SIMULTANEOUS OBLIQUE SOUNDING WITH DOPPLER ANALYSIS AND LOW-PROBABILITY-OF-INTERCEPT COMMUNICATIONS USING DIGITAL IONOSONDES

This paper describes a waveform, modulator, and demodulator for which a U.S. patent has recently been granted. Using a programmable ionosphe ric sounder (the Digisonde portable sounder, or DPS), the composite sp read-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 d efined by a unique position in an 8-D orthogonal signal space, providi ng a symbol set of over 250 million symbols. Four dimensions are spann ed by modulating time delay, amplitude, frequency, and spreading code, while a second subcarrier in phase quadrature to the first nearly dou bles the capacity by providing four more orthogonal dimensions: time d elay, amplitude, code, and phase modulation. This large symbol set all ows the signal to carry 10 s of data bits simultaneously in the same r adio bandwidth that conventional direct sequence (DS) spread-spectrum signals use to convey only 1 bit of information while, like convention al DS, providing in excess of 30 dB of signal processing gain. The ben efits provided by this new waveform are (1) higher data rates without channel equalization; (2) lower interference to other users of the rad io spectrum; and (3) low power or extended range operation.