(GLAS) PRECISION ATTITUDE DETERMINATION - Center for ...

Table 3.3: Some noise characteristics for the HD-1003 star tracker and the HRG [47]MeasurementHD-1003dim starsbright starsmagnitudeHRGNoise (1)7.3 arcseconds4.5 arcseconds0.2 magnitudeRate White Noise (RWN, v ) 0.05 arcsec/sec 1 2Bias Stability (BS, b )1.33e-03 arcsec/secRate Random Walk (RRW, u ) 3.19e-05 arcsec/sec 3 2Then, the two measurement angles and in the SCF, analogous to and in theCRF, are dened byx mm = tan f (3.6)y mm = tan fcos() : (3.7)From the known noise characteristics of the CCD star tracker, noise components are addedto and . A measurement noise is also added to the magnitude. A set of realistic noisecharacteristics for the GLAS star tracker are given in Table 3.3. The noisy position anglesand magnitude in the SCF are considered to be the simulated measurements obtained bya star tracker. The purpose of the attitude determination that will be discussed in laterchapters is to nd input Euler angles which were used for data simulation. As mentionedin Chapter 2, quaternions will be used instead of Euler angles. The owchart showing theprocedure to get simulated star tracker data from the stars which were selected in a starcatalog is illustrated in Figure 3.3.3.2 Hemispherical Resonator Gyro (HRG)Gyros provide the spacecraft angular rate, even though several independent technologiesare used to design dierent gyros, such as the traditional mechanical gyro, laser gyrosand the HRGs. The ICESAT will use a set of HRGs mounted on the instrument opticalbench. An HRG is based on the inertial sensing property of a standing vibration wave ona hemispherical body [53]. It was noted that the location of a vibration pattern at therim of a hemispherical shell precesses relative to a reference on the shell when the shellis rotated about its axis of symmetry. The vibration pattern precession was observed tobe a constant fraction of the inertial input. Measuring the amount of precession providesthe inertial rotation of the input axis. A carefully designed IRU consisting of a few HRGsprovides the inherent small size, high reliability characteristics and precision performancewith relatively long life time expected in space applications.26