One of the most sensitive methods for detecting transient changes in the ionosphere is the HF Doppler technique operating in the 3-10 MHz band. HF Doppler systems have advantages over all other techniques for the measurement of TID characteristics. They are more amenable to analysis than data from ionosonde chains, and their time resolution (30 sec) is much higher than that of ionosondes . Unlike total electron content (TEC) methods, which respond to height-integrated TID effects, the HF Doppler radar responds to TIDs at the altitude of the radio reflection point. Finally, HF Doppler systems have low power consumption, so that both spatial and temporal resolution can be maintained for many days without the costs that would be associated with an incoherent-scatter radar.
ASTRA has designed, built and deployed TIDDBIT radars (TID Detector Built In Texas) in Texas, Virginia, and Peru to monitor the properties of bottomside F-region undulations that are thought to trigger ionospheric instabilities in low latitude regions. Each system consisted of three transmitters and a receiver. HF Doppler sounders together with modern data analysis techniques provide both horizontal and vertical velocities across the entire TID spectrum. We provide a synoptic survey of the TID characteristics observed over Texas during January-March 2002, and compare these data with newer results from the three systems.