1. ATTITUDE CONTROL SYSTEM FOR BALLOON-BORNE EXPERIMENTS A.Boscaleri (1), E.Pascale (2) (1)Istituto di Ricerca sulle Onde Elettromagnetiche “Nello Carrara (IFAC-CNR) Florence Italy –Gruppo Stratosfera (2)Dep. Of Astronomy and Astrophysics of University of Toronto (CA) (1) a.boscaleri@ifac.cnr.it (2) enzo@physics.utoronto.ca Star Tracker (Maxima, Boomerang 1997, Boomerang 1998 and 2002) The star tracker we developed has to give positional information measuring the orientation of the platform with respect to the stars. Different classes of star trackers are needed in order to work either during the day or during the night or both, continuously, day and night. The main sensor is a ccd array and the accuracy of the positional detection depends from the pixel dimensions in sky both through the lens focal length and through the pixel physical dimensions. During the night, a simple bore sight Cohu 1910 camera with a 50mm f 0.8 lens is able to detect up to mag. 7 stars with an accuracy of 1 arc minute rms and 33 msec integration. The field of view of 6X7 degrees guarantees at least two stars detection and the full attitude determination. During day-time the light scattered from the atmosphere at float altitude is less than one hundredth of the light at sea level. This background has to be limited on to each pixel to avoid saturation of the ccd. This is obtained both using ccds with pixels dimensions that are small enough and/or using lenses with long enough focal length. The Pivot is designed to hang the full payload and scan or point it in azimuth. 1)Flight Chain 2)Reaction Wheels 3)Tachos 4)Gondola 5)Motors Servoed Star Tracker A Cohu 1910 camera in mounted on to a motorized two axis platform. Each axis is controlled through a stepper motor and a 16 bits encoder. The camera is used with a 500 mm f 4.6 spotting scope and it is high pass filtered in wave length at 715 nm. As it can be seen from the bottom plot, the back ground at float is as bright as a mag. 4 star. The star tracker in this configuration is able to detect bright stars and lock on them. As the payload scans and swings, the lock is kept moving the axis of the motorized platform. The accuracy obtained is the one of the encoders (~20 arc-seconds). Only one star can be locket at once and no roll information can be obtained. This design as been especially developed for day- time use, but the system can works also during the night since the use of the filter result in a lost of only 0.8 magnitude and the full system can be run at 33Hz Bore Sight Star Tracker The main requirement is to have a large field of view so that at least two stars can be detected at once and without saturating the ccd with the light coming from the scattered background. Field subtraction is used to extract from the background as may stars as possible: a frame containing only the background signal is subtracted from each frame containing the stars image on top of the background. The plot at the side shows an example based on the Kodak KAF-3200 CCD sensor (pixel surface is 16 times smaller than the Cohu ccd). The blue line is the signal from a magnitude 7 star while the red line is the noise after subtraction. It is possible with this kind of sensors to detect up to mag. 7 stars with a 150 mm lens and a field of view of 2-3 degrees both in azimuth and elevation. Detection of two stars at once permits full attitude determination (azimuth, pitch and roll). M I T O Millimeter and IR Testa Grigia Observatory Kodak CAF-3200 CCD chip Two axis motorized platform controlled by step motors and 16 bits encoders drive the star tracker keeping it locked on to a star while the payload is scanning in azimuth and swinging Lower Plot: Signal of the noise on each pixel after frame subtraction (red line) is plotted vs. the focal length of the lens. The blue line is the signal of a mag. 7 star. With a 150mm lens is the possible to detect up to mag. 7 stars at float altitude (~35 Km). Upper Plot: Signal from the scattered sun light background (red line) at float altitude (~35K) is plotted versus different focal length lenses and compared with the signal of mag. 4 star (blue line). All signals are high passed in wave lengths with a filter cut-on at 715nm. A R G O M A X I M A Millimeter Amisotropy eXperiment Imaging Array B O O M E R an G Balloon Observation of Millimetric Extragalactic Radiation and Geophysics Precision Fiber Optic rate Gyros (Boomerang, Maxima) FOGs are the main sensors in pointing reconstruction. The signal of a 3 axis gyro is integrated over one of the available absolute position sensors to remove offsets and long term drifts. We use QVH Industries FOGs that have a noise of about 5’/sqrt(hour) Single Axis Stabilization (SAFIRE-B) It gives angular displacements from the local zenith with an accuracy below 1 arc-minute rms. It is based on an integrating gyro and uses the natural swinging of the balloon together with a pendulum to calibrate offset and long term gyro drifting. S A F I R E Spectroscopy of the Athmosfear using Far InfraRed Emission ACS Sensors Coarse Sensors: Magnetometer, GPS attitude solution Fine sensors: CCD Linear arrays, Star Tracker, Gyros, Inclinometers Magnetometer (ARGO 1993, Maxima 1996, Boomerang 1997, SAFIRE-B 1999-2002) It is used to measure azimuth angles from the direction of the local Magnetic Field Vector. Advantages: This sensor at mid latitude is the less expensive absolute azimuth sensor. It is easy to use and can achieve, according to the signal to noise ratio, a relative accuracy of tens of arc-seconds. Disadvantages: It has to be mount far from the strongest magnetic disturbances strictly connected to high current devices and it is no easy to find the true heading due to the gondola influence. However the analog sine-cosine voltage output, after a 16 bit digital conversion, can be processed in order to produce a linear correspondence between a pure binary code and the true azimuth. This operation, called Tracking Converter (TC), is able of nulling the actual magnetic declination as well as the possible built-in coil quadrature errors. The Attitude Control System Project The Attitude Control System for stratospheric experiments, designed at IROE, counts three main parts: The Pivot is capable of drive payloads up to 2500 Kg in weight with NASA-NSBF safety factors. It houses two DC torque motors with a torque sensitivity a 1.4 NW.M/AMP and a peak armature current of 13 AMP as well as two tachos devote to the control of the angular speed of the upper and lower flywheels. The mechanical design of the pivot can perform different strategy while pointing sources or scanning large sky regions according to the interaction of the upper reaction wheel with the flight chain. The hardware that controls the gondola movements is built around a commercial PC 386 CPU board (AMPRO). Some custom boards interface analog and digital sensors as well as drive motors by PWM technique at 28 Khz. The Pointing system is fully digital and can be controlled by telemetry link and can send data for post processing via different protocol (NRZL or BI-PHASE) to GSE. An onboard system backup based on dedicated PC card can store the same down-link matrix according to a safety strategy of not loosing data due to possible radio link failures. The system is open to accept any kind of sensor to perform an accurate reconstruction of the telescope line of sight as magnetometer, sun and star sensor, GPS attitude solution and three axis rate gyros. The software can take advantage of a PC architecture and controls the Transfer Function of the motor (PID) for different weigh-inertia moment combinations of the payload as well as any communication links with other part of the experiment in order to output ACS data consistent with the scientific down link stream. Sensor Under developing Sun sensor based on RAD-HARD CMOS SENSOR (FILLFACTORY) with 1024 x1024 pixel array with 15  m square pixel and electronic exposure control TRIP A Cooled Telescope for Measurements of the Near infrared Cosmological Background