Assisted GPS
Assisted Global Positioning System (A-GPS, dt assisted Global Positioning System) is a method for transmitting auxiliary data on a different GPS transmission network for more accurate and faster GPS fix.
Disadvantages of the conventional GPS
The satellite-based positioning with GPS was primarily designed to determine the position of a continuously active receiver outdoors. For frequent interruptions in the satellite receiver connected to a temporary change of location ( such as airplanes, public transport, large building complexes, through tunnel), the GPS system was not designed. Therefore, the recalculation of the current geographic position for such abrupt 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 offer there location-based services ( LBS ) can.
In the conventional GPS the following problems arise:
- The time to first fix is dependent on the timeliness of the almanac stored in the receiver, which is transmitted to the GPS signal, and in which the satellite their orbital data (ephemeris ) list. If the device was so long time not active, relatively much information must be received before a fix is possible ( after more than 2 to 6 hours: about 45 seconds, after several days or if the unit moves without receiving more than 300 km has been up to 12.5 minutes).
- In urban areas the clear view to the GPS satellites are often severely limited, if not impossible in closed rooms and not already given in tunnels.
- The power consumption of the satellite receiver is relatively high, a disadvantage of the small battery capacity of current mobile devices.
Operation of A-GPS
A-GPS reduces these problems by, for example, uses the GSM network to the receiver to transmit auxiliary data by which the position determination leads to success faster.
Isolation of the radio cell
For mobile phones, the approximate whereabouts is based on the radio cell, which serves the phone already known. This place can be clarified by measurements of the signal propagation times of another adjacent cell towers on. The reception of at least three base stations is required in order to calculate the location to clear this way. The amount can not be determined in this procedure by using three base stations.
This roughly determined position can also be used to restrict the search space for the satellite signals ( identity of the currently visible satellites, approximate duration, Doppler shift ) and thus to speed up the processing of the measurement.
Tracking via mobile phone
Via a separate data connection, e.g., a mobile communications network, the receiver is supplied with additional auxiliary information, such as the approximate location of the receiver ( for example, determined by mobile positioning ).
Almanac update
In the conventional GPS receiver has two tasks. He measures the travel time of the signals, and it reads the data transmitted by the satellites, which among other orbital parameters and error corrections included (almanac ). When A-GPS almanac of reference receivers is read which are stationary placed in locations with a good view of the sky and their exact position data has already been measured. The mobile receiver no longer needs to update lengthy on the satellite signal the almanac. Furthermore, it affects the GPS receiver of the mobile receiver to predict the Doppler of the GPS signals better, so for Phase Lock Loop ( PLL) and delay lock loop (DLL) narrower bandwidths can be used. As a result, the GPS receiving section by up to 30 dB weaker use GPS signals as they occur, for example, in buildings. In order to reduce errors in GPS position considerably, error corrections can be transmitted according to the principle of differential GPS in addition.
Variants
Depending on where the actual calculation of the position takes place, one distinguishes the network-based and terminal-based mode. In network-based A- GPS ( Mobile Station Assisted, MSA) sends the terminal ( ie, for example, the mobile phone ), the measured signal propagation times of the respective satellite signals (ie ultimately the distances ) to a server in the mobile network, which calculates the exact position and back to the requesting terminal transmits. The auxiliary data in this case are only of the search space parameters. When terminal-based mode ( Mobile Station Based MSB), the terminal receives only the satellite and auxiliary data and calculates its position by the measurement itself
For A-GPS various signaling by 3GPP and OMA have been standardized. A common method is for example SUPL ( Secure User Plane ).
If there is just no contact with GPS satellites or lacks a GPS receiver at all, so also only the signal propagation times can be used to calculate the position. This is then, however, usually relatively inaccurate. Since there is no GPS signal is used, such a method, however, can not be referred to as A -GPS.
On the other hand, an implementation of A-GPS completely without support of the network operator is conceivable. Even the transmission of almanac a faster and more robust channel as the GPS signals in many cases leads to a considerably accelerated position determination. This method is also supported by some car navigation devices. The data is downloaded by the user from the internet and copied to the device.
Mobile phones such as the Nokia N8 can determine their position based on several sources ( methods of determination ) more or less accurate even in buildings. Are available including: assisted GPS, integrated GPS, external Bluetooth, GPS and Wi-Fi.
A WLAN - based location works like a radio cells, but can be much more accurate if more wireless networks are in range, and they are now stored with exact position in a database. For this purpose, a software that is usually already integrated into the wireless LAN adapter, permanently looking for MAC addresses and wireless network name. Both position and movement direction can be determined, for example, about the signal propagation times and quality. The software detects multiple networks, evaluates their quality signal and compares the information to a database.
Although the Wi-Fi based location just as the position detection via network-based services uses a GPS signal, it contributes as an additional method of determination for GPS positioning.
Support
A-GPS needs the support of the GSM network operator in order to function optimally. The network elements can provide assistance in scope information for each mobile phone ( via "Service Mobile Location Center "). In addition, support is needed in the mobile phones, which can also evaluate the assistant information in addition to a GPS tracking chip. As part of the Enhanced 911 program in the U.S., all providers of wireless services had to upgrade their infrastructure so that since the end of 2005 emergency calls from mobile phones can be located precisely - in this second phase was it mostly with triangulation, the results also to the mobile station available. In Japan, even to all mobile phones sold after April 2007 the third generation have the A- GPS functionality for emergency calls, which can evaluate the support information of the network. A similar regulation is expected in the EU for the year 2014, when the European Galileo satellite system is to determine the position available.
A-GPS is used at least since 2009 in all GPS - enabled mobile phones and in all GSM -enabled navigation devices. So that is possible than with conventional devices using this a substantially quicker activation of the GPS function. A- GPS is available in most GSM networks in Europe throughout.
Criticism
For A-GPS usage fees may apply (usually in the form of connection charges of the provider). Alternatively, sometimes also reaches a more sensitive GPS receiver, such as an external GPS mouse out to quickly determine yourself in the position of buildings, provided current almanac data is available.