How many mobile beacons do I need per drone?
Can we only have one mobile beacon per autonomous drone?
One mobile beacon is enough for basic 3D (XYZ) tracking
If you want to have just a basic 3D (XYZ) tracking, a single mobile beacon, such as the Mini-TX (for NIA), the Mini-RX with External Microphone (for IA), or Super-Beacons (with or without additional microphones, and suitable for IA, NIA, and MF NIA), is enough.
See more details in the Placement Manual.
There are numerous examples of basic indoor drone tracking. To start tracing a drone indoors, you need to:
- Install four stationary beacons high on the wall or on the ceiling and face them down towards the drone
- Attach one mobile beacon to the drone and face it towards the stationary beacons
- Connect one modem to your computer with the Dashboard via a USB cable
- Build the map in 5-7 seconds automatically without any manual measurements
- Save the map and start tracking the mobile beacon on the drone, i.e., track the drone, in 5-7 seconds.
Can we fly autonomously indoors with only one mobile beacon or not?...
Yes, it is possible to fly autonomously indoors with only one beacon. For example, our DJI-based solution uses this approach.
However, it is challenging to fly autonomously indoors with only one mobile beacon, and we don’t recommend doing so unless necessary, for example, when your drone is small and there is no space or insufficient base for a second mobile beacon.
If you absolutely need to fly the indoor drone with a single beacon, then the algorithm can be like this:
- Keep the take-off area 1m or without any obstacle horizontally
- Take off
- Start flying any direction for 50-100 cm, but keep movement exactly linear using the gyro readings – that is easy
- Then, by knowing your take off XYZ coordinates (point 1) – before the linear and horizontal flight – and XYZ coordinates after the linear flight (point 2), you can calculate the direction of the drone against the indoor positioning system and your waypoints that you also know in terms of your indoor positioning system. Suppose your indoor positioning system is geo-referenced to other coordinates, such as indoor coordinates within the building or outdoor coordinates, like GPS. In that case, you will know XYZ and the direction against them as well. It is still easy if you haven’t hit anything during the initial linear flight. It is easy because the IMU can be assumed ideal during this time – no drift – because it is pretty short
- Then, you start flying according to your waypoints
- This flight is not short anymore. Thus, you need to compensate for the yaw drift of the gyro. When you have two mobile beacons, the system automatically handles this, and you receive the IMU data without drift. But with a single mobile beacon, it is your task. It is doable. Not highly complex but not too simple either. It is solvable when the drone is moving and locations are updated along the way. But in static, particularly, if you haven’t locked or created a special mechanism to remember your direction when you landed, it is impossible to know your direction after some time because you don’t have a mechanism to compensate for the yaw gyro drift. At this point, you return to the initial phase – take off and fly horizontally somewhere and pray that you don’t hit anything
Is it a viable solution in some cases, particularly when using tiny drones? – Sure, it is!
Do we recommend it to novice users of our system? – No, it is too complex and less robust at the same time
But what is the problem and why do we need the second mobile beacon on the drone?
The task of knowing where you are facing is not as simple as one may think. Note that the drone’s gyroscope doesn’t provide direction. The gyro gives you the change of direction, which is not the same thing.
But why not to use compass/magnetometer?
You can try using compass/magnetometer but you risk failing your autonomous flight at the most critical moment, because magnetometers/compasses don’t work indoors reliably:
- There are a lot of ferromagnetic materials around that distort the Earth’s magnetic field
- There are lot of wires with currents that produce their own magnetic fields thus distorting the Earth’s magnetic field even more
What is the recommended solution for autonomous indoor drones?
The recommended solution for indoor drones to have Location + Direction is to use two mobile beacons per drone, with as large a base as possible between the mobile beacons, based on the drone’s size.
Configurations may vary slightly depending on your specific cases. Examples for autonomous indoor drones:
- Typical PixHawk-based drone: 2 x Super-Beacon in NIA configuration if there is only 1-2 drones
- 2-8 x PixHawk-based drones: 2 x Super-Beacon in MF NIA configuration
For smaller 1-2 drones:
- 1-2 x Mini-TX per drone flying in NIA, depending on whether you want Location only or Location+Direction
- Typically, if the drone is large enough to carry 2 x Mini-TX, it is large enough for the Super-Beacons
For tiny drones, weighing under 100 grams, it is possible to remove the housing and replace the default battery with something very lightweight, such as 30-50 mAh, or even power it directly from the drone. Remember to account for voltage drops, electric noise from the motors, etc. It may seriously affect. If not, you may easily save several very valuable grams.
In IA, each mobile beacon calculates its own position independently. Thus, there could be thousands of mobile beacons in the same swarm. The location of each of them will be calculated simultaneously, and the update rate per drone doesn’t directly depend on the number of mobile beacons, unlike in NIA.
- 2 x Super-Beacon, if propellers are facing down, for example, like in the DJI Matrice, and the mobile beacons are installed on top of the motors and facing up. In this case, the beacons’ body will be an ultrasound shield against the propellers’ noise
- 2 x (Mini-RX + External Microphone + Ultrasound deflector) – similar to this case but with two mobile beacons and a base between them of 30+ cm
Summary
- Can we track an indoor drone in XYZ with only one mobile beacon? – Yes, it is easy
- Can we fly autonomously indoors with only one mobile beacon? – Yes, it is possible, but more challenging
- What is the easy and recommended configuration for indoor autonomous drones? – two mobile beacons/drone