# [time-nuts] GPS receiver vs local oscillator

Attila Kinali attila at kinali.ch
Thu Feb 2 19:02:48 UTC 2012

```On Thu, 02 Feb 2012 10:49:22 -0800
Hal Murray <hmurray at megapathdsl.net> wrote:

> > Yes, it comes out in the calculations.
> > That's why you need 4 satellites.  You solve for x,y,z and local clock
> > offset. (and, in reality you also have to solve for xdot,ydot,zdot, and
> > fdot)
>
> I thought the 4th satellite was needed to determine the time.  Wouldn't it
> take a 5th satellite to also determine the frequency of the local clock?

Not really. There are two ways to get the postion and time derivatives.
One is to either use two fixes which give you each a (x,y,z,t) tuple,
while you know what your expected delta-t is, you can calculate the
"real" delta-t and get from that your frequency offset.

The other way is to use the doppler shifts of the satelites.
You know what position and speed relative to you the satelites
have and can from this calculate what your speed, respektive frequency
is.

Attila Kinali

--
Why does it take years to find the answers to
the questions one should have asked long ago?

```