The subject of this paper is the merchant marine user requirements for position fixing systems (a merchant marine with approximately 40000 ships operating). These requirements are varied and can be summarized as follows:

(i) accuracy

(ii) availability (possible fix frequency)

(iii) reliability

(iv) coverage

(v) user skill and workload

(vi) presentation of information

(vii) ambiguity

(viii) environmental constraints

(ix) operational constraints

(x) system control

(xi) system user costs.

These eleven requirement elements should together serve the two basic purposes of merchant marine navigation: (a) provide a service adequate for safety and (b) enhance economic performance/benefit.