Chris Thompson - AC2CZ - Homebrew Amateur Radio Station

Measuring the Signal to Noise ratio of the downlink

May 2016 - FoxTelem Skyplots

This is a new feature in FoxTelem Version 1.04.

Below is a plot of RF Signal Strength vs Azimuth and Elevation from my QTH. The shading shows the signal strength and darker is a stronger signal. It is then plotted in two dimensions showing the whole sky as viewed from my antenna.

The plot shows data averaged over many passes. Basically every pass since launch. Click on the plot for a larger version. The big white empty section that is from 0-25 degrees Azimuth and 0 to 18 degrees elevation is the Con-Ed power station on the East River right by me. You get a map of the obstacles around your ground station together with a feeling for how the signal strenth of the spacecraft varies in the sky. It will be really nice to see these plots for other ground stations, so email me with examples!

The plot is followed by a plot of RF Signal to Noise Ratio and then Bit Signal to Noise Ratio. It's interesting to see the differences.

There is a bit of history to these plots and the reason I spent an evening coding them. I produced very similar plots in 1990 when I was working on my final year project at the University of Southampton. Those plots were of the UoSATs and the data was the S-Meter of the FT-736R, read via the CAT interface. I was supposed to be measuring the signal to noise ratio with a very new DSP board that my supervisor had. But I never managed to do that. It was way beyond me then.

Instead, a friend of mine worked out how to substitute ASCII characters for a grey scale box and we plotted these charts directly on the dot matrix printer. There were no graphics on the computer screen. The program crunched the data and output it directly to the printer port. Everyone was suitably impressed with the charts, one for each satellite we were tracking, and I passed. But I never did measure the signal to noise ratio. So it is nice to revisit this and finally finish my degree project :)

Copyright 2001-2018 Chris Thompson
Send me an email