Apparatus for passively measuring the velocity of a ferrous vehicle along a path of travel

Apparatus for passively measuring the velocity of a ferrous vehicle along a path of travel, comprising:

• a) first and second magnetometers located adjacent said path of travel and spaced from one another by a distance dx along a line essentially parallel to said path of travel with each said magnetometer being operable to produce a plurality of magnetometer signals indicative of the magnetic induction of said vehicle along respective magnetometer axes, as said vehicle travels along said path in the vicinity of said magnetometers;
• b) signal processing means responsive to said magnetometer signals to derive respective simultaneous first and second resultant magnetic induction indicative signals, B.sub.1 and B.sub.2, indicative of the magnetic induction of said vehicle at the respective location of said first and second magnetometers, and being further operable to derive the difference, B.sub.1 -B.sub.2 =dB.sub.space, between said signals;
• c) said signal processing means being operable to repetitively sample said magnetometer signals to derive said magnetic induction indicative signals;
• d) said signal processing means being operable to drive the difference, dB.sub.time, in magnetic induction indicative signals from one said sampling to a subsequent sampling, for the first of said magnetometers;
• e) memory means for storing derived signals;
• f) said signal processing means being responsive to said stored signals and operable to derive an indication of the velocity of said vehicle from the ratio of:
• dB.sub.time with respect to the sampling period, and dB.sub.space with respect to the distance between said magnetometers.