Differentiate Precision From Accuracy

GPS Interference Testing

Interference can pose a threat to the reception of GNSS signals in a variety of ways. Even low-level signals have the potential to interfere with GNSS receivers, which require very high sensitivity for acceptable performance due to the extremely low received GPS signal power at the Earth’s surface.

Recently, a new potential interference threat has emerged and has attracted much attention in the United States and elsewhere. In January 2011, the U.S. Federal Communications Commission (FCC) granted satellite broadband provider LightSquared Subsidiary LLC a waiver to operate a terrestrial-only Long Term Evolution (LTE) network that would use L-band spectrum adjacent to the L1 frequencies occupied by GNSS.

The FCC’s waiver included a condition that the company must prove that its signals cause no interference to GPS. The commission set a June 15, 2011 deadline for LightSquared to submit a final report on the issue.

This very short timescale drove an urgent need for subject matter experts within the GPS community to form a GPS Technical Working Group (TWG). The TWG would appoint dedicated expert teams to conduct a comprehensive test campaign to investigate the potential for interference with all categories of GPS receivers. This was especially challenging because, with the exception of the cellular industry, few standardized industry approaches to GPS receiver performance testing exist, especially with regard to interference.

This article uses this test campaign, the methodology and results of which are now in the public domain, as a framework for discussion because it presents an ideal opportunity to review the methods used to quantify interference effects on GPS receivers, especially since the scope included the use of both live and synthesized laboratory signal environments.

Although this article explores these methods in the context of the LightSquared testing campaign, they are applicable to a much wider range of potential interference sources for GNSS. It is not intended to express an opinion on the part of the authors about the impact of LightSquared signals, nor on whether LightSquared should be allowed to deploy. It does, however, present a selection of the results from the TWG report, which was released on June 30, 2011, to illustrate the various methods employed and in particular to describe the value of laboratory testing.

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