I'm curious if anyone has any ideas of how to perform color calibration for very far targets (~ a mile or so). My concern is that the spd of the illuminant will be different at the camera so using a MacBeth chart will not provide accurate colors. Any ideas? Does it matter?
I doubt you need a color calibration of that kind.
Let me assume that as the target is far away, this is some kind of landscape photography, even cityscape, so simply assign one preset on the camera, sunny, cloudy or something like that. Remember to shot in RAW if you can.
You can also try to find a target that you consider to be neutral enough, a wall, a car and use it as a reference later.
The main idea of a Macbeth target is on a controlled scenario, mainly photo on a studio.
On a landscape, the idea is not necessary to neutralize the light casted by the illuminants but to reflect their color as part of the scene.
You do not want to neutralize the golden light of a sunset, or the fiery red of a faraway volcano.
The atmosphere will throw your measurements off, irrespective of the illumination conditions. Without a good model of atmospheric attenuation, it is simply not possible. Akkaynak and Treibitz (2019) have published a paper of interest about colour correction of underwater images, your context is different but the challenges are very similar.
Does colour correction matter in your context? It really depends on what you are trying to achieve. Content with scientific accuracy required, it does matter, hence the referenced publication, artistic content, probably less. In any case, you should consider bringing the chart closer to the camera.