What Does It Do
The picture below show the panel of a glider with a FLARM display (barely visible) with the display indicating the closest threat with one red LED indicating a moderate threat. There would one beep alerting the pilot to this threat (see Classic FLARM Indicators).
The following description is taken from the FLARM User's Manual
FLARM receives your position and movement information from an internal GPS receiver. Each second, FLARM calculates your predicted flight path from the GPS information, and transmits this predicted flight path via radio, as a low power digital signal. Provided they are within receiving range, this signal is received by other FLARM-equipped aircraft. Your FLARM compares your predicted flight path with those received from other FLARMs. At the same time, FLARM compares your predicted flight path with known obstacle data, including electric power lines, radio masts and cable cars.
If FLARM determines you are at risk of colliding with another aircraft or an obstacle, the unit warns you of the greatest danger at that moment. The warning is given by a whistle sound (beep) and bright light emitting diodes (LED). The display also gives indication of the threat level, plus the horizontal and vertical bearing to the threat.
The operating range is very dependent upon the antenna installation in the aircraft. The range of 'Classic' FLARM is about 2 km, but up to 5 km may be achieved in individual cases. PowerFLARM will have twice the range. The effective range can easily be verified with an online tool. The GPS and coordinates of the other aircraft can also be made available for use in other systems (e.g. external display, speech synthesizer, PDA) via a serial data output. There are a number of manufacturers of equipment which displays FLARM data.
In addition to implementing the FLARM framework described above (FLARM to FLARM), the new PowerFLARM (available in US April 2011) also detects (but is not be detected by) nearby aircraft with transponders and ADS-B transmitters.
A Layman's Description
The write-up below attempts to explain in layman's terms some of the unique aspects of FLARM in achieving its very powerful collision detection and warning capabilities, in particular for gliders that fly in very close proximity to each other. Other pages provide more technical details on some of the features described below.
Every second, FLARM goes through a cycle of steps involving data acquisition from its own GPS and from other aircraft around. The FLARM performs an analysis of all that data and then displays the results. This provides the pilot with information about nearby traffic without the need to rely on an Air Traffic Controller (looking at a screen full of little dots trying to figure out which dots may represent a threat to others and then radioing the info back to the aircrafts concerned, and therefore to everybody on the frequency). In this sense, FLARM implements the basic idea behind ADS-B of automated transmission of "position and track" directly between aircraft and, on that basis, autonomous collision threat analysis performed within each aircraft.
Step 1: The first thing that FLARM does in each cycle is to get your absolute position in space (lat, long, altitude) and the aircraft track from the GPS. It uses this GPS information to project the expected flight path, i.e. where you are expected to be over the next 20-seconds. Its smart about this--it knows if you are a glider thermalling and such. See the figure to the right for an example of a projected track.
Step 2: The next step is for FLARM to broadcast this information about itself on a common radio channel used by all other FLARM devices, and to receive similar information from all other FLARM devices that are nearby, i.e., within typically 2 to 4 or more Kms (about 1.5 to 2.5 or more sm). The upper distance depends mostly on the power and efficiency of each transceiver (for similar devices of identical power and antenna design the efficiency depends mostly on how properly positioned and installed your antenna is - just like for your voice radio). In any case, the closer range is the important one here, and the good thing is that the closer the other systems are, the greater the certainty of receiving clear messages. One of the keys features of the broadcast algorithm is the protocol which minimizes systems from transmitting simultaneously and "stepping on each other."
Step 3: The next step is for FLARM to use the information received from all nearby aircraft to check if their projected path intersects with your projected track, an easy task for microprocessors (done much more reliably every second than the ATC guy looking at a screen full of dots could). The results indicate what aircraft represent a potential threat during a future time window, and what aircraft definitely don't.
In the sketch of the figure, aircraft "A" which is moving away is clearly not a threat, although it is very close (say, 100 yards behind me). "B" and "C" who are also very close, maybe hundred yards away turning at the same altitude as me in the same thermal, are also no threat in the next few seconds. "D" is not a threat because 1500 ft below, whereas "E" much further away than either A, B, and C, and at an altitude below mine, is a threat because its future path intersects the volume of space where I could possibly be in the future. Sure, if he has seen me, he will likely join the thermal smoothly and safely, but that is what I can't be sure of! In the case of the figure at right, if I am banking and turning left, I cannot see E because he is right under the belly of my fuselage (additionally, my eyes are likely looking toward the inside of the circle at all the other guys already turning with me and therefore I may not catch all those gliders that are or will be arriving from maybe 300 ft below and pulling up to join the thermal). If E hasn't seen me, I sure will be glad for him to get a "heads-up" or "warning" from his FLARM about me being there, and for me to get information about him with a quick indication of where this threat is with respect to me... all the while not getting alarms for the other guys in the gaggle or thermal who, despite being very close, are not threats to me as long as they remain "polite" and well behaved. FLARM will give you an alarm, and rightly so, for the "eager-beaver" at nearly your altitude who suddenly decides to go look for the core hundred yards away and whose circle will now intersect everybody else's.
Using its database of fixed obstacles (cables, tall antennas, etc.), FLARM also checks for potential collision with any that may be nearby, and alerts the pilot if necessary.
In doing its analysis of potential collisions, FLARM analyzes future time windows, up to 18 seconds. [Note that 18 seconds, even for gliders cruising at high speed and closing in heads-on at a relative speed of something like 240 mph, represents a distance of about one mile.] For each of these time windows, FLARM calculates the volume where you could possibly be during that future time window and checks whether the expected path of any of the other nearby aircrafts could intersect your volume during that time window. So, not only does FLARM detect potential future collision threats, but it also knows when the time of a potential collision is, and therefore how imminent the threat is. Because the uncertainty on future possible presence and expected paths decreases with the closer in time you get, FLARM also generates an indication of how serious the situation is, and obviously the most imminent and serious threat is the one requiring highest levels of alarm. Because FLARM knows which aircraft are threats and their exact current location in space (from the messages received), it can calculate their position relative to you (heading, distance, altitude above or below) and display the information in a very intuitive manner. Look at the simulation of the Parowan event to see how the relative position and intensity/seriousness of a threat is updated every second and displayed using both the sound and LEDs of the "classic" FLARM systems. For the PowerFLARM the screen will provide a map-like display using colors as additional information means.
Speaking about PowerFLARM (available in the US late 2010) will have all the FLARM-to-FLARM capabilities discussed above, plus it will also be able to receive messages on another frequency, namely the ADS-B 1090 MHz frequency on which all transponders reply to interrogations from ATC-based traffic control or from airborne TCAS type systems (see page on "Transponders and TCAS" for details of these systems). So, PowerFLARM will be able to do the very advanced FLARM-to-FLARM collision analysis that is so special and specific to gliders (e.g., gaggles, thermals, etc.), and will also be able to display some collision-potential data about nearby transponder-equipped commercial and GA (General Aviation) power planes. In addition, PowerFLARM receives ADS-B messages for further information about potential threats (see ADS-B page for more information). Thus, the PowerFLARM unit should answer the concern and now-stale question: Should I protect myself mostly from other gliders in contests (where near-proximity and the very special cases of turnpoints, gaggles, circling in thermals, similar route under the same cloud streets, etc. represent unique danger situations and threat-detection challenges) or from other power traffic because I fly in an area where there is a high density of these aircrafts around? PowerFLARM will do both.
This has been a top-level, maximally simplified, layman's description of what FLARM does and how the overall technology works, in an attempt to explain why it is so powerful for us, the gliding community, and so well adapted to our very particular type of flying. If you want even more details on how predictive calculations can be made, check US patent 6201482 or US patent 6438492, for example, that illustrate two different methods to do that.
The FLARM software is proprietary and there have been various versions of the software, each with more advanced, refined, and precise algorithms. It will continue to evolve and get better and be upgraded, etc. For now, the important point for us, the glider community, is that PowerFLARM can give you very precise information about the potential of collision with other nearby FLARM-equipped gliders and about nearby transponder-equipped power planes; and do that in a way that is very specific to gliders and that no other system can do today and not likely to do in the foreseeable future. That information could allow you to avoid an impending collision and maybe save your life. What will you do when it will be available in the US in the next few months?