Abstract:
The present invention provides an acquisition sensor and system to align the communication lasers of two satellite communication terminals so that the satellites acquire one another to allow laser communication. The communications lasers of both satellites are scanned over their respective pointing uncertainty regions. Each satellite&#39;s acquisition sensor detects the presence of the scanned beam of the other satellite and provides positional resolution of the other satellite. Each satellite than adjusts its scanning to conform with the new positional data. of the other satellite. The acquisition sensor is a quadrant InGaAs photo-detector and accompanying monolithic acquisition processing circuitry sealed in a hermetic package with an optical window. Each quadrant of the sensor is responsive to the beam of the other satellite, thus providing positional resolution of the other satellite to the space of a single quadrant. Once the Field of View (FOV) of the acquisition sensor has realigned, the positional resolution repeats until the other satellite&#39;s tracking sensor is illuminated.

Description:
BACKGROUND OF THE INVENTION 
     The present invention generally relates to an optical communication system for acquiring an optical link between terminals. More particularly, the present invention relates to an acquisition sensor for such a system including a multi-channel photodetector for use in acquiring an optical link between terminals. 
     At least two communication terminals are involved in laser communications, a transmitting terminal and a receiving terminal. The transmitting terminal transmits the optical signal (such as laser energy) which is received by the receiving terminal. The receiving terminal receives the optical signal with a detector such as a photodetector. 
     One of the difficulties in long-distance laser communication between a transmitting terminal and a receiving terminal is the initial alignment of the optical transmitting source and the receiving detector of the two communication terminals. For example, if the optical transmitting source is a laser, the laser from the transmitting terminal must be pointed so that the laser is incident on the detector of the receiving terminal. When the separation between satellites is great (for instance, thousands of kilometers), this initial alignment and acquisition may be quite challenging. In addition to the wide separation between terminals, the laser beam itself may be quite narrow, further adding to the challenge. The narrowness of the laser beam arises because of the power constraints inherent in satellite communications. Wider beams require more power which in turn adds to satellite weight, cost and size. 
     Many prior systems used one or more laser beacons to align communication satellites with respect to each other. Multiple laser beacons increase cost in terms of both the size and weight of the satellite and the power consumption of the beacon. Other systems relied on hyper-accurate initial positioning, which may not be achievable when the separation between terminals is large and may be easily disrupted by spacecraft jitters. 
     Thus, a need has long existed for an acquisition system and sensor that minimizes power consumption and additional weight and size while providing reliable and fault tolerant acquisition within a short time. 
     SUMMARY OF THE INVENTION 
     One object of the present invention is to provide an optical acquisition sensor for use in an acquisition system that eliminates the laser beacon signal that must be provided in several prior systems. 
     Another objective of the present invention is to minimize the cost, complexity, size, weight, and power consumption of the hardware used to provide an acquisition system and sensor. 
     One or more of the foregoing objects are met in whole or in part by the inter-satellite optical link acquisition sensor of the present invention. The present invention provides an acquisition sensor and system for acquiring an optical beam transmitted by a source located in a region of uncertainty. A source (transmitted) optical signal is scanned and the acquisition sensor is employed on a receiving terminal to look for the scanning beam. Based on the information from the acquisition sensor, the Field of View (FOV) and region of uncertainty of the receiver can be adjusted accordingly to establish the communication link. 
     The acquisition sensor includes a multi-channel photodetector, channel circuitry, a threshold circuit, a comparator and filtering. The received optical signal is converted to an electrical signal and supplied to channel circuitry. The channel circuitry includes a threshold circuit supplying a threshold signal. The received electrical signal is compared to the threshold signal and, if the received signal exceeds the threshold, a hit is detected. 
     These and other features of the present invention are discussed or apparent in the following detailed description of the preferred embodiments of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates the Gimballed Telescope Assembly (GTA) of the present invention. 
     FIG. 2 illustrates the Beam Expansion Telescope (BET) of the Space Telescope Subassembly (STS). 
     FIG. 3 illustrates the optical bench of the STS. 
     FIG. 4 is a representational drawing of the acquisition sensor  350 . 
     FIG. 5 illustrates a detailed schematic of a preferred embodiment of the acquisition sensor. 
     FIG. 6 illustrates the scanning patterns of the transmitted laser beam. 
     FIG. 7 shows the normalized intensity of the laser energy delivered by the spiral pattern  600 . 
     FIG. 8 shows the Field of View (FOV) of the transmitter and receiver terminals during the first step in the exemplary acquisition process of the present invention. 
     FIG. 9 shows the FOV of the transmitter and receiver terminals during the second step in the exemplary acquisition process of the present invention. 
     FIG. 10 shows the FOV of the transmitter and receiver terminals during the third step in the exemplary acquisition process of the present invention. 
     FIG. 11 shows the FOV of the transmitter and receiver terminals during the fourth step in the exemplary acquisition process of the present invention. 
     FIG. 12 shows the FOV of the transmitter and receiver terminals during the fifth step in the exemplary acquisition process of the present invention. 
     FIG. 13 is a flowchart  1300  illustrating the acquisition system of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a Gimballed Telescope Assembly (GTA)  100  of the present invention. The GTA  100  includes a Space Telescope Subassembly (STS)  125 , 1 Gimbal Subassembly  150 , and an Acquisition Pointing and Tracking (APT) Electronics Subassembly  175 . The preferred embodiment of the GTA includes a two-axis gimballed 15 cm diameter telescope and associated sensors, mechanisms, and control electronics required for acquisition, pointing and tracking. 
     The STS  125  contains the telescope and an optical bench holding the transmit, receive, acquisition, and tracking components. The gimbal subassembly  150  provides the required azimuth and elevation range of motion and tracking slew rate to the STS  125 . The transmit and receive signals are carried to the STS  125  through the gimbal subassembly  150  on optical fibers. The APT electronics subassembly  175  provides acquisition/pointing/tracking control. 
     As will be explained in greater detail below, incident laser energy passes through the protective window  130  of the STS  125  to the interior of the STS  125  where it is received and processed. Additionally, the satellite upon which the GTA  100  is mounted may transmit laser energy through the protective window  130  of the STS  125 . The protective window serves to isolate the interior of the STS  125  from the harsh space environment. 
     The STS  150  is comprised of a Beam Expansion Telescope (BET)  200 , and the associated transmit, receive, acquisition, and tracking components mounted on an optical bench. FIG. 2 illustrates the BET  200  of the STS  125 . The BET includes a first mirror  210 , a second mirror  220 , and a third mirror  230 . The three mirrors are positioned in a Three Mirror Anastigmat (TMA) configuration relative to each other and the incident laser energy. 
     In operation, the BET  200  expands the outgoing laser beam and collects the incoming light. The outgoing laser beam enters the BET  240  and is incident upon the third mirror  230 . The path and orientation of the outgoing beam is shown by rays  240 . The outgoing beam is reflected from the third mirror  230  and redirected to the second mirror  220 . The second mirror  220  reflects the incident beam onto the first mirror  210 . The first mirror  210  reflects the incident beam through the protective window  130  and into the space environment. The path of the laser beam as it exits the BET  200  through the protective window  130  of the STS  125  is indicated by rays  250 . Because of the relative focal radii and optical properties of the mirrors ( 210 - 230 ) the outgoing beam is expanded to a desired size. 
     An incoming laser beam follows the same path as the outgoing laser beam, but in reverse. The incoming laser beam passes through the protective window  120  to the first mirror  210  where it is reflected to the second mirror  220  which reflects the incoming laser beam to the third mirror  230  which reflects the beam out of the BET  200 . 
     The preferred embodiment of the BET  200  uses an all-reflective Three Mirror Anastigmat (TMA) with an on-axis magnification of 7.5 and an effective collection area of 177 cm 2 . The optical components are preferably diamond-turned, enabling reliable low-cost volume fabrication and delivering superior wavefront quality. Additionally, the protective window  130  preferably has a coating that admits radiation only above 1500 nm and provides solar and contamination control. 
     FIG. 3 shows the optical bench  300  of the STS  125 . The optical bench  300  includes a Fine Track Mechanism (FTM)  305 , a Transmit/Receive Dichroic Beam Splitter  310 , transmit segment  301 , a receive segment  302 , an acquisition segment  303 , a tracking segment  304 . The transmit segment  301  includes a Point Ahead Mechanism (PAM)  312 , Transmit Collimation Optics  314 , and a transmit Fiber Laser Source  316 . The receive segment  302  includes a Narrow Band Pass Filter (NBPF)  320 , a Track/Receive Beam Splitter  325 , Receive Collimation Optics  330 , an Annular Coupler Mirror  335 , and a Receive Fiber or Communications Detector  340 . The acquisition segment  303  includes acquisition refocusing optics  345 , acquisition sensor  350 , and acquisition processing electronics  335 . The tracking segment  304  includes track focusing optics  360 , a track sensor  375 , track processing electronics  380 , and is connected to the acquisition processing electronic  355  similar to the acquisition processing electronic  355  in the acquisition processing segment  303 . 
     In operation, received laser energy travels from the BET  200  of FIG. 2 to the FTM  305 . The FTM  305  reflects the laser energy onto the dichroic beam splitter  310 . The dichroic beam splitter  310  is substantially transparent to laser energy at the wavelength of the receive laser energy. Thus the received laser energy passes through the dichroic beam splitter  310  to the receive segment  302 . 
     Transmitted laser energy travels from the transmit segment  301  to the dichroic beam splitter  310 . The dichroic beam splitter  310  is reflective to laser energy at the wavelength of the transmitted laser energy. Thus, the transmitted laser energy is reflected from the dichroic beam splitter  310  to the FTM  305 . The FTM  305  reflects the incident laser energy to the BET  200 . 
     Both the received and transmitted laser energy thus occupy the same optical pathway from the dichroic beam splitter  310  to the FTM  305  and through the optics of the BET  200 . The received and transmitted laser energy do not interfere with each other because they are at different wavelengths. Thus, both the received and transmitted laser energy may use the same optical pathway without interference and the dichroic beam splitter  310  may appear simultaneously reflective to the transmitted laser energy and substantially transparent to the received laser energy. Preferably, the FTM  305  is an electromagnetically driven, flexure-mounted mirror with integral angle sensing, providing 700 to 800 Hz bandwidth pointing control over +/−0.5 degree mechanical travel. 
     During operation of the transmit segment  301 , the transmit fiber laser source  316  emits laser energy which passes through the transmit collimation optics  314  to the PAM  312 . The PAM  312  reflects the laser energy to the dichroic beam splitter  310  where the laser energy is further reflected to the FTM  305  and then to the BET and eventually into the space environment. Both the PAM  312  and the FTM  305  may be mechanically gimbaled to provide steering of the laser energy. For fine steering and pattern steering, the PAM  312  is preferred. 
     Preferably, the PAM  312  is similar to the FTM  305  except for an increased field of regard to accommodate the scan angle acquisition. The scan angle required for acquisition is driven primarily by the 0.1 degree uncertainty factor in the spacecraft attitude. Preferably, the PAM  312  can be mechanically repositioned by +/−2.25 degrees to accommodate the scan angle for acquisition. 
     During operation of the receive segment  302 , laser energy entering the BET  200  is reflected by the FTM  305  and passes through the dichroic beam splitter  310 . The received laser energy then passes through the NBPF  320 . The NBPF  320  is centered on the wavelength of the received laser energy and serves to reduce the level of non-signal optical noise entering from the external space environment. While the preferred embodiment of the protective window  130  of FIG. 1 preferably has a coating that admits radiation only above 1500 nm and provides solar and contamination control as noted above, the NBPF operating in conjunction with the protective window  130  may yield a more refined and band-centered filtering then the protective window  130  alone. 
     After passing through the NBPF  320 , the laser energy impinges upon the track/receive beam splitter which redirects a portion of the total laser energy into the tracking segment  304 . The remainder of the total laser energy passes through receive collimation optics  330  and impinges upon the annular coupler mirror  335 . The annular coupler mirror  335  is a reflective disk with a centered circular portion of the disk removed. Laser energy impinging on the annular coupler mirror  335  within the center circular portion passes through the plane of the mirror and impinges upon the receive fiber or communications detector  340 . Laser energy impinging on the annular coupler mirror  335  outside of the center circular region is reflected into the acquisition segment  303 . 
     Turning now to the acquisition segment  303 , laser energy reflected from the annular coupler mirror  335  passes through the acquisition refocusing optics  345  and impinges upon the acquisition sensor  350 . The acquisition sensor  350  transforms the laser energy impinging upon it to an electrical signal. The electrical signal generated by the acquisition sensor  350  is further processed in the acquisition processing electronics  355 . 
     Turning now to the tracking segment  304 , laser energy reflected from the track/receive beam splitter  325  passes through the track focusing optics  360  and impinges upon the tracking sensor  375 . Like the acquisition sensor, the tracking sensor  375  transforms impinging laser energy to an electrical signal which is further processed in the track processing electronics  380 . Additionally, the electrical signal may be passed to the acquisition processing electronics  355  for further processing. 
     FIG. 4 is a representational drawing of and acquisition sensor  350 . The acquisition sensor  350  is hermetically sealed inside a hermetic package  420 . The hermetic package  420  includes an optical window  440  and a number of electrical leads  460 . As a representational drawing, FIG. 4 is not to scale, nor are the relative sizes of the optical window  440 , the hermetic package  420  and the electrical leads  460  constrained to be as they appear in FIG.  4 . Nor is the number of leads of the essence. During operation, laser energy focused by the acquisition refocusing optics  345  of FIG. 3 focuses laser energy through the optical window  440  and onto an optical sensor as will be discussed below. 
     A detailed schematic of a preferred embodiment of an acquisition sensor  500  is shown in FIG.  5 . The acquisition sensor  500  includes an InGaAs photodetector  505  and a monolithic detection circuit  510  with five channels  511 - 515 . The monolithic detection circuit  510  also includes bias circuitry  507  for biasing the photodetector  505  and a digital to analog converter (DAC)  520 . Each channel  511 - 515  of the monolithic detection circuit  510  includes a Low Pass Filter Trans-impedance Amplifier (TIA)  525 , an amplifier  527 , a High Pass Filter (HPF)  532  including an external capacitor  530 , a second Low Pass Filter (LPF)  535 , a comparator  540 , a latch  545 , and a TTL buffer  550 . 
     The photodetector  505  is preferably circular in aspect and partitioned into four contiguous quadrants  505   a-d  as shown. However, for the purposes of the present invention, the photodetector may be partitioned into any number of channels. Each of the quadrants  505   a-d  is electrically connected to a single channel  511 - 515 . For example, quadrant A  505   a  may be electrically connected to channel  1   511 . Each quadrant  505   a-d  of the photodetector  505  responds to incident laser energy by transmitting an electrical signal to its respective channel  511 - 515 . For example, when laser energy is incident upon quadrant A  505   a  of the photodetector  505 , an electrical signal will be generated and sent to channel  1   511 . The electrical signal has an amplitude which includes a signal component corresponding to the optical signal and a noise component corresponding to background noise. Bias circuitry  507  provides higher bandwidth performance to the photodetector  505 . 
     Using channel  1   511  as an example, the electrical signal, in the form of an electrical current pulse, is transmitted from quadrant A  511  to the input  526  of channel  1   511 . The signal is then passed through the TIA  525  to convert it to a voltage signal and to remove any undesired high frequency response such as high frequency noise, for example. The signal is then amplified through the amplifier  527 . Next, the signal is passed through the HPF  532  including the external capacitor  530 . The HPF  525  requires a fairly large capacitance value which is provided by the external capacitor  530 . The capacitor  530  is preferably external because its physical dimensions would reduce space efficiency if included directly in the monolithic detection circuit  510 . 
     After passing through the HPF  532 , the electrical signal is then passed through a second LPF  535  to remove undesired high frequency responses such as high frequency noise, for example. The electrical signal is then passed to the comparator  540 . The comparator compares the electrical signal with a threshold provided by the DAC  520 . The DAC receives the threshold level command from an external threshold select  521 . The threshold command is expressed as a multi-bit digital signal. The DAC  520  converts the multi-bit digital threshold command to a corresponding analog threshold level. 
     At the comparator  540 , when the electrical signal is less than the threshold, no action is taken. When the electrical signal exceeds the threshold, a signal is transmitted to the latch  545 . The latch  545  also includes an external reset  546 . The output of the latch  545  is passed to the TTL buffer  550 . The output of the TTL buffer  550  is provided to output  560  which may be electrically connected to further processing circuitry. 
     Each of the channels  1 - 4   511 - 514  connected to quadrants A-D  505   a - 505   d  of the photodetector  505  has a similar structure to the channel discussed above. However, the sum channel  515  is slightly different. For the sum channel  515 , each of the HPF  532  from channel  511 - 514  is connected to its input to be summed and the threshold level is correspondingly changed to reflect the connection. 
     The acquisition sensor  500  along with the BET  200  and optical bench  300  of the STS  125  may be used to acquire a communications link between two satellites. Such an acquisition system is disclosed below. While the system below is presented in terms of a communications link between two satellites, those skilled in the art will readily appreciate that the disclosed system may be expanded to any optical link communications system such as links between, for example, multiple satellites (such as in a satellite network), a satellite and a ground station, or between ground stations. 
     Initially, the two satellites are installed in space. When installed, the laser communication beams of the two satellites point generally at their desired location. However, each beam is subject to a certain unknown pointing error. At installation, this pointing error may be reduced to preferably less than 0.1 degrees. However, due to the wide separation between satellites, the accuracy of the beam pointing must be increased for communication to occur. 
     As shown in FIG. 6, each of the satellites begins its acquisition by sweeping its transmitted laser (from transmit laser fiber source  316  of FIG. 3) through a spiral pattern  600 . The sweep is started at the center of the uncertainty area. The spiral pattern  600  is generally circular with a diameter of about 2 miliradians (about 0.11 degrees) as shown. At the center of the spiral  600 , the laser beam may be swept in a rosette pattern such as the wide rosette  625  or narrow rosette  650  of FIG.  6 . 
     FIG. 7 shows the normalized intensity of the laser energy delivered by the spiral pattern  600 . The pattern of the spiral with respect to the beamwidth of the laser is such that the entire region of uncertainty receives at least half-power from the laser beam. Thus, the amount of laser energy incident on any point within the region of uncertainty is sufficient to trigger a response from an acquisition sensor that may be located within the region of uncertainty. 
     The angular velocity of the beam as it is swept through its pattern is maintained at a substantially constant rate to maximize reception by the receiving acquisition sensor. Although the spiral pattern is the preferred pattern, other patterns such as larger rosettes and more liner patterns are possible. The rosette pattern may be altered by modifying an on-board database. The rosette size, density, and linear rate may all be altered. 
     FIG. 8 shows the first step  800  in an exemplary acquisition process. The Field of View (FOV)  810  of the Beam Expansion Telescope  200  of terminal  1  is shown. The FOV is what is seen looking out of the telescope into space. The location of terminal  2  can be seen in Terminal  1 &#39;s FOV  810 . The FOV  820  of the acquisition detector of terminal  1  is also shown. The location of terminal  2  is not detected by terminal  1 &#39;s acquisition detector because the laser beam of terminal  2  is not yet incident on terminal  1 . 
     The FOV  830  of terminal  2  shows the location of terminal  1 . However, the FOV  840  of the acquisition sensor of terminal  2  does not register the presence of terminal  1  because terminal  1 &#39;s laser beam is not yet incident upon terminal  2 . 
     The first step is an example of the relative positions of the terminals with respect to each other at installation. For communication to take place, the transmitted beams of the terminals must be centered in the FOVs of the receiving terminals. To accomplish this centering, at some time after installation both terminal  1  and terminal  2  begin to sweep their laser beams in the spiral or rosette patterns shown in FIG.  6 . The starting time of the sweeps of terminals  1  and  2  need not be synchronized, but similar starting times may yield a faster acquisition. 
     Preferably, the PAM  312  of the optical bench  300  may be used to sweep the uncertainty region with the narrow communication beam of approximately 11 microradians half beam width. This beamwidth yields about 30 seconds to completely sweep the initial uncertainty area. The commanded pattern is a uniform spiral which may be corrupted by spacecraft jitters. The beam irradiance in the far field is nearly Gaussian. The result after the spiral is that the irradiance applied to each point in the uncertainty area is not constant (as shown in FIG.  7 ), but is more evenly illuminated than a single diffracted beacon covering the same uncertainty area. Even though the beam is narrow, it is very bright compared to the background because it contains the full output power of the terminal. 
     As. the lasers are swept through their respective patterns, at some point one of the lasers will sweep over the other terminal and illuminate the acquisition sensor of the other terminal. In operation, whether terminal  1  illuminates terminal  2 &#39;s acquisition sensor or terminal  2  illuminates terminal  1 &#39;s acquisition sensor is irrelevant. 
     FIG. 9 shows a second step  900  of the acquisition system. In FIG. 9, the two terminals have begun to sweep their respective lasers. As the lasers are swept through their patterns, the laser of terminal  1  illuminates the acquisition sensor of terminal  2 . In practice, because of the uncertainty of positioning, terminal  2 &#39;s laser could have illuminated terminal  1 &#39;s acquisition sensor first, but for this exemplary acquisition process terminal  1  illuminates terminal  2 . 
     As terminal  2  is illuminated, terminal  1 &#39;s FOV  910  and terminal  1 &#39;s acquisition sensor&#39;s FOV  920  remain unchanged. Terminal  2 &#39;s FOV  930  also remains unchanged. However, the illumination  950  from terminal  1  illuminates terminal  2 &#39;s acquisition sensor. The spot illumination will flash with a time period corresponding to the scan rate and beamwidth. The acquisition sensor of terminal  2  is shown in FIG.  5 . The illumination from terminal  1  falls upon quadrant B  505   b  of the photodetector  505 . The spot produces a pulse of current from the photodetector  505  that is converted to a voltage pulse by TIA  525 , amplified and passed through filtering  530 ,  535 . The output from the filter  535  is compared to a threshold and if the output exceeds the threshold, a detection or hit is registered as having occurred in that quadrant. The registered hit signal is passed out of the acquisition sensor to control and processing circuitry (not shown). 
     FIG. 10 illustrates a third step  1000  in the acquisition process. The acquisition sensor is divided into quadrants which correspond with the quadrants of the telescope&#39;s FOV. If a hit is detected in a certain quadrant, the detecting terminal knows that the transmitting terminal must be. in that quadrant somewhere. The uncertainty area in the transmitting terminal&#39;s location is thus reduced from four quadrants to a single quadrant. Once the hit has been detected, the terminal reorients its FOV to center on the detecting quadrant. The FOV is narrowed (the radius of the FOV is decreased) to encompass only that quadrant. Because the region of uncertainty in the FOV remains circular, the reduction in the region of uncertainty is 50% rather than the 75% reduction in uncertainty that may be expected by identifying the location of the transmitting terminal within a single quadrant. 
     In FIG. 10, terminal  1 &#39;s FOV  1010  and terminal  1 &#39;s acquisition sensor&#39;s FOV  1020  remain unchanged. However, terminal  2 &#39;s FOV  1030  has been reoriented on the detecting quadrant as shown. Terminal  2 &#39;s FOV remains circular, but the radius of the FOV is decreased to encompass only the detecting quadrant as shown. Terminal  2  is not aware of the location of terminal  1  in this new, decreased uncertainty area. Terminal  2  begins to sweep this new quadrant with the same spiral pattern, again beginning at the cenqter. 
     The center of terminal  2 &#39;s FOV is controlled by the FTM  305  which may be rapidly redirected to the center of the new, smaller uncertainty area. The STS  125  is also redirected to the new uncertainty area, but the FTM  305  may be redirected more quickly. Thus, the FTM  305  may immediately begin scanning the new uncertainty area beginning at the center while the STS  125  need not complete its reorientation until the spiral pattern passes outside the area of the previous uncertainty area. 
     Meanwhile, terminal  1  merely keeps performing its spiral pattern while terminal  2  is detecting terminal  1 &#39;s transmission and reorienting. Terminal  1  is unaware that is has been detected by terminal  2 . Because terminal  1 &#39;s laser hits and then keeps moving along, terminal  2 &#39;s acquisition sensor&#39;s FOV  1040  no longer detects terminal  1 . 
     FIG. 11 shows a fourth step in the acquisition process. Terminal  2 &#39;s FOV  1130  has now been centered on the detected quadrant and the uncertainty area of the FOV (reduced scan FOV after first detection)  1135 . The location of terminal  1  remains within this new, smaller uncertainty area, but terminal  2  is unaware of the location because terminal  1 &#39;s laser has not yet illuminated it again. Because terminal  1 &#39;s laser has not again illuminated terminal  2 , terminal  2 &#39;s acquisition sensor&#39;s FOV  1140  has not recorded a hit. 
     However, the laser of terminal  2 , while sweeping the new reduced region of uncertainty in the spiral pattern, has illuminated terminal  1 . Thus, a spot corresponding to the location of terminal  2  in terminal  1 &#39;s acquisition sensor&#39;s FOV  1120  is detected. 
     As shown in a fifth step  1200  of FIG. 12, once the spot appears in terminal  1 &#39;s acquisition sensor&#39;s FOV  1220 , terminal  1  reorients its FOV  1210  to coincide with the detection quadrant. As above, the region of uncertainty is reduced and terminal  1  begins scanning the new region of uncertainty with the spiral pattern. Meanwhile, terminal  2 , unaware that it has been detected by terminal  1 , continues its spiral pattern in its reduced region of uncertainty. 
     The sum channel of the acquisition track sensor  350  operates similarly to the quadrant sensors. Preferably, when a detection occurs in the sum channel, the FOV of the terminal is not changed, but the region of uncertainty is reduced by at least 50%. Scanning continues in this reduced uncertainty region like in the quadrant-centered regions above. 
     These successive steps complete the first stage in the acquisition system for both terminals. The area of uncertainty for each terminal has been reduced by 50%. In the next stage, these steps are repeated, again reducing the area of uncertainty by 50%. The stages continue until the track sensor becomes illuminated. 
     The acquisition process continues in a series of stages, with each stage resulting in reduced uncertainty area, and a resulting increase in the pulse rate at which the terminal illuminates an opposing terminal. During the final phases of the acquisition process, the scan area is small, preferably about 30 to 40 microradians. This yields a pulse rate of 300 Hz to 400 Hz which is sufficient to adjunct the coherence of the reference communication laser. When indicated by the communications electronics, the PAM  312  stops the scan motions and transmits a continuous beam based on the information from a track sensor. The PAM closes on the track signal and communication commences. 
     The acquisition process may be more generally described as acquiring a source located in a region of uncertainty, the initial region of uncertainty being the initial pointing error. Successive stages of the acquisition process minimize this region of uncertainty. Each uncertainty region may be divided into subdivisions, each corresponding to a channel of the photodetector as well as a region in space. That is, the channel receiving an optical beam depends on the location of the terminal in space. 
     FIG. 13 is a flowchart  1300  illustrating the acquisition system. The satellite is initialized at initialization step  1305 . The satellite is provided with its location, the general location of the opposing terminal, the initial uncertainty region, and the expected power. The satellite then positions its transmit beam to the desired opposing terminal location and begins scanning. The expected power information is used to determine the threshold for received laser energy to determine when a hit has occurred. 
     Next, the terminal keeps scanning unless a greater than maximum number of pulses is detected at override step  1310 . If a greater than maximum number of pulses is detected, then the process proceeds to stage succeeded step  1315 . If a greater than maximum number of pulses is not detected the terminal determines if the stage is complete at step  1320 . The stage is complete, even if greater then the maximum number of pulses have not been detected, if the time allocated for the stage has expired. If the stage is complete, the process proceeds to stage succeeded step  1315 . If the stage is not complete, the process proceeds to step  1325  and the scan pattern is calculated and superimposed on the PAM  312  and scanning continues. 
     A timed stage runs for the allocated time or until override. Updates to opposing terminal information are applied at the end of a stage. A real time stage updates the opposing terminal positions whenever a pulse detection occurs. Thus, at step  1330 , if the stage is timed, the process proceeds back to step  1310 . If the stage is not timed, the process proceeds to step  1335  and a real time update is calculated and applied to the opposing terminal estimate. 
     The average hit rate is then calculated at step  1340 . At step  1345 , the average hit rate is compared with the go forward threshold. If the hit rate is above the go forward threshold, the process proceeds to step  1350 . At step  1350 , the process determines if the current stage is the final real time stage. If so, the process proceeds to the tracking stage  1355 . If not, the process proceeds to step  1360  and the stage number is increased by one and the opposing terminal estimate is adjusted. The process then proceeds back step  1325 . 
     At step  1345 , if the hit rate is not above the go forward threshold, the process proceeds to step  1365 . At step  1365 , if the hit rate is below the go back threshold, then the process proceeds to step  1370  and the stage is decreased by one and the opposing terminal location is re-estimated. The process then proceeds to step  1325 . If the hit rate is not below the go back threshold then the stage is continued at step  1375 . The process then proceeds to step  1325 . 
     Going back to step  1315 , if the minimum number of pulses are detected and the pulses are consistent, then the stage has succeeded and the process proceeds to step  1380 . At step  1380  the stage is increased by one and the opposing terminal estimate is readjusted. The process then proceeds first to step  1381  where the FOV is centered and then to step  1325 . If the minimum number of pulses are not detected or the pulses are not consistent, then the stage has failed and the process proceeds to step  1385 . 
     At stage  1385 , if less than the minimum number of pulses are detected then the stage has failed and the process proceeds to step  1390 . At step  1390 , the stage is decreased by one and the opposing terminal estimate is readjusted. The process then proceeds first to step  1391  where the FOV is centered and then to step  1325 . 
     At stage  1385 , if more then the minimum number of pulses have been detected but the detections are inconsistent, the process proceeds to step  1395  and the stage is re-tried. The process then proceeds to step  1325 . 
     While particular elements, embodiments and applications of the present invention have been shown and described, it is understood that the invention is not limited thereto since modifications may be made by those skilled in the art, particularly in light of the foregoing teaching. It is therefore contemplated by the appended claims to cover such modifications and incorporate those features which come within the spirit and scope of the invention.