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1.INTRODUCTIONStar trackers are fundamental devices for determining the attitude of a satellite platform in the (inertial) frame reference of fixed stars. Satellite orientation with respect to the Celestial Sphere is extrapolated from the image of the observed star field. In general, there are more than a single star tracker installed on a platform, in order to guarantee a continuous availability of satellite attitude data, both in case of failure and partial/total field of view occupation by large objects, such as the Earth or the Moon. The validation of the star trackers operation is therefore a crucial step for any space mission. Star trackers are usually tested by using Optical Ground Support Equipment (OGSE) and Electrical Ground Support Equipment (EGSE). OGSE are “optical stimulators” used to test both the optical characteristics of the star trackers and their internal algorithm for star recognition. EGSE are devices which permit the integration and validation of the electrical and software functions of the star tracker. While EGSE are able to test only some parts of the star tracker, OGSE can be used to verify the performance of the system as a whole. Most OGSE are designed for testing star trackers performance in the laboratory, before their installation on the platform. These systems present a static or a dynamic simulation of the scene observed by the star tracker so that the output of the star tracker can be compared with the input data provided by the OGSE1, 2. Some laboratory facilities rely on the use of laser or light emitting diodes (LEDs) to mimic a static scene that portraits the brightest stars of the night sky. An example of such facilities is the Celestial Object Simulator at John Hopkins Applied Physics Laboratory3, which uses a large number of LEDs to simulate the night sky of the northern hemisphere. Another approach is the one that relies on the use of a large LCD screen showing the simulated star-field viewed by the star sensor. In this case, the laboratory facility can perform either static or dynamic simulations. During static simulations the LCD screen mimics a static star constellation, while during the dynamic simulation LCD screen displays time series of star field scene that takes into account the apparent motion of the observed scene. All these methods are based on some simplifications that can limit the accuracy of the test. Efforts have been made to overcome some of these limitations, such as the radiometric modulation of the source used in the simulation4. The validation of star trackers after their assembly on the platform is more challenging and, under some aspects, is still an open problem. A first step forward was provided by the implementation of test systems able to simulate dynamic star fields as those observed during the on-board operational activity. A further improvement was the development of miniaturized test systems to be installed directly on the star tracker. Despite the growing interest aroused by such miniaturized devices, their availability on the market is still very limited. Presently, only few miniaturized devices are available, e.g. the STOS (Star Tracker Optical Stimulation for Sensors) manufactured by Airbus Space Equipment5 and the Optical Sky Stimulator (OSI) for ASTRO APS star sensors by Jena-Optronik6. Both systems simulate a dynamic scene of a star field. These devices use a miniaturized LCD monitor showing the simulated image in apparent motion to the star tracker and an optical system projecting the simulated image in the star tracker optics. These systems can simulate large non-star objects and different disturbances (like stray light and the effects due to charged particles). They can be used either in open-loop configuration for sensor test or in close-loop configuration to test the behavior of the star tracker operating in a realistic scenario. In this paper, we present a new OGSE prototype, the MINISTAR, recently developed by a consortium of Italian enterprises and the Applied Physics Institute of the National Research Council. The MINISTAR is a miniaturized electro-optical device able to generate synthetic images of dynamic star fields. Its innovative design permits the simultaneous test of multiple star trackers, both for optics, electronics and on-board attitude software. The MINISTAR is able to perform a dynamic simulation of the apparent motion of the observed scene in order to test the star tracker in a realistic working scenario. It can be placed directly on the star tracker under test and, thanks to its reduced dimensions and weight, the test and validation phase can be performed while the star tracker is assembled on the satellite platform. The MINISTAR is also able to simulate the presence of large objects such as the Sun, the Earth and the Moon, custom objects and disturbances such as cosmic rays and stray light effects. 2.THE MINISTAR PROTOTYPE2.1Prototype’s descriptionA block diagram of the MINISTAR prototype is outlined in Figure 1. The prototype consists of the main following blocks:
Figure 2 shows the assembled MINISTAR prototype (Figure 2A) and the monitor with the rendered star-field (Figure 2B). 2.2Main technical features of the prototypeThe main requirements taken into account for the design of the MINISTAR were: 1) fast computation, 2) system miniaturization, and 3) system capability of testing different types of star trackers commercially available. The request for a computationally fast algorithm lead to the choice of a modular structure for the design of the simulation model and control interface. The MINISTAR simulation model provides a real time computation of the dynamic synthetic scenes. It also permits the simulation of both stellar and non-stellar objects, stray light and other disturbance effects (like SEU (Single Event Upset). It also renders the selected scenarios in the Field of View (FOV) with a custom radiometric dynamics. The MINISTAR software was also designed using a modular approach: the MINISTAR is controlled through a graphic user interface that permits the simulation of both the HIPPARCOS7 star catalogue and non-star objects. The software was also designed for the simultaneous test of multiple-head star tracker. Miniaturization of the final device was one of the key for the mechanical design of MINISTAR. Its construction was carried out using innovative materials for in order to obtain a light-weight, miniaturized opto-mechanical assembly (weight: < 1 kg; dimensions: 175 mm (∅) x 150 mm). These characteristics makes it possible the mounting directly on the star tracker’s baffle. Compliancy with several star tracker models available on the market was also a key parameter for the optical design of the collimating optics, with particular reference to the pupil and the FOV. In particular, the MINISTAR was designed so as to have a large pupil and FOV in order to be compatible with most of the star trackers on the market; Table 1 shows the main technical data of the MINISTAR prototype, together with the main characteristics of other two miniaturized test systems (STOS and OSI) presently available on the market. Table 1.Comparison between the main specifications of three different miniaturized devices for star tracker testing.
2.3Functioning verificationEach functional block of the MINISTAR prototype was individually verified. The simulation model and the system software were tested after the integration of the star field simulation library with the application software. As an example, Figure 3 shows the results of the test performed to verify the capability of the system to display an image with 2000 stars. For this purpose, a logic signal analyzer was used to check the timing: the total time measured for the display of 2000 stars is about 1.2 msec. Figure 3.Test results for verifying the capability of the system to display an image with 2000 stars.  The quality of the optical system was tested. The test included the surface imperfection tolerance, the surface form tolerance, the centering tolerance, the lens thickness and the spectrophotometric response. The final alignment after the mechanical assembly of the lenses was also verified by means of interferometric measurement. 3.CONCLUSIONSA miniaturized lightweight prototype of a multi-head test equipment for star trackers was constructed. The main features of the device include:
ACKNOWLEDGEMENTSMINISTAR was supported by Regione Toscana (MINISTAR project - POR_FESR_2014-2020- Bando n.2 “ Progetti di ricerca e sviluppo delle MPMI”). REFERENCESRufino G., Accardo D., Grassi M., Fasano G., Renga A., and Tancredi U.,
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