Communications: directive radio wave systems and devices (e.g. – Directive – Including a satellite
Reexamination Certificate
2000-03-09
2002-01-08
Camby, Richard M. (Department: 3661)
Communications: directive radio wave systems and devices (e.g.,
Directive
Including a satellite
C342S357490, C342S357490, C701S213000, C701S214000
Reexamination Certificate
active
06337657
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to methods and apparatuses for improving the accuracy of, and reducing errors in, the position measurements of stand alone navigation and differential navigation receiver system. The present invention is oriented to the improvement of characteristics of a navigational receiver working with navigational Satellite Positioning System signals (SATPS, in particular GPS and/or GLONASS), and using the code measurements and the integrated carrier phase measurements.
BACKGROUND OF THE INVENTION
For the positioning of a navigational roving receiver, one uses radio signals received from a plurality of Satellite Positioning System (SATPS) satellites, and in particular GPS and/or GLONASS satellites. The receiver's position is known if its coordinates in a particular coordinate system (for example, Earth-Centered-Earth-Fixed coordinate system) are known.
The time scale of any receiver (including a roving receiver) usually has some casual offset with respect to the SATPS system time. The value of this time offset is determined simultaneously with the determination of the position coordinates. In this case, for the determination of three coordinates of a roving receiver, it is usually necessary to receive navigation signals from not less than four navigational satellites. However if one of the receiver coordinates is considered to be known (e.g., a two-dimensional solution is used and the height is known or assumed), it is usually necessary to receive navigational signals from not less than three navigational satellites.
FIG. 1
shows a simplified form of the Satellite Positioning System (SATPS) operation in the Stand Alone Navigation (SAN) mode. (Stand Alone Navigation corresponds to an absolute positioning system, i.e. a system that determines a receiver's position coordinates without reference to a nearby reference receiver). The roving receiver
1
and its corresponding antenna
3
are located on the ground surface, or in the near Earth space, and can receive signals from navigational satellites
7
,
9
,
11
, and
13
. The receiver position is defined by its coordinates in a particular coordinate system, for example the Earth-Centered-Earth-Fixed (ECEF) system. It is supposed that the exact position of the receiver is unknown, and that the receiver has the task of generating an estimate of the receiver's position within a given accuracy. That accuracy depends upon the number of available satellites, the configuration of the satellites, the quality of the receiver, and a number of other factors.
FIG. 2
shows a simplified form of the SATPS operation in the Differential Navigation (DGPS) mode. In the differential mode, the reference receiver
15
evaluates/estimates the slowly varying components of-measurement errors and generates scalar or vector corrections for each visible navigational satellite. These corrections are sent to the roving SATPS receivers which are configured to use the corrections and which are near enough to the reference receiver for the corrections to be useful. (This explained in greater detail at page 4 of Parkinson, et al,
Global Positioning System: Theory and Applications, Volume II
, American Institute of Aeronautics and Astronautics, Inc., 1996, herein referred to as “Parkinson Vol. II”)
In one implementation, the roving receiver
1
receives, by means of the modem
5
, differential correction signals generated and sent by the reference receiver
15
, by means of the modem
19
. By means of the antenna
3
, the roving receiver
1
receives signals from navigational satellites
7
,
9
,
11
, and
13
, and processes them together with the differential corrections. In other implementations, other communications means besides modems
5
and
19
may be used, and more than four satellite signals may be used.
The navigational signals broadcast by navigational satellites
7
,
9
,
11
, and
13
, are received by the antenna
3
of the roving receiver
1
and by the antenna
17
of reference receiver
15
. (The number of navigational satellites in the general case must be four or more, but if one of the coordinates of the roving receiver is known, the number of navigational satellites must not be less than three.) Both of receivers
1
and
15
:
separate the received signals and identify the navigational satellites (define the satellite number or its corresponding index) for each received signal;
a determine the current position (coordinates) of each of the observed navigational satellites
7
,
9
,
11
, and
13
, using the transmission delay of the satellite's code signal and the information on satellite's Ephemeris, which is conveyed by a low frequency (50 Hz) signal which is modulated onto the satellite's code signal; and
track the code signal delay over time and determine the integrated carrier phase for each navigational signal (the satellite's code signal is modulated onto a higher frequency carrier signal).
The antennas
3
,
17
and the receivers
1
,
15
of navigational signals are the user portion of the SATPS (in particular GPS and/or GLONASS).
Snapshot Position Solutions
A “snapshot position solution”, or simply “snapshot solution”, is a determination of the receiver's position coordinates at a particular instant of time using the pseudorange code measurements from the satellites at that particular instant of time. The snapshot position solution does not use information from previous time points (e.g., previous epochs). In addition to the code delay information, a snapshot position solution may also use carrier phase information. A snapshot solution of the receiver coordinates is usually formed by application of the least squares method (LSM) to the pseudorange code measurements, obtained for one epoch (for a certain moment of time). The LSM method is explained in greater detail at pages 412-413 of Parkinson, et al.,
Global Positioning System: Theory and Applications, Volume I
, American Institute of Aeronautics and Astronautics, Inc., 1996, herein referred to as “Parkinson Vol. I”, as well as by other references.
The magnitude of error in the snapshot solution is related to the errors in the pseudorange code measurements, the number of satellites used in the snapshot solution, and the geometry of navigational satellites. The snapshot error is reduced by reducing the errors in the code measurements, by increasing the number of satellites in the solution, and by having the satellite's configuration near to the optimal constellation configuration (which is well known to the GPS art). Errors in the pseudorange code measurements are caused by the presence of thermal and other noises at the receiver's input, as well as by a number of reasons, amongst which the following ones are the most significant (see Parkinson Vol. I, page 478):
Errors in the transmitted Ephemeris data;
Errors in the satellite (transmitted) clock, including those caused by Selective Availability;
Ionospheric refraction;
Tropospheric refraction;
Multipath reflections.
The error in the receiver coordinate estimates caused by the number of satellites and by their specific geometry is characterized by the value of a geometric factor called the geometric dilution of precision (GDOP) (see Parkinson Vol. I, pages 413, 420, 474.). When the number of observed navigational satellites is reduced, which is often caused, for example, by the short-term shadowing or blocking of one or more of the satellites, the value of the dilution of precision can sharply increase, which in turn results in a substantial increase in the errors of the coordinate estimates.
As is typically done in the global positioning art, the errors in the receiver's position and time scale caused by the above-identified error sources are estimated by statistical methods using a simulation model of the receiver and the satellites. Each error source, such as thermal noise in the receiver or clocking errors in the satellite signals caused by Selective Availability, is modeled as a random noise source having a represen
Ashjaee Javad
Zagnetov Peter P.
Zhodzishsky Mark I.
Camby Richard M.
Coudert Brothers
Topcon Positioning Systems, Inc.
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