A-GPS – What does it mean?


If you’ve been browsing GPS devices you might have seen some of them, particularly GPS-enabled smartphones, boasting A-GPS. What is A-GPS? What are the benefits of Assisted GPS, and why is it a good feature to have?

A-GPS, which stands for Assisted GPS (what is GPS?), helps a standalone GPS unit to lock on to a satellite signal. Known as the TTFF (time-to-first-fix), this startup period can be challenging for GPS units where the satellite signal is weak or distorted by surrounding buildings.

It’s important to note that AGPS will only work if the device has a GPRS (data) connection, normally using a 3G connection.

How Assisted GPS helps speed things up

The first time a device attempts to lock on to a GPS satellite signal (known as a ‘cold start’) it has to download three sets of data (GPS satellite signals, almanac data, and ephemeris data) to calculate its position. A device may do a cold start if it has been switched off for a long period of time, has moved a considerable distance or the cache reset.

For standalone GPS, a cold start can take up to 12 minutes. If the signal is interrupted during that initial startup (due to a weak signal, conditions in the atmosphere, walls or trees) then it has to restart, further delaying the TTFF.

Assisted GPS can help avoid this, solving the data connection issue by linking to a web-based internet server (known as an assistance server) that already holds the current satellite information. This information is relayed to the device using GPRS (cellphone data connection service) rather than using the much slower, and possibly unreliable, radio signal from the satellite.

Even for devices doing a warm start, studies showed that using AGPS saved, on average, at least a minute.

What A-GPS won’t do

While helpful, there are some things Assisted GPS can’t do, no matter how much you want to believe it can. It doesn’t possess some magical ability to lock on to a satellite signal through everday obstructions. GPS won’t work underground, inside a building (away from glass windows) or underwater, and will be affected by dense tree cover.

AGPS won’t change that. As the name suggests it is Assisted-GPS – if you don’t have the GPS part of A-GPS then it just won’t work. All that AGPS does is significantly improve the startup time when first locating your position.

A-GPS is not to be confused with WiFi positioning or cellphone tower triangulation, methods where position can be determined when GPS is unavailable.

Some devices will use a combination of these methods to determine your position, using what is known as hybrid-positioning systems.

Integrated systems the way forward

Under ideal conditions GPS (or more correctly GNSS) is the best (fastest, most accurate) method to determine your location. However, there are several situations where GPS signals don’t work (underground, underwater, inside buildings or poor atmospheric conditions) so the future of device positioning lies in providing users with an integrated approach.

As discussed at the FIG Congress in 2010, the holy grail of device location is ‘ubiquitous positioning’ or for devices to be constantly location-aware. The only way, it appears, to achieve this is by using several different technologies to converge – GPS, WiFi positioning and cell tower triangulation.

Combining the advantages of different location technology will allow devices to automatically switch based on current conditions without requiring the user to interfere. With growing interest in location intelligence it won’t just be commercial fleet management systems that will benefit from accurate and reliable positioning systems.

Reduce fleet miles

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