GB2077546A - Locating and tracking satellites - Google Patents
Locating and tracking satellites Download PDFInfo
- Publication number
- GB2077546A GB2077546A GB8113662A GB8113662A GB2077546A GB 2077546 A GB2077546 A GB 2077546A GB 8113662 A GB8113662 A GB 8113662A GB 8113662 A GB8113662 A GB 8113662A GB 2077546 A GB2077546 A GB 2077546A
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- GB
- United Kingdom
- Prior art keywords
- satellite
- imaging device
- radiation
- view
- field
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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Classifications
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S3/00—Direction-finders for determining the direction from which infrasonic, sonic, ultrasonic, or electromagnetic waves, or particle emission, not having a directional significance, are being received
- G01S3/78—Direction-finders for determining the direction from which infrasonic, sonic, ultrasonic, or electromagnetic waves, or particle emission, not having a directional significance, are being received using electromagnetic waves other than radio waves
- G01S3/782—Systems for determining direction or deviation from predetermined direction
- G01S3/785—Systems for determining direction or deviation from predetermined direction using adjustment of orientation of directivity characteristics of a detector or detector system to give a desired condition of signal derived from that detector or detector system
- G01S3/786—Systems for determining direction or deviation from predetermined direction using adjustment of orientation of directivity characteristics of a detector or detector system to give a desired condition of signal derived from that detector or detector system the desired condition being maintained automatically
Abstract
An earth satellite whose approximate trajectory is known is tracked by- (a) moving a scanning device across the sky in accordance with the known approximate trajectory, (b) imaging temperature generated radiation from the scanned region onto a screen whereby the satellite, if detected, will appear substantially stationary against a moving background of stars, (c) evaluating the deviation of the satellite from a reference point on the screen, and (d) moving the scanning device in order to reduce the deviation to zero. In this way the beam of a steerable radio antenna may be aligned on a low orbit communication satellite. <IMAGE>
Description
SPECIFICATION
Locating and tracking satellites and the like
This invention relates to methods of locating and tracking satellites and like objects moving across the sky.
The invention is particularly, though not exclusively, concerned with locating and tracking low orbit earth satellites which characteristically undergo frequent orbit corrections due for example to varying ionospheric conditions and variations in the geological structure of the earth's surface. These orbit corrections present obvious difficulties when using tracking systems based upon long range forward predictions of the satellite's trajectory. Other techniques, such as locking onto a radio frequency signal transmitted by the satellite, and radar location are also often impracticable on the one hand because the satellite may not be continuously transmitting and on the other because radar location requires an already accurate knowledge of the satellite's position.
According to the present invention, a method of locating and tracking a satellite or other distant object moving across the sky, of which time and position coordinates relating to its approximate path are known, comprise panning an imaging device along the approximate path of the satellite or object as determined by said coordinates such that temperature-generated radiation emanating from the satellite or object appears as a substantially stationary spot within the field of view of the imaging device, and aligning the imaging device with the observed stationary spot whereby to locate and track the actual position and path of the satellite or object.
Panning the imaging device along the approximate path in this manner enables the "target" satellite or object to be distinguished from other stars or satellites with which it might otherwise be confused, as these will appear as a moving background against the stationary spot image produced by the target.
satellite.
The coordinates relating to the approximate trajectory of the satellite or object, particularly in the case of an earth satellite, may be derived from forward predictions based on earliertracking information, for example from one or more previous orbits.
The radiation may be infra-red (IR) radiation but is not necessarily so, as explained in the last paragraph of the present description.
It will be apparent that for maximum sensitivity the field of view of the imaging device should be kept as small as possible consistent with ensuring with a reasonable degree of certainty that the satellite or object to be tracked will fall within the field of view when centred on any approximate or predicted track position. This will in turn depend upon the accuracy with which the coordinates of the path can be predicted.
Taking the case of infra-red radiation for example, the inventor has realised that it is possible to detect on the earth's surface the small amounts of infra-red radiation emanating from a low orbit satellite and transmitted through the earth's atmosphere in certain wavelength bands. The various mechanisms which contribute to the total IR radiation emanating from such a satellite are dependent upon variations in the orbital position and orientation of the satellite, both in relation to the observation point and the position of the sun, and the strength of the radiation emanating from the satellite, and the wavelength band within which it occurs will vary accordingly.
Further, the IR spectral transmission and scattering characteristics of the earth's atmosphere are also dependent upon variations in the weather conditions, seasonal effects and day/night states.
Accordingly, the imaging device may be selectively responsive only to radiation in a wavelength band, or a combination of such bands, selected in dependence upon the prevailing spectral radiation characteristics of the satellite or object, in combination with the prevailing spectral transmission and scattering of the earth's atmosphere whereby to tend to maximise its sensitivity to the radiation emanating from the object or satellite and to minimise its sensitivity to background radiation levels.
In orderto achieve this, the imaging device may comprise a plurality of sensing devices sensitive to radiation in different wavelength bands, and means for selecting or selectively combining signals from one or more of the sensing devices for imaging purposes.
The invention may conveniently be employed to align the beam of a steerable radio antenna with the satellite or object for communication purpose. In such an application, the imaging device may be fixedly aligned with the beam of the antenna such that when a reference point in the field of view of the imaging device (normally its central point) coincides with the stationary spot image produced by the satellite within its field of view, the beam of the antenna is automatically fixed on the satellite's position.
The invention will now be described in greater detail, by way of example only, with reference to the accompanying drawings in which:
Figure 1 is a schematic diagram of satellite tracking apparatus embodying the present invention;
Figure 2 illustrates typical IR spectral transmission characteristics of the earth's atmosphere;
Figures 3 and 4 illustrate typical IR spectral characteristics of the radiation observed at the earth's surface under different conditions; and
Figure 5 illustrates the spectral radiation characteristics produced by black body radiators at different temperatures:
Referring to the drawings, the apparatus shown in figure 1 comprises a steerable parabolic radio reflector antenna of conventional form comprising a parabolic reflector 1 and a feed 2 located substantially at the focus of the reflector to produce a focused antenna beam along the focal axis or boresight AB of the antenna. The antenna also includes means 4 for
steering the antenna beam in response to signals
from a control circuit 5 whereby the antenna can be
made to track along any desired orbital path.
Mounted on the reflector 1 of the antenna, and
accurately aligned with the boresightAB thereof through an aperture in the reflector, is a high sensitivity IR camera 7 of any suitable known form adapted to produce a video output signal for generating a visual display on a display unit 8 ofthe
IR image of the part of the sky within its field of view.
The field of view of the camera 7 displayed on the
CRT raster-type display unit 8 is thus centred on and tracks with the boresight of the antenna system as the beam is steered along an orbital path underthe control of the control circuit 5.
In the present example, the IR camera comprises a mechanically scanning device in which a rotating reflector in combination with a tilting reflector mechanism sequentially directs light from different parts of the field of view onto one or a combination of several individual detectors each sensitiveto a selected wavelength band within the IR spectrum.
The detector or detectors may comprise or include thermocouples, bolometers, photo-detectors or
Golay cells at least sufficiently sensitive to detect radiation power of 1 x 10-10 watts/cm2 or less.
The selection of the band or bands within the IR spectrum to which the IR detector or detectors of the camera are sensitive depends upon the radiation and transmission characteristics of the earth's atmosphere as well as on the wavelengths ofthe IR radiation emanating from the satellite.
Figure 2 illustrates the IR transmission characteristics of the earth's atmosphere and it will be seen that although there are several substantially transparent bands or windows, two major bands exist extending from wavelengths of about 3.5 to 4 microns and 8.5 to 12 microns, and within these bands up to 900 of the incident radiation is transmitted.
However, while the earth's atmosphere is effectively transparent within certain IR wavelength bands further difficulties are presented by sources of background radiation which may tend to obscure the small amounts of radiation from a satellite and which are also affected or caused by various weather conditions, seasonal effects and day/night states.
Examples of these are illustrated in Figures 3 and 4.
Figure 3 illustrates the typical (worst case) spectral radiation of the sky both during the day (solid line) and night (broken line), from which it will be seen that the lowest levels of radiation occur in the wavelength region 2.5 to 4.0 microns during the day, and below 4 microns during the night. These regians coincide to some extent with the lower of the two major atmospherictransmission bands lying at 3.5 to 4 microns. Similarly Fig. 4 shows that the lowest
levels of incident radiation from beneath a cloud covering lie approximately in the same region, ie
between 2.5 and 4 microns. From these observations
it can be concluded that the lower of the two major transparent bands is the more suitable over a wider
range of conditions, although in some cases the
higher band may additionally or alternatively be
used with advantage.One further transparent band
shown in Figure 2 lying just above 2 microns wavelength may also be used as, in certain conditions, the overall background radiation at this wavelength is relatively low.
Having established those IR bands in which radiation may most usefully be detected in the IR camera underthe various atmospheric conditions, it is also necessary to consider the various forms of radiation which are likely to be emanating from the satellite to enable itto be detected.
Although there isa comparatively large number of different mechanisms by which IR radiation may either be generated or reflected by a typical low orbit satellite, only four major source are' considered for the purposes of the present application. These major sources comprise reflection of solar energ-incident on the satellite; reflection of solar energy irtddenton the satellite; reflection of albedo energy (Te solar energy reflected from the'earth onto--the-satefli-teY;:- internal heating of the satellite; andSreflectiorrof heat from theN earth acting as a black body radiator.The total energy received by the camerau7 will apzoxi-- mately bathe sum of these four coribution-s..
Considering first the direct reflection of solar energy incident on the satellite, this will have a simi lar spectral content to that of the original solar energy, assuming the satellite is a good IR reflector, and this is characteristic of a black body radiator. at 6000"K as illustrated in Figure 5. For a given satellite, the strength of this radiation will depend upon the inclination of the satellite with respect to the sun and the earth, and at times will fall to zero when the satellite is in the earth's shadow. From Fig. 5 it can be shown that a greater proportion of this energy contribution (0.5%) will lie in the 3.5 to 4 microns wavelength band than in the 8.5-12 micron band (0.01%).
Reflected albedo radiation from the satellite will again have a spectral characteristis corresponding-ro a 6000"K black body radiator since it also originates from the sun. It will also vary with the inclination of the satellite's orbit with respect to the observation point, and with the extent of illumination of the earth's surface facing the satellite..
In the absence of any external heating, the internal heat generated by a typical low orbitsatejlite will cause itto radiate substantially asa.blacltbody at about 2800K Apart from variationsin the. range of the sateilitefrom the observationi position, this can.- tribution will remain substantially' constant proved ing the internal power consumption at the satellite also remains constant It can shown from Figure 5 thatabout0.4% of radiation frem a bliacI:bodyat 280' 'K falls within the 3.5 to 4 micron wavelength band, while this figure increase to 4, in the 8.5 to 12 micron band.
Finally, reflection by the satellite of black body radiation from the earth will have a spectral characteristic corresponding to that of a body at 290"K and accordingly a far greater proportion (4%) will lie within the 8.5 to 12 micron band than in the 3.5 to 4 micron band (0.4%). Further, apart from variations in the range of the satellite from the observation position, this contribution will remain substantially constant during both day and night The above discussion assumes that the surface area of the satellite observed remains fixed during the orbit although in practice this will not be the case, and additional variations in the intensities of the various contributions considered will also occur as a result.
The relative magnitudes of these various contributions, and the amount of background radiation which will tend to obscure the relatively small amounts of energy emanating from the satellite, will all vary substantially under different conditions and in different wavelength bands. For example, estimates indicate that in the wavelength band from 3.5 to 4 microns, the contribution from the reflection of the earth's black body radiation accounts, on average, for approximately 50% of the total radiation emanating from the satellite, while in the wavelength band from 8.5 to 12 microns, the contributions from the direct solar, and albedo reflections become insignificant in relation to the reflected black body radiation.
From the above it will be apparent that to obtain an acceptable contrast ratio in the thermal image produced by the camera between satellite and the background radiations, it is desirable to confine camera observations to one or more of the wavelength bands to which the earth's atmosphere is transparent, in which the prevailing conditions do not produce a significant level of background radiation or interference, and in which the satellite is reflecting or generating a reasonable amount of radiation. While this can usually be achieved by selecting only one of the wavelength bands to which the earth's atmosphere is substantially transparent, in most cases the 3.5 to 4 micron band, there will often be some advantage to be gained in selectively combining thermal information collected in a number different wavelength bands to derive the final display.This may be achieved by employing a number of different cameras, each sensitive to radiation in a respective one of the bands, and additively combining their outputs on the display unit 8. Alternatively radiation received by a common camera may be selectively directed onto a number of individual IR detectors each sensitive to a respective one of the bands or arranged, eg by using filtering techniques, to receive energy in only one such band.
In use of the apparatus to track a low orbit earth satellite from a ground station, the antenna beam AB is directed to follow the approximate predicted orbital path of the satellite derived from forward predictions based on information from previous orbits. The field of view of the camera 7, as displayed on the display unit 8 is thus centered on the predicted instantaneous position of the satellite. However, the actual position of the satellite is likely to be different from this due to course perturbations which cannot be fully accounted for in the prediction computations. These perturbations-may result from variations in the geological structure of the earth's surface and varying ionospheric conditions which have an unpredictable influence on the orbital path.The size of the field of view of the camera 7 is chosen to accommodate the estimated errors in the predicted position of the satellite, so that there is a reasonable likelihood that the satellite will in fact fall within the field of view when centred on its predicted position.
Typically a field of view of 5" x 5may be used in the case of a satellite orbiting at an average height of 50 miles.
As the antenna beam, and hence the field of view of the camera 7, track along the predicted path of the satellite, the latter, providing it does in fact lie within the camera's field of view, will appear as a substantially stationary spot on the image displayed on the display unit 8. Other IR images caused for example by stars or possibly other satellites also falling within the camera field of view, and which might otherwise be confused with the spot produced by the target satellite will not be stationary and this provides a means by which the target satellite can be readily identified.
Having identified the target satellite in this way, corrections to the tracking path of the antenna can be made to cause the displayed satellite bright spot image to be brought into alignment with the centre point of the camera's field of view represented for example by cross-wires 10 superimposed on the display screen, thereby accurately aligning the antenna beam onto the actual orbital path of the satellite.
This adjustment of the tracking path of the antenna may be effected manually by an operator monitoring the display unit, or automatically by a computer programmed to locate the position of the stationary bright spot produced by the satellite on the display screen, or a stored representation thereof, and to carry out the necessary antenna tracking corrections to bring this spot into, and maintain it in register with a central reference point representing the centre of the camera's field of view.
It will be appreciated that although the present invention has been particularly described in its application to aligning the beam of a radio antenna on a low orbit satellite for tracking purposes, it may nevertheless be used to simply locate such a satellite. In such applications the directional orientation of the camera 7 when its field of view is centred on the stationary satellite bright spot as observed on the display, would provide sufficient means for determining the coordinates of the satellite.
It will be further appreciated that while the invention is primarily applicable to the accurate location and tracking of low orbit satellites which are generally difficult or impossible to track by other means due to their relatively high orbital speed and susceptibility to orbital path perturbations, it may nevertheless also find applicatibns in the passive location and tracking of other distant objects moving across the sky, such as aircraft and free flight rockets.
Although the invention has been described in relation to an embodiment which images infra-red radiation from the distant object, other embodiments may be arranged to image ultra-violet (UV), visible and millimetre radiation therefrom. As will be seen from
Fig. 5, solar radiation covers the UV-to-lR range, and the solar spectrum also includes millimetre radiation (not shown in Fig. 5). Radiation in the whole UV-tomillimetre range is also generated in consequence of the temperature of the satellite itself, and is also reflected therefrom as a result of generation by the earth. Thus the four major sources mentioned earlier
in relation to IR radiation, also produce radiation in the UV, visible and millimetre regions. An embodiment of the invention may comprise imaging devices adapted to receive any of these radiations, or any combination of two or more of them, the latter arrangement enabling the most effective radiation to be selected in the prevailing conditions, eg time of day, season and weather.
Claims (14)
1. A method of locating and tracking a satellite or other distant object moving across the sky, of which time and position coordinates relating to its approximate path are known, comprising panning an imaging device along the approximate path of the satellite or object as determined by said coordinates such that temperature-generated radiation emanating from the satellite or object appears as a substantially stationary spot within the field of view ofthe imaging device, and aligning the imaging device with the observed stationary spot whereby to locate and track the actual position and path ofthe satellite or object.
2. A method as claimed in claim 1 wherein the imaging device is selectively responsive only two radiation in a wavelength band, or a combination of such bands, selected in dependence upon the prevailing spectral radiation characteristics of the satellite or object, in combination with the prevailing spectral transmission and scattering of the earth's atmosphere, whereby to tend to maximise its sensitivity to the radiation emanating from the object or satellite and to minimise its sensitivity to background radiation levels.
3. A method as claimed in claim 2 wherein the imaging device comprises a plurality of sensing devices sensitive to radiation in different wavelength bands, and means for selecting or selectivelycombining signals from one or more of the sensing devices for imaging purposes.
4. A method as claimed in any preceding claim wherein the imaging device comprises a raster-type scanning camera.
5. A method as claimed in any preceding claim wherein the imaging device is fixedly aligned with the beam of a steerable radio-communication antenna such that when a reference point in the field of view of the imaging device coincides with the stationary spot image produced by a communication satellite within its field of view, the antenna beam is automatically aligned on the satellite's position.
6. A method as claimed in any preceding claim wherein the imaging device is sensitive to infra-red radiation emanating from the satellite or object.
7. A method of locating and tracking a satellite or other object moving across the sky substantially as hereinbefore described with reference to the accompanying drawings.
8. Apparatus for locating and tracking a satellite or other distant object moving across the sky, of which time and position cordinates relating to its
approximate path are known, comprising means for
panning an imaging device along the approximate
path of the satellite or object as determined by said
coordinates such that temperature-generated radiation emanating from the satellite or object appears as a substantially stationary spot within the field of view of the imaging device, and means for aligning the imaging device with the observed stationary spot whereby to locate and track the actual position and path of the satellite or object.
9. Apparatus as claimed in claim 8 wherein the imaging device is selectively responsive only to radiation in a selective wavelength band or combination of such bands
10. Apparatus asclaimed in claim 9 wherein the imaging device comprises a plurality of sensing devices sensitive to radiation in different wavelength bands, and means for selecting or selectively combining signals from one more ofthe sensing devices for imaging purposes.
11. Apparatus as claimed in any of claims 8 to 10 wherein the imaging device comprises a raster-type scanning camera.
12. Apparatus as claimed in any of claims 8 to 11 wherein the imaging device is ffxedly aligned with the beam of a steerable radio-communication antenna such that when a reference point in the field of view of the imaging device coincides with the stationary spot image produced by a communication satellite within its field of view, the antenna beam is automatically aligned on the satellite's position.
13. Apparatus as claimed in anyofclaims8to 12 wherein the imaging device is sensitive to infra-red radiation.
14. Apparatus for locating and tracking a satellite or other object moving across the sky substantially as hereinbefore described with reference to the accompanying drawings.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB8113662A GB2077546B (en) | 1980-05-08 | 1981-05-05 | Locating and tracking satellites |
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB8015227 | 1980-05-08 | ||
| GB8113662A GB2077546B (en) | 1980-05-08 | 1981-05-05 | Locating and tracking satellites |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| GB2077546A true GB2077546A (en) | 1981-12-16 |
| GB2077546B GB2077546B (en) | 1984-01-18 |
Family
ID=26275437
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| GB8113662A Expired GB2077546B (en) | 1980-05-08 | 1981-05-05 | Locating and tracking satellites |
Country Status (1)
| Country | Link |
|---|---|
| GB (1) | GB2077546B (en) |
Cited By (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB2318236A (en) * | 1983-10-01 | 1998-04-15 | British Aerospace | Radiation detector arrangements |
| GB2344951A (en) * | 1998-12-17 | 2000-06-21 | Starlight Xpress Limited | Distant object tracker |
| US11618594B2 (en) | 2019-02-07 | 2023-04-04 | The Government Of The United States Of America As Represented By The Secretary Of The Air Force | System and method for daylight imaging of high altitude objects |
-
1981
- 1981-05-05 GB GB8113662A patent/GB2077546B/en not_active Expired
Cited By (5)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB2318236A (en) * | 1983-10-01 | 1998-04-15 | British Aerospace | Radiation detector arrangements |
| GB2318236B (en) * | 1983-10-01 | 1998-09-02 | British Aerospace | Radiation detector arrangements |
| GB2344951A (en) * | 1998-12-17 | 2000-06-21 | Starlight Xpress Limited | Distant object tracker |
| GB2344951B (en) * | 1998-12-17 | 2003-07-30 | Starlight Xpress Ltd | Method and apparatus for tracking a selected distant object |
| US11618594B2 (en) | 2019-02-07 | 2023-04-04 | The Government Of The United States Of America As Represented By The Secretary Of The Air Force | System and method for daylight imaging of high altitude objects |
Also Published As
| Publication number | Publication date |
|---|---|
| GB2077546B (en) | 1984-01-18 |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| PCNP | Patent ceased through non-payment of renewal fee |