Satellite navigational systems provide positional and timing information to earth-bound receivers. Each system has its own constellation of satellites orbiting the Earth, and, in order to calculate its position, a receiver on Earth uses the satellites “in view” (i.e., in the sky above) from that system's constellation. Global Navigational Satellite System (GNSS) is often used as the generic term for such a system, even though such navigational satellite systems include regional and augmented systems—i.e., systems that are not truly “global.” The term “GNSS,” as used herein, covers any type of navigational satellite system, global, regional, augmented or otherwise, unless expressly indicated otherwise.
The number of GNSS systems, both planned and presently operational, is growing. The widely-known, widely-used, and truly global Global Positioning System (GPS) of the United States has been joined by other global systems: Russia's GLObalnaya NAvigatsionnaya Sputnikovaya Sistema (GLONASS), Europe's Galileo system, and China's BeiDou system—each of which has, or will have, its own constellation of satellites orbiting the globe. Regional systems (those that are not global, but intended to cover only a certain region of the globe) include Japan's Quasi-Zenith Satellite System (QZSS) and the Indian Regional Navigational Satellite System (IRNSS) currently being developed. Augmented systems are normally regional as well, and “augment” existing GNSS systems with, e.g., messages from ground-based stations and/or additional navigational aids. These include the Wide Area Augmentation System (WAAS), European Geostationary Navigation Overlay Service (EGNOS), Multi-functional Satellite Augmentation System (MSAS), and GPS Aided Geo Augmented Navigation (GAGAN). Regional GNSS systems, such as QZSS, can also operate as augmented systems.
A GNSS receiver may be implemented in a mobile terminal, a tablet computer, a camera, a portable music player, and a myriad of other portable and/or mobile personal consumer devices, as well as integrated into larger devices and/or systems, such as the electronics of a vehicle. The term “GNSS receiver” as used herein, covers any such implementation of GNSS capabilities in a device or system.
Broadly speaking, the reception/processing of GNSS signals involves three phases: acquisition, tracking, and positional calculation (producing a “navigation solution” or “position solution”). Acquisition is the acquiring or identifying of the current satellites in view (SVs), which means satellites that are “visible” overhead, i.e., the satellites from which the GNSS receiver can receive signals. Acquisition can be understood as “finding” the SVs, while tracking is the fine tuning of the signals received from the acquired SVs and keeping track of the acquired SVs over time. Once acquired and adequately tracked, the SV's signals are processed to extract the navigational, positional, timing, and other data transmitted in each SV's signal, and the data from all the SV's being tracked is then used to calculate the GNSS receiver's position. Of course, there are further complexities to the actual reception and processing of GNSS signals, such as various loops feeding back information between these phases for further correction and adjustment of data, as is known to one of ordinary skill in the art.
However, there are times when most or all of the satellites overhead are completely blocked and/or only weakened versions of their signals reach the GNSS receiver, such as when the GNSS receiver is travelling through a tunnel. This is referred to, among other things, as being “offline” or in a “dead zone”, “null zone”, or “signal null”. Signal nulling may be long and expected, such as the GNSS receiver entering a tunnel, or for short unexpected periods.
Sometimes GNSS signals from specific satellites continue to be tracked while in a null zone, and these GNSS signals lead to bad measurements that are used in position fixing when the GNSS receiver exits the null zone. This is due to insufficient satellite signal strength to maintain a substantially aligned signal track, in terms of automatic frequency control (AFC) carrier tracking and code loop tracking—i.e., the signal energy available for tracking drops below the amount required to maintain substantial alignment with incoming satellite signals. When receiving too few and/or too weak GNSS signals while in a dead zone, the tracking loops are driven in large part by random noise formed by the noise floor of the GNSS receiver, thereby causing the signal tracking to drift randomly. Because the GNSS receiver continues to provide measurements to the navigation engine, large measurement errors (e.g., in position) are observed.