Abstract:
An apparatus for determining the pointing uncertainty of a satellite communications system comprises a communications terminal coupled to a pointing mechanism which is operative to move and position the communications mechanism. A data storage device is configured to store data from which an actual position of a preselected light source which radiates a reference light can be determined. An acquisition sensor is positioned to move with the communications terminal and is configured to focus incident light onto the acquisition sensor. A control mechanism is coupled to the pointing mechanism and is operative to move the pointing mechanism to a position which would focus the reference light source at a preselected position on the acquisition sensor if the pointing uncertainty were substantially zero. The reference light focusing at a measured position on the acquisition sensor. A processor is coupled to the acquisition sensor and is operative to calculate the difference between the preselected position and the measured position, whereby the pointing uncertainty is calculable from the difference.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to lasercom acquisition and tracking systems and more particularly to an apparatus an method for determining a pointing uncertainty of a satellite communications systems. 
     Space based communications systems are mounted on spacecraft and provide communications between two terminals on separate spacecraft. For high data rate transfers, some spacecraft systems use Laser signals for communications, herein referred to as Lasercom. Lasercom communications beams are typically very narrow and as such, must be precisely directed towards the opposing terminal to communicate. Factors such as uncertainty in attitude and position of the terminals as well as the uncertainty of the attitude and position of each spacecraft itself can result in the Lasercom signal being mispointed. To solve this problem, each satellite which has a Lasercom system typically provides hardware and software which enables one satellite to acquire the opposing terminal. 
     One method used to acquire the opposing satellite in a lasercom system is to provide a separate mechanism on one spacecraft which is used for acquiring the opposing spacecraft. To do so, a beacon having a beamwidth which is as broad as the pointing uncertainty is mounted on each spacecraft. In addition, a sensor having a field-of-view which encompasses thehhe field-of-view increases. As such, the beamwidth of the beeeacon and the sensor must be increased to cover the wider field-of-view. This is undesirable since a wider beamwidth beacon requires additional power which is a scarce commodity on a spacecraft. 
     Another method used to acquired an opposing satellite in a lasercom system is to mount a beacon having a narrow beamwidth and a sensor having a wide field-of-view on one spacecraft. The narrow beacon is scanned in the vicinity of the opposing satellite and eventually, the opposing terminal is acquired. This is referred to as the narrow beacon approach. A downfall of this approach is that the field-of-view which must be scanned increases with an increase in the uncertainty. As such, as the uncertainty increases, the field-of-view which must be scanned increases in an amount proportional to the uncertainty squared since the field-of-view which must be scanned is a two dimensional grid with each dimension increasing in size an amount equal to the uncertainty. This increased scan area means that the amount of time required for acquiring increases. This increase time to acquire can cause problems for some communications systems since the time devoted to scanning and acquiring is time away from the communications function. This can typically be tolerated if acquisition is only required once in a satellites service life, such as in a geosychronous system where the satellites move together and acquisition is typically only required at the beginning of the mission. However, in a system where acquisition is required more often, such as in a non-geosychronous communications system where satellites are moving with respect to each other and move into an out of each other&#39;s view as often as once every ½ hour, a long acquisition time can be tolerated. In such a system, each time a satellite moves out of and then back into the view of the acquiring satellite, reacquisition must take place. The scanning process requires several minutes of missed communications for each and every reacquisition which is unacceptable for most communications systems. Since continuous or virtually continuous communications are desirable for many system, it is important to reduce the amount of time needed for acquisition in a lasercom system. 
     What is need therefore is an apparatus and a method which reduces the field-of-view which must be scanned to acquire an opposing terminal and does not require a wide field-of-view sensor. 
     SUMMARY OF THE INVENTION 
     The proceeding and downfalls of the prior art are addresses by the present invention which provides, in a first aspect, an apparatus for determining the pointing uncertainty of a satellite communications system. The apparatus comprises a communications terminal coupled to a pointing mechanism which is operative to move and point the communications mechanism. An acquisition sensor is configured to move with the communications terminal and is configured so that an incident light signal will focus on the acquisition sensor. 
     A data storage device is configured to store data from which an actual position of a preselected light source which radiates a reference light can be determined. A control mechanism is coupled to the pointing mechanism and is operative to move the pointing mechanism to an angle which would focus the reference light source at a preselected position on the acquisition sensor if the pointing uncertainty were substantially zero, whereby the reference light focuses at a measured position on the acquisition sensor. 
     A processor is coupled to the acquisition sensor and is operative to calculate the difference between the preselected position and the measured position, whereby the pointing uncertainty is calculable from the difference. 
     In a second aspect, the present invention provides a method for determining an initial pointing uncertainty in a satellite communications system having a communications terminal which is pointed by a pointing mechanism. Actual angles for a reference source which radiates a reference light is determined. A sensor is mounted to the satellite in a preselected position so that that sensor is moveable with the communications terminal. 
     The communications terminal and the sensor are pointed to a position which would focus the reference light onto a preselected position on the sensor if the initial pointing uncertainty were substantially zero. The reference light is focused onto the sensor at a measured position and, the initial pointing uncertainty is calculated from the measured position and the actual position. 
     In a third aspect, the present invention provides a method for acquiring an opposing terminal having a reference position in a satellite communications system having a communications terminal which is pointed by a pointing mechanism. An actual position for a reference source which radiates a reference light is determined. A sensor is mounted to the satellite in a preselected position so that that sensor is moveable with the communications terminal. 
     The communications terminal and the sensor are pointed to a position which would focus the reference light onto a preselected position on the sensor if the initial pointing uncertainty were substantially zero. The reference light is focused onto the sensor at a measured position and, the initial pointing uncertainty is calculated from the measured position and the actual position. The reference position of the opposing terminal together with the calculated initial pointing uncertainty are used to point the communications terminal towards the opposing terminal, and, the communications beam is scanned over a preselected field-of-view until the opposing terminal is acquired. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram depicting a communications and acquisition system which provides a novel approach to Lasercom acquisition in accordance with the present invention; and, 
     FIG. 2 is a schematic drawing depicting a satellite communications system in which the present invention may be used. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIGS. 1 and 2, the present invention provides a communications and acquisition apparatus  10  for acquiring an opposing terminal  11  and communicating with the opposing terminal  11 . The communications apparatus  10  includes an acquisition sensor  13 , which is operative to acquire the opposing terminal  11 ; and, a communications terminal  16  which is operative to communicate with the opposing terminal  11 . The apparatus  10  is preferably mounted to a spacecraft  18  and is operative to decrease the uncertainty in pointing the communications terminal  16  towards the opposing terminal  11  so that the field-of-view over which the communications terminal  16  must scan to acquire and communicate with the opposing terminal  11  is reduced thereby reducing the time required for acquisition. The apparatus  10  is particularly applicable in systems which use laser beams for communications between spacecraft  18 ,  19 , herein referred to as a Lasercom communications system. 
     Spacecraft  18 ,  19  which intend to communicate with each other are typically programmed with information indicating the position of the spacecraft  18 ,  19  with respect to each other. As such, if uncertainties do not exist in either spacecraft  18 ,  19 , the beam from the communications terminal  16  can be pointed directly at the opposing terminal  11  and communications can commence. Unfortunately, uncertainties exist which result in the communications beam from the communications terminal  16  being mispointed. The present invention provides an apparatus  10  and a method for reducing the amount of the pointing uncertainty of the communications terminal  16  by measuring the position of a fixed, known reference source  20 , and using that measurement to correct for the pointing uncertainty. 
     The first step in the process is to select a fixed, known reference source  20  and place data from which the known position of the reference source can be determined in a data storage device  26 . The position of the source  20  should be known and substantially fixed. As such, it is preferred that a star  20  be used as the reference source since the positions of many stars are well known. An additional advantage of using a star is that stars are already present in space. As such, using a star  20  as the fixed reference source takes advantage of an already present naturally occurring reference source. Any star  20  which radiates light and has a substantially known position can be selected. Data which indicates the position of the selected star  20  is preferably stored in a data storage device  26  located on the host spacecraft  18  but can alternatively be located in a ground terminal (not shown). 
     The next step in the process is to configure the communications terminal  16  and the acquisition sensor  13  to move together and be pointable to a preselected location. To do so, the communications terminal  16  is mounted on a pointing mechanism  27 , which preferably includes a gimbal mechanism  27 . It is preferred that the acquisition sensor  13  be mounted directly to the communications terminal  16  but can also be mounted on the moveable yolk section  28  of the gimbal  27  or on any other component or structure which moves with the communications terminal  16 . The acquisition sensor  13  can alternatively be mounted on a separate gimbal mechanism, however, this would require the additional weight of another pointing mechanism and the added complication of calibrating the sensor  13  to the communications terminal  16 . As such, it is preferred that the communications terminal  16  and the acquisition sensor  13  be coupled together and moved together with a single gimbal mechanism  27 . 
     The next step in the process is to calculate the amount of the pointing uncertainty. To do so, the gimbal  27  is moved to a position which would focus the reference light signal at an expected location on the sensor  13  if no uncertainty existed in the communications terminal  16 . The gimbal  27  is coupled to a controller  29  which provides a control signal  30  to the gimbal  27 . The gimbal  27  is responsive to the control signal  30  and is operative to move either a preselected amount or to a preselected position in response to the control signal  30  to point the acquisition sensor  13  in the proper direction so that the light radiating from the reference star  20  is focused onto the sensor  13 . 
     The expected location at which the reference light is focused on the sensor  13  is determined from the known reference position of the star  20 . Since uncertainties exist, the star light will focus on the acquisition sensor  13  at a measured location which is different than the expected location. The sensor  13  can be any sensor known to one skilled in the art to measure the position of a light source  20  but preferably includes an optical member and a sensor array which are configured so that a light signal originating from a location and illuminating the optical member at an angle will be focused on the sensor array at a position corresponding to the angle of illumination. The type of sensor  13  and material for the sensor  13  are selected so as to be responsive to star light. Selecting a star  20  for the reference source allows for a variety of sensor choices since advantage can be taken of the stars spectral broadness. Silicon and InGaAs are two possible materials for the sensor  13  and, since the sensor  13  is pointed at the star  20  by the gimbal  27  or a pointing mirror, a nulling sensor such as a quad cell could be used as well as two dimensional array such as a charge coupled device (CCD). Using a CCD for the sensor  13  minimizes the expense of the apparatus  10  since a CCD is typically inexpensive and does not require a significant number of optical devices or extensive data processing to measure the incoming angle or location of a light source  20 . The CCD is preferably mounted onto the side of the communications terminal  16  and the line of sight of the CCD is coaligned with the line of sight of the communications terminal  16 . The accuracy of the alignment between the CCD and the communications terminal  16  is not critical since the intent of the present invention is to simply reduce the amount of the uncertainty so that acquisition can proceed more quickly. Even a non-perfect alignment would reduce the uncertainty from a large amount, such as 5 milliradians, to a tolerable uncertainty such as a 0.5 milliradian, and, since the area which must be scanned to acquire the opposing satellite  19  is a function of the uncertainty squared, even a non-perfect alignment would reduce the scanning area from, for example, 25 sq. milliradians to 0.25 sq. milliradians. 
     The difference between the measured location of the focused starlight and the expected location provides sufficient data to compute the amount of the pointing uncertainty. Once the difference between the measured position and the actual position is known, the amount of the uncertainty is known and the angle  32  between the star  20  and the line-of-sight  34  of the opposing terminal  11  can be calculated. The gimbal  27  can then be commanded to move the calculated angle  32  so as to initially point the communications terminal  16  towards the opposing terminal  11  in a direction which is much closer to the opposing terminal  11  than the communications terminal  16  would be pointed absent the uncertainty measurement. Thus, by obtaining knowledge of the uncertainty, the mispointing can be reduced. 
     For the preferred embodiment of the invention, the process further includes measuring a second star  40  and using that measurement to further refine the pointing of the communications terminal  16 . Using a single star  20  for the reference source is acceptable if the uncertainty in the communications terminal  16  is not too great and there is an acceptable known star  20  reasonably close in angular distance. In such a situation, as soon as the reference star  20  is acquired and measured, the communications terminal  16  can be repointed to acquire the opposing terminal  11 . Note that if the host satellite  18  has significant attitude rate uncertainty, the reference star  20  is tracked for a sufficient time to reduce the rate uncertainty to an acceptable level. 
     However, if the host satellite  18  has a significant amount of attitude uncertainty, measuring a single star  20  alone will not provide enough information to repoint the communications terminal  16  since measuring a single star  20  will provide only two angular measurement where three angular measurements must be made to adequately compensate for the attitude uncertainty. To obtain three angular measurement, the position a second star  40  is measured and the actual position of the second star  40  is compared to the measured position of the second star  40  in the same manner as described above. The second star  40  is selected to be separated from the first star  20  by a reasonable angular amount. Measuring two stars  20 ,  40  spaced at least preferably 30 degrees apart will provide information for three angular degrees of freedom, and, spaced substantially 90 degrees apart will produce the best accuracy. 
     Using the data obtained from measuring both the first  20  star and second  30  stars to point the communications terminal  16  in effect calibrates the on-board reference system for those errors which are attributable to the on-board reference system. As such, the attitude reference error of the communications terminal  16  is calibrated out. However, other errors can still exist which may result in the communications terminal  16  and the opposing terminal  11  being misaligned. As such, the next step in the process is to scan a preselected field-of-view to acquire the opposing terminal  11 . As previously mentioned, the field-of-view over which the beam from the communications terminal  16  must be scanned is proportional to the square of the uncertainty amount, which has been reduced by the measurement process detailed above. Thus, the field-of-view over which the beam from the communications terminal  16  must be scanned to acquire the opposing terminal  11  is greatly reduced when compared to the field-of-view which must be scanned absent the knowledge of the uncertainty. 
     As is apparent from the above discussion, the present invention in effect uses the narrow beacon approach but uses information gained from measuring fixed, known stars to reduce the field-of-view or area over which the communications terminal must scan to acquire the opposing terminal. For a satellite platform which is stable and has some uncertainty, the present invention can be used to quickly reduce the pointing uncertainty and reduce the amount of time required for acquisition from, for example, 30 minutes to between 10 and 30 seconds thereby greatly lowering the amount of time required for acquisition. 
     The main objective of the present invention is to reduce the amount of the uncertainty so that the field-of-view over which the communications terminal  16  must be scanned is reduced, as such, the measurement of the uncertainty does not have to be perfect or even very accurate and can still reduce the uncertainty. As such, even a low sensitivity, inexpensive sensor such as a CCD can be used and will provide sufficient information to at least lower the uncertainty below that which was present prior to using the above method and apparatus. 
     The present apparatus and method significantly decreases the area to be scanned reducing the acquisition time, does not require a separate beacon on the spacecraft, and can be accomplished with a relatively inexpensive acquisition sensor thereby saving weight, cost and time. The present invention also takes advantage of the already present stars and the availability of relatively cheap sensors such that the present invention in effect is operative to calibrate the on-board reference system with an inexpensive visible sensor and the stars. It will be appreciated by persons skilled in the art that the present invention is not limited to what has been shown and described hereinabove. The scope of the invention is limited solely by the claims which follow.