Global Positioning System Assistance Data Message

Description

The GPS Assistance Data Message is a broadcast message defined in 3GPP specifications 44.035 and 44.071, which are part of the GERAN (GSM/EDGE Radio Access Network) series. Its primary purpose is to deliver GPS differential corrections to mobile stations (MS) to enhance the accuracy of location estimates. This message is a key component of Assisted GPS (A-GPS) technology, which combines information from the cellular network with GPS satellite signals to improve positioning performance, especially in challenging environments such as urban canyons or indoors.

In a typical A-GPS architecture, the network provides assistance data to the MS to reduce the time to first fix (TTFF) and improve sensitivity. The GPS Assistance Data Message specifically contains differential corrections, which are adjustments to the GPS pseudorange measurements to correct for atmospheric delays, satellite clock errors, and other inaccuracies. These corrections are typically generated by a reference GPS receiver network and are valid for a certain period and geographic area. By broadcasting these corrections, the network enables multiple MSs to benefit without requiring individual point-to-point connections, thus saving signaling resources.

The message is broadcast on a common control channel, such as the Broadcast Control Channel (BCCH) or a dedicated broadcast channel for location services. The exact mapping and scheduling are defined in the specifications. The MS receives the message, decodes the differential corrections, and applies them to its own GPS measurements to compute a more accurate position. This process is transparent to the user and can significantly improve the accuracy from tens of meters to a few meters.

The GPS Assistance Data Message is part of a broader set of assistance data messages defined for A-GPS in GERAN, which may include ephemeris data, almanac data, ionospheric models, and time assistance. However, the differential corrections are particularly important for high-accuracy applications like E-911 (emergency calling) and location-based services. The message structure includes fields for the correction data, reference time, and possibly integrity information. The specifications 44.035 and 44.071 detail the format and encoding of these messages, ensuring interoperability between different network equipment and mobile devices.

Over the releases from Rel-8 to Rel-19, the definition of the GPS Assistance Data Message has remained unchanged, indicating its stable role in the GERAN location services framework. However, the surrounding ecosystem has evolved, with improvements in GPS technology, the introduction of other global navigation satellite systems (GNSS) like Galileo and GLONASS, and the integration of A-GPS with other positioning methods in later 3GPP releases. Despite these advancements, the GPS Assistance Data Message remains a fundamental building block for location-based services in GSM/EDGE networks.

Purpose & Motivation

The GPS Assistance Data Message was created to address the limitations of standalone GPS in mobile devices, particularly in terms of accuracy, time to first fix, and power consumption. Standalone GPS receivers require a clear view of the sky and can take several minutes to acquire satellite signals and compute a position, which is impractical for emergency services and many commercial applications. By providing differential corrections via the cellular network, the GPS Assistance Data Message enables the MS to correct errors in the GPS measurements, leading to faster and more accurate positioning.

This message solves the problem of how to efficiently distribute GPS correction data to a large number of users without overwhelming the network with individual data sessions. Broadcasting is an efficient one-to-many distribution mechanism that leverages the inherent broadcast nature of the radio interface. It allows all MSs in a cell to receive the same corrections simultaneously, reducing signaling load and latency. This is especially important for emergency call location, where many users might need assistance at the same time.

The motivation for including GPS differential corrections in a broadcast message also stems from regulatory requirements, such as the FCC's E-911 mandate in the United States, which requires accurate location information for emergency calls. To meet these requirements, 3GPP developed A-GPS and the associated assistance data messages. The GPS Assistance Data Message is a critical piece of this puzzle, as it provides the corrections needed to achieve the required accuracy. Over time, the concept has been extended to support other GNSS systems, but the original GPS Assistance Data Message remains a testament to the importance of network-assisted positioning in mobile communications.