Formation flying constellations perform an intricate choreography,
where each spacecraft is required to precisely know the position
and orientation of its partners. Direct sensing of relative
range and bearing is essential to enable position and attitude
control of each spacecraft in the formation, especially in deep-space
scenarios, where the benefit of the GPS is not available. Such
relative sensors need to have an operating range from few meters
to at least few kilometers. Advanced optical ranging techniques
are uniquely positioned to provide this information in a manner
that is non-interfering to the primary data acquisition goals
of the formation mission, due to the well defined paths of an
Prototype transmit/receive sensor node
The formation metrology task is developing
a range and bearing metrology system that operates in the
near-infrared spectral region. This system will allow precise
determination of range to much better than one millimeter
and relative angular attitude to better than one arc-minute.
As a bonus, the same optical ranging beam can also carry bi-directional
data for inter-spacecraft communications.
The optical formation metrology system senses inter-spacecraft range
and bearing by measuring the time-of-flight and relative phase
of a coded laser beam between spacecraft. Each spacecraft
transmits its own coded sequence, which is reflected from
multiple retroreflectors mounted on another spacecraft. Normally,
the multiple retroreflectors, which are required to determine
bearing, would generate ambiguous overlapping return signals.
However, each retroreflector is equipped with a high-speed
modulator based upon a multiple quantum well (MQW) device
developed by the Navel Research Laboratories, converting it
to a Modulated Retro-Reflector (MRR). Each MRR then effectively
functions as a ‘dynamic’ bar code, allowing the
return beams to be selectively decoded. With the knowledge
of the mounting geometry of the transmit laser and the reflecting
MRR devices, and by measuring the round trip time-of-flight
coding correlations, inter-spacecraft range and bearing can
be computed in real time.
We are presently working on a laboratory demonstration of this
concept operating near 980 nanometers using two MRR targets.
The laser transmitter is modulated with a Pseudo-Noise (PN)
pattern and the MRR targets are asynchronously modulated with
orthogonal identification codes The retroreflected return signals
are collected by an off-axis parabolic reflector and imaged
onto a large-area photodetector. The receive signal is then
amplified, digitized and processed to extract the range and
one-axis bearing measurements.
As the next step, a fully
integrated system will be demonstrated in a dynamic test environment:
the TPF Formation Control Testbed. Here, multiple spacecraft
simulators with a full six degrees of freedom will be equipped
with the optical MRR formation metrology system and integrated
into the control system to show real time range and bearing
in a realistic dynamic setup.
FCT Optical Hardware
Provide the FCT robots a direct sensing capability of their relative positions with more accuracy.
The Optical Pointing Sensor is a laser based sensor system that
gives relative position information to the FCT robots. The sensor
system consists of two components. A laser scanner is placed on
one robot and a passive retroreflector is placed on the other robot.
The position information is provided in the form of range and bearing
to this retro target. The system uses two laser sources. An off
the shelf IR laser rangefinder is used to give range measurements
based on time of flight. A second 633nm source is used in conjunction
with a fast steering mirror (FSM) and shear sensor on the backend
of the optical path to control and detect, respectively, the shear
at the retro target. Bearing information is sampled from a local
angle sensor mounted on the gimbal of the FSM.
Models of the FSM, optical path and shear sensor have been developed.
These models have been used to demonstrate an end to end simulation
of the system. This simulation testbed is also used for compensator
design and code development. Both simulated and benchtop experimental
data have demonstrated an open loop spiral search of the retro
target, detection and lock up of the pointing system and tracking
of a moving retro target.
This sensor system is capable of providing centimeter level range
accuracy and 3.0 arcminute bearing accuracy.