The Nokia N78 is an interesting piece of kit with some very cool features. One of the overlooked ones seems to be the inbuilt FM transmitter so you can beam your music from the handset to the nearest radio tuner. This makes it one hugely featured phone, what with a 3.2 megapixel camera (plus a second camera), HSDPA, WiFi, FM radio, MP3 player, video recording, FM transmitter and A-GPS.
And it was actually the A-GPS that got me thinking as to what exactly that meant. It’s clearly featured on many more handsets nowadays, but how does it fit into the scheme of satellite navigation exactly? Well I did a bit of digging around and asked a few questions and came up with this little set of facts relating to A-GPS and GPS. I’m hoping I understand it all correctly.
- Conventional GPS uses a dedicated satellite system to triangulate (through a minimum of 3 satellites) your position down to very accurate distances like 50 metres. It can take up to 3 minutes for a satellite lock to happen (and never indoors).
- A-GPS (or aGPS, Assisted GPS) also uses the same satellite system, but also accesses a remote server via the cell phone network to download satellite location data and so drastically speed up the TTFF (time to first fix) of standard GPS. This means the handset can lock on to your position in seconds rather than minutes, which also reduces the battery drain. Also A-GPS can often obtain a cell based location fix indoors.
- Most handsets in the world look as though they will eventually contain A-GPS chipsets. This is primarily for emergency services location purposes.
- Some A-GPS systems require that you have cellular network connections or they will not work (nor work when you roam internationally). Other A-GPS systems will work in standalone mode when there is no network coverage.
There are currently 5 different types of A-GPS:
- Autonomous. The handset or device can operate independently of a cellular network. This means that you can use the navigation anywhere in the world there is satellite access as long as you have the right map.
- Enhanced Autonomous. Predicted satellite orbit data is transferred to device over a standard network environment (e.g. WiFi or cellular?) at regular intervals (e.g. each week) to speed up TTFF lock-on times.
- MS (Mobile Station) Based. The handset uses both the remote cellular connected server and standard GPS satellites to provide navigational position for TTFF, and does the position computation itself after receiving a small amount of satellite data from the server.
- MS Assisted. The server does the positional computation itself and delivers constant positional updates to the handset. This is useful for simple ‘one-shot’ location tasks where accuracy is not critical and the handset is not moving quickly.
- MS Assisted/Hybrid. Combines additional cellular network data with GPS satellite data to improve positional reliability in difficult GPS areas.
That’s as far as I can make out right now, if anyone knows more, or indeed better, please speak up in the comments.