A very important device on any spacecraft is the one which determines the spacecraft’s attitude. There are many different methods by which the attitude of a satellite can be determined, but one of the mostly widely used is the star tracker.

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Star tracker specifications

A very important device on any spacecraft is the one which determines the spacecraft’s attitude. There are many different methods by which the attitude of a satellite can be determined, but one of the mostly widely used is the star tracker. Other methods for determining attitude exist, but none can reach the precision of a star tracker (arc-seconds accuracy) most of the time being just complementary of it. Therefore, the importance of star trackers on spacecraft is difficult to estimate. An ideal star tracker should be able to find the attitude instantly without chance of error in lost in space conditions and without aid from some other sensors. This should be done in parallel with minimal resources consumed (storage and CPU speed) but the performances versus resources are conflicting constraints and in the end everything is a compromise based on the mission requirements.
The star trackers are functioning by identifying the stars within the field of view and based on this the attitude of the star tracker can be determined, which then can be transformed to the attitude of the spacecraft itself.
Usually each sensor available on the market has special features adapted to its unique mission and must be manufactures conform to the specific requirements of the particular mission on which will fly.
However there are a number of characteristics that should be considered when evaluating the performances of a star sensor:

• Field of view
In general by decreasing the FOV it can be achieved a better accuracy, however as this implies as well an increase of the number of stars that should be stored in the onboard database for successful of the identification process, it means that a more processing power will be demanded.


• Sensitivity to star magnitude
Increasing the number of detectable magnitudes produces as well a better accuracy, but as for the previous topic, this means as well an increase of the storage and computing consumption.

• Accuracy
The accuracy of the star sensor is highly dependent on the operating conditions on board. For such a reason the operating software is differentiated in several so called “operating modes” such as pointing mode, scanning mode, calibration mode, software maintenance mode or tracking mode.

• Noise equivalent angle
This is a direct indication over the influences of the star sensor’s errors on the final measurements.

• Update period
As the star sensor is the most accurate sensor of the attitude control system, the updating periods on board (i.e. update of the star sensor firmware, update of the onboard star catalog etc), will directly affect the performance of the spacecraft. Therefore is very important that these periods to be shortened as much as can be.

• Star acquisition time
This is an indication of how fast is the attitude determination process on board of the star sensor. Star acquisition and star patter recognition are the most time consuming routines in the star tracker operation, therefore is very important to speed up these algorithms.

• Tracking capacity
This is related with the umber of stars simultaneously tracked by the star sensor.

• Spatial separation

• Star motion
The accuracy of the star sensor directly depends on the spacecraft angular velocity.

• Reliability
Star sensors are very delicate and complex instruments, so as their reliability should be comparable with the rest of the components on board the spacecraft, the technical problem of surmounting the operational conditions is often hard to resolve.

• Power consumption
As the sources of energy on board of the spacecraft limits to the solar panels (special conditions should be as well considered i.e. like eclipse periods), it is therefore very constraining to minimize the power consumption.

• Weight
In general for the space field the weight is very important for costs of the launching considerations (typically a star sensor weights some kilograms).