The MAPSTAR imaging Doppler interferometer (IDI) radar: description and first results

Abstract MAPSTAR is a medium-frequency imaging Doppler interferometer (IDI) radar. In this paper we describe the radar and present results from 34.5 h of data taken during the AIDA campaign in Puerto Rico during April 1989. The IDI method uses several independent antennas and receivers, pulsed sounding, range-gating, Doppler sorting and spatial interferometry to determine a three-dimensional Doppler image of the rf scatterers within the volume being illuminated. The analysis characterizes any perturbation in the index of refraction that returns rf energy (clear-air turbulence, scattering from striations, meteor trails, etc.) in terms of the three-dimensional locations, Doppler velocities and scattering amplitudes and phases of a number of apparent points in space, called ‘scattering points’. Scattering points are defined as those Fourier voltages whose phases on the several antennas agree on the direction in the sky of the source of that spectral power. We find that most of the power scattered from the mesosphere and E region can be described in this fashion. This allows us to replace, say, 10 complex voltages by two voltages and two coordinates locating the source. The IDI process is, en passant, a data-compression technique, reducing the data volume by a factor of 25–30 without losing any significant information. However, if we can so easily characterize the data as if discrete scattering points were responsible, the obvious question is: what physical processes do these scattering points actually represent? We don't complete an answer to this question here, but we do begin an answer with a description of some of their properties and behavior. We present in Section 2 a description of the hardware as designed and in Section 3 a description of its deployment in Puerto Rico, where the radar was tuned to 3.175 MHz and used the Arecibo Heating Facility's transmitters and transmitting antennas. In Section 4 we show the time domain data, in which the E region and several distinct mesospheric regions can be seen. We show the power vs altitude profile, which is typical of medium-frequency returns and describe the phase behavior of the returns, which can be used for tracking discrete targets such as TIDs and meteors. We describe in Section 5 the IDI algorithm, which involves Fourier transforming the several independent data streams and examining (for each pulse series, at each range-gate and at each Fourier frequency) the phases of the 10 complex Fourier voltages. When it is sufficiently accurate to represent the phase variations along the two linear distributions of antennas as linear with distance, we replace the 10 complex voltages with two voltages and two phase gradients. The data are thus cast into a form that is most easily interpreted as the result of scattering from a number of simultaneous discrete points. In Section 6 we describe some of the properties of these scattering points. The polarization properties of the points are shown first, since this is used for filtering the results that follow. We show the distribution of radial velocities with altitude for the ordinary, extraordinary and linear modes. The radial velocities show the characteristic mesospheric-scattering distribution of the ordinary-mode points (which was the transmitted mode), contrasted with the near-random distributions of the linear and extraordinary-mode points. This justifies the use of polarization filtering and introduces a powerful new filtering technique. Next we show the combined power of the scattering points (the ‘recovered’ power) and its variation with altitude and time. The recovered power looks very much like the raw power as usually seen by medium-frequency radars (one to several mesospheric regions plus the E region), except that it has somewhat better altitude resolution (being in altitude rather than range) and better low-altitude coverage due to ≈ 10 dB better signal/noise. Skymaps of the scattering points (i.e. their locations in the horizontal plane) are shown at several altitudes, showing the varying spread and sometimes asymmetry, in their distribution. We then show the variation of the recovered power with zenith angle, demonstrating the aspect sensitivity of the returns, from nearly specular at 60 80 km, spreading out to a maximum around 90 100 km and contracting sharply to very mirror-like above that. The motions of the scattering points are explored with contour maps of horizontal layers and by calculating their three-dimensional mean motions. Estimates of the calcultional uncertainties are made for the locations and motions of the points.