Astronaut photographs are acquired using handheld, commercial off-the-shelf digital cameras; as such, the images are not suitable for spectral analysis in most cases.
The ISS orbit is constrained to within 51.65 degrees N and S latitudes; nadir viewing opportunities are likewise restricted to this range.
Given the handheld nature of data collection, and the variation in ISS altitude, astronaut photographs may have a wide range of viewing geometries, sun illumination, and image pixel size; for these same reasons, it is difficult to obtain repeat imagery of a given area in the sense of a polar-orbiting, sun-synchronous, nadir-viewing platform such as Landsat.
Crew Earth Observations targets are provided to the crew on a “task-listed” basis. This means that the crew has the option of whether or not to respond to a given target callout, based on their daily schedule and other needed duties on ISS. For the requestor, this means that we cannot guarantee collection of data for a specific target at a specific date or time. In some cases, it may be some time before useful data of a given target is collected.
