Motivated by a satellite communications problem, we consider a generalised colouring problem on unit disk graphs. A colouring is k -improper if no vertex receives the same colour as k +1 of its neighbours. The k -improper chromatic number chi_k (G) is the least number of colours needed in a k -improper colouring of a graph G. The main sub ject of this work is analysing the complexity of computing chi_k for the class of unit disk graph and some related classes, e.g. hexagonal graphs and interval graphs. We show NP-completeness in many restricted cases and also provide both positive and negative approximability results. Due to the challenging nature of this topic, many seemingly simple questions remain: for example, it remains open to determine the complexity of computing k for unit interval graphs.