Brussels / 4 & 5 February 2017


Monitoring the ionosphere altitude variation with a sound card

software defined radio processing of DCF-77 signals

DCF-77 is a German very low frequency (VLF) emitter locked on the Cs clocks of PTB used for synchronizing radiofrequency-disciplined clocks found, for example, in low-cost weather stations. VLF signals propagate through the waveguide whose boundary conditions are defined on the one hand by ground, and on the other hand by the ionosphere (charged) layer altitude. In addition to the amplitude modulation of the Cs-locked DCF-77 frequency standard, a phase modulation is imprinted on the carrier for time of flight measurement, allowing for "precise" time of flight measurements. The topic of the presentation is

  1. reception of the DCF-77 VLF signal using a coil antenna, with enough power to feed a lock-in amplifier and extract phase and magnitude information. When the reference signal of the lock-in is referred to a local Cs primary standard, day/night ionosphere altitude variations are readily observed -- here using the carrier phase analysis,

  2. extraction of the time of flight through CDMA processing of the phase output from the lock-in amplifier, recorded on a (low frequency) oscilloscope, emphasizing the time resolution improvement of the phase modulation with respect to the amplitude modulation gained from the increased signal bandwidth,

  3. replace the lock-in amplifier with software-defined radio processing of signals recorded with a personal computer sound card or the analog to digital converter (RTL2832U) of a DVB-T receiver dongle,

  4. refer the second channel of the stereo sound card/DVB-T to a low-cost GPS 1-PPS time reference for local oscillator drift compensation, and hence measure the ionosphere altitutde variation through time of flight variation by comparing the phase-encoded cross-correlation peak position with the 1-PPS position, yielding results consistent with the lab-grade instrumentation.

Although the whole processing chain is trivial, various issues making its practical implementation challenging will be emphasized. Most significantly, the processing chain is very similar to those applied to GPS signals, yet easier to grasp with the strong VLF signal. We aim at analyzing the time stability of the phase modulated signal -- allowing for sub-100 us timing resolution -- and correlated observed phase fluctuations with ionosphere altitude


Photo of Jean-Michel Friedt Jean-Michel Friedt