why 3 sats to give a fix?

[toonboy]

Hi everyone, my first post

I have recently bought an Ipaq 5550, Nexipac, Pretec GPS-LP CF Receiver and 256Mb CF card.

I am a total newbie to GPS, but would like to know if it is possible to get an estimated fix from either 1 or 2 sat's. I get very good results (5 - 8 sats) when outside in the open air but would just like to know if there is any software that would give me an estimated fix when I'm just sitting in the house (1-3 sats)?

Also, can anyone explain why 3 sats to get a fix? or point me to a faq?

Cheers
Marc

[kartracer]

From the GPS FAQ at http://vancouver-webpages.com/peter/gpsfaq.txt:

"The receiver measures the time required for the signal to travel
from the satellite to the receiver, by knowing the time that the
signal left the satellite, and observing the time it receives the
signal, based on its internal clock.

If the receiver had a perfect clock, exactly in sync with those on
the satellites, three measurements, from three satellites, would
be sufficient to determine position in 3 dimensions.
Unfortunately, you can't get a perfect clock that will fit
(financially or physically) in a $300 (or even $3000) receiver, so
a fourth satellite is needed to resolve the receiver clock error."

[Hunnymonster]

In effect it's because you need to solve a 4 variable equation (to give answers for latitude, longitude, altitude and time) - so you will need at least 4 satellites to enable an answer satisfying all 4 variables (rather than a number of possible answers if you have 3 or less) - if you have 5 or more satellites then either a better estimate of the correct answer, or a check of the final answer can be calculated.

We see you’re using an ad-blocker. We’re fine with that and won’t stop you visiting the site.
Have you considered making a donation towards website running costs?.
Or you could disable your ad-blocker for this site. We think you’ll find our adverts are not overbearing!