Modern over-the-horizon (OTH) radars currently being developed for coastal surveillance incorporate antenna arrays which are to be erected quickly with minimal site preparation. For such arrays, array calibration is necessary. Meteors, noise sources and beacons (special sources) may be used for calibrating these OTH radars. In this paper we discuss some properties of these sources from an array calibration perspective, with the major emphasis on meteors. For calibrating a 16 element OTH radar array, one requires 20-30 plane-wave sources for each frequency of interest within the operating frequency band. The sources need to be stationary over the observation interval, which must consist of at least 5-10 snapshots. Further, the sources must have signal-to-noise ratio (SNR) in excess of 20-25dB.