[time-nuts] Warped back to 1993

[Magnus Danielson]

On 08/11/2013 07:07 AM, Hal Murray wrote:
>> With the wrong date and time, the GPS should not find almanac data,  so will
>> not lock. 
> I don't think that's the right way of describing the problem.
>
> The satellites broadcast on a known frequency, but that gets shifted all over 
> the place by Doppler.   ("All over" means a big shift relative to the 
> bandwidth of the signal.)
>
> If you have a recent almanac and you know the date/time and location, then 
> you can compute the Doppler and look in the right frequency and find the 
> satellites quickly.  In this context, "find" means hearing a signal at an 
> expected frequency.  If you don't hear anything where you expect it, then you 
> get to check nearby frequencies.  If you don't find anything nearby, you get 
> to give up and start searching the whole Doppler range.  It's the difference 
> between warm start and cold start.
>
> Once you do find several satellites, you can figure out the date/time and 
> location and after a while get a new almanac.
You only need to find a single satellite to get rough GPS time and the
full almanac.

You don't need the almanac, time or position to search the full doppler
and code bins for all 32 codes, it just takes time. Once you got the
first bird, you start to get the almanac, if the user supplies a rough
guess of position (or it has been stored) you can speed up the process,
if you have a recent enough almanac you can also speed things up using a
rough position, as you stop wasting time on birds you likely won't see.
But really, with completely wiped memory, you can get lock, it's more an
issue of start-up time.

Cheers,
Magnus