Communications: directive radio wave systems and devices (e.g. – Directive – Including a satellite
Reexamination Certificate
2002-02-05
2003-05-27
Issing, Gregory C. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Directive
Including a satellite
Reexamination Certificate
active
06570534
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a GPS receiver, and in particular to a solid state GPS receiver that provides continuous carrier phase tracking. The GPS receiver of the invention maybe used, by way of example, to provide real time control or guidance of heavy machinery, including farm equipment.
2. Description of Related Art
This section begins with a background discussion of the current Global Positioning System (GPS), which is followed by a discussion of the requirements of, and problems associated with, application of GPS to automatic control of heavy machinery and in particular farm equipment, and finally a discussion of specific previously-proposed solutions to some of the problems addressed by the present invention. Those skilled in the art will appreciate that although the present invention addresses a number of problems related to specific applications, the solutions presented may be applicable to a wide variety of different GPS applications, in contexts not specifically discussed herein. The discussion of related art is therefore not intended to be limiting, nor should the related art discussed herein be considered the only prior art having relevance to the present invention.
The Global Positioning System (GPS)
GPS is a satellite constellation originally developed by the U.S. military for real-time 3 dimensional position fixing worldwide. GPS nominally employs 24 satellites in circular 26,000 km orbits. Each satellite carries an atomic frequency standard tied to GPS time as maintained by the U.S. Naval Observatory. The radio signal propagates from the satellite to the user at the speed of light. The ability to determine range to the satellite is therefore governed by one's ability to measure time.
The normal operation of GPS is to broadcast a timing signal using a known biphase pseudo random noise (PRN) code so that a receiver can lock up to this pre-defined sequence. If a user had a perfect clock, ranging to 3 separate sources (one for each dimension of latitude, longitude, and altitude) would be sufficient to determine one's position. To allow for low-cost receiver sets that utilize a simple quartz oscillator time base, the design of GPS includes an additional 4
th
ranging source. Therefore, every position fix includes a solution for a users position and clock bias with respect to GPS time.
GPS broadcasts on two frequencies, L1 and L2. We can define a basic frequency reference of GPS to be f
0
(1.023 MHz). L1 is 1540 f
0
(1,574.42 MHz), and L2 is 1,200 f
0
(1,227.60 MHz). L1 provides a known C/A code that is provided for civil use worldwide at all times. This is a PRN code that is modulated at f
0
. There is also a P(Y) code provided on both L1 and L2 that is modulated at 10 f
0
. The default operational mode is that a known P code is broadcast. The U.S. Military may decide from time to time to switch to an alternate unknown (encrypted) Y code. This mode is called “Anti-Spoofing”, or AS. Because this code is not known a priori to an enemy in a time of military conflict, an enemy cannot generate a false signal that could be used to mislead an allied receiver about its own true position.
GPS is now used in a wide variety of applications, including transportation, recreation, scientific research, and industry. The specific applications contemplated herein are general and could pertain to a broad range of applications, including surveying, GIS, natural resources, and mapping. Furthermore, GPS is now starting to be used as a guidance and control sensor in aviation, agriculture, construction, and mining.
Precision Control Applications
Precision automatic control of heavy machinery places stringent requirements on the navigation sensors that are used for guidance. Examples of such machinery and applications are aircraft automatic landing, automatic steering of farm tractors, and autonomous operations of mining and construction equipment, including haul trucks. Sometimes the heavy equipment made from steel and hydraulics, such as tractors, haul trucks, dozers, drills, shovels, road graders, etc. are known as “heavy iron”.
Farm Tractors
Automatic guidance of farm tractors is starting to provide major new efficiencies to farmers. Some of the first gains are being realized in row crops, such cotton and vegetables. In row crops, it is very important to create straight furrows for planting and subsequent operations. If rows are too close, the cultivating process will shred the farmer's crop. If the rows are too widely separated, a farmer loses valuable surface area that could be used for planting.
Prior to automatic guidance, a farmer could only carry out operations in good visibility. Fog, dust, or darkness meant an automatic pause or end to operations. Now, with a GPS-based tractor autosteering system, a farmer can operate 22 hours per day—something he could never do before. An example of such equipment is the AF5001 manufactured by IntegriNautics in Menlo Park, Calif. Such a unit determines the position of a tractor to a centimeter of accuracy, then uses that information to directly control the steering of the tractor to follow straight rows. The lateral accuracy is one inch.
A human operator is still used. After a 12 hour shift, chronic fatigue is effectively eliminated because the operator now spends his time managing the overall quality functioning of the tractor process-especially the appropriate functioning of the implement itself.
Other gains are being recognized for major commodities, such as corn, wheat, and soybeans. By eliminating the row overlap that commonly occurs, farmers will see 10% reductions in fuel, fertilizer, time, and pesticide use.
Cycle Ambiguity Resolution
For precision positioning using the GPS carrier, centimeter-level accuracy is possible only after the integer cycle ambiguities are resolved between the baseline separating each pair of antennas projected into the line of sight to each satellite.
Some methods of cycle ambiguity resolution include Cohen et al. (U.S. Pat. No. 5,572,218), Zimmerman et al. (pending application filed by IntegriNautics), and Rabinowitz et al. (pending application filed by Stanford). Other methods employ “wide-laning” using dual frequency GPS carrier phase measurements. Because the two GPS frequencies when differenced have an effective wavelength that is much longer than that of either band by itself, it is possible to use that information to systematically search for the correct set of cycle ambiguities that form the smallest residual solution error. Various algorithms have been developed that sequentially throw out the “image” solutions. The reference (Instantaneous Ambiguity Resolution, by Ron Hatch, Paper Presented Sep. 11, 1990 at Banff, Canada at KIS Symposium 1990) describes one such technique. “Wide-laning” is a generic approach known in the art.
Precision Requirements
It is envisioned that this invention will be broadly applicable. However, it is also strongly desired that the invention be useable as a guidance sensor of high quality to control heavy machinery to stringent standards of precision, safety, and reliability. Suitability for machine control implies the potential to be integrated into a feedback control loop system controlling a vehicle or machine to 1 inch lateral accuracy that weighs typically in excess of 20,000 lb. In our invention GPS becomes capable of providing the necessary levels of performance, but requires specific augmentation.
The general requirements for precision control of heavy machinery are as follows:
Accuracy (system deviation from truth): Must be 1 cm (one sigma) or better.
Integrity (system ability to provide timely warnings of hazardous readouts): Probability of hazardously misleading information must be better than one failure in a billion landing approaches for civil aviation. Goal for autonomous applications in heavy iron is better than one failure in a million operational per equivalent exposure time of 150 seconds.
Continuity (probability of operating continuously for entire landing approach—150 second exposu
Cobb Stewart
Cohen Clark
Gutt Gregory
Lawrence David
Melton Walter
Bacon & Thomas PLLC
IntegriNautics Corp.
Issing Gregory C.
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