- I don’t understand why we can’t measure the distance to a specific point (surface) with just one SAR image.

- If SAR knows the time when the microwaves were transmitted and received, and the location of the satellite, can’t we know the absolute distance between the satellite and the point (surface) without phase information?

(The content below is what I understand about SAR.)

- SAR imagery is data visualized as a raster by transmitting microwaves in a pulse form and detecting the received signal.
- Therefore, when each pixel of the SLC includes signal strength (amplitude) and phase information, I understand that the phase information contains distance information.
- But since SAR originally knows the time of transmission and reception for each pixel, and knows the location information of the satellite, shouldn’t we be able to know the distance from the satellite to that point (surface, pixel)? Can’t we get the distance from the satellite to the point (surface) without using phase information, just by multiplying the speed of light by time? (I think I have this question because I don’t understand SAR data… I’m curious what elements I’m missing)

- If the baseline of the two images used when applying InSAR is not zero, that is, if the images are not taken from the exact same location, can’t we apply InSAR?

- Even if the band used in SAR is as long as 70cm, if there is even a slight difference in the location of the two phases, it will have a big impact on the results. Isn’t this error considered?
- The information contained in the interferogram is simply the phase difference between the two wavelengths. Isn’t it impossible to know how many more repetitions the slave has made based on the master?
- Therefore, how do we know whether the pixel determined to be farther from the satellite due to the interferogram is actually a result of the surface being eroded or the satellite moving farther away?
- Of course, Sentinel-1 adjusts its orbit to be located within a tube of up to 100m for InSAR, but if the baseline differs by about 10m, wouldn’t it be impossible to determine whether the satellite has moved away or subsidence has occurred?

For example, suppose no subsidence occurred at point A, the master satellite is 100m away from point A, and the slave satellite is 110m away from point A. (band length is 20cm)

- If InSAR is applied using the two SAR images, it will determine that A point is 10m away, and fringes will be created based on that determination.
- However, the interferogram is only displayed as the difference in phase (-pi ~ pi), not the absolute distance (10m), so it will appear as pi on the interferogram.
- But if you just look at the value of the fringe without knowing the answer, how can you tell whether it is a phase difference after one repetition, or a phase difference after 100 repetitions (10m / (20cm/2)) like in the example above?
- Also, how do we know whether actual erosion has occurred or whether the satellite is farther away?

I’ve worked hard to write down the questions, but in fact, there is a part to correct the orbit when applying InSAR with SNAP, and I think that such errors might be removed in that part.

In summary,

- Why can’t we calculate the absolute distance from the SAR satellite to the surface with just one SAR image? Don’t we know the position of the satellite and the time it took for the microwave signal to be transmitted and received? Isn’t it just a matter of multiplying the time and the speed of light?
- When analyzing ground subsidence with InSAR, how do we distinguish whether subsidence has actually occurred or whether it’s simply a result of an increased distance between the two satellites?
- In the interferogram, how do we know how many oscillations the slave’s wave has made compared to the master’s, based on the phase difference that is shown? The method of counting the number of fringes and multiplying it by the wavelength to calculate the total displacement assumes that the slave and the master have oscillated the same amount, but how do we know that this is the case?

Sorry for my English and long questions.