| From the beginning of the practice of ionospheric sounding the instruments used, with a few notable exceptions, have incorporated powerful pulse transmitters. Due to the extremely high external noise in the HF spectrum and the large variability of propagation conditions, it is hard to set a reasonable upper limit on the amount of power required to provide quality measurements. However, balanced against this need for high power is a desire on the part of radio operators as well as government agencies and the scientific community to have a small portable monitoring capability to get the measurement instrument and the data provided by it closer to the end user, especially at remote locations. These motivations initiated an effort at the University of Massachusetts Lowell to apply modern radar signal processing techniques and technology in order to provide a high quality measurement capability in a small, inexpensive, low power system. This new instrument was named the Digisonde Portable Sounder or DPS.; Techniques incorporated in the DPS system which provide the basis for its measurement capabilities include coherent digital pulse compression, coherent Doppler integration, interpulse phase coding, digital beamforming, stepped frequency high range resolution processing, d{dollar}phi{dollar}/df precision ranging, super-resolution angle-of-arrival processing, rapid switching of transmitter and receiver polarization, and multiplexed integration for simultaneous measurement on several antennas, frequencies and polarizations. Technology involved includes parallel phase coherent receivers, direct digital frequency synthesis, 12-bit quadrature digitization at 1MHz, DSP/microcomputer parallel processor architecture, interrupt driven real-time multitasking software, MOSFET solid-state transmitter, and synchronization to a GPS satellite receiver.; The pulse compression technique with the widest acceptance in the radar community has historically been FM/CW "chirp" waveforms. The waveform used in the DPS is fundamentally different (PSK/CW) using ideal sets of maximally orthogonal phase modulation codes. Pulsed and CW techniques were developed which realize a pulse compression with absolutely no spurious time domain response (i.e. time domain sidelobes, or leakage). Although this is mathematically trivial with infinite bandwidth signals, its realization in a physical, bandlimited system is subtle and unique. |
