Autonomous Spin Axis and Spin Rate Control of HAMSAT

Paper number

IAC-07-B4.3.02

Author

Mr. P. Natarajan, Indian Space Research Organisation (ISRO), India

Coauthor

Mr. P.N. Pandey, India

Coauthor

Dr. V.K. Agrawal, Indian Space Research Organisation (ISRO), ISAC, India

Coauthor

Mr. N.K. Malik, Indian Space Research Organisation (ISRO), India

Year

2007

Abstract

HAMSAT is a microsatellite for providing satellite based Amateur Radio Services to the Indian as well as the international community of Amateur Radio Operators. HAMSAT is a spin stabilized satellite with a nominal spin rate of 4 rpm. Autonomous control of spin rate and spin axis is desirable for small satellites in order to reduce ground support. First time, the Negative Bdot control law has been used to align the spin axis of HAMSAT along the orbit normal. As the variation of earth’s magnetic field along orbit normal is nearly zero, the spin axis is maintained to the orbit normal within very good accuracy. The spacecraft has been made totally autonomous with Spin axis orientation by Negative Bdot control law and Spin rate control using Cross product law. This paper describes Autonomous Attitude Control System (AACS) of HAMSAT and On-Orbit results of AACS

The sensors used in HAMSAT are of two types namely Magnetometer and Twin Slit Sun Sensor (TWSS). Magnetometer provides the local magnetic field measurements. TWSS provides Sun Transit Pulse and Sun Aspect angle.   
Two magnetic torquers with a capacity of 9Amp-m2 are used as actuators. 

The HAMSAT was launched on 05-05-2005 aboard PSLV-C6 along with CARTOSAT-I into a sun synchronous orbit of 624 Km altitude. With snap, Detumbling and Spin up mode was selected automatically by the onboard sequencer. The transverse rates were damped out using Negative Bdot control law by energizing the spin axis torquer. Simultaneously spin up was carried out using the transverse torquer. In the next visible orbit, it was observed that the spin rate has reached 4.3 rpm and the spin axis was almost normal to the orbit as indicated from the TWSS. Further the spacecraft mode was commanded to Auto Normal mode to maintain the spin axis along the orbit normal and spin rate within + O.5 rpm of the nominal rate. The satellite has been functioning normally for the last 21 months unattended except for monitoring the health of the satellite once in a while. 

HAMSAT being small satellite, the magnetometer output is influenced by the magnetic torquer activity because of close mounting. Bdot control gets affected by the magnetometer output. A suitable scheme has been worked out to overcome this limitation. 

This paper describes in detail, the control laws for autonomous spin rate control and spin axis orientation control, computer simulation results and on-orbit results for all the autonomous modes.

Abstract document

IAC-07-B4.3.02.pdf

Manuscript document

IAC-07-B4.3.02.pdf (🔒 authorized access only).

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