NRSC Attitude Fusing Using Digital Sun Sensor

8/20/2019 NRSC Attitude Fusing Using Digital Sun Sensor

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Attitude Fusing using Digital Sun Sensor &Magnetometer for RISAT Satellite

Amit Maji

Laboratory of Electro-Optics System (LEOS)

E-mail: [email protected]


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Introduction

Attitude of Spacecraft: Orientation of Spacecraft relative tosome references or object of interest such as Sun, Earth,Star or earth magnetic field

Involved several types of sensor likes Sun Sensor, Earthsensor, Magnetometer and Star Sensor

Absolute Attitude sensor for Indian Remote SensingSatellite:

Star Sensor(SS), 4PI Sun Sensor(4PISS), Digital Sun Sensor(DSS)& Magnetometer


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Attitude Sensors

Star Sensor : Provides the complete attitude of the spacecraft Accuracy : 10“(Detector Plane) & 40“(bore sight Axis)

4PI SS : Provides Sun Vector in sensor body frame Accuracy : ±5˚

DSS: Provides Sun Vector in sensor body frame Accuracy : ±0.125˚

Magnetometer: Provides three axes earth magnetic field in sensor body frame Accuracy : ±500nT


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Attitude Fusion

Normally SS provides the absolute attitude of thespacecraft with the accuracy of few arc sec.

Many satellites, coarse attitude of the satellite is calculatedusing sun vector and earth magnetic field vector usingattitude fusion algorithm

Attitude Fusion: Determination of Absolute attitude of thesatellite using two or more sensor data

The attitude accuracy is depends on the accuracy of thesensor

Algorithm Used: TRIAD


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TRIAD Algorithm

Required two vector(sun vector & magnetic field vector) in

sensor body frame –Sb & Mb Same vector in Inertial frame - Si & Mi Sun vector is selected as first vector in triad since it

provides higher accuracy

First vector in Triad Frame: t1b =Sb & t1i =Si Second Triad Vector: perpendicular to the two observations

t2b = (Sb X Mb ) /| Sb X Mb | & t2i = (Si X Mi ) /| Si X Mi |

Third Vector

t3b = (t1b X t2b ) & t3i = (t1i X t2i ) Triad Matrix: [ t1b t2b t3b ] & [ t1i t2i t3i ]

Attitude Matrix:

R bi=[ t1b t2b t3b ] [ t1i t2i t3i ]T


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Attitude Fusion using 4PI & Magnetometer

In many satellites, coarse attitude is calculated using 4PISSand Magnetometer data when high accuracy attitudesensors are not available

But 4PISS is a low accuracy analog sensor whose accuracy is

affected by the Earth Albeo, reflected sun lights from theearths’ surface

The accuracy of the magnetometer is also affected by thebiased magnetic field of the spacecraft, internalmisalignment of magnetometer system, externalmiscalibration of magnetometer system

The attitude error: ±5˚


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Attitude Error


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Proposed Attitude Fusion using DSS & Calibrated

magnetometer

DSS is a two axes high accuracy sun sensor providesthe sun aspect angle w.r.t sensor body frame

No Earth Albedo Effect

α

β

P

Y

R

DSS

Sun Vector

l

m

n

= *

Cont..


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DSS output Vs Reference Output

Cont..


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Proposed Attitude Fusion using DSS & Calibrated

magnetometer

Magnetometer is calibrated using the bias determinationalgorithm

A(t) =3x3 attitude matrix at time t that transforms vectors from

reference to body co-ordinate B0(t) =Earth’s Magnetic field in reference co-ordinate B(t)=Earth’s Magnetic field in S/C body frame co-ordinate D(t)=The spacecraft control magnetic dipole(Magnetic torque )

which has linear effect on the magnetometer data b= Bias Vector, to be determined S=3x3 scale factor/ misalignment matrix, to be determined T=3X3 matrix relating the control vector to the magnetometer

data, to be determined I= 3x3 identity matrix

Cont..


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Magnetometer Data


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Attitude Error


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Conclusion

The determination of satellite attitude using DSS and calibratedMagnetometer is more accurate compare to 4PI & Magnetometer

The roll and yaw attitude error of the satellite using calibratedmagnetometer & DSS by fusion algorithm are within ±1° for fullorbit and for pitch is ±2°.

The attitude error is within ±0.5° during pole crossing wheremagnetometer error is less

The attitude accuracy can be further improved filtering usingkalman filter

The attitude obtained by fusion of DSS & Magnetometer canform a backup option for satellite independent of prime sensors.


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Thank You

NRSC Attitude Fusing Using Digital Sun Sensor

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