RC Vario Altimeter


This page is just somewhere I can make notes during development.

Having been thinking about it for a long time I have designed a small (45x25mm) circuit board for a barometric altimeter.

The board includes PPM-in, PPM-out, a auxiliary/spare output and memory for recording flight altitude, which should hold about 8 hours at 1 sample per second.

A 433MHz transmitter will send vario tones to the same receiver I use for lost model tracking. It could act as a lost model locator but relies on the RC battery; unlike our dedicated tracker which is self contained and low-power.

Uses include ALES, radio-vario, recording altimeter, altitude limiter, lost model locator, and probably other things.

Now the complex part; software….

Most of the software is just a minor variation on what I’ve already done. Converting and calculating altitude from the pressure¬† sensor will be a challenge.

The pressure sensor produces a number between 0 and about 200000 representing altitude between about -1000 and 14000 meters, giving a theoretical resolution of better than 80mm. Trouble is it’s not linear and affected by temperature. There is a temperature sensor on-board.

General Notes

I have a example of pressure/temperature conversion to altitude. The tricky part is to implement this in a form that along with all the other stuff will fit into the small micro. Handling it all as 32-bit integer math in the 8-bit micro eliminates floating point, reduces the code size and since there’s no on-board display, format conversion can be done after extracting the data to a PC.

Altitude is determined by reading the pressure and temperature and working from a reference pressure and altitude. The reference is created when the unit is switched on and can be offset later for graphing relative to zero or ground level.

Not knowing the atmospheric pressure at the time of use, it won’t know exact altitude above sea level. But knowing temperature and assuming a starting altitude it should be able to work up from there with good accuracy.

It’s probably good enough to read the pressure at start-up and assume altitude based on a guesstimate of average atmospheric pressure.

Existing ALES devices may provide a different 200 meters at sea level to say a flying field at 1000 meters ASL. But everyone is affected the same, it’s relatively small and there’s no way to compare numbers so it doesn’t matter.


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