In connection with the increased interest in observations from space, I propose to consider what is at stake and what it is capable of. The materials in this article are mainly excerpts from this book . In the next (if any), I will try to describe modern technology based on my knowledge and experience.

A spacecraft (SC) of observation usually flies in a circular orbit around its object.

In fact, this is a camera with a good lens.

This means that the person from above will be visible as a point, the tank is already about 50 points. There are no readings of newspapers and car numbers from orbit (and this is generally not required).
Typical apparatus structure:

For each specialist, his own system is the main one. The motion control system seems to me the main thing:

On past and still present devices, all this is compiled something like this:
Yes, these are huge devices weighing several tons. Requiring appropriate media and working relatively shortly. Modern microelectronics can significantly reduce the size and everyone already knows about cube-sat. Devices for communication or some kind of test are collected by students and even schoolchildren. But observation devices were some fundamental limitations of space:
- must be a precise definition of the orientation. As a rule, these are three or more devices.
- quite accurate stabilization is required when shooting an object. You can put power gyroscopes, but you can not accelerate them to infinity. After some time, it is necessary to reset the kinetic moment. In a magnetic field, electromagnetic systems are already used for this, but without a field, only motors.
- more shooting quality - more focal length of the lens. On modern devices, these are essentially telescopes.
- the transfer of large amounts of information (images) requires significant radio link power, especially at a distance from the Earth.
- more than about 500 watts per square meter of the solar panel does not get.

That's all for now. Next, we will consider what three devices are needed for orientation, how much fuel takes a couple of shots and how much a square meter of a solar panel weighs.

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