Assisted Global Positioning System
Assisted Global Positioning System ( A-GPS , German supported global positioning system ) is a process for the transmission of auxiliary data via a transmission network other than GPS for more precise and faster GPS position determination.
Disadvantages of traditional GPS
The satellite-based position determination with GPS was mainly developed to determine the position of a continuously active receiver outdoors. GPS was not designed for frequent interruptions in satellite reception combined with a temporary change of location (e.g. in airplanes, public transport, large building complexes, tunnel passages). Therefore, the recalculation of the current geographical position after such sudden changes of location takes a long time. Due to the high accuracy of the method GPS is still interesting for use in mobile phones to there location-based services (LBS) offer.
The following problems arise with conventional GPS:
- The time until the first position is determined depends on the currentness of the almanac stored in the receiver , which is transmitted with the GPS signal and in which the satellites list their orbit data ( ephemeris ). If the device has not been active for a long time, a relatively large amount of information must be received before a position can be determined (after more than two to six hours: around 45 seconds; after several days or if the device has been moved more than around 300 km without reception : up to 12.5 minutes).
- In urban surroundings, the free view of the GPS satellites is often severely restricted, in closed rooms even impossible and in any case not given in tunnels.
- The power consumption of the satellite receiver is comparatively high, a disadvantage given the low battery capacities of current mobile devices.
How A-GPS works
A-GPS reduces these problems by e.g. As the GSM - mobile network used to transmit to the receiver auxiliary data by which the position determination quickly leads to success.
Location using the cellular network
In the case of cell phones, the approximate location is already known based on the radio cell that the phone operates. This location can be made more precise by measuring the signal propagation times from other neighboring cell phone towers. Reception from at least three base stations is required in order to be able to clearly calculate the location in this way. With this method, the altitude cannot be determined even with three base stations.
This roughly determined position can, however, be used to limit the search area for the satellite signals (identity of the currently visible satellites, approximate transit time, Doppler shift ) and thus to accelerate the processing of the measurement.
Almanac update
With conventional GPS, the receiver has two tasks. It measures the transit time of the signals and it reads the data sent by the satellites, which include orbit parameters and error corrections ( almanac ). With the A-GPS, the almanac is read by reference receivers that are stationary in locations with a good view of the sky and whose exact position data has already been measured. The mobile receiver then no longer has to tediously update the almanac via the satellite signal. Furthermore, the GPS receiving part can thereby better predict the frequency shift of the GPS signals due to the Doppler effect, so that narrower bandwidths can be used for phase lock loop (PLL) and delay lock loop (DLL) . This means that the GPS receiver can use GPS signals that are up to 30 dB weaker, such as those used in buildings.
variants
- 1. Ground-based positioning
The position is calculated for the first FIX using cellular or WiFi location (for example with Apple devices).
(There are different connections between GPS and GSM, so that some devices also work with GSM switched off, others only with GSM switched on but without SIM or GSM with SIM and network.)
- 2. Offline I
Almanac data and / or NTP time corrections for the first fix are loaded from the Internet and / or from the mobile station (for example AGPS cell phones, Android devices, older PNA).
(Often the data is deleted as soon as the device is switched off, or it expires after a few hours.)
- 3. Offline II
Pre-calculated almanac and path data as well as basic time corrections are loaded from the Internet (no data from the mobile stations) and stored on the device for 3 to 14 days (e.g. AGPS devices with Windows Mobile 6).
- 4. Mixture of 1, 2 and 3
Depending on where the actual calculation of the position takes place, a distinction is made between the network-based and the terminal-based mode. With network-based A-GPS (Mobile Station Assisted, MSA) the terminal (e.g. mobile phone) sends the measured signal propagation times of the respective satellite signals (i.e. ultimately the distances) to a server in the cellular network, which calculates the exact position and then sends it back to the person making the request Terminal transmitted. In this case, the auxiliary data consist only of the search space parameters. In the terminal-based mode (Mobile Station Based, MSB), the terminal only receives the satellite and auxiliary data and calculates its position itself after the measurement.
A-GPS are different signaling of 3GPP and OMA been standardized. A common method is, for example, Secure User Plane Location (SUPL).
If there is currently no contact with GPS satellites or if there is no GPS receiver at all, the signal propagation times alone can be used to calculate the position. However, this is usually relatively imprecise. However, since no GPS signal is used here, such a method cannot be referred to as A-GPS.
On the other hand, an implementation of A-GPS without the support of the network operator is also conceivable. Even the transmission of the almanac via a faster and more robust channel than the GPS signals leads in many cases to a considerably accelerated position determination. This method is supported by some car navigation devices, among others. The user downloads the data from the Internet and transfers it to the device.
Cell phones such as the Nokia N8 can determine their position more or less precisely inside buildings using several sources (determination methods). The following are available among others: supported GPS, integrated GPS, external Bluetooth, GPS and WLAN.
A WLAN-based location works in a similar way to one via radio cells, but can be much more precise if several WLANs are within range and these have been saved with their exact position in a database. For this purpose, software that is usually already integrated into the WLAN adapter can permanently search for MAC addresses and WLAN names. Both position and direction of movement can be determined, for example, via the signal propagation times and quality. The software recognizes several networks, assesses their signal quality and compares the information with a database.
Although Wi-Fi and cellular-based location do not use GPS signals, they are additional methods of determining your position.
support
A-GPS needs the support of the GSM network operator in order to function optimally. The network elements can provide various amounts of assistance information for the individual mobile devices (via “Service Mobile Location Center”). In addition, support is needed in the devices, which can evaluate the assistance information in addition to a GPS tracking chip. As part of the Enhanced 911 program in the USA, all wireless service providers had to retrofit their infrastructure in such a way that, since the end of 2005, emergency calls from mobile phones can be precisely localized - in this second phase it was mostly done with triangulation, the results of which were also transmitted to the mobile phone To be available. In Japan, all third-generation cell phones sold after April 2007 must have A-GPS functionality for emergency calls that can evaluate the network's assistance information.
A-GPS has been used in all GPS-enabled mobile phones and in all GSM-enabled navigation devices since 2009 at the latest. This enables the GPS function to be activated much faster with these devices than with conventional ones. A-GPS is available in most GSM cellular networks, throughout Europe.
criticism
A-GPS may incur usage fees (mostly in the form of connection fees from the provider). Alternatively, a more sensitive GPS receiver, such as an external GPS mouse , is sometimes sufficient to quickly determine the position even inside buildings, provided current almanac data is available.
If the A-GPS receiver sends personal information, such as B. the IMSI to the A-GPS server, the operator of the A-GPS server can track the location of the user.
Web links
See also
Individual evidence
- ↑ kowoma: The structure of the GPS signal, September 16, 2008
- ↑ "A-GPS should facilitate navigation and location services" , Marie-Anne Winter, Teltarif.de, March 18, 2005
- ^ "How SUPL Reveals My Identity And Location To Google When I Use GPS" , Martin Sauter, wirelessmoves.com, August 31, 2014