ACQUISITION AND TRACKING TERMINAL FOR OPTICAL INTER-SATELLITE by jxc15528

VIEWS: 7 PAGES: 10

									                                ACQUISITION AND TRACKING TERMINAL
                    FOR OPTICAL INTER-SATELLITE COMMUNICATIONS

                  Kazuhiko AOKI,* Hideaki MIKAMI,** Toshihiro KURII** and Ryutaro SUZUKI#
                   *
                    Functional Devices Research Laboratory, NEC Corp., Kawasaki 216-8555, JAPAN
                           **
                             Space Systems Division, NEC Corp., Yokohama 224-8555, JAPAN
                                        #
                                    Next-generation LEO System Research Center,
                       Telecommunications Advancement Organization, Kawasaki 210-0005, JAPAN

ABSTRACT

A small acquisition and tracking terminal has been developed for use in optical inter-satellite communications. This
terminal contains both a gimbal that employs an Active Universal Joint (AUJ), and a Wide-range Fine Pointing
Mechanism (WFPM). In the acquisition mode, as the WFPM performs its main role of high-speed acquisition by
exploiting its wide-angle range and fine tracking accuracy, the role of the AUJ gimbal itself is limited to adjusting for
any WFPM acquisition error. These relaxed demands on the gimbal with respect to high speed and accuracy have
made possible a smaller and lighter terminal. Acquisition and tracking tests show fast acquisition (80 to 160 ms) and
high accuracy, with tracking-error angles of less than +/-3 micro-radians (even when the received optical power is only
–67 dBm). Since the terminal is small and yet covers the azimuth through 360 degrees, it is suitable for
communications among low-earth-orbit satellites.

Keywords: Optical Communications, Inter-Satellite Links, Fast Acquisition, Highly Precise Tracking, Active
Universal Joint, Fine Pointing Mechanism, Wide Scanning Range


1. INTRODUCTION

Optical inter-satellite communications systems appear to offer great promise for a new, post-microwave high-speed
network infrastructure [1], and acquisition and tracking terminals will be one of the most important parts of such
systems. For practical optical systems, however, there is a need for terminals that is smaller and lighter than those
currently available. Most acquisition and tracking terminals consist of two pointing mechanisms, a coarse pointing
mechanism (CPM) and a fine pointing mechanism (FPM). In acquisition using a conventional narrow-range FPM,
since the CPM has to conduct all the initial acquisition, it must be extremely fast and highly precise requirements that
have resulted in larger and heavier terminals.

In response to this, we have developed a smaller, lighter acquisition and tracking terminal, one that employs a gimbal
equipped with an Active Universal Joint, as well as a Wide-range Fine Pointing Mechanism. This paper describes the
unique design concept of the terminal and discusses its experimental performance.

2. STRUCTURE OF ACQUISITION AND TRACKING TERMINAL

2.1 Total System
The primary components of the acquisition and tracking terminal are a gimbal that employs an Active Universal Joint
(AUJ), and a Wide-range Fine Pointing Mechanism (WFPM) (Fig. 1). The AUJ gimbal is able to conduct coarse
pointing over a 360 degree azimuth. The main function of the WFPM is fast acquisition and fine tracking. The AUJ
gimbal employs a coarse pointing sensor (CPS), and the WFPM a fine pointing sensor (FPS).

Because the WFPM features both wide-range and fine pointing, performance requirements for the AUJ gimbal can be
relaxed. The basic acquisition motion of the WFPM is high-speed centering. In acquisition, the function of the AUJ
gimbal is limited to adjusting WFPM tracking angles to zero. Such relaxed speed/precision demands on the AUJ
gimbal have made it possible to design a smaller and lighter terminal (Fig. 2).


""""""""""""""""""""""""""""
*
 Tel: +81-44-856-8482, Fax: +81-44-856-2227, E-mail: k-aoki@dc.jp.nec.com

                                                                 !
2.2 AUJ (Active Universal Joint) Gimbal
Here we have applied a unique Active Universal Joint mechanism to a coarse pointing, two-axis gimbal [2] (Fig. 3).
The mechanism connects inner and outer components through a universal joint. Two-axis rotations generate motion
about both azimuth and elevation axes.

One important feature of the AUJ gimbal is that both its inner and outer motors are attached to a fixed component,
whereas in a conventional gimbal at least one motor will be attached to a movable component. This offers significant
advantages in weight and size because the AUJ gimbal does not require a main-axis structure to support an entire
subordinate-axis structure, nor does it need any special transmission lines, slip rings, or other devices to supply a
subordinate-axis motor with electrical power and control signals. Further, it is able to rotate continuously and
limitlessly around its azimuth axis. Table 1 lists the AUJ gimbal specifications.

2.3 WFPM (Wide-range Fine Pointing Mechanism)
The WFPM is a mirror scanning mechanism composed of four electromagnetic actuators and a unique flexible support
system [3] (Fig. 4). The actuators are moving-coil-type Voice Coil Motors (VCMs) with long strokes to ensure a wide
scan range (+/-4 degrees). The support system, which consists of a metal center pivot and four thin springs, allows a
mirror to be rotated freely about a vertically constant point; the mirror can be rotated about the x- and y-axes
simultaneously. The system is balanced so that the center of rotation will be fixed on the mirror surface, thus keeping
beam path-length constant.

The WFPM is a very small, 40 x 40 x 25 mm3 (Fig. 5). Its wide scanning range is made possible by the high-
flexibility of the support structure and the long VCM stroke. Highly precise pointing is achieved by means of its
frictionless support structure and the high linearity of the VCMs in micro-actuation. Table 2 lists WFPM
specifications.

2.4 CPS (Coarse Pointing Sensor)
The Coarse Pointing Sensor (CPS) we employ is an area sensor that detects the center of gravity of the surface area
covered by a received beam. The size of the active field of the CPS is 7.6 x 7.6 mm, which is equivalent to a field of
view of +/- 0.3 degrees. The CPS detects angles in 0.0029-degree units, and its sampling frequency is 33 Hz. It is an
InGaAs device having a wide sensitivity range (from 0.98 to 1.60 microns).

2.5 FPS (Fine Pointing Sensor)
The Fine Pointing Sensor (FPS) is a quadrant detector, and if the area covered by a received beam extends over more
than one quadrant, it is able to specify a highly precise position. The size of the active field of the FPS is 1.0 mm in
diameter, which is equivalent to a field of view of +/- 0.048 degrees through an optical antenna with a magnifying
power of 10. The FPS is also an InGaAs device.

2.6 Fine Pointing Control System
A digital control system using the FPS beam position has been designed for fine beam pointing control. It features a
two-degrees-of-freedom robust controller comprising one feedback loop and two feed-forward loops (Fig. 6). The
feedback loop has a phase compensator and a low-frequency integrator. The phase compensator recovers both sensing
and sampling delays and thus secures the system’s margin of stability. The low-frequency integrator improves
tracking stability by suppressing low-frequency tracking error. One of the feed-forward loops is an acceleration feed-
forward type that makes it possible to conduct reference-model-following that is independent of the feedback loop.
The other is a disturbance-cancellation type which contains a disturbance observer and which cuts off vibrations from
the satellite body for more highly precise tracking.




                                                          #
3. ACQUISITION AND TRACKING SEQUENCE

The acquisition and tracking sequence is as listed below (note, also, that Figure 7 illustrates the motion of a beam
detection point across the CPS and the FPS in acquisition).

     Step 1: The CPS detects a beam. The WFPM moves at high speed over a wide range to get the beam into the
     FPS field of view. The AUJ gimbal slowly moves to get the beam into the center of the CPS.
     Step 2: The FPS detects a beam. The WFPM is driven by a fast search algorithm to get the beam into the exact
     center of the FPS.

     Step 3: The center of the FPS detects a beam. The mode is changed from acquisition to tracking, and the beam is
     directed to a fiber collimator.

     Step 4: The WFPM moves to compensate for any perturbations caused by external disturbances, helping to
     maintain highly precise tracking, with the beam being accurately delivered to a fiber core that is only 10 microns
     in diameter.

In acquisition, because the wide scan range of the WFPM (+/-4 degrees) covers the whole of the CPS field of view (+/-
3 degrees), the WFPM is able to start acquisition motion as soon as the CPS detects a beam, and little work is required
of the gimbal.

In tracking, the WFPM performs most of the fine-tracking function. WFPM motion angles, which are large in the
early stages of acquisition, statically converge to within a small range around zero degrees as the AUJ gimbal slowly
adjusts the WFPM angle to zero. Average power consumption of the WFPM is extremely low.

4. EXPERIMENTAL RESULTS
We used a laser beam from a fixed laser source to test the performance of the acquisition and tracking terminal.

4.1 Fine Pointing Loop Test
A fine pointing loop employing the FPS exhibited a highly satisfactory control-bandwidth, and the frequency at –3 dB
down was 580 Hz (Fig. 8). Further, the loop showed high error-suppression. The rate at 1 Hz was –57 dB, and at
10Hz it was –39 dB (Fig. 9).

4.2 Acquisition Test
The terminal achieved acquisition times ranging from 80 ms to 160 ms (acquisition time here is defined as the time
from initial beam-detection by the CPS to the change to the tracking mode). The average acquisition time obtained
from a hundred trials was 104 ms. It is particularly notable here that acquisition time has been shortened by 69 percent
(from 340ms to 104ms) over that a previous model. This was a result of the wide scan range of the WFPM.

Best case and worst cases for acquisition time are seen in Figure 10. In the figure, beam positions along the x- and y-
axes on the FPS are illustrated separately. Optical power values represent the normalized power of a beam as received
by the FPS.

In the best case (1), the terminal switched from the acquisition mode to the tracking mode and was stably tracking the
beam after only one acquisition sequence. By way of contrast, in the worst case (2), although acquisition was initially
successful, the beam later moved far enough off the center of the FPS to require a second acquisition. This movement
of the beam appears to have been caused by limit-cycle vibration of the gimbal (note the vibration of approximately 10
Hz clearly seen in case (2), which occurs after acquisition has been completed. The frequency of such limit-cycle
vibration depends on the gimbal attitude.




                                                          $
4.3 Tracking Test
In the gimbal-off state after acquisition, a tracking error of less than 3 micro-radians is maintained as long as the
received optical power is over –67 dBm (Fig. 13). Increases in tracking error exhibit clear linearity with respect to
decreases in received power. This is because such decreases in received power allow high-frequency noise to exert a
stronger influence (see Fig. 11). Such noise has two main constituents: 1) FPS heat noise, and 2) noise from the
analog-to-digital (AD) converters in the WFPM control circuits. As both received power and FPS S/N ratio decrease,
high-frequency noise results in increased tracking error. Noise from the AD converter, the larger of the two
constituents, clearly appears, however, to be easily reducible.

In the gimbal-on state, tracking error is from +/-2.8 to +/-7.4 micro-radians (Fig. 13), and tracking error and received
power appear independent. These differences between the gimbal-on and gimbal-off state are mainly attributable to
low-frequency periodic vibration (Fig. 12). That is to say, limit-cycle vibration of the gimbal induces significant
tracking error, sufficient to mask any error resulting from decreased received power. Such error might be reduced by
improvements in the tracking performance of the AUJ gimbal and in the error suppression performance of the WFPM.
We intend to work toward such improvements as a next step, and we feel confident of eventually achieving tracking
error equivalent to that for gimbal-on state.

5. CONCLUSION

We have combined an AUJ gimbal and a Wide-range FPM in the development of a small, light acquisition and
tracking terminal. Experimental results show that the terminal achieves fast acquisition (104 ms on average) and high
tracking precision (under +/-3 micro-radians in the gimbal-off state). It appears promising for direct application to
high-speed switchover of target satellites and to the maintenance of accurate tracking needed for optical
communications networks that employ multiple low-earth-orbit satellites, thus helping to make high-speed inter-
satellite communications and broadband communications networks in outer space a practical reality.

ACKNOWLEDGMENTS

The authors would like to express their deep gratitude to Prof. Haruhiko Yasuda in Waseda University, the leader of
the next-generation LEO system research project, for his extremely valuable suggestions, and we are grateful as well
to the many members of the project team who offered their very useful ideas.

REFERENCES

[1] R. Suzuki, K. Sakurai, S. Ishikawa, I. Nishiyama and Y. Yasuda, "A Study of Next-Generation LEO System for
    Global Multimedia Mobile Satellite Communications," 18th AIAA International Communications Satellite
    Systems Conference, AIAA-2000-1102, 2000
[2] N. Takanashi, S. Yashima, K. Aoki and T. Nishizawa, “Complete Modular Links for Hyper Redundant Robots,”
    COE Super Mechano-Systems Workshop, 1999
[3] K. Aoki, H. Kuroda, S. Yashima and A. Satoh, "Wide and Fine Pointing Mechanism with Flexible Supports for
    Optical Inter-satellite Communications," Free-Space Laser Communication Technologies XI, SPIE Proceedings,
    vol. 3615, pp. 222-229, 1999




                                                          %
                                   AUJ Gimbal
           Laser Beam



                  Optical
                  Antenna


               Beam Splitter                  CPS


                                                        Fiber           Pointing
                   WFPM                               Collimator       Controller

                                                    Optical Receiver
                                        FPS




                            Fig. 1 Acquisition and Tracking System




                  Table 1 AUJ Gimbal Specifications

        Item                                  Specifications
Size                        220 mm (diameter) x 235 mm (height)
Inside Diameter             70 mm
Weight                      12.0kg
Actuator                    Direct-Drive Motor
Angle Detector              Resolver
Travel Range                Azimuth : 360 degrees
                            Elevation: +/-20 degrees




                        Fig. 2 Acquisition and Tracking Terminal

                                               &
Fig. 3 Mechanical Structure of Two-axis Gimbal with Active Universal Joint




               Table 2 WFPM Specifications

           Item                     Specifications

  Size                     40 x 40 x 25 mm
  Mirror Diameter          15 mm
  Weight                   110 g
  Actuator                 Voice Coil Motor
  Angle Detector           Gap Sensor
  Steering Axes            Two Orthogonal (x-, y-axes)
  Travel Range             X-axis: +/-4.0 degrees
                           Y-axis: +/-4.0 degrees
  Control Bandwidth        380 Hz
  Power Consumption        2.5 W max




                                   '
Fig. 4 Mechanical Structure of Wide-range Fine Pointing Mechanism




    Fig. 5 Wide-range Fine Pointing Mechanism (cover-opened)




                               (
                          Acceleration
                          Compensator                                            WFPM
                                                            Control
Target                                                       Input
Angle    +        Low-Frequency           Phase          ++        Moving-Coil
             -      Integrator          Compensator       +          Driver



                                  Disturbance
                                   Observer
             Detected
              Angle
                                                                          FPS


                             Fig. 6 Fine Pointing Control System

                                                Y        CPS Detection




                                                                 Step 1
                                         FPS Detection

                                                            Step 2        X
                                                FPS
                                                          FPS Center
                                                           Detection



                                  CPS




                    Fig. 7 Motion of Beam Detection Point in Acquisition




                   Fig. 8 Fine Pointing Closed-Loop Frequency Response




                                                    )
              Fig. 9 Fine Pointing Error-Suppression Frequency Response




Case (1) Acquisition Time 80ms                Case (2) Acquisition Time 160ms

                         Fig. 10 Acquisition Time Responses




    (a) Optical Power: 64dBm                     (b) Optical Power: 53dBm

               Fig. 11 Tracking Error Time Responses (Gimbal-Off State)




                                         *
(a) Optical Power: 64dBm                   (b) Optical Power: 53dBm

          Fig. 12 Tracking Error Time Responses (Gimbal-On State)




           (a) Gimbal-Off State            (b) Gimbal-On State

           Fig. 13 Tracking Error Dependence upon Optical Power




                                   !+

								
To top