IE50678B1 - Ground station antennae for satellite communication systems - Google Patents

Abstract

1. An antenna for ground radio stations (E) whose position can be set with respect to a geostationary earth satellite (Sat) which is yaw-axis-stabilised in respect of the polarisation of its transmitted wave relative to the geographical longitude of its position, the antenna having a pivot bearing about two pivot axes (O/W ; N/S) mutually at right angles, of which one (O/W) serves for East-West follow-up and runs parallel to the earth's axis, characterised in that the other pivot axis (N/S) serves for North-South follow-up and is at right angles to the meridian plane in which the satellite is positioned.

Description

The invention relates to a ground radio station antenna for use with a geostationary satellite, and comprising a mounting which is pivotable about two axes at right angles to one another to effect position tracking. Known antennae of this kind are normally mounted on gimbals in order to facilitate pivoting about an azimuth axis and an elevation axis. Furthermore, a feed waveguide is normally provided which is rotatable in order to be able to follow variations in the polarisation of transmissions from the satellite, which changes with any movement thereof. Consequently both the position of the satellite and the polarisation plane require to be constantly monitored to provide control information, which can only be acquired with a substantial outlay in radio technological equipment.

In the case of geostationary satellites, the apparent movement of the satellite relative to the earth is limited to a few angular degrees, and therefore the required antennae mountings need permit only a limited angle of pivot about the declination axis and the elevation axis and for this reason the mountings may be of relatively simple construction. However, if the polarisation plane is also to be monitored, then even in these simplified constructions the feed waveguide must be designed to be rotatable and have an associated control system.

Thus, for example, an adjusting frame is known for use with ground radio station antennae, in which an antenna is mounted at three points. One point is located in the apex of the frame and contains a double joint which allows the antenna to pivot about an hour axis and a declination axis which are arranged mutually perpendicular in a plane which is normal to the antenna axis.

If such an antenna is arranged at the degree of longitude of the satellite, additional polarisation following can be dispensed with, as is discussed in the German Patent Specification laid open to inspection under serial No. 2,454,830.

One object of the present invention is to provide a ground radio station antenna of the aforementioned construction, or of a simplified construction, with means by which it. may be simply controlled, independently of location, and thus independently of any displacement of the geographic longitude of the ground station from that of the satellite, and to dispense with separate means for monitoring the polarisation plane of the satellite transmissions.

It is to be assumed that the geostationary satellite has means provided by which the polarisation of the wave which it emits is yaw-axially stabilised relative to the geographic longitude of its position.

The invention accordingly provides an antenna for ground radio stations (G) whose position can be set with respect to a geostationary earth satellite (Sat) which is yaw-axis-stabilised in respect of the polarisation of its transmitted wave relative to the geographical longitude of its position, the antenna having a pivot bearing about two 678 pivot axes (E/W) N/S) mutually at right angles, of which one (E/W) serves for East-West follow-up and runs parallel to the earth's axis, characterised in that the other pivot axis (N/S) serves for North-South follow-up and is at right angles to the meridian plane in which the satellite is positioned.

The term north-south tracking axis is used in place of the term elevation axis in view of the special alignment and function of this axis in embodiments of the invention.

Whereas known antenna for ground radio stations provide a main radiation direction that is normal to the elevation axis, in an exemplary embodiment of the present invention the relationship of the main radiation direction to this axis differs from a right angle in accordance with the displacement of the degree of longitude of the ground radio station from that of the satellite. As a result, during the north-south tracking motion the antenna undergoes not only a change in elevation, but also a slight rotation. This rotation will be the greater, the more the angle between the north-south tracking axis and the main radiation direction of the antenna differs from 90°. The above described slight rotation in the radiation axis which occurs during the north-south tracking motion exactly compensates the rotation of the polarisation plane which is caused by the unavoidable motion of the satellite, and as a result any additional and elaborate S0678 ' polarisation tracking means can be dispensed with.

The invention will now be described with reference to the drawings, in which:Figures 1.1, 1.2 and 1.3 show the earth and a geostationary satellite in elevation, plan and side views respectively; Figures 2.1, 2.2 and 2.3 show the relationship between the two tracking axes, viewed from the east, the south and in a plan view respectively; and Figure 3 schematically illustrates one exemplary embodiment having a simplified antenna mounting.

Figure 1.1 shows in elevation the earth, with the lines of latitude and longitude marked at intervals of 30°, a satellite Sat located precisely in front of the centre of the earth, and a ground station G at a latitude Of approximately 50° north and a longitude approximately 45° east of the satellites normal position, this position being by way of example.

Four arrows forming,a figure-of-eight schematically indicate a' typical deviation of the satellite’s position during one rotation of the earth. The east-west tracking axis of a ground station antenna is located, as in known antenna tracking constructions, to lie parallel to the earth's axis, whereas in accordance with the invention, the north-south tracking axis is at right angles to the plane containing the meridian of the satellite's normal position, i.e. the mean position shown in Figure 1.1. The antenna has a main direction of radiation H.

Figure 1.2 illustrates a plan view of the same arrangement, viewed from above the North Pole. The main radiation direction H of the ground station antenna lies at an angle°£ to the north-south tracking axis N/S, and the angle oC differs from 90°. The distance of the satellite Sat is represented to scale. Zig-zag lines W - E indicate the deviation motion illustrated by a figure-of-eight in Figure 1.1. The areas located symmetrically opposed on the satellite Sat schematically indicate solar generators.

In Figure 1.3 the globe has been rotated by 90° for convenience of illustration. As can be seen from Figure 1.3, from any ground station G the geostationary satellite Sat appears to describe a small arc about the north-south tracking axis. The east-west displacement shown in Figure 1.2 has been omitted from this initial discussion, but it will be appreciated that the deviations which it produces are compensated by east-west tracking. If the antenna is rotated about this north-south axis during the position tracking the beam from the ground radio station passes through a surface sector M of an extremely flat cone. Figure 1.3 illustrates this sector M of the conical surface which is passed through by the beam from the ground station antenna during the north25 south tracking motion. This motion contains a linear and a rotating component. That part of the satellite beam which reaches the ground station remains coincident with the beam from the ground station and experiences the same motion components. Thus, polarisation tracking can be derived from the position tracking by virtue of the special alignment of the north-south tracking axis.

Since, as indicated in Figure 1.2, the satellite can also move in the east-west direction within specific tolerances, it is necessary to design the entire system to be rotatable in a plane parallel to the equatorial plane. By virtue of the rotation of the above mentioned north-south tracking axis with the retention of the parallelism to the plane of the Equator, the east-west tracking compensates the error occasioned by the east-west deviation of the satellite which is represented in Fig. 1.2.

The use of a north-south tracking axis physically aligned in accordance with the technical theory of the invention can be simulated when using a differently mounted antenna, if the control means incorporate a computer, such as a micro-processor, which controls the tracking device of the differently mounted antenna to act as if the antenna were mounted in the novel manner described above.

The erection and adjustment of a ground radio station provided with combined position and polarisation tracking in accordance with the invention is dependent upon the knowledge of the following data: (i) the ground station latitude (ii) the difference X between the longitude of the ground station and the longitude of the satellite's normal position; (iii) the north-south direction; and (iv) the horizontal plane G' at the ground station G In the northern hemisphere the east-west tracking axis is inclined to the north at an angle β relative to the horizontal axis G' , so that it runs parallel to the axis of the earth. In the southern hemisphere this tracking axis is inclined to the south in a similar fashion.

The north-south tracking axis is at right angles to the east-west tracking axis, and is elevated with respect to the horizontal plane G' by the angle in a direction which, for a ground station in the northern hemisphere that is eastwards of the satellite's position, differs from the southerly direction eastwards by the angle arctan 1/(tan Λ . sin p ), whilst if westwards of the satellite's position differs 20 westwards. Analogous conditions apply for stations in the southern hemisphere.

Figure 2.1 illustrates the relationship of the two tracking axes, N/S andE/W, which are mutually perpendicular, and intersect above the horizontal plane G' at the location of the ground station, the illustration showing the axes viewed from the east, and assuming a geographic position that corresponds to that illustrated in Figure 1, by way of example. The northerly direction N lies to the right-hand side of the drawing plane. The axis E/w lies in the drawing plane, and its angle of elevation β is that of the latitude, a value of 50°. The axis N/S points approximately to the south-west, with an angle of elevationtf that is projected towards the drawing plane, and therefore cannot be represented in its true size.

Figure 2.2 shows the same axes viewed towards the north N, as represented by the arrows tail referenced N; in this illustration neither of the axes are arranged in the drawing plane.

Figure 2.3 illustrates the same axial intersection in a plan view, with the northerly direction N at the top. The angle Yindicates the deviation of the tracking axis N/S from the north-south direction and the earth's axis. An arrow A represents the direction of view used for the representation of the exemplary antenna embodiment which is illustrated in Figure 3, now to be described.

In Figure 3 an exemplary embodiment of the invention is illustrated schematically in simplified form. On the earth's surface, represented by the horizontal plane G’ , a directional antenna C, in the present example a Cassegrain antenna provided with a main reflector and an auxiliary reflector (not shown in detail), is mounted in movable manner at two points K and D on rigid supports, which have not been shown in detail. A point K represents a ball and socket joint which permits a rotation out of the drawing plane about the eastwest axis E/W in the manner indicated by the circular arrow E/W on a segment of a circle that is limited to a few angular degrees by the length of a sliding mount D’ that is provided at the point of attachment D. The ball and socket joint K also allows the antenna C to pivot about the north-south axis in a manner indicated by the circular arrow N/S, the pivot forces acting upon a centre of gravity S. The two axes which are at right angles to one another are arranged in accordance with the angular illustrations shown in Figures 2.1 to 2.3. The antenna C is mounted on the north-south tracking axis N/S in such manner that its main radiation direction H' forms an angle CC with the north-south tracking axis, where:15 = ,arctan^(r-R . Cos^. cos'?')2 + R^ . sin^''/R.cosy^. sin?\ .

(Tie. main radiation direction H1 illustrated in Figure 3 is not identical to the actual main radiation direction H shown in Figure 1, as the angle °C could not be represented in its natural size) .

In the above description: R = the earth's radius; r = the distance of satellite from the centre of the earth; °C = the angle of the main radiation direction (H) of the antenna to the north-south tracking axis (N/S); the latitude and the angle of elevation of the axis angle relative to the main radiation direction of the antenna. If these three angular conditions are fulfilled, then any pivoting motion about the north-south tracking axis causes the antenna to experience a linear motion component and a rotating motion component, which latter compensates the apparently circular motion of the satellite. 5067 east-west tracking axis (E/W); Ύ = direction deviation of the north-south tracking axis (N/S) from the south or north as the case may be; of = the angle of elevation of the north-south tracking axis (N/S); and 7\ = the difference between the longitudes of the ground station and the satellite normal position.

To summarise,, ground station antennae are proposed comprising means for controlling pivoting movement about two axes at right angles to one another for position tracking a geostationary satellite that is yaw-axially stabilised in respect of the polarisation of its transmissions relative to its longitude.

In such an antenna, polarisation tracking must be provided in addition to position tracking or alternative means provided to effect correction. Separate polarisation tracking can be dispensed with if the antenna is pivotable about a north-south tracking axis which is at right angles to the plane containing the meridian of the satellite) The east-west tracking axis runs parallel to the earth's axis, in known manner.

Such an alignment of the north-south tracking axis requires that three angular conditions should be fulfilled, which relate firstly to the axis angle relative to the horizontal axis at the location, secondly any deviation relative to the longitude of the location, and thirdly to the

Claims (4)

1. CLAIMS:1. An antenna for ground radio stations (G) whose position can be set with respect to a geostationary earth satellite (Sat) which is yaw-axis-stabilised in 5 respect of the polarisation of its transmitted wave relative to the geographical longitude of its position, the antenna having a pivot bearing about two pivot axes (E/W); N/S) mutually at right angles, of which one (E/W) serves for East-West follow-up and runs 10 parallel to the earth's axis, characterised in that the other pivot axis (N/S) serves for North-South follow-up and is at right angles to the meridian plane in which the satellite is positioned.

2. , An antenna as claimed in Claim 1, characterised 15 in that the North-South follow-up axis (N/S): a) forms the angle δ = arcsin (sin λ · cos β) relative to the horizontal plane (G 1 ) at the location of the antenna; 20 b) is inclined in a direction which deviates from the meridian in the direction towards the equator by the angle γ = arctan_1_ tan λ · sin β and in fact East of the satellite position towards the East and West of the satellite position towards the West; and c) forms the angle l 9 9 Ο a = arctan ’(r—R · cos β · cos λ) + R · sin β 5 R cos β · sin λ with the antenna main beam direction; where in all the formulae: R = the earth's radius , r = the distance between the satellite and the centre of the earth; β = the geographical latitude of the location (G) of the ground radio station, and λ = the difference in longitude between the location (G) of the ground radio station and the theoretical position of the satellite (Sat) (Fig. 2, Fig. 3).

3. , An antenna as claimed in Claim 1 and 2, characterised in that the East-West follow-up axis (E/W) forms an angle β with the horizontal plane, which angle 20 corresponds to the latitude of the location (G), and is inclined towards the North in the northern hemisphere and inclined towards the South in the southern hemisphere

4. A ground station antenna substantially as described with reference to Figures 1, 2 and 3.