a specific Galileo signal structure sometime this summer, and needs to know how China’s satnav
project will unfold in order to do so. China has filed frequency registrations with the International Telecommunication Union,
and announced plans to grow its current barebones regional system to a full global constellation.
As reported by GPS World from the Munich Satellite Navigation Summit in February, Jing Guifei of the National Remote Sensing
Center of China and the Ministry of Science and Technology said the Compass frequency design plan calls for four carrier
frequencies and bandwidth of downlink signals:
B1: 1561.098 plus or minus 2.046 Mhz
B1-2: 1589.742 plus or minus 2.046 Mhz
B2: 1207.14 plus or minus 12 Mhz
B3: 1268.52 plus or minus 12 Mhz
“We have also tested a new satellite, the Compass-m1. The first medium-Earth orbit satellite, launched April 2007, is for system
in-orbit validation and to secure the frequency filings,” Guifei asserted.
Knowledgeable observers privately express uncertainty about China’s true intentions. The country certainly feels a regional
system to be an absolute necessity for military reasons, but has not made a clear case for a global system, and may be angling
for some kind of bargaining chips instead.
Meanwhile, the potential global Compass signals as currently specified would interfere with Galileo’s signals, throwing a monkey
wrench into Galileo plans to set its signal specs firmly before soliciting build bids from industry in July, en route to final
contracts in December.
Compass proposes to use frequencies planned for Galileo’s Public Regulated Service (PRS) — and for the GPS military code —
meaning that in an emergency, Europe could not jam the Chinese signal without also jamming its own encrypted, security-related
signals as well. The same problem holds for the U.S. military.
Diplomacy remains a delicate matter for the United States with China, particularly in the Taiwan Strait. The U.S. State
Department has been in discussions regarding Compass with the Chinese but has likewise found uncertainty, not only in program
direction within China, but in knowing who there really makes such decisions.
GLONASS Faces Budget Doubters
In mid-February, the Russian Audit Chamber board assessed the country’s space program, Roskosmos. It stated GLONASS was unlikely
to offer serious competition to the U.S. GPS, annulled the results of a 2006 tender for developing a new spacecraft to replace
aging Soyuz launchers, and cast doubts on any potential commercial aspects of GLONASS. While Russian armed forces require their
own navigation systems, heavy worldwide reliance on GPS, including within Russia by Russian airliners, means that Roskosmos
programs are not market-oriented nor destined for market success.