The Terahertz band is at the Wavelength range of 1 mm to 100 μm, which is very good for synthetic aperture radar aka SAR since the shorter the wavelength the better angular resolution you get. Unfortunately, there are a few problems when it comes to using the Terahertz band, but for the sake of my question lets assume that generating coherent terahertz radiation is no longer difficult. The other problem is that Terahertz radiation is quickly absorbed by the atmosphere. Although some Terahertz wavelengths can penetrate through the atmosphere (https://en.wikipedia.org/wiki/Submillimetre_astronomy). And there are some papers that claim that imaging from up to hundreds of meters with Terahertz is possible (https://pubmed.ncbi.nlm.nih.gov/20721012/). So could it be possible to image the Earth with Terahertz radiation and would it have a greater resolution?
1 Answer
Would it have greater resolution? Yes: the resolution limit of SAR is independent of range (really), and is on the order of $0.5-1$ times the wavelength (if you can synthesize the aperture: to get 100 micron resolution on the moon, you need a really big aperture).
The atmosphere is opaque at 0.1 - 1.0 mm, so that will ruin space borne systems.
If you had airborne system (say a drone, so close range), you still need to know the position of the phase-center of your antenna down to a fraction of a wavelength to do coherent processing.
You also need a lot of bandwidth to get 1mm resolution (150 GHz)...how do you digitize that data and downlink it?
Wikipedia says there have been experimental THz SARs on a lab bench providing sub millimeter resolution, with no reference. One thing to note that the "Doppler" part of the SAR is not caused by platform velocity Doppler shifting the reflected signal at the carrier frequency (that is a tiny shift, and is ignored in normal processing, but can be exploited to improve azimuth resolution in "doppler beam sharpening").
Rather, it is a phase shift (e.g. for a point target) that is caused by the change in distance to the target between pulses (at the mixed down frequency) So: you don't need a moving platform to do THz SAR in the lab, you could just move the antenna between well known fixed positions, taking a pulse at each position, and then synthesize your aperture in post.
That would be a proof of principle, putting it in motion is a lot harder. You need to know whence you transmitted and received with sub wavelength accuracy to focus the image.
