I am currently in Ukraine studying avionics and stuff about ballistic missiles for some time. I have a pretty good university knowledge of DSP  and just  started learning about control engineering. I would like to know whether my knowledge of DSP is useful for control engineering (in e.g. aviation and/or ballistics)?

Not only is knowledge in DSP **very** useful designing digital controllers, but Control Theory has some concepts that are useful for DSPers that never got paid for designing a controller.  A couple are:

1. State-Variable LTI system model.  Ya know, that thing with the A, B, C, D matrices and states.  And the notions of controllability and observability is useful.  Sometimes DSPers design filters that have pole/zero cancellation (like a moving sum or moving average filter) and you could have an unstable pole that puts in an unstable mode you cannot see in the output because the system is not completely observable.  The state-variable model is straight-forward to code, but you analyze it and understand it from what we learn in Control Theory.

2. I have once had to design a miniature PID controller in an Asynchronous Sample Rate Converter alg in a DSP.  When you have two independent and asynchronously-sampled signals, if you want to line the signals up in real time (which includes times in between samples), you may need a "hurry up" or "slow down" controller on the sample pointers in memory.

Other than that, I haven't used what I learn **solely** from Controls in DSP, **but** there is a lot of overlap because of all of this LTI Systems and Signals theory we get as a prerequisite to Controls, Communications, Electronic Circuits, DSP, etc. In both Controls and DSP you want to be very familiar with transforms, transfer functions, and poles and zeros.  What you learn in one will help you in the other.

Yes, it's very useful.  Think of the frequency response of the control system - that's just signal processing.  All you learned about filters applies to sensor feed back to the control system.  You should have a very easy time with control systems - you already know the hard part!

Here's a link to an article back in 1978 by MIT Prof. Alan S. Willsky, one of the authors of the "Oppenheim Signals and Systems" bible:

Relationships Between Digital Signal Processing and Control and Estimation Theory