The GNU Chirp Sounder is a software defined radio based receiver for monitoring ionospheric sounders (ionosondes) and over-the-horizon radars that use linear frequency sweep FM-CW transmissions. The software is based on gnuradio and relies on Ettus research USRP2 and USRP N210 based digital receivers. The receiver can be used to receive the whole HF band (typically at 25 MHz bandwidth) simultaneously, and to receive multiple sounders simultaneously. The current receiver can be used to perform single or dual polarization (channel) soundings. The dual channel recorded can be used to determine the polarization form vertical soundings, or for angle of arrival measurements. Multiple independent receivers could also be used for imaging purposes.

The program consists of a data recorder that performs chirp downcoversion band-pass filtering and decimation. The chirp downconversion multiplies the received signal with the undelayed analytic chirp waveform. Another program can be used to visualize the recorded data files, essentially performing a windowed spectrogram of the data file and scaling the axes correctly.

An example oblique sounding of a Cyprus based over the horizon radar measured in Sodankylä. The digital receiver was tuned to 22.5 MHz, but due to the help of aliasing, the full band between 8 and 35 MHz can be measured.

A sounding from Puerto Rico demonstrating the new adaptive filtering feature that removes HF broadcast stations.

An example dual polarization vertical sounding of the Sodankylä ionosond located 1 km from the receiver. The plot shows the phase difference between the North-South and East-West polarized incident plane waves.

An example O-mode vertical sounding of the Sodankylä ionosond located 1 km from the receiver.

An example X-mode vertical sounding of the Sodankylä ionosond located 1 km from the receiver.

6 days of the Cyprus 8-35 MHz OTHR measured up to 25 MHz with 5 minute time resolution.

2 days of oblique soundings in Boston using the new HF broadcast removal feature added in 1.23.

6 days of the Sodankylä ionosonde measured 1 km from transmitter (vertical sounding).


The latest version of GNU Chirpsouder is available on Github. The newest version doesn't require knowledge of the timings of the chirp sounders, it will discover the chirp timings on its own. A GPS locked receiver is also no longer needed.

Example hardware

The receiver requires a USRP2 or USRP N200 series, equipped with a GPS stabilized reference 10 MHz and 1 PPS synchronization signal (internal GPSDO is recommended). The receiver also requires at least one antenna that covers the frequency range of interest.

Example hardware configuration for dual polarization soundings using the GNU Chirp Sounder. A single channel receiver can be done with just one usrp2, n210 or n200.


The GNU Chirp sounder has been developed by Juha Vierinen. You can contact me by e-mail: j (AT)