PASSIVE IONOSONDE WITH SOFTWARE DEFINED RADIO
We at BHT have built a passive ionosonde using a Software Defined Radio. Even though the system is currently running full time, we will improve it in the future when time allows. The project has been motivated by our personal enthusiasm and to serve as a proof of concept. It has not been a commercial project.
The above ionogram has been drawn based on the received signal at BHT headquarters (61.4926N, 21.8075E) located in Pori in southwest Finland. The receiver tracks SGO’s (Sodankylä Geophysical Observatory) transmission and the resulting ionogram is updated every odd minute.
Click the link to download a 24-hour period as a gif animation. Three previous days are available. Above files are large, so download time might be high.
ARCHITECHTURE AND DESCRIPTION OF THE SYSTEM
Ionosonde is based on chirp modulation: a frequency sweep transmission scans the ionosphere to chart which portions are reflective at which frequencies. Active ionosonde, such as SGO’s ionosonde, has both the transmitter and receiver at the same location, whereas passive ionosonde has only the receiver which is synchronized to a some specific active ionosonde. Ours as mentioned is a passive one.
At the moment we’re tracking transmissions originating from Sodankylä Geophysical Observatory (67.3644N, 26.6303E) located in Sodankylä in north Finland. You can find SGO and its latest ionogram here: https://www.sgo.fi/Data/RealTime/ionogram.php
SGO’s ionograms tend to differ quite substantially in comparison to ours. This is because an active ionosonde scans the ionosphere directly above the system antennas, whereas our passive ionosonde scans the ionosphere for the whole length of Finland as Pori and Sodankylä locate at almost the pole opposite latitudes within the country.
PROOF OF CONCEPT
As an all-in-house project, the ionosonde provides an excellent proof of BHT’s know-how.
- The system has to be time synchronized within a fraction of millisecond. For this we’ve built a GPSDO – a GPS locked time reference and precision oscillator. The work comprised schematics, PCB design and casing
- We have built an SDR and implemented the signal processing the reception requires
- We’ve built an active loop antenna suitable for the receiver frequency range of 1 MHz to 16 MHz
- Tracking of other active ionosondes and drawing corresponding ionograms
- Building cross polarized active loop antennas (currently there’s just one)
- Improving image processing to get rid of interferences and to produce less noisy ionograms overall
- Calibration for better estimation of altitudes of reflections