The present invention relates to terrain navigation wherein an
earth-reflected satellite signal is received from at least one reference
signal source whose position is known (120), for example a navigation
satellite. Based on information contained in the received signal a
measured estimate of a shortest reflection distance (d.sub.1(t.sub.b)) to
the satellite (120) is derived. A relative spatial movement (.DELTA.x,
.DELTA.y, .DELTA.h) of a relevant craft (110) since a previous
positioning instance (t.sub.a) is registered onboard the craft (110).
Moreover, stored surface information is retrieved pertaining to an
elevation (h.sub.1b) of the earth surface (130) in relation to the mean
sea level (z.sub.0) at at least one geographical position (X.sub.1b,
Y.sub.1b). Based on the relative spatial movement (.DELTA.x, .DELTA.y,
.DELTA.h) and the retrieved surface information at least one calculated
estimate of the shortest reflection distance (d.sub.1(t.sub.b)) to the
satellite (120) is derived. Finally, a geographical position (X.sub.1a,
Y.sub.1a, h.sub.1a) is determined on basis of the measured and calculated
estimates of the shortest reflection distance (d.sub.1(t.sub.b)). Thus, a
highly reliable and entirely passive positioning may be accomplished on
basis of external signals from as few satellites (120) as one.