GPS overdetermined positions and better?

Question

Hello i know all about GPS and how it works etc....My question is when the GPS unit is using 5 satellites at one time to determine its position its called overdetermined mode because it is extra accurate...If you are using more than 5 satellites (ie 6,7,etc) does the accuracy get better or is it the same as 5? please dont just guess or state it please include links or other references Thanks!!

Answer 1

Wow, there's a lot of misinformation here!!!

The accuracy of GPS is not dependent on the number of satellites but rather the accuracy of the measurements to the satellites.  

A minimum of 4 satellites is needed to get a GPS position solution (contrary to what Freelance Physicist says).  This is because there are 4 unknowns (x, y, z, and time).  The reason 5 or more satellites becomes overdetermined is because you have more than the minimum it needs.  When an overdetermined solution is possible, most GPS receivers will either do a weighted least-squares solution using all the data or employ a kalman filter.  The extra satellites don't really provide more accuracy per se.  What they do provide is redundancy and the ability to detect any bad measurements from other satellites.  If you only had 4 satellites but had perfect measurements, that position solution would be no better or worse than perfect measurements from 5 satellites.  Now as Freelance Physicist stated, there is no perfect measurement.  So the advantage of the overdetermined solution allows measurements to be weighted based on signal strength and satellite geometry.

As far as what Charlie O said, ignore it.  His information is just outright incorrect.  5 satellites is not the maximum in view.  The constellation was designed such that there would always be at least 4 satellites in view (assuming no obstructions).  However, there is often 8 or more in view, at any point on the earth.  I've seen as many as 12.  You'll more likely to be limited by the number of tracking channels in your receiver.

Answer 2

The more satellites visible to a receiver, the more accurate the position readout will be.  Only three satellites are necessary for a unique position determination, but because no measurement is without uncertainty, an average of multiple position measurements will be more accurate.

Answer 3

I'm glad you know all about GPS... Then it would be remiss of me to tell you that reception from 5 satelites is the maximum any GPS unit can access, as there are a fixed number in orbit (24 in all).

The plane of the earth, the old curvature thing, well your GPS unit can only recieve information for computation from the available number or resources.  Which in this case the maximum anyone can access from any point on planet (except the extreme polar regions) is 5.

If we put more up, it wouldn't improve or diminish your accuracy.

Want a link?  ok

http://msl.jpl.nasa.gov/Programs/gps.htm...