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Formation Metrology

Task Objective

Prototype transmit/receive sensor node
Prototype transmit/receive sensor node
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 optical beam.

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.

Task Description

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.

Packaged MMR
Packaged MMR
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
FCT Optical Hardware

Task Objective

Provide the FCT robots a direct sensing capability of their relative positions with more accuracy.

Task Description

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.

OPL Benchtop




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