Data processing: vehicles – navigation – and relative location – Navigation – Employing position determining equipment
Reexamination Certificate
2000-11-28
2002-09-10
Cuchlinski, Jr., William A. (Department: 3661)
Data processing: vehicles, navigation, and relative location
Navigation
Employing position determining equipment
C701S214000, C701S215000, C701S208000, C342S357490, C342S357490
Reexamination Certificate
active
06449558
ABSTRACT:
The present invention relates generally to positioning systems in which an object or user at an unknown location receives signals from a plurality of sources and uses information derived therefrom to determine the object's or user's current position. More particularly, the present invention relates to a positioning system which utilises a network of self-integrating positioning-unit devices, synchronised to a Global Navigation Satellite System (GNSS), for high accuracy position determination in satellite obscured environments.
BACKGROUND OF THE INVENTION
The need to locate exactly where someone or something is on the world's surface has constantly preoccupied humans. In fact, the precision and predictability with which location can be derived is a yardstick by which a civilization's technological refinement can be judged. Over time, man has improved terrestrial location and navigation, progressing through sextant and chronometer, inertial systems, LORAN, TRANSIT and, most recently, GPS.
The GPS constellation of 24 satellites created by the United States Government broadcasts precise timing signals locked to on-board atomic clocks. Using precise, well-developed formulae, a user receiver that picks up signals from 3 or more satellites simultaneously can determine its position in absolute global co-ordinates, namely latitude and longitude. GPS has proven to be a boon to location determination because it is globally available, it is reasonably precise, and it is free to the end user.
Despite its technological sophistication, GPS still suffers from several critical limitations that impede its wide adoption at consumer level. Firstly, GPS signal strengths require satellites to be “in view” relative to the receiver. This means that no substantial obstruction can exist between the satellites and the receiver. Secondly, GPS formulae require at least 3 satellites to be in view for determination of a 2-dimensional location (i.e., latitude and longitude), and at least 4 satellites to be in view for determination of a 3-dimensional location (i.e., latitude, longitude and altitude). In combination, these two major shortcomings severely disrupt GPS reliability in built-up areas such as “urban canyons”, and they ensure that standard GPS will not function at all inside buildings or in shielded environments. GPS is therefore of extremely limited use in metropolitan environments where a large part of the world's population lives.
Surprisingly, further “consumer” limitations of GPS arise from its global availability and its potential for reasonably high precision. In its innate form GPS has the potential to deliver an accuracy of approximately 15 meters. The United States Government became concerned with the possibility that their own satellite system could be used against the United States for accurate delivery of enemy weapons payloads. For this reason, signals broadcast by the GPS network for civilian use are intentionally degraded relative to the more accurate, encrypted U.S. military signals. This degradation, commonly called Selective Availability (SA), reduces the raw accuracy available to civilians to approximately 100 meters 2 dRMS.
In an effort to overcome SA, a system known as Differential GPS (DGPS) was developed for civilian users in a localized area. DGPS is capable of giving accuracy of several meters to a mobile user. However, DGPS demands the establishment of an expensive local broadcasting station. It also necessitates the mobile consumer to purchase additional equipment, in the form of a radio receiver, to acquire DGPS corrections for their GPS receiver. A further recent development called Real Time Kinematic (RTK) allows accuracy from the GPS system to be improved to approximately one centimeter. Whilst this degree of accuracy is highly desirable for many possible applications, RTK is almost wholly the province of highly technical and skilled disciplines such as geodesy, surveying, and physics. RTK receivers are commonly an order of magnitude more expensive than standard GPS receivers are. RTK systems require uncommon local transmitters, and, depending upon the level of complexity can take up to 10 hours of motionless signal acquisition before RTK-accurate positions can be determined. The level of expense necessary for RTK, along with the specialized equipment and skills required, strongly militate against RTK being considered for consumer or commercial use.
In summary: GPS is a marvelous boon to modem location and navigation needs. However, GPS is optimally employed in open field, desert or high-seas environments. Its usefulness is severely compromised in urban canyons, and it was never designed to work indoors. Moreover, should sufficient GPS signals ultimately be acquired in built-up areas, the resultant position solution is so highly degraded by SA that it may prove of little use in restricted areas. If a consumer in this situation, looks to improved accuracy via DGPS or RTK methods, it is only achievable with considerable effort, expense and relatively complex infrastructure.
Attempts to overcome these difficulties, are described in prior art. Hybrid systems have been developed which incorporate an absolute positioning system (e.g., GPS) plus a relative positioning system. Such methods include inertial sensor systems that incorporate “dead reckoning” when satellites are obscured (U.S. Pat. No. 5,311,195) or commercial radio broadcast transmissions performing “delta phase positioning” when satellites are obscured (U.S. Pat. No.
5,774,829).
Unfortunately, these prior art systems have several drawbacks. Dead reckoning exhibits cumulative error with extended use and both dead reckoning and delta phase position accuracy is limited to initial absolute position accuracy. Any initial position ambiguity will therefore be carried on through the subsequent position solutions. Delta phase position accuracy will be constrained by pre-existing geometry of commercial radio broadcast transmission sites. Poor geometry as seen by the roving receiver will produce poor position solutions. In addition, delta phase position accuracy is constrained by the frequency/wavelength of the transmission signal whereby lower frequencies (i.e., longer wavelengths) produce decreased accuracy. Moreover, delta phase roving receivers need a pre-existing knowledge of commercial radio broadcast transmission site co-ordinates. Finally, delta positioning requires a reference receiver and data link in addition to the commercial radio broadcast transmissions. U.S. Pat. No. 5,774,829 suggests that this data link be placed as information on the commercial radio broadcast transmission signal SCA channel. This would potentially require co-operation with thousands of commercial broadcasters, bringing about substantial logistics problems.
Also known in the art are attempts to use pseudo-satellites, or “pseudolites”, to enhance or augment the standard GPS constellation. Pseudolites are ground based transmitters that emit GPS-like signals. Pseudolites were first used in 1977 by the US Department of Defense for Phase I GPS testing at the Yuma Proving Ground in Arizona. They were used to augment the GPS constellation for testing user equipment before there were sufficient satellites for navigation. In 1984 Klein and Parkinson were the first to point out that pseudoiites could be a useful adjunct to the operational GPS system, improving navigation availability and geometry for critical applications such as aviation. In 1986 Parkinson and Fitzgibbon developed and demonstrated a procedure for finding the optimal location for a ranging pseudolite. Also in 1986 the RTCM-104 committee, which developed the first standard for local area DGPS systems, proposed a method for transmitting DGPS information by pseudolite.
Pseudolites are currently expensive devices and are manufactured in extremely small quantities. They generally transmit their signals on the GPS L
1
and L
2
frequencies, so they normally need regulatory approval to operate. Experimental groups within Universities, government agencies, the m
Cuchlinski Jr. William A.
Marc-Coleman Marthe Y.
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