Method for controlling an antenna of an earth station for telecommunication via satellites

Method for controlling an antenna of an earth station for telecommunication via satellites, which antenna is provided with means to determine the position of the antenna and with means to ascertain the strength of the received signal, the direction of each change of position to be made by the antenna being derived from the change of the signal strength and the attendant change of position in consequence of the uncontrolled changes of position made by the antenna with respect to the position chosen.

BACKGROUND OF THE INVENTION 
The invention relates to a method for controlling an antenna of an earth 
station for telecommunication via satellites, which antenna is provided 
with means to determine the position of the antenna and with means to 
ascertain the strength of the received signal. 
Various known methods make use of the so-called hill-climbing method, so 
named because in this method there is a continuous search for a position 
of the antenna in which the received signal is stronger than it was in the 
preceding position; in other words: in the hilly landscape of signal 
strengths one always tries to climb. One of the ways to realize this 
method is the so-called "step-track" method. This "step-track" method is a 
simple and relatively cheap solution for keeping an antenna pointed at a 
satellite. The position of the antenna is changed step by step with equal 
steps of for example 0.01.degree., both in elevation and in azimuth, it 
being always tried to find a position in which the received signal is as 
strong as possible. 
The tracking system using the "step-track" method causes in an arbitrary 
direction a step-by-step change in for example the direction of elevation. 
If due to this change of position the strength of the received signal 
measured increases, a next step is made in the same direction, and so 
forth. When after a number of steps a signal decrease caused by the last 
step is observed, then one step is made backwards, after which a similar 
step-by-step change of position is started in the azimuth direction. As 
the received signal can be subject to strong fluctuations due to 
atmospheric influences, measuring of the signal strength has to take place 
over a longer time, for example some minutes, after which the average 
value is determined. Each change of position of the antenna is 
time-consuming because of the great slowness of the antenna. Each first 
step is arbitrary with the risk of a decrease of signal. 
The object of the invention is to overcome the drawbacks of the 
"step-track" method. 
Another object of the invention is to provide a method in which the 
correction of the position of the antenna is effected in such a way that 
the antenna makes the smallest possible number of steps, because steps 
calculated beforehand as to their direction and size are made. This object 
is attained because the direction and the magnitude of each change of 
position to be made by the antenna are derived from the change of the 
signal strength and the attendant change of position in consequence of the 
uncontrolled changes of position made by the antenna with respect to the 
position chosen.

DETAILED DESCRIPTION 
FIG. 1 shows a base 1 provided with a support 2, the support 2 being 
capable of rotating around a vertical axis relative to the base. The 
support 2 has a horizontal spindle 4 on which an antenna 3 is mounted. The 
angular position of the support 2 with respect to the base 1 can be 
determined by means of an angular position indicator, which is not shown; 
likewise the angular position of the antenna 3 with respect to the support 
2 can be determined by a second angular position indicator, which is also 
not shown. A satellite, for example a geo-stationary one, is represented 
by a point 5, the line 6 designating the optimum position of the center 
line of the antenna 3. The actual center line of the antenna 3 is 
designated by a line 7, which generally forms an angle with the line 6. 
Due to all sorts of influence, the position of the satellite 5, even in 
the case of a geo-stationary satellite, is not always the same with 
respect to the earth and, consequently, not with respect to the antenna 
either. In order to maintain a maximum reception level of the antenna, the 
antenna, however, has to be kept pointed at the satellite 5 in the best 
possible way. In FIG. 1 this means that it attempted to always have the 
lines 6 and 7 coincide. 
One of the ways in which the above can be attained is the so-called 
"step-track" method. This known method consists in that the strength of 
the received signal is determined at any time during a certain time 
period, after which, when not reaching a certain minimum value, the 
position of the antenna is changed step by step. As the direction in which 
the first step has to be made is unknown, it is chosen arbitrarily. All 
this is explained by FIG. 2, in which the azimuth is plotted along the 
horizontal axis and the elevation along the vertical axis. The center line 
7 of the antenna cuts a plane 8 (FIGS. 1 and 2), which is perpendicular to 
the line 6, at point A; this should, however, be at point E' (FIGS. 1 and 
2). If during a certain time priod, for example some minutes, the signal 
remains lower than a certain fixed value, the antenna makes a fixed step 
of for example 0.01.degree. in an arbitrary direction; according to FIG. 2 
upwards along the elevation axis to point J. If it appears now that the 
received signal has increased, a next step (to point C) is made in the 
same direction. After the third step the center line of the antenna 
coincides with the point B, but now the signal measured is weaker than 
that of the preceding position, so that the controlling system causes the 
antenna to make one step backwards to the point C. After this a step is 
made in an arbitrary direction along the azimuth axis, to, for example, 
point K. In this case too the steps are repeated in the same direction 
until a signal is measured which is weaker than the signal obtained after 
the preceding step (point D), after which a step is made backwards (to 
point E). 
The procedure described above takes a lot of time. Because of the fact that 
the received signal is not constant in consequence of atmospheric 
influences, measuring of the signal strength has to take place for some 
time, so that a reliable average of the strength of the signal can be 
obtained. 
A further drawback is that the direction chosen for the first step is 
arbitrary and that the direction once chosen is continued as long as after 
each step the received signal is stronger than the signal measured after 
the preceding step. If, for example, during the stepping movement from 
point A to point E the satellite has moved to point H', so that point H 
would be the most proper point to point the antenna axis at it, then more 
gain of signal could be obtained by stopping already for the first time at 
point E coming from the direction of point C along the elevation axis. As 
described, the controlling system, however, will cause line 7 to reach 
point E for the third time via the points D and F to cause line 7 to then 
make steps according to the elevation axis only. Moreover, FIG. 2 shows 
how point H is now reached, and how from point H the controlling system 
keeps searching both in azimuth and in elevation (FIG. 1) for a direction 
with a stronger signal. 
According to the invention the aforesaid drawbacks are overcome and the 
number of steps to be made are considerably reduced. As a consequence a 
certain desired correction is effected in a much quicker way, whereas 
thanks to a greater precision a higher average signal is obtained. 
Under the influence of a number of circumstances, such as small 
instabilities of the controlling system (so-called "limit-cycles"), wind 
forces, gravity, thermal expansion and shrinking, an antenna for 
communication by satellites will make, within certain limits, uncontrolled 
movements with respect to the position chosen. The driving system is 
continually active in readjusting the antenna to the set value. 
Measurements on a test antenna have proved that the course in the time of 
the uncontrolled movements in azimuth is almost as represented in FIG. 3a; 
the course of those movements in elevation, seen by the digital angle 
position indicators, is as represented in FIG. 3e. The result in both 
cases is 0.016.degree. from top to top. The less regular course of the 
movement in elevation is caused by the unbalance of the antenna. 
It can be ascertained that both for azimuth and for elevation the antenna 
is in one of the outermost positions for the greater part of the time 
(i.e. 96% of the time). This is caused because in the case of a standstill 
the coefficient of friction at the points of suspension of the antenna is 
much greater than when moving. In consequence of this it requires 
relatively much driving power to cause the antenna, when being in one of 
its two outermost positions, to move. However, once such a movement has 
started, the speed quickly increases, so that the desired position is gone 
beyond, after which the controlling system slows down the movement of the 
antenna and stops it. Then the whole procedure starts again. If these 
movements are regarded in a plane 9 (part of plane 8 in FIGS. 1, 4), then 
the antenna axis cuts this plane at one or another of the four points LO, 
RO, LB, RB for about 96% of the time and it is only about 4% of the time 
somewhere between these points. 
As shown in the diagram of FIG. 5, the antenna 3 can be pointed towards any 
point of the upper hemisphere by a combined movement about two orthogonal 
axis, the so called elevation 4 and azimuth 10 axis. Each axis is coupled 
with an angle position indicator 11 resp. 12 (Manufacturer HEIDENHAIN, 
Type ROD 7/98.1, angle resolution 0.002.degree.), a tacho generator 13 
resp. 14 and a drive unit 15 resp. 16. The upper branch 17 shows the 
signal flow of the satellite signal from antenna feed to the computing 
system (type PDP 11) 18. The digital signals delivered by the position 
indicators 11, 12 are also led to the computer 18. The jitter movement of 
the antenna and the small antenna beamwidth causes the signal received 
from the satellite to fluctuate. Thanks to these fluctuations it is 
possible to determine the position of the center line 19 of the antenna 
(FIG. 5) with respect to the optimum position (i.e. center line directed 
to the satellite). A special program is implemented in the computing 
system to compute this position. The program takes into account the 
influence of (thermal and quantization) noise and the occurrence of 
fading. With the help of these computations the computing system instructs 
a control circuit to readjust the antenna position. In the block diagram 
the control circuits are shown in more detail to reveal the application of 
the feedback. In the described two loops for azimuth and two loops for 
elevation are to be distinguished. The first loop in both cases contains 
the tacho generator 13, 14 to realize velocity feedback i.e. the relation 
between antenna velocity and the control variable becomes less dependent 
on torques delivered by wind and friction. The second loop in both cases 
is formed by the chain; (a part of) the computing system 18, D/A convertor 
20,21, integrator 22,23, difference amplifier 24,25, control amplifier 
26,27, drive motor 15,16, position indicator 11,12. The purpose of this 
loop is to obtain a zero error in angular position of the antenna in the 
constant velocity mode and a small error in the constant acceleration 
mode. 
A numerical example showing the search for the desired antenna position is 
given hereafter. 
An extensive computer simulation based on the measured values of essential 
antenna parameters has been carried out. The results of this simulation 
are representative for the practical situation because every factor 
influencing the positioning procedure is taken into consideration. For 
this reason the measured antenna parameters (in particular the antenna 
diagram and the behaviour of antenna jitter i.e. the uncontrolled changes 
of position made by the antenna) are supplemented by measurements of 
receiver noise and fading characteristics. The simulation is carried out 
as follows. 
A computer generates each second a sample representing the amplitude of the 
received signal from a satellite and with the same statistical parameters, 
the signal is received with an antenna position supposed to be in (x,y) 
with respect to the optimum position (0,0). After the generation of about 
twenty samples the computing procedure is started. In this computation the 
relatively slow signal attenuation due to fading is approximated by a 
higher order polynomial function. Regression is executed to find the best 
fit radiation diagram matching to the collected twenty samples of signal 
strength as a function of position and time. The regression calculates the 
starting position (x,y) with a certain inaccuracy. After this, again 
twenty samples are generated for the same position (x,y) and the 
computation is restarted. For the purpose to determine the accuracy of the 
method applied the simulation is extended for a 5 minute period. After 
this 5 minutes period the procedure is stopped and the results are 
evaluated. 
NUMERICAL EXAMPLES 
The following examples are restricted to some striking results of 
investigation for the following cases: 
Concerning position: 
(1) the antenna is pointed in the optimum position; 
(2) the antenna has a deviation with respect to the optimum position of 
0.18 degrees in elevation (=y) and 0.18 degrees in azimuth (=x); this 
means that the (x,y) position is at a -20dB point of the main beam. 
Concerning fading type: 
(A) fading with a rather smooth characteristic (FIG. 6); 
(B) fading with a ramp characteristic (FIG. 7) and 
(C) fading with a step characteristic (FIG. 8). 
The values of the calculated position (with respect to the optimum 
position) averaged over a 5 minutes period for both directions (azimuth: 
.mu..sub.x, elevation: .mu..sub.y) are tabulated together with the values 
of the standard deviation calculated over the same period and based on a 
20 sec cycle time (Ox, Oy). In the table (FIG. 9) three successive periods 
of 5 minutes are reported. The conclusion can be made that after a 20 sec 
cycle the tracking is maintained (pos. 1) and considerably progression 
towards the optimum position is made (pos. 2). Finally the results are 
given likewise for a 3.times.5 minutes period, considering all fading 
types. Over each 5 minutes period and for each fading model the maximum 
value for .vertline..mu..sub.x -X.vertline. and .vertline..mu..sub.y 
-Y.vertline. is chosen. The worst case error .sqroot..mu..sub.x -X.sup.2 
+.mu..sub.y -Y.sup.2 is calculated and plotted in FIG. 10. After one 
period the antenna is moved to a point B inside circle I, for a second 
period computations are continued for this point and successively 
following steps are made until the antenna is in the tracking region. It 
turns out to be that the antenna remains pointed inside a circle with a 
maximum radius of 5.9 m.degree., the last value may be interpreted as a 
rest deviation. The value being smaller than the jitter value of 8 
m.degree.. 
As it has already been said the method according to the invention makes use 
of the uncontrolled movement of the antenna as described above, which 
movement is registered by the angular position indicators. Together with 
each registration of the antenna position the attendant signal strength is 
registered as well. Now the method is such that the optimum antenna 
position is calculated by means of a regression technique from the 
measuring data collected during a certain period of time, use being made 
of the direction diagram of the antenna. In the simplest regression 
technique the signal strength is regarded as a function of time, 
approximated in the best possible way by a constant to be determined. In 
that case the period of measuring has still to be rather long. The gain of 
time as compared with the step-track technique is in this case only 
obtained because better and sometimes larger steps can be made towards the 
optimum. After having calculated the optimum antenna position, if 
necessary by means of a number of separate calculations, and by making use 
of one of the regression techniques, the controlling system can then point 
the antenna at that point. 
Although there has been question of a geo-stationary satellite in the above 
the method according to the invention can also be employed for tracking a 
non-stationary satellite having a mainly known orbit. In the latter case 
the method leads to a somewhat deviating orbit, which ensures a stronger 
received signal. 
The invention offers a method for the precise and quick tracking of a 
satellite, which is no more expensive than the known antenna controlling 
methods. 
A more refined and quicker method is obtained if the signal fluctuations in 
consequence of atmospheric influences are approximated over a rather short 
time interval in the best possible way by a higher order polynomial (with 
constants still to be determined) as time function. Said regression 
technique offers at the same time the possibility to eliminate unreliable 
results which will occur in the case of very strong fluctuations. 
By means of simultation techniques it has been proved that an antenna in a 
position at 20 dB from the optimum has reached the top after two steps.