What is Assisted GNSS?

Assisted GNSS or Augmented GNSS is a type of Global Positioning System, which takes the assistance of mobile network towers to reduce time to fix position or TTFF. In this setup, Mobile towers have GNSS receivers that continuously receive signals from GNSS satellites and have powerful servers to process this received information. This information is then sent to the GNSS receiver via the mobile network. The receiver can use this data to calculate the position when the Satellite Signals are weak or can use this data for error correction to improve positioning accuracy. Assisted GNSS has a faster start-up time compared to a standalone GNSS system. The start-up time is the time required to establish a reliable connection between the receiver and satellites.

The assisted GNSS signal that is received at the mobile tower consists of almanac and ephemeris data.

• Almanac data contains satellite vehicle orbital information and allows GNSS receiver to predict which satellites are overhead when they’re looking for GNSS signals. Using almanac data saves time as the receiver can concentrate on those satellites that it can see and ignore others that are out of its view.
• Ephemeris data is the orbital position of each satellite. This data contains information on week number, satellite accuracy, and health, age of data, satellite clock correction coefficients, orbital parameters. It guarantees real-time satellite coordinate computation which is required in position computation.

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 ephemeris and the almanac data 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. But in assisted GPS the orbital information from the satellite can be stored in the database of the assisted GPS server. This information can be accessed using mobile-network radio bearers such as GSM, CDMA, WCDMA, LTE. Usually, the data rate of these bearers is high, hence downloading orbital information takes less time.

Assisted GNSS has two modes of operation:

1. Mobile Station Assisted (MSA): In MSA mode, the assisted GNSS capable device receives acquisition assistance, reference time, and other optional assistance data from the mobile network. The mobile service provider continuously logs GNSS information (almanac and ephemeris data) from the GNSS satellites using an assisted GNSS server in its system. With the help of the above data, the assisted GNSS server calculates the position and sends it back to the assisted GNSS device.

2. Mobile Station Based (MSB): In MSB mode, the assisted GNSS capable device receives ephemeris, reference location, reference time, and other optional assistance data from the assisted GNSS server. With the help of the above data, the assisted GNSS device receives signals from the visible satellites and calculates the position.

This protocol is part of the Positioning Protocol defined by two different standardization bodies -  3GPP and Open Mobile Alliance (OMA).