measuring doppler in ionosphere

Home Up December 2018 Januari 2019 Februari 2019 Maart 2019

During propagation measurements on the 3.555555 MHz beacon from PA0RYL we found periods
in which the centre frequency off the beacon fell out of my SDR pass band which is 0,5 Hz

At first we searched for a problem in the beacon. Although it uses a Rubidium reference oscillator
 we suspected the stability of this reference

As a follow up on this we used another reference and improved the measurement method by adding a
GPS stabilised reference in the receive chain to monitor receiver LO stability

Then we measured for a longer period the signal from the beacon. The result is the picture below.

In this measurement we can see a resolution of about 10 milli-Hertz

The pink line on the left side is the local GPS disciplined oscillator at 3.555545 MHz,
if this lines moves it means that the local oscillator from the SDR receiver has drifted, during the measurement
 this line is quite stable so LO stability is not causing the effects we see on the beacon signal.

In the middle we see the signal of the beacon with its modulation products.
As reference look at the white line in the frequency scale (below).
The length of the white line corresponds with 1 Hz.
The red line has a length of 0,5 Hz.

The beacon signal shows all kinds off small deviations, at some periods not as strong but especially
around sunset the received frequency off the beacon shows large and unstable drifting.

This drift most probably (almost certain) is caused by Doppler caused by moving layers off ionisation in the ionosphere moving overhead at some speed relative to the trajectory between observer and the beacon.

Measured variations off plus and minus 0,2 Hz are visible in the plot.

I done some calculations on Doppler speeds needed to reach this drift, and for a Doppler off 0,2 Hz the speed of the reflecting cloud layer
should be at 60 km/hour which seems quite reasonable and acceptable to understand.

Although this measurement and explanations of the results look quite acceptable this theory still is a bit pre-mature
 and we will try to find more prove off this theory

Although existence off Doppler shift caused by atmosphere is well known, just listen to some short wave radio stations and you will hear it, I am not sure if it has been measured before like this measurement, by radio amateurs.

Most probably this will have some follow up measurements.

Then next picture is the measurement from 23-10

On the right you can see the received signal strength yellow line the beacon, light blue line the noise level

At about 16.00 UTC I place a small white line in the waterfall representing the length of 1 Hz step.
At this point the beacon frequency suddenly changes by 0.8 Hz caused by ionospheric Doppler.

In december 2018 I started using SBSpectrum software.
This is a special software for making dopplergrams, it's easy in use and the main advantage is that is very easy in use.

It will automatically store the BMP file when it is ready and will continue the measurement.

Take a look at the measurements I done using SBSpectrum
 but be aware these pages are very large about 30 Mb so these will take some download time.

Home Up December 2018 Januari 2019 Februari 2019 Maart 2019