The Global Positioning System (GPS) provides the following: (1) position and velocity determination to support vehicle GN&C, precise orbit determination, and payload pointing; (2) time reference to support onboard timing systems and data time tagging; (3) relative position and velocity determination to support cooperative vehicle tracking; and (4) attitude determination to support vehicle attitude control and payload pointing

Saunders, Penny E.

English

NASA Technical Reports Server

needs. What is the range accuracy? Two (2) percent of the range. Can tilt and roll could be
determined? Yes, it can, using the Synthetic Estimation Filter.
,,,'/r3:0:'_/,;/ Office of Spaceflight
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Standard Spaceborne Global Positioning System
User Equipment Project
by Penny E. Saunders, NASA, JSC
(GPS)
The Global Positioning System (GPS) provides: 1) position and velocity determination to support
vehicle GN&C, precise orbit determination and payload pointing; 2) time reference to support
onboard timing systems and data time tagging; 3) relative position and velocity determination to
support cooperative vehicle tracking; and 4) attitude determination to support vehicle attitude
control and payload pointing.
The expected GPS performance depends on system implementation, hardware design, software
design, and GPS service. The standard spaceborne GPS user equipment project was begun by
JSC to identify the benefits of a standard GPS receiver development for the STS, SSF, and ACRV
and the potential benefits to other NASA programs. The Office of Space Flight then recommended
the collection of NASA-wide GPS requirements and preparation of a project plan including cost
and schedule estimates. A common GPS system can reduce development, maintenance, and
logistics costs.
There are two services provided by GPS, standard and precise. Some commercial applications
have achieved centimeter level accuracy using GPS. However, commercial off-the-shelf systems
cannot handle the high velocities and Doppler shift. Nevertheless, ACRV, CTV, SSF, and STS
can use militarized systems that use Coarse Acquisition, the standard service.
A task team of representatives from JSC, MSFC, GSFC, and JPL was formed to develop a
Standard Spaceborne GPS Receiver. The design approach is to maximize the use of available
hardware and software, minimize user program integration cost, and provide upgrade options for
unique requirements and anticipated future requirements. The design also should reflect: a
modularized approach in receiver architecture for incorporation of user-unique requirements,
addition of NASA unique command, control and data interfaces, and modified software to
accommodate vehicle dynamics; addition of NASA's safety, reliability, and quality standards; and
addition of radiation protection.
The project consists of three phases: the definition phase, development phase, and production
phase. The definition phase should be completed in September of 1993, the development phase
completed in December of 1995, and the production phase starting in January of 1996 and
continuing until completion of the last flight unit that is needed.
The GPS application to AR&C consists of placement on both the chaser vehicle and the target
vehicle. It includes a communication link between the vehicles so that relative position data can be
determined. The expected performance depends on: (1) number of common satellites; (2) receiver
measurement accuracy and measurement types; (3) relative state processing algorithms, (4)
receiver/processor commonality; and (5) type of GPS service used.
In summary, standard GPS User Equipment development ensures commonality and coordination
between user programs, thus providing overall NASA cost reduction and improvement in relative
tracking capability. The cost reduction is due to the one-time versus multiple nonrecurnng
engineering efforts, quantity purchases of flight units, and consolidated engineering supporting
development, logistics, operations, and maintenance. The use of a common hardware and
software design results in the improvement in relative tracking capability and also improves
accuracy and simplifies interfaces.
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Concernsdiscussedfollowing the presentation: When the author mentionedthat there is an
agreementfor scarring SSFfor a GPSreceiver the audiencerespondedenthusiastically. A
question from the floor cameup regardingthe issueof putting GPSreceivers in a vacuum
environmentandwhether the issuewasaddressedin the standardGPSrequirements. It was
answeredthat this issue had not been addressedbut the presentrequirements assumean
atmosphericenvironment.A commentfrom thefloor wasthattheExplorerplatformwill fly next
yearandit will haveaGPSreceiveronboardoperatingin a vacuum.A final questionwas:Are the
differentaccelerationsof SSFandthechasevehicleaproblem?Theanswerwasthatacceleration
is generallynot aproblem.Velocity is thedriverbecauseof theDopplershift.
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Autonomous Pre-alignment of a Docking Mechanism /_" _' 1_"1
by Monty B. Carroll and John A. Thompson N 9 3 "/_ _'_--_9, ,,I" 8
Lockheed Engineering & Sciences Co.
The subject project can be described as the development and testing of a digitally controlled t
docking mechanism. The mechanism consists of a 6 DOF parallel manipulator for docking r"
interface pre-alignment, and a machine vision sensor for real-time target tracking. The parallel
manipulator also can be used for capture/latching, energy attenuation, and structural rigidization of
docking, but the scope of this paper is the proof-of-concept demonstration of autonomous pro-
alignment of a docking mechanism using machine vision.
The docking mechanism incorporates 8 linear actuators in tandem attached to a lower and an upper
ring. The lower ring is stationary while the upper ring maneuvers the mechanical docking interface
in three-dimensional space. Each linear actuator of the manipulator is position controlled by a
dedicated digital servo controller and receives actuator length commands from a central IBM PC
based master controller. The master controller oversees operation of the linear actuators and
receives real-time position and orientation data from either a machine vision processor or a manual
6 DOF input device. The machine vision system, a gray-scale and binary vision system, senses
an optical target on the passive docking interface via a CCD camera on the docking mechanism's
stationary base. It then provides real-time position and orientation commands to the master
controller. The master controller kinematically resolves position lengths of each actuator and
moves the docking interface to align with the approaching target. A host or user interface to the
master controller provides the start, stop, reset, or mode commands, and displays current
manipulator position.
The demonstration of the autonomous prealignment of a docking mechanism was a success,
showing a full scale working prototype utilizing machine vision. System response was estimated
at 5 hertz. Docking interface alignment was accurate to approximately 20 mm (0.75 inches). It is
believed that these system parameters could be considerably enhanced by several minor hardware
and software upgrades. The machine vision accuracy was dependent upon the distance of the
target from the CCD camera. The positional accuracy obtained for each of target sphere was about
one percent of the actual distance. Additionally, it was noted that lighting condition variations
greatly influenced vision tracking accuracy. This did not affect the demonstration greatly, since
constant lighting could be maintained. However, a more robust tracking algorithm, less sensitive
to lighting, was determined possible.
The technology demonstrated in this project has the potential to improve the efficiency and
configuration of a docking system. A sensor controlled docking mechanism could have smaller
capture guides and shorter attenuator strokes, thereby reducing weight. The capture and
attenuation loads would be reduced because of the precise method of alignment. Future work is to
include the study of the remaining phases of docking; i.e., capture/latching, attenuation, and
structural rigidization. In addition, upgrades to hardware and software are being considered.
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Office of Spaceflight Standard Spaceborne Global Positioning System (GPS) user equipment project

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