Chapter 6 Part 3. Attitude Sensors. AERO 423 Fall 2004

Transcription

1 Chapter 6 Part 3 Attitude Sensors AERO 423 Fall 2004

2 Sensors The types of sensors used for attitude determination are: 1. horizon sensors (or conical Earth scanners), 2. sun sensors, 3. star sensors, 4. magnetometers, 5. inertial reference units (IMU or attitude reference units ARU), and 6. GPS receivers. Horizon sensors measure pitch and roll to an accuracy of about deg. An accuracy less than 0.05 deg can be achieved by extensive calibration and accounting for the equatorial bulge. Sun and star sensors provide directions. An horizon sensor does not provide yaw information (for momentum bias systems it is not necessary to measure yaw). One wide-fov or two narrow-fov star sensors are needed to provide attitude. Since the star sensors cannot provide continuous attitude measurements an IMU/ARU is needed to provide the attitude between measurements. IMUs suffer from drift and biases and need frequent updates which are provided by the star sensors. Magnetometers measure Earth's magnetic. Accuracies no better than 1 deg can be obtained. The GPS receivers are used in an interferometer mode to determine attitude. Accuracies as good as 0.01 deg are expected using GPS. 2

3 Sun Sensors Most common because they are light weight, not expensive, limited power required and because they provide an accuracy which is acceptable for most of the missions. Sun is the principal source of energy (exemption: interplanetary spacecraft). Solar flux goes as 1/r 2. Sun (in the IRF) is evaluated by using interpolating functions (armonic series expansion) which least square fit the Sun positions recorded in the star atlas. They are used to synchronize command with spin period, and they are used to protect star sensors. At 1AU the sun may be considered as a point (0.267 deg.). For GEO there is the parallax error (0.03 deg as max values). 3

4 Presence Sun Sensors Sun rays Photocell Shadow rod Grid slit Shadow area Entrance slit Presence sun sensors provide the sun crossing time only, or the sun presence within the sensor FOV Used to synchronize pulsed command (spin-up, spin-down) to maneuvre, and to turn on/off onboard experiments and instrumentation. Photocell at the 4 base (below slit)

5 Presence Sun Sensors Based on Snell refraction law. nsin ϑ= sin ϑ nsin γ= 1 ' Aperture Lens Image plane mask Photocells Photo cell 2 Photocell 1 Sun image Mirror 5

6 Analogic Sun Sensors Reference axis Normal Sun rays Normal Sensor 2 Sensor 1 Photocell T I( ϑ ) = I(0) d n= I(0)cos ϑ Normal Normal Sun rays Mask Photocells Photocell #2 Photocell #1 6

7 Output current Sensor #1 Sensor #2 Angle (deg) 7

8 Measurement Command Digital Sun Sensors Command: sun presence Measurement (4 parts): 1) ATA, 2) Sign bit, 3) Code bits, 4) Interpolating bits. Command Measureme nt 8

9 Digital Sun Grid slits Photocells sensor 9

10 Sun angle Binary code Gray code 10

11 Binary and Gray code The gain of the Gray code consists of the fact that the bit string, which represents the angle measure, changes one bit only at each digital step. Decimal Binary Gray Decimal Binary Gray

12 Double Digital Sun Sensor Photocells 12

13 Earth horizon sensor Best range: 14µ -16 µ of CO 2 (less high altitude clouds contr.) Relative radiance 400 (IR) and 30,000 (visible) 13

14 Earth horizon sensor Measure two crossing times t and t 1 2 With angular speed ω, the measured angle is ϕ =ω( t t ) Two measured angles, ϕ and ϕ 1 2 allow to evaluate the Nadir direction

15 Hot Moon Moon horizon sensor Relative radiance Hard horizon Moon temperature range: 240 deg to +30 deg Hot area for cold Moon Hot area for hot Moon Cold Moon Cold horizon position Moon center distance Hot horizon position 15

16 Earth/Sun sensors for spinning S/C Meridian slit Inclined slit Displacement angle (deg) Satellite equator 16

17 Star sensor for spinning S/C Optical axis Elevation slit 17 Azimuth slit

18 Star Trackers Electronics Photomultiplier Optical system Grid Baffle 18 Star

19 Magnetometers They may be used at altitudes less than 1,000 Km, only. (Earth magnetic field decreases with radius as 1/r 3 ) V = Edl = dφ dt B Small precision is associated with the fact that the EMF Models are not so accurate. Sensors Biases not negligeable (residuals of magnetic fields caused by onboard circuits) Electronic unit Signal processing and Analog to digital converter Telemetry Link 19

20 Typical GN&C Sensors (Wertz) 20

21 Attitude Determination Systems 21