Automated Rendezvous and Capture requires the determination of the 6 DOF relating two free bodies. Sensor systems that can provide such information have varying sizes, weights, power requirements, complexities, and accuracies. One type of sensor system that can provide several key advantages is a binocular stereo vision system

Fisher, Timothy E.

Smith, Alan T.

English

NASA Technical Reports Server

is described in detail in the presentation showing that this sensor is outstanding for space use. It
has, indeed, already gone through most of the space qualification criterion.
The Spine-Locking Screw is central to robotic (and astronaut hand tool) fastening. A fundamental
change to the basic machine screw, it permits robots to fasten objects (and themselves in a walking
mode) to other objects with high preload forces from small input torques and without any
possibility of cross-threading. In addition, this fastener can be slightly modified to provide an
outstanding electrical connection along with the mechanical. Like the "Capaciflector," this fastener
already has been recognized as an outstanding system for use in space.
Taken together, the "Capaciflector" and the "Spine-Locking" Screw extend the state-of-the-art in
automated berthing technology. It is now possible to have smart instrumented payloads with a
sensing capability that can easily be fastened by robots with unprecedented control and safety
throughout.
A Binocular Stereo Approach to AR&C
at the Johnson Space Center
by Timothy E. Fisher and Alan T. Smith
Johnson Space Center and Lockheed Engineering and Sciences Company
Automated Rendezvous and Capture requires the determination of the 6 DOF relating two free
bodies. Sensor systems that can provide such information have varying sizes, weights, power
requirements, complexities, and accuracies. One type of sensor system that can provide several
key advantages is a binocular stereo vision system.
Binocular stereo uses two video cameras on the rendezvous vehicle viewing the target vehicle. The
target vehicle is equipped with a passive target that allows for easy and robust identification of the
target. Range to the target is inferred from the different bearings in the left and right camera views
of the target points. When the target has three or more identifiable points in a known geometry, the
attitude of the target can be inferred from the bearings and ranges to the three points.
One of the main advantages of such a binocular stereo vision sensor is the simplicity of the
hardware. The system uses standard video cameras and digital processing hardware. These items
are likely to be present already on the rendezvous vehicle. The other major hardware components
required include image digitizing electronics and possibly camera lens and pointing controls.
Another advantage of this system is that it uses only passive sensors, limiting the amount of power
required. Also, the system requires only a passive target so that rendezvous is possible with non-
powered or dysfunctional satellites and systems.
Another advantage of the stereo vision approach is that the system designer has many design trade-
offs that can be used to "scale" the system to a particular application (specific baseline, range
accuracy, etc.). For example, the field of view of the cameras can be narrowed to increase the
operational range of the sensor at the expense of limiting the minimum range at which the system
can operate. Other variables that can be altered include camera resolution, baseline distance
between the cameras, and vergence angle of the camera pair. Also, the system processor can be
replaced with a more powerful one if faster update rates are necessary.
w
Binocular stereo has some limitations that need to be addressed when considering the sensor
requirements. One of the key limitations of this system is the limited operational range of the
sensor and the limited range accuracy at longer distances. Range is calculated using a triangle
whose vertices are the two carner_ and the target point. As the target point moves far away from
the cameras, the triangle is elongated, and the errors in measuring the beating angles dominates the
measurement and accuracy suffers. Another reason for the limited operating range of the sensor is
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https://ntrs.nasa.gov/search.jsp?R=19930012232 2020-03-17T06:31:04+00:00Z
thatthepassivetargetmustsubtendseveralcamerapixelsbeforethetargetidentificationalgorithms
canwork. Theselimitationscanbecounteractedby properdesignof thesystem,but thetradeoffs
necessaryto achievethedesiredresultsatlongrangesmayadverselyaffectperformanceat shorter
ranges.
At JSC,wehavedevelopedanengineeringdevelopmentplatformto investigatethe issuesinvolved
in usingstereo-basedsensorsfor AR&C. We have successfully demonstrated a binocular stereo
based automated rendezvous operation using a mobile robot and a simulated earth background.
Preliminary results show range precisions on the order of 0.25% of range at close ranges. Detailed
error calculations will be done in the near future.
2
A Comparison of Laser-Based Ranging Systems for AR&C
3.4 2 2 bys ph ,J.K = rgandPl0  an,W. rvLangley Research Center
The three most common types of laser target ranging (time-of-flight, tone modulated, and FM-CW)
are discussed first as far as principle of operation is concerned. The first, time-of-flight, is shown
to depend more on the ability of support electronics than the fundamentals of the concept.
Examples in the literature are cited to show the remarkably good performance of time-of-flight
systems. A system developed in Finland has been shown to have an incremental ranging capability
of a few millimeters. The system is notonly robust; it does not include additions such as reference
legs and the like, which are expected, for improvement in performance.
Though not immediately obvious, the second technique, tone modulated ranging is somewhat of an
extension of the time-of-flight technique. Essentially the tone systems use the phase shift of the
reflected laser (or LED, for that matter) power to determine the range. Since phase shift is just time
shift times the "two PI per period" of the modulation, then this technique is directly analogous to
the time-of-flight method already discussed, Limitations on the use of very high modulation
frequencies for improved incremental accuracy in this technique arise from the ambiguity in total
phase after one period. Tone modulated systems are fairly common, commercial versions have
been developed. A simple analysis is presented to show the kind of performance that might be
expected from the tone modulated sYS_tems.
Third, the FM-CW technique is presented. This is completely different from the tone or time-of-
flight techniques, but is a direct translation of the same-named technique used in radar ranging
systems. One major advantage for this application that lasers have over the RF version is precise
aiming and lack of multipath interference. In this technique, the source must be a rapidly tunable
laser source without mode hops of the main oscillating mode. The laser is chirped over its useful
tuning range and transmitted to the target. After return from the target, the laser is heterodyne
detected and processed. Since the round trip time to the target causes a delayed replica of the
transmitted wave, heterodyne detection gives a beat frequency whose value is related to the tuning
rate of the laser and the round-trip time. Knowledge of the tuning rate of the laser gives the range
through utilization of frequency counters or detectors. It is noted that the FM-CW technique
benefits from two factors: high signal-to-noise in heterodyne detection and much higher accuracy
from the lack of phase ambiguity limitations. Performance analys_s isgiven for comparison wi_
the other techniques and some typical results are given to show how very accurate the FM-CW
laser radar concept can be. As is typically the case, there are competing factors that limit the actual
performance possible for FM-CW laser radars can achieve. One of the most important is the effect
of lack of coherence length in the types of lasers capable of the wide tuning ranges needed. The
effect manifests itself, not only in the loss of signal from coherence loss, but more importantly, the
appearance of quantum phase noise associated coherence loss. Quantum phase noise represents a
jitter in the phase part of the laser output, and although it is only phase and not amplitude, the effect
is the same. Moreover, the phase noise is proportional to laser field amplitude squared, as
15


A binocular stereo approach to AR/C at the Johnson Space Center

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