How do military fighter aircraft identify themselves and each other?

I live in an area where there is much NATO military activity. Often there are various fighter aircraft fly-overs. But none of the common websites such as flightradar24, flightaware and planeflighttracker etc. used for identifying aircraft using ADS-B and ACARS etc, are able to "see" them.

1. So apart from ground radar, how does the military aircraft identify themselves?
2. Are there other type of transponders used in these?
3. Has there been any known attempts at building some amateur radar network to track these unidentified aircraft?

There was little useful information in this SE answer. I also found after posting a very similar SE question and answers. However, it still doesn't fully address the questions.

EDIT: As an update for question (3), there seem to have been some success on using SDR's as passive radars, as shown in these links:

• What makes you think something other than ground radar is needed? Ground radars (mainly SSR but also PSR) are used to track the large majority of all flights today, mostly based on mode A/C and mode S transponders. – J. Hougaard Jul 6 '17 at 10:12
• As for amateur radar networks, you have to be a filthy rich "amateur" if you want to install one of these in your back yard: radartutorial.eu/19.kartei/14.ssr/pic/img3091.jpg – J. Hougaard Jul 6 '17 at 10:13
• The civilian SSR transponder (mode 3/A, 3/C or 3/S) is derived from the military one (IFF) which is being upgraded to Mark XII. IFF has 5 interrogation modes, mode 3 is the civilian mode. When you mention amateur radar network, I suppose you mean transponder receiver network like ADS-B receivers used by the sites you list in the question, not the SSR stations with directional antennas. – mins Jul 6 '17 at 10:38
• I think the latest update is to Mark XIIA according to STANAG 4193 it seem to use 1030 MHz (ground-to-surface) interrogation and then 1090 MHz ADS-B (answer). – not2qubit Jul 6 '17 at 12:51
• A surprisingly good source for the different modes used. – not2qubit Jul 6 '17 at 13:02

Military aircraft, like civil aircraft have transponders. Military aircraft are assigned military codes, and their transponders have special military modes.

An aircraft doesn't need a transponder to be "illuminated" by radar, but the transponder enhances the "blip" and modern transponders transmit additional aircraft information such as altitude (Mode C).

See the "Interrogation Modes" of transponders in this link.

Notice that the Military modes include "cryptographically" secured version of Mode S and ADS-B GPS positioning.

• I believe the secure mode is called Mode 4, if I remember correctly. – Steve Jul 6 '17 at 16:54
• @Steve correct, and also Mode 5, which has Mode S and ADS-B GPS position information. – Devil07 Jul 6 '17 at 17:22
• Seem that Mode 4/5 are also waveform based. – not2qubit Jul 6 '17 at 17:55
• @not2qubit as opposed to PCM? – Devil07 Jul 6 '17 at 18:04
• @Devil07 Yes, I suppose, since it is considered a NATO secret and not officially mentioned anywhere, while every product brief mention "waveform". I take it to believe they are using some more common way of spread-spectrum modulation such as: Frequency-hopping spread spectrum (FHSS), direct-sequence spread spectrum (DSSS), time-hopping spread spectrum (THSS) or chirp spread spectrum (CSS), as they provide greater resistance to narrow-band jamming. – not2qubit Jul 7 '17 at 21:01