I refer to the explanation I picked up from a pdf below. Please note a person who already has knowledge on A-GPS and CGEE and SGEE does not have to read the explanation provided for those below.
I know SGEE is downloading the Ephemeris information from the GPS Vendors server. Which I think is also scary in some aspects. (if anyone had any comments with regard to that kindly discuss)
My second Question is if a GPS is saving the used Ephemeris data in RAM and using the same in next startup. Is that called A-GPS or CGEE (Client generated EE)?
Explanation of Technologies
What is Assisted GPS?
Assisted GPS or A-GPS uses some other communication channel such as WiFi or the mobile phone network to assist the GPS receiver by delivering current time and ephemeris data for all the satellites in the system. Having the current ephemeris data allows the receiver to be more precise in it’s searching, which makes the receiver sensitivity effectively the same as the tracking sensitivity. That is, it makes any start, a hot start. With A-GPS, the network provides the Ephemeris data to the GPS receiver to improve sensitivity the TTFF. Assisted GPS or A-GPS is also used to improve the performance within buildings where the GPS signals are by 20 dB or possibly more. Again by providing information to the GPS receiver in the
mobile it is able to better correlate the signal being received from the satellite when the signal is low
in strength. Using this technique it is possible to gain considerable increases in sensitivity under
regular use-cases.
Client Generated Extended Ephemeris
Client Generated Extended Ephemeris (CGEE) is calculated on the client using broadcast ephemeris data downloaded for visible satellites. CGEE provides three day Extended Ephemeris (EE) models for visible satellites and stores the data in local memory. With CGEE no network connection is required.
Server Generated Extended Ephemeris
With Server Generated Extended Ephemeris (SGEE), a server obtains information from world-wide tracking stations and calculates the EE data for all GPS satellites at least once a day. To receive the data a network connection is required to download the data. Typical size for a one day payload to cover the entire constellation of satellites is around 11 Kbytes. These files are typically provided by the GPS supplier and are available to download at no charge.
Best Answer
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.