[ntp:questions] Sure gps looses all sattelite fixes

A C agcarver+ntp at acarver.net
Tue Feb 28 06:56:28 UTC 2012

On 2/27/2012 22:13, unruh wrote:

> I assume that the receiver is still obtaining the orbital elements from
> the sattelites and that is how the receiver figures out where in the sky
> they are. Some say it remembers the orbital elements forever, until the
> sattelite changes them. Maybe, I do not know. I was assuming that it was
> receiving the obits from the sattelites even though it could not send it
> reliable timing data-- since noise could well mess up the time far more
> easily than it does the digital data.

The ephemeris data does come from the satellites but it is transmitted 
only once every 12.5 minutes (subframes 2 and 3).  It is the 
responsibility of any and all GPS receivers to discover and retain the 
ephemeris data unless another ephemeris transmission is received.  If 
the GPS does not receive another ephemeris, it is supposed to maintain 
the current one until such time that it expires according to the 
ephemeris message as encoded by the USAF ground station.

If the ephemeris expires or somehow no longer exists (e.g. the battery 
backup on the receiver fails), all GPS receivers are then supposed to 
scan all possible PRNs (the satellite number) and begin trying to decode 
a satellite message.  Once a successful lock on a satellite is made, the 
GPS enters a holding mode until the ephemeris is received (elapsed time 
can be up to 12.5 minutes depending on the moment the satellite signal 
is acquired).

As for the ephemeris message, every GPS satellite transmits the full 
ephemeris data for the entire constellation at once.  That way it 
doesn't matter which PRN the receiver locks on to, it will still obtain 
a full constellation ephemeris.

See http://www.gps.gov/technical/icwg/IS-GPS-200E.pdf for the nitty-gritty.

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