foreign
for years
the dream of conquering the space
frontier has continued to become reality
an increased level of operations in this
challenging environment will require
frequent docking of orbiting vehicles
to achieve this
a safe rendezvous and docking mechanism
is essential
an accurate docking sensor would
minimize plume impingement
increase fuel efficiency
simplify docking mechanisms
and increase safety and reliability
more importantly
the system would be capable of operating
in a fully autonomous mode
this would minimize the necessary crew
interaction during docking
various optical docking sensors have
been proposed which provide information
on range bearing angles
attitude and their respective rates
accuracies for these sensors are fairly
stringent
as a result
a system is being designed by the
tracking and communications division at
nasa's johnson space center to test the
accuracies of these docking sensors
this system
designated the six degree of freedom or
six doff system
provides six degrees of movement by
using five rotational stages
and one linear stage
range
bearing angles
attitude and their respective rates are
provided
range is furnished by a granite rail 12
meters in length with a carriage
floating on three air bearings
the maximum change in range travel
distance is 10.5 meters while rates can
vary from 5 millimeters per second to
400 millimeters per second
mounted inside the granite rail is a
linear encoder with a resolution of 5
microns
the gratings on the encoder are
photoelectrically scanned and counted to
keep track of absolute position
two rotational stages on top of the
range carriage provide azimuth
and elevation
referred to as the sensor gimbal mount
this entire assembly can safely support
a sensor weighing up to 40 kilograms
here a laser is mounted on the sensor
gimbal mount to simulate a docking
sensor prototype
a separate movable granite table
designated the target gimbal mount
is offset from the granite rail
three rotary stages provide yaw
roll
and pitch
all five rotational stages contain
rotary encoders that possess a
resolution of one one thousandth of a
degree
a retro reflector assembly would be a
typical target attached to the target
gimbal mount
a 19-inch rack houses the equipment
necessary to operate the sixth off
stages
included is a computer containing the
control software which drives the stages
through a central interface
the six dof subsystems are also
contained in this rack
including a global positioning system
time receiver and a rate meter
these instruments provide time tagging
and calculation of absolute and relative
rates of each of the stages
okay
operation of the sixth system is based
on knowing the initial coordinates of 14
reference points on the two gimbal
mounts and commanding the stages to move
to a desired range and attitude at a
specified rate
in order for all position and rate
measurements to be relative to each
other
a common coordinate system was defined
a digital metrology system is used to
shoot individual targets and calculate
position coordinates
these coordinates are then entered into
the control computer software
the stages are commanded to move
the laser spot searches for the target
the proximity of the laser spot to the
desired target determines the accuracy
of the sensor prototype
in a typical testing scenario
static and dynamic accuracies will be
tested on a candidate optical docking
sensor
position information taken from the
sensor will be compared to the true six
dot position
dynamic information is provided by
comparing the tracking capability of the
docking sensor to that of the more
accurate six dof system
future enhancements of the six dof
system
include a proposed expansion into an
autonomous rendezvous and docking test
facility
here docking scenarios will be set up to
gain knowledge on docking sensors
this will enable development of an
optimum docking sensor that could be
used in a multitude of docking
applications
a potential application of the six dof
system would support the shuttle and
space station docking simulator
currently astronauts practice docking
the shuttle to the space station by
using visual cues
if this system was connected to the
simulator it could move using the
relative range
bearing angles and attitude as
calculated by the six dof system
this would enable the simulator to
operate in a completely autonomous mode
from enhancing astronaut training
to docking with space station freedom
to aiding orbital assembly
the accuracy of the six degree of
freedom system will provide an important
service
as we continue to reach out beyond
earth's boundaries
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