27 Jan 2014
article de recherche
Aérospatiale et transport terrestre
Universal Acquisition and Tracking Apparatus for Global Navigation Satellite System (GNSS) Signals: Research Patent Introduction (RPI)
An RPI is a blog article introducing Research Patent Application Publications and Research Papers done by researchers from École de technologie supérieure (ÉTS) de Montréal.
Over the years and especially since 2000 when the Selective Availability (SA) feature of the Global Positioning System (GPS) was deactivated, satellite based positioning has become a widely used technique in a variety of application fields. However, its use remains limited in terms of availability, integrity, accuracy and resistance to interference . These limitations indicate areas where current GPS receivers have exhibited a lack of robustness. The present invention relates to the construction on-the-fly of a Global Navigation Satellite System (GNSS) receiver according to different configurations including signal standards.
The modernization of existing Global Navigation Satellite Systems and the arrival of new systems have diversified to a great extent the range of navigation signals available for civil use. The additional signals address the four traditional weaknesses of the GPS, namely availability, accuracy, integrity and resistance to interferences. This justifies the importance of implementing new robust acquisition and tracking architectures capable of harvesting all the new signals power in a compact design.
Availability (and continuity) refers to in-view satellites continuously broadcasting signals. Integrity refers to the reliability of the system and of its compliance with specifications, or that signals are as they should be and any anomaly should be promptly identified. Accuracy refers to the resolution of the navigation solution and the precision of the computed position. This depends on both the Dilution Of Precision (DOP), which models the satellites’ geometry, and the User Equivalent Range Error (UERE). Interference resistance is an important characteristic since interference events, whether they are intentional (i.e. jamming) or not, could compromise the raw observation measurements (i.e. code and carrier phase measurements). Unintentional sources of interference include harmonics of other frequency bands, amplifier non-linearities, and multipath, which causes superposed reflections to be added to a direct line of sight (or direct path) signal. Jamming can take the form of narrow- or wide-band, constant or pulsed, fixed or sweeping sinusoidal waves. More sophisticated jammers, such as “spoofers”, could also mimic and alter the true GPS signal by broadcasting another at higher power.
With the advent of more recent Global Navigation Satellite Systems (GNSS) including modernized GPS, GLONASS, Galileo and Beidou (a.k.a COMPASS) systems, new types of signals are now broadcast or at least should start being transmitted soon. These signals help resolve the above limitations of GPS.
One contribution of the new signals is that their higher signal bandwidths will increase the resistance to interference effects by diluting the impact of a narrow band interference over a larger bandwidth .The new signals should also provide better positioning accuracy and resistance to multipath since the chip period is shorter , thus requiring smaller correlator spacing and a higher sampling rate. Longer codes will increase cross-correlation protection of the signals and their robustness in weak signal environments. The higher number of satellites will increase availability and increase accuracy by reducing DOP impact. Integrity should also be improved through more detailed navigation messages.
Currently, the most economical way to produce a navigation receiver is through an Application Specific Integrated Circuit (ASIC), which provides low-cost devices at high volumes. Therefore, hardware resource use of a GNSS channel is still an important consideration, despite the recent trend for pure software receivers or Software Defined Radios (SDR). Indeed, in ASIC designs that are based on signal-specific channels, chances are good that high percentages of the chip will not be used most of the time. Also, populating many dedicated channels drives IC cost up. This is an important consideration going forward, as increasing blocks of functionality i.e. such as GPS, or GNSS receivers are being implemented as IP cores and are therefore expected to occupy less of the overall available ASIC real estate.