Jane C. Pavlich

Dynamic simulation and validation of a satellite docking system

In recent years, Michigan Aerospace has approached the problem of gentle autonomous spacecraft rendezvous and docking using a flexible soft-dock cable that is extended from the docking spacecraft to the target spacecraft. Because of the nature of a soft-dock cable, testing and validation of the technology is difficult in normal gravity. To properly emulate the behavior of this soft-dock cable, we have performed dynamic computer simulations so that the effects of micro-gravity could be simulated. The Autonomous Satellite Docking System (ASDS) was initially prototyped and tested at Marshall Space Flight Center’s air-bearing floor facility. The test data was compared to the simulations and used to validate the model. Once a good correlation between the simulation’s predicted results and the actual data was shown, the model was used to predict future performance of the ASDS mechanism on several potential spacecraft for the Orbital Express program. A new dynamic simulation model was created and compared to test data from a recent KC-135 flight test to further validate the modeling approach used. This paper will describe the methodology used in modeling and simulating the ASDS mechanism. Correlation between the models and the test data will be discussed.

Advancements in KC-135 microgravity testing of an autonomous satellite docking system

During the development of an autonomous spacecraft docking mechanism, one of the primary areas of interest in the way the mechanism will behave in a micro-gravity environment. This issue is of particular interest when a flexible soft-dock cable is used to make initial capture, because ground-based testing does not adequately represent the environmental conditions that will be seen on orbit. To this end, Michigan Aerospace Corporation has recently conducted flight tests of its prototype autonomous satellite docking system in a micro-gravity environment on the KC-135 in conjunction with the Air Force Research Laboratory Space Vehicles Directorate and Microcosm, Inc. Though the first flight was primarily for the purpose of testing the core operating principles of the docking mechanism, several lessons were learned that will be applied toward developing a second, more advanced prototype and experimental setup intended for a second series of flights on the KC-135. Areas of improvement for the new flight test will be in the physical operation of the experimental apparatus and the data collection methods used. The use of redundant sensors as a means of eliminating noise will be explored, as will the merits of using a combination of coarse and fine sensors to collect data over a broader measurement range.

Performance and comparison of 532-nm and 355-nm groundwinds lidars

GroundWinds 2nd Generation (2nd Gen.) New Hampshire (NH) and GroundWinds Hawaii (HI) are direct detection Doppler LIDAR instruments that operate at 532nm and 355nm, respectively. These ground based incoherent LIDARs utilize backscatter from Rayleigh and Mie scattering to measure Doppler shifts in the atmosphere. The NH and HI instruments routinely make wind measurements from 0.5 to 15 kilometers and achieve sub-meter per second accuracies in the lower troposphere. This paper will provide a brief review of each instrument, and detail the instruments performance and achievements in wind measurement.

GroundWinds New Hampshire and the LIDARFest 2000 campaign

The GroundWinds New Hampshire instrument is a direct detection Doppler LIDAR system that utilizes backscatter signal from both Rayleigh and Mie scattering to measure Doppler shifts in the atmosphere from the ground. This system is the first of two planned systems that will be used to validate the technology and improve the design for other potential implementations. As a means to that end, a validation campaign was conducted in September 2000 to compare the GroundWinds measurements to that from four other systems. These were the GLOW instrument, the NOAA Mini MOPA system, and a Microwave sounder from the National Weather Service. This paper will review the design of the GroundWinds instrument, as well as summarize some of the preliminary GroundWinds results from the field experiment.

Advancements in design of an autonomous satellite docking system

The past five years has witnessed a significant increase in the attention given to on-orbit satellite docking and servicing. Recent world events have proven how we have come to rely on our space assets, especially during times of crisis. It has become abundantly clear that the ability to autonomously rendezvous, dock, inspect and service both military and civilian assets is no longer a nicety, but a necessity. Reconnaissance and communications satellites, even the space shuttle and International Space Station, could benefit from this capability. Michigan Aerospace Corporation, with funding from the Defense Advanced Research Projects Agency (DARPA) and the Air Force Research Laboratory (AFRL), has been refining a compact, light, compliant soft-docking system. Earlier prototypes have been tested on the Marshall Space Flight Center (MSFC) flat-floor as well as on the Johnson Space Flight Center (JSC) KC-135 micro-gravity aircraft. Over the past year, refinements have been made to the mechanism based on the lessons learned from these tests. This paper discusses the optimal design that has resulted.

Space-based direct detection wind mission design

There is an important need for accurate measurements of tropospheric wind altitude profiles. These wind systems have long been recognized as one of the primary unknowns limiting weather forecasting over timescales of several days. Typical measurement architectures have focused primarily on space-based approaches, using a high-powered and highly effective Light Detection and Ranging (lidar) system. This paper discusses architectures for low-altitude space missions. The architectures are analyzed in the context of a weather forecasting system for the Gulf of Mexico region during hurricane season. The architecture studies were developed by collaboration between a class of engineers who are part of the University of Michigans new Space Engineering program and Michigan Aerospace Corporation, a University of Michigan spin-off company specializing, in part, in lidar systems.

An imaging neutron spectrometer

We have developed, fabricated and tested a prototype imaging neutron spectrometer designed for real-time neutron source location and identification. Real-time detection and identification is important for locating materials. These materials, specifically uranium and transuranics, emit neutrons via spontaneous or induced fission. Unlike other forms of radiation (e.g. gamma rays), penetrating neutron emission is very uncommon. The instrument detects these neutrons, constructs images of the emission pattern, and reports the neutron spectrum. The device will be useful for security and proliferation deterrence, as well as for nuclear waste characterization and monitoring. The instrument is optimized for imaging and spectroscopy in the 1–20 MeV range. The detection principle is based upon multiple elastic neutron-proton scatters in organic scintillator. Two detector panel layers are utilized. By measuring the recoil proton and scattered neutron locations and energies, the direction and energy spectrum of the incident neutrons can be determined and discrete and extended sources identified. Event reconstruction yields an image of the source and its location. The hardware is low power, low mass, and rugged. Its modular design allows the user to combine multiple units for increased sensitivity. We will report the results of laboratory testing of the instrument, including exposure to a calibrated Cf-252 source. Instrument parameters include energy and angular resolution, gamma rejection, minimum source identification distances and times, and projected effective area for a fully populated instrument.

A Portable Neutron Spectroscope (NSPECT) for detection, imaging and identification of nuclear material

We have developed, fabricated and tested a prototype imaging neutron spectrometer designed for real-time neutron source location and identification. Real-time detection and identification is important for locating materials. These materials, specifically uranium and transuranics, emit neutrons via spontaneous or induced fission. Unlike other forms of radiation (e.g. gamma rays), penetrating neutron emission is very uncommon. The instrument detects these neutrons, constructs images of the emission pattern, and reports the neutron spectrum. The device will be useful for security and proliferation deterrence, as well as for nuclear waste characterization and monitoring. The instrument is optimized for imaging and spectroscopy in the 1-20 MeV range. The detection principle is based upon multiple elastic neutron-proton scatters in organic scintillator. Two detector panel layers are utilized. By measuring the recoil proton and scattered neutron locations and energies, the direction and energy spectrum of the incident neutrons can be determined and discrete and extended sources identified. Event reconstruction yields an image of the source and its location. The hardware is low power, low mass, and rugged. Its modular design allows the user to combine multiple units for increased sensitivity. We will report the results of laboratory testing of the instrument, including exposure to a calibrated Cf-252 source. Instrument parameters include energy and angular resolution, gamma rejection, minimum source identification distances and times, and projected effective area for a fully populated instrument.

3D sensor algorithms for spacecraft pose determination

Researchers at the Michigan Aerospace Corporation have developed accurate and robust 3-D algorithms for pose determination (position and orientation) of satellites as part of an on-going effort supporting autonomous rendezvous, docking and space situational awareness activities. 3-D range data from a LAser Detection And Ranging (LADAR) sensor is the expected input; however, the approach is unique in that the algorithms are designed to be sensor independent. Parameterized inputs allow the algorithms to be readily adapted to any sensor of opportunity. The cornerstone of our approach is the ability to simulate realistic range data that may be tailored to the specifications of any sensor. We were able to modify an open-source raytracing package to produce point cloud information from which high-fidelity simulated range images are generated. The assumptions made in our experimentation are as follows: 1) we have access to a CAD model of the target including information about the surface scattering and reflection characteristics of the components; 2) the satellite of interest may appear at any 3-D attitude; 3) the target is not necessarily rigid, but does have a limited number of configurations; and, 4) the target is not obscured in any way and is the only object in the field of view of the sensor. Our pose estimation approach then involves rendering a large number of exemplars (100k to 5M), extracting 2-D (silhouette- and projection-based) and 3-D (surface-based) features, and then training ensembles of decision trees to predict: a) the 4-D regions on a unit hypersphere into which the unit quaternion that represents the vehicle [QX, QY, QZ, QW] is pointing, and, b) the components of that unit quaternion. Results have been quite promising and the tools and simulation environment developed for this application may also be applied to non-cooperative spacecraft operations, Autonomous Hazard Detection and Avoidance (AHDA) for landing craft, terrain mapping, vehicle guidance, path planning and obstacle avoidance.

Instrument specifications and performance prediction for 2005 high altitude (30 km) balloon demonstration of GroundWinds fringe imaging Doppler lidar

The GroundWinds direct detection Doppler wind LIDARs located in NH and HI are operational, ground based, multi-order fringe imaging systems capable of detecting Doppler shifts in both Aerosol and Molecular backscatter from 0.25 km to 18 km. The technology developed through these GroundWinds programs will be incorporated and flown on a high altitude (30km) balloon in 2005. The demonstration of GroundWinds Fabry-Perot based incoherent LIDAR technology from a high altitude, downward looking platform to measure winds throughout the entire troposphere and boundary layer will be a significant milestone toward the validation of this technology. Key questions will be answered about the phenomenology of direct detection LIDAR, especially its effectiveness in the optically thick boundary layer. The extensive characterization of the 532nm GroundWinds NH and 355nm GroundWinds HI LIDARs serve as excellent reference points from which performance estimates and technology requirements can be determined to ensure a successful balloon mission. This paper will describe the baseline BalloonWinds instrument specifications; including etalon specifications, system component transmissions, transmit/receive specifications, required laser power, and detector characteristics. This paper will also present performance estimates based on model simulations that employ the baseline system specifications.