The paper discusses the design, implementation, and test of a software-defined radio (SDR) based receiver to acquire HRPT (high resolution picture transmission) and APT (automatic picture transmission) signals of NOAA weather satellites simultaneously, recognizes modulation types, and demodulates the signals. The captured HRPT RF signal is downconverted to the 137.5 MHz band in the RF (radio frequency) front-end and then combined with the APT RF signal. The combined signals are then downconverted and digitized at the programmable receiver platform. The digitized data are spectrally analyzed with short-time FFT (fast Fourier transform) to obtain an instantaneous spectrum for adjustment of the adaptive digital band-pass filter. The modulation recognition block identifies split-phase PSK (phase shift keying) for HRPT and FM (frequency modulation) for APT. The HRPT signal is then demodulated with a Costas loop because the two side bands have more power than the carrier. The APT signal is demodulated in an FM demodulator with Zoom-FFT and automatic gain control (AGC) to maintain the brightness of the whole weather map according to the brightness of the synchronization mark. The system functions are developed and realized in an SDR software platform. The resulting SDR-based receiver is then used to receive and process real signals.
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