TIDDBIT HF Doppler Radar
Gravity waves (Classification of Gravity waves/TIDS) occur everywhere in the atmosphere all the time, yet their characteristics and climatology are not well known or understood, due to the difficulty of detecting and measuring their properties.
ASTRA is using the TIDDBIT (Traveling Ionospheric Disturbance Detector Built in Texas) radar to investigate the sources, propagation and dissipation of gravity waves, and their effects on the ionosphere. ASTRA has extensive experience (e.g. Crowley et al., 1987; Crowley and Williams, 1987; Williams et al., 1988; Rice et al., 1988; Crowley and McCrea, 1988) with HF systems, and is currently building Doppler sounders in Texas, Virginia, New Mexico, and Peru (Jicamarca). The TIDDBIT systems in Virginia and Peru measure waves thought to generate ionospheric irregularities that cause scintillation in GPS and other radio signals. The system in Peru will also provide electric field estimates.
HF Doppler sounders (HF Doppler Sounder Principle) represent a low-cost and low-maintenance solution for monitoring gravity wave activity in the F-region ionosphere. Together with modern data analysis techniques, they provide both horizontal and vertical velocities across the entire Traveling Ionospheric Disturbance (TID) spectrum.
A TIDDBIT Doppler system consists of three continuous wave (CW) radio transmitters and a receiver, have a high- frequency stability (1 part in 107), together with some kind of recording device (e.g. Crowley, 1985). The CW signals are typically transmitted in the HF band between 3-10 MHz with a power of about 20-100 watts. The transmitters are placed about 100 km from the receiver. When a HF radio wave is reflected from the ionosphere, movement of the reflection point during passage of a TID produces a change in phase path and a Doppler shift proportional to the time rate of change of the phase path. If three or more spatially separated propagation paths are monitored, the time difference between the wave signatures from the three reflection points yields the speed and direction of the TID by triangulation. The TIDDBIT radar provides the TID parameters listed in Table 1 (period, horizontal and vertical velocities and wavelengths). The table also indicates how the measured TID parameters relate to the properties of the underlying acoustic gravity wave (AGW) propagating through the neutral gas, as explained in more detail under “ HF Doppler Sounder Principle ”.
Figure 1 depicts typical TIDDBIT data from the Texas system. The Doppler shifts on three transmission paths are plotted as a function of time for about 6 hrs. The frequency perturbations are well correlated on three channels, and wave properties such as velocity (Fig 2) can be obtained as a function of period.
TABLE 1. TID Parameters Provided by TIDDBIT radar versus Gravity Wave Parameters