The IRIS system at NAL produces a two-dimensional 8x8 narrow-beams (about 11° half-power beam-widths) by using a two-dimensional 8x8 half-wavelength dipole array (Plate 1), Butler matrix and 8-ch radio receivers (Plate 2). System specifications are as follows:
Operating Frequency 30.0 MHz Bandwidth 100 kHz Sensitivity 0.1 dB Phasing system Butler matrix Location 78.9°N, 11.9°N L value L = 16 Sample interval 4 s ADC system 12 bits Data storage Magneto-Optical disk
The spacing between each dipole is 0.65-wavelength at 30 MHz. The projection of -3 dB power-widths of the two-dimensional 8x8 beams projected to the ionospheric plane at 90 km altitude is shown in Figure 1 (Yamagishi et al., 1992). The north-south dipole array is aligned with the invariant magnetic north-south direction. In the Figure 1, beam numbers are given along the North-South and East-West directions by marks of N1, N2, ···, N8 and E1, E2, ···., E8, respectively. The specific pencil beam near the zenith is, for example, marked by N4E4, corresponding to the cross-section of N4 column and E4 array. The field-of-view of the IRIS is approximated by a square of about 200 km side at the 90 km altitude. Detailed performances of the IRIS system is reported by Yamagishi et al. (1992) and Sato et al. (1992) and initial observational results is presented by Nishino et al. (1993).
