Data processing: vehicles – navigation – and relative location – Navigation – Employing position determining equipment
Reexamination Certificate
2000-10-24
2002-05-28
Nguyen, Tan (Department: 3661)
Data processing: vehicles, navigation, and relative location
Navigation
Employing position determining equipment
C701S207000, C701S214000, C342S357490
Reexamination Certificate
active
06397147
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates generally to Global Positioning System (GPS) receivers and more particularly to a method for computing a precise relative location using a single differential GPS receiver that acts as a reference station by generating differential correction terms, and then subsequently as a remote receiver unit by using the differential correction terms.
2. Background
The Global Positioning System (GPS) was established by the United States government, and employs a constellation of 24 or more satellites in well-defined orbits at an altitude of approximately 26,500 km. These satellites continually transmit microwave L-band radio signals in two frequency bands, centered at 1575.42 MHz and 1227.6 MHz., denoted as L1 and L2 respectively. These signals include timing patterns relative to the satellite's onboard precision clock (which is kept synchronized by a ground station) as well as a navigation message giving the precise orbital positions of the satellites, an ionosphere model, and other useful information. GPS receivers process the radio signals, computing ranges to the GPS satellites, and by triangulating these ranges, the GPS receiver determines its position and its internal clock error.
In standalone GPS systems that determine a receiver's position coordinates without reference to a nearby reference receiver, the process of position determination is subject to errors from a number of sources. These include errors in the GPS satellite's clock reference, the location of the orbiting satellite, ionosphere induced propagation delay errors, and troposphere refraction errors. A discussion of these sources of error is given in more detail in U.S. Pat. No 5,828,336 by Yunck, et al.
To overcome the errors of the standalone GPS system, many kinematic positioning applications, including applications of GPS to precision farming, have made use of data from multiple GPS receivers. Typically, in such applications, a reference receiver, located at a reference site having known coordinates, receives the GPS satellite signals simultaneously with the receipt of signals by a remote receiver. Depending on the separation distance between the two GPS receivers, many of the errors mentioned above will affect the satellite signals equally for the two receivers. By taking the difference between signals received both at the reference site and at the remote location, the errors are effectively eliminated. This facilitates an accurate determination of the remote receiver's coordinates relative to the reference receiver's coordinates.
The technique of differencing signals from two or more GPS receivers to improve accuracy is known as differential GPS (DGPS). Differential GPS has been well described in literature with two examples cited here: 1) “Global Positioning System: Theory and Applications”, volume 2, pp. 3-49, 81-114, edited by Parkinson and Spilker; and 2) “GPS Theory and Practice”, Third edition, pp. 132-143 by Hofiann-Wellenhof, Lichtenegger and Collins. Differential GPS has taken on many forms and has been an object of a number of patents (for example, see U.S. Pat. No. 4,812,991 by Hatch, U.S. Pat. No. 5,148,179 by Allison, U.S. Pat. No. 5,155,490 by Spradley, Jr., et al., U.S. Pat. No. 5,361,212 by Class, et al., U.S. Pat. No. 5,495,257 by Loomis, U.S. Pat. No. 5,596,328 by Stangeland, and U.S. Pat. No. 5,638,077 by Martin). It includes local DGPS systems that utilize a single reference receiver delivering corrections to one or more remote receivers and it includes Wide Area Differential GPS (WADGPS) where differential correction terms are generated by combining data from multiple reference GPS receivers spread geographically over a region of intended coverage. Several methods of WADGPS appear in U.S. Pat. No. 5,323,322 by Mueller, et al., U.S. Pat. No. 5,621,646 by Enge, et al., U.S. Pat. No. 5,828,336 by Yunck, et al., and U.S. Pat. No. 5,899,957 by Loomis. In all forms of DGPS, however, the positions obtained by the end user's remote receiver are relative to the position(s) of the reference receivers). Thus, absolute accuracy of any DGPS system depends heavily on the accuracy at which the reference receiver locations were determined when installing or implementing the DGPS system.
Relative accuracy is often all that is desired in many applications involving GPS and in these cases, the reference location need not be extremely accurate relative to any one particular coordinate system. That is, it is not a question of determining so much exact position, but position relative to some starting point with a high degree of accuracy. For example, the primary need for swathing applications that guide farm vehicles applying pesticides or fertilizer is to be able to guide the vehicle so that, relative to an initial swath, the subsequent swaths are at a series of prescribed offsets from the original swath (or from each other). There is often no accuracy requirement on the initial swath, only that subsequent swaths be accurate relative to the initial swath.
Prior to May 1, 2000, the largest portion of the positioning error associated with non-differentially corrected (standalone) GPS resulted from the purposeful dithering of the GPS satellite's clock; a process known as Selective Availability (SA). The U.S. Department of Defense relied on SA as a means to limit GPS accuracy of non-authorized users. Typical position errors resulting from were SA 20 to 60 meters but these errors could be more than 100 meters at times. On midnight of May 1, 2000, the intentional degradation of the Global Positioning System signals was discontinued, perhaps indefinitely.
Even without SA, absolute positioning errors caused by the ionosphere can be tens of meters and orbit-induced errors can be several meters. But these errors tend to be slowly changing, sometimes taking more than 2 hours before a stationary receiver reports a position that significantly different from past reports of position. In some applications, such as the GPS guidance of agricultural aircraft used in applying chemicals, the need for relative accuracy may be well matched to a two-hour duration. In fact, it may take less than an hour to complete a typical spraying job. Thus with SA disabled, employing the method described herein, standalone GPS systems may be used where DGPS systems (with multiple GPS receivers) were once required. For example, in certain precision agriculture applications and other arenas such as surveying or Global Information Systems (GIS) DGPS may no longer be required.
Although it is true that existing standalone GPS technologies can achieve high degrees of relative accuracy for several hours, it cannot be guaranteed. Satellites that are tracked by the GPS receiver and are used in computing the receiver's position may set below the horizon; rise above the horizon; or they may be temporarily blocked from view by an obstruction. Such transitions often cause jumps in the receiver's computed position that maybe larger than a particular application can tolerate. To appreciate why, it is important to understand how the GPS position is computed. In the over-determined case (which is the preferred approach to solving the GPS receiver's position) each of the GPS satellite's range measurements contributes to the position reported by the GPS receiver because each range measurement contains information that is mathematically combined, using a Least-Squares or similar technique, to compute the GPS receiver's position. Furthermore, each satellite's range measurement is corrupted by errors, such as ionosphere induced delay, and each error has a different effect on the final position. The ionosphere model most often used for standalone GPS is the Klobuchar model that is broadcast in the GPS Navigation message arriving from each GPS satellite and this model is often in error by one to several tens of meters. As a satellite range measurement is, for example, removed from the Least-Squares position solution, the computed pos
CSI Wireless Inc.
Nguyen Tan
Tran Dalena
LandOfFree
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