Radio Science, Volume 32, Number 5, Pages , September-October 1997

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1 Radio Science, Volume, Number, Pages 87-88, September-October 997 Azimuth spread of two-hop ionospheric modes W. M. Sherrill and Bradley C. Brown Southwest Research Institute, San Antonio, Texas Abstract. We use direction finding ionogram data to analyze the azimuth spread of twohop F region modes in which ground scatter occurs () in the Gulf of Mexico and coastal areas of southern Louisiana and () in rough terrain in the Painted Desert region of Arizona. Ocean surface/coastal ground scatter are observed to be quasi-specular within the resolution of these measurements. Rough terrain ground scatter imposes a nominal. ø azimuth spread in addition to that introduced by the two traverses of the ionospheric F region. Introduction published by Ratcliffe [96], Bramley [967], and The azimuth angle of arrival of ionospheric modes others; experimental studies go back to Eckersley varies as a function of time (illustrated schematically [98] and include Rice [97] and, in this laboratory, in Figure ) in response to refractive changes due to Sherrill and Smith [ 977], and Black et al. [ 99]. traveling ionospheric disturbances and other sources In this paper, we analyze direction finding (DF) of ionospheric variability. This phenomenon was sounding measurements originally reported by Black intensively studied in the 9s to 96s, beginning et al. [99] on one- and two-hop modes, comparing with the influential summary published by Ross one-hop angular spread resulting from a single tra- [97], and typically characterized in terms of the verse of the ionosphere with two-hop angular spread RMS bearing error with respect to the Great Circle including two ionospheric traverses plus a ground bearing. The angular spread of single modes relates to scatter at the path midpoint. We use this data to the intrinsic spread of the mode and is characterized separate the ionospheric from the ground scatter by the standard deviation (SD) about the (time vary- components of azimuth spread and estimate the ing) mean angle of arrival as shown in Figure. ground scatter component separately for two types of Over the decades, theoretical treatments have been terrain. SD, -..- iii I Illl II Ill ß ß./'-;.:.... &, %. -% RMS,....,.,... io == ß p,',. d GCB ERROR Figure. Single mode azimuth versus time illustrating standar deviation (SD) with respecto the trend line and RMS error with respecto the Great Circle Bearing. Copyright 997 by the American Geophysical Union. Paper number 97RS /97/97RS

2 876 SHERRILL AND BROWN: AZIMUTH SPREAD OF IONOSPHERIC MODES ß ß ee o Sweep Freq Synth o o Sweep Control GPS Clock 7-Channel Receiver AD Converter DSP Rb Freq Std Host Computer Direction Finding lonesend Intrumentation Parameters Parame r V ue Disk Tape Work Station Computer Swe khz/= FFT eornl x blrm,s mple Id.lz Bin re duf(xt Hz Time cl ay re olution j o RF ( weep) rea Xt on khz Ps oontmi (so. s preci ion) DF 8- MHz for seven-eikeme : -r L 8fi y -m epmluro Figure. Direction finding sounder block diagram. DF Sounding and Path Geometry Figure shows a simplified block diagram of the DF ionosonde receiver. Each of seven coherent receiver channels is connected to an element of a We have analyzed -hour sequences of DF ionograms obtained on paths between 876 and km range from San Antonio, Texas, as listed in Table. The two-hop midpath from Isabela, Puerto Rico, is in the Gulf of Mexico; the two-hop midpaths from Table. D F Sounding Paths Path Bay St. Louis, Mississippi Panama City, Florida Range, km McClellan Air Force Base, California Isabela, Puerto Rico Mississippi and Florida are in low-lying areas along the Gulf coast. Only the two-hop path from McClellan Air Force Base, near Sacramento, California, entails ground scatter in rough terrain, i.e., near the Painted seven-element crossed baseline interferometer array to Desert region of Arizona. Figure shows the path produce frequency modulation/continuous wave geometries. The inset figures show a -km square (FM/CW) oblique ionograms in group delay, azimuth, centered on the path midpoint from McClellan and and elevation [Black et al. 99]. DF array and from Panama City, Florida. The difference in ground typical sounder operating parameters are summarized irregularity is obvious, in particular as the vertical in the inset table. axis on the McClellan path encompasses - 89 m; the Gulf coast midpath encompasses - m. Ground Scatter Effects on Mode Azimuth Hourly F and F azimuth spread for each of the four paths between 876 and km is compared in Figure, obtained by computing the standardeviation from the azimuth trend line (illustrated in Figure ) for all frequenciesupported by each mode/path. Each plot of hourly standar deviation shows a single -hour period. However, for the McClellan path, we show results obtained in two different years. The time axis increases from right to left beginning at 8 UT, proceeding through midnighto 8 UT the following day. For further comparative analysis, we take the afternoon period from 8 to UT where F

3 SHERRILL AND BROWN' AZIMUTH SPREAD OF IONOSPHERIC MODES ' "',."-'"' ",.. " ', ,.....'.....'.- 7O.... '....., --. ',..?.-.,-,.'.,-, '...,....? Mc Clellan Mid Path Panama City, FI Mid Path Figure. MidPath topography illustrating rough terrain path midpoint (McClellan Air Force Base) and smooth path mid-point (Panama City, Florida). (From: Map -6, U.S. Dept. of the Interior, U.S. Geological Survey.)

4 , 878 SHERRILL AND BROWN' AZIMUTH SPREAD OF IONOSPHERIC MODES Table. Afternoon Bearing Standard Deviation (8- UT) Path Range, km RMSD +o, deg RMSD+ o, deg Bay St. Louis, Mississippi Panama City, Florida McClellan Air Force Base (99) McClellan AFB (99) 876. _+..7 _ Isabela, Puerto Rico azimuth spread is consistently the lowest value for the entire day (subject, as always, to hourly variations), representing a time when ionosphericonditions tend to be most stable. Table lists the mean and standard deviation of azimuth (SD+o), averaged over the 8 to UT afternoon period for the F and F modes, respectively. The F modes show nominal. ø-.6 øaverage standardeviation (SD for all ranges, representing a single pass through the ionosphere. The corresponding two-hop mode (SD ) is larger than the F modes, reflecting the cumulativeffects of two passes through the ionosphere plus a single ground scatter near the path midpoint. However, (SD) for the McClellan path is significantly greater than (SD) on the other paths. To investigate the relative contributions of ionosphere and ground scatter to single-mode azimuth spread, we estimate the two-hop bearing variance (SD) using the one-hop bearing variances (SD ) as follows: sot - si () Thus SD SDI = ;q/ = F elevation () COS lilt Measured F low ray (F[o) and F low ray (F o) standardeviations obtained from DF ionogram data are shown in Figure. These SD values were extracted by selecting the azimuth measurements for the low ray branch of the oblique ionograms where resolved from the high (or Pedersen) ray branch. In Table, we estimate SD based on two ionospheric traverses and a quasi-specular ground scatter (SDo- ) and compare with measured afternoon SD. The measured DF ionogram data are shown in Figure. The agreement is good (i.e., within. o) on all but the McClellan path. If the excess azimuth spread is attributed to ground scatter, SDo can be computed by SD G = SD SD COS lilt () where SDo = AZ standard deviation due to ground The result is an estimated excess azimuth standard scatter and SD I =AZ standard deviation due to one deviation of.ø(observed in the two campaigns a ionosphere traverse. In using equation (), we make year apart), attributable to the irregular ground scatter the assumption that the azimuth spread contributions zone on the McClellan path. from each pass through the ionosphere (SDx) and from ground scatter (SDo) are independent. In estimating Summary SDi, we take the F SD to represent basic azimuth We have presented measuremeres of single-mode spread for one ionosphere traverse, increased by the azimuth standar deviation, illustrating the quamita- F elevation factor, which takes into account increas- tive increase in azimuth spread for two-hop modes ing geometric spread toward the zenith. compared with one-hop modes. For ranges of

5 , SHERRILL AND BROWN: AZIMUTH SPREAD OF IONOSPHERIC MODES 879 F Bay St. Louis, MS F 876 km $ o :.an,, np.,,l-!, I., ri-p., n Panama City, FL 9 km McClellan AFB km McClellan AFB km 8 nn,rnnlqonnllnnnbnfi[ 6 8 Ilnnn Isabela, PR = km 8 oonnnonnnbilllnnnnj 6. ô ' Figure. Hourly azimuth standar deviation (876- km range) of F and F modes.

6 88 SHERRILL AND BROWN: AZIMUTH SPREAD OF IONOSPHERIC MODES Table. Estimated F Standard Deviation From F Standard Deviation Mode F o F o Estimates F o SD Path Range, km (measured) : deg (SD Estimated) - SD COS / SD (measured) Bay St. Louis, Mississippi Panama City, Florida McClellan Air Force Base (99) McClellan Air Force Base (99) Isabela, Puerto Rico km or more, the F mode consistently produces Eckersley, T. L., A wireless interferometer, Nature,, azimuth standard deviation of the order of.øunder 69-7, 98. stable afternoon conditions, increasing to about Ratcliffe, J. A., Some aspects of diffraction theory and their application to the ionosphere, Rep. Prog..8øunder nighttime conditions. Hourly variation in Phys.,9, 88-67, 96. azimuth standard deviation, however, is significant Rice, D. W., Phase characteristics of ionospherically and most pronounced during nighttime hours. Ground propagated radio waves, Nature,, 86-88, 97. scatter appears quasi-specular for sea or flat, water- Ross, W., The estimation of probable accuracy of HF saturated terrain. Two-hop F modes with ground radio DF bearings, J. Inst. Electr. Eng. Part IIIA, scatter in mountainous, rough terrain consistently 9(), 7-76, 97. show nominally. ø azimuth standardeviation in Sherrill, W. M., and G. A. Smith, Directional disperaddition to that incurred by two traverses of the sion of sporadic E modes between 9 and MHz, Radio Sci., (), ,977. ionosphere. References Black, Q. R., J. F. Wood Jr., A. G. Sonsteby, and W. M. Sherrill, A direction finding ionosonde for ionospheric propagation research, Radio Sci., 8() 79-89, 99. Black, Q. R., J. F. Wood Jr., and W. M. Sherrill, Mode angles of arrival at to km range, Radio Sci., () 69-7, 99. Bramley, E. N., Diffraction of an angular spectrum of waves by a phase changing screen, J. Atrnos. Terr. Phys., 9(), -8,967. B. C. Brown and W. M. Sherrill, Signal Acquisition and Radiolocation Department, Signal Exploitation and Geo-location Division, Southwest Research Institute, 6 Culebra Road, P, O. Drawer 8, San Antonio, TX ( bbrown@swri. otg; wsherrill@swri. org) (Received December 6, 996; revised March, 997; accepted April, 997.)