I think Majenko's answer misses the mark a bit.
In order to get a rapid first fix, a GPS receiver needs to figure out quickly which satellites are in view, and their approximate Doppler shifts. In order to accomplish this, it needs two pieces of information: It needs to know roughly where in the world it is, and it needs to know roughly where the satellites are as well.
A-GPS refers to the former: by some means other than GPS, the receiver gets an approximate idea of where it is located. There are a number of ways of doing this; some WiFi access points broadcast their own geographic coordinates, which gives a pretty good idea of the receiver's loocation, since the coverage area of any particuar access point is fairly limited. Somewhat looser precision is available from cell towers, since their "footprint" is generally larger.
CGEE and SGEE are two methods of getting the second piece of data, the "ephemeris", which is a bunch of numbers that describe the orbits of all of the satellites and where they are currently located in those orbits. Without this information, the receiver must do a "blind search" until it finds at least one satellite, and then wait until that satellite broadcasts the ephemeris data for the rest of the constellation. Note that finding a satellite also helps narrow down the receiver's own position, since it must be somewhere within the "ground footprint" of that particular satellite, along an arc defined by the particular Doppler shift found.
CGEE basically means that the receiver extrapolates forward in time from information it had when it was last operating. Obviously, if the receiver is off for a long time, this information will become stale and relatively useless.
SGEE means that it gets current ephemeris information from an exernal server of some sort. Obviously, this requires that the receiver must have regular access to a network connection of some sort, which is readily achieved when it is embedded in a cell phone, for example.
The system defined frequencies for GPS and GLONASS are the center frequencies. The data and signals aretransmitted with different bandwiths. Since the GPS and GLONASS frequencies are close together, you can desing the antenna to cover both frequencies. It is possible that a single GPS antenna can 'see' GLONASS frequencies and push them forward as observations to the receiver-unit (processor). BUT (!) Although the antenna seems to 'see' the GLONASS satellites, the problem is that the data has to be prepared in a appropriate way! This is carried out by the LNA (low noise amplifier) of the antenna.
Why?
This has to do with the bandwith of the antennas LNA and how sharp the edges of the filter inside the antenna front end are designed. As long as the LNA is not optimized for GLONASS reception, you will never be sure to use the correct data for your receiver.
Solution
Check the quality of the incomming data (signal strength, how stable is the code or carrier solution) and the repeatability of a single GLONASS solution compared to a GPS-only solution. But the best way will be to use an antenna with multi-constellation support (GPS+GLONASS+derivatives) - also named as Global Navigation Satellite Systems (GNSS). Therefore you need to collect some information of the antennas LNA.
further reading:
- Ivan G. Petrowski and Toshiaki Tsujii (2012): Digital Satellite Navigation and Geophysics, Cambridge University Press, Cambridge
- Philip Mattos and Fabio Pisoni (2014): Quad Cosntellation Receiver, GPS World 25(1):34-63, online
Best Answer
The GPS receiver projects the XYZ coordinate that it finds from trilateration (or quadrilateration) onto a oblate spheroid model of the earth to find the latitude and longitude. Some fancier chipsets even have a (very coarse) height model of the earth that it uses to back-calculate latitude and longitude.