Chapter 6 – Part 3 - Aerospace Engineering Courses pageaeweb.tamu.edu/mortari/aero423/Chapter 6 -...
Transcript of Chapter 6 – Part 3 - Aerospace Engineering Courses pageaeweb.tamu.edu/mortari/aero423/Chapter 6 -...
Chapter 6 – Part 3
AERO 423 – Fall 2004
Attitude Sensors
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SensorsThe 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), and6. GPS receivers.
Horizon sensors measure pitch and roll to an accuracy of about 0.1-0.5 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.
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Sun SensorsMost 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: interplanetaryspacecraft). Solar flux goes as 1/r2.
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).
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Presence Sun Sensors
Entrance slit
Grid slit Photocell at the base (below slit)
Sun rays Photocell
Shadow area
Shadowrod Presence sun sensors provide
the sun crossing time only, or the sun presence within the
sensor FOVUsed to synchronize pulsed
command (spin-up, spin-down) to maneuvre, and to turn on/off
onboard experiments and instrumentation.
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Presence Sun SensorsBased on Snell refraction law.
Photocells
Mirror
Image planemask
Sun image
LensAperture
Photocell 1Photocell 2
'sin sinsin 1
nn
ϑ = ϑγ =
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Sun rays Normal
Photocell
T( ) (0) (0)cosI I d n Iϑ = = ϑ
Analogic Sun Sensors
Photocell #2 Photocell #1
Normal
Sun rays
Normal
Photocells
Mask
Referenceaxis
Normal
Sensor 1
Sensor 2
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Sensor #2
Sensor #1
Angle (deg)
Out
put c
urre
nt
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DigitalSun Sensors
MeasurementCommand
Measurement
Command
Command: sun presence
Measurement (4 parts):1) ATA,2) Sign bit,3) Code bits,4) Interpolating bits.
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DigitalSun
sensorPhotocells
Gridslits
10Binary code
Gray code
Sun
ang
le
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Decimal Binary Gray Decimal Binary Gray
0123456789
10
01
1011
100101110111
100010011010
01
1110
110111101100
110011011111
1112131415161718192021
10111100110111101111
100001000110010100111010010101
11101010101110011000
110001100111011110101111011111
The gain of the Gray code consists of the fact that the bit string, whichrepresents the angle measure, changes one bit only at each digital step.
Binary and Gray code
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Photocells
Double Digital Sun Sensor
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Earthhorizonsensor
2Best range: 14 -16 of CO(less high altitude clouds contr.)
µ µ
Relative radiance400 (IR) and 30,000 (visible)
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Earth horizon sensor1 2Measure two crossing times and t t
2 1With angular speed , the measured angle is ( )t tω ϕ = ω −
1 2Two measured angles, and allow to evaluate the Nadir direction.
ϕ ϕ
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Hot Moon
Hard horizon
Hot area for cold Moon
Hot area for hot Moon
Cold Moon
Hot horizon positionCold horizon position
Moon center distance
Rel
ativ
e ra
dian
ceMoon horizon
sensor
Moon temperature range: –240 deg to +30 deg
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Meridian slit
Inclined slit
Satelliteequator
Dis
plac
emen
t ang
le (d
eg)
Earth/Sun sensors for spinning S/C
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Star sensor for spinning S/C
Optical axis
Elevation slit Azimuth slit
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StarTrackers
ElectronicsPhotomultiplier
Optical system
Baffle
Star
Grid
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Sensors
Electronic unit
Signal processing
andAnalog to
digitalconverter
TelemetryLink
MagnetometersThey may be used at altitudes less than 1,000 Km, only.
(Earth magnetic field decreases with radius as 1/r3)
Small precision is associated with the factthat the EMF Models are not so accurate.
Biases not negligeable(residuals of magnetic fieldscaused by onboard circuits)
V E dlddtB= =∫
Φ
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Typical GN&C Sensors (Wertz)
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Attitude Determination Systems