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[aprssig] Re: D700A gps interface problem

Gerry Creager N5JXS gerry.creager at tamu.edu
Wed Jul 21 13:30:43 UTC 2004


Scott Miller wrote:
{other info deleted}...
> 
> I haven't used the Deluo, but I've got DigiTravelers that will sometimes
> take 15 minutes on cold start to get a good lock.

Just a gentle reminder to the list that if a GPS receiver loses its 
almanac, by design (some receivers prefer a cold start every time while 
others prefer to lock on to SOMETHING even if it's not ideal) or via 
backup battery failure... or via glitch erasing or invalidating the 
almanac, the entire almanac must be reacquired before the receiver can 
perform a sky-search to acquire satellites, according to ICD-GPS-200 
(all versions).  If the receiver retained information about its last 
coordinates, it will initiate the sky search there.  If the last known 
position is not available, a generic sky search originating at some 
arbitrary point will ensue, and if that fails to acquire any satellites, 
it will fail to a general search looking to match the Gold Codes.  That 
will succeed but may take a while.

Generally, it takes a minimum of 11 minutes to acquire a new almanac. 
Once a receiver is running it will acquire an almanac as a matter of 
course, without interfering with other positioning operations, 
periodically (every 6 hours).

Recall also that each satellite sends its own, and ONLY its own 
ephemeris data for position determination and trilateration.  It can 
take up to 6 minutes to acquire the ehpemerides of a single satellite. 
In general, ephemeris data can be downloaded in parallel, and as a new 
satellite comes into view, its ephemeris will be incorporated on the 
fly, with no loss of receiver performance.

There will be a quiz next period.
-- 
Gerry Creager -- gerry.creager at tamu.edu
Network Engineering -- AATLT, Texas A&M University	
Cell: 979.229.5301 Office: 979.458.4020 FAX: 979.847.8578
Page: 979.228.0173
Office: 903A Eller Bldg, TAMU, College Station, TX 77843





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