Over the last few years, we have seen different technology labels from different GPS and mobile device makers. InstantSIRF Fix, Enhanced GPS, aGPS, are essentially the same, or commonly known as Asssisted GPS (or AGPS). The technology has been introduced in the market for a few years, but it continues to confuse alot of people, thinking that it’s another variant of LBS (location-based service). When HP first introduced built-in GPS in their flagship iPAQ product, iPAQ 6515, it has not received much attention other than it’s just a phone with some GPS chipset. HP had not marketed the benefit of A-GPS then. Neither were the reviewers, who probably don’t understood the purpose of A-GPS, judging by the number of reviewed articles on this topic.
The fact is AGPS is essentially still a GPS solution, with added technology to speed up the location acquisition fix process (commonly known as Time to First Fix, or TTFF), which has always been the most time consuming process of GPS.
Despite the recent introduction of sensitive GPS chipset such as SIRFIII, it does not necessary address the inherent challenge of a GPS solution, i.e. Time to First Fix. In order for a GPS receiver to triangulate its position, it must first obtain a couple of data broadcasted from the satellites. The first is almanac, which from layman perspective, documents the health of all the available satellites. Almanac data would require approximately 12.5 minutes to download, so that explains why a receiver that’s fresh from the factory, could take a long while to acquire a fix. Luckily, this data is valid for several months, so we are spared from such painful process most of the time.
The other satellite broadcasted data that the receiver needs, is the ephemeris data. This is a more precise orbital path of the satellite, and the data usually lasts no more than 4 hours. Ephemeris data are broadcasted by the Satellite every 30 sec, and download takes 30sec. Not only the receiver needs a minimal signal strength to receive the data, the download process must not be interrupted. Otherwise the whole download process must be repeated again. That’s why it is always recommended that you should stay stationary (obviously only if the receiver is facing the open sky) during the TTFF process. When you have not used your receiver for more than 4 hours, this data download process must be repeated, and that explains why the TTFF for cold start vs hot start is so different.
The assisted GPS technology, attempts to improve TTFF. It attempts to address two weak points. Firstly, the ephemeris data that’s received from the satellite is only valid for 4 hours. The technology involves in projecting the orbital path of the satellite that could be valid for a few days, up to a maximum of 10 days. While the length of the validity usually implies a lesser accurate path, the technology is actually maturing and a 5-7 days of projected path is actually quite accurate. The time consuming part of the problem, is addressed by making the data available through internet. That’s why some people confuse such data download with data received from cell station.
This technology is available mostly in mobile devices, mainly because of the lack in proper GPS antenna, plus download of the ephemeris data is convenient in a mobile device. It was meant as a supplement, rather than a substitute, to the GPS chipset. A GPS chipset is still required, as long as you need a constant (every millisecond) tracking of one’s precise (in terms of metres) position. There’s no way you can achieve this just by cell station.
Hopefully this clears up the misconception of AGPS, and dismiss the myth that a AGPS is not a real GPS.