The Complete Handbook of Altimeters

Chapter 1: Understanding Altimeters

You are standing at the bottom of a deep canyon. Your GPS receiver tells you the time and your horizontal position to within a few meters, but the altitude reading it provides is bouncing around with an error margin that can be larger than a two-story building. Inside your jacket, a mechanical pressure altimeter, no larger than a pocket watch, drives a needle across a dial. It is reading a steady value, derived from the weight of the air above you. These two devices are supposed to do the same job, yet they are giving you wildly different answers. This is the core reality of altimetry: it is not a single measurement, but a set of different technologies, each with its own physical basis, limitations, and applications.

An altimeter is any instrument that measures altitude-- the vertical distance between a point and a reference level, usually mean sea level. The term is broad cover for three distinct families of technology. You cannot make an intelligent purchasing decision or use any of them with confidence until you understand the fundamental differences in how they work. The first family is the pressure altimeter, which infers height from barometric pressure. The second is the radio altimeter, which measures the actual time it takes a radio signal to travel to the ground and back. The third is the satellite-based (GNSS) altimeter, which uses triangulated signals from orbiting satellites to calculate a three-dimensional position that includes height. Each of these is