GB2173643A - Automatically tracking satellite by receiving antenna - Google Patents

Abstract

In a device and method for automatically tracking a communication satellite by a parabolic antenna 10 installed on a moving body such as ship, which can compensate for directional deviation caused by movement of the moving body, an auxiliary radiator 16' is used in addition to the main radiator 16 for detecting the sense of the azimuthal deviation in order to increase efficiency of the compensation search. <IMAGE>

Description

SPECIFICATION Device and method for automatically tracking satellite by receiving antenna This invention relates to a device and a method for tracking a communication satellite by a parabolic 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 used for satellite communication which is installed on a moving body such as ship or floating platform is generally provided with elevational and azimuthal driving means, elevation and asimuth detecting means, means for detecting incrination of the moving body in the direction of elevation, means for detecting received signal level and means for controlling the elevational and azimuthal driving means in accordance with a predetermined program based upon detected current elevation, azimuth, incrination and received signal level data. The inventor previously provided an improved control program for driving such an antenna system in U.S. Patent Application No.

735,278 filed May 17, 1985, Canadian Patent Application No. 482,273 filed May 24, 1985, Australian Patent Application No. 42621/85 filed May 17, British Patent Application No. 8513227 filed May 24, 1985, French Patent Application No. 8507881 filed May 24, 1985, German Patent Application No.

P3518587.2 filed May 23, 1985, and Dutch Patent Application No. 85.01494 filed May 24, 1985, respectively. In this program, the antenna is first caused to scan the whole azimuth about a starting or reference elevation obtained from a commercially available table or chart for this purpose until the received signal level exceeds a first reference level. Thereafter, the antenna is driven so that its axis drafts a rectangular spiral on the celestial sphere until the received signal level exceeds a second reference level which is higher than the first reference level. This spiral scan search is repeated as correction search when the received signal level drops below the second reference level due to movement of the moving body.

The spiral correction search is effected about the original direction at which the maximum signal level had been obtained. In other words, it is effected to cover the same width of aximuth in each side of the original direction, though the target direction should have changed to one sense of aximuth. Accordingly, if the sense of azimuthal change of the target direction can be detected, it is possible to double the search efficiency by limiting the search only in the detected sense.

An object of this invention is to provide an improved method of tracking a communication satellite by a parabolic antenna carried on a moving body, which can significantly improve the tracking accuracy in accordance with the abovementioned principle.

Another object of this invention is to provide a novel tracking device for realizing the inventive method.

In accordance with this invention, there is provided a device for automatically tracking a communication satellite by a parabolic antenna, which includes a main primary radiator fixed at the focus said parabolic antenna and directed to its axial direction, first antenna driving means for changing the azimuth of said antenna, second antenna driving means for changing the elevation of said antenna, main level detecting means for detecting the level of the signal received from the main radiator, and first antenna control means responsive to the main signal level from the main level detecting means to control the first and second antenna driving means for causing the detected level to exceed a predetermined level.As a feature of this invention, the device further comprises an auxiliary primary radiator fixed with respect to said main primary radiator at a slight azimuth deviation with respect to the main radiator, auxiliary level detecting means for detecting the level of the signal received from the auxiliary radiator, second antenna control means responsive to the main and auxiliary signal levels to control the first antenna driving means for causing the difference between the main and auxiliary signal levels to have a predetermined polarity, and third antenna control means responsive to the difference of first and second signal levels to control the first antenna driving means for causing the main signal level to exceed the predetermined level when it drops below the predetermined level.

These and other objects and features of this invention will be descreibed in detaii below with reference to the accompanying drawings.

In the drawings: Figure 1 is a biock diagram representing a summary of the device of this invention; Figure 2 is a block diagram representing an embodiment of the device of this invention; Figures 3(a) and 3fbJ are flow charts representing an example of the program embodying the method of this invention; Figure 4 is a diagram representing directive characteristics of main and auxiliary radiators given for an aid of explanation of this invention; and Figures 5 and 6 are diagrams representing two tracking traces drafted during execution of the program of Figure 3.

As shown in Figure 1, the device of this invention comprises a parabolic antenna 10 coupled mechanically to first and second driving means 12 and 14 for driving the antenna 10 about its azimuth and elevation axes, respectively. The antenna 10 is provided with a pair of similar primary radiators 16 and 16' fixed with respect to the antenna body through a suitable support frame or arm (not shown). The first radiator 16, which is referred to as "main radiator", is located at the focus of the parabolic antenna 10 and directed to its axial direction, while the second radiator 16', which is referred to as "auxiliary or sub radiator", is located close to the main radiatgor 16 but deviated slightly in the azimuth direction.Signals received by the radiators 16 and 16' are converted suitably in frequency by frequency converters 18 and 18' and supplied to main and sub level detectors 26 and 26', respectively. The main level detector 26 detects the input main signal level and supplies corresponding level indicating signal to first, second and third control means 20, 22 and 24, while the sub level detector 26' detects the input sub signal level and supplies corresponding level indicating signal to the second and third control means 22 and 24. The first control means 20, which is common to the prior art device, is responsive to the current main signal level to provide a control signal to the first and second driving means 12 and 14 for directing the antenna 10 for causing the main -signal level to exceed a predetermined level.

The second control means 22, which is peculiar to the inventive device, is responsive to the current main and sub signal levels to provide a second control signal to the first drive means 12 for changing the azimuth of the antenna 10 for causing the difference of the main and sub signal levels to have a predetermined polarity. The third control means 24, which is also peculiar to the inventive device, is responsive to the difference of the main and sub signal levels to provide a third control signal to the first drive means 12 for changing the azimuth of the antenna 10 for causing the main signal level to exceed the predetermined level when it has dropped below the predetermined level.

Although the control means 20, 22 and 24 have been shown in Figure 1 as separate units, they may be unified in practice as a microcomputer, for example, as described in conjunction with the embodiment of Figure 2.

In Figure 2, a parabolic antenna 10 for receiving an electric wave from a broadcasting satellite (not shown) is supported on a platform 32 fixed to a moving body (not shown) such as navigating ship.

The antenna 10 is arranged to be triven rotationally about a horizontal axis for elevational change by an elevation driving unit 36 and also about a vertical axis for azimuthal change by an aximuth driving unit 38. The driving units 36 and 38 are controlled respectively by elevation and azimuth control units 40 and 42 in their angle and sense of rotation. The driving units 36 and 38 may be electric motors controlled by switching devices as the control units 40 and 42, or may be hydraulic catuators controlled by electromagnetic valve devices.

Elevation and azimuth sensors 44 and 46 are coupled respectively to elevation and azimuth driving units 36 and 38 for detecting magnitude and sense of variation in elevation and azimuth from a selected reference direction of the antenna 10 to produce elevation and azimuth signals VEL and VAz, respectively. The sensors 44 and 46 may be potentiometer devices, for exampie. An inclination sensor 48 is coupled to a second platform 34 driven by the azimuth driving unit 38 and adapted to detect magnitude and sense of inclination of the platform 34 in a vertical plane including the axis of the parabola 10 to produce an inclination signal VINC The inclination sensor 48 may include orthogonal levels.

The antenna 10 is provided with a pair of primary radiators 16 and 16' which are arranged as described with reference to Figure 1 with respect to the parabola 10. In this embodiment, it is assumed that the sub radiator 16' deviates leftwards from the main radiator 16, as shown, when looking from the front side of the antenna. The main radiator 18 is coupled through a frequency convertor 18 to a main tuner 52 and the sub radiatgor 16' is coupled through a frequency converter 18' to a sub tuner 52'. The frequency converters 18 and 18' convert a microwave frequency received from the broadcasting satellite into a low frequency signal.

The main tuner 52 re-converts this low frequency signal into an optimum frequency signal for a conventional television receiver 54 and also detects the input signal to produce a main level signal M indicative of the signal level currently received by the main radiator 18. The sub tuner 52' detects the input signal from the convertor 18' to produce a sub level signal S indicative of the signal level currently received by the sub radiator 18'.

The signals M, S VEL, V,,, and VINC are supplied through a input regulator 56 to an analog-to-digital convertor 58 and converted thereby into digital form to be processed in a central processing unit (CPU) 60, such as microcomputer, as described later with reference to Figures 3 and on. The CPU 60 provides a set of command signals for controlling the elevation and azimuth of the antenna 10 to a driver unit 32 for amplification and level control.

The output of driver 32 is divided into two control signals KIEL and K',, by a solid state relay 64 and these signals are converted into d.c. control signals KEL and K,, by a rectifier 66 and applied respectively to elevation and azimuth control units 40 and 42. CPU 30 is provided with a digital switch or keyboard 68 for inputting an optional value of elevation, as described below.

Now, the description will be made with reference to Figures 3(a) and 3(b) about the method of track- ing a satellite according to this invention. The first step of the method is to direct the parabolic antenna 10 to the satellite correctly so as to receive the signal at the maximum level, when the platform 32 is in a stationary state (or the ship is in a stop).

In step 100, an initial value o of elevation 0 is written into CPU 60 by the digfital switch 68. This value e0 represents approximate altitude of the satellite at the geographical position of the ship and can be obtained from a commercially available table or chart. X=1 and N=2X-1 are also written in CPU 60 in this step 100. X and N are variables having a unit of degree. In step 102, CPU 60 and the succeeding control system are actuated to set the antenna 10 actually at elevation 00. Thereafter, the antenna 10 is turned to its predetermined zero aximuth direction in step 104. Next, in step 106, the antenna 10 is turned successively rightward to increase aximuth value ç and it is inquired, in step 108, whether the signal level M exceeds a predetermined level p or not. If not, it is further inquired whether azimuth ç reaches 360 degrees or not in step 110. If not, steps 106, 108 and 110 are re peated and, when azimuth reaches 360 , elevation of the antenna 10 is changed to 0+0.5N degrees in step 112. In this case, N=1 since X=l, and, therefore, the new elevation û=0o+0.5 degrees. This means that the axis of the antenna 10 has drafted a trace 0-1-2 as shown in Figure 5 on the celestial sphere during steps 104 through 112.

In the next step 114, the antenna 10 is turned successively in opposite direction or leftward. During this azimuthal revolution, it is inquired, in step 116, whether the signal level M exceeds the value p or not and, in step 118, whether the azimuth has returned to the initial zero value or not. When the azimuth has returned to zero before the signal level M reaches the value p, the elevation e of the antenna 10 is decreaserd by -X, which is one (1) degree in this case, in step 120 and the value of X is then increased by one (1) degree and changed into two (2) degrees in step 122. It should be understood that the trace of the antenna 10 on the celestial sphere has been extended from point 2 to point 4 during steps 112 through 122.

In the next step 124, it is inquired whether the value of X has reached five (5) degrees or not and, if not, the the process returns to step 106 and the antenna 10 changes the direction of azimuth revolution and similar steps are repeated until the value X reaches five (5) degrees. When it has reached five degrees, the antenna has reached the end point 19 of its trace as shown in Figure 5, and the process is returned to the start point to correct the initial elevation Oc.

If the signal level M has exceeded the value p in step 108 or 116, the process is taken over a correction process of this invention starting from step 126. In this process, the sub signal level S is used together with the main signal level M. As shown in Figure 4, directive characteristics of the radiators 16 and 16' are substantially same but displaced slightly in horizontal direction. The abovementioned predetermined level p is a signal level required for comfortable reception of the satellite broadcasting and is selected slightly below the intersection of the both characteristic curves M and S. Therefore, the difference M-S is above zero or positive when the main signal level M is higher than the value p (for range A of Figure 4).As seen from Figure 4, there are two unallowable ranges E and D in which the main signal level M is lower than the value p. As also seen from the drawing, the difference M-S is negative in range E, while it is positive in range D. In other words, when the main signal level M drops below the value p, the antenna 10 deviates leftward from the satellite if M-S is negative and it deviates rightward if M-S is positive. The method of this invention utilizes this feature.

In step 126, it is inquired whether M-S is positive (including zero) or not, that is, whether the satellite is within range C or not. If "NO", this means that the satellite is within range B or E. (If it is sure that M is higher than p, it is within range B.) Therefore, in order to correct the azimuth, the antenna is turned leftward in step 131 and it is inquired whether M is higher than p or not. This inquiry is also made for confirmation when the answer is "YES" in step 126. If it has been confirmed in step 128 that M is higher than p, the antenna is stopped in position in step 130.

After stopping the antenna in step 130, it is again inquired whether M-S is positive or not. If not, the antenna is turned leftward in step 136 as in step 131 and it is reconfirmed in step 128 that M is higher than p. If "YES", the current level "a" of the inclination signal VINC is written in the CPU 60 and then steps 128, 130 and 132 are executed similarly.

By and during such repetition of steps 128 and 136, the antenna catchs the satellite within the desired range C of Figure 4.

If, in the meantime, the antenna has failed to catch the satellite in range A of Figure 4 due to movement of the ship, the answer in step 128 becomes "NO" and step 138 of Figure 3(b) is taken over. In step 138, it is inquired whether the difference M-S is positive or zero or negative. If it is positive, this means that the satellite is within range D and, therefore, the antenna is turned rightward in step 140 and step 142 is entered. If it is negative, this means that the satellite is within range E and, therefore, the antenna is turned leftward in step 144 and step 142 is entered. If the level difference is zero, step 142 is entered directly.

(When the directive characteristics M and S have a relation as shown in Figure 4 with respect to the level p, the difference M-S should never become zero when M is below p. However, such chance where M-S=0 and, at the same time, M < p may be obtained when the ship is at the end of broadcasting area or in the shade of island and the received signal levels have been lowered.) In step 142, the current level "b" of the inclination signal VINC is written in the CPU 60 and, in step 146, it is inquired whether the difference a-b is positive or zero or negative. This difference represents an elevational deviation of the antenna from the desired value in step 134 due to pitching and/ or rolling of the ship.If the difference is positive, this means reduction of elevation and, therefore, the elevation o is increased in step 148 and step 150 is entered next. If it is negative, this means increase of elevation and, therefore, the elevation is reduced in step 152 and step 150 is entered. If it is zero, this means no deviation caused and, therefore, step 150 is entered directly. In step 150, it is inquired whether M is higher than p or not. If "YES", this means the satellite is within range C of Figure 4 and, therefore, the program returns to step 128 to repeat the abovementioned steps.

The abovementioned program is satisfactory in the normal receiving condition where the intersection of two characteristic curves is above the predetermined level p. In other words, when the main signal level M drops below the level p, the antenna may be turned rightward so long as the difference M-S is positive. However, when the received sig nai level has dropped extraordinarily for some reason as aforementioned and the above intersection has come below the level p, as shown by p' in relative fashion. In such condition, no optimum reception can be obtained by turning the antenna rightward when the satellite is within range F or Figure 4, though M-S is positive.Therefore, if the answer of step 150 is "NO", a speciai program is executed as the under.

In step 154, the value of X is reset to one (1) degree and, in step 156, the antenna is turned rightward to increase its azimuth. In the next step 158, it is inquired whether the main signal level M has reached the level or not. If "YES", the program is returned to step 128 and the abovementioned steps are repeated to maintain the optimum condition for reception. If "NO", however, it is inquired, in step 160, whether the increment of azimuth has reached five (5) degrees or not and steps 156, 158 and 160 are repeated until this increment reaches five degrees. When it has reached five degrees, the elevation of the antenna is increased by 0.5N degree, that is, 0.5 degree in this case since N=2X-1 and X=1 , in step 162.Then, the antenna is turned inversely or leftward to decrease the azimuth in step 164 and it is again inquired whether the level M has reached the level p or not in step 166. If "YES", the program is returned to step 128 and the abovementioned steps are repeated for maintaining this receiving condition. If "NO", however, it is inquired, in step 168, whether the decrement of azimuth has reached five (5) degrees, that is, the antenna has turned leftward by ten (10) degrees from step 164 or not, and steps 164, 166 and 168 are repeated until such azimuth is reached. When it has been reached, the elevation of antenna is reduced by X degree, that is, one degree in this case, in step 170. Then, the value of X is increased by one degree in step 172 and it is inquired whether X has reached five (5) degrees or not in step 174. If "NO", the program is returned to step 156 and steps 156 to 174 are repeated similarly until X reaches five degrees. It will be understood that, when X has reached five degrees, the antenna axis has drafted such a trace as shown in Figure 6 on the selestial sphere. As seen from this drawing, this search is made over the range of 10 degree azimuth about the initial direction, including the lefthand side in order to avoid the aforementioned fault.

If the value of X has reached five degrees in step 174, this means that the satellite has deviated too excessively to correct the error and, therefore, the program is returned to the start point for resetting the initial elevation 00.

Claims (1)

1. (1) A device for tracking a communication satellite by a parabolic antenna comprising a parabolic reflector, a primary radiator located at the focus of said reflector, a first drive unit for rotating said antenna about its axis of azimuth, a second drive unit for rotating said antenna about its axis of elevation, a main level detector coupled to said main primary radiator for detecting a main signal level received by said main radiator, and first control means responsive to said main signal level for causing said first and second drive units to drive said antenna so that said main signal level exceeds a predetermined level; characterized in that said device further comprises an auxiliary primary radiator located close to said main radiator so as to have a slight azimuthal deviation from said main radiator, an auxiliary level detector coupled to said auxiliary primary radiator for detecting an auxiliary signal level received by said auxiliary radiator, second control means responsive to the difference of said main and auxiliary signal levels for causing said first drive unit to change the azimuth of said antenna so that said difference exhibits a predetermined polarity, and third control means responsive to the difference of said main and auxiliary signal levels to cause said first drive unit to change the azimuth of said antenna, when said main signal level has dropped below said predetermined level, so that said main signal level exceeds said predetermined level.

   (2) A device, according to Claim 1, characterized in that said first, second and third control means are included in a single microcomputer.

   (3) A method for tracking a communication satellite by a parabolic antenna comprising a parabolic reflector, a main primary radiator located at the focus of said reflector, an auxiliary primary radiator located close to said main radiator so as to have a slight azimuthal deviation from said main radiator, and main and auxiliary level detectors coupled respectively to said main and auxiliary primary radiators for detecting main and auxiliary signals levels received by said main and auxiliary radiators, respectively; characterized by steps of adjusting the azimuth and elevation of said antenna to allow said main signal level to exceed a predetermined level, adjusting the azimuth of said antenna to allow the difference of said main and auxiliary signal levels to exhibit a predetermined polarity, and adjusting the azimuth of said antenna, when said main signal level has dropped below said predetermined level, in accordance with the polarity of said signal level difference, whereby restoring the predetermined signal level.

   (4) Method and apparatus for tracking a communication satellite substantially as described with reference to the accompanying drawings.