Re: [gpsd-dev] Parallel build broken?

[Greg Troxel]

"Gary E. Miller" <address@hidden> writes:

> If we are going to get really technical, then yes "almanac" is sent
> every 12.5 minutes, but the almanac is not what we want.  As the spec
> says: 20.3.3.5.2.1 "The almanac is a subset of the clock and ephemeris
> data, with reduced precision."

Sure -the almana is intended to be good enough to know which SVs are
above the horizon and compute doppler shift.  All SVs transmit the same
almanac data.

> So you really want the ephemeris, not
> the almanac.  And the ephemeris needs two hours:
>
> 30.3.3.1.1 "The ephemeris parameters in the message type 10 and type 11
> describe the orbit of the transmitting SV during the curve fit interval
> of three hours. The nominal transmission interval is two hours, and
> shall coincide with the first two hours of the curve fit interval."

My understanding is that each satellite transmits an ephemeris for
itself (only) every 30s, and the two hours refers to how often a new
ephemeris message is uploaded to each satellite.

So when you turn on a receiver that has been off for a day, it finds
satellites within seconds and then after 30s of listening has ephemeris
and can compute a solution.   The first time it is turned on, it has to
search.  And if it's only been off for 10 minutes, it can compute a
solution without receiving ephemeris because it already has it.

All that said, I think people said that the GPS-UTC offset is in the
almanac; it's not a per-satellite parameter, and one only needs it to
compute UTC for external use; the navigation solutions are in GPS time
and don't need it.


How many GPS receivers supported by gpsd send time to the computer in
GPS time, rather than UTC?  Is that the only reason gpsd needs to have
leapsecond information?

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