Almost 2 months after GPS IIF-2 was set healthy, its ephemeris/clock
performance is still way behind IIF-1 (PRN25).

PRN1 performs (around 70cm RMS URE) in the worse 10 GPS satellites
PRN25 performs (around 40cm RMS URE) in the top 5 GPS satellites
PRN2 performs around 25cm RMS URE, PRN23 performs around 30cm RMS URE.
Both are IIR birds.

Now, its clear PRN1 performance is firmly within the official
performance specs, however its performing around IIA satellite
average. If this is a IIF exception, its not a real issue, however
since IIF-2 represents half of the operational IIF satellites so far,
it does raise the concern if this might repeat in the future.

I'd say this is a big reason not to abandon the GPS III program.
Neither IIF birds are the best performing satellites. So far IIF clock/
ephemeris performance is far from originally projected.

Marcelo Pacheco

On Dec 12, 3:52*pm, macpacheco <marc...@macp.eti.br> wrote:
> Almost 2 months after GPS IIF-2 was set healthy, its ephemeris/clock
> performance is still way behind IIF-1 (PRN25).
>
> PRN1 performs (around 70cm RMS URE) in the worse 10 GPS satellites
> PRN25 performs (around 40cm RMS URE) in the top 5 GPS satellites
> PRN2 performs around 25cm RMS URE, PRN23 performs around 30cm RMS URE.
> Both are IIR birds.
>
> Now, its clear PRN1 performance is firmly within the official
> performance specs, however its performing around IIA satellite
> average. If this is a IIF exception, its not a real issue, however
> since IIF-2 represents half of the operational IIF satellites so far,
> it does raise the concern if this might repeat in the future.
>
> I'd say this is a big reason not to abandon the GPS III program.
> Neither IIF birds are the best performing satellites. So far IIF clock/
> ephemeris performance is far from originally projected.
>
> Marcelo Pacheco

I've also been following the performance of GPS IIF-2 from data
provided by the GPS Operations Center:

https://gps.afspc.af.mil/gpsoc/

Although it's Signal In Space error has been much larger than GPS
IIF-1, it has been slowly but steadily decreasing. GPS IIF-1
consistently exhibits an error of slightly less than 0.5 meters.
Recently, GPS IIF-2 exhibits an error of about 0.7 meters. That is
much improved in comparison to errors greater than a meter when the
satellite was set usable.

I suppose there's some 'fine tuning' as the Kalman filter refines
ephemeris while the system operators understand how the satellite is
affected to solar pressure, eclipse related events and the overall
environment of space.

Then, there appears to be an issue with the GPS IIF clocks. Evidently
the cesium clock of GPS IIF-2 is not working properly. The rubidium
clocks, which are provided by Perkin Elmer, were redesigned to
overcome parts obsolesce manufacturing issues. Obviously, if there
are issues with the redesigned clocks, they must be resolved before
any future satellites are launched.

--- CHAS