## 1. Introduction

[2] Traveling ionospheric disturbances (TIDs) remain a troubling cause of coordinate registration (CR, geolocation) errors for over-the-horizon (OTH) radar. They are also the principal remaining mechanism limiting the performance of SIFTER, an enhanced target detection algorithm for OTH radar [*Fridman and Nickisch*, 2004], because the associated range/azimuth deflections spread the target return in the SIFTER computational space. In SIFTER, a portion of the effect of TIDs is mitigated by ionospheric-induced Doppler (IDOP) correction. Here the Doppler shifts imposed by the moving ionosphere are removed by aligning the Bragg line signatures in the radar returns. (These Bragg line signatures are enhancements in radar backscatter corresponding to coherent ocean wave scatter from wave components with half the radar's operating wavelength that are moving toward and away from the radar.) IDOP correction, however, does not affect the range and azimuth domains of the data. In a series of measurements where the U.S. Navy Relocatable Over-the-Horizon Radar (ROTHR), AN/TPS-71, dwelled on a fixed beacon for long periods of time, the beacon appeared to move several tens of kilometers in times as short as a few minutes. These apparent motions are clearly due to TIDs. An example of this is shown in Figures 1 and 2, which show the measured group range (or group path, denoted *P*′, which is the speed of light times propagation delay) and azimuth of the fixed beacon over a period of about 3 hours. The azimuthal swing near 0450 UT, for example, corresponds to an apparent motion of 80 km over a time span of only 15 min. It is easy to see that such artificial motion will be detrimental to trackers tuned for slow-moving maritime targets. Furthermore, since OTHR CR algorithms are not currently designed to account for TID apparent motion, very large CR errors will inevitably result.

[3] So far it has been impossible to mitigate the effect of TID-induced motions on OTH radar return signals. Conventional OTH radar soundings have neither the spatial nor temporal resolution to resolve TIDs. In a generalized CREDO approach, we augmented ROTHR soundings with total electron content data derived from low Earth orbit satellite beacons and were thus able to “image” TID structure and account for it in the CREDO ionosphere model [*Fridman and Nickisch*, 2001]. But augmenting ROTHR with a suitable satellite beacon data source is impractical in the near term.

[4] We know that to a large extent, IDOP is caused by TID activity. We also know a lot about the physics of TIDs. They are driven by acoustic gravity waves (AGWs), which are buoyancy waves in the neutral atmosphere akin to water waves in the ocean. Because of the exponentially decreasing density of the neutral atmosphere with altitude, the energy from fairly minor disturbances in the lower atmosphere turns into large-amplitude waves at altitude, and these drive the ionosphere up and down the geomagnetic field lines; the geometry of TID undulations is constrained by the geomagnetic field. There is potential, then, to correlate IDOP with TID-induced deviations and to model this relationship. Recent studies of TIDs have been done by *Kirchengast et al.* [1996] and *Ma et al.* [1998].

[5] We report here on a study of TID-induced apparent range deflections of HF sky wave signals. This study is based on both numerical simulation and on real OTH returns of a fixed beacon (transponder). For the simulations, we apply ray tracing in modeled TID environments using an implementation of the Bottone AGW/TID model [*Bottone*, 1992], which includes geomagnetic constraints and temporal evolution of the effects. Using these numerical simulations, we analyze IDOP–group range rate correlations with the goal of eventually developing a mitigation strategy for TID wander. The idea is to use the indication of TID activity present in normal OTH radar IDOP measurements to estimate the associated apparent wander of the OTH radar signals and to mitigate this wander. Previous studies of the HF signatures of TIDs were done by *Georges and Stephenson* [1969] and *Earl* [1975].

[6] In section 2 we present theoretical expectations for the correlation of group range rate with Doppler shift in TID environments. Results obtained in our numerical simulations are presented in section 3, and in section 4 we present a preliminary analysis of ROTHR fixed-beacon measurements. Concluding remarks are given in section 5.