Communications: directive radio wave systems and devices (e.g. – Directive – Including a satellite
Reexamination Certificate
2003-12-11
2004-11-30
Gregory, Bernarr E. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Directive
Including a satellite
C375S224000, C370S249000
Reexamination Certificate
active
06825801
ABSTRACT:
BACKGROUND
a. Field of Invention
The invention relates to the equipment and process for testing the capacity of a global positioning system (GPS) receiver to lock on to the transmitted signal of a particular satellite and properly process the signal.
b. Background of the Invention
The Global Positioning Satellite (GPS) system is widely used by civilian and military personnel to obtain a precise determination of position on or near the surface of the earth. The system is comprised of a constellation of satellites in earth orbit and positioned such that a sufficient number of satellites (typically 4) will be in range to communicate on a line of sight path with a receiving unit anywhere on the surface of the earth. The constellation of satellites is monitored and maintained from a number of earth based stations. The earth based stations send data to the satellites, to be stored and subsequently transmitted to GPS receivers, as needed. The information includes the satellites' orbital elements, almanac information containing abbreviated orbital elements, ranging measurement corrections and status flags. A user of the GPS must establish communication between his or her receiver and a sufficient number of satellites with up-to-date data and in working order. The receiver must receive the satellite communications including time of transmission and navigation data message, and triangulate the position of the receiver by solving the position equation. Although the system is generally reliable, it depends on the proper functioning of multiple satellites in earth orbit as well as the proper functioning of the receiver.
The primary signal transmitted by the satellites is known as L-1 and is a biphase shift keying modulator modulated with a 1.023 MHz pseudo random noise coarse acquisition code. The coarse acquisition code repeats once each millisecond. The GPS receiver demodulates the received code from the L-1 carrier and compares the transmitted coarse acquisition code with coarse acquisition codes generated by the receiver. The receiver mimics the code of each satellite until it reproduces one that matches the transmission coming from the satellite and thereby identifies the correct satellite. The system currently in use provides for 36 separate coarse acquisition codes. The coarse acquisition code is the modulo-2 sum of two 1023 bit linear patterns designated as G-1 and G-2. The G-2 pattern is selectively delayed by an integer number of chips, which number varies for each of the 36 separate and unique variations. The result is that each satellite has a unique time delay in its G-2 signal such that when the G-2 is modulo-2 added to the G-1 signal a unique one of the 36 possible coarse acquisition codes is produced.
The navigation message is a 1500 bit data word transmitted at a rate of 50 bits per second. The message contains the time of transmission, the satellite position, satellite health, satellite clock correction, propagation delay effects, time transfer to UTC and constellation status. The navigation message effectively modulates the coarse acquisition code and is transmitted along with the L-1 carrier.
In many locations such as underwater, underground or inside a metal building, a GPS satellite signal cannot be received by a GPS receiver. In order to receive the signal, the receiver must be moved to an exposed position where the signal is accessible. In military (and in some other) situations there may be a need for covert operation and the time of exposure must be minimized. It helps in this regard to know in advance that the GPS equipment is fully functional before moving to the exposed location to communicate with the satellites. This avoids downtime while exposed, but it also requires pre-testing of the equipment out of range of the satellites. The basic operation of the receiver can be tested, by known methods, but the capacity of the receiver to receive GPS data cannot be easily confirmed. The equipment which generates the satellite signals can be reproduced in the laboratory but the size of this equipment makes it impractical for use in the field.
The GPS receiver can be tested by other methods currently known in the art. There are a number of existing systems that test, calibrate, and/or otherwise assist GPS receivers in acquiring/locking onto signals from GPS satellites (e.g. shortening the time to “first fix”). For example, U.S. Pat. Nos. 6,400,314, 6,064,336, and 5,841,396 to Krasner disclose the use of a precision carrier frequency, emanating from a base station, for calibrating local oscillators (i.e. GPS receivers). The GPS receiver disclosed in U.S. Pat. No. 6,320,536 to Sasaki utilizes signals from a stationary satellite to shorten the time required to lock onto the signals from the target GPS satellite. The GPS receiver disclosed in U.S. Pat. No. 5,663,735 to Eshenbach utilizes information derived from a standard time and/or standard frequency radio signal for the purpose of pre-tuning to the carrier frequency of the target GPS satellite. Finally, U.S. Pat. No. 6,289,041 to Krasner discloses a GPS receiver that utilizes a psuedo-random noise matching filter method, requiring the processing of a plurality of GPS satellite-generated signals, to achieve both fast signal acquisition and a high degree of sensitivity. Unfortunately, none of these four apparatus incorporate a fully self-contained design . . . all require one or more signals to be received from some remote source. It would be greatly advantageous to provide a fully self-contained design for a device to test the capacity of the receiver to lock on the signal of particular satellites and to properly process their signals (“self-contained” meaning that the signals required to test/calibrate the operation of the GPS receiver's outer loop antenna path are generated internally by the device).
SUMMARY OF THE INVENTION
It is an object of the invention to provide a compact, low physical profile and affordable GPS satellite emulator which can produce the L-1 signal of each of the GPS satellites within the constellation of the system.
It is an object of the invention to provide a satellite emulator which can quickly test the outer loop antenna path of a GPS receiver to confirm the capability to receive and process a GPS satellite signal.
It is a further object of the invention to provide an emulator equipped with switches to allow a user to vary the output of the emulator to match the signal of a specific satellite.
It is yet another object of the invention to provide an emulator equipped with a main oscillator having a relatively broad range of accuracy, to reduce the size and cost of the device of the present invention.
According to the present invention, the above-described and other objects are accomplished by a GPS satellite emulator that uses a single oscillator to provide a 10.23 MHz clock signal and 1.57542 GHz signal to a biphase shift keying modulator, which outputs the emulated satellite signal. The oscillator is selected to have a frequency accuracy of +/−10 KHz and is connected through an attenuator to the biphase shift keying modulator. The attenuator reduces the signal power of the output to −70 dBm. The relatively wide range of frequency accuracy allows for the selection of a relatively small sized oscillator and the attenuator reduces the signal power to a value near the typical signal strength of a satellite, which is approximately −120 dBm. The oscillator, attenuator and biphase shift keying modulator comprise the radio frequency components of the satellite emulator.
The GPS receiver is sufficiently sensitive to discern a signal at the power level output by the satellites and is susceptible to interference from radio frequency leakage. The present invention includes a metal case enclosing the oscillator to minimize the interference from radio frequency leakage. Additionally, the reduced power output of the satellite emulator acts to reduce the amount of radio frequency leakage interference.
The digital circuitry divides the 10.23 MHz signal by a factor of 10 to p
Coldiron, Sr. Gary L.
Dove John W.
Gregory Bernarr E.
Mull Fred H
Tarlano John L.
The United States of America as represented by the Secretary of
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