Abstract: A passive data determination system (10) for use in a traveling projectile (12) is provided including a logarithmic receiver (20) for receiving a power signal P.sub.r from a source target (14) and emitting in response thereto an output voltage signal E.sub.o which is proportional to the logarithm of P.sub.r. A clock pulse signal is provided, such as by an oscillator (22) to measure a change in output voltage .DELTA.E.sub.o. Means such a microprocessor (28) provides a signal R which is a function of Kv.multidot.(.DELTA.t/.DELTA.E.sub.o), where K is equal to -2 (log e) and V is a known relative velocity between the projectile (12) and the target (14) and which indicates the range of the projectile to the target.