Interference between satellites

This is quite a complex thing to explain, especially as I don't fully understand it myself, however, here is my attempt at it, the main carrier frequency could be modulated in various different ways, it could have several 'sub carriers' on different frequencies as they do in stereo FM radio, this is called Frequency Division Multiplexing.

Another method would be for each user of the common carrier frequency to take turns in using the carrier, this is called Time Division Mulitiplexing, however the method that GPS uses allows the entire frequency spectrum to be used by all users at the same time, it is called Code Division Multiplexing.

The best analogy I have seen to describe the different modulation types are :-
FDM = multiple people in a room talking in diferent tones
TDM = multiple people in a room taking turns to speak
CDM = multiple people in a room talking in the same tone at the same time, but in different languages

To take this any further you would need to understand how CDM / CDMA / DSSS works, Google is as good a way as any, you could also take a look at :-
Understanding GPS Principles and Applications section 4.2.1
But it's quite hard to follow, (for me anyway)