Daytime VFR flight legality depends on FAR 91.205, requiring specific functioning instruments (air speed indicator, altimeter, compass, etc.). A helpful mnemonic is "A TOMATO FLAMES" to remember them.

Here is a neat project I found at Matjaž Vidmar’s Website. The purpose of this project was to design an accurate radio altimeter with a dual-channel (quadrature) homodyne receiver. This new design ...

Barometric altimeters are most common, and measure altitude above sea level using atmospheric pressure. Modern commercial ...