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  • Philly_Phan Level 6 Level 6 (12,370 points)
    Italics and bolding are mine.

    According to NASA, Assisted GPS is a "service" provided by Differential GPS.

    +Assisted-GPS (A-GPS) data – provides data and information about the GPS orbit and clock states that allow cellular phones to pinpoint their location on phone map applications and when making emergency 911 calls.+

    +A DGPS reference station is used to continuously monitor GPS signals in a given area. Since the position of the reference station has been precisely surveyed, *any errors in the satellite signals themselves can be calculated and corrections broadcast to users in the area.*+

    The following is possibly one source of the confusion.
    +Another form of GPS augmentation uses a transmitter on the ground to transmit GPS-like signals. These transmitters act like pseudo-GPS satellite or "pseudolites." In situations where signals from GPS satellites are blocked or obscured, such as in urban canyons or deep valleys, pseudolites can provide an additional source of GPS signals to improve the availability of GPS service. Pseudolites may be placed in fixed or mobile locations or mobile and transmit on frequencies close to or far from GPS itself depending on local conditions.+

    References:
    http://scienceandtechnology.jpl.nasa.gov/newsandevents/newsdetails/?NewsID=1063
    http://gpshome.ssc.nasa.gov/content.aspx?s=aug
  • Dah•veed Level 7 Level 7 (32,335 points)
    The conversation is not about GPS per say. It is about the wifi iPad. I was explaining that the wi-fi version of the iPad does not have what it takes to have the form of GPS that Apple explains the iPads have, assisted GPS, which in addition to the GPS chip, which communicates with the network of satellites in earth orbit, but would also require the cellular capabilities of the 3G version of the iPad and why.
  • David M Brewer Level 6 Level 6 (9,270 points)
    Wifi model has no GPS
    Wifi+G3 model has GPS...

    How does the GPS work? 2by4, rubber bands and duct tape.
  • Chris CA Level 9 Level 9 (77,695 points)
    A-GPS and DGPS are not the same thing.
    DGPS is a ground station (there are only 100) which checks the GPS signal coming from the satellite and adjusts for anomalies in the signal.
    It then sends this signal out to GPS receivers in the area, which can use it. The iPad does not use it.

    Cell towers locations are already known and these signals are sent out over the cell networks.

    When using GPS, it needs to know where it is to begin with, in order to know which satellites can be seen by the receiver. It'll start looking but takes while to get data from the satellites, which has data on location of all the other satellites.
    A-GPS receivers can get A-GPS data quicker and then it will know what satellites to look for instead of waiting for the actual GPS satellite data.
  • Philly_Phan Level 6 Level 6 (12,370 points)
    Per NASA:

    +The GDGPS System is supported and funded by its many civilian and military users who benefit from its high accuracy correction message, as well as other real-time GPS data and positioning services it provides. *Some examples of GDGPS services include:*+

    +GREAT ALERT – a natural hazard alert system, which was recently successful in predicting the size of the tsunami triggered by a magnitude 8.8 earthquake off the coast of Chile in February 2010.+

    +*Assisted-GPS (A-GPS) data – provides data and information about the GPS orbit and clock states that allow cellular phones to pinpoint their location on phone map applications and when making emergency 911 calls*+

    +Real-Time positioning – with the user module of the RTG software, users can determine their precise location and time whether they are in space, in the air, or on the ground.+
  • Chris CA Level 9 Level 9 (77,695 points)
    Just to clarify. Not looking to argue about it.
    A-GPS system uses GDPS but it is not GDPS. Cell towers (A-GPS) do not send out GDPS data.
    My GPS unit receives DGPS data but does not use A-GPS just like the iPad/iPhone uses A-GPS but does not receive DGPS data.

    A-GPS is transmitted over a cell signal. DGPS is transmitted over the GPS signal.
    Cell phones will send a GPS location to the cell tower which then uses the DGPS data as error correction to better locate the device. The cell phone does not receive DGPS data.

    A-GPS on the iPad is for a quicker location.
    -> iPad: Understanding Location Services
    Improving GPS Accuracy (iPad Wi-Fi + 3G)
    GPS accuracy varies depending on the number of GPS satellites visible to the iPad. Locating all visible satellites can take several minutes, with accuracy gradually increasing over time. Use these tips to improve GPS accuracy:

    Ensure the date, time, and timezone are correctly set on the device in Settings > General > Date & Time.
    Important: Incorrect settings on your computer can sync to your device. Verify the date, time, and timezone on any computer that syncs with your device.
    *Verify that you have a cellular or Wi-Fi network connection. This allows the Assisted GPS (A-GPS) on the device to locate visible GPS satellites faster, in addition to providing initial location information using the Wi-Fi or cellular networks.*
  • Philly_Phan Level 6 Level 6 (12,370 points)
    Chris CA wrote:
    A-GPS is transmitted over a cell signal. *DGPS is transmitted over the GPS signal.*
    Cell phones will send a GPS location to the cell tower which then uses the DGPS data as error correction to better locate the device. *The cell phone does not receive DGPS data.*


    If DGPS is transmitted over the GPS signal and cell phones receive GPS signals, why do they not receive DGPS data? I'm seeing the DGPS ground stations as "fake satellites."
  • Chris CA Level 9 Level 9 (77,695 points)
    If DGPS is transmitted over the GPS signal and

    Correction to my previous comment.
    DGPS is not the same GPS signal, however, it does require a separate receiver and antenna.
    I'm seeing the DGPS ground stations as "fake satellites."

    Sorta, kinda but not really.
    The GPS receiver reads the satellite data and plots a position X/Y/Z.
    The DGPS will do the same then figures the anomolies of the satellite signal and sends out correction info (based on the location of the DGPS station). DGPS station A correction data will likely be different than Station B.
    The GPS receiver gets this corrected data (if it receives DGPS) and "fixes" it's own position. So the position calculated is now X+2 meters/Y+1 meter/Z.

    All GPS receivers start looking for satellite data when first turned on but this takes a bit of time.
    A-GPS will start looking but if it has not acquired all the data, it will look for A-GPS data from cell towers. Once it gets this data, it will know approximately where it is and which satellites should be in view and can then get a faster fix on the satellites.
  • Michael Black Level 6 Level 6 (18,395 points)
    Glorfindeal wrote:
    Even though you have no data plan, the 3g still connects to the cellular service and can use assisted gps.

    Glor


    No it does not. Without a data plan, no carrier will allow a connection to their network by a device (do you really think any carrier is going to let you use their network for free?). Without a data plan, the "assisted" part of the iPad's aGPS is useless, but the GPS still works fine if it can get a satellite signal. Assisted GPS will use cell tower triangulation if possible, but it does not depend on it. The aGPS in the iPad is fully capabale of working as a straightforward, satellite only GPS device.

    The iPhone is exactly the same - turn cellular data off, turn wifi off, open the compass app and as long as you have clear sky above you, you will get a GPS location fix just fine.
  • Chris CA Level 9 Level 9 (77,695 points)
    Assisted GPS will use cell tower triangulation

    A-GPS has nothing to do with cell tower triangulation.
    They did cell tower triangulation long before cell phones had GPS.
  • Philly_Phan Level 6 Level 6 (12,370 points)
    It's making more sense, now. Thanks.

    A major cause of the confusion is semantics, which is not all that unusual. That's the main reason why I did not refer to your device as a "GPS System." It's correctly a "Navigation System that uses GPS." Picky, yes but incorrect semantics result in misunderstandings. I actually found a NASA web page that identified DGPS as "Global DGPS." Gee, "Global Differential Global Positioning System!"

    Chris CA wrote:
    The DGPS will do the same then figures the anomolies of the satellite signal and sends out correction info (based on the location of the DGPS station).


    Read the following and tell me if I'm correct. The DGPS station receives GPS data and calculates its position. However, since it knows its own location, it can then determine the X/Y error in the calculated values. That is the error information that it transmits to other receivers. Naturally, if I'm unable to read DGPS signaling or if I'm out of reach of the signal, I must rely on GPS with the accompanying error.
  • Chris CA Level 9 Level 9 (77,695 points)
    A major cause of the confusion is semantics, which is not all that unusual

    So true.
    The DGPS station receives GPS data and calculates its position. However, since it knows its own location, it can then determine the X/Y error in the calculated values. That is the error information that it transmits to other receivers. Naturally, if I'm unable to read DGPS signaling or if I'm out of reach of the signal, I must rely on GPS with the accompanying error.

    Correct.
    Depending on the use, DGPS may be of little consequence. A 20' error will likely not cause anyone (using the iPad) any problems. If it would cause a problem, they should not be using an iPad for whatever critical application they are doing.
  • Glorfindeal Level 6 Level 6 (9,330 points)
    As far as the iphone is concerned, by law they must allow the connection for 911 services. I would assume since I do get messages from at&t on my iPad when I don't have a data plan, that my ipad is indeed connected to their service.

    Glor

    Message was edited by: Glorfindeal
  • Philly_Phan Level 6 Level 6 (12,370 points)
    Chris CA wrote:
    Depending on the use, DGPS may be of little consequence. A 20' error will likely not cause anyone (using the iPad) any problems. If it would casue a problem, they should not be using an iPad for whatever critical application they are doing.

    Agreed. I'm more concerned about errors in the maps which, of course, have absolutely nothing to do with our discussion.

    Now, what is the signal flow for A-GPS? I'm thinking that the DGPS ground station transmits not only the GPS error information but also its own location. This is received by the cell towers and, after signal conversion, is retransmitted out to the cell phones and similar devices. As soon as I turn my device on, it takes the A-GPS information to use as a "ballpark" estimate of its own location. That, in turn, makes it easier to locate the visible satellites. Close?
  • Dah•veed Level 7 Level 7 (32,335 points)
    According to Apple, assisted GPS is not using cellular triangulation. The AGPS device uses cell towers to more quickly know the location of the GPS satellites. This allows the device to know its location more quickly that waiting for it to connect with the actual satellites, which can actually be difficult for the device to do even with "clear sky above." Being able to see the unobstructed horizon in several directions is a lot more important than the sky above.
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