More Openness for Airspace Safety: The FLARM FAMP Public Protocol Is Here
We are pleased to announce a step that many in the community have long been waiting for: as of today, the FLARM FAMP Public Protocol (FFPP) is publicly available as a free download. With it, we are opening up the specification of the part of the FLARM network responsible for traffic safety.
Safety thrives on trust, and trust grows through transparency. With this publication, we want to give everyone interested a reliable insight into how FLARM works – documented, tested and first-hand, rather than based on guesswork.
What is FAMP?
FAMP stands for FLARM Aircraft Motion Prediction – the protocol through which FLARM participants exchange their position, movement vector and identity. It forms the basis for situational awareness, traffic monitoring, collision avoidance and tracking. To do so, FLARM uses an efficient radio system in license-free bands (868 MHz and 915 MHz, respectively).
FAMP is the result of decades of experience and development. In the following, we give an overview of the methods used in FAMP.
Four methods that make FAMP efficient and future-proof
Extended Range Encoding (ERC). Every FAMP packet has to pack a great deal of information into a very small space – position, altitude, speed, climb and turn rate, track, accuracy estimates and more. Instead of encoding values linearly, FAMP therefore uses a non-linear encoding. This makes it possible to represent a very large range of values with just a few bits – for example, a climb rate of ±95 m/s in only 9 bits. Precision stays high where it is needed (around zero) and degrades in a controlled way at larger values. The method requires only simple integer operations and thus runs even on resource-constrained hardware such as FLARM devices of the Classic generation.
Adaptive Coordinate Compression (ACC). Transmitting a global position in full would likewise not be an efficient use of the available bits: by their very nature, sender and receiver are within radio range of each other, which means the distance between the two is physically limited. ACC exploits this locality: it divides the world into a grid and transmits only the position within the cell. Using its own position, the receiver selects the only physically possible solution. To make this work at higher latitudes as well, ACC dynamically adapts the grid width for longitude to the geographic latitude – even via a clever approximation of the optimal function that works exclusively with bit shifts and therefore remains computable even on 8-bit systems.
Enhanced-Privacy Random Address (EPRA). Privacy and safety are not a contradiction. EPRA changes the sender address continuously and at random, and transmits the information needed for a continuously receiving participant to still attribute the sender correctly. Anyone who loses reception loses the trail – the identity remains protected, while the collision-avoidance function is preserved.
Dynamic Message Versioning (DMV). This is perhaps the most important conceptual innovation. FAMP devices are permanently backward-compatible and can receive and send messages of older versions. The protocol version used is chosen dynamically based on the devices in the vicinity. The protocol can therefore evolve without older devices having to be taken out of service by a fixed expiry date. With this, we resolve a limitation of earlier generations that has been discussed for years – in the interest of users.
How can I use FFPP?
The specification is available under clear, fair terms (see Annex 1 of the document). We grant a worldwide, perpetual, royalty-free license for non-commercial purposes – specifically for academic and research purposes as well as amateur and private use, limited to receive-only.
Deliberately excluded are transmitting on the FLARM radio network as well as commercial and military use. There is a good reason for this: in the interest of flight safety and interference-free network operation, every transmitting device requires validation and an assigned, unique FLARM radio address. Anyone who wants to develop products, offer services or transmit themselves can obtain a separate commercial license for this – we welcome any inquiry at licensing@flarm.com.
For everyone who wants to get started right away, the document also includes a representative test data set that can be used to verify your own receive implementations.
Who is this for?
Universities and research groups can now study FLARM on a documented basis instead of relying on assumptions. Developers of ground receivers and analysis systems get a reliable reference. And technically minded pilots get an honest look at how much engineering work goes into each individual 24-byte packet that helps make the cockpit safer.
With the FAMP Public Protocol, we are taking a deliberate step toward greater transparency – while staying true to what FLARM has always stood for: reliable, responsible technology for safety in the air.
The full document is available for download here. We look forward to the exchange with the community – and to what comes of it.
