Locating an object using GPS with additional data

Details Locating an object using GPS with additional data Locating an object using GPS with additional data

2001-06-22

2003-12-02

Phan, Dao (Department: 3662)

Communications: directive radio wave systems and devices (e.g.,

Directive

Including a satellite

C342S357490, C342S357490

Reexamination Certificate

active

06657584

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed to a system for determining the position of an object.
2. Description of the Related Art
Technologies for tracking moving objects are in demand. For example, systems are used to track airplanes, automobiles, persons, objects at sporting events and other objects of interest. Examples of tracking objects at sporting events include systems for tracking hockey pucks during a hockey game using infrared technology, tracking objects using RF signals and tracking objects using pattern recognition technology.
One technology that has become popular for tracking objects is the use of the Global Positioning System (GPS). GPS is a satellite based navigation system operated and maintained by the U.S. Department of Defense. GPS consists of a constellation of 24 GPS satellites providing worldwide, 24 hour, three dimensional navigational services. By computing the distance to GPS satellites orbiting the earth, a GPS receiver can calculate an accurate position of itself. This process is called satellite ranging. The position being tracked is the position of the antenna of the GPS receiver. In one implementation, a GPS receiver receives signals from a set of satellites, determines its location and outputs a derived position (longitude and latitude), GPS derived velocity, Dilution of precision (DOP) and a list of the members of the satellite constellation. The satellite constellation includes a list of satellites for which the GPS receiver has received data. Experimentation has shown that GPS data is more reliable when the GPS receiver is receiving data from the same satellites for an extended period of time. When the satellite constellation changes, the GPS data tends to be less reliable.
The basis of satellite ranging is multi-lateration. A GPS receiver measures distance using the travel time (or differences in travel time) of radio signals. To measure travel time (or differences in travel time), GPS receivers need very accurate timing. Along with distance, a GPS receiver needs to know exactly where the satellites are in space. Finally, the GPS receiver can correct for any delays the signal experiences as it travels through the atmosphere. A calculation of a three dimensional location requires valid data from four satellites. GPS receivers can also provide precise time information.
In general, satellite ranging can be thought of as follows. If the receiver knows the distance from the satellite to the receiver, then the receiver can symbolically draw a sphere with the center of the sphere at the satellite's location and the radius of the sphere equal to the distance from the satellite to the receiver. The receiver is somewhere on the surface of the sphere. If the receiver knows the distance from three satellites, the receiver draws three spheres. These three spheres will intersect at two points. The receiver is located at one of the two points. In many cases, it is possible to eliminate one of the points because one of the points is a ridiculous answer. For example, one of the points can be too far from earth. A fourth measurement is typically used to determine which of the two points is the true location of the receiver. The fourth measurement is also used for timing purposes, as described below.
A GPS receiver measures the distance from a satellite to the receiver by measuring the time it takes for a signal sent from the satellite to arrive at the receiver. Because the time of travel from the satellite to the receiver is extremely short, the receiver must have precise clock information. A GPS satellite emits a pseudo-random code. The receiver will also generate the same pseudo-random code and compare the generated code with the received code to determine the shift between the two codes. That shift is used to determine travel time. One key to the above computation is extremely accurate timing. Satellites have incredibly precise atomic clocks on board. However, it is not cost effective for the receivers to also have such atomic clocks. To compensate for not having an atomic clock, the receivers need data from an extra satellite (e.g. the fourth satellite) in order to determine time values.
As stated above, a GPS receiver must know the location of the satellite. The GPS satellites have been injected into very precise orbits. GPS receivers have an almanac programmed into their computers that tell them where in the sky each satellite is, moment by moment. Neverth 12,2000eless, a satellite can deviate from the perfect orbit due to such errors induced by gravitational pulls from the moon and sun, and by the pressure of solar radiation on the satellite. These errors are called ephemeris errors because they effect the satellites orbit or ephemeris. The Department of Defense measures satellites' exact position and relays that information back to the satellite itself. The satellite then includes this new position correction information in its transmitted signal.
There are a number of errors that are associated with GPS ranging, including errors due to the Earth's ionosphere and atmosphere. Additionally, basic geometry itself can magnify these errors. The configuration of the satellites in the sky can also magnify the errors. The dilution of precision, a measure of error, is a description of the uncertainty of particular GPS data.
One enhancement to standard GPS technology includes the techniques of differential GPS, which involves a reference GPS receiver that is stationary and other GPS receivers that are moving and making position measurements. To understand differential GPS, it is important to know that the satellites are so far out in space that the little distances traveled on Earth are insignificant in comparison. So, if two receivers are fairly close to each other, say within a few hundred kilometers, the signals that reach both of them will have traveled through virtually the same slice of atmosphere, and will have virtually the same errors. With differential GPS, one stationary reference receiver is used to measure the timing errors. The reference receiver then provides error correction information to the other receivers (e.g. mobile receivers). This way, systemic errors can be effectively eliminated from the system, even the Selective Availability errors. In order to work properly, the reference receiver is placed at a location that is accurately surveyed. The reference receiver receives the same GPS signals as the moving receivers. Thus, instead of working like a normal GPS receiver, it attacks the equations backwards. Instead of using timing signals to calculate its position, it uses its known position to calculate timing. It figures out what the travel time of the GPS signals should be, and compares it to what they actually are. The difference is the error correction factor or reference error information. The reference receiver then transmits this reference error information to the moving receivers in order to correct the measurement of the moving receivers. Since the reference receiver has no way of knowing which of the many available satellites a moving receiver might be using to calculate is position, the reference receiver quickly runs through all the visible satellites and computes each of their errors. Then it encodes this information in standard format and transmits it to the moving receivers. The information can be transmitted using RF or other technologies. The moving receivers get the complete list of errors and apply corrections to the particular satellites they are using.
It is possible for an individual entity to set up its own reference receiver. Alternatively, there are public agencies which already have reference receivers that transmit corrections which can be received for free. It is also possible, that the reference receiver can send its information to a central computer and the moving receivers can also send information to the central computer. The central computer uses the information from the moving receiver, taking into account the errors from the reference receiver,

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