Assisted GPS, A-GPS
With the proliferation of smartphones, iPhones, and iPad devices with GPS capabilities, many people are asking about Assisted GPS, also known as A-GPS. Does it use the “regular” GPS satellites? Is it as good as GPS? Since it is “assisted” is it better than GPS?
Before we begin, you might want to check out a primer on how a GPS figures out your location in this Acquiring Satellites article.
GPS Chips, Power
Not all GPS chips that go into various devices are created equal. Some GPS chips drain power from the battery faster than others. Some chips are better at pulling in weaker signals than others. Some occupy a smaller size inside your device than others. Manufacturers chose a GPS chipset for their product based on the size of space available, the amount of power/battery life available, and to a degree the amount of accuracy desired.
When dealing with devices like a smartphone, you typically have a very tiny amount of space to work with. Designers also have very small power requirements to work with. A dedicated handheld GPS might only get eight hours of battery life which is unacceptable on a smartphone, especially since it needs to power many more chips than a typical handheld GPS. So when picking a GPS chipset for a smartphone, you want something tiny, and easy on power. This poses a problem, as the chipsets that are low on power and size are also typically the chips that take a longer time to acquire your location and can’t pull in weaker signals that high power chips can.
Assisted GPS, AGPS
The “assisted” part of A-GPS means that instead of leaving the GPS chipset to figure out your location on its own, another part of the device will give the GPS chipset a little hint where to start looking. This typically comes via knowing the location of nearby wifi stations or nearby cellular towers. These other location technologies are assisting the GPS to get your position faster.
So is it better?
People often ask if this makes A-GPS “better” than traditional, dedicated GPS. The question has multiple answers. From the perspective of a smartphone it is “better” in the sense that your phone will figure out its location faster, and with less sacrifice in battery power than if it used a more traditional GPS only chip. There is a downside though– absent a cellphone signal and nearby wifi locations a device with A-GPS might take much longer to find your location than a traditional GPS.
On a handheld GPS you don’t see A-GPS technologies because to keep costs down those devices don’t have wifi radios nor cellular radios, yet they have “stronger” GPS chips since battery life is slightly more expendable.
So A-GPS gets your device a location fix faster and with less power, pending you are within range of a cellphone tower. Traditional GPS gives you relatively quick acquisition times without needing any terrestrial based assistance and can pull in weaker GPS signals, at the expense of battery life.