Why does my satellite have geometric & radiometric anomalies?

Behind every Earth observation (EO) mission is a team of scientists ensuring consistent, accurate image quality over time. No satellite sees Earth perfectly!

Let us explore common causes of anomalies that must be accounted for in your mission. Most anomalies can be corrected with scientific calibration. Other anomalies will be random and unpredictable and will require engineering to compensate for.

Physical causes

The Earth is not round, but rather an oblate spheroid. The surface is highly irregular, for example, mountains and valleys. Because of this, your satellite’s altitude will differ with each pass in orbit. Remember, your camera’s focus is designed for a specific altitude. Your satellite’s altitude will degrade over time. This is normal!

When you look at a mountain from the East versus the West, your perspective shifts. If you look straight down from a satellite at a mountain, or at an angle (off-nadir acquisition), your perspective shifts.

All the above make it difficult to determine where each pixel in your image must be. This affects geolocation accuracy.

Satellite causes

No satellite knows exactly where it is, and exactly where it is pointing. It is likely that your satellite cannot control its orientation and vibration perfectly.

The above, together with jitter, introduces unpredictable randomness in geolocation.

Did you know: if you have an Attitude Determination and Control (ADCS) with an accuracy of 0.01 degrees, the point on earth can still be off by 90 meters!

Timekeeping onboard satellites is not perfect. There are multiple clocks that do not perfectly synchronise. Remember, your satellite is moving around 8 kilometres per second. It is important to know the exact time down to a microsecond to align the GPS and ADCS data to the imager and between multiple imagers.

Optical instrument

No camera lens can be perfect and will distort an image to some degree.

Inside the imager are tiny, extremely sensitive detectors. These detectors catch the photons reflected from the Earth. Detectors are not identical and will have slightly different responses to the same amount of light. This variability is usually stable once compensated for.

The camera's temperature will change in a single orbit. This affects the detectors' sensitivity. The temperature also affects the camera’s focus. Thermal noise is introduced by electronics. Detectors also age over time, and this changes their response.

Sometimes things can or will go wrong during assembly, integration, launch, or operation. This can cause fixed or variable deviations that need to be compensated for.

In the next blog in this series, we will explore how FarEarth applies corrections for deviations.