Determination of star sensor magnitude

What is sensitivity magnitude?

Magnitude refers to the relative brightness of a star in the sky. Also refers to apparent magnitude, which is the brightness of a star as seen from the Earth. The brighter a star appears from Earth, the lower its apparent magnitude. Under common circumstances, people use visible light to measure the apparent magnitude. The magnitude of a star depends on multiple factors such as its distance from the earth, its own luminosity, and interstellar dust obscuration. The limit that the average human eye can distinguish is approximately 6.5 magnitude.

Star sensor requirements for the number of stars

The accuracy of the star sensor mainly depends on the pixel size and focal length of the sensor. In theory, the smaller the pixel size and the longer the focal length, the higher the accuracy of the star sensor and the smaller the corresponding field of view.

Star sensor star map identification requires a certain number of stars in the field of view. There are too many stars in the field of view, which requires the star sensor to have a higher visual magnitude, which brings great difficulties to the design of the optical system and exposure time of the star sensor; while the number of stars is too small and cannot meet the requirements of the star sensor. Requirements such as image recognition have an impact on the functions of star sensors.

To achieve reliable star map identification, a single field of view must contain at least 4 stars when using the angular distance method. Star map identification using the grid method usually requires that the field of view preferably contains 7 navigation stars. In order to ensure that star map recognition can be achieved when the star sensor points to any sky area, the star sensor needs to be sensitive to as many navigation stars as possible, that is, the sensitive magnitude must be high enough. The higher the magnitude, the more navigation stars there are.

The relationship between star sensor accuracy and number of stars

The greater the number of stars, the higher the accuracy of the star sensor. However, as the number of stars increases, the amount of calculation required for the system to track all navigation stars also increases accordingly. When the number of stars is greater than 15, the accuracy increase of the star sensor is not particularly obvious. When the average number of stars in the star sensor reaches more than 13, the probability that the number of stars in the field of view is less than 3 has reached the level of one thousandth. The increase in the number of stars is not very significant in increasing the capture rate.