Simultaneous oblique sounding with Doppler analysis and low-probability-of-intercept communications using digital ionosondes


  • D. Mark Haines,

  • Bodo W. Reinisch,

  • George P. Cheney


This paper describes a waveform, modulator, and demodulator for which a U.S. patent has recently been granted. Using a programmable ionospheric sounder (the Digisonde portable sounder, or DPS), the composite spread-spectrum radio signal has been generated, and preliminary on-the-air tests have been made to verify its efficacy. The new radio signal uses an eight-dimensional (8-D) set of digital message symbols, each defined by a unique position in an 8-D orthogonal signal space, providing a symbol set of over 250 million symbols. Four dimensions are spanned by modulating time delay, amplitude, frequency, and spreading code, while a second subcarrier in phase quadrature to the first nearly doubles the capacity by providing four more orthogonal dimensions: time delay, amplitude, code, and phase modulation. This large symbol set allows the signal to carry 10 s of data bits simultaneously in the same radio bandwidth that conventional direct sequence (DS) spread-spectrum signals use to convey only 1 bit of information while, like conventional DS, providing in excess of 30 dB of signal processing gain. The benefits provided by this new waveform are (1) higher data rates without channel equalization; (2) lower interference to other users of the radio spectrum; and (3) low power or extended range operation.