Communications: directive radio wave systems and devices (e.g. – Directive – Including a satellite
Reexamination Certificate
2002-07-15
2004-01-27
Blum, Theodore M. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Directive
Including a satellite
C342S357490, C342S357490, C342S358000, C701S213000, C701S226000
Reexamination Certificate
active
06683563
ABSTRACT:
BACKGROUND OF THE INVENTION
(a) Field of the Invention
The present invention relates to a system for determining a precise orbit of a satellite and a method thereof. More specifically, the present invention relates to a system for determining a precise orbit of a satellite and a method thereof, in which the system and method are applied to a mission requiring position information that is accurate to within 1 m, such precise position information being obtained by using only L
1
carrier phase data and applying a proportional coefficient of the total number of electrons of the ionosphere to an estimation parameter in order to minimize the errors in an ionospheric model.
(b) Description of the Related Art
A global positioning system (GPS) provides three-dimensional information such as the position, the speed, and the direction of a moving object. The GPS is now used for various applications such as in geodetic surveys, general surveying, scientific surveys, and visual synchronization, as well as in moving object navigation systems for the sea, the earth, and the air.
Taken as a whole, the GPS consists of three units: a space unit, a control unit, and a user unit. The transmitter of the GPS satellite generates a C/A (coarse/acquisition) code and a P (precision) code, that are pseudo noise codes, to thus modulate a carrier. The signal modulated in this manner is propagated to a user through an amplifier and an antenna.
The frequency of a carrier for a standard lateral position is 1575.42 MHz (L
1
band) and the frequency of a carrier for a lateral position is 1227.6 MHz (L
2
band). The L
1
band carrier and the L
2
band carrier are phase-modulated by the C/A code and the P code. The L
1
band frequency uses the P code and the C/A code. The L
2
band frequency uses only the P code.
An algorithm for determining a precise orbit of a satellite using the GPS data includes precise orbit dynamics models, measurement models, and an estimation algorithm.
Therefore, according to the algorithm for determining the precise orbit of the satellite using the GPS data, after predicting the orbit for each measurement time by applying precise orbit dynamics models from an initial orbit element, calculated measurement data are obtained by applying measurement models.
An orbit of a satellite is determined through an estimation algorithm using the differences between actual measurement data received from an on-board receiver of a satellite and GPS receivers of ground stations and calculated measurement data obtained by applying the measurement models.
When the on-board GPS receiver uses only received L
1
single frequency GPS carrier phase data in a process of determining the precise orbit of the satellite, a path error of a signal through the ionosphere is not completely removed, and accordingly, significant errors exist in the measurement data calculated by the measurement models.
Data errors caused by the path error of a signal must be reduced in order to determine the precise orbit of the satellite.
In the conventional method for determining the precise orbit of a satellite using GPS data, the precise orbit is determined using two different techniques when the on-board GPS receiver generates only the GPS data of the L
1
single frequency.
According to the first technique of determining the precise orbit of the satellite using the GPS data, an ionospheric error is removed by combining pseudo range data of a C/A code or a P code with the L
1
single frequency GPS carrier phase data.
The GPS satellite broadcasts signals with the C/A code or the P code loaded on the L
1
band frequency. The GPS receiver generates the same code, compares the generated code with the code of the received satellite, and measures the time it takes for a signal of the satellite to leave the satellite and to reach the receiver.
Therefore, a distance between the satellite and the receiver is measured by multiplying the speed of light (the speed of the satellite signal) to the time elapsed. The C/A code is formed of a pseudo random noise code that is almost actual noise. Because the obtained distance includes an error, the distance is called a pseudo range.
At this time, because the noise of a code pseudo range is larger than the noise of carrier phase data by 1000 times in the case of the C/A code pseudo distance and by 100 times in the case of the P code pseudo distance, new errors caused by pseudo range noise are added instead of removing the ionospheric error. A degree of precision in the orbit determination deteriorates due to such errors.
In the second technique of determining the precise orbit of the satellite using the GPS data, the total number of electrons of the ionosphere is estimated using the IRI-95 ionospheric model. The path error caused by the ionosphere is calculated using the estimated total number of electrons of the ionosphere.
However, according to the above methods, because only about 60% of the error of an actually existing ionosphere is corrected using only the ionospheric model, the degree of precision in the orbit determination deteriorates.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a system for determining a precise orbit of a satellite and a method thereof, in which the system and method are capable of improving the degree of precision in determining a satellite orbit by determining a precise orbit of a low orbit satellite using L
1
carrier satellite data received from a satellite and by removing a path error caused by the ionosphere by applying a proportional coefficient of the total number of electrons of the ionosphere to an estimation parameter.
In order to achieve the above object, there is provided a system for determining a precise orbit of a satellite including a satellite for receiving GPS data from GPS satellites, an international GPS service for geodynamics (IGS) for collecting and processing L
1
/L
2
carrier phase data of reference ground stations of the GPS satellites distributed all over the world, a satellite control system for monitoring and controlling a state of a satellite by receiving telemetry data and transmitting telecommand data through an antenna and for achieving the L
1
/L
2
carrier phase data of the IGS, and an image processing system for processing image data collected by the satellite.
The satellite control system comprises a tracking, telemetry and command (TTC) module for receiving the telemetry data from the satellite, tracking the satellite, and performing a link to the satellite; a satellite operations sub module for extracting the L
1
carrier phase data by processing and analyzing the telemetry data received by the TTC module, monitoring the state of the satellite, generating telecommand data to be transmitted to the satellite, and controlling and operating the satellite; and a mission analysis and planning subsystem (MAPS) for determining the precise orbit of the satellite using the L
1
carrier phase data extracted by the satellite operations sub module, the L
1
/L
2
carrier phase data of the reference ground stations of the GPS satellites collected by the IGS, and a path error caused by the ionosphere of data, and for analyzing and planning a mission of the satellite.
The MAPS comprises a first data generator for generating double differenced actual measurement data with respect to the L
1
carrier phase data extracted by the satellite operations sub module and the L
1
/L
2
carrier phase data of the reference ground stations of the GPS satellites collected by the IGS by pre-processing processor; a second data generator for generating predicted precise orbit data of a satellite by applying precise orbit perturbation models from a priori orbit and attitude elements at the measurement time with respect to the L
1
carrier phase data extracted by the satellite operations sub module and the L
1
/L
2
carrier phase data of the reference ground stations of the GPS satellites collected by the IGS; a third data generator for calculating measurement errors through GPS measurement models and generating calculated measurement data
Choi Kyu-Hong
Kim Jae-Hoon
Lee Byoung-Sun
Lee Jeong-Sook
Lee Seong-Pal
Blum Theodore M.
Electronics and Telecommunications Research Institute
Jacobson & Holman PLLC
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