It is observed that an ionospheric volume in the F layer subjected to high power HF illumination becomes an effective scattering medium for radio signals in the VHF/UHF frequency range. The experimental results are representative of a field‐aligned scattering geometry for which the first such observations of VHF/UHF scattering from a heated ionospheric volume are presented. Two distinct scattering modes are observed, center‐line and plasma‐line scattering. Center‐line scattering is observed at the transmitted radar frequency f; plasma‐line scattering is observed as a pair of sidebands at f ± fh where fh is the heater frequency. The two scattering modes are observed to have markedly dissimilar characteristics. Center‐line scattering is highly aspect sensitive with respect to the direction of the geomagnetic field, B; plasma‐line scattering is found to be much less aspect sensitive, if at all. The region of maximum backscatter for the center‐line mode is found from these measurements to consist of the locus of points within the heated volume over which perpendicularity between the radar line of sight and B is achieved, independent of the location of maximum heating. The backscattering region for the plasma‐line mode is found from these measurements to be determined by the altitude of maximum heating, independent of geometrical considerations involving B. The longitudinal coherence length, L, for center‐line scattering is found from these measurements to be greater than the maximum antenna diameter of 85 ft; no more exact estimate for L is possible. A striking reversal in frequency dependence is found between the center‐line and plasma‐line modes. The per‐unit‐volume center‐line backscatter cross section is found to be about 7 db greater at VHF than at UHF; the per‐unit‐volume plasma‐line backscatter cross section is found to be at least 11 db less at VHF than at UHF. Preliminary results concerning time‐dependent behavior are presented. For both modes the scattering cross section is found to be effectively turned on and off very rapidly in response to the heater excitation; the spectral width of the scattering for both modes is found to be quite narrow (about 10 Hz). The spatial configuration of the heated volume is investigated; significant differences are observed depending on whether fh/f0F2 is greater or less than unity.

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ance connected to terminals of negative resistance. I 2 -r , -2 X Fig. 2-Wes equivalent circuit of Fig. 1. R I ' E i -< Fig. + -s e r i e s model of tunnel diode amplifier. G =(equal source and load impedances yield maximum gain for a given bandwidths).The gain a t wo is 2Ro ' Go = ( z R , r i i a ; > In order to calculate the bandwidth, we will make the assumption that (cw-l/LW)' <

<p>This manuscript corrects a longstanding formulation of the precision of interferometric measurements of the height of topographic features.</p> <p>This communication presents a corrected formulation, differing significantly from a traditional and widely-employed one, for the precision of topographic measurement estimates by means of space-based or airborne radar interferometry. Comparisons between the different formulations are exhibited for interferometer baseline tilt angles relative to the horizontal of five and sixty degrees. The reasons for the differences are explained. The results are applicable to both downlooking topography and conventional geometries for interferometric synthetic aperture radar. The discrepancies are especially wide at near-nadir viewing, which is the case for a current satellite program. The corrected formulation for estimation precision is presented in both its near-exact form and a convenient approximate expression, the two differing by less than 0.1 percent for the parameters illustrated for a spaceborne interferometer.</p>