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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.

13 Responses

  1. Nice post and good explanation. The question seems to come up more often these days, so it’s great that you’ve taken the time to explain it in an easy to follow way.

    Todd - May 10th, 2010
    • Thanks, Todd!

      Tim - May 10th, 2010
    • Good review, I totally agree with Todd. Easy read.

      Pay - April 4th, 2011
  2. You don’t mention accuracy. I am assuming that a dedicated GPS with WAAS will generally provide a more accurate location than using cell phone towers or wifi locations?

    Steve - May 13th, 2010
    • Not necessarily– Mobile phones and other devices that are using A-GPS are still getting the majority of their data from the same GPS satellites. And due to technical things like “multipath errors” sometimes being able to pull in weaker satellite signals (like a dedicated GPS is better at) can make the accuracy worse. (Though most of the time it will be better.) Overall, yes, I’d trust a dedicated GPS to have better accuracy, especially with WAAS, but not by much. We also have a similar article talking about WAAS.

      Tim - May 13th, 2010
  3. One other thing to note, “assisted” GPS often results in additional phone charges unless you already have a data plan with your cell carrier. Many GPS apps often turn it on by default. You can quickly rack up charges if you are not aware and do not have a data plan.

    Patruns - May 17th, 2010
    • Out of interest, does A-GPS consume a significant amount of data? Does anyone have any idea on this?


      Mark - June 4th, 2010
  4. I have a Blackberry Curve with GPS. I am very happy with its performance, both response and its sensitivity. I have compared its performance to several dedicated hand held GPSs and I feel it outperforms. I have found it, the GPS, is more accurate than Gen 2 Iphone.

    I am considering a new I phone, Gen 3 or 4 soon to be released. Aug feed back on GPs performance with the new Iphones? Does the Iphone have the SiRFstarIII GPS chipset, like my Blackberry?

    Tom - June 3rd, 2010
  5. Can I make a assumption that AGPS is a traditional GPS plus?

    selim - September 5th, 2010
  6. The explanation of AGPS at the top of this link is partial. The problem GPS receivers have is they need to work out or know the exact satellite Doppler frequency to receive the satellite data signal and decode data for computation of a range (pseudorange). Think of the change in sound of a passing car or train travelling at 30mph; that’s changing Doppler frequency. Now think of a satellite circling the earth every 11 hours 58 minutes at about 6000 mph; it’s Dopper frequency shift could be huge if the satellite was going directly away from the receiver. If received signal strength is low, intermittent and of incorrectly computed Doppler frequency it’s quite easy for the GPS receiver to miss it completely and fail to download the ephemeris or orbital data. This is a part of a 1500 bit navigation message transmitted by the satellite at 50 bps thereby being repeated every thirty seconds. The complete message is essential to the receiver for a fast start. In an urban location particularly where signals to a moving receiver can easily be obstructed by a building, tree, bridge sometime during the thirty seconds this just isn’t going to happen easily for the four or more satellites necessary to compute a position. Drop one bit of data and the receiver needs to go through the cycle again…and again, if you keep obstructing the signal. Take note that decoding a GPS signal is equivalent to detecting someone turning a household light on and off at 20,000km distance. A receiver in sky search mode flips through the frequencies looking for satellites. A broken signal during a receiver’s RF deck passage through a frequency search pattern often equates to no signal in some receivers and it logically, but rather stupidly (as we see it), switches to the next Doppler frequency.

    So what AGPS does is download the satellite ephemeris data at a telephone exchange where the GPS antenna has a good steady view of the satellite and passes the last received message over the GSM telephone’s carrier frequency to the mobile GPS handset in a fraction of a second. Other data downloaded is current local celltower position (or similar) and precise time.

    Now the handset GPS knows enough to compute satellite positions (not your own yet) and with the celltower position and precise time compute an accurate Doppler frequency with sufficient confidence to know it should only search a particular frequency for a particular satellite. This is excellent news for the GPS receiver’s RF deck which can now dedicate RF channels to satellites and KNOW that it’s going to find a signal if it keeps on looking. If it gets a clean signal on at least four satellites for as littel as just a second now the GPS will get a position .

    So a GPS receiver needs initialising before it can measure pseudoranges to satellites and compute a position. Initialising by satellite signal takes 30 continuous seconds of viewing a lightbulb at 20,000km for at least four different satellites. Any lost data means you need a new 30 second signal to be read. However it’s possible to use data gathered by another nearby receiver thats in constant unbroken satellite view, e.g. a GPS receiver at the telephone exchange and transmit that last thirty second burst from every satellite over GSM into your handset GPS in a fraction of a second. At that point the GPS receiver’s cooking on gas and can take a set of pseudorange measurements within a second and give the user a position.

    That’s real AGPS as used on many European, Asian, Australasian and American phone networks.

    Charles - November 7th, 2010
    • But will AGPS drain my phone battery faster than GPS or no GPS at all? Droid X.

      Rosemary - February 20th, 2011
    • Thanks for the clear explanation. I guess A-GPS will use a little more battery power during the time a fix is acquired, since it downloads “the satellite ephemeris data at a telephone exchange” requiring the system to use the radio. But if say, you only want location information in the sort term or even once-off, this will save power since you don’t have to keep GPS on for 2-3 minutes, instead only a few seconds.
      Once a fix is acquired, I think there is no difference between A-GPS and normal GPS.

      Riaan - May 25th, 2011
  7. I’m interested in the new iPad and want GPS. I don’t have a phone plan,and don’t want a phone plan. I’m looking for a satellite based tablet to use for lake charts (no wifi).
    Will it do the job?

    Lou - March 14th, 2012

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