Scanned acquisition using pre-track data

A scan acquisition technique for acquiring terminals (62, 64) that does not rely on precise alignment between a sensor (66, 70) and a transmitter (68, 72) associated with the terminals (62, 64). The terminals (62, 64) separate uncertainty regions (76, 78) into a plurality of sections (88, 90). Scan beams (82, 84) include encoded information of what section (88, 90) the scan beam (82, 84) is currently scanning. Each terminal (62, 64) will eventually receive the scan beam (82, 84) of the other terminal (62, 64). When it does, it will encode its scan beam (82, 84) with both the outgoing code and the return code for that section (88, 90), so that when it's scan beam (82, 84) is received by the other terminal (62, 64), that terminal (62, 64) will know what scan section (88, 90) the other terminal (62, 64) is located.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to a system and method for acquiring a distant terminal for transmitting an optical data beam thereto and, more particularly, to a system and method for mutually acquiring two distant terminals, where each terminal simultaneously scans an uncertainty region in which the other terminal is located, and where the scan beams include both encoded information about which section of the uncertainty region the scan beam is currently scanning and the position in the uncertainty region decoded from the other scan beam at the time the terminal of origin was last illuminated.

2. Discussion of the Related Art

Optical beams are sometimes employed to transmit digital data between two distant sources or terminals, such as space-to-space and space-to-ground optical communications, at very low power levels. The optical data beams have low divergence, and thus have extremely narrow beamwidths (for example 1-20 microradians) when they reach the target terminal to operate at the desired low power level. The optical data beam impinges on a collection aperture on the terminal. Conventional optics are used to focus the optical beam onto a detector to extract the data.

When the terminals wish to exchange data, and currently aren't tracking each other, an acquisition technique is employed to align the terminals so that the optical beam is transmitted in the proper direction with high accuracy. First, both terminals are informed of the general vicinity of the other terminal, defined herein as an uncertainty region. The uncertainty region is much larger than the angular beamwidth of the communications data beam. The terminals then initiate the acquisition process so that at the conclusion, their beam is pointed directly at the other terminal's telescope to exchange the data. During the acquisition process, information is exchanged between the two terminals until the uncertainty region is reduced to less than half the angular beamwidth of the data beam. If the incoming and outgoing beams are perfectly co-aligned, tracking can then commence to maintain the alignment. Because the information extracted by a local sensor from the arriving beam only tells the direction of the incoming beam, a misalignment of greater than one-half of the beamwidth between the incoming and outgoing beam will result in a failure to achieve track even if the incoming beam knowledge is perfect.

FIG. 1is an illustration of a communications system10for transmitting optical data between a first terminal12and a second terminal20some distance apart. The terminal12includes a telescope having a sensor14for receiving optical beams from the terminal20and a transmitter16for transmitting optical beams to the terminal20. Likewise, the terminal20includes a telescope having a sensor22for receiving optical beams from the terminal12and a transmitter24for transmitting optical beams to the terminal12.

The terminal12gives its best estimate of its location to the terminal20, and the terminal20gives its best estimate of its location to the terminal12for subsequent data transmissions. However, neither of the terminals12and20will give their location to the other terminal12or20with a high enough accuracy. Therefore, the actual location of the terminals12and20is unknown to the other terminal12or20before a signal beam is acquired. The given position of the terminals12and20is shown here as terminals12′ and20′, which is some unknown distance from the actual location of the terminals12and20. Thus, an uncertainty region28is defined for the terminal12in which the terminal12is located, and an uncertainty region30is defined for the terminal20in which the terminal20is located. For example, the uncertainty region28or30may be 100 times the diameter of the data beamwidth, which provides a factor of 10,000 times of total area. The expected location of the terminals12and20is set at the center of the uncertainty regions28and30, respectively, in two dimensions.

The uncertainty region28is shown relative to the sensor22of the terminal20′, and the uncertainty region30is shown relative to the sensor14of the terminal12′. The uncertainty defined by the uncertainty regions28and30includes both positional uncertainty and angular uncertainty. It is the angular uncertainty that causes the uncertainty regions28and30to be shown relative to the sensors14and22for the expected locations of the terminals12′ and20′.

In one known acquisition technique, the uncertainty regions28and30are flooded with a beacon of light from the terminals12and20, respectively, to determine the position of the other terminal12or20by looking for the direction of the other terminals flood beam. This technique requires a separate beam than the data beam, and is typically relatively slow at providing acquisition if the power levels are low. Particularly, because of the distance between the terminals, the sensors14and22see the other terminals flood beam as a point source on its detector, such as a charge coupled device (CCD) array. It may take a significant amount of time for the CCD array receiving the low level flood beam to integrate enough charge to provide an indication of the direction of the other terminals flood beam.

Some of the problems with beacon type acquisition have been alleviated by employing scan beams that scan the uncertainty regions28and30, where the beams are detected by the sensors14and22, respectively.FIG. 2shows the system10where only the actual locations of the terminals12and20are provided for illustrating a scan acquisition technique. The terminal12transmits a scan beam34from the transmitter16, having the same beamwidth as the data beam, that is scanned across the uncertainty region30to illuminate the terminal20. At the same time, the terminal20transmits a scan beam36from the transmitter24that scans across the uncertainty region28to illuminate the terminal12. Each time the scan beam34or36is received by the sensor22or24of the terminal12or20, that terminal12or20knows the approximate direction of the other terminal12or20because of where the beam impinges the sensors field-of-view. Thus, the terminals12and20can home in on each other by receiving the other terminals scan beam34or36until acquisition is completed. This occurs when the uncertainty regions28and30are reduced to less than half the beamwidth of the scan beams34and36.

FIG. 3depicts the terminals12and20after being acquired by the scan beams34and36, after which the terminals12and20can track each other to maintain the pointing in the event that one or both of the terminals12or20is moving. Because the terminals12and20are simultaneously scanning for the other terminal12or20, the average acquisition time can be reduced because once one terminal12or20is illuminated by the other terminal12or20its uncertainty area is reduced allowing it to acquire faster.

Recent improvements have been made in the known scan acquisition techniques to more quickly acquire the terminal of interest. Particularly, U.S. patent application Ser. No. 09/481,924 titled “Satellite Optical Communication Beam Acquisition Techniques,” filed Jan. 13, 2000, assigned to the Assignee of this application and herein incorporated by reference, discloses one improvement. In this scan acquisition technique, the position of one terminal is determined by subdividing a sensor of the other terminal into sensor quads, and then continually subdividing each sensor quad after the terminal receives the scan beam until acquisition.

The operation of this acquisition technique will be discussed herein with reference to the terminal12, the sensor14and the scan beam36from the terminal20.FIG. 4shows a sensor40, representing the sensor14that is applicable for this purpose. The '924 application used a sensor divided into sensor quads for two-dimensional acquisition. However, for illustration purposes, this technique can be shown in only one-dimension with the sensor40. The sensor40is separated into a first sensor half42and a second sensor half44separated by a line46. The sensor halves42and44only determine if the scan beam36arrives through the portion of the uncertainty region28being watched by that sensor half. Because there are only two cells, high performance materials and electronics can be used to maximize sensitivity and minimize noise at reasonable cost. A CCD with the same level of performance would be extremely expensive. One example of a sensor suitable for this purpose is an InGaAs cell, well known to those skilled in the art.

As the scan beam36scans the uncertainty region28, the scan beam36will eventually impinge the sensor14. The sensor40includes suitable circuitry to determine which sensor half42or44is illuminated by the scan beam36(referred to as a “hit”). The sensor40will then adjust its field-of-view so that the line46falls half-way through the portion of the uncertainty region28that was previously covered by the sensor half42or44that was “hit” by the scan beam36. For example, if the field-of-view of the sensor40is 16°, and the scan beam36is detected by one of the sensor halves42or44, the sensor40will then move the center of its field-of-view to bisect the portion of the original field-of-view covered by the particular sensor half42or44that detected the beam36.

By positioning the sensor40at the field-of-view for the sensor half42or44, the uncertainty region28is cut in half, and now this half is covered by both of the sensor halves42and44around the line46. When the sensor40is illuminated by the scan beam36again, it will again hit one of the two sensor halves42or44, and thus the field-of-view can be divided in half again. This process is continued until the scan beam36is simultaneously detected by both sensor halves42or44at the line46. For a two-dimensional scan, a quad cell sensor would be employed in this manner.

Once the uncertainty region28is reduced to half of the beamwidth of the scan beam36, the transmitter16of the terminal12should be aligned with the sensor22of the terminal20. However, the process described above requires that the sensor14be accurately aligned with the transmitter16because the transmitter16transmits the scan beam34in the direction just determined by the sensor14. Misalignment between the sensor14and the transmitter16will induce an undetectable bias in uncertainty region28which may be greater than one-half of the beamwidth. Therefore, it is necessary to precisely align the transmitted beam to the center of the sensors field-of-view using precise alignment devices or applying an extremely stable structure.

SUMMARY OF THE INVENTION

In accordance with the teachings of the present invention, a scan acquisition technique is disclosed for acquiring two communications terminals that does not rely on precise alignment between a sensor and a transmitter associated with each terminal. The terminals are located within known uncertainty regions. The uncertainty regions are separated into a plurality of scan sections where each section is designated by a code. The scan beams include “forward” encoded information about what scan section the scan beam is currently scanning within the uncertainty region. As one terminal scans the uncertainty region trying to illuminate the other terminal, it will eventually receive the scan beam from the other terminal. When it does receive the other terminals scan beam, it will then decode the “forward” encoded information. The scan beam will contain two pieces of information. The first is the forward encoded information and the second is “return” information containing the decoded data from the terminal of origin. Thus, when the terminal of origin receives the scan beam, that terminal will know what scan section of the uncertainty region it was in when it successfully illuminated the other terminal.

Additional advantages and features of the present invention will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The following discussion of the embodiments of the invention directed to a scan acquisition technique for acquiring optical terminals is merely exemplary in nature, and is in no way intended to limit the invention or its applications or uses.

According to one embodiment of the present invention, the sensors14and22are separated into a plurality of sensor sections to decrease the scanning acquisition time. For example,FIG. 5shows a sensor50including eight separate sensor sections52separated by lines54. As above with the sensor40, when the sensor14or22receives the scan beam36or34, respectively, the particular sensor section52that receives the beam36or34provides an indication of which direction the scan beam came from. Therefore, the uncertainty region28or30for the particular terminal12or20can be reduced to the size of that section52, here ⅛ of the original size of the uncertainty region28or30. For example, if the original uncertainty region28or30is 16°, the first hit allows the uncertainty region28or30to be reduced to 2°. Thus, the first time that the sensor50receives the scan beam34or36, the uncertainty region28or30can be considerably reduced beyond that for the sensor40including the two sensor halves42and44.

The sensor field-of-view of the sensor50is then moved so that one of the lines54falls at the center of that section52that previously received the scan beam34or36. Therefore, only two of the sensor sections52are employed and the remaining sensor sections52are not used. Also, once the first section52is determined, then that section52can only be divided in half for each subsequent hit. If too many sensor sections are employed, then the same problem of using a sensor that is relatively unsensitive occurs. Also, the several circuits required to monitor more sensor sections becomes more complex. After the first hit, the many circuits that are required for the several sensor sections would not be used. This process continues until the uncertainty region28or30is reduced to one-half of the beamwidth of the scan beam34and36. The improvement comes as a result of the reduction in the size of the uncertainty region28or30after the first initial detection of the scan beam34or36.

According to the invention, a scan acquisition technique is disclosed that eliminates the requirement that the sensor and the transmitter be precisely aligned.FIG. 6depicts a communications system60employing a first terminal62and a second terminal64that wish to communicate with each other through optical data beams. The terminal62includes a sensor66and a transmitter68, and the terminal64includes a sensor70and a transmitter72. At the initiation of the acquisition, it is known that the terminal62is positioned within an uncertainty region76, and the terminal64is positioned within an uncertainty region78. The transmitter68of the terminal62transmits a scan beam82that scans the uncertainty region78to be received by the sensor70of the terminal64. Likewise, the transmitter72of the terminal64transmits a scan beam84that scans the uncertainty region76to be received by the sensor66of the terminal62.

According to the invention, the uncertainty region76is separated into a plurality of contiguous scan sections88, here fifteen. Likewise, the uncertainty region78is separated into a plurality of contiguous scan sections90, also fifteen sections. Each scan section88and90is identified by a particular code, here a number from one to fifteen for simplicity purposes. Each section88and90represents the steering angle of the respective scan beam82and84. The scan beams82and84are encoded with the specific scan section that they are currently scanning through. The scan beam84sequentially scans through all of the scan sections88of the uncertainty region76, and the scan beam82sequentially scans through all of the sections90of the uncertainty region78.

In this example, the terminal62is positioned within scan section1of the uncertainty region76and the terminal64is positioned within scan section15of the uncertainty region78. However, this is for illustration purposes only. The scan beam84starts with scan section1of the uncertainty region76, sequentially scans to scan section15, and then returns to scan section1. Likewise, the scan beam82starts with scan section1of the uncertainty region78, sequentially scans to scan section15, and then returns to scan section1. The direction of the scan is also for illustration purposes.

Each time the scan beam84or86moves from one scan section88or90to the next scan section88or90, it is encoded with the code for that section. For example, when the terminal64begins its scan in scan section1of the uncertainty region76, the scan beam84is encoded with the number for that section88. Because the terminal62is located in scan section1, the sensor66of the terminal62receives the scan beam84and decodes the section code therefrom. The terminal62then encodes its scan beam82with both the outgoing section code and the decoded section code1just received from the scan beam84. However, the terminal64does not receive the scan beam82until it reaches scan section15of the uncertainty region78.

When the scan beam82does reach scan section15of the uncertainty region78, it is still coded with both the outgoing section code15and the return scan section code1from the uncertainty region76. Because the terminal64is located in scan section15, it receives and decodes the scan beam82and now knows which section88the terminal62is located. Further, when the sensor70of the terminal64receives the scan beam82, the terminal64encodes its scan beam84with both the outgoing section code1and the return section code15of the uncertainty region78. Thus, when the scan beam84returns to scan section1of the uncertainty region76, the terminal62will now know that the terminal64is in scan section15.

Each time the terminals62and64know what scan section88or90the other terminal62or64is located, the uncertainty regions76and78are reduced to the size of that section88or90. That section88or90is then subdivided fifteen times where each scan section88or90is again designated by the particular code. Thus, each time the data makes a round trip, the uncertainty regions76and78are reduced by 1/15. This process of decreasing the uncertainty regions76and78continues until the uncertainty regions76and78are one-half the size of the beam width of the scan beams82and84. Thus, the relative orientation between the sensors66or70and the transmitters68or70is not important. This is because the information being exchanged is not just the arrival angle of the incoming beam, but is also the departure angle necessary to illuminate the other terminal. Once the uncertainty region76or78is less than half of the beamwidth, it is certain that a track can be entered since co-alignment will be automatically compensated.

The percentage reduction of the uncertainty regions76or78for a single round trip scan piece of data is limited by the amount of data that can be packed into a hit.

The example given above separates the uncertainty region76and78into fifteen scan sections. However, this is by way of a non-limiting example for illustration purposes. In a practical example, ten bits can be provided on the scan beams82or84for coding. This provides 1024 scan sections for each of the uncertainty regions76and78. Further, each time a scan section is subdivided, it can be subdivided by a different number of times than was previously done. Further, the two uncertainty regions76and78can be subdivided into different numbers of scan sections. Also, the field-of-view of the sensors68and70can be the entire field-of-view for the current uncertainty region76or78, and thus do not need to be subdivided as was done in the embodiment discussed above. The example discussed above is a one-dimensional example. However, those skilled in the art will readily recognized that the scan can actually be a two-dimensional scan, where the invention can be used for that type of scan.