CA1236916A

CA1236916A - Method of automatically tracking satellite by receiving antenna

Info

Publication number: CA1236916A
Application number: CA000482273A
Authority: CA
Inventors: Sumitaka Ueshin
Original assignee: DX Antenna Co Ltd
Current assignee: DX Antenna Co Ltd
Priority date: 1984-05-25
Filing date: 1985-05-24
Publication date: 1988-05-17
Also published as: AU558265B2; GB2159335B; GB2159335A; FR2569308B1; NL188817B; GB8513227D0; NL8501494A; NL188817C; AU4262185A; FR2569308A1; DE3518587C2; DE3518587A1

Abstract

ABSTRACT OF THE DISCLOSURE
A method of automatically tracking a communication satellite by a parabolic antenna installed on a moving body such as ship, which can compensate for directional deviation caused not only by inclination of the body itself but also by its navigation, using a relatively simple and economical tracking system. The antenna is adjusted previously in its elevation and azimuth so that a received signal level above a predetermined level is obtainable when the moving body is in its stationary state. The method is characterized by a step of changing elevation and azimuth of the antenna within a predetermined range of variation, when the received signal level has lowered due to movement of the moving body, so as to trace a rectangular spiral on the celestial sphere by the axis of the antenna, until the received signal level exceeds the aforementioned predetermined level.

Description

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This invention relates to a method of tracking a communication satellite by a parabol;c antenna installed on a moving body, such as ship, for receiving an electric wave efficiently therefrom, by automatically changing elevation and azimuth of the antenna in response to movement of the body.
A parabolic antenna installed on a moving body, such as a ship or floating platform, for satellite communication is generally provided with an automatic tracking device for directing a main beam direction of the antenna to a satellite. The automatic tracking device adopts typically a step tracking system in which elevation and azimuth axes of the antenna are driven intermittently in order to keep the maximum reception field from the satellite. Such tracking system requires an expensive gyroscope unit for establishing a reference direction for detecting directional change of the antenna.
The opened Japanese utility model specification No. 59-46010 of March 27, 1984 discloses an improved tracking device having simple and economical construction. This device includes elevation and azimuth driving units for controlling the elevation and azimuth of the antenna, respectively, a control circuit for intermittently actuating both driving units in order to maximize the reception field from the satellite, and means for detecting inclination of a platform on which the antenna is installed to provide a control signal to the control circuit. The detecting means include two inclination detectors for separately detecting pitching angle and rolling angle of the platform, respectively, and outputs of these detectors are coupled to an interface circuit including a microcomputer for storing inclination of the platform at the time of interruption of the s36~

intermittent operation of elevation and azimuth driving units and comparing the stored inclination with the detected inclination to provide an operation resuming signal to the control circuit when the difference of both inclinations exceeds a predetermined value. In this device, however, the inclination of the platform must be corrected during interruption of operation of the driv;ng units and this may cause some tracking error.
Accordingly, an object of this invention is to provide an improved method of tracking a communication satellite by a similar economical device at higher accuracy.
According to this invention, a method of automatically tracking a satellite by a receiving antenna installed on a moving body and adjusted previously in its elevation and azimuth so that a received signal level above a predetermined level is obtainable when the moving body is in a stationary state is provided. In one respect of the invention, the method includes a step of changing elevation and azimuth of the antenna when the signal level has lowered due to movement of the moving body, so as to trace a rectangular spiral on the celestial sphere by the axis of the antenna, until the maximum signal level is restored, within a predetermined range of variation of the elevation and azimuth. In another respect of the invention, the elevation of the antenna is separately controlled in response to inclination of the moving body. Thus, this method enables automatic tracking for the antenna even while the moving body is in a moving state.
These and other objects and features of this invention will be described in more detail below with reference to the accompanying drawings.
In the drawings:
Figure 1 is a block diagram illustrating a device used for , ~.3~

embodying the tracking method of this invention;
Figures2 and 3 are flow charts representing a program for executing a provisional adjustment of an antenna in a stationary state of the moving body;
Figures 4 and 5 are flow charts representing a program for executing a complimentary adjustment of the antenna in a moving state of the body; and Figure 6 is a diagram illustrating a rectangular spiral drafted on the celestial sphere during execution of the program of Figures 4 and 5.
Referring first to Figure 1, a parabolic antenna 2 for receiving an electric wave from a broadcasting satellite tnot shown) is supported on a platform 4 fixed to a moving body (not shown) such as a navigating ship.
The antenna 2 is arranged to be driven rotationally about a horizontal axis of elevation by an elevation driving unit 6 and also about a vertical axis of azimuth by an azimuth driving unit 8. These driving units 6 and are controlled respectively by elevation and azimuth control units 10 and 12 in their angle and sense Oc rotation. The driving units 6 and may be electric motors controlled by switching devices as the control units 10 and 12, or may be hydraulic actuators controlled by electromagnetic valve devices. An elevation sensor 14 and an azimuth sensor 16 are coupled respectively to elevation and azimuth driving units 6 and 8 for sensing magnitude and sense of variation in elevation and azimuth, respectively, from a selected reference direction of the antenna 2 to produce elevation and azimuth sense signals VEL and VAz, respectively. The sensors 14 and 16 may be potentiometer devices, for example. An inclination sensor 20 is also coupled to platform 4 for sensing magnitude, sense and direction of ~6~6 variation in inclination of the platform 4 from its reference position at which the reference direction of the antenna 2 was selected, to produce an inclination sense signal VINc. The inclination sensor may be a device using orthogonal levels, which is substantially cheaper than the gyroscope device used ln the prior art units.
The antenna 2 is provided with a frequency converter 22 for converting a microwave frequency received from the broadcasting satellite into a low frequency which is then re-converted by a tuner 24 into an optimum frequency for a conventional television receiver 26. The tuner 24 also produces a signal VR indicative of the signal level currently received by the antenna 2.
g R~ VEL~ VAz and VINc are converted into digital form by an analog-to-digital convertor 28 to be processed in a central process-ing unit (CPU) 30 such as a microcomputer, as described later with reference to Figure 2 and others. The CPU 30 provides a set of command signals for controlling the elevation and azimuth of the antenna 2 to a driver unit 32 for amplification and level control. The output of driver 32 is divided into two control signals S'EL and S'Az by a solid state relay 34 and these signals are converted into d.c. control signals SEL and SAz by a rectifier 36 and applied respectively to elevation and azimuth control units 10 and 12.
CPU 30 is provided with a digital switch or keyborad 3S for inputting an optional value of elevation, as described below.
Now, the description will be made with reference to Figures 2 and 3 about a method of directing the parabolic antenna 2 correctly to the satellite for reception at the maximum signal level when the platform 4 is in a stationary state (or the ship is stopped).

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In the first step 40, a value ~0 of elevation is written into CPU 30 by digital switch 38. This value ~0 represents approximate elevation of the satellite at the time of this operation and can be obtained from a commertially available tale or chart. In step 42, CPU 30 and the succeeding control system are actuated to set the antenna 2 actually at elevation ~0. Thereafter, the antenna 2 is turned to its predetermined zero azimuth position in step 44. Next, in step 46, the antenna 2 is turned successively rightward to increase azimuth value and it is enquired whether the current value of azimuth is equal to or greater than 360 degrees in step 48. Until the value reaches 360 degrees, the answer is "NO" and another inquiry is made in step 50 on whether the current reception level VR has reached a predetermined level V1 or not. The level Vl is selected as the minimum level at which the television receiver 26 can display at least a visible image. If VR reaches Vl before reaches 360 degrees, the answer is "YES" and the antenna 2 is stopped in place where the elevation and azimuth values are l and l respectively, in step 52.
However, if "N0", step 46, 48 and 50 are repeated until "YES" is obtained.
If azimuth has reached 360 degrees before VR reaches Vl, the answer of step 48 is "YES" and the process is led to step 54 in which it is enquired whether this is the first rightward turn or not. At first, the answer is "YES" and elevation change ao is specified as 0.5 degree in step 56. This velue, "0.5 degree", has been selected only for an illustrative purpose and can be changed arbitrarily. In step 58, the antenna 2 is moved to set its elevation at ~0 0.5 and then turned leftward in step 60 to decrease the azimuth value, I. During this azimuth rotation, it is enquired whether the azimuth value has reached zero or not in step 62. Until ^:

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reaches zero, the answer is "N0" and it is further enquired in step 64 whether the receptlon level VR has reached the allowable level Vl or not.
If VR reaches Vl bcfore reaches zero, t'he answer is "YES" and the process is led to step 52 to stop the antenna in place. However, if "N0", step 60, 62 and 64 are repeated until "YES" is obtained.
If azimuth has returned to zero before VR reaches Vl, the answer of step 62 is "YES" and the process is led to step 66 in which the antenna 2 is moved to change its elevation to ~0 - 0.5. Thereafter, the process returns to step 46 and a second ri~htward azimuth scanning is executed similarly in steps 46, 48 and 50. If no allowable signal level is obtained during this scan, a second leftward scan is initiated from step 54.
This scan is similar to the first leftward scan, except that steps 68 and 70 are passed instead of step 56 since the answer of step 54 is "N0". In step 70, the elevation change ~3 is increased by another 0.5 degree and, therefore, the second leftward scan is effected at elevation 30 + 1Ø If no allowable signal level is obtained, a third rightward scan is effected at elevation aO 1.0 and followed by a third leftward scan at elevation ~0 +
1.5. As readily understood from the above description, similar reciprocal azimuth scannings are repeated with successive elevation increments of 0.5 degree until VR reaches Vl or exceeds five degrees. The antenna is stopped in place in step 52 in the former case, while the search is unsuccess-fully ended in the latter case. The above limit of I, "5 degrees", is selected only for an illustrative purpose and can be changed arbitrarily.
Next, the description will be made about a process of fine search following step 52 of Figure 2, with reference to Figure 3. In the first step 80, the elevation change with respect to the resultant elevation l -~69~

obtained by the above-described rough search is specified as 0.5 degree and, in the next step 82, the antenna is moved to sst its elevation to l + 0 5- Then, the azimuth change with respect to the resultant azimuth l obtained by the rough search is specified also as 0.5 degree in step 84 and the antenna is moved to azimuth l + 0 5 in step 86. In step 88, it is enquired whether the current reception level VR has reached a second allowable level V2 or not. This level V2 is selected as the minimum level at which the television receiver 26 can display an image of satis-factory quality and, of course, V2 is higher than the aforementioned first allowable level Vl. If the answer is "N0", A is increased to 1.0 degree in step 90 and the antenna is moved to azimuth l + 1.0 in step 86 since is yet less than 10 degrees and the answer of step 92 is "N0". Unless the allowable reception level V2 is reached, similar operation is repeated as increasing the azimuth by 0.5 degree every cycle until exceeds 10 degrees. This limit is selected only for an illustrative purpose and can be changed arbitrarily.
If Q~ has exceeded 10 degrees and the answer of step 92 becomes "YES", is returned to 0.5 degree in step 94 and similar operation is repeated through steps 96, 98, 100 and 102 as decreasing the azimuth by 0.5 degree every cycle until exceeds 10 degrees. During the above-described search operation, if the allowable reception level V2 is reached in step 88 or 98, the antenna is stopped in place (at the current elevation and azimuth ~2 and ~2 ) to complete the antenna directing operation.
If has exceeded 10 degrees in step 102, the sense of elevation change is enquired in step 106. In this first approach to step 106, the elevation l + 0 5 is maintained as it was set in step 82 and the enquired ~L~3~

sense is positive or upward. Therefore, the elevation is reset at l O.S in step 108 and similar operation is resumed from step 86 at this new elevation. At the end of this operation, if step 106 is reached again, the answer of this step should be "NO" and the elevation change is increased by O.S degree or to 1.0 degree in step 110. Then, the process is returned to step 82 to resume operation at increased elevation l 1.0, since is yet less than 5 degrees in step 112. This limit value, "5 degrees", is also illustrative and can be changed arbitrarily.
As readily understood from the above, similar search operation is repeated as increasing elevation and azimuth of the antenna by 0.5 degree every cycle until the successful end of step 104 is reached or exceeds 5 degrees in step 112. If Q~ has exceeded 5 degrees in step 112, the search operation is interrupted in step 114 and, after a lapse of one hour, it is redone from step 42 of Figure 2. This one-hour interruption is based upon the consideration that the broadcasting program transmitted through the satellite may be interrputed unfortunately during this search in spite of the fact that the system is operating normally.
After the antenna is directed correctly to the satellite in step 104 of Figure 3 when the ship is stopped, a satellite tracking operation according to this invention is initiated with movement of the ship. This operation will be described below with reference to Figures 4, 5 and 6. The same control system as shown in Figure 1 is used also in this operation.
Referring to Figure 4, N, no and no counters built into CPU 30 (Figure 1) are cleared in starting step 120 and sense of the inclination signal VINc is enquired in step 122. In accordance with the answer or sensed inclination of the ship, elevation of the antenna is changed in steps 124 "go and 126 so as to cancel the inclination and restore the allowable reception level V2 in step 128.
If V2 is not restored and the answer is "N0" in step 128, the N-counter is advanced to count one in step 130 and it is enquired whether this count is odd or not (even) in step 132. of course, the answer at this time is "YES", so that the antenna is turned rightward to increase its azimuth by 0.5 degree in step 134 and the n~-counter is advanced to count one in step 138. In step 140, it is enquired whether has exceeded 10 degrees or not. Similarly, is the magnitude of azimuth change from the value ~2 attained in step 104 of Figure 3 and is now 0.5 degree. Accordingly, the answer of step 140 is "N0" and it is enquired whether the allowable level V2 has been attained or not in step 142. If the answer is "N0", it is further enquired whether count no is equal to count N or not in step 144. As N = no = 1 at this time, the answer is "YES" and step 146 of Figure 5 follows to enquire whether has exceeded 5 degrees or not. Similar to is the magnitude of elevation change from the value ~2 attained in step 104 in Figure 3 excepting the compensatory change in steps 124 and 126 and is now zero. Accordingly, the answer is "N0" and it is enquired whether count N is odd or not in step 148. As N is now one, the antenna is turned upward to increase its elevation by 0.5 degree in step 150 and the n~-counter is advanced to count one in step 154. In step 156, it is enquired whether the allowable level V2 has been restored or not. If not, it is enquired whether count no is equal to count N or not. As no= N = 1 at this time, the answer is "YES" and the process is resumed from step 122 after clearing the no and no counters in step 160.
In the resumed operation, count N is advanced to two in step 130.

~3~ 6 Therefore, the answer of step 132 is "N0" and the antenna is~turned leftward to decrease its azimuth by 0.5 degree. As the n~-counter has been cleared, count no is newly advanced to one in step 138. Therefore, the answer ox step 144 is "N0" and a similar operation is repeated in steps 132 and 134 to decrease the azimuth of antenna again by 0.5 degree. As count no is advanced to two in step 138, the answer of step 144 becomes "YES" and steps 146, 148 and 152 follow to move the antenna downward to decrease its elevation by 0.5 degree. As the no - counter has bcen cleared, count no is advanced newly to one in step 154. Therefore the answer of step 158 is "N0" and a similar l operation is repeated in steps 146, 148 and 152 to decrease the antenna elevation again by 0.5 degree. As coùnt no is then advanced to two in step 154, the answer of step 158 becomes "YES" and the process is resumed again from step 122 after clearing the nO and no counters.
As described above, the antenna azimuth and elevation are sequentially increased by N times 0.5 degree each when N is odd, and then decreased sequentially by N times 0.5 degree each when N is even, until exceeds 5 degrees in step 146 and, thereafter, exceeds 10 degrees in step 140, unless the allowable reception level V2 is attained in step 128, 142 or 156. With increase of count N, such operation of the axis of the antenna will trace a rectangular spiral as shown in Figure 6 on the celestial sphere.
When the allowable level has been attained during such spiral tracking, the process returns to "START'7 position for repeating the same process or awaiting the next tracking command. If A has exceeded 10 degrees in step 140, the antenna directing operation need be resumed from step 42 of Figure 2. In Figure 6, natural numbers put sequentially on the "spiral" trace correspond to count N and black dots on this trace correspond to steps 144 and 158 of Figures 4 and 5, respectively. While the values of and are limitted to 5 and 10 of 3~ 6 degrees, respectively, they can, of course, be selected arbitrarily. Moreover, the interval of "black dots", which is 0.5 degree in this embodiment, can also be selected arbitrarily.
According to the tracking method of this invention it is possible to find a lost satellite relatively quickly, since its search is advanced radially outwards from its initial position. As described above, the inventive method does not use the original co-ordinate of the moving body or platform as a reference of measuring its directional change and, therefore, the tracking device need not be provided with an expensive gyroscope unit for conserving the reference direction as in the prior art methods.
The compensation for elevation change due to inclination of the platform executed in steps 122 to 128 i5 not always necessary, since it can be done by the "spiral" tracking only. However, this additional process may contribute to facilitating the tracking operation.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method of automatically tracking a satellite by a receiving antenna installed on a moving body and adjusted previously in its elevation and azimuth so that a received signal level above a predetermined level is obtainable when said moving body is in its stationary state, characterized by a step of changing elevation and azimuth of said antenna within a pre-determined range of variation, when said received signal level has lowered due to movement of said moving body, so as to trace a rectangular spiral on the celestial sphere by the axis of said antenna, until said received signal level exceeds predetermined level.

2. A method, according to Claim 1, characterized by another step of sensing inclination of said moving body to control the elevation of said antenna to cancel said inclination.