Telecommunications – Carrier wave repeater or relay system – Portable or mobile repeater
Reexamination Certificate
1999-09-17
2004-10-19
Trost, William (Department: 2683)
Telecommunications
Carrier wave repeater or relay system
Portable or mobile repeater
C455S012100, C455S502000
Reexamination Certificate
active
06807398
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a time synchronization technology using a satellite. More specifically, this invention relates to a time synchronization system for synchronizing the time in the satellite system with the time in the ground system, a satellite system applied in the time synchronization system, a ground system applied in the time synchronization system, a time synchronization method, and a computer-readable recording medium with a program for making a computer execute the method.
BACKGROUND OF THE INVENTION
When astronomical events are observed using a remote system such as a satellite, it is required to synchronize the time in the satellite system with that of the ground time in order to know the time of generation of observed events. With recent advance in the field of computers, high speed data processing has become possible in the satellite system, and also an advanced protocol such as packet telemetry based on recommendation by CCSDS (Consultative Committee for Space Data Systems) for communications between a satellite system and a ground station are used.
Because of the necessity of data processing in a satellite or employment of packet telemetry, ambiguous delay in time interval is generated from the time when data is acquired until the time when the data is transmitted to the ground. Therefore, it is difficult to estimate the time of generation of an event from a time when the data is received at the ground station. On the other hand, it is required to know the accurate time of generation of data while realizing a high speed data processing in the satellite system or an advanced protocol such as packet telemetry. Especially in astronomical observation, in order to verify the result of observation to that acquired by other satellites or that obtained on the ground, precision in time measurement of the order of microseconds is required.
In the conventional technology, observed data is sampled according to the timing generated by an apparatus for controlling the timing of operations of the entire satellite system. The data is inserted at a fixed location in the transfer frame and the time at which the data was generated is determined from the time when the transfer frame is received. On the other hand, a system in which request for the current time is made when required to a device which controls the system time, or a system in which standard time is determined by using data input time into a device generating a transfer frame is employed in a satellite system employing the packet telemetry therein.
In recent years, time is determined by using the GPS (Global Positioning System).
FIG. 15
shows the commonly used GPS system. The system shown in
FIG. 15
is a satellite system in which an orbiting satellite
105
acquires the signals, namely the observation data, from four GPS satellites
101
to
104
. In this satellite system, the satellite
105
acquires apparent distances between itself and the GPS satellites
101
to
104
based on the acquired observation data, and obtains four unknown parameters i.e. its own position (x, y, z) and the difference between its own time and the time in the GPS satellites. With this method, an accurate time can be acquired in the satellite
105
.
In the satellite system described above, in association with the advance in the computer technology, sophisticated processing such as data compression or data extraction has become possible, so that data length of the observed data or the like changes, and sometimes waste of resources occurs in data transfer when a fixed data format like that in the conventional technology is used. Efforts have been made in order to improve the efficiency in data transmission by employing packet telemetry such as CCSDS.
However, in the satellite system described above, because the complicated data processing such as the packet telemetry like CCSDS is employed, a delay is generated until the acquired data is packetized, or ambiguous delay is generated until a packet including data is actually edited into the transfer frame. Further, fluctuations in time from generation up to transfer of data to the ground station becomes larger, so that it is difficult to decide the time at which the data was generated from the time at which the data is received.
In a system in which time is required to be acquired by the a device which manages the time in the satellite, non-uniformity of around a couple of tens of microseconds is included in association with realization of a protocol for acquiring time, so that an error which is not desirable in a system requiring accurate time may be generated.
High precision time determination can be realized with GPS shown in
FIG. 15
having been employed and becoming popular in recent years. However, the system configuration is very complicated. Further, in the example shown in
FIG. 15
, because the satellite
105
itself rotates around the Earth at a high speed a Doppler shift is generated. This Doppler shift makes the use of GPS on the ground difficult.
SUMMARY OF THE INVENTION
To solve the problems as described above in the conventional technology, it is an object of the present invention to provide a time synchronization system capable of determining time of generation of data with high precision and also with a simple configuration. It is also an object of the present invention to provide, a satellite system applied in the time synchronization system, a ground system applied in the time synchronization system, a time synchronization method, and a computer-readable recording medium with program for making a computer execute the method recorded therein.
In the present invention, the satellite system inserts a time frame to be used for establishing a time correlation between the satellite system and the ground system between the transmission frames at an arbitrary timing, and the ground system acquires the time of generation of data in the satellite system from this time frame. Thus, a time correlation between the time in the satellite and that on the ground can be established only by using the time frame, so that time of generation of data can precisely be determined with a simple configuration.
Further, the transmission frame is generated by packetizing the data generated in the satellite system, so that even complicated data processing can be executed.
Further, the satellite system generates the transmission frame utilizing the observation data generated in a plurality of equipments mounted thereon, so that the ground system can acquire a result of observation in the satellite.
Further, the satellite system distributes a time clock to each of the equipment and synchronizes the internal time in satellite (satellite time) with of the internal time in each of the equipments, so that a centralized time management in the satellite system can be realized.
Further, the satellite system appends the satellite time to the data generated in each of the equipment, so that time management of data can be realized in the satellite system.
Further, the satellite system generates the satellite time by counting clocks which are synchronized to a bit rate of the transmission frames, and set an entire portion of the satellite time below the time required for transmission of one frame to zero at the head of a transmission frame, so that a fraction of satellite time is eliminated and precision in time synchronization can be improved.
Further, in the satellite system, satellite time is corrected depending upon the changes in the temperature, so that time error due to temperature conditions inside the satellite system can be suppressed.
Further, the satellite system inserts satellite time at two different timings between the transmission frames, while the ground system acquires an average frequency of satellite time from the time interval between these two satellite times inserted between the transmission frames and a time interval between the ground times corresponding to these two satellite times. Then, the ground system corrects the time of generation of data in the
D'Agosta Stephen
Fujitsu Limited
Staas & Halsey , LLP
LandOfFree
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