# How Many Satellites . . .just Wondering

## Recommended Posts

When my Extrex Yellow is searching for the satellites there are 4 dots on the screen representing satellites and four lines connecting the dots to a figure representing me and my Etrex. Do the 4 dots on the screen represent exactly 4 satellites that I'm connected with, or is it possible that I'm linked to some number fo satellites other than 4? Just wondering.

The screen with the 4 satellites is a generalization.

If you go to the sky view (press the enter button with on the satellite page and select sky view), you can see the locations of all satellites and their signal strengths.

(Instructions from my feeble memory, may be off).

The reason that it shows four sats on your etrex is that four sats is considered the minimal requirement for good 3D positioning. You can determine your position in three dimensional space with some accuracy with 3 known distances. Three known distances narrows your position down to 2 possible locations. The GPS usually throws out one as being ludicrous (moving too fast, etc.).

If you've got four it knows which of those two is the right one. The more satellites, the more accurate the fix on your position. Satellite geography counts too. You get a better fix on your position if the sats are at wide angles close to the horizon (at least according to the Garmin rep I talked to).

3 satellites will not give a position in three dimensional space. 3 sats will give a two dimensional fix with the Z being assumed and if Z is assumed incorrect then the 2D fix isn't worth much either.

It needs 4 to compute all unknowns X, Y, Z and Time. 3 sats will generally solve for X, Y and Time.

Catch 22 with low horizon sats is that these introduce the most error due to atmospheric influences as the signal has to pass through more of the atmosphere the lower the satellite is to the horizon.

Ideally receivers should cut-off these low horizon sats or at least allow the user to define a cut-off angle.

Cheers, Kerry.

On the eTrex, pressing Enter on that page should take you to the advanced screen.

http://www.garmin.com/manuals/eTrex_OwnersManual.pdf (page 10 aka page 12 of 56)

This tells you how many satellites, which ones, signal strength, WAAS, etc.

It still doesn't make any sense to me, but it's pretty to look at.

## Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

×   Pasted as rich text.   Paste as plain text instead

Only 75 emoji are allowed.