Phase-coded pulse aperiodic transmitter coding

Both ionospheric and weather radar communities have already adopted the method of transmitting radar pulses in an aperiodic manner when measuring moderately over- spread targets. Among the users of the ionospheric radars, this method is called Aperiodic Transmitter Coding (ATC), whereas the weather radar users have adopted the term Si- multaneous Multiple Pulse-Repetition Frequency (SMPRF). When probing the ionosphere at the carrier frequencies of the EISCAT Incoherent Scatter Radar facilities, the range extent of the detectable target is typically of the order of one thou- sand kilometers - about seven milliseconds - whereas the characteristic correlation time of the scattered signal varies from a few milliseconds in the D-region to only tens of mi- croseconds in the F-region. If one is interested in estimat- ing the scattering autocorrelation function (ACF) at time lags shorter than the F-region correlation time, the D-region must be considered as a moderately overspread target, whereas the F-region is a severely overspread one. Given the technical restrictions of the radar hardware, a combination of ATC and phase-coded long pulses is advantageous for this kind of target. We evaluate such an experiment under infinitely low signal-to-noise ratio (SNR) conditions using lag profile inversion. In addition, a qualitative evaluation under high- SNR conditions is performed by analysing simulated data. The results show that an acceptable estimation accuracy and a very good lag resolution in the D-region can be achieved with a pulse length long enough for simultaneous E- and F- region measurements with a reasonable lag extent. The new experiment design is tested with the EISCAT Tromso VHF (224 MHz) radar. An example of a full D/E/F-region ACF from the test run is shown at the end of the paper.