Why do unit GPS units pick up different satellites?

[GeoBobC]

OK, I just had to ask the experts out there. I have identical (I think they are) Garmin 60Cx GPS units. They are configured with the same software and firmware versions. They have both been used extensively.

 

I wonder why they do not always pick up the same satellites. Case in point: the other day I had the units side by side, and one unit displayed a solid lock on satellite #10 on the satellite page. The second unit showed a hollow bar for satellite 10, and the bar as high as possible in the display. Both units had been on for more than 30 minutes. However, even after leaving the units stationary for more than two minutes, the second unit never locked on #10. Why?

 

Also, I find that both units rarely lock on the same WAAS satellite. Why?

 

Why don't two "identical" units pick up the same satellites?

[+TJ Willy]

OK, I just had to ask the experts out there. I have identical (I think they are) Garmin 60Cx GPS units. They are configured with the same software and firmware versions. They have both been used extensively.

 

I wonder why they do not always pick up the same satellites. Case in point: the other day I had the units side by side, and one unit displayed a solid lock on satellite #10 on the satellite page. The second unit showed a hollow bar for satellite 10, and the bar as high as possible in the display. Both units had been on for more than 30 minutes. However, even after leaving the units stationary for more than two minutes, the second unit never locked on #10. Why?

 

Also, I find that both units rarely lock on the same WAAS satellite. Why?

 

Why don't two "identical" units pick up the same satellites?

 

I don't know but will stab at it.

 

Perhaps the battery strength? Also, the point in the horizon for number 10 bird?? Where was its position on the globe??

 

I don't know man. This is interesting though.

[GeoBobC]

Battery strength was strong on both units. Regardless of where #10 was (I'm not certain), it was in the same location for both units, and one locked on immediately. And this is just one example.

[rwsmith123]

Battery strength was strong on both units. Regardless of where #10 was (I'm not certain), it was in the same location for both units, and one locked on immediately. And this is just one example.

 

My guess is that one unit got a lock on #10 when you were at a different location and when you were looking at both you were in an unfavorable location so the second unit was having problems locking on to it, the other unit already had a lock on it and was able to maintain it.

[+webscouter.]

The way I understand it is this. A hollow bar means that the GPS has not downloaded the almanac from the satellite. Once it has that data it will keep itself in synch as long as it has a signal from the satellite. Could be that the first GPS started downloading that data first and was the only one that completed the download for some reason.

[Grasscatcher]

It all has to do with almanac and chip sensitivity.

 

Go onto Trimble's site and download the free Trimble Planning software, get the latest almanac file, plug in your location and from inside the house with your GPS unit off, you can tell exactly which satellites your unit(s)will see when you turn them on outside for any given day/location. (non-WAAS only)

 

If there is still a difference, then one chip is probably slightly more sensitive and the "different" satellite is probably sending the weakest signal.

 

WAAS difference is just whichever one(s) get locked onto first.

Edited January 20, 2008 by Grasscatcher

[+apersson850]

Almanac data is coarse orbital information for the GPS satellites, something that is not required, but beneficiary to get a first lock onto the satellite signals. Almanac data is valid for months and is transmitted for all satellites by all satellites. If proper almanac data is available, and the unit is in about the same place as it was last time, that's enough for the GPS to properly place the satellites on the skyview and start trying to correlate the signals, so it can lock on to the signals from a particular satellite.

Almanac data is repeated in such a way, that complete almanac information is transmitted with no longer interval than five minutes.

 

Ephemeris is precise orbital information, something that's required for the PVT calculation. Ephemeris is transmitted by each satellite for itself, in a block consisting of three six second long segments, starting every tenth second. The segments can be received in any order, but the whole block must be completed, before the satellite can be used.

 

When either almanac data, or hard work (autolocate) has lead to that the GPS has initially found the signal from a GPS, a hollow signal strength bar will appear.

When a complete ephemeris data block has been received (all three segments), the signal strength bar becomes filled/colored, and the satellite is available for the PVT calculation.

There's no special indication of when a satellite is actually used in the calculation, although it is of course known to the navigation device.

 

Coincidences may now interfere with the reception of a segment on one unit, or it may have found information regarding the code phase of that satellite, that set it on the wrong track or it may have got a renewed almanac data the other one missed or it may have got a "don't use" message in error or whatever may have happened, that made one unit do one thing, but another do something else.