BACKGROUND OF THE INVENTION
This invention relates to satellite communications and
more particularly relates to such communications employing optical beams, such as
The beams used for space-to-space and space-to-ground optical
communications have extremely narrow beam widths that require high bandwidth, closed
loop control for pointing and tracking to maintain adequate signal power for communications.
The beam widths are so narrow (on the order of 1-20 microradians) that methods are
needed to initially acquire the communications beams from the usual 0.1-0.3 degree
pointing knowledge uncertainty of current spacecraft. The acquisition method must
be highly robust and minimize total weight and power requirements for the optical
Beam acquisition methods have been described in the past.
For example, in columns 9-11 and Figure 5, U.S. Patent No. 3,504,182 (Pizzurro et
al., issued March 31, 1970) describes an acquisition method in which a first beam
of a first satellite dwells at one point in a field of view while a second beam
of a second satellite scans the entire field of view. When the beams illuminate
their respective satellites, the acquisition terminates.
U.S. Patent No. 3,511,998 (Smokier, issued May 12, 1970)
describes an acquisition method employing slow oscillatory scan motion limited by
limit switches. Receipt of a second beam signal during the slow scan motion terminates
the acquisition (Column 11).
U.S. Patent No. 5,060,304 (Solinsky, issued October 22,
1991) describes an acquisition method relying on beam reflection (Abstract).
U.S. Patent No. 5,282,073 (Defour, et al., issued January
25, 1994) describes an acquisition method in which the width of the beam is altered
during acquisition (Columns 5-6).
U.S. Patent No. 5,475,520 (Wissinger, issued December 12,
1995) describes an acquisition method in which multiple transmitted beams are defocused
to provide wide area coverage during acquisition (Column 2).
U.S. Patent No. 5,592,320 (Wissinger, issued January 7,
1997) describes an acquisition method in which a beam is
modulated with time or location information during the
acquisition (Column 3).
U.S. Patent No. 5,710,652 (Bloom et al., issued January
20, 1998) describes an acquisition system employing an array of a CCD acquisition
camera (Column 5).
Document EP 0 317 373 A2 discloses a laser beam communication
system for communication between two spacecrafts wherein, for link acquisition,
the initiating spacecraft scans a projected laser beam over an area expected to
contain the position of the target spacecraft (i.e. the one with which communication
is to be set up) and meanwhile also scans the filed-of-view of a detector over the
same area and along the same scan path.
In US 4,621,893 an optical scan device for use in a satellite
orbiting the earth for meteorological radiometer applications is described. The
flexibility of the employed scanning mechanism offers diverse scan modes and different
scan patterns for scanning the surface of the earth.
"A Twice Synchronous Range GEO to GEO Optical Intersatellite
Link", Dreisewerd D. W. et al., Military Conference, 1992, MILCOM '92, Conference
Record, Communications-Fusing Command, Control and Intelligence, IEEE San Diego,
CA, USA, 11-14 October 1992, New York, NY, USA, IEEE, US, 11 October 1992, pages
177-182 discloses a method for aligning opposing terminals in a satellite communication
system using a spiral pattern.
US 4,009,393 discloses an optically guided target seeker
scanning the entire field of view with a rosette or spiral scanning pattern.
Each of these prior methods and systems have limitations
which decrease its usefulness.
BRIEF SUMMARY OF THE INVENTION
The object of the invention is achieved by an apparatus
according to claim 1 and a method according to claim 10.
The invention is useful in a communication system employing
an optical beam suitable for transmission of data between a first terminal located
on an earth orbiting satellite and a second terminal remote from the first terminal.
In such an application of the invention, the beam is transmitted from the first
terminal for alignment with a beam receptor located on the second terminal. According
to a preferred embodiment, the beam is first generated. The beam then is transmitted
toward the second terminal, preferably by optics. During transmission, the beam
is scanned over a controlled uncertainty region defining an outer perimeter beginning
at a starting scan point with a first scan pattern and continuing at another scan
point with a second scan pattern different from the first scan pattern. The scanning
preferably is conducted by a positioning mechanism and a controller.
By using the foregoing techniques, terminal weight and
power can be minimized and the design of the positioning mechanism is simplified,
because the degree of acceleration required at the central portion of the scan is
BRIEF DESCRIPTION OF THE DRAWINGS
DETAILED DESCRIPTION OF THE INVENTION
- Figure 1a is a schematic block diagram of a pair of earth orbiting satellites
incorporating terminals which transmit and receive optical beams, such as laser
beams, which may use the acquisition techniques of the present invention.
- Figure 1b is a schematic block diagram of an earth orbiting satellite and a
ground station incorporating terminals which transmit and receive optical beams,
such as laser beams, which may use the acquisition techniques of the present invention.
- Figure 2 is a perspective view of a housing suitable for enclosing one embodiment
of the invention.
- Figure 3 is a general schematic block diagram of one embodiment of the invention.
- Figure 4 is a schematic block diagram of a preferred form of an optics module
made in accordance with the invention.
- Figure 5 is a schematic block diagram illustrating one embodiment of a beam
expansion telescope suitable for use with the optics module shown in Figure 4 in
which the telescope is rotated 90 degrees from the position shown in Figure 4.
- Figure 6 is a fragmentary perspective view of one embodiment of a positioning
module for moving the telescope shown in Figure 5 and a schematic block diagram
of a preferred form of control circuitry for controlling the positioning module
shown in Figure 6, the sensors shown in Figure 4, the point ahead mechanism shown
in Figure 4 and the fine track mechanism shown in Figure 4.
- Figure 7 is a schematic block diagram of a preferred form of servo loops for
controlling the positioning module shown in Figure 6, the point ahead mechanism
shown in Figure 4 and the fine track mechanism shown in Figure 4.
- Figure 8 is a flow diagram illustrating a preferred form of operation of the
apparatus shown in Figure 4.
- Figures 9-13 are schematic diagrams illustrating the fields of view of an acquisition
sensor (e.g., shown in Figure 4) and a telescope (e.g., shown in Figure 5) in various
modes of operation located in different terminals shown in Figure 1.
- Figure 14 is a diagram illustrating exemplary output vectors of a beam transmitted
by the apparatus shown in Figure 4 versus time.
- Figure 15 is a diagram illustrating exemplary output vectors of a beam transmitted
by the apparatus shown in Figure 4 during an exemplary acquisition process with
one of the terminals shown in Figure 1 moving at 7.5 kilometers per second.
- Figure 16 is diagrams of the spiral and rosette pattern used to transmit a beam
by a portion of the apparatus shown in Figure 4.
- Figure 17 is an enlarged view of one quadrant of the spiral scan shown in Figure
Referring to Figure 1a, the present invention allows optical
beams used for communication to be aligned with communication terminals located
on earth-orbiting satellites. The alignment process is generally known as acquisition.
As shown in Figure 1a, earth orbiting satellite 10 carries a communication terminal
12 which includes a telescope 14 for sending and receiving optical beams 16 and
17. Another earth orbiting satellite 20 carries a communication terminal 22 including
a telescope 24 for sending and receiving beams 16 and 17. Beam 17 is aligned with
a beam receptor 25. Although the beams 16 and 17 are shown separated in Figure 1,
in fact the beams follow a common path through telescopes 14 and 24. The beams are
transmitted using complimentary frequencies. For example, beam 16 is transmitted
at 1.554 microns and beam 17 is transmitted at 1.546 microns. Terminals 12 and 22
provide full duplex operation at 6.75 Gbps. Beams 16 and 17 are both diffraction
limited. They are not spread or defocused and are modulated to transmit data after
acquisition is completed.
Referring to Figure 1b, the present invention allows optical
beams used for communication to be aligned with a communication terminal located
on an earth-orbiting satellite and with a terminal located on a ground station.
Ground station 26 includes a terminal 28 with a telescope 30 which transmits and
receives beams 16 and 17 previously described. Each of terminals 12, 22 and 28 is
identical. As a result, only one such terminal is described in this specification.
Referring to Figures 2, 3 and 7, a preferred beam acquisition
system for terminal 12 made in accordance with the invention basically comprises
an optics subassembly (STS) 50, a gimbal positioning subassembly (GS) 150, an electronics
control subassembly (AES) 200, a remote electronics assembly 250, and pointing and
tracking control loops 300.
Referring to Figure 5, optic subassembly 50 includes telescope
14 (Figure 1) which comprises a protective window 51 and mirrors 53-55 arranged
as shown. Telescope 14 is coursely positioned using a means known to those skilled
in the art so that it is pointed in the direction of terminal 22 on satellite 20
within a first region of uncertainty. Telescope 14 is a silicon carbide, off-axis,
three-mirror anastigmat system, made of silicon carbides, which can be assembled
without alignment. Silicon carbide systems also are advantageous because they inherently
have less sensitivity to temperature changes than other types of telescopes. Window
51 has a silicon substrate with a silicon dioxide anti-reflective coating on the
exterior surface to protect the interior optical surfaces from particulates, micrometeorites,
and ion exposure. Window 51 has a bandpass AR coating which provides attenuation
of solar radiation above 1550 nanometers. Sapphire is also an option for the substrate
if warranted by the environmental effects. If sapphire is employed, a silicone substrate
bandpass filter is sandwiched with the sapphire window. An especially preferred
form of telescope 14 is described in the commonly assigned application entitled,
"Optical Inter-Satellite Link (OISL) Gimbal," Application No. 09/346052, filed on
July 7, 1999 in the names of Dan R. Johnson, Mark A. Carroll and Daniel R. Sherman,
which is incorporated by reference.
Telescope 14 defines a field of view 56 having a center
point 57 (Figure 9), and defines a common beam path 58 for beams 16 and 17 (Figure
1A). Path 58 is centered on center point 57 which defines an axis 59 passing through
the center point and parallel to beam path 58 (Figure 5).
Referring to Figure 4, optics subassembly 50 also includes
a fine track mechanism 60 which comprises an electromagnetically driven two axis
gimbal assembly 62 flexure-mounted to a mirror 64 that deflects optical beams 16
and 17. Mechanism 60 is provided with integral angle sensing and a high bandwidth
pointing control over plus or minus 0.5 degrees of mechanical travel. Control signals
are transmitted over a cable 65. An especially preferred form of mechanism 60 is
described in the commonly assigned application entitled "Fine Pointing Assembly
Configuration", docket no. 11-1040, filed on the same date as this application in
the names of Ashley, C. Danial and Arthur P. Balz, which is incorporated by reference.
Optic subassembly 50 also includes a dichroic beam splitter
66 which divides the optical path into a transmit path 68 and a receive path 70.
A narrow band pass filter 72 allows passage of the receive beam and discriminates
against the transmit beam which uses a different frequency than the receive beam.
A track/receive beam splitter 74 diverts about 10% of the beam in receive path 70
to a track sensor path 76. The portion of the beam in path 76 is focused by focusing
optics 78 (including one or more lenses) on a track sensor 80.
Track sensor 80 defines a field of view 82 located on a
photo detector comprising four tracking quadrants TQ1-TQ4. Both the field of view
and the tracking quadrants define a common center point 84. The photo detector within
track sensor 80 is fabricated from InGaAs. Track sensor 80 is associated with two
sets of electronics. The first set causes a detector to respond to optical pulses
of a particular temporal nature. In addition, the first set of track sensor electronics
processes the sum of all four quadrants in the photodetector as well as each quadrant
individually. The second set of electronics generates track error signals in order
to keep beams 16 and 17 on track after the acquisition phase is completed.
A particularly preferred form of tracking sensor 80 is
described in the commonly assigned application entitled "Inter-Satellite Optical
Link Track Sensor," Application No. 09/301494, filed on April 28, 1999 in the names
of Chie W. Poon, Robert C. Carden, and Robert M. Englekirk, which is incorporated
The 90% of the receive beam transmitted through beam splitter
74 is received by collimation optics 86 which includes one or more lenses or mirrors.
Optics 86 focuses a portion of the receive path beam on an annular mirror 88 which
defines a central hole 90. A portion of the beam transmitted through the central
hole is focused on a fiber coupler 92 that transmits the portion of the beam to
an optical fiber 93. Coupler 92 and fiber 93 serve as a beam receptor for the beam
in receive path 70. Fiber coupler 92 and fiber 93 are the eventual recipients of
the beam 16 energy directed toward terminal 12 by terminal 22. After acquisition,
communication signal information impressed on beam 16 received from terminal 22
is decoded by remote electronics assembly 250. The portion of the beam falling on
annular mirror 88 is reflected through an acquisition refocusing optics 94 (including
one or more lenses or mirrors) and is focused onto an acquisition sensor 96.
Beam splitter 74 and mirror 88 are arranged such that about
10% of the beam in receive path 70 is focused on track sensor 80, with the remainder
of the beam in receive path 70 focused either on acquisition sensor 96 or the fiber
coupler 92, depending on the tilt angle of the collimated beam. Techniques for focusing
an optical beam on a fiber coupler are known to those skilled in the art and are
described in U.S. Patent No. 5,062,150 (Swanson, issued October 29, 1991). Techniques
for transmitting and receiving beams and aligning them with sensors, a laser diode
and mirror are described in U.S. Patent No. 5,390,040 (Mayeax, issued February 14,
Acquisition sensor 96 includes a photo detector divided
into four acquisition quadrants AQ1-AQ4 which define a field of view 98. Both the
field of view 98 and the photo detector have a common center point 100. Acquisition
sensor 96 comprises a quadrant detector sensitive to the wavelength used for communication
together with electronics that processes the signals from each quadrant designed
to detect pulses of optical energy of beam 16 with a temporal nature to be described
later. The acquisition sensor has a field of view suitable for covering the entire
first uncertainty region, i.e., if the uncertainty region is 0.25°, the acquisition
sensor field is 0.25°. Center point 100 of acquisition sensor field of view
98 is aligned with center point 84 of field of view 82 of track sensor 80. Similarly,
center point 100 of field of view 98 of acquisition sensor 96 is aligned with center
point 91 of fiber coupler 92. An especially preferred form of acquisition sensor
96 is described in the commonly assigned application entitled "Inter-Satellite Optical
Link Acquisition Sensor," Application No. 09/301297, filed on April 28, 1999 in
the names of Chie W. Poon, Robert C. Carden and Robert M. Englekirk, which is incorporated
The optics 78, 86 and 94 are designed so that the field
of view of acquisition sensor 98 is about 20 to 50 times larger than field of view
of track sensor 82. Field of view 98 is substantially the same size as field of
view 56 of telescope 14. The size of field of view 98 is controlled by optics 94,
and the size of field of view 82 is controlled by optics 78.
The various outputs from track sensor 80 are transmitted
over a cable 85 for further processing. Various outputs from acquisition sensor
96 are transmitted over a cable 102 for further processing. Cables 85 and 102 are
combined into a cable 103.
Fine track mechanism 60 is used to simultaneously direct
the center points 100 and 84 of the fields of view of the acquisition sensor 96
and track sensor 80 and transmit beam 17 toward the estimated position of the opposing
terminal (e.g., terminal 22) as determined by acquisition sensor 96.
Optics subassembly 50 also includes a point ahead mechanism
110 which is identical to fine track mechanism 60 except for an increased field
of regard to accommodate the scan angle required for acquisition (driven primarily
by the 0.1 degree uncertainty in spacecraft attitude and position). Point ahead
mechanism 110 includes a two axis gimbal assembly 112 which moves a mirror 114 through
about plus or minus 2.25 degrees of mechanical travel. Point ahead control signals
are transmitted over a cable 113 which is combined with cable 65 to form a cable
115. Point ahead mechanism 110 directs transmit beam 17 along transmit path 68 relative
to center point 100 of field of view 98 of acquisitions sensor 96.
Point ahead mechanism 110 is designed with sufficient range
to allow the transmit beam 17 to be directed anywhere within the field of view 98
of acquisition sensor 96. This range is extended over that normally required for
transmit/receive point ahead during tracking and communication. Point ahead mechanism
110 has the range, bandwidth, resolution, and accuracy required to direct transmit
beams 17 over the acquisition sensor field of view 98 in the manner to be described
later and corrects for the apparent location of the opposing terminal, (e.g., terminal
22) due to the opposing terminal's velocity and light travel time. An especially
preferred form of point ahead mechanism 110 is described in the commonly assigned
application entitled "Fine Pointing Assembly Configuration," filed on the same date
as this application in the names of Ashley C. Danial and Arthur P. Balz which is
incorporated by reference.
Included in transmit path 68 is transmit collimation optics
116 (which includes one or more lenses) and which collimates the transmit beam 17
propagated from a transmit coupler 118 which receives the beam over an optical fiber
119. The transmit beam 17 is steered differently from the receive beam 16 line of
sight by the point ahead mechanism 110 to compensate for beam travel time to a remote
terminal, such as terminal 22 (figure 1A). Point ahead mechanism 110 also scans
beam 17 during the acquisition process.
Referring to Figure 6, positioning subassembly 150 comprises
an elevation drive 152 which moves telescope 14 through approximately -3 to +26
degrees of elevation. The change in azimuth of telescope 14 is accomplished by a
yoke 154 which is driven by an azimuth drive 156 through approximately ±80
degrees of azimuth field regard.
Positioning subassembly 150 is a two-axis gimbal for course
pointing of telescope 14. Both the elevation and azimuth gimbal axes use permanent
magnet brushless motors (i.e., drive 152 and drive 156) and a rotary variable differential
capacitive angle sensor. A particularly preferred embodiment of positioning subassembly
150 is shown in the commonly assigned application entitled, "Optical Inter-Satellite
Link (OISL) Gimbal," Application No. 09/346052, filed on July 7, 1999 in the names
of Dan R. Johnson, Mark A. Carroll and Daniel R. Sherman which is incorporated by
reference. A particularly preferred embodiment of the capacitive angle sensor is
described in the commonly assigned application entitled "Capacitive Resolver," Application
No. 09/310365, filed on May 12, 1999 in the names of Dan R. Johnson, Daniel R. Sherman
and Paul A. Franson which is incorporated by reference.
Elevation drive 152 and azimuth drive 156 are supported
by an isolator interface ring 158. The ring reduces pointing disturbances from satellite
10 (Figure 1A). Isolator 158 consists of six passively damped spring elements arranged
in a Stuart Platform configuration which provides the same fundamental frequency
in all six degrees of freedom. The isolator spring elements are highly damped using
acrylic visco-elastic material. Isolator 158 is designed to have a 10-15 Hz. corner
frequency and to provide greater than 10 decibels attenuation of satellite disturbances
at 100 Hz.
Positioning subassembly 150 also includes an azimuth cable
wrap 160 and a base 162.
Still referring to Figure 6, control subassembly 200 comprises
terminal controller electronics 202 which performs computing functions for the control
subassembly. For example, electronics 202 provides the command/telemetry interface
to the satellite payload processor, performs internal digital processing for control
of fine track mechanism 60 and point ahead mechanism 110 during the acquisition
process, and implements control for remote electronics assembly 250. The internal
processing of electronics 202 include sensor digitization and control for a track
loop, and unloading loop, a fiber alignment loop, and a point-ahead loop, along
with the required module-to-module communication. Electronics 202 also receives
power on/off commands and performs power commanding for remote electronics assembly
Control sub-assembly 200 also includes mechanical drive
electronics 204 which provides drive and position control functions for fine track
mechanism 60 and point ahead mechanism 110. Electronics 204 also provides analog
to digital and digital to analog functions as needed.
Control subassembly 200 also includes gimbal drive electronics
206 which provides the drive electronics for elevation drive 152 and azimuth drive
Control subassembly 200 also includes sensor processing
electronics 208 which process the outputs from track sensor 80 and acquisition sensor
Control subassembly 200 also includes a power converter
210 which supplies separate analog and digital power to various components of the
control subassembly 200. The various electronics modules of control sub-assembly
200 are connected through a conventional back plane 212.
Referring to Figure 3, remote electronics assembly 250
comprises a master oscillator/modulator (Mo/M) 252 which receives a 6.75 Gbps serial
data and clock from the satellite 10 payload on differential lines. After the acquisition
phase, Mo/M 252 modulates beam 17 with communication data. The encoded data modulates
the output of a continuous wave master oscillator using a dual-electrode push-pull
Mach-Zehnder low-biased to operate as a phase modulator using an active control
loop. The distributed feedback master oscillator laser is wave length controlled
via active temperature control to within the tracking range of the optical modulator
on the receiver end. The deskew, scrambler/differential encoder, and driver amplifier
for the modulator are mounted on the front side of the module.
Assembly 250 also comprises a transmit amplifier (TA) 254
which boosts the low-level modulator output to about 300 milliwatts for transmission.
Assembly 250 also includes a low-noise amplifier (LNA)
256 which comprises a low-noise Er fiber amplifier. The signal is filtered before
demodulation and detection. Amplifier 256 also includes a tunable filter that closely
matches the optical bandwidth to the signal bandwidth. The filter center frequency
tracks the optical frequency of the input signal to compensate for Doppler shift
or master oscillator wavelength drift.
Assembly 250 also comprises a demodulator bit synchronizer
(DBS) 258. After the acquisition phase, communication data in beam 16 is demodulated
by the combination of an asymmetric Mach-Zehnder interferometer and a balanced photo
detector/differential transimpedence amplifier. The demodulator splits the optical
signal into two paths with a differential delay of 1-bit. The paths are then recombined
to form a sum and difference output. When a "zero" bit is transmitted, the phase
of the optical carrier is left unchanged from the previous bit by the DPSK modulator.
Optical signals from the two paths add constructively on the sum output and destructively
on the difference output, resulting in a positive voltage at the transimpedence
amplifier output. When a "one" is transmitted, the phase of the optical carrier
changes by 180° relative to the previous bit and the opposite occurs, resulting
in a negative voltage at the transimpedence amplifier output.
The bit synchronization recovers a data clock and detects
the bits from the low pass filtered analogue wave form. Since the modulation receiver
uses a balance detector, no threshold control is necessary. The bits synchronization
output is de-scrambled and output on a serial 6.75 Gbps differential interface along
with the recovered clock.
Assembly 250 also comprises a power converter 260 which
includes three commercially available converter modules and filters.
Referring to Figure 7, pointing and tracking control loops
300 comprise a track loop 310 including a compensation circuit 312 which provides
a signal to a fine tracking mechanism mechanical loop including a compensation circuit
314, a torquer circuit 316 and a sensor 318 connected as shown. The fine tracking
mechanism mechanical loop controls the operation of fine tracking mechanism 60 (Figure
4). Track loop 310 tracks the angular position of an opposing terminal (such as
terminal 22) (Figure 1A)) to maintain coupling of the received optical energy into
receive fiber 93 (Figure 4), the error signal from the tracking sensor 80 measurements
is used to adjust the fine track mechanism 60 pointing angle. The loop bandwidth
is about 300 Hz.
Control loops 300 also include an unloading loop 340 (Figure
7) which comprises a compensation circuit 342 that feeds a signal into a gimbal
loop which includes a compensation circuit 344, a torquer 346 and a sensor 348 connected
as shown. The gimbal loop drives elevation drive 152 and azimuth drive 156 (Figure
6). The unloading loop transfers the fine track mechanism 60 angular position to
elevation drive 152 and azimuth drive 156 to keep the fine track mechanism within
its mechanical range. That is, center point 57 is aligned with center points 84
and 100 (Figures 9 and 4) .
Control loops 300 also comprise a fiber alignment loop
360 (Figure 7) which comprises low noise amplifier 256 (Figure 3) and a compensation
circuit 364 which provides a signal to the fine track mechanism mechanical loop
previously described. The output of the fine track mechanism alters the relationship
of the receive beam to receive fiber coupler 92 as shown in Figure 7. The fiber
alignment loop is a low-bandwidth loop to correct alignment errors between fiber
receive coupler 92 and track sensor 80. Fine track mechanism 60 applies a small
tilt dither in the receive beam; variation in power on target telemetry from the
remote electronics assembly low noise amplifier 256 then corrects to the track sensor
80 angular bias.
Control loops 300 also comprise a point ahead loop 370
(Figure 7) which includes a compensation circuit 372 that provides a signal to a
point ahead mechanism mechanical loop that includes a compensation circuit 374,
a torquer 376 and a sensor 378 connected as shown. The point ahead mechanism mechanical
loop controls the operation of point ahead mechanism 110 (Figure 4). The point ahead
loop 370 continually corrects for point ahead misalignment. Initial point ahead
is based on pointing angles derived from satellite ephemeris. After acquisition,
power on target measurements from the opposing terminal (e.g., terminal 22, Figure
1A) communicated across the optical link in optics assembly 50 produce corrections
to the position of point ahead mechanism 110. Opposing terminals, (e.g., terminals
12 and 22) dither at different frequencies (nominally 5 and 7 Hz) so that point-ahead
error can be distinguished from fiber alignment error.
The apparatus described in Figures 1-7 is operated during
the acquisition procedure as illustrated in Figure 8. During an initialization step
performed on satellite 10, the approximate position of terminal 22 on satellite
20 is received within an initial uncertainty region. Since satellite 10 knows its
approximate current location, it can anticipate the power of beam 16 when it is
received from terminal 22.
During step S10, the acquisition logic of terminal 12 in
satellite 10 is loaded with information on the location in space of terminal 22
on satellite 20 within an initial uncertainty region RU1 (Figures 14 and 15), the
expected power level of beam 16 to be received from terminal 22, and data base parameters
defining conditions for transitions between acquisition stages. Once this information
is loaded, the acquisition process is commanded to start by external means. No further
coordinating messages by external means between terminals 12 and 22 are required.
Each of terminals 12 and 22 directs the center of its acquisition
sensor (e.g., center point 100) and track sensor (e.g., center point 84) towards
the estimated position of the opposing terminal by its fine track mechanism (e.g.,
fine track mechanism 60). The fine track mechanisms in terminals 12 and 22 are continually
updated to maintain the center points of the fields of view of the respective sensors
towards the estimated position of the opposing terminal. Each terminal then uses
its point ahead mechanism (e.g., point ahead mechanism 110) to scan out the uncertainty
region RU1 with a transmitted beam (e.g., beam 17) using a spiral scan pattern with
As shown in Figure 16, the starting point of the scan is
at the center of uncertainty region RU1 to optimize the time required for acquiring
the opposing terminal, since the opposing terminal is less likely to be located
at the extremes of the uncertainty region due to the processes that drive the uncertainty
estimates of the opposing terminal. Near the center of the scan, where high acceleration
would be required by point ahead mechanism 110 to maintain the equivalent area scan
velocity, the spiral scan transitions to a cycloidal pattern that limits the required
acceleration to that realizable by the point ahead mechanism 110 while still maintaining
coverage of the uncertainty region. The cycloidal pattern occupies the central 40
microradians of the scan. The size of the pattern is dependent on the acceleration
characteristics of the point ahead mechanism.
One example of the cycloid pattern is the rosette pattern
After the area of the uncertainty region near the center
of the scan has been covered, the rest of the uncertainty region to outer perimeter
OP is covered by a spiral scan 600 (Figures 14 and 16).
The spiral scan sweeps out the uncertainty region at a
constant velocity with a distance between the arms of the spiral set to minimize
the time to cover the uncertainty region while maintaining probability of adequately
covering the complete region in the presence of terminal based motion due to satellite
vehicle disturbances. The velocity is chosen to minimize the time required to cover
the uncertainty region while generating a pulse of the appropriate power and time
interval as described later.
Both the spiral scan 600 and the cycloid pattern 650 are
generated based on the bandwidth of the point ahead mechanism 110 to result in optimized
coverage of the uncertainty region. At the perimeter of the uncertainty region OP,
the outward spiral transitions to a spiral in an inward direction (Figure 14), or
the spiral can be restarted from the center with the choice made to minimize the
overall acquisition time. Whether the scan is being accomplished with the rosette
pattern 650 or with the spiral pattern 600, the velocity of the scan is maintained
Figure 17 illustrates a normalized intensity distribution
for a portion of one quadrant of transmitted beam 17. The spiral convolutions, such
as 610-612, overlap slightly as shown in Figure 17 so that the entire uncertainty
region is covered. When the transmit laser source sweeps by the location of the
opposing terminal (e.g., when beam 16 sweeps by terminal 12), the near gaussian
shape of beam 16, as well as the constant velocity scan, results in a pulse of optical
energy with a characteristic time interval and intensity envelope. The electronics
of track sensor 80 and acquisition sensor 96 are highly sensitive to pulses of energy
with this characteristic time interval and intensity envelope while being relatively
insensitive to signals with other characteristics, such as those signals resulting
from solar, planetary, and stellar bodies and body motion and internal optical signals
and sensor noise. When a signal with the appropriate characteristics is received,
and the signal is above the pre-determined power level threshold, acquisition sensor
96 defines the detector quadrant in which the pulse was detected.
When a pulse from beam 16 transmitted by terminal 22 is
detected, the acquisition logic can continue down different paths. The first path
involves step S12 shown in Figure 8. Control subassembly 200 includes a logic which
counts the number of pulses detected by acquisition sensor 96 over time (i.e., the
number of times beam 16 enters telescope 14). In step S12, if less than the maximum
number of pulses has been detected, the logic moves to step S14 which includes a
time allocated for stage expiration. After the time has elapsed, the logic moves
onto step S16 which determines whether the minimum number of pulses has been detected
and whether the pulses are consistent. If less than a set number of pulses has been
detected, the acquisition stage can transition to a previous acquisition stage as
indicated by steps S18 and S20, or if in the first acquisition stage, the first
stage can be re-tried as indicated in step S22. If the number of first stage re-tries
is above a predetermined number, satellite 10 can be notified that the acquisition
did not succeed. If more than the minimum number of pulses was detected, but the
pulses were not consistent (i.e., the pulses were detected in quadrants on opposing
sides of acquisition sensor 96 or in inconsistent quadrants between the acquisition
sensor 96 and track sensor 80, indicating that one or more of the pulses was a false
detection), the acquisition stage can be re-tried as indicated in step S22. If more
than the minimum number of pulses was detected and if the pulses were consistent,
the logic can transition to the next acquisition stage as indicated in step S24.
The first acquisition stage is illustrated in Figures 9
and 10. The numbers with an A suffix refer to like numbered parts found in terminal
22. Figure 9 illustrates the field of view 56 of telescope 14 and the field of view
98 of acquisition sensor 96. As shown in Figure 9, fields of view 56 and 98 are
substantially the same size. Figure 9 also illustrates a field of view 56A with
center point 57A of telescope 24 of terminal 22, and a field of view 98A with center
point 100A of the acquisition sensor within terminal 22. An exemplary location of
terminal 22 within field of view 56 is indicated by L22. An exemplary location of
terminal 12 within field of view 56A is indicated by L12.
As shown in Figure 10, a pulse of receive beam 16 from
terminal 22 is detected in quadrant AQ1 of the photo detector of acquisition sensor
96. As a result of this pulse, the position of mirror 64 of fine track mechanism
60 is altered in step S26 (Figure 8) so that center point 100 of field of view 98
of acquisition sensor 96 is pointed toward the region in space represented by quadrant
AQ1 in which the location of terminal 22 was detected. In order to illustrate this
point, in Figure 11, the re-directed field of view 98' with center point 100' of
acquisition sensor 96 is superimposed on the original field of view 56 of telescope
The same mode of operation is illustrated in Figure 14
which shows the initial region of uncertainty RU1 and the resulting scan of beam
17 by point ahead mechanism 110 in a spiral pattern. The scan pattern defines a
spiral locus of scan lines having a center scan line along horizontal axis 0 at
the origin of the scan.
The illumination of terminal 12 by beam 16 illustrated
in Figure 10 occurs at time T10 shown in Figure 14. At time T10, the initial region
of uncertainty RU1 is reduced to a second region of uncertainty RU2 also illustrated
by new scan field of view SC2 (Figures 11 and 12). Point ahead mechanism 110 and
fine track mechanism 60 point the center line of the scan into region RU2 as determined
by the pulse detected in detector quadrant AQ1. Point ahead mechanism 110 begins
to scan transmit beam 17 in region RU2 beginning at center point 100'. As shown
in Figure 14, uncertainty region RU2 is smaller than uncertainty region RU1. The
scanning of transmit beam 17 in region RU2 is the same as the scanning previously
described in region RU1, except that the diameter of the region is smaller.
Referring again to Figure 8, if step S20 results in a decrease
in the stage, then in step S28 the field of view 98 for the preceding stage is altered
in a manner which reverses the order of Figures 10 and 11.
As shown in Figure 12, telescope 14 is reoriented using
unloading loop 340 (Figure 7) so that center point 57 of field of view 56 again
is aligned with the new field of view of acquisition sensor 96. That is, center
points 57, 84, 91 and 100 are aligned.
To summarize the transition of operation from the initial
stage to the next stage, the estimate of the opposing terminal location (e.g., L22,
Figures 9-12), the uncertainty region and the scan pattern are adjusted. When a
pulse from terminal 22 is detected by terminal 12 on a particular quadrant (e.g.,
quadrant AQ1), this means that terminal 22 is located within the intersection of
uncertainty region RU1 and quadrant AQ1. The new uncertainty region RU2 (Figure
14) is taken to be this intersection. The estimated location of terminal 22 is taken
to be the center of the new uncertainty region RU2. The radius of the scan pattern
is adjusted to encompass the new uncertainty region RU2, and the center point 100
of the acquisition sensor field of view 98 and then the center point 57 of the telescope
field of view 56 are pointed at the center of the new uncertainty region RU2.
Fine track mechanism 60 is used to point center point 100
of the acquisition sensor 96 toward the region of space indicated by the detector
quadrant (e.g., quadrant AQ1) in which a pulse is detected. The acquisition logic
delays the pointing of the center point of the field of view of the acquisition
sensor to account for light travel time between terminals 12 and 22 and to allow
fine track mechanism 60 to settle on the new location.
Point ahead mechanism 110 is committed to follow the new
scan pattern so that the center point of the spiral scan of beam 17 and center point
100 are aligned. Telescope 14 then follows the movement of point ahead mechanism
110 so that center points 57 and 100 again are aligned. Point ahead mechanism 110
then begins the previously described scan pattern for transmit beam 17.
The result of the transition to the new acquisition stage
is that the estimate of the opposing terminal position, (e.g., terminal 22) is improved
and the uncertainty region is reduced. Due to the reduced uncertainty region, it
takes less time for the scan pattern to be completed resulting in an increasing
pulse rate detected by acquisition sensor at the opposing terminal. Thus, each acquisition
stage takes less time to complete than the previous acquisition stage.
The number of acquisition stages required is dependent
on the size of the initial uncertainty region. After transition through a number
of acquisitions stages, the remaining uncertainty and the estimate of the opposing
terminal location is less than the transmit laser source beam diameter or area.
This uncertainty region is sufficient to enable tracking sensor 80 to continuously
track beam 16 from terminal 22.
Figure 14 illustrates acquisition stages involving uncertainty
regions RU1-RU6 which terminate at times T10-T15, respectively, when a pulse from
beam 16 is detected by acquisition sensor 96. Each of uncertainty regions RU1-RU6
is smaller than the previous regions. The transaction from one region to the next
can be understood from the description of the transition from region RU1 to RU2.
Thus, the size of regions RU1-RU6 successively approaches the size of region TU
at which tracking can commence.
The final stages of acquisition, illustrated by uncertainty
region FS in Figure 14, drop the spiral scan described previously in favor of a
modified cycloid scan pattern like pattern 650 shown in Figure 16. Such patterns
are designed to sweep the transmit laser beam across the opposing terminal position
a smaller number of times at the edge of the scan pattern 630 as opposed to the
central portion of the scan pattern 632. At the opposing terminal, this operation
results in a lower pulse rate at the edge of the uncertainty region 630 and a higher
pulse rate at the center of the uncertainty region 632.
During the final stages of acquisition, the acquisition
logic is modified to move the estimate of the opposing terminal position, and therefore
the center point 84 of track sensor 80, a fixed amount in the direction appropriate
for the quadrant of track sensor 80 in which the most recent pulse of beam 16 was
Since the effect of a false detection is less critical
during the final stages of acquisition, the power level at which the pulses are
detected can be adjusted to minimize overall acquisition time. The threshold is
adjusted in the circuitry for acquisition sensor 96.
A transition to a previous acquisition stage occurs if
less than the predetermined average number of pulses per second are received or
if a stage time has been exceeded. This indicates that the opposing terminal has
a larger uncertainty region and therefore needs more time to improve its estimate
of the current terminal location. The stage times account for light travel time
and are selected to minimize the possibility that the two terminals (e.g., terminals
12 and 22) will cycle back and forth between stages by adding a random time interval
to each stage time. If greater than a specific average number of pulses is received,
the terminal transitions to the next acquisition stage in a smaller cycloidal pattern
that results in a higher detection pulse rate at the opposing terminal.
At the final cycloidal acquisition stage, illustrated by
uncertainty region TU in Figure 14, when the average pulse rate exceeds a predetermined
threshold, the cycloidal pattern is removed from the commands sent to the point
ahead mechanisms (e.g., point ahead mechanism 110) resulting in a constant power
beam on the opposing terminal track sensor (e.g., track sensor 80). On the opposing
terminal, the second set of track sensor electronics uses this constant power beam
to generate track error signals that are used to further correct the opposing terminal
tracking. Acquisition is completed when the second set of track electronics on both
terminals indicate the reception of sufficient signal power to maintain tracking.
If after a predetermined stage time, the second set of track electronics does not
indicate sufficient signal power, the acquisition logic drops back to the previous
cycloidal acquisition stage for that terminal. The stage times account for light
travel time and are selected to minimize the possibility that the two terminals
cycle back and forth between stages by adding a random time interval to each stage
Referring to Figure 8, the previously described final stages
of acquisition are entered through step S32. Since the final stages of acquisition
preferably are not timed, the logic proceeds to step S34 in which terminal 12 calculates
a real time update and applies it to the estimate for terminal 22. The logic then
proceeds to step S36 in which circuitry calculates the average rate at which beam
16 is striking terminal 12. If the strike or hit rate is above a predetermined threshold,
step S40 transfers the logic to step S42 which determines whether the hit rate is
sufficient to enter the tracking stage at step S44. If the hit rate is insufficient,
the logic proceeds to step S46 in which the region of uncertainty stage is increased
by 1 and the estimate of the opposing terminal position is modified by a predetermined
Returning to step S40, if the hit rate is not above the
go forward threshold, then the logic proceeds to step S48 to determine whether the
hit rate is below the go back threshold which requires a decrease in the stage.
If the hit rate is below the threshold, then the stage is decreased by 1, and the
adjustment of the opposing terminal position estimate is also decreased so that
the region of uncertainty is enlarged by a predetermined amount in step S50.
Returning to step S48, if the hit rate is not below the
return threshold, then the current stage is continued in step S52.
A comparison of Figures 14 and 15 shows that the number
of stages can vary depending on the circumstances, such as the size of the initial
region of uncertainty RU1. As shown in Figure 14, there are six progressively smaller
regions of uncertainty before terminal 12 enters the final stages of acquisition
FS through steps S32 and S34 (Figure 8). When the tracking stage is entered at step
S44, the region of uncertainty is the tracking region of uncertainty TU shown in
Figures 14 and 15 in which the uncertainty region is less than the diameter or area
of the received beam width. Figure 15 illustrates an acquisition in which only three
regions of uncertainty (RU1-RU3) are successively entered before the final stage
FS is entered.
While terminal 12 is being aligned with beam 16, terminal
22 simultaneously is being aligned with beam 17 in the same manner. Figure 13 illustrates
the change in pointing of the center point of the field of view of the acquisition
sensor on terminal 22 when transmit beam 17 strikes that terminal. The portions
of the field of views defined in terminal 22 are identified by like numbers used
in connection with the terminal 12 apparatus, but are given the suffix "A." As a
result, the operation in terminal 22 can be understood from the preceding discussion
of the operation in terminal 12 provided in connection with Figures 11 and 12. More
specifically, as shown in Figure 13, the re-directed field of view 98' with center
point 100A' of acquisition sensor 96A is superimposed on the original field of view
56A of telescope 24.
If during the tracking mode of operation, the signal power
drops below that required for tracking, terminal 12 automatically transitions to
the final stage of the acquisition logic and follows the acquisition logic either
to notification that the acquisition and therefore the link has failed or that the
track has been re-established. This results in robust performance and minimizes
the impacts of interruptions in the link between the terminals.