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Method and system for a data interface for aiding a satellite positioning system receiver

Method and system for a data interface for aiding a satellite positioning system receiver description/claims

This application is a non-provisional application claiming benefit of priority under 35 U.S.C. 119(e) of U.S. Provisional Patent Application Ser. No. 60/509,163, filed Oct. 6, 2003, entitled “Distributed GPS/DR Navigation System,” by Jaime B. Colley and Lars Boeryd, and U.S. Provisional Patent Application Ser. No. 60/509,186, filed Oct. 6, 2003, entitled “Integrated GPS and Map-Matching Navigation System,” by Jaime B. Colley and Lars Boeryd, both of which are incorporated herein by reference in their entirety.

This application is related to copending U.S. patent application Ser. No. ______, filed concurrently herewith, entitled “A System and Method for Augmenting a Satellite-Based Navigation Solution”, by Jaime B. Colley and Lars Boeryd, which is incorporated by reference herein in its entirety.

The invention described herein relates to location and navigation systems using Satellite Positioning System (SPS) data combined with various other data sources, to aid in locating or navigating a platform, for example, an automobile, ship, aircraft, or any other object that can generate data.

SPS receivers, such as, for example, receivers using the Global Positioning System (“GPS”), also known as NAVSTAR, have become commonplace. It is appreciated by those skilled in the art that GPS systems include Satellite Positioning System “SPS” and/or Navigation Satellite Systems. In general, GPS systems are typically satellite (also known as “space vehicle” or “SV”) based navigation systems. Examples of GPS systems include but are not limited to the United States (“U.S.”) Navy Navigation Satellite System (“NNSS”) (also know as TRANSIT), LORAN, Shoran, Decca, TACAN, NAVSTAR, the Russian counterpart to NAVSTAR known as the Global Navigation Satellite System (“GLONASS”) and any future Western European GPS such as the proposed “Galileo” program. As an example, the US NAVSTAR GPS system is described in GPS Theory and Practice, Fifth ed., revised edition by Hofmann-Wellenhof, Lichtenegger and Collins, Springer-Verlag Wien New York, 2001, which is fully incorporated herein by reference.

GPS is funded by and controlled by the U.S. Department of Defense (DOD). While there are many thousands of civil users of GPS worldwide, the system was designed for and is operated by the U.S. military. GPS provides specially coded satellite signals that can be processed in a GPS receiver, enabling the receiver to compute position, velocity, and time. At least four GPS satellite signals are used to compute positions in three dimensions and the time offset in the receiver clock.

GPS position determination is based on a simple mathematical principle called trilateration. In order to solve for user position, the GPS receiver must determine two things: the location of at least three satellites above the user, and the distance between the user and each of those satellites. The GPS receiver solves these variables by analyzing high-frequency, low-power radio signals from the GPS satellites.

At a particular time every day, the GPS satellite or Space Vehicle (SV) begins transmitting a long, repeating, digital pattern called a pseudo-random code. The GPS receiver begins running the same digital pattern also at exactly the same time. When the satellite's signal reaches the receiver, its transmission of the pattern will lag slightly behind the receiver's running of the pattern. The length of the delay is equal to the signal's travel time. The receiver multiplies this time by the speed of light to determine how far the signal traveled. Assuming the signal traveled in a straight line, this is the distance from the receiver to the satellite.

In order to make this measurement, the GPS receiver and satellite both need clocks that can be synchronized down to the nanosecond. To make a satellite positioning system using only synchronized clocks, one would need to have atomic clocks not only on all of the satellites, but also in the GPS receiver itself. Atomic clocks are not an inexpensive consumer product. However, the Global Positioning System uses a clever, effective solution to this problem. Every satellite contains an expensive atomic clock, but the receiver itself uses an ordinary quartz clock, which it constantly resets. In summary, the receiver looks at incoming signals from four or more satellites and gauges its own inaccuracy, but, of course, the GPS clock is a source of errors too.

To determine location using four satellites, the GPS receiver mathematically requires (for three-dimensional positioning) that four spheres having a radius equal to the distance from an SV to the GPS receiver, all intersect at one point. Three spheres will intersect even if there are inaccuracies, but four spheres will not intersect at one point if the GPS receiver has measured incorrectly. Since the GPS receiver makes all its distance measurements using its own built-in clock, the distances will all be proportionally incorrect.

The GPS receiver can easily calculate the necessary adjustment that will cause the four spheres to intersect at one point. Based on this, it resets its clock to be in sync with the satellite's atomic clock. The GPS receiver does this constantly whenever it is on, which means it is nearly as accurate as the expensive atomic clocks in the satellites.

In order for the distance information to be of any use, the GPS receiver also has to know where the satellites actually are located. This is not particularly difficult because the satellites travel in very high and predictable orbits, the GPS receiver simply stores an almanac in memory describing where every satellite should be at any given time. Gravitational forces like the pull of the moon and the sun do change the satellites' orbits very slightly, but the Department of Defense constantly monitors their exact positions and transmits any adjustments to all GPS receivers as part of the satellites' signals.

This system works well, but inaccuracies are present. For example, this method assumes the radio signals will make their way through the atmosphere at a consistent speed (the speed of light). In fact, the Earth's atmosphere slows the electromagnetic energy down somewhat, particularly as it goes through the ionosphere and troposphere. The delay varies depending on where you are on Earth, which means it is difficult to accurately factor this into the distance calculations. Problems can also occur when radio signals bounce off large objects, such as skyscrapers, giving a receiver the impression that a satellite is farther away than it actually is. This phenomenon is sometimes referred to as multipath. Furthermore, satellites sometimes transmit inaccurate almanac data, misreporting their own positions.

Differential GPS (DGPS) helps correct these errors. The basic idea is to gauge GPS inaccuracy at a stationary receiver station. Since the DGPS hardware at the station already knows its own position, it can easily calculate its receiver's inaccuracy. The station then broadcasts a radio signal to all DGPS-equipped receivers in the area, providing signal correction information for that area. In general, access to this correction information makes DGPS receivers much more accurate than ordinary receivers.

Three binary codes shift the satellite's transmitted L1 and/or L2 frequency carrier phase. The C/A Code (Coarse Acquisition) modulates the L1 carrier phase. The C/A code is a repeating 1 MHz Pseudo Random Noise (PRN) Code. This noise-like code modulates the L1 carrier signal, “spreading” the spectrum over a 1 MHz bandwidth. The C/A code repeats every 1023 bits (one millisecond). There is a different C/A code PRN for each SV. GPS satellites are often identified by their PRN number, the unique identifier for each pseudo-random-noise code. The C/A code that modulates the L1 carrier is the basis for the civil uses of GPS.

The GPS receiver produces the C/A code sequence for a specific SV with some form of a C/A code generator. Modern receivers usually store a complete set of precomputed C/A code chips in memory, but a hardware shift register implementation can also be used. The C/A code generator produces a different 1023 chip sequence for each phase tap setting. In a shift register implementation the code chips are shifted in time by slewing the clock that controls the shift registers. In a memory lookup scheme the required code chips are retrieved from memory. The C/A code generator repeats the same 1023-chip PRN-code sequence every millisecond. PRN codes are defined for 32 satellite identification numbers. The receiver slides a replica of the code in time until there is correlation with the SV code.

Receiver position, that is, the end user position, is computed from the SV positions, the measured pseudo-ranges (corrected for SV clock offsets, ionospheric delays, and relativistic effects), and a receiver position estimate (usually the last computed receiver position). This is illustrated in the following pseudo-range navigation solution example, where three satellites are used to determine three position dimensions with a perfect receiver clock. In actual practice, three SVs are used to compute a two-dimensional, horizontal fix (in latitude and longitude) given an assumed height. This is often possible at sea or in altimeter equipped aircraft. Five or more satellites can provide position, time and redundancy. More SVs can provide extra position fix certainty and can allow detection of out-of-tolerance signals under certain circumstances.

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