5.4 Attitude Sensors

5.4 Attitude Sensors

Attitude control of a satellite is classified by two methods ; spin control and three axis control. The former method is usually adopted for geosynchronous meteorological satellites which itself rotates itself together with rotating scanner. The latter method is mainly adopted for earth observation satellites such as Landsat which needs accurate look angle in the direction of the earth.

The spin control is rather simple but has a low S/N ratio, while the three axis control is more complex, but has a high S/N.

Figure 5.4.1 shows the typical types of attitude measurement sensors, which are used for different purposes.

A gyro-compass is used for measurement of attitude variation over a short interval. Earth sensor detects the radiation of CO2 with in the wavelength range of 14 - 16 mm emitted from the rim of the earth, from which two axis attitude of roll and pitch can be measured with an accuracy of 0.3 - 1 degree, as shown in Figure 5.4.2. If the earth sensor is combined with a sun sensor and gyro-compass, the three axis attitude can be measured with higher accuracy of 0.1 - 0.3 degree. Magnetic sensors can measure the three axis attitude but with a slightly low an accuracy. The responsivity of the above sensors is 2 Hz at maximum. If the high frequency attitude such as jitter is to be measured, the angular displacement sensor (ADS) is necessary.

The angular displacement sensor of Landsat 4 and 5 has a responsivity of 2 - 18 Hz. The highest accuracy of attitude can be achieved by the star sensor. For example, the standard star tracker (SST) on board Landsat 4 and 5 will measure an accurate attitude from the image of stars acquired by an image dissector with a reference of about 300 star catalogue up to the sixth grade stars stored in an on board computer. The accuracy of SST is about +- 0.03 degree (3).(3 standard deviations).

In case of the space shuttle, the star sensor has a lower accuracy with only about a 50 star catalog compared to the SST, because the space shuttle does not need the higher attitude control when it returns to the troposphere.