SANAE HF Radar
The SANAE (South African National Antarctic Expedition) HF radar is located in Antarctica (71.68 deg S, -2.85 deg E) and looks over a section of ionosphere poleward of 65 deg S that covers much of East Antarctica. It has operated since 1997. The facility is part of the SuperDARN network of HF radars that extends from Alaska to Scandinavia in the Northern hemisphere and covers much of Antarctica in the Southern hemisphere. The SuperDARN radar for the most direct 2-D merging of SANAE velocity data is located at Halley Station.
The radar forms and steers its beam by the phasing of transmissions from 16 elements in a linear antenna array. It has no second array for deriving elevation angle information. The basic radar operation consists of the following steps: i) selection of a beam position (0-15) ii) search for the quietest 5-kHz channel about the assigned transmitting frequency (8-20 MHz) iii) repeated transmission of a multi-pulse sequence over the selected integration period and reception of backscattered signal with gating in range (1-70) iv) calculation of the auto-correlation functions (ACFs). These operations are usually carried out in sequence for the 16 beam positions, which collectively constitute 1 scan. The beam integration period is usually 7 sec, and the scan repeat time is then 120 sec (2 min). Within a scan, beam 0 corresponds to the most westward beam position and beam 15 to the most eastward. The azimuth sector scanned is approximately 50 deg wide and is centered on 107.0 deg E of N.
Returned signal is generated primarily by two mechanisms i) coherent backscatter from small-scale (decameter) field-aligned irregularities in the E and F regions, and ii) backscatter due to reflection from the earth's surface after reflection from the ionosphere ("groundscatter"). For both types of scatter, analysis of the ACFs gives estimates of the backscattered power, line-of-sight Doppler velocity, and spread in the Doppler velocity (i.e. spectral width). In the case of irregularity scatter, the motion is primarily due to the convection of ionospheric plasma across geomagnetic field lines, thus the Doppler velocity characterizes one component of the convective drift. In the case of groundscatter, information on the motion of the ionospheric layers can be inferred from the imposed Doppler shift.
In cases when the ionospheric scatter is noisy, improved estimates of the velocity can be obtained by filtering. An example: examine the data in blocks of 5 range gates, 4 beams, and 3 scans. This amounts to sampling 5x4x3 = 60 times in a block. Require that a certain % of the samples deliver good signal (e.g., 15-50%), then do a median filter to generate a best velocity estimate. The good signal could be selected on the basis of reasonable velocity (e.g, |vel|<2000 m/s) and significant power (e.g., dB>3).
For these clock-dial summary plots of the presumed line-of-sight ion drift data, all groundscatter data were eliminated, and averages were made over 4 azimuths, 4 ranges, and 3 scans approximately every 6 minutes. Data were eliminated if they were at altitudes less than 200 km (could be 150 km), if the velocities were less than 25-35 m/s (usually ground scatter) or greater than 2000 m/s, and if the signal-to-noise ratio was less than 2 dB (or 3 dB). In addition, if a bin was less than about 20% (10-35%) full, which is the occurance percentage, then the values were also discarded. Tick marks on the outer circle show the approximate time of UT 0, 6, 12 and 18. The occurance percentage used is listed on the plots. These data are plotted from files made from the original data for AMIE (Assimilative Mapping of Ionospheric Electrodynamics) runs.
Greenwald, R. A., et al., DARN/SUPERDARN, A Global View of the Dynamics of High-Latitude Convection, Space Science Reviews, 71, 761-796, 1995.
Summary Plots for SANAE HF Radar
Other Summary Plots at British Antarctic Service site
Color plots of the line-of-sight velocity (where negative flows are away from the radar) are available from May 18, 1997.
Daily Listing for IS/HF Radars
-Revised 02 Jun 2001 by Barbara Emery