Cause Identified for Pseudorange Error from New GPS Satellite SVN49

Cause Identified for Pseudorange Error from New GPS Satellite SVN49
http://sidt.gpsworld.com/gpssidt/content/printContentPopup.jsp?id=3D61=
0789

Jul 13, 2009
By: Richard B. Langley

with an additional note by Oliver Montenbruck


The GPS Wing and its contractors have traced the cause of pseudorange err=
ors on L1 and L2=20
broadcast by the newest GPS satellite, SVN49, to the manner in which the =
L5 signal=20
demonstration payload was added to the satellite. Signal leakage between =
the two input=20
ports of the antenna coupler network for the satellite=E2=80=99s array of=
12 helical antenna=20
elements, reflected from the L5 filter and then transmitted, creates a se=
cond signal with=20
a delay of approximately 30 nanoseconds, and the appearance of a multipat=
h component.

While testing an adjustment to the signal-in-space to minimize the effect=
of the problem=20
on receiver navigation solutions on Earth, the GPS Wing is interested in =
hearing from=20
manufacturers and the user community concerning the different impacts of =
SVN49 signals on=20
the wide range products and applications in operation, before reaching a =
final decision on=20
what to do with the satellite prior to setting it healthy.

The seventh modernized GPS Block IIR satellite was launched on March 24, =
2009. Called=20
SVN49, its sequence number in the long line of GPS satellites, or PRN01, =
after its=20
pseudorandom noise code identifier, this satellite is special. In additio=
n to the=20
equipment required to transmit the legacy GPS C/A-code and P(Y)-code sign=
als and the new=20
civil L2C-code and military M-code signals on the standard L1 (1575.42 MH=
z) and L2 (1227.6=20
MHz) frequencies, SVN49 carries an L5 demonstration payload. L5 is the ne=
w civil signal to=20
be transmitted on 1176.45 MHz by Block IIF and succeeding generations of =
GPS satellites.

See: http://sidt.gpsworld.com/gpssidt/content/printContentPopup.jsp?id=3D=
610789

I read this article, and it's really settling in. The Wing, ITT, LM and
especially Aerospace (they're paid millions to look over contractors'
shoulders to try to pick nits) have some significant egg on their faces.
Some careful measurements with a network analyzer (the RF kind, not the IP
kind) should have caught this in the factory without unduly disturbing the
bird. Like I said in one of my earlier posts, you can specify impedances and
try to match them up, but especially when you are dealing with high power
components (this hybrid coupler is likely more a waveguide the size of a
small lunchbox to handle the power), this is easier said than done so you
better test to verify. Ah, hindsight is 20/20.

The receiver dependencies will give Trimble something to crow about. "See,
our Everest(TM) technology (or whatever they're currently pitching as their
technique) rejects close multipath that degrades other receivers..."

I bet LM/ITT are internally smarting over whatever trade caused them to need
to use a more than 4 meter cable from the L5 box to the antenna deck (of
course all the prime real estate was already taken by the primary payload).
If that cable were half the length the effect would have been buried in the
noise. If they could have squeezed the L5 filter right by the port (on the
other end of the cable) that would have prevented it as well.

In the end the Air Force may abandon its earlier ephemeris hack and settle
on setting that bird's URA a notch higher (remember, it's still within
spec). Local area DGPS will work, but WADGPS (WAAS, WAGE) won't. The RTK and
IGS crowd will have to come up with their own fixes to not have the code
errors screw up the carrier phase ambiguity fixing process (e.g., use the
4th order model suggested by Tim Springer). Again, wider area network RTK
systems will have the harder time since the errors start to decorrelate
between the references and the users. I'm not an expert on their algorithms
so I won't go further (Tim?).

Looking forward to further analyses (I sure hope I make it to Savannah in
September). They may need to add a special session on this topic.

-Marty

wrote:
610789
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Marty,
The "within spec" line is a ruse. The accuracy of the GPS system
exceeded specification back in the early 80's when it was first
deployed. There has been a long standing effort to steady improve the
accuracy of the GPS system which has got the constellation average
down to less than 0.8 meters signal in space pseudorange error. This
is the de facto standard that any new satellite has to hit.

"It is not possible to fully fix the problem."

This is something that I have been saying all along. Multipath is
viewed by a receiver as noise. The ten million or so existing
receivers have algorithms to deal with noise but their algorithms
assume a coherent signal. SVN49's signal is generating about twice
the noise as other GPS satellites and the receiver's noise reduction
algorithms (except apparently for one isolated brand) does not handle
it correctly.

Is "within spec" going to be the party line when the OCX Kalman Filter
fails to match the current system's <0.8 meter SIS PR? Grrrrrrr.

--Mike Jr