Hi again David,
This is a very complicated subject, but I will endeavour to explain it in as simple terms as possible.
Mode C transmissions should be 'altitude' responses from a 'Pure' Mode C or from a Mode S transponder - but can in some circumstances also be 'ID Squawk' responses from either of the above. Due to limitations in what is in fact a relatively old and technically very complicated system, transponders sometimes use the same response 'code' to report a 'Squawk' of for example 7000
or an altitude of 7000 ft. There are numerous similar code 'duplications'. The interrogating Radar, of course, knows
which type of response it is 'expecting' from the transponder, based on the interrogation it has sent out, and advises the radar operator accordingly, but we (unfortunately) don't have the benefit of this 'interrogation' information - which is sent out on a completely different frequency (1030MHz). Mode C responses are therefore
extremely difficult to 'pin down' - unless they include an ICAO Hex Identifier which ties them to
a specific Mode S transponder - though these identifiers aren't even present in every Mode S response packet - and they simply don't exist in the older 'Pure Mode C' transponders.
In order to be able to report the presence of 'Pure Mode C' aircraft therefore, PilotAware has to assess each individual 'potential Mode C' response it receives and try to work out whether it is from a 'known' Mode S or ADSB aircraft (including an 'altitude' response from
your own Mode S), from the aircraft's 'own' Mode C transponder (if you are running one of those), or from a separate genuine Mode C 'target' aircraft, and then work out what level of 'risk' the target poses to your aircraft and report this accordingly.
In order to do this, the system has to monitor
all potential Mode C responses over a fairly brief period of time and look for common
or varying factors (if a series of received packets
aren't varying (except in signal strength), they are almost certain to be an ID squawk) and then try to match and group each individual response to an aircraft, then eliminate it as being from your 'own transponder' or report it to your EFB as 'traffic'. This process is going on constantly and at high speed, with the results reported in the lower part of the PilotAware 'Traffic Screen' - primarily for system monitoring purposes - with system generated 'C' codes allocated to each data packet to allow them to be tracked and compared through the process. These codes are then used to report 'genuine' Mode C targets to your EFB as 'Traffic' and to generate associated Audio Warnings direct from your PilotAware.
In your screenshot, PAW has obviously assessed received reports and identified a Mode C transponder which it is reporting as 'High Risk' or 'Danger' (identified by the Red Circle). This means that it is likely to be within a
relatively short horizontal distance from your aircraft (the
actual distance will depend on the power of the transponder and your selected Bearingless Target Reporting 'Range' in PAW Configure). Note that we
DON'T try to work out an
actual distance as this can't be done accurately and - depending on your selected Bearingless Traffic Range Setting and the output power of the transponder, the aircraft
may, or may not be inside the circle and is
extremely unlikely to be concentric with your own aircraft - at least at the point where the warning first appears. The circle is purely indicative of the degree of perceived risk (see below*) and denotes the importance of trying to identify the aircraft position visually - especially if it is at or near your own level, or is climbing or descending towards your level. PilotAware
has however been able to determine the actual altitude of the aircraft from the received data packets - which it is reporting in this case as 3,300 ft
below your current level (
-3.3). Note: if the reported aircraft is near your own level and the risk level is increasing, you need to try to locate the aircraft visually as a priority, or be prepared to initiate measures to increase separation (e.g by climbing or descending as appropriate, or turning away from your present course while keeping a
very careful lookout for the reported aircraft), especially if the relative altitude continues to close. Be aware that PilotAware NEVER reports aircraft that don't exist (but see also below re high power transponders)*
* Any avoiding action will of course depend on your location and circumstances and must be decided on and implemented
by the pilot after interpreting what the system is reporting and all other relevant issues - including the proximity of controlled airspace.
NEVER 'knee-jerk' react to a PilotAware warning without considering ALL relevant factors. It is important to 'Learn' in advance what your display (and associated audible warnings) are telling you and think ahead as to what you could do in each of the various situations you might come across. In making your decision, you should also be aware that a high power transponder (such as a CAT Mode S),
may report straight to 'Danger'
or extremely quickly through the Notice / Alert / Danger stages (Progressively smaller Green then Amber then Red Circles) when the transponding aircraft may still be outside your visual range. This often occurs in the vicinity of major airports and the specific warning pattern or 'signature' once experienced is relatively easy to recognise. This is specifically why the use of Bearingless Target Reporting - whilst it is an extremely useful tool due to the high percentage of 'Pure' Mode C or Mode S equipped aircraft around - is usually not recommended until you have 'learned' how to use and recognise what it is telling you. If you aren't sure about this please read the information available on the website at
https://www.pilotaware.com/knowledge-base - and if you are still not sure,
please come back on here and ask!
In closing, a huge amount has been done over the past year or two to improve the accuracy and reliability of both Mode S and Mode C detection and reporting, so it goes without saying that users should always avail themselves of the most recent PilotAware software updates as soon as practicable after they are released. Recent developments - including the availability of the PilotAware Firmware Updater APP for both Apple and Android devices makes this a 'no brainer' and developments are even in hand (currently being tested in Beta) to introduce automatic downloading of latest updates in the background via iGRID - where installed (though you do still need to manually prompt the actual install once notified of the availability of the update).
I hope this helps clarify the situation.
Best Regards
Peter
