Unixman:
Let's try an example: Assume that the satellite is directly above the equator, also that the a/c is north of the equator and flying south.
As the a/c approaches the equator, doppler effect will cause an increase in measured frequency of the return signal in relation to the transmitted as the a/c effectively becomes closer to the satellite. This drops to zero offset as the a/c passes under the equator. Past the equator, the frequency will decrease as the effective distance from the satellite increases.
You can only get a 1d point fix from one satellite, but a 2d fix may be possible with two, depending on available data...