Showing papers on "Assisted GPS published in 1984"

The Use of Pseudo‐Satellites for Improving GPS Performance

Abstract: The NAVSTAR Global Positioning System has proven itself to be extremely accurate under tests of normal operations conducted at the Yuma proving ground. During the same tests of several years ago, a special technique called Differential GPS was also evaluated and shown to have even more startling accuracies. The basic concept of Differential GPS is to have a local receiving station continuously calibrate the remaining bias in the GPS signal and transmit it to the local users. This allows aircraft, ships or land vehicles to refine their estimate of position to the last meter or two. One technique for transmitting such a correction is to use a GPS-like transmitter itself. The exact form of the signal used may be quite significant to operations and accuracy enhancement. The “pseudo-lite” or GPS ground transmitter also provides powerful geometric leverage for the total navigation solution. A pseudo-lite will be called a PL in this paper. This paper will address the geometric problems encountered by the current GPS satellite constellation. Simulation results will show the value of PLs to deal with both poor satellite geometry and potential satellite outages (in the event of failure). Quantitative examples of the performance improvements are included. This paper will also outline potential design alternatives for the PLs.

Sensitivity of GPS Acquisition to Initial Data Uncertainties

Abstract: To acquire a GPS satellite signal, it is necessary to estimate the Doppler frequency shift due to the motion of the satellite relative to the receiver. This estimate is sensitive to uncertainties in the receiver position and in the GPS “Time-of-Week,” which is used to compute the satellite's position and motion. Knowing this sensitivity and the uncertainties in the initial data, the receiver can determine the number of frequency bins that must be searched, which directly impacts the “Time-to-First-Fix.” Ignoring this sensitivity and searching the wrong number of bins can result in missing the signal on the one hand or excessive delays on the other. Since the Doppler shift is proportional to the range rate, its sensitivity to position errors is maximized by taking the gradient of range rate, and its sensitivity to time errors is given by the time derivative. Somewhat surprisingly, the Doppler estimate is most sensitive to both kinds of initial errors when the satellite is directly overhead, implying that the common practice of acquiring the highest satellite first needs to be reexamined. Also, the elevation angle alone does not determine sensitivities, since it takes two angles to specify the orientation of the satellite's orbit to the receiver—even when earth rate is neglected. In addition, numerical results for position and time sensitivities are tabulated for the entire spectrum of receiver/satellite orientations. These show that in the worst case, an error of one second in time causes nearly as large an error in estimated Doppler as does an error of one kilometer in position, roughly one hertz. One implication of this result is that a continuous, battery-powered “Real-Time-Clock” can significantly improve the Doppler estimate and reduce the acquisition time.

Audio verification system and technique for GPS receivers

Abstract: There is disclosed a system and technique for providing an audio verification of Global Positioning System (GPS) receiver signal acquisition. In a GPS receiver, the correlated output of the GPS signal is generally represented as a data modified CW signal with a Doppler component caused by the movement of a GPS satellite with respect to the receiver. Due to the Doppler component, this signal output provides a signature which is indicative of the particular satellite from which the signal is being received. The signal output is mixed with a local oscillator to produce a beat frequency in the audio band which may be filtered and coupled to an audio amplifier to produce an audio tone. The presence of the audio tone indicates the acquisition of a selected satellite signal and its audio frequency is indicative of the particular satellite signal being received.

Gps navigation device

Abstract: PURPOSE:To measure continuously a position without any break by estimating the position by using azimuth data and range data from a magnetic goniometer and a range finder and position measurement data right before the interruption of reception. CONSTITUTION:When a radio wave from a GPS (global positioning system) satellite is cut off by a shield and its reception is interrupted, an estimated position computing element 2 receives a position measurement failure alarm signal sent out of a GPS receiver 1 and starts operating to estimate the position until the reception is restarted on the basis of the azimuth data and range data from the magnetic goniometer 7 and range finder 8 and the measured position right before the transmission of a position measurement failure alarm of a receiver 1. This estimated position information is outputted to a position display device 4 through a data switch 3 which is operated to the side of the computing element 2 with the position measurement failure alarm signal outputted from the receiver 1, so that the information is displayed numerically. When the receiver 1 restarts the reception and begins to measure the position, the switch 3 is changed over to the side of the receiver 1 and the display device 4 displays the position measurement result of the receiver 1.

Automatic pilot device

Abstract: PURPOSE:To obtain an automatic pilot device having high accuracy and a long lifetime with a low cost by making to have automatic sailing after obtaining the azimuth, etc of the bow, etc by using only the radio wave radiated from a GPS/NAVSTAR satellite CONSTITUTION:A scheduled course of a ship, etc including both start and destination points is fed previously to an arithmetic controller 5 from an input device 6 The position, etc of a ship are obtained from reception of the GPS radio wave sent from a GPS/NAVSTAR satellite This obtained position and said scheduled course are calculated 5 Then a steering engine 7 and a steering wheel 8 are controlled based on the result of the calculation 5 Thus the ship sails along the scheduled course For the position detection, etc of the ship, the above-mentioned radio wave is received by a position measuring antenna 1 and an azimuth measuring antenna 3 respectively A position measuring receiver 2 receives and outputs A the position, the speed and the altitude of the moving azimuth At the same time, an azimuth measuring receiver 4 receives and outputs B the PN code of the output B of the receiver 2, the timing output, the present satellite azimuth and the bow azimuth of the own ship

Specification of a NAVSTAR Global Positioning System (GPS) receiver for a differential GPS ground system

Abstract: One step towards the successful completion of a functional ground unit for the Differential Global Positioning System (DGPS) will be in choosing a currently available GPS receiver that will accurately measure the propagation times of the satellite signals and have the capability to be electrically interfaced with and controlled by a Digital Equipment Corporation (DEC) PDP-11/34A computer. The minimum requirements and characteristics of a NAVSTAR Global Positioning System (GPS) receiver are described. The specific technical specifications addressed include data accuracies and resolutions, receiver interface/external control, enclosure dimensions and mounting requirements, receiver operation, and environmental specifications.

Marine positioning with a GPS-aided inertial navigation system

Abstract: The accuracy requirements for precise horizontal positioning of a moving vessel in the offshore and open ocean are expected to approach the 3m to Sm level within this decade Previous simulation studies and lan d-based tests have shown that such a level of accuracy can be achieved by a combination of inertial navigation and GPS satellite positioning techniques A Kalman filter and an optimal smoother have been developed to integrate an inertial navigation system with a slow switching GPS satellite receiver for marine positioning purposes The Kalman filter and optimal smoother were tested on the Canadian east coast in November 1982 The paper outlines first the operational principle of the GPS aided inertial navigation system and the development of the Kalman filter and smoother It then presents the results and a detailed error analysis of the offshore tests

Apparatus for measuring azimuth of stem

Abstract: PURPOSE:To improve the accuracy of the titled apparatus and to make it possible to use the same even in the polar regions, by calculating a stem azimuth from the azimuth of a GPS satellite with respect to the stem azimuth obtained by detecting the phase difference of the cycle of the phase change of the received radio wave from the GPS satellite and a cylce for successively changing over antenna and the azimuth of the GPS satellite obtained from a GPS receiver CONSTITUTION:In a place detector 42, the phase difference of the output of a changed-over wave generator 44 and the output of a second band pass filter 40 is taken out and the signal thereof is subsequently introduced into the one input terminal of a stem azimuth operator 46 On the other hand, the azimuth signal of a GPS satellite containing the PN code of a GPS receiver 48 is supplied to the other input terminal of the stem azimuth operator 46 As a result, in the operator 46, operation is performed so as to subtract the output of the phase detector 42+(90 deg-360 deg/16) and, therefore, the stem azimuth is calculated to be displayed by a display device 50

Apparatus for measuring stem azimuth

Abstract: PURPOSE:To improve the accuracy of the titled apparatus and to make it possible to use the same even in the polar regions, by calculating a stem azimuth from the azimuth of a GPS satellite with respect to the stem azimuth obtained by detecting the phase difference of the cycle of the change in the distance to an antenna from the GPS satellite and a cycle for successively changing over the antenna and the azimuth of the GPS satellite obtained from a GPS receiver. CONSTITUTION:The phase difference of a change-over signal and the output signal of a second band pass filter 44 is calculated in a phase detector 46 and the signal thereof is introduced into one input terminal of a stem azimuth operator 50 while an azimuth signal related to a GPS satellite containing the PPN code supplied to a PPN code generator 30 from a GPS receiver 52 is supplied to the other input terminal of said stem azimuth operator 50. therefore, the stem azimuth is calculated by subtracting the output of a place detector 46-(90 deg.+360 deg./ 16) from a GPS azimuth signal in the stem azimuth operator 50 to be displayed by a display device 54.