GPS World: GPS Embarrassment of Riches

The System =E2=80=94 A Healthy Constellation
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6615

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=20
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 GLONA=
SS satellites are set healthy, for a total combined=20
constellation of 47 satellites.

SVN48/PRN07 is the sixth modernized Block IIR-M satellite to join the con=
stellation. It has assumed the plane A, slot 4=20
position, replacing SVN27, the U.S. Air Force said. Two further IIR-M sat=
ellites are scheduled to be launched this year, in June=20
and September, respectively.

The IIR-M satellites carry an upgraded antenna panel that provides increa=
sed signal power to both military and civilian=20
receivers on the ground, two new military signals for improved accuracy, =
enhanced encryption and anti-jamming capabilities for=20
the military, and a second civil signal, L2C.

The IIR-M to launch in June will broadcast the first L5 signal by a GPS s=
atellite (WAAS satellites have been transmitting L5).=20
GPS World plans to publish observations and analyses of these first L5 si=
gnals at the earliest opportunity.

Musical Chairs. The earlier setting of Space Vehicle Number (SVN) 32, bro=
adcasting on channel Pseudo Random Noise (PRN) 01, to=20
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=20
number is fixed per satellite; the PR number, the satellite=E2=80=99s uni=
que PRN code, can be changed among satellites at the discretion=20
of the GPS Wing and the 2nd Space Operational Squadron (2SOPS).

GPS administrators had issued a Notice Advisory to NAVSTAR Users (NANU) o=
n March 13, advising that satellite SVN32, transmitting=20
L-band code as PRN01, would soon be set unusable no earlier than March 17=
=2E Some observers suspected the change in SVN32=E2=80=99s status=20
may have been related to the GPS control segment and another satellite, S=
VN23, recently set to healthy and broadcasting as=20
PRN32, which had caused problems for some older receivers.

The Air Force stated that SVN32=E2=80=99s status was unrelated to SVN23=E2=
=80=99s status, however. In response to a story published March 14 in=20
the Navigate! daily e-newsletter (the confusion in similar numbers led to=
the error in the earlier Navigate! story,=20
necessitating the correction), the GPS Wing e-mailed the following:

=E2=80=9CSpace Vehicle Number (SVN32/PRN01 was unusable beginning 17 Mar =
08 at 1953 Zulu and removed from the Broadcast Almanac at 2200=20
Zulu. With SVN32/PR-01 in this configuration the performance of the clock=
can still be monitored by Architecture Evolution Plan=20
(AEP). How it behaves may help determine whether SVN32 is a good candidat=
e for long-term storage or not. The aforementioned=20
SVN32/PRN01 actions are unrelated to SVN23/PRN32 which was set healthy on=
26 Feb 08.=E2=80=9D

The GPS Wing did not elaborate as to exactly why it is evaluating SVN32 f=
or long-term storage. Some have suggested that the GPS=20
control segment may have difficulty addressing more than 31 healthy satel=
lites.

Civilian observers report that prior to it being set unusable, SVN32 was =
behaving nominally; as of the time this story was=20
published =E2=80=94 as the GPS Wing indicated =E2=80=94 the satellite=E2=80=
=99s L-band transmitters were still on and it was transmitting a navigati=
on=20
message with orbit and clock data, even though it had been removed from t=
he broadcast almanac.

Backstory. According to unconfirmed reports, the atomic clock on PRN01/SV=
N32 (a Cesium clock, the last operational clock=20
on-board) has been in use since 1996 and is finally failing. 2SOPS had be=
en keeping the satellite=E2=80=99s user range error (URE) to a=20
minimum by uploading clock updates several times a day, averaging six upl=
oads in a 24-hour period, and this task became a major=20
burden for the 2SOPS crews since they have 32 satellites to keep updated.=
The plan is to set SVN32/PRN01 to test mode once=20
removed from the broadcast almanac. In test mode, the performance of the =
atomic clock will still be monitored by the AEP. If GPS=20
Wing determines that SVN32 is a good candidate for long-term storage, it =
could become a standby asset in space, to be called=20
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=20
receivers when there are 32 PRNs in the almanac and broadcasting. Some of=
it is due to older equipment that was never designed=20
for 32 PRNs and some is because the military signal specifications were n=
ot followed correctly or completely. There are also=20
some military systems that have problems with 32 PRNs for various reasons=
and that has caused consternation in the military GPS=20
receiver world. A GPS World editor has spoken several times with represen=
tatives of Rockwell Collins, who maintain that none of=20
their systems (PLGRs and DAGRs) have problems with 32 PRNs being broadcas=
t. So it is equipment built by other manufacturers, of=20
which there are several.

Technically some of the problems are because many GPS receiver manufactur=
ers used scientific notation when counting PRNs; they=20
started with zero and went through 31. However, the military started with=
one and went through 32. The result is the same number=20
of satellites received, but obviously problems with the almanac and inter=
nal nomenclature.

Currently neither the GPS Wing nor the 2SOPS will release information abo=
ut the systems affected. It is not classified but=20
considered for official use only (FOUO). They say they are working to fix=
the problem.

=E2=80=9CIn my opinion,=E2=80=9D stated one anonymous source, =E2=80=9Cwe=
will not see 32 PRNs until the problem is resolved or the military decid=
es to=20
live with it and replace the affected systems.=E2=80=9D

Single-String Mode. In further complications, by October 2009 the GPS con=
stellation could lose as many as nine satellites that=20
are now in single-string failure mode (one of the critical systems is run=
ning on back-up and there is no further back-up when=20
that system fails). Such problems may or may not have anything to do with=
the GPS payload; they can be batteries, solar cells,=20
reaction wheels, receivers, and so on. =E2=80=9CIn my opinion, this circu=
mstance allows the GPS Wing and the 2SOPS time to determine a=20
fix for the 32 PRN problem,=E2=80=9D stated the source. =E2=80=9CBoth org=
anizations have assured me that the problem is not with AEP, although=20
it is true that the older OCS could not handle more than 29 PRNs.=E2=80=9D=


Galileo, Compass on Collision Course

As Galileo looks forward to its second satellite launch (see Expert Advic=
e), program officials fret over possible technical and=20
security-related threats posed by the nascent Compass system touted by Ch=
ina. Talks during the week of April 21 between the=20
European Space Agency (ESA) and European Commission (EC) and the Chinese =
vice minister for science and technology may or may not=20
resolve the impasse.

The EC says it must commit to a specific Galileo signal structure sometim=
e this summer, and needs to know how China=E2=80=99s satnav=20
project will unfold in order to do so. China has filed frequency registra=
tions with the International Telecommunication Union,=20
and announced plans to grow its current barebones regional system to a fu=
ll global constellation.

As reported by GPS World from the Munich Satellite Navigation Summit in F=
ebruary, Jing Guifei of the National Remote Sensing=20
Center of China and the Ministry of Science and Technology said the Compa=
ss frequency design plan calls for four carrier=20
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
=E2=80=9CWe have also tested a new satellite, the Compass-m1. The first m=
edium-Earth orbit satellite, launched April 2007, is for system=20
in-orbit validation and to secure the frequency filings,=E2=80=9D Guifei =
asserted.

Knowledgeable observers privately express uncertainty about China=E2=80=99=
s true intentions. The country certainly feels a regional=20
system to be an absolute necessity for military reasons, but has not made=
a clear case for a global system, and may be angling=20
for some kind of bargaining chips instead.

Meanwhile, the potential global Compass signals as currently specified wo=
uld interfere with Galileo=E2=80=99s signals, throwing a monkey=20
wrench into Galileo plans to set its signal specs firmly before solicitin=
g build bids from industry in July, en route to final=20
contracts in December.

Compass proposes to use frequencies planned for Galileo=E2=80=99s Public =
Regulated Service (PRS) =E2=80=94 and for the GPS military code =E2=80=94=
=20
meaning that in an emergency, Europe could not jam the Chinese signal wit=
hout also jamming its own encrypted, security-related=20
signals as well. The same problem holds for the U.S. military.

Diplomacy remains a delicate matter for the United States with China, par=
ticularly in the Taiwan Strait. The U.S. State=20
Department has been in discussions regarding Compass with the Chinese but=
has likewise found uncertainty, not only in program=20
direction within China, but in knowing who there really makes such decisi=
ons.

GLONASS Faces Budget Doubters

In mid-February, the Russian Audit Chamber board assessed the country=E2=80=
=99s space program, Roskosmos. It stated GLONASS was unlikely=20
to offer serious competition to the U.S. GPS, annulled the results of a 2=
006 tender for developing a new spacecraft to replace=20
aging Soyuz launchers, and cast doubts on any potential commercial aspect=
s of GLONASS. While Russian armed forces require their=20
own navigation systems, heavy worldwide reliance on GPS, including within=
Russia by Russian airliners, means that Roskosmos=20
programs are not market-oriented nor destined for market success.