GPS and A-GPS

As a few questions were posted earlier regarding A-GPS (assisted GPS). I will try to outline the differences between regular GPS and A-GPS.

A GPS module is dependent on the GPS satellite signals to find a fix. The weaker this signal, the longer it will take to get a fix. If too weak, no fix will be found at all.

• When a GPS module is first turned on, it will usually have no satellite position data (ephemeris). This is what is called a cold start of the module (assuming the module does have a valid almanac stored). It is then necessary to place the module in open sky to have it acquire satellite data. This can take anywhere from a minute to several minutes, sometimes even longer, depending on the satellite signal strength, surroundings and the sensitivity of the GPS module. This sensitivity of the GPS module typically ranges from -140dBm to -146dBm for a cold start (the more negative the number, the more sensitive the module is).
  
• Once a GPS lock has been obtained, the GPS module will store this recent satellite data (ephemeris, almanac and time) in its memory and the module is said to be in tracking mode. When in tracking mode, the sensitivity increases as the module now has satellite data available. Typical tracking sensitivities are from -155dBm to -160dBm.
• If power is cycled off on the GPS module, then fairly immediately back on, the ephemeris data is still "fresh" (but the time might not be), and a GPS lock will be very quick since the GPS module does not have to collect new ephemeris data. This is called a warm start and the time to fix is typically less than half a minute.
• If the GPS module also still has an accurate time, the fix will be even faster (usually on the order of a second) and this is called a hot start.
  

In general, if ephemeris data is not available or outdated, obtaining a GPS lock can take some time. This is where A-GPS can help.

An A-GPS receiver can help getting ephemeris, almanac and time data from an assistance server. The assisting data can either be downloaded every time the GPS module is turned on (e.g. through the GSM network or WiFi) or in advance (e.g. once every 14 days at home from the internet). In the first case, ephemeris data, almanac and time are downloaded. In the latter case, differential almanac correction data is downloaded.

In order to be able to use A-GPS, we need three things:

1. A chip that is A-GPS compatible, i.e. the chipset allows for download of assisting data to the GPS module,
2. An assistance server from where the assisting data can be downloaded. Assistance servers are usually provided by GPS chipset manufacturers and need to be mirrored for client usage. (Apple is probably mirroring Broadcom's servers for use on the 3G iPhone).
   
3. A way to upload the A-GPS data to the GPS chipset. This can usually be done through a software download daemon which both acquires the data from the assistance server and sends it to the GPS chip at certain time intervals.

So A-GPS can make your GPS module find a faster lock, typically increasing the cold start sensitivity to about the level of a hot start.

It needs to be mentioned however that if a satellite signal is simply too weak (e.g. <-160dBm in parking structures or big concrete buildings), you won't see any improvement at all in the performance of your GPS module, even if you have an A-GPS enabled chipset as the GPS receiver won't be able to receive any signal even though you tell it were the satellites are.

Regarding the GPS chipset that I am using, the chipset can be enabled for A-GPS use and assistant data could be downloaded to the module through the serial port if the data can be obtained from an assistance server.