Monitoring the ionosphere altitude variation with a sound card software defined radio processing of…
Channel: FOSDEM
Date: 06/03/2018

Monitoring the ionosphere altitude variation with a sound card software defined radio processing of DCF-77 signals
by Jean-Michel Friedt

At: FOSDEM 2017

[DCF-77](https://www.ptb.de/cms/fileadmin/internet/fachabteilungen/abteilung_4/4.4_zeit_und_frequenz/pdf/2011_PTBMitt_50a_DCF77_engl.pdf)is a German very low frequency (VLF) emitter locked on the Cs clocks of PTBused for synchronizing radiofrequency-disciplined clocks found, for example,in low-cost weather stations. VLF signals propagate through the waveguidewhose boundary conditions are defined on the one hand by ground, and on theother hand by the ionosphere (charged) layer altitude. In addition to theamplitude modulation of the Cs-locked DCF-77 frequency standard, a [phasemodulation](https://www.ptb.de/cms/en/ptb/fachabteilungen/abt4/fb-44/ag-442/dissemination-of-legal-time/dcf77/dcf77-phase-modulation.html) is imprinted on the carrierfor time of flight measurement, allowing for "precise" time of flightmeasurements. 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 itspractical implementation challenging will be emphasized. Most significantly,the processing chain is very similar to those applied to [GPSsignals](https://archive.fosdem.org/2015/schedule/event/xcorr/), yet easier tograsp with the strong VLF signal. We aim at analyzing the time stability ofthe phase modulated signal -- allowing for sub-100 us timing resolution -- andcorrelated observed phase fluctuations with [ionospherealtitude](https://www.funkamateur.de/tl_files/downloads/hefte/2006/DG9WF_Iono_lang.pdf)


Room: AW1.120
Scheduled start: 2017-02-04 17:15:00