JPH04369934A - Doppler frequency compensation method - Google Patents

Abstract

PURPOSE:To allow out of synchronization of a receiver to hardly take place by correcting a fraction of a Doppler frequency before changeover of a changeover switch so as to decrease discontinuity of the frequency at changeover. CONSTITUTION:A frequency difference [fdown.dop(tn)- fnirx(tn)].221/240 between a unidirectional Doppler measurement mode and a two-way Doppler measurement mode for the frequency to change over a changeover switch 23 is corrected by a ground station 1 and the changeover switch 23 is switched afterward to switch a fixed oscillator 22 or an external synchronization oscillator 21. After the changeover switch 23 is switched, the frequency is restored continuously to the nominal frequency fnirx and the nominal frequency is selected, then the two-way Doppler measurement mode is started. Thus, the frequency of the fixed oscillator 22 is matched with the frequency of the external synchronization oscillator 21 before the changeover of the changeover switch 23. Moreover, the frequency after changeover is swept and the sweeping is implemented by a triangle wave. Thus, the re-acquisition when out of synchronization takes place in a receiver 17 of the ground station 1 is facilitated.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] This invention is applicable to a tracking control system in which a communication satellite has a radial velocity relative to a ground station.
The present invention relates to a Doppler frequency compensation method for compensating for the Doppler frequency.

0002

2. Description of the Related Art FIG. 6 is an explanatory diagram showing a frequency sweep waveform in a conventional Doppler frequency compensation method, and FIG. 7 is an explanatory diagram showing changes in the reception frequency of a ground station. Further, FIG. 5 is a block diagram showing a transmitting/receiving device of a tracking control system to which the present invention and the conventional Doppler frequency compensation method are applied.

In FIG. 5, reference numeral 1 is a ground station installed on the earth, and reference numeral 2 is a communication satellite that is launched on an elliptical orbit around the earth and communicates with the ground station 1.

[0004] In the ground station 1, 10 is the ground station 1
11 is a modulator that modulates the carrier wave of the frequency determined by the sweep circuit 10 using a command signal or the like.

Reference numeral 12 denotes a transmission frequency converter that converts the output frequency of this modulator 11 into an RF frequency specified by the satellite communication system, and 13 denotes this transmission frequency converter 1.
This is a high power amplifier that amplifies the output of 2. 14 is a ground station antenna of this ground station 1 that performs transmission and reception with the communication satellite 2.

[0006] Within the communication satellite 2, 20 is a satellite receiving antenna that receives a transmission signal from the ground station 1 which has undergone a Doppler frequency shift in the uplink. 21 is a frequency multiplier that multiplies the frequency of the signal received by the satellite receiving antenna 20 by a specified multiplication ratio (240/221 times in this case) and outputs synchronization/asynchronous information between its input and output. It acts as an externally synchronized oscillator synchronized to the transmission frequency from station 1.

Reference numeral 22 indicates a fixed oscillator of the communication satellite 2 which oscillates a predetermined frequency in a fixed manner, and 23 indicates one of the oscillation frequency of the fixed oscillator 22 and the output frequency of the frequency multiplier 21 at the ground station 1. This is a selector switch that is selected according to commands from 24 is a satellite transmitting antenna that transmits the signal selected by the changeover switch 23 to the downlink.

Again, in the ground station 1, 15 is a low-noise amplifier that amplifies the transmission signal of the communication satellite 2 received by the ground station antenna 14 after receiving a Doppler frequency shift in the downlink, and 16 is a low-noise amplifier. RF output from amplifier 15
This is a reception frequency converter that converts a frequency into an IF frequency suitable for a receiver, which will be described later.

A receiver 17 performs tracking, demodulation, and decoding based on the received signal converted to an IF frequency, and sends synchronization/asynchronous information of the communication satellite 2 to the sweep circuit 10. 1
8 is a Doppler frequency calculation circuit that predicts and calculates the Doppler frequency in the signal received by the ground station antenna 14 based on the orbit information and provides the result to the receiver.

Next, the operation will be explained. In the tracking control system configured in this way, the communication satellite 2 is connected to the ground station 1.
, the frequency of the received signal at the ground station 1 will have a Doppler frequency shift. In this case, change the changeover switch 23 to select the frequency multiplier 2.
By selecting either the fixed oscillator 1 or the fixed oscillator 22, the Doppler frequency shift is compensated for.

First, in the communication satellite 2, the changeover switch 2
The explanation will be given starting from the state where 3 selects the fixed oscillator 22 side. This state is called one-way Doppler measurement mode. In this case, the frequency received by the ground station 1 is only one way downlink from the communication satellite 2, so it is called unidirectional.

[0012] Now, if the frequency of the fixed oscillator 22 of the communication satellite 2 deviates from the specified value (nominal frequency) fN1RX by △fN1RX due to changes over time, temperature changes, setting errors, etc., △fN1RX will appear on the ground as if it were a Doppler frequency. Since it is visible, it is characterized by an error in the distance change rate. The frequency relationship is shown in FIG.

Consider changing to the two-way Doppler measurement mode in this state. First, sweep circuit 1
0 is operated to sweep the transmission frequency and phase synchronize the frequency multiplier 21 of the communication satellite 2. The frequency sweep waveform at that time is shown in FIG.

Note that the synchronization frequency range of the frequency multiplier 21 is narrow, and since the sweep circuit 10 is located at the ground station 1 and not mounted on the communication satellite 2, it is necessary to sweep from the ground station 1. Therefore, before this synchronization information is determined on the ground station 1 side, there is a waiting time depending on the transmission time between the communication satellite 2 and the ground station 1, and the bit sheet and frame structure of the transmission method, so as shown in FIG. The synchronization information from the communication satellite 2 is waited for Tw seconds after the completion of the triangle sweep.

Here, if synchronization is achieved at the (+) side frequency as shown in FIG. 6(a), no further sweeping is performed. Furthermore, when synchronization is not achieved at the (+) side frequency as shown in FIG. 6(b), sweeping is also performed at the (-) side frequency after the Tw seconds have elapsed.

Although the sweep waveform is triangular here, other waveforms such as a trapezoidal wave may be used. However, after the sweep is completed, it is necessary to reach the specified frequency fN1TX in order to make Doppler measurement accurate.

In this way, when the uplink is established by the sweep with the synchronization of the frequency multiplier 21, it becomes possible to send commands from the ground station 1 to the communication satellite 2, and the changeover switch 23 of the communication satellite 2 is activated. Frequency multiplier 2
Switching to the ground station 1 side can be controlled from the ground station 1 side.

FIG. 7 shows how the reception frequency at the ground station 1 changes when the changeover switch 23 is switched in this manner. FIG. 7 shows a case where the distance R' to the communication satellite 2 (that is, f' of the Doppler frequency f) is changing, and the frequency is increasing to the right. Also, at the bottom of the figure,
The Doppler measurement mode and the operation of the changeover switch 23 are also shown.

At the timing of time tX, the selector switch 2
3, set the Doppler measurement mode to bidirectional mode, and set the frequency reference oscillation source to the fixed oscillator 2 of the communication satellite 2.
2 to a frequency multiplier 2 based on a highly stable oscillation source of the ground station 1
Switch to side 1. The stability (Δf/f) of this highly stable oscillation source of the ground station 1 is about 1×10 −11 , which is extremely low as a Doppler measurement accuracy.

By switching the selector switch 23, the receiving frequency is changed to the following equation 1 before switching.
After switching, it becomes the same equation (2).

[0021]

[Equation 1] Before switching: fDOWN・DOP (tX) + △f
N1RX(tX)
…■ After switching: 2・fDOWN・DO
P(tX) + fDOWN・DOP−DOP(tX)
...■fDOWN・DOP(tX
); Downlink Doppler frequency shift amount at time tX fDOWN・DOP-DOP(tX); One-way Doppler reception frequency shift corresponding to the uplink Doppler shift frequency at time tX

[0022] Here, for example, if the transmission frequency is 21
When the distance change rate is 10km/sec at 20MHz,
fDOWN/DOP is approximately 76.8KHz, 2fDOWN
・DOP is approximately 53.6KHz, fDOWN・DOP・D
OP is about 5Hz, and △fN1RX is about 3KH.
It will be less than or equal to z.

[0023] Therefore, the receiving frequency before switching is 79.8.
KHz, and the receiving frequency after switching becomes 153.6 KHz, and a frequency discontinuity of about 70 KHz occurs at the moment of switching operation of the changeover switch 23.

[0024]

[Problem to be Solved by the Invention] Since the conventional Doppler frequency compensation method is configured as described above, when switching between the unidirectional Doppler measurement mode and the bidirectional Doppler measurement mode, the received frequency jumps significantly and the frequency There is a problem in that discontinuities may occur and the receiver 17 may become out of synchronization.

The present invention has been made in order to solve the above-mentioned problems, and provides a Doppler frequency compensation method that suppresses frequency discontinuity when switching Doppler measurement modes to a small level and makes it difficult for receivers to lose synchronization. The purpose is to obtain.

[0026]

[Means for Solving the Problems] The Doppler frequency compensation method according to the invention set forth in claim 1 is a method for compensating a Doppler frequency by transmitting signals from a ground station before switching between a fixed oscillator and an external synchronous oscillator using a changeover switch in a communication satellite. The transmission frequency is controlled to match the frequencies of the fixed oscillator and external synchronous oscillator.

Furthermore, the Doppler frequency compensation method according to the second aspect of the invention further sweeps the frequency after switching the changeover switch, and the Doppler frequency compensation method according to the third aspect further comprises the following steps: The frequency is swept in a triangular waveform.

[0028]

[Operation] The Doppler frequency compensation method according to the invention described in claim 1 corrects the difference between the one-way equivalent and the two-way equivalent Doppler frequency before switching the changeover switch, thereby changing the Doppler measurement mode. A Doppler frequency compensation method is realized in which the frequency discontinuity in time is kept small and the receiver is less likely to lose synchronization.

Furthermore, the Doppler frequency compensation method according to the invention described in claims 2 and 3 further sweeps the frequency after switching the change-over switch, so as to compensate for the occurrence of synchronization in the receiver of the ground station. A Doppler frequency compensation method that is easy to reacquire is realized.

[0030]

【Example】

Example 1. Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is an explanatory diagram showing a frequency sweep waveform in an embodiment of the invention as set forth in claim 1, and FIG. 2 is an explanatory diagram showing changes in the receiving frequency of the ground station 1. In FIG. Note that the Doppler frequency compensation method according to the present invention is applied to the transmitting/receiving device of the tracking control system shown in FIG. 5, etc., as in the conventional case.

In this embodiment, the changeover switch 23
The frequency at which switching is performed is the conventional nominal frequency fN1.
Not RX, but the difference between unidirectional Doppler measurement mode and bidirectional Doppler measurement mode [fDOWN・DOP(tN)−△
fN1RX(tN)]·221/240 frequency is corrected on the ground station 1 side, and then the changeover switch 23 is switched.

After the changeover switch 23 is switched, the frequency is continuously returned to the nominal frequency, and after being set to the nominal frequency, the bidirectional Doppler measurement mode is started.

Note that the amount of frequency deviation of the oscillator 22 of the communication satellite 2 can be known from operational data. From this, the Doppler frequency in one direction can be measured.

Furthermore, since the Doppler frequency can be calculated from the orbit prediction data of the communication satellite 2, the above correction can be made. Here, the double Doppler shift amount fDOWN・DOP・D
Since OP is about 5 Hz, it can be ignored compared to the reception bandwidth of the receiver 17 of the ground station 1.

According to the frequency sweep waveform and switching timing shown in FIG. 1, the reception frequency of the ground station 1 changes as shown in FIG. Here, the frequency of the frequency multiplier 21 and the frequency of the fixed oscillator 22 are
Match before switching.

As a result, the frequency before switching of the selector switch 23 is expressed by the equation (2) shown below, and the frequency after switching is also expressed by the equation (2).

[0037]

[Equation 2] Before switching; fNIRX (tN) + fDOWN・D
OP(tN)+△fN1RX(tN)
…■ After switching; fNIRX(
tN)+△fNIRX(tN)+2fDOWN・DOP
(tN)+fDOWN・DOP−DOP(tN)
−[f′D
OWN・DOP(tN)+△f′NRX(tN)]
…■ =
fNIRX(tN)+fDOWN・DOP(tN)
+[{ △fNIRX(tN
)−△f′NRX(tN)}
+{fDOWN・DOP(tN)−f′DOW
N・DOP(tN)} +
fDOWN・DOP−DOP(tN)]

…■

[0038] Here, the formula (■) shown in the above equation 2 is
This is a modified version of the formula. The part enclosed in square brackets in this formula (■) is the error frequency component, and if the error frequency component is made smaller than the band of the receiver 17 (approximately 3000Hz to 300Hz), it is possible to prevent the receiver 17 from losing synchronization. can.

Example 2. In the above embodiment, the corrected frequency after switching indicated by "A" in FIG. 1 is shown as a linear waveform, but this portion may be further swept. FIG. 3 is an explanatory diagram showing a frequency sweep waveform in one embodiment of the invention described in claims 2 and 3, and FIG. 2 is an explanatory diagram showing changes in the reception frequency of the ground station 1.

In this embodiment, as indicated by "B" in both figures, the frequency is swept in the form of a triangular wave after the changeover switch 23 is switched. By sweeping the frequency after switching in this way, ground station 1 can
Even if the period of the receiver 17 is lost, synchronization can be easily reacquired.

[0041]

As described above, according to the invention set forth in claim 1, the difference between the one-way equivalent and the two-way equivalent of the Doppler frequency fraction is corrected before switching the changeover switch. , it is possible to suppress frequency discontinuity when switching Doppler measurement modes to a small level, and it is possible to obtain a Doppler frequency compensation method in which loss of synchronization of the receiver is less likely to occur.

Further, according to the invention as set forth in claims 2 and 3, since the frequency after switching the changeover switch is configured to be swept, for example, in the form of a triangular wave, synchronization can be easily recaptured. This has the effect of providing a possible Doppler frequency compensation method.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram showing a frequency sweep waveform in a Doppler frequency compensation method according to an embodiment of the invention as set forth in claim 1;

FIG. 2 is an explanatory diagram showing changes in reception frequency at the ground station.

FIG. 3 is an explanatory diagram showing a frequency sweep waveform in an embodiment of the invention according to claims 2 and 3.

FIG. 4 is an explanatory diagram showing changes in reception frequency at the ground station.

FIG. 5 is a block diagram showing a transmitting/receiving device of a tracking control system to which the present invention and the conventional Doppler frequency compensation method are applied.

FIG. 6 is an explanatory diagram showing a frequency sweep waveform in a conventional Doppler frequency compensation method.

FIG. 7 is an explanatory diagram showing changes in reception frequency at the ground station.

[Explanation of symbols]

1 Ground station 2 Communication satellite 21 External synchronous oscillator (frequency multiplier) 22 Fixed oscillator 23 Changeover switch

Claims (3)

[Claims] Claim 1. A tracking control system in which a communication satellite is equipped with a fixed oscillator that oscillates at a predetermined frequency and an external synchronous oscillator that is synchronized with a transmission frequency from a ground station, wherein the communication satellite is connected to the ground station. By switching between the fixed oscillator and the external synchronous oscillator of the communication satellite using a changeover switch operated by a command from the ground station, the Doppler frequency deviation of the reception frequency of the ground station can be changed. In the Doppler frequency compensation method for compensating for Frequency compensation method. 2. The Doppler frequency compensation method according to claim 1, further comprising sweeping the frequency after switching the changeover switch. 3. Claim 2, wherein the frequency sweep after switching the changeover switch is performed in a triangular wave shape.
The Doppler frequency compensation method described in .