Maurizio Betto

The Bering autonomous target detection

An autonomous asteroid target detection and tracking method has been developed. The method features near omnidirectionality and focus on high speed operations and completeness of search of the near space rather than the traditional faint object search methods, employed presently at the larger telescopes. The method has proven robust in operation and is well suited for use onboard spacecraft. As development target for the method and the associated instrumentation the asteroid research mission Bering has been used. Onboard a spacecraft, the autonomous detection is centered around the fully autonomous star tracker the Advanced Stellar Compass (ASC). One feature of this instrument is that potential targets are registered directly in terms of date, right ascension, declination, and intensity, which greatly facilitates both tracking search and registering. Results from ground and inflight tests are encouraging, both with respect to robustness, speed and accuracy, and demonstrates the span and range of applications of this technology.

Star tracker and vision systems performance in a high radiation environment

A part of the payload of the second Ariane 5 prototype vehicle to be launched by Arianespace, was a small technology demonstration satellite. On October 30/sup th/, 1997, this test satellite, dubbed Teamsat, was launched into Geostationary Transfer Orbit and would as such pass the Van Allen radiation belts twice per orbit. One of the experiments onboard Teamsat was the so-called Autonomous Vision System (AVS). The AVS instrument is a fully autonomous star tracker with several advanced features for non-stellar object detection and tracking, real-time image compression and transmission. The objectives for the AVS in Teamsat were to test these functions, to validate their autonomous operation in space, and to assess the operational constraints of a high radiation environment on such processes. This paper describes the AVS experiment, and the radiation flux experienced onboard TEAMSAT. This overview is followed by examples of the radiation impact on the AVS instrument flown onboard the TEAMSAT, and finally the operations of the various countermeasures are discussed.

Radiation impacts on star-tracker performance and vision systems in space

Abstract CCD-chips are widely used in spacecraft applications, due to their inherent high resolution, linearity, sensitivity and low size and power-consumption, and irrespective of their rather poor handling of ionizing radiation. One of the experiments onboard the Teamsat satellite, the payload of the prototype Ariane 502, was the Autonomous Vision System (AVS), a fully autonomous star-tracker with several advanced vision features. The main objective of the AVS was to study the autonomous operations during severe radiation flux and after appreciable total dose. The AVS experiment and the radiation experienced onboard Team-sat are described. Examples of various radiation impacts on the AVS instrument are given, and compared to ground based radiation tests.

The Bering target tracking instrumentation

The key science instrument on the Bering satellite mission is a relative small telescope with an entrance aperture of 300 mm and a focal length between 500 and 1000 mm. The detection of potential targets is performed by one of the target scanning advanced stellar compasses (ASCs). This procedure results in a simple prioritized list of right ascension, declination, proper motion and intensity of each prospective target. The telescope itself has a dedicated ASC Camera Head Unit (CHU) mounted on the secondary mirror, largely co-aligned with the telescope. This CHU accurately determines the telescopes pointing direction. To achieve fast tracking over a large solid angle, the telescope pointing is achieved by means of a folding mirror in the optical pathway. When a prospective target approaches the telescope FOV, the ASC on the secondary will guide the folding mirror into position such that the target is inside the telescope FOV. During the telescope observation time, the ASC will constantly control the folding mirror to correctly position the target at the center of the telescope, basically performing a standard telescope tracking service. The telescope will alter the initial target acquisition track and observe the object of interest. To achieve milliarcsecond accuracy the telescope is equipped with a tip-tilt system on the secondary. The performance of the acquisition and telescope guidance has been tested and excellent noise, acquisition and settling time performance has been achieved. The operations have been verified for telescope focal lengths of 250 and 8000 mm.

The Bering Small Vehicle Asteroid Mission Concept

Abstract: The study of asteroids is traditionally performed by means of large Earth based telescopes, by means of which orbital elements and spectral properties are acquired. Space borne research, has so far been limited to a few occasional flybys and a couple of dedicated flights to a single selected target. Although the telescope based research offers precise orbital information, it is limited to the brighter, larger objects, and taxonomy as well as morphology resolution is limited. Conversely, dedicated missions offer detailed surface mapping in radar, visual, and prompt gamma, but only for a few selected targets. The dilemma obviously being the resolution versus distance and the statistics versus ΔV requirements. Using advanced instrumentation and onboard autonomy, we have developed a space mission concept whose goal is to map the flux, size, and taxonomy distributions of asteroids. The main focus is on main belt objects, but the mission profile will enable mapping of objects inside the Earth orbit as well.

The autonomous vision system on TeamSat

The second qualification flight of Ariane 5 blasted off-the European Space Port in French Guiana on October 30, 1997, carrying on board a small technology demonstration satellite called TeamSat. Several experiments were proposed by various universities and research institutions in Europe and five of them were finally selected and integrated into TeamSat, namely FIPEX, VTS, YES, ODD and the Autonomous Vision System, AVS, a fully autonomous star tracker and vision system. This paper gives short overview of the TeamSat satellite; design, implementation and mission objectives. AVS is described in more details. The main science objectives of the AVS were to verify, in space, multiple autonomous processes intended for spacecraft applications such as autonomous star identification and attitude determination, identification and tracking of non-stellar objects, imaging and real-time compression of image and science data for further ground analysis. AVS successfully determined the attitude and attitude dynamics of TeamSat.

Advanced stellar compass - Onboard autonomous orbit determination, preliminary performance

Abstract: Deep space exploration is in the agenda of the major space agencies worldwide; certainly the European Space Agency (SMART Program) and the American NASA (New Millennium Program) have set up programs to allow the development and the demonstration of technologies that can reduce the risks and the cost of deep space missions. From past experience, it appears that navigation is the Achilles heel of deep space missions. Performed on ground, this imposes considerable constraints on the entire system and limits operations. This makes it is very expensive to execute, especially when the mission lasts several years and, furthermore, it is not failure tolerant. Nevertheless, to date, ground navigation has been the only viable solution. The technology breakthrough of advanced star trackers, like the advanced stellar compass (ASC), might change this situation. Indeed, exploiting the capabilities of this instrument, the authors have devised a method to determine the orbit of a spacecraft autonomously, onboard, and without a priori knowledge of any kind. The solution is robust and fast. This paper presents the preliminary performance obtained during the ground testing in August 2002 at the Mauna Kea Observatories. The main goals were: (1) to assess the robustness of the method in solving autonomously, onboard, the position lost‐in‐space problem; (2) to assess the preliminary accuracy achievable with a single planet and a single observation; (3) to verify the autonomous navigation (AutoNav) module could be implemented into an ASC without degrading the attitude measurements; and (4) to identify the areas of development and consolidation. The results obtained are very encouraging.

The determination of the attitude and attitude dynamics of TeamSat

Abstract The second qualification flight of Ariane 5 was launched from the European Space Port in French Guiana on October 30, 1997. It carried on board a small technology demonstration satellite dubbed TeamSat into which five experiments, proposed by various universities and research institutions, were integrated. Among them, the Autonomous Vision System, AVS, a fully autonomous star tracker and vision system. This paper gives a short overview of the TeamSat satellite design, implementation and mission objectives. The AVS is described in more details. The main science objectives of the AVS were to verify, in space, multiple autonomous processes intended for spacecraft applications such as autonomous star identification, attitude determination and identification and tracking of non-stellar objects, imaging and real-time compression of image and science data for further ground analysis. AVS successfully determined the attitude and attitude dynamics of TeamSat.

Operations of a non-stellar object tracker in space

The ability to detect and track non-stellar objects by utilizing a star tracker may seem rather straight forward, as any bright object, not recognized as a star by the system is a non stellar object. However, several pitfalls and errors exist, if a reliable and robust detection is required. To test the operation, performance and robustness of such a function, the Autonomous Vision System (AVS), a fully autonomous star tracker that has flown onboard the Teamsat was equipped with several advanced features. These features included a non-stellar object detection and tracking module and an image and science data compression module. This paper describes the AVS, sensitivity, and possible detection ranges for various objects. The general description is followed by examples of images and tracking series obtained by the AVS on Teamsat.

Autonomous target ranging techniques

For the deep space asteroid mission, Bering, the main goal is the detection and tracking of near Earth objects (NEOs) and asteroids. One of the key science instruments is the 0.3-m telescope used for imaging and tracking of the detected asteroidal objects. For efficient use of the observation time of this telescope, a fast determination of the range to and the motion of the detected targets are important. This is needed in order to prepare the future observation strategy for each target, i.e. when is the closest approach where imaging will be optimal. In order to quickly obtain such a determination two ranging strategies are presented. One is an improved laser ranger with an effective range with non-cooperative targets of at least 10,000 km, demonstrated in ground tests. The accuracy of the laser ranging will be approximately 1 m. The laser ranger may furthermore be used for trajectory determination of nano-gravity probes, which will perform direct mass measurements of selected targets. The other is triangulation from two spacecraft. For this method it is important to distinguish between detection and tracking range, which will be different for Bering since different instruments are used for detection and tracking. Also, the baseline distance between the two spacecraft will provide two different (close and far) scenarios of observation. The limiting range and the relative range accuracies of the triangulation method are discussed.