AGPS – not a real GPS?


how-gps-works.jpg

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.

35 thoughts on “AGPS – not a real GPS?

  1. Howard says:

    So if you have Tmobile, and get a phone with AGPS, can you:

    1) turn off AGPS on the phone? (i know depends on the phone but for most phones)

    2) Do i need a Data plan to access AGPS?

  2. Although I can’t say for specific telco, but with my knowledge of AGPS and how it function,

    yes, you can turn off AGPS (a few phones allow you to do that, e.g. Samsung i780) and you don’t need a data plan to download AGPS data, WIFI or Activesync can also do the job, but the option could vary based on the manufacturer’s implementation.

    FWIW, a few folks I know turned off AGPS data, then realised how much they have missed in terms of the super fast GPS acquisition.

  3. Howard says:

    awesome! thats great news. thanx for responding quickly. but now i got another question…

    Will any phone gps (and agps) work for the usa?

    Will toshiba portege G810 (unlocked) work for Tmobile USA (GPS and AGPS)?

  4. any phone with GPS will work in USA. Satellites hover around the earth, so you will not be discriminated in certain region unless you are under a tree canopy 😛

    With AGPS, again, the telephony is in the equation simply because it’s a medium to receive the AGPS data through the internet. So again, it will work in USA.

    Have never come across Toshiba portege G810, is it a laptop that has GPS function?

  5. Howard says:

    the G810 is Toshiba’s new phone line… been seeing it everwhere online. looks very slick, and supposedly comes out Q3 this year.

    thanx for all the information!

  6. Asssisted GPS? what does it mean? Can REGULAR GPS device uses this technology? or you have to throw the OLD GPS to the trash bin and buy a new device to get this AGPS?

  7. Hey,

    This a great description of “Enhanced GPS”. But Assisted GPS and Enhanced GPS are two different things.

    What you’ve described – the “back channel” downloading of almanac/ephemeris data is “Enhanced GPS”. It reduces your TTFF by improving the download speed for these required chunks of data.

    Assisted GPS is a bit different, and more “hands-on” by the network. There’s actually two main flavors:

    MS-Based: The network provides an initial gross estimate (via cell ID-based geolocation) of the handset’s location, and the handset does its own geolocation using this initial estimate and the received satellite data.

    MS-Assisted: The handset just pipes its received satellite data to the network; the network geo-locates the handset and sends the location coordinates back to the handset.

    Both of these methods can be faster, more accurate, and even less of a battery hog than Standalone/”Enhanced” GPS.

  8. Hi Andrew,

    Thanks for your sharing. I agreed there’s other flavour of Assisted GPS, or perhaps that’s the true definition of Assisted GPS, whereas the one I had described might be better called as Enhanced GPS. However, I don’t believe there’s a standard terminology for all these, and the terms are used inter-changably, and therefore, I just wanted to clarify most of the A-GPS implementation (or what you called Enhanced GPS) are not cell-based GPS solution, and more importantly, they are standalone GPS solution that has its own GPS chipset.

    BTW, do you know which gadgets or devices today implemented MS-assisted and MS-Based? And what does MS stand for?

  9. Cooks says:

    Here’s Sprint’s definition:

    1) MS-BASED LOCATION: The location is calculated by the Mobile Station (MS) with initial help in acquiring GPS satellite data provided by the Sprint Network Position Determination Entity (PDE). The MS may continue to provide location fixes for a period of time without use of the PDE until the MS determines that more current satellite information is required from the PDE. MS-based location can only be used through a certified Java application and not through a browser session. MS-based location will not work where GPS satellite data cannot be received by the MS (e.g. indoors).

    2) MS-ASSISTED LOCATION: The location is calculated in the Sprint Network with assistance from the Mobile Station (MS). The PDE will provide the MS with help in acquiring GPS satellite data. The MS will provide back to the Sprint Network any available GPS and/or CDMA network information that can assist the PDE in calculating the location. MS-assisted location can be used through a certified Java application or through the Sprint Business Mobility Framework, but not through a browser session. MS-Assisted location works best where the device can receive GPS satellite data (e.g. outdoors), but may still provide location of a degraded accuracy without GPS data (e.g. indoors).

  10. HJ says:

    Hello
    I’m looking to buy and unlocked Nokia phone to use with T-mobile. The nokia E71 says it has GPS: integrated a-GPS.
    My question is, will this GPS technology be able to help me figure out simple compass directions like 26 degrees W?
    I need this for work purpose so that i don’t have to always carry a compass.

    Thanks
    HJ

  11. thanks Cooks, the thing is in today’s world, we don’t really have MS-assisted solution, at least implemented in the consumer market, or maybe it’s more prevalent in US but not the rest of the world. But still, it’s good to understand other technologies, and we never know when these technologies become popular again.

    HJ,

    GPS technology (by then, I meant to include both the satellites and client software) can serve as a compass, however, if you are static, it may not know where you are, only when you move, then will the combination of satellite paths and the triangulation process at the software/client end, will know where you are heading to.

  12. isim yok says:

    Does a-gps use only the network data or use both of network data and satellites? I asked this question because, in some phone specifications, I see that there is only a-gps. Does “a-gps phone” mean that the phone has a gps chipset? or does a-gps phone needs network data always?

  13. Hey neat site.. I got this E71.. and tried everything but i can never achieve a Satellite lock.. i tried with Nokia Maps 2.0 and also Google Maps.. but it dosen’t seem to work. Once i kept it on in ma car for about an hour while i drove.. nothing.. nxt i kept it on overnight in ma home beside ma bed while i slept.. lol nothing?.. what gives?.. yah Google Maps used WiFi for aGPS and trangulated ma position without using any GPS lock but that is different. Why i never get any GPS lock?

  14. Google Maps do make use of the LBS to locate your proximity if you do not turn on your GPS.

    if you are placing your phone inside the car (that’s heavily tinted), or inside a room, you will never get a lock, at least not a fast TTFF.

    Try this, take your phone out to an open space where you/your phone can have clear view to the sky. turn on your GPS, and let it get a fix. You should be able to get a fix within 2 minutes.

  15. Shailesh says:

    If u hv heard about Google Navigator (Not By google) it downloads Google maps on ur mobile..and can be used for gps
    in such case will AGPS work properly with Google navigator…..

    • zen says:

      why not. Lets be clear again, A-GPS (or more commonly known as Q-GPS now adays) is just a complimentary tool to achieve faster TTFF for a GPS receiver, it does not interfere, or override whatever position that the GPS receiver has triangulated, which is used for the relevant GPS activities such as navigation as you have quoted as an example.

  16. youpi says:

    does a-gps gives live data, meaning can trace a traffic jam few km in front of us? or is it just a static map?

    can we use it in the mountain and have the idea of 3 dimension of the mountain or it only useful in the plain area like streets?

    thanks

    • zen says:

      no, A-GPS is only orbital path of the satellites. It doesn’t contain anything relating to the map or street directory, that’s a function of navigation software.

      your second question, should be asking about GPS in general, not A-GPS. Again, A-GPS is meant to complement your GPS receiver. By itself, it’s not going to give you any location acquisition or anything meaning to you.

  17. Toni says:

    Hello, I read all this about the GPS & AGPS and I have two questions.
    The AGPS it is integrated only in the mobile phones with GPS support or it is integrated in the PDA too? and the seccond did I have to pay to my mobile provider for the AGPS because my phone use their antenna or this is free (I want to by mobile phone with integrated GPS Nokia 5800, maybe I must to pay for the GPS, and please tell me advice that maybe it’s better to by PDA or Mobile with integrated GPS)
    Thanks

  18. Toni says:

    Hello, I read all this about the GPS & AGPS and I have two questions.
    The AGPS it is integrated only in the mobile phones with GPS support or it is integrated in the regular GPS device too? and the seccond did I have to pay to my mobile provider for the AGPS because my phone use their antenna or this is free (I want to by mobile phone with integrated GPS Nokia 5800, maybe I must to pay for the GPS, and please tell me advice that maybe it’s better to by regular GPS device or Mobile with integrated GPS)
    Thanks

    • zen says:

      @Toni,

      AGPS is usually integrated with phone due to its built-in internet connectivity that’s required to download the almanac and ephemesis data.

      To me, buying a regular GPS device or Mobile with integrated GPS shouldn’t be driven by the download cost, because the latter can be operated even without downloading the GPS data, as the latter is meant to complement, not overtaking the operation of the GPS. GPS location acqusition (i.e. the ability to identify your position based on satellite information) is free, but ability to tell your location with respect to a map is not, directly or indirectly. So your question should be more whether you want a standalone GPS device for a specific purpose, e.g. you would mostly used when driving, or you want the convenience of turning on your GPS capability as and where you are, which is where a mobile device with integrated GPS will be useful since a mobile device is something you would carry most of the time, not a dedicated GPS device.

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