A method is described whereby the directly observable radial velocities of a missile relative to a ground receiver—both in a radio-Doppler electronic system of tracking—may be compared to radial velocities determined by vacuum-trajectory formulae and the differences converted to ionization densities at the levels through which the rocket is passing. Basically, DOVAP1 is an instrumentation system which continuously compares the phase of two radio-frequency signals transmitted from a ground station to another ground station—one signal directly, the other by way of the rocket. The continuous variation in phase difference as a function of time is the well-known Doppler effect and is proportional to the missile velocity in the transmitter-missile-receiver path. If the transmitter and receiver are coincident, the measured velocity is twice the radial velocity of the missile relative to the receiver. 1Doppler velocity and position. The fact that the DOVAP system depends upon phase comparison for the dat... Abstract A method is described whereby the directly observable radial velocities of a missile relative to a ground receiver—both in a radio-Doppler electronic system of tracking—may be compared to radial velocities determined by vacuum-trajectory formulae and the differences converted to ionization densities at the levels through which the rocket is passing. Basically, DOVAP1 is an instrumentation system which continuously compares the phase of two radio-frequency signals transmitted from a ground station to another ground station—one signal directly, the other by way of the rocket. The continuous variation in phase difference as a function of time is the well-known Doppler effect and is proportional to the missile velocity in the transmitter-missile-receiver path. If the transmitter and receiver are coincident, the measured velocity is twice the radial velocity of the missile relative to the receiver. 1Doppler velocity and position. The fact that the DOVAP system depends upon phase comparison for the dat...