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