Wannberg, G.1, Westman, A.1, Pellinen-Wannberg, A.2, Kero, J.2 and Szasz, C.2
1 EISCAT Scientific Association, Kiruna, Sweden
2 Swedish Institute of Space Physics, Kiruna, Sweden
The EISCAT UHF system is uniquely well suited to meteor orbit determination through head echo Doppler velocity estimation. Since only a few good Doppler fits from each site are required to determine the orbital parameters, even meteors crossing the beams at very large angles and/or spending very short times in the common volume produce analysable events. The technique can therefore be used to map meteors arriving from arbitrary directions over a very large solid angle.
During 2001/2002, several versions of a radar code optimised for this application were developed, culminating in the code version used for all EISCAT meteor observations since and presented here. In this code, the transmitter is BPSK modulated by a low-sidelobe 32-bit pseudo-random sequence with 2.4 us baud-length, for a total pulse length of 76.8 us. Pulses are repeated every 1656 us. The receiver -3 dB bandwidth is set to 1.6 MHz to accommodate both the modulation bandwidth and the target Doppler shift. The receiver output voltage is sampled at 0.6 us intervals, corresponding to 90-m range resolution.
First-order target range and Doppler are extracted through a multi-step matched-filter procedure. A signal power vs. range function computed from the complex-amplitude data is convolved with a normalised infinite-SNR echo. This convolution peaks at a fixed offset relative to the start of an echo, whose range is thereby determined to within one or two samples. Next, the power-frequency spectrum of a 128 sample subset of the data vector is convolved with the theoretical spectrum of the transmitted pulse and the center-of-gravity of the convolution taken as the first-order Doppler estimate.
A fine-tuning procedure follows: A complex sinusoid at the first-order Doppler frequency is BPSK modulated with a range-shifted replica of the transmitted code and cross-correlated with the raw data vector. The pulse compression ratio is then computed from the correlation function. A robust gradient-search routine varies range and Doppler until the compression ratio maximises; their values at maximum are taken as best estimates.
For strong (SNR>5) events this code achieves 100-150 m/s Doppler velocity standard deviation. Its effective range resolution of about 30 m also allows very accurate time-of-flight velocity estimates. A statistical analysis of the entire database shows Doppler and TOF velocities agreeing to within about one part in 103. A unique feature is that the code can resolve two or more simultaneous targets separated by < 300 m in range; a beautiful VHF event will be used to exemplify.