International GNSS Service

Description

The International GNSS Service (IGS) is not a 3GPP-internal technology but an external, real-world service whose data products are critical for 3GPP conformance testing of positioning features. The IGS is a voluntary federation of more than 200 worldwide agencies that pool resources and permanent GNSS station data to generate the highest quality data and products for the GNSS community. These products include precise satellite ephemerides (orbits), satellite clock corrections, Earth rotation parameters, and global ionospheric and tropospheric models.

Within the 3GPP ecosystem, the IGS's role is defined in test specifications, particularly for Assisted GNSS (A-GNSS). When testing a UE's positioning capabilities, a simulated or real GNSS scenario is created. The 'true' or reference position of the satellites and the theoretical signal propagation conditions must be known with extreme accuracy to calculate the expected pseudorange measurements and, consequently, the expected UE position fix. IGS's precise orbital and clock data provide this gold-standard reference. Test systems use this data to generate the simulated GNSS signals that the UE under test will receive, or to calculate the expected results from real-signal tests.

The architecture involves the IGS data centers and analysis centers that process data from a global network of reference stations. These processed products are made publicly available with specific latencies (rapid, final, etc.). 3GPP test equipment and simulators integrate software that downloads and utilizes these IGS data products. For example, in a test case validating UE-assisted GNSS performance, the test system uses IGS precise ephemeris data to model the satellite positions precisely. It then compares the position calculated by the UE using received assistance data and satellite signals against the 'true' position derived from the IGS reference. This ensures the UE's implementation meets the stringent accuracy requirements specified in 3GPP standards for emergency services, location-based services, and other applications.

Purpose & Motivation

The IGS is referenced in 3GPP specifications to provide an unambiguous, high-accuracy external reference frame for positioning performance validation. Before the widespread adoption of standardized high-precision references, testing GNSS capabilities in UEs could yield inconsistent results because different test labs might use different sources for satellite orbit and clock data, which vary in accuracy. This made it difficult to objectively verify if a UE met the mandated performance standards for location accuracy.

The integration of IGS into 3GPP test specs solves the problem of test reproducibility and accuracy benchmarking. It establishes a common, globally recognized 'ground truth' that all parties—chipset vendors, UE manufacturers, and test laboratories—can use. This was motivated by regulatory requirements for emergency caller location (e.g., E911 in the USA, eCall in Europe) and the commercial need for reliable location-based services. Using IGS data ensures that 3GPP positioning tests are rigorous, repeatable, and based on the best possible scientific data, which in turn guarantees that UEs deployed in networks provide reliable and accurate location information to end-users and network services.