Hi Adam:

Some corrections. 

All GPS satellites broadcast on the same frequencies (CDMA).  The thing 
that differentiates one satellite from another is the pseudo random code 
(PRN) it uses.  So you can not use a spectrum analyzer to see which 
satellites are up there.

When doing a cold start (the receiver does not know where it is and does 
not have any satellite data) it needs to search a number of spaces such 
as trying different PRNs, different doppler shifts, different Lat-Lon 
combinations.  Once the receiver has found a satellite it can download 
the almanac data for all the satellites.  Note that the almanac is used 
for finding which satellites are visible, not for navigation.  Now the 
receiver can do a smart search to find more satellites.  Almanac data is 
good for maybe some months.

Once the receiver is locked to a satellite it can start downloading the 
ephemeris for that satellite.  The ephemeris contains data similar to 
the almanac, but to a higher precision and including some corrections 
specific to that satellite.  Once 4 ephemeris sets have been downloaded 
the receiver can compute a fix. ephemeris data is good for maybe a day.  
The receiver can use data that's out of date by a few updates and get a 
rough position and at the same time be downloading the current version.  
It takes about 12 minutes to download the needed data so for a true cold 
start you can not do it faster.

The old Trimpack GPS receivers used the DS1221S and a Lithium primary 
battery to make all the RAM into static RAM that retains everything when 
the power switch was turned off (think core memory).  This way when 
power is turned on (and the receiver is within a few hundred km of it's 
last fix) the Time To First Fix is much shorter than for receivers that 
need to do a cold or warm start.
http://www.pacificsites.com/~brooke/Trimpack.shtml

There are a number of GPS receivers made specifically for timing 
applications.  The Motorola M12+T is the best one I have and it's been 
replaced by a newer M12M.  Trimble makes the Resolution-T.  These are 
what's called code receivers and can provide a 1 Pulse Per Second that's 
good for single digit nano seconds.  They have a "Zero-D" mode for 
timing that will give timing information when tracking only one 
satellite (need to know Lon, Lat & Elev).  There's development work on 
"carrier phase timing" where the accuracy of the 1 PPS is in the pico 
second area.

Have Fun,

Brooke Clarke

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