Abstract: 1. The method of determining the trajectory of a ballistic missile having trajectory substantially defined by the formula:y = - g + .rho..infin. g(y).sup.2 /2 .beta. sin .gamma.x = .rho..infin. g (x).sup.2 /2 .beta. cos .gamma.comprising the steps of measuring the velocity and flight path angle of the missile at a reference altitude, measuring the radiation emitted by the gases in the proximate environment of the missile, measuring the time required for the missile to travel from the reference altitude to the occurrence of maximum radiation, measuring the altitude at which maximum radiation occurs, computing the value of "a" from the following formula: ##EQU1## COMPUTING THE VALUE OF ".beta." FROM THE FORMULA:A = .rho..sub.O G/2 C .beta. SIN .gamma.whereV = velocity at altitude yV.sub.e = velocity at altitude y.sub.o.gamma. = flight path anglee = base of natural logarithmsc = constanta = .rho..sub.o g/2 c .beta. sin .gamma..rho..sub.o = density of the atmosphere at sea level.rho..infin.