If the FPV community had a motto, this would be a strong contender. FPV drone pilots will go to absolute extremes to rescue a beloved four-rotored friend from an uncertain fate in the jaws of a particularly greedy tree or equally sinister location. The first step in making a rescue, however, is to track down the lost FPV drone. To do so, the RSSI function native to video receivers and radio transmitters can be used to hasten the search.
What is RSSI and why is it Useful?
RSSI is an acronym for Received Signal Strength Indicator, essentially, RSSI measures the quality of communication between a radio transmitter and receiver pair. The unit for RSSI is decibels, which denote the power level of an electrical signal. In FPV, RSSI readouts are primarily used during flight to accurately measure the health of the radio link between the radio receiver within the quadcopter and the handheld radio transmitter. An interesting alternative use of RSSI is for finding downed FPV drones.
How can RSSI be Used to Find a Lost Drone?
There are two potential sources in an FPV quadcopter from which a signal can be read; the video transmitter and the radio receiver. Most radio transmitters have an RSSI functionality built in, in this guide the Taranis X9D+ will be used on stock internal RF. Additionally, some video receivers also have an RSSI function that can be used to track the whereabouts of your multicopter. An advantage of using video RSSI is that a directional antenna can be fitted to improve accuracy.
Antenna Radiation Patterns and Gain
Before RSSI strength can be reliably used to relocate a missing model, a basic understanding of antenna radiation patterns must first be obtained. There are two types of antenna; directional and omnidirectional, categorised based on their respective radiation patterns. Directional antennas have a beam that is greatly focused in one direction, omnidirectional antennas have multiple beams. The narrower the beam width, the more useful it will be for searching purposes as the RSSI will indicate most strongly when the on-board receiver/transmitter antenna are within the range of this narrow beam.
Another factor that affects the usefulness of an antenna for quadcopter locating is the gain of the antenna. Gain is a measurement of how focused an antennas beam width is, increasing the gain value of an antenna further narrows and extends the beam, increasing directionality of the antenna. To learn more about FPV antennas, visit GetFPV’s All About Multirotor Drone FPV Antennas article.
Monopole Antenna (Radio Transmitter)
The monopole antenna is a simple omnidirectional antenna, as the name suggests it consists of a singular active element. Monopole antennas are widely found on radio transmitters, such as the Taranis X9D+ used in this demonstration. Being omnidirectional, radio wave projection is concentrated about the sides of the antenna, with transmission strength being null at the tip and tail of the antenna.
Directional Video Receiver Antennas
There are a variety of directional video receiver antennas in common use, the three most notable are the patch, crosshair and helical. Each of these antennas have a singular beam that projects directly outward from the antenna surface.
Using FrSky Telemetry to Locate a Lost Model
To get audible RSSI readouts from your Taranis, the first step is to ensure that RSSI information is being transmitted to the radio as a telemetry value. The process varies depending on which receiver, flight controller and flight controller firmware you are using. See GetFPV’s FrSky RSSI setup guide for more information.
The stock Taranis X9D+ antenna is a low gain monopole antenna. Keep in mind that RSSI values will read highest when the sides of the antenna is directed towards the lost drone, although this is not necessarily in one direction. Hold your radio as you normally would and fold the antenna sideways. This will ensure that some of the signal exiting the antenna is blocked by your body, essentially making the monopole somewhat more directional.
Once the monopole antenna is correctly positioned, slowly turn yourself around in a circle while carefully observing the RSSI readout. Take note of which direction returned the highest RSSI readout value, this is the direction you will start searching in.
Begin walking in the designated direction, chances are that your lost multirotor will turn up shortly. If, however, RSSI begins to droop quickly come to a halt and regain your bearings with another slow circular turn. It’s possible that you may have misjudged the whereabouts of the quadcopter on your first RSSI check and wandered astray. Repeat this general method until the missing drone is located.
Lost Model Tracking with Video RSSI
There are a range of video receivers with a built in RSSI function, pictured is the FuriousFPV Tru-D V3.6. Although on-screen RSSI will make the hunt easier, the amount of snow in the received image can also be used to locate the quadcopter fairly accurately. The locating process is dictated heavily by the directionality of the antenna being used.
Omnidirectional Video Antenna
Firstly, mount the omnidirectional antenna pointing flat along the goggles. This is done to increase the directionality of the antenna, as the omnidirectional antenna generally have an ‘RSSI deadzone” at the tip and base of the antenna. Once again, begin to turn a slow circle while observing the RSSI readout from the video receiver. Remember the video signal will be received strongest by the exposed frontward side of the antenna. When you believe you have obtained the general direction of the quadcopter advance forward while monitoring the RSSI value, be prepared to change your course if RSSI reduces.
Directional Video Antenna
Directional video antennas don’t require much explanation to use for lost model tracking. Simply point the active zone of the directional antenna away from the goggles and perform the aforementioned slow spin to define the general location of the hidden quadcopter.
RSSI is an excellent function to take advantage of to rapidly locate a downed FPV drone. Be sure to hustle, you’ve only got as long as your battery lasts and don’t forget to look up! Airmode bounce can be a horrid thing.