Communications: directive radio wave systems and devices (e.g. – Directive – Including a satellite
Reexamination Certificate
1999-05-27
2001-01-30
Tarcza, Thomas H. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Directive
Including a satellite
C342S357490, C342S357490
Reexamination Certificate
active
06181275
ABSTRACT:
The present invention relates to a receiver for a navigation system comprising a constellation of satellites transmitting signals. The invention also concerns a method of locating a receiver in a navigation system comprising a constellation of satellites transmitting signals. It applies in particular to satellites in low Earth orbit.
BACKGROUND OF THE INVENTION
The invention concerns satellite navigation systems; these systems, using a constellation of satellites transmitting known signals, enable a receiver to compute its position within a geocentric frame of reference. These systems are used to navigate vehicles of all kinds, in particular ships and aircraft. The systems in use at present are the GPS and GLONASS systems, which comprise constellations of satellites at an altitude of 20 000 km. These systems are based on measuring pseudo-distances between the satellites and the receiver. A pseudo-distance is evaluated from the transmission time of a signal transmitted at a particular moment by the satellite and received by the receiver. For this purpose, the receiver measures the arrival time of the encoded signal transmitted by a satellite of the constellation and reads the time at which the signal was transmitted by the satellite in the received message. The receiver evaluates the transmission time by determining the time the message was received or by computing the phase of the signal.
Knowing the position of the satellites makes it possible to determine the position of the receiver by triangulation on the basis of the pseudo-distances. Existing prior art systems use four satellites to compute the position of the receiver; four measured pseudo-distances are thus used to determine the time and the position of the receiver station.
One of the main features of the above systems is that there is no limit on the number of users, in other words the system cannot be saturated; the satellites merely transmit and position is computed at each receiver. The receiver is therefore passive, operating in “listening” mode. This also implies that the pseudo-distances are measured on the one-way satellite-to-receiver path. Other details on the structure and operation of the above navigation systems can be found in the prior art documents cited below.
U.S. Pat. No. 5,420,592 describes the use of a GPS navigation system for positioning meteorological balloons; it proposes separating the GPS receiver into a mobile part carried by the balloon and a fixed part on the ground. A Kalman filter is used to predict the position of the balloon and enables positioning even with reduced visibility of the constellation of satellites. The system corrects the offsets induced by the Doppler effect in the signals from the satellites to enable correction of the signal from the satellite for fast computation of the pseudo-distance.
EP-A-0 460 862 describes a GPS type global positioning system receiver that can be implemented in a VLSI circuit. The receiver includes means for evaluating the pseudo-distance and the variation in the pseudo-distance in order to eliminate the Doppler effect from the carrier from the satellites.
U.S. Pat. No. 5,436,632 proposes to use the Doppler effect to compute corrections to be applied to the rate of variation of the pseudo-distances in a system for monitoring the integrity of the signals from a GPS type system.
U.S. Pat. No. 5,343,209 proposes to evaluate the Doppler shifts using an estimate of the speed of the receiver and the variations of the line of sight (LOS) of a GPS type positioning system. That evaluation produces an estimate of the phase of the carrier, the frequency of the carrier and the phase modulation of each of the received signals which is better than could be provided by each signal independently.
EP-A-0 518 146 describes a multi-point positioning system for a ship and towed buoys. That system uses a plurality of GPS receivers and Kalman filters to evaluate the quality of each position indication. It is again proposed to correct the Doppler shift for evaluating the pseudo-distances.
U.S. Pat. No. 5,594,453 describes a GPS receiver with a waiting mode to reduce power consumption; in order to enable rapid acquisition of the position on leaving the waiting mode, that document proposes to correct the influence of temperature on the clock frequency. The Doppler effect is used to compute the estimate of the frequency of the satellite in the new position, on leaving the waiting mode.
U.S. Pat. No. 5,557,284 proposes a system for detecting spoofing (the transmission of fake signals) in a global positioning system; it proposes to use the differences in the Doppler effect between two receivers on an aircraft or on the vehicle to detect fake signals.
In all the above documents, as in the prior art in general, the Doppler effect embodied in the signals from the satellites is seen as a drawback; it is proposed to correct the signals received to eliminate the Doppler effect or to allow for the Doppler effect in predicting the position of the receiver. None of the above documents proposes to use measurement of pseudo-speeds by the Doppler effect to improve the accuracy of the position determined. Also, all the above documents concern constellations of satellite in medium Earth orbit (MEO), such as the GPS or GLONASS systems.
OBJECTS AND SUMMARY OF THE INVENTION
In contrast, the invention proposes to use the pseudo-speed information provided by evaluating the doppler effect to compute the position of the receiver. It applies in particular to a constellation of satellites in low Earth orbit (LEO) for which the Doppler effect and the geometrical variability of the visible satellites are greater.
To be more precise, the invention proposes a receiver for a navigation system comprising a constellation of satellites transmitting signals, the receiver comprising means for measuring pseudo-distances between the receiver and the satellites from signals received from the satellites, means for measuring pseudo-speeds between the receiver and the satellites from signals received from the satellites, and means for computing the instantaneous position of the receiver from the combination of measured pseudo-distances and pseudo-speeds.
The pseudo-speed measuring means advantageously measure the pseudo-speeds on the basis of the Doppler shift of the signals received from the satellites.
In one embodiment, the system further comprises means for computing the speed of the receiver from the measured pseudo-speeds.
The computing means preferably compute the position or the speed of the receiver by a least squares method.
In one embodiment, the computing means compute the position or the speed of the receiver by minimizing the matrix of covariance of the measured pseudo-distances and pseudo-speeds.
The computing means preferably further comprise means for predicting the position of the receiver, for example using Kalman filters.
The constellation of satellites is advantageously a constellation of satellites in low Earth orbit.
The invention further proposes a method of locating a receiver in a navigation system comprising a constellation of satellites transmitting signals, comprising the steps of:
measuring pseudo-distances between the receiver and the satellites from the signals received from the satellites,
measuring pseudo-speeds between the receiver and the satellites from signals received from the satellites, and
computing the position of the receiver from the combination of measured pseudo-distances and pseudo-speeds.
The step of measuring the pseudo-speeds advantageously includes measuring the Doppler shift of the signals received from the satellites and computing the pseudo-speeds from the measured shift.
In one embodiment, the step of computing the position of the receiver also includes a step of computing the speed of the receiver from the measured pseudo-distances and pseudo-speeds.
The computation step preferably comprises computing the position or the speed of the receiver by a least squares method.
The computation step advantageously comprises computing the position or the speed of t
Chenebault Jean
Journo St{acute over (e)}phane
Lobert Bruno
Alcatel
Phan Dao L.
Sughrue Mion Zinn Macpeak & Seas, PLLC
Tarcza Thomas H.
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