September/October 2007

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Time is a crucial factor in satellite-based radionavigation. The elapsed time between the transmission of a GNSS signal and its reception by a receiver, multiplied by the speed of light, provides the basis for calculating ranges to the satellites. However, GPS and Galileo will use different reference time systems creating a time offset, which can complicate the positioning computations in user equipment employing signals from both satellites. A team of researchers at the French space agency CNES describe the effect of this time offset on positioning accuracy and examine three methods proposed to eliminate the offset.

A 2004 agreement between the European Union and the United States — an unprecedented cooperation in GNSS affairs — established a common baseline signal BOC(1,1) for the Galileo Open Service and the modernized civil GPS signal on the L1 frequency (L1C). The agreement also allowed the opportunity for improvements on that signal design, which a bilateral working group subsequently proposed in 2006: the multiplexed BOC or MBOC. Under the terms of the 2004 pact, the EU had the right to decide whether to implement the BOC(1,1) or MBOC as the common baseline. This article describes the process leading up to the recent decision to implement MBOC and provides an overview of the final Galileo signal and frequency plan.

Who helped design all of the Navstar GPS navigation signals, keeps the GPS Interface Control Documents, and patented the innovation that allows an unmanned aircraft to land itself? Karl Kovach, the “GPS guy.”

How familiar is this? You switch on your GNSS receiver. It lights up but won’t acquire satellites and begin tracking signals. Is the problem in the receiver, the signals, or perhaps the operating environment? This article describes a simple but effective receiver-based method for detecting radio frequency interference (RFI) or jamming that can help answer this question. The design does not require the receiver to be tracking for it to reliably provide RFI situational awareness. Instead, the solution uses measurements obtained from the automatic gain control to help determine the jamming-to-noise power ratio, which serves as the key metric for assessing the RF environment.

Securing GNSS systems against unauthorized use and false signals (spoofing) is a matter of growing concern for GNSS operators and users. In this column, the second and final part of a series, the authors explore a variety of methods for user and signal authentication and discuss their application in GNSS.

The first time I heard the term “system of systems” applied to GNSS, I thought to myself, “Yeah, a catchy phrase, but that won’t really happen.” After all, much of the last 15 years has been spent accentuating the differences, divisions, and mutually exclusive competition among the existing and proposed GNSS systems.