Autonomous on-board orbit control was developed, first and foremost, to improve orbital
operations, principally by reducing operations costs. It accomplishes this improvement
by eliminating the planning burden on the mission operations team. In addition, because
the orbit phase and other orbital elements are held invariable, the position of the
spacecraft is known at all future times, and a new array of mission applications is opened
up, especially related to communications, mission planning, installation, maintenance,
and hazard avoidance. Another significant benefit is a reduction in orbit maintenance
The MicroMak device is a new, high-precision, very compact star sensor weighing less than 100 grams, with three independent 4-degree square fields of view. The collection telescope is a Maksutov design that incorporates three telescopes into a single sensor head. The sensor is designed for star identification and spacecraft attitude determination with a device that offers unprecedented low cost, volume and mass. While star trackers have achieved sub-arcsecond accuracy by utilizing sophisticated algorithms and complex hardware, the MicroMak sensor must rely on fairly efficient algorithms that utilize data from only the image sensor. This paper will discuss the attitude determination algorithm as well as a complete end-to-end simulation of the system that was used to optimize the design and predict performance. This simulation accepts various star and sensor parameters as inputs, and generates error estimates of attitude of the sensor. The inputs include color temperatures and magnitudes of stars, focal length, receiver aperture, reflectivity curves of mirrors, modulation transfer function of the telescope system, vignetting effects, jittter characteristics, spacecraft spin rate and spin axis, detector pixel size, read noise, dark noise, sensor update rate, quantum efficiency as a function of wavelength, and detector fill factor. A complete forward model of the optical train has been built, and used with a maximum likelihood estimator to generate estimates of sensor attitude. A Monte Carlo algorithm was used to generate error distributions on the attitude error given the noise and distortions injected into the measurement.
Tropospheric wind measurements are of great meteorological and tactical value, but are presently not available on a global basis. The primary obstacle to a space-based Doppler wind LIDAR mission capable of obtaining these measurements has been the cost and risk associated with flying high power lasers and large telescopes in low-earth orbit. This paper presents an alternative approach that would result in a low-cost, low-risk responsive approach to deploying a global tropospheric wind measurement system.
In general, autonomous rendezvous and docking requires that two spacecraft start at a remote distance (i.e., out of sight of each other), come together into a common orbit, rendezvous, dock, and control the new combined spacecraft in both orbit and attitude. Doing this requires developing and testing a variety of new technologies including absolute and relative autonomous navigation, autonomous rendezvous and docking hardware and software (both sensors and actuators), and autonomous control of a "new" spacecraft with different mass and inertia properties than either of the two original spacecraft. While these are very workable technologies, they do require a significant change in mindset -- turning over control of thrusters and other actuators to an on-board computer. While there is substantial potential for cost savings, risk reduction, and new mission modes by use of these technologies, there is a very strong reticence to allowing operational spacecraft to control their own destiny, particularly in firing thrusters.
This paper summarizes work at Microcosm and elsewhere in each of the above technologies. Autonomous navigation and absolute orbit control have been demonstrated on orbit. In conjunction with Michigan Aerospace, autonomous rendezvous and docking hardware and algorithms have been demonstrated in parabolic flights and zero-g simulations. Approaches have been proposed for more precise and robust autonomous navigation and autonomous on-orbit estimation of combined mass and inertia properties, leading to efficient orbit and attitude control of the combined spacecraft. Many of these technologies can be tested at low cost in parabolic flights, suborbital flights, and evaluation of data from existing or planned missions. Thus, a "coordinated attack" on the complete problem of fully autonomous rendezvous and docking is both feasible and potentially very low cost.
Conference Committee Involvement (2)
Enabling Sensor and Platform Technologies for Spaceborne Remote Sensing
9 November 2004 | Honolulu, Hawai'i, United States