Luleň University of Technology, Luleň, Sweden
In order to provide truly instantaneous three-dimensional radar measurements spanning the entire vertical extent of the ionosphere, the planned EISCAT 3D incoherent scatter system includes multiple receive-only antenna arrays, situated at 90-280 km from the main transmitting/receiving core site. These will employ band-pass sampling at ~80 MHz, with the input signal spectrum contained in the 6th Nyqvist zone. Digital beamformers realized in Field Programmable Gate Arrays (FPGAs) will generate five or more simultaneous beams that intersect the transmitter beam at different altitudes. Among key requirements is frequency-independent beam direction over a 30 MHz band with correct reconstruction of pulse-lengths down to 200 ns.
This talk presents simulations and methods used to investigate how well these requirements can be met by a Fractional-Sample-Delay (FSD) system. To reconstruct short pulses the beamforming must be a true time-delay system since the whole array not will be illuminated simultaneously. Subsequent demands are put on this system: The clock jitter from sample-to-sample must be extremely low for the integer sample delays; the FSD must be able to delay the 30 MHz wide signal-band 1/1024th of a sample without introducing phase shifts; and it must all be done in the base-band. The simulation system, LAARSE, is implemented in Matlab to provide cross-platform compatibility and can be applied to any similar system. Performance degrading aspects such as noise, jitter, bandwidth and resolution are included.
The use of Finite Impulse Response (FIR) filters in the base-band of a band-pass sampled signal to apply true time-delay beamforming is shown not only possible but also well behaved. The simulations done with LAARSE have resulted in better knowledge of the necessities of the FIR-filters as well as a relaxation of the preliminary timing demands on the EISCAT 3D system.