The aim of this open source project is to develop a system to monitor the movement/location of small animals using radio transmitters and receivers. A main focus is, that non-technical users can assemble and operate it via a standard web browser.
First we need an animal, for example a bat.
On the bat we mount a radio transmitter. Those transmitters are nothing new and are distributed by many manufactures. They are transmitting one “beep” or a set of four short “beeps” on a predefined radio frequency . The bat can thus be identified with the preknowledge of the transmitting frequency and the time intervall between two “beeps” or the set of “beeps”. If you use transmitters with a set of four “beeps”, the lag also helps to identify the transmitter.
To receive the signals of the radio transmitter, we need an antenna and a receiver. The antenna might be either a directional antenna (receives better in a specific direction) or a dipole antenna (no preferred direction). The advantage of a directional antenna is, that you can identify the direction where the bat might be.
The receiver is now the new part in the setup. Instead of using the typical distributed receivers, we use a SDR (Software Defined Radio) USB dongle and connect it to the antenna. Those dongles are cheaper an can be directly connected to a computer.
For the field application it is easier to take a Raspberry Pi mini computer than a desktop pc or a laptop. It is cheaper and it’s power consumption is lower. And again, the setup is easy. You just need to assemble the parts, download the image file, copy it on the SD-Card and plug it into the Raspberry Pi. To start the receiver, you just have to connect to the wifi hotspot and tune to the frequency of you radio transmitter using your browser.
There are several options on how you can find the animal’s location using radio transmitters. The possibility to use GPS-trackers won’t be discussed since the weight is too high if they transmit the position. In our project we are first using the 3rd of the following options, as it is the simplest to implement.
There are three possible ways to calculate the animals position:
- Time Difference of Arrival (TDOA): An electromagnetic wave travels with the speed of light, which means that depending of the distances between the transmitter and the receivers it takes different times till the signal is received. This can be used to calculate the exact position of the transmitter.
- The phase of the electromagnetic wave can be used. The incoming wave will be received with several antennas which have a know distance to each other. If you take a look at the phase difference of the incoming signal on the different antennas, you can calculate the origin’s direction of the wave. Taking the bearings from several positions, the transmitters location can be derived as the intersection of two lines.
- Directional Antennas have an preferred amplification direction. If the antenna is targeted directly on the transmitter, the signal strength of that direction will be highest. If several directional antennas are taken, the direction of the origin can be calculated. Taking again the bearings from several positions, the location can be derived.