slide 55

Interference relies on coherence between two phases. In the IMSA system, coherence is measured by calculating the complex cross-correlation between two interferences. Due to limitations, only one sun image can be obtained at a time. To address this, the assumption is made that sun pixels are spatially correlated. Instead of calculating the expectation value, a small region is used to approximate the average coherence. Coherence values range from 0 to 1, with lower values indicating greater differences between phases. When phases are more different, their random components cannot be reduced through interference. The superposition of random components remains random in space and may have a larger magnitude than the phase of interest. Coherence is key to interference. If you read the definition of interference, you will see it mentions interference should be implemented in coherent phases. Coherence is the similarity between two phases. In IMSA, the coherence is estimated by calculating the complex cross-correlation between two interferences. The formula is shown here. The definitions of symbols are shown below. In theory, it has to calculate the expectation value. However, we only can obtain one sun image at one time. We cannot obtain several sun images at the same time. To solve this issue, it assumes the sun pixels are spatially correlated. So the near-beam sun pixels bear similar characteristics. So instead of calculating the expectation value, it is approximated by calculating the values in a small region. It is just like calculating the average coherence in space. The value of coherence is between 0 and 1. Lower coherence means two phases are more different. If two phases are more different, their random components may not be similar. So they cannot be reduced through interference. The superposition of two random components are still random in space. And the magnitude may be larger than the phase of interest.