Martin W. Regehr

The formation control testbed

Terrestrial planet finder (TPF) is a space telescope mission which performs spectral analysis of the infrared emissions from extrasolar planets, and which searches for carbon-based life on such planets. One configuration being considered for this mission is a stellar interferometer with several collectors and a combiner on separate spacecraft flying in a tightly controlled formation. The distance to earth for this mission are sufficiently great that having ground in the loop for reconfiguration or collision avoidance maneuvers impractical. Moreover, because of constraints in the orientation of the spacecraft relative to the sun, limitations on the field of view of relative range and bearing sensors, and restrictions on the orientations of thrusters, both the attitude and the relative position of each spacecraft in the formation must be taken into account in the event of a temporary sensing or control fault during maneuvers. These maneuvers include initial deployment of the formation, reconfiguration, and collision avoidance maneuvers. The formation algorithms and simulation testbed (FAST) and the formation control testbed (FCT) at JPL are being built to simulate and demonstrate 6 degree of freedom, autonomous formation flying and reconfiguration for TPF. The testbeds are complementary. Control algorithms simulated in the FAST are tested in the FCT in order to validate the FAST. This paper describes the design and construction of the formation control testbed. The FCT consists of three robots navigating on an air bearing floor, propelled by cold gas thrusters. Each robot contains an attitude platform supported on a spherical air bearing which provides three rotational degrees of freedom. The sixth degree of freedom, vertical translation, is provided by a powered vertical stage, actively controlled to provide a simulated zero-g environment for the attitude platform.

ADAPT demonstrations of onboard large-divert Guidance with a VTVL rocket

The Autonomous Ascent and Descent Powered-Flight Testbed (ADAPT) is a closed-loop, free-flying testbed for demonstrating descent and landing technologies of next-generation planetary landers. The free-flying vehicle is the Masten Space Systems Xombie vertical-takeoff, vertical-landing suborbital rocket. A specific technology ADAPT is demonstrating in the near-term is Guidance for Fuel-Optimal Large Diverts (G-FOLD), a fuel-optimal trajectory planner for diverts during powered descent, which is the final kilometers of descent to landing on rocket engines. Previously, ADAPT used Xombie to fly optimal large-divert trajectories, extending Xombies divert range to 750 m. However, these trajectories were planned off-line with G-FOLD. This paper reports the successful Xombie flight demonstrations of large diverts using G-FOLD on board to calculate divert trajectories in real time while descending. The culminant test flight of the last year was an 800 m divert that was initiated at an altitude of 290 m while moving away from and crosswise to the landing pad. Hence, G-FOLD had to calculate a constrained divert trajectory that reversed direction, was fully three-dimensional, with horizontal motion nearly three times the initial altitude, and it did so in ~100 ms on board Xombie as it was descending. Xombie then flew the divert trajectory with meter-level precision, demonstrating that G-FOLD had planned a trajectory respecting all the constraints of the rocket-powered vehicle. The steps to reach this flight demonstration of on-board generation of optimal divert trajectories and the system engineering for future ADAPT payloads are also presented.

Micro-arcsecond metrology (MAM) testbed overview

One of the most critical technology requirements for the Space Interferometry Mission is that the difference in pathlength traveled by the starlight through each arm of the instrument be known with picometers of precision. SIM accomplishes this by using an internal laser metrology system to measure the optical path traveled by the starlight. The SIM technology program has previously demonstrated laser gauges with measurement accuracy below 10 picometers. The next challenge is to integrate one of these gauges into a full interferometer system and demonstrate that the system still operates at the required level. For SIM, the ultimate requirement is that the internal metrology system be able to give an accurate measure of the starlight internal path difference to about 150 picometers over its narrow-angle field, with a goal of 50 picometer accuracy. This accuracy must be maintained even as SIMs various active systems articulate the SIM optics and vary the SIM internal pathlengths. The Microarcsecond Metrology Testbed (MAM) is a full single-baseline interferometer coupled with a precision pseudostar, intended to demonstrate the level of agreement between starlight and metrology phase measurements needed to make microarcsecond-level measurements of stellar positions. MAM has been under development for several years and is now producing picometers-level consistency that translates into microarcseconds-level performance. This paper will present an overview of the MAM Testbed, together with recent results targeting the 150 picometer performance level required by SIM.

Automatic alignment of a displacement-measuring heterodyne interferometer

A technique to align automatically the beams of displacement-measuring interferometric gauges is described. The pointing of the launched beam is modulated in a circular pattern, and the resulting displacement signal is demodulated synchronously to determine the alignment error. This error signal is used in a control system that maintains the alignment for maximum path between a pair of fiducial hollow-cube corner retroreflectors, which minimizes sensitivity to alignment drift. The technique is tested on a developmental gauge of the type intended for the Space Interferometry Mission. The displacement signal for the gauge is generated in digital form; and the lock-in amplifier functions of modulation, demodulation, and filtering are all implemented digitally.

Flight Testing of Terrain-Relative Navigation and Large-Divert Guidance on a VTVL Rocket

Since 2011, the Autonomous Descent and Ascent Powered-Flight Testbed (ADAPT) has been used to demonstrate advanced descent and landing technologies onboard the Masten Space Systems (MSS) Xombie vertical-takeoff, vertical-landing suborbital rocket. The current instantiation of ADAPT is a stand-alone payload comprising sensing and avionics for terrain-relative navigation and fuel-optimal onboard planning of large divert trajectories, thus providing complete pin-point landing capabilities needed for planetary landers. To this end, ADAPT combines two technologies developed at JPL, the Lander Vision System (LVS), and the Guidance for Fuel Optimal Large Diverts (G-FOLD) software. This paper describes the integration and testing of LVS and G-FOLD in the ADAPT payload, culminating in two successful free flight demonstrations on the Xombie vehicle conducted in December 2014.

Autonomous access links using laser communications

We have validated an autonomous acquisition scheme that is critical for achieving data transfer over proximity links with ranges up to a few thousand kilometers. The sun-illuminated International Space Station (ISS) against a dark sky background during terminator passes over Southern California was used to validate the autonomous acquisition and tracking scheme. A root mean square (rms) accuracy of 83 μrad was achieved.

Emulating an Optical Planetary Access Link with an Aircraft

Video imagery was streamed from the ground to an aircraft using a free-space laser communication link. The link operated at 270 Mb/s over slant ranges of 5-9 km in day and night time background conditions. The experiment was designed to demonstrate autonomous link acquisition and served as a first proof-of-concept for a planetary access link between a surface asset and an orbiter at Mars. System parameters monitored during the link demonstration including acquisition and tracking and communication performance are discussed.

Pointing performance of an aircraft-to-ground optical communications link

We present results of the acquisition and pointing system from successful aircraft-to-ground optical communication demonstrations performed at JPL and nearby at the Table Mountain Facility. Pointing acquisition was accomplished by first using a GPS/INS system to point the aircraft transceivers beam at the ground station which was equipped with a wide-field camera for acquisition, then locking the ground station pointing to the aircrafts beam. Finally, the aircraft transceiver pointing was locked to the return beam from the ground. Before we began the design and construction of the pointing control system we obtained flight data of typical pointing disturbances on the target aircraft. We then used these data in simulations of the acquisition process and of closed-loop operation. These simulations were used to make design decisions. Excellent pointing performance was achieved in spite of the large disturbances on the aircraft by using a direct-drive brushless DC motor gimbal which provided both passive disturbance isolation and high pointing control loop bandwidth.

Dynamic characterization of a prototype of the Thirty Meter Telescope primary segment assembly

Finite element models (FEMs) are being used extensively in the design of the Thirty Meter Telescope (TMT). One such use is in the design and analysis of the Primary Segment Assembly (PSA). Each PSA supports one primary mirror segment on the mirror cell, as well as three actuators, which are used to control three degrees of freedom - tip, tilt, and piston - of the mirror segment. The dynamic response of the PSA is important for two reasons: it affects the response of the mirror to fluctuating wind forces, and high-Q modes limit the bandwidth of the control loops which drive the actuators, and impact vibration transmissivity, thereby degrading image quality. We have completed a series of tests on a prototype PSA, in which the dynamic response was tested. We report on the test methods used to measure the dynamic response of the PSA alone and with candidate actuators installed, and we present comparisons between the measured response and FEM predictions. There is good agreement between FEM predictions and measured response over the frequency range within which the dynamic response is critical to control system design.

Optical path control in the aiam testbed

818-354- 1693 Abstract-Future space-based optical interferometers will require control of the optical path delay to accomplish some or all of three objectives: balancing the optical path in the two arms to within a tolerance corresponding to the coherence length of the star light being observed, modulating the optical path in order to observe the phase of the star light interference fringe, and modulating the path length in order to reduce the effect of cyclic errors in the laser metrology system used to measure the optical path length in the two arms of the interferometer. In the Micro- Arcsecond Metrology (MAM) test bed, three types of actuator are used to control the optical path delay: a coarse actuator consisting of a stepper-motor-driven translation stage supporting a corner cube, a balanced voice coil modulator driving a flat mirror, and two flat mirrors mounted on tripod PZT actuators. This paper describes the mechanical and electronic designs used in these actuators and the software algorithms developed to control them. The coarse actuators primary function is to search for the point at which the optical path length in the two arms is balanced. This is accomplished using a software algorithm, which spirals the optical path back and forth over an ever-increasing range until fringes are observed on a CCD detector, which monitors the interfered star light. The control algorithm also provides means for moving rapidly to either limit of the translation stage, to its home position (sensed by a dedicated home sensor), or to the position at which fringes were last observed. The voice coil actuator provides modulation for phase detection and it provides fine control of the optical path difference. The desired path-length modulation is a triangle wave, which is rounded slightly at the points where the velocity of the mirror changes sign. This modulation is approximated using a drive waveform tailored to counteract the internal forces in the modulator and to produce no net acceleration except during the turnarounds. Two mirrors on tripod PZT actuators are driven with commands intended to provide alignment control, together with additional path length modulation over exactly one wavelength of the laser metrology system. The signals for these two functions are combined using sohare running in a real-time computer.