How many satelites?

[+rogueleader1977]

I just got an explorist 300 to replace my lost 210. I am thinking that there is something wrong with the explorist 300. Today it led me within 20 feet of one geocache, which was good. But later it was having problems. It could only lock onto 3 or 4 satelites. It kept bouncing around from 2d to 3d fix and none of the locks seemed steady. It got up to 6 once but never higher. I don't know if it's a bad unit or was just a bad constelation day. When I went to the satelite screen across the bottom of the screen it shows a list of satelites. Not necesarily ones it had a lock on. Are these the satelites the unit is looking for based on time/ date/ location? In other words are these the satelites it should be able to see in ideal conditions i.e. no trees, mountains, clouds? It was only showing six, so does that mean there were only six 'possible' satelites anyway? I don't know if I should return this or not, I was always happy with my 210 but I didn't want another because the color makes it to hard to see when I drop it.

[+Fhantazm]

Sounds like normal non-SirfIII operation. Were you in tree cover when you experienced this? The "hollow" bars you mention are satellites that the GPSr sees but has not yet downloaded the almanac data. The Magellan GPSr's are known to have good reception under tree cover but there are still times ALL GPSr's are affected by cover.

 

I just got an explorist 300 to replace my lost 210. I am thinking that there is something wrong with the explorist 300. Today it led me within 20 feet of one geocache, which was good. But later it was having problems. It could only lock onto 3 or 4 satelites. It kept bouncing around from 2d to 3d fix and none of the locks seemed steady. It got up to 6 once but never higher. I don't know if it's a bad unit or was just a bad constelation day. When I went to the satelite screen across the bottom of the screen it shows a list of satelites. Not necesarily ones it had a lock on. Are these the satelites the unit is looking for based on time/ date/ location? In other words are these the satelites it should be able to see in ideal conditions i.e. no trees, mountains, clouds? It was only showing six, so does that mean there were only six 'possible' satelites anyway? I don't know if I should return this or not, I was always happy with my 210 but I didn't want another because the color makes it to hard to see when I drop it.

Edited August 20, 2006 by Fhantazm

[+rogueleader1977]

Well, I was not in heavy tree cover all day. At time I was but I was also in a parking lot that thad some tree cover but not a ton. I don't recall my 210 having this much trouble. The satelites I was questioning had no bar nothing at all above them. These are the satelites that the gps 'knows' should be there but can't yet see, right?

Edited August 20, 2006 by rogueleader1977

[+rogueleader1977]

What's non-SirfIII operation?

[+Fhantazm]

Something like that. As I said above, the hollow bars are satellites that the GPSr "sees" but has not yet downloaded the almanac data required for a lock to that satellite.

SiRf III is a chipset in the new GPSMAP "x" series that's highly sensitive and does very well under heavy cover.

Edited August 20, 2006 by Fhantazm

[+badlands]

On a great day, I'll lock on to 11 birds but 6-8 is more typica and some days I'll only get 3 or 4. Oddly enough I don't seem to get any closer to the cache with 11 than I do with 6. (Explorist 500)

[+The Jester]

On a great day, I'll lock on to 11 birds but 6-8 is more typica and some days I'll only get 3 or 4. Oddly enough I don't seem to get any closer to the cache with 11 than I do with 6. (Explorist 500)

The GPSr only use 4 birds at a time for location fixes (best is one overhead, and three scattered around the horizon). Multiple fixes allows it to choose the best set to use - at that time. As you move and signals get blocked (trees, mountains, you...) it can switch to another set and still keep your position. Also the birds are moving which just adds to to the fun.

 

What may of happened to the OP is a line of birds that the GPSr was reading. This gives some of the worst fixes. I believe the Explorist uses a patch antenna that is the best of birds on the horizon, so it may not have been able to catch those low off the horizon.

[+GPSlug]

The GPSr only use 4 birds at a time for location fixes (best is one overhead, and three scattered around the horizon). Multiple fixes allows it to choose the best set to use - at that time. As you move and signals get blocked (trees, mountains, you...) it can switch to another set and still keep your position. Also the birds are moving which just adds to to the fun.

This is an all too common misconception. A GPSr will use all the sats it's able to for each solution, not just 4.

[8mmag]

This is an all too common misconception. A GPSr will use all the sats it's able to for each solution, not just 4.

 

To elaborate a little further, to solve the mathematics involved in determining your position in 3 dimensions requires 4 equations and 3 unknowns. One satellite is used to form each equation, the 3 unknowns are latitude, longitude, and elevation. The sumultaneous solution of these equations determines your position.

 

That is the minumum number of satellites needed to determine a 3D fix. I know at least Garmin, and probably all the others too, will use as many satellites as the receiver has available to 'overcalculate' the solution to increase precision, since atmospheric condition variables result in inexact solutions. So in reality, the more the merrier.

[+EScout]

Something like that. As I said above, the hollow bars are satellites that the GPSr "sees" but has not yet downloaded the almanac data required for a lock to that satellite.

SiRf III is a chipset in the new GPSMAP "x" series that's highly sensitive and does very well under heavy cover.

The eXplorists use the TrueFix technology, introduced a year before the Garmin's latest. It does very well under cover, and some argue better than the Garmins because of the later's wandering position reporting. The signal bars change color or shade as they are used. You can count and see the satellites in the current constellation. The OP might have had a bad constellation at the time. You can get the free software from Trimble called "Planning" that will calculate in your location the quality of the constellation at any given time, past or future.

 

Explorists receive 12 satellites plus 2 WAAS. I usually get 7 to 9 plus a WAAS. Occasionally like last Sunday, it showed 14 received at full S meter on all (12 signal bars are shown, and the regular sats will move the 2 WAAS off the S meter.)