# Social Thing: Physical Computing _ Research 5 _ radio direction finding (RDF)

## What is the Radio Direction Finding (RDF) ?

radio direction finding (RDF), is the measurement of the direction from which a received signal was transmitted. This can refer to radio or other forms of wireless communication, including radar signals detection and monitoring (ELINT/ESM). By combining the direction information from two or more suitably spaced Receivers (or a single mobile receiver), the source of a transmission may be located via Triangulation. Radio direction finding is used in the navigation of ships and aircraft, to locate emergency transmitters for search and rescue, for tracking wildlife, and to locate illegal or interfering transmitters.

## Using Triangulation, it would be possible to track the location who has short-wave radio

What is triangulation?

triangulation is the process of determining the location of a point by forming triangles to it from known points.

Trilateration can be performed with a single receiver if it can move and the transmitter is stationary. One would have to compare the received signal strength at 3 or more (multilateration) locations. This comparison must be done to signals transmitted at the same (or known) power. If the walkie-talkie pair uses amplitude modulation, this means the transmitter needs to send the same tone and amplitude — no sound input might work. Frequency modulation schemes can include a carrier or squelch tone, transmitted at a constant amplitude. This method can be simplified if the transmitted power is known.

Radiated radii is about r=Pavg‾‾‾‾√10dBμ/20+1r=Pavg⋅10dBμ/20+1. By overestimating the transmitted power by a bit one can graph the trilateration circles. This example uses a 22mW estimation for a 15mW transmitter.

This graphical method can be extended to multilateration, using a least squares technique on an overdetermined system to estimate the transmitter location.

Using trilateration with received power readings won’t give accurate results, especially when line-of-sight is obstructed, but will give a general area in which to search. How accurate is it? Maybe one of these papers can tell you, as that question is complicated:Efficient Solution and Performance Analysis of 3D Position Estimation by Trilateration,A Simple and Efficient Estimator for Hyperbolic Location, A Passive Localization Algorithm and Its Accuracy Analysis, Recursive Position Estimation in Sensor Networks.