Maintaining the 1-dB standard

How do we ensure that GPS is protected from harmful interference?

By J. David Grossman, guest columnist

J. David Grossman

Debates in Washington over harmful interference and the coexistence of divergent services are raging. Nowhere are the differences more apparent than when comparing radio navigation services such as GPS to radio communications systems used in wireless communications networks.

How do we ensure that a satellite-based radionavigation service like GPS, which by design operates below the ambient noise floor, is protected from harmful interference? The International Telecommunications Union’s (ITU) definition of harmful interference provides a starting point, by defining harmful interference as a level that “endangers the functioning of a radionavigation service.”

With this foundational definition, the internationally established criterion of a 1-decibel (dB) increase in the noise floor, otherwise known as the 1-dB standard, provides the answer, offering a readily identifiable, objective and predictable metric.

The 1-dB standard uses a 1-dB increase in the noise floor as the distinction between the onset of interference that can be detected by a GPS receiver and harmful interference. (This can be reliably measured by a 1-dB decrease in the carrier-to-noise ratio, C/N₀, reported by the receiver). Thus, the 1-dB standard provides a definitive way to protect GPS receivers from harmful interference. Adherence to this standard helps ensure that systems operating in an adjacent spectrum band do not interfere with GPS.

Why use the 1-dB standard instead of other metrics? The 1-dB standard is based upon well-understood GNSS engineering considerations and is associated with quantifiable changes in the overall noise to which GNSS receivers are subject, with equally well-understood effects on receiver operation. (The 1-dB standard enables system designers and spectrum regulators to carefully assess interference from various sources and analyze their net effect on GNSS receivers).

It also has been adopted internationally and has a long and well-established proven history of protecting GPS operations from harmful interference in both international and domestic regulatory proceedings.

So-called “alternatives” to 1 dB, which may be appropriate in the context of radio communications systems, fail to recognize that the accuracy, integrity and reception (availability) of GPS signals used by a receiver can be degraded by interfering noise in ways not immediately apparent to an end user. This means that the effects of degraded service of GPS signals can still be detrimental well before the user loses position accuracy or experiences complete loss of position.

Additionally, C/N₀ is computed at the entry point of a GPS receiver, such that a 1-dB decrease serves as an early warning of interference potentially becoming harmful. Other metrics, computed further downstream, may be indicative of harmful interference already occurring.

GPS has become a fundamental part of our lives and is an integral engine of the U.S. economy, creating new jobs, and unlocking innovation. Maintaining the 1-dB standard ensures that the GPS success story and American innovation will continue for decades to come.

J. David Grossman is executive director of the GPS Innovation Alliance.