Assisted GPS
Posted by Doug Smith on 08/7/07 in Cell Carriers, GPS, General, Pocket PC Phones, PocketPC, Smart Phones, Ultra Mobiles, Windows Mobile 6
With new Cell Phones being released with GPS, I was curious to see how they worked. I especially wondered what AGPS is that will be on the new HTC TyTN II or HOPEFULLY the at&t 8925. AGPS is simply Assisted GPS (Global Positioning Sensor) that makes use of not only Satellites, but also the Cell Phone Carriers Centralized Server. Whereas I have not found out any more information on the type of GPS integrated into the TyTN II (SiRF III), I was able to find a very informative write up in Wikipedia and on Andrew.com. Click below to read more on the AGPS from both sites.
Photo Source: www.andrew.com
Source: Wikipedia
GPS is a satellite based positioning system. Assisted GPS, or A-GPS was introduced to enhance performance. The development of A-GPS was accelerated by the U.S. FCC’s E911 mandate requiring the position of a cell phone to be available to emergency call dispatchers. [1]
Conventional GPS had difficulty providing reliable positions in environments surrounded by tall buildings - the so-called ‘urban canyon’, as well as indoors and under heavy tree cover.
Under these conditions, the GPS signal is often of very poor quality, making it hard for receivers to obtain a position. In addition, when first turned on in these conditions, a non-A-GPS may not be able to download the orbital information from the GPS satellites, rendering it unable to function until it has a clear signal for around one minute.
An A-GPS receiver can address these problems in several ways, using an Assistance Server:
The Assistance Server has a good satellite signal, and lots of computation power, so it can compare fragmentary signals relayed to it by cell phones, with the satellite signal it receives directly, and then inform the cell phone or emergency services of the cell phone’s position.
It can supply orbital data for the GPS satellites to the cell phone, enabling the cell phone to lock to the satellites when it otherwise could not, and autonomously calculate its position.
It can have better knowledge of ionospheric conditions and other errors affecting the GPS signal than the cell phone alone, enabling more precise calculation of position.
Some A-GPS solutions require an active connection to a cell phone (or other data) network to function, in others [2] [3] it simply makes positioning faster more accurate, but is not required.As an additional benefit, it can reduce both the amount of CPU and programming required for a GPS Phone by offloading most of the work onto the assistance server. (This is not a large amount for a basic GPS - many early GPSs ran on 386/16 or similar hardware).
Source: www.andrew.com
Mobile Position Determining Technologies
Assisted GPS (A-GPS)
Assisted-GPS (A-GPS) is a handset-based mobile locating method fundamentally based on GPS locating technology but functionally distributed between the mobile and a centralized server. Mobiles must be specially equipped to receive GPS signals from multiple GPS satellites in order to make measurements of the signals from GPS satellites and relay this information or their calculated position through the mobile network to the SMLC/SAS/SLP software (iSAS: Control Plane, SUPL: User Plane implementations). In cases where the mobile does not compute its own location from GPS data, the SMLC/SAS/SLP uses GPS data relayed from the target mobile to compute the mobile’s position. Once determined, the position can be sent to a gateway mobile location center (GMLC) or SUPL Location Platform (SLP) for distribution to the application that originated the location request. A-GPS is a supplemental form of GPS whereby GPS satellite position data is sent from the SMLC/SAS/SLP through the mobile network to the mobile to assist the mobile in determining which GPS satellites it should attempt to acquire. This supplemental data is intended to:Speed the mobile’s satellite signal acquisition process.
Increase the mobile’s ability to receive signals from the target GPS satellites.
A-GPS is intended to reduce latency and improve other location performance when a mobile’s view of GPS satellites is obscured. That can occur when the caller is in a building, in a vehicle, or in an “urban canyon.” Although A-GPS does improve overall locating performance, accuracy, latency, and yield performance can be subject to degradation if a mobile’s view of GPS satellites is obscured.A-GPS has been used in public safety and commercial applications in CDMA networks where it has been supplemented with AFLT (Advanced Forward Link Trilateration) to enhance location reliability. Since AFLT requires the underlying mobile network to be synchronous, AFLT is not available for wireless networks other than CDMA. Geometrix MLC offers the option of Enhanced CID (using TA/NMR) for GSM network applications as a supplement to A-GPS.
Location request originated by MSC or LBS application
SMLC determines from GPS Worldwide Reference Network which GPS satellites are in range and sends this assistance information to the target mobile
Mobile measures arrival of signals from multiple satellites and may compute, depending on MS capabilities, its position
Mobile sends measured GPS data or computed position to SMLC
SMLC computes mobile’s position if not provided by mobile
Location Center sends position data to GMLC (through BSC and MSC)
GMLC sends position data to LBS application
A similar procedure occurs with A-GPS Operation in SUPL (User Plane) operation, In SUPL, however, the LBS Application forwards a Location Request to the SUPL Location Platform (SLP), instead of the GMLC. Since SUPL is a data service, the MSC would be replaced by a SGSN, and the A-GPS hub would communicate directly with the SLP.
Written by Doug Smith · Filed Under Cell Carriers, GPS, General, Pocket PC Phones, PocketPC, Smart Phones, Ultra Mobiles, Windows Mobile 6
Tagged:
Did you like this post?
Share your vote!
(No Ratings Yet)
Leave a Reply