Communications: directive radio wave systems and devices (e.g. – Directive – Including a satellite
Reexamination Certificate
2000-04-20
2001-12-11
Cuchlinski, Jr., William A. (Department: 3661)
Communications: directive radio wave systems and devices (e.g.,
Directive
Including a satellite
C342S357490, C342S357490, C342S406000, C342S455000, C701S213000
Reexamination Certificate
active
06329945
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a satellite navigation receiver and, more specifically, to a receiver that weights range information from satellites according to their directions from the receiver.
BACKGROUND OF THE INVENTION
In a satellite navigation system, multiple satellites, of known positions encode their transmissions in a manner that permits a receiver to calculate its “pseudo-ranges” to the transmitting satellites. The receiver then uses the calculated pseudo-ranges from several satellites to resolve its own position. Examples of such systems are the Global Positioning System (GPS) and Global Navigation Satellite System (GLONASS). The range measurements, and the resulting calculation of a receiver's position, vary in accuracy, due to the adverse effects of multipath and other interference on the received signals.
The accuracy of range measurements from the satellites generally decreases as the satellites approach the horizon. This is primarily due in part to the increased magnitude of reflected or refracted signals that result in multipath interference. Some current systems thus provide for rejection of signals from all satellites having an elevation below an operator-controlled cutoff. However, measurements do not consistently degrade with decreasing satellite elevation. A measurement from a satellite having five degrees elevation may be more precise, or exhibit less variability, than a measurement from a satellite having thirty degrees elevation. The accuracy of the measurements also varies according to azimuth, because of differences in the terrain features that cause multipath interference and also because of the effects of interfering signals from various other sources.
SUMMARY OF THE INVENTION
The invention makes use of the fact that in many situations the relative reliability of satellite signals received from various directions substantially repeats appreciably over a substantial period of time, e.g. 24 hours. Accordingly data collected over a period of that length can be used to assign reliability values to the respective directions. Satellite signals are then weighted in accordance with the directions from which they are received.
For example a differential GPS ground station, which is at a fixed location, will usually be subject to much the same distortion characteristics from day to day. Another example is a ship at sea, which primarily experiences distortion from reflections from the surface of the ship itself. The reflection characteristics thus change appreciably only from substantial changes in the surface of the vessel. An airplane more than 300 meters above ground does not experience multipath distortions from the earth's surface. Accordingly, multipath is due entirely to reflections from the surface of the airplane. It therefore varies only in response to movement of the control surfaces, e.g. ailerons or if structural changes are made to the surface of the airplane. Even closer to the ground the multipath interference is due primarily to reflections from the surface of the aircraft.
According to the invention, the receiver divides the sky into direction bins, each of which comprises a small range of azimuth and elevation angles. For example, the size of each bin may be five degrees in both azimuth and elevation, in which case there will be 1296, bins (72-azimuth times 18 elevation). The receiver generates a sky map reflecting the reliability of the measurements made as satellites pass through the respective bins. It uses this data to assign a reliability factor to each bin. This factor is used to assign weights to the pseudo-ranges measured for satellites in those bins and thereby improve the accuracy of the determination of receiver position.
In the simplest implementation of the invention, the receiver ignores all measurements of satellites passing through bins in which the reliability factor is less than a predetermined level. Alternatively, measurements obtained from all bins may be used, with the relative weights depending on the respective reliability factors.
The data used to assign the reliability factors may be the variations in signal-noise ratio from the satellites passing through each bin. In the absence of distortion, the signal-noise ratio will vary by a relatively small amount as a satellite passes through a bin and the variation will generally be monotonic. On the other hand, with multi-path reflections or refractions, for example, the phase of the multi-path signals relative to that of the direct signal from the satellite will vary with satellite position. Thus, the signal-noise ratio of the resultant signal will also vary, usually periodically. The standard deviation of the signal-noise ratios of the signals received from a satellite passing through a bin will thus vary in accordance with the strength of the multi-path signal and will therefore be a measure of the reliability of the signals received from that bin.
In the preferred embodiment of the invention, the receiver uses code-minus-carrier measurements to assign reliability factors for the respective directional bins. Specifically, as a satellite passes through a bin, the receiver makes successive measurements of the satellite's pseudo-range, based on the timing of the pseudo-random code received form the satellite. At the same time, it measures the amount by which the carrier phase changes for the corresponding pseudo-range measurements. For each pseudo-range measurement it then subtracts to the carrier range from the code range, thereby generating a code-minus-carrier (CMC) value. These values are collected during the interval that the satellite passes through the bin and the standard deviation of these values is accumulated for each satellite passing through the bin for a predetermined period, for example, 24 hours. A large standard deviation signifies that the range measurements for the bin are less reliable than those corresponding to a small CMC standard deviation. At the end of the predetermined period, all of the standard deviations accumulated for each bin are averaged to obtain a reliability factor for that bin. Preferably further improvements are achieved by correcting for ionospheric divergence, using different carrier frequencies from each satellite in accordance with a well-known procedure.
More specifically, multi-path signals adversely affect pseudo-range measurements, which depend on the timing of the received pseudo-random code. That is, they shift the peak of the correlation curve and/or distort the curve, thereby causing an error in pseudo-range measurement. The magnitude of the error depends on such factors as the timing of the multi-path signal relative to the direct signal from the satellite and also on the strength of the multi-path signal. On the other hand, because the strength of the multi-path signal is substantially less than that of the direct signal, it does not have a material effect on the overall phase of the resultant carrier. Accordingly, the accumulated phase change from measurement to measurement in the absence of multi-path will be close to that encountered in the presence of multi-path. Variations in the code-minus-carrier values are therefore a measure of the reliability of the pseudo-range measurements.
The invention is applicable primarily to fixed stations, i.e., stations in which the surrounding objects that are a source of multi-path distortion do not move appreciably relative to the receivers. These stations include base stations for differential-GPS systems, receivers used in control survey arrangements, ships at sea and airplanes in flight. Of course there may be changes in the surrounding environment from time to time and a feature of the invention is that major changes can be detected and the sky map altered accordingly. Moreover, if an intruding source of reflections, such as an automobile, is the cause of multi-path degradation of pseudo-range measurements, the cause can be identified to the operator and then removed.
For receivers located on the surface of the earth, the surface of the sky
Cesari & McKenna LLP
Cuchlinski Jr. William A.
Hernandez Olga
NovAtel Inc.
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