Ionospheric heating experiments have begun to examine interactions at altitudes where the heater HF wave frequency matches the local electron plasma frequency. We discuss the frequency-shifted signature observed by radiowave receivers on a rocket launched into the HF beam and approaching the interaction altitude. The frequency shifts associated with the upgoing (transmitted) and the downgoing (reflected) waves arise from the Doppler effect and changes in the wave vector. The characteristic spectral signature provides a method to locate the reflection altitude accurately. Generally, the ratio of intensities of upgoing and downgoing waves decreases as the reflection altitude is approached.¿ 1997 American Geophysical Union