A-GPS(Assisted GPS)

 Assisted GPS (A-GPS) :-  is used to speed up start-up times of GPS-based positioning systems. GPS may have problems getting a lock when the signal is weak and in such a case A-GPS would assist in getting a lock.

This, however, is achieved by the use of an Assistance Server, so a data connection is required and charges may apply for the data transfer.

Salient features :-

● A-GPS takes data from satellites which are used for GPS.

● A-GPS gives correct navigational feedback even in the city where direct contact to satellite is not possible.

● A-GPS works in smart phones with the help of Internet and additional charges may apply when using it.

Basic concepts

Standalone GPS provides first position in approximately 30–40 seconds. A standalone GPS needs orbital information of the satellites to calculate the current position. The data rate of the satellite signal is only 50 bit/s, so downloading orbital information like ephemerides and the almanac directly from satellites typically takes a long time, and if the satellite signals are lost during the acquisition of this information, it is discarded and the standalone system has to start from scratch. In A-GPS, the network operator deploys an A-GPS server, a cache server for GPS data. These A-GPS servers download the orbital information from the satellite and store it in the database. An A-GPS-capable device can connect to these servers and download this information using mobile-network radio bearers such as GSM, CDMA, WCDMA, LTE or even using other radio bearers such as Wi-Fi or LoRa. Usually the data rate of these bearers is high, hence downloading orbital information takes less time.

Modes of operation

A-GPS has two modes of operation:

Mobile Station Assisted (MSA)

In MSA mode A-GPS operation, the A-GPS capable device receives acquisition assistance, reference time and other optional assistance data from a mobile service provider. The mobile service provider continuously logs GPS information (mainly the almanac) from the GPS satellites using an A-GPS server in its system. With the help of the above data (the data received from the mobile device and the data already present in an A-GPS server) the A-GPS server calculates the position and sends it back to the A-GPS device.

Mobile Station Based (MSB)

In MSB mode A-GPS operation, the A-GPS device receives ephemeris, reference location, reference time and other optional assistance data from the A-GPS server. With the help of the above data, the A-GPS device receives signals from the visible satellites and calculates the position.