Most of this stuff is common knowledge, perhaps we miss a few things here and there. When exactly do you squawk 2000? What does that former Air Force guy mean when he says IFF? What is a secondary target? More recently: where do you find your Mode S squawk?
What follows are quotes from the relevant regulatory documents, listed below, as well as my comments in blue.
[DoD Dictionary of Military and Associated Terms, pg. 123] Identification, friend or foe — A device that emits a signal positively identifying it as a friendly.
You will often hear "IFF" from ex-military types when what they mean to say is "transponder." While the IFF found in many military aircraft are much more than just transponders, for our purposes the terms can be considered synonymous.
[Air Traffic Organization Policy Order JO 7110.65U, pg. PCG P-4] PRIMARY RADAR TARGET− An analog or digital target, exclusive of a secondary radar target, presented on a radar display.
[Air Traffic Organization Policy Order JO 7110.65U, pg. PCG S-2] SECONDARY RADAR TARGET− A target derived from a transponder return presented on a radar display.
The primary target is the "blip" on the controllers radar screen that is nothing more than the aircraft's radar return, while the secondary target is the signal and information beamed from the aircraft's transponder to the radar.
Figure: AN/APX-100 IFF Transponder, from Eddie's sordid past.
This was a two digit code we could set depending on mission.
I've heard this code told the good guys what your mission is and was set for each aircraft, though aircraft I've flown allowed us to change this in flight.
This is the 4-digit code we set in the cockpit as assigned by ATC or what we are doing at the time, often called "Mode 3/A" and usually combined with Mode C to provide altitude information.
Provides the aircraft's pressure altitude, sometimes called "Mode 3C."
Provides a 3-pulse reply to crypto coded challenge.
Provides a cryptographically secured version of Mode S and ADS-B GPS position.
Provides multiple information formats to a selective interrogation. Each aircraft is assigned a fixed 24-bit address.
Photo: CMS Page 17, from Eddie's aircraft.
The G450 Aircraft Operating Manual mentions the Mode S code in §2B-21-00, page 33, saying it can be found on the MCDU DLK (AOC) STATUS page 3, but that does not appear to be true. I have found the code on the CMS, page 17. (I erased our code from the photo.)
The code is simply a hexadecimal conversion from your registration. You can check this at: http://www.avionictools.com/.
[Aeronautical Information Manual, ¶4-1-17.g.] Transponder Operation Under Visual Flight Rules (VFR): Unless otherwise instructed by an ATC facility, adjust transponder to reply on Mode 3/A Code 1200 regardless of altitude.
[Transport Canada Aeronautical Information Manual, ¶1.9.4] During VFR flight in low-level airspace, adjust your transponder to reply on the following unless otherwise assigned by an ATS unit: (a) Mode A, Code 1200, for operation at or below 12 500 ft ASL; or (b) Mode A, Code 1400, for operation above 12 500 ft ASL
There may be other examples, but for at least in the U.S. and in Canada below 12,500 feet, squawk 1200 when VFR.
[ICAO Doc 8168, Vol I, §III-3-1-1, ¶1.1.2] Except in case of emergency, communication failure or unlawful interference (see 1.4, 1.5 and 1.6), the pilot shall:
a) operate the transponder and select Mode A codes as directed by the ATC unit with which contact is being made; or
b) operate the transponder on Mode A codes as prescribed on the basis of regional air navigation agreements; or
c) in the absence of any ATC directions or regional air navigation agreements, operate the transponder on Mode A Code 2000.
[ICAO Doc 7030, §AFI, ¶18.104.22.168.] Africa-Indian Ocean Regional Supplementary Procedures. Unless otherwise directed by air traffic control, the last assigned SSR (Mode A) code shall be retained. If no SSR code has been assigned, Mode A code 2000 shall be selected and retained.
[NAT Doc 007, ¶6.8.1] All aircraft operating as IFR flights in the NAT Region shall be equipped with a pressure- altitude reporting SSR transponder. Unless otherwise directed by ATC, pilots flying in the NAT FIRs will operate transponders continuously in Mode A/C Code 2000, except that the last assigned code will be retained for a period of 30 min after entry into NAT airspace or after leaving a radar service area. Pilots should note that it is important to change from the last assigned domestic code to the Mode A/C Code 2000 since the original domestic code may not be recognised by the subsequent Domestic Radar Service on exit from the oceanic airspace.
[AC 91-70A ¶5-5.b.] In airspace controlled by Oakland Center or Honolulu Center, accomplish normal VHF communication. In airspace controlled by Oakland Oceanic (KZAK), CPDLC or HF voice backup accomplishes ATC communication (including en route requests) and position reporting. You can also use HF as primary communication for the aircraft not equipped with data link. When reaching oceanic airspace, squawk 2000 and monitor VHF 121.5 and the pacific air-to-air frequency 123.45.
[AC 91-70A ¶8-16.b.] In South America, there are no alternate instructions in the AIP. Therefore, use code 2000 when beyond radar coverage if there is no specification for another code.
[AC 91-70A ¶13-9.b.] Special Requirements For Flights Transiting Iceland. Pilots will operate SSR transponders continuously on Mode A, Code 2000, except that departing aircraft will retain the last assigned code for 30 minutes after entry into NAT oceanic airspace unless otherwise instructed by ATC.
[AC 91-70A ¶14-2.d.] On polar routes beyond areas of radar coverage, squawk 2000.
[AC 91-70A Appendix 2, ¶2.g.] Thirty minutes after oceanic entry, crews should Squawk 2000, if applicable. There may be regional differences such as Squawking 2100 in Bermuda’s airspace or maintaining last assigned Squawk in the West Atlantic Route System (WATRS). Crews transiting Reykjavik’s airspace must maintain last assigned Squawk.
In general you are going to squawk 2000 when oceanic, waiting 30 minutes after the entry waypoint is required over the North Atlantic and doesn't hurt elsewhere. There are exceptions so make sure you view the regional pages before entry.
[ICAO Doc 4444, ¶22.214.171.124] Code 7500 shall be reserved internationally for use by pilots encountering unlawful interference.
[ICAO Doc 8168, Vol I, §III-3-1-2, ¶1.6]
[ICAO Doc 8168, Vol I, §III-3-1-1, ¶1.1.5] When requested by ATC to CONFIRM SQUAWK (code), the pilot shall:
a) verify the Mode A code setting on the transponder;
b) reselect the assigned code if necessary; and
c) confirm to ATC the setting displayed on the controls of the transponder.
[ICAO Doc 4444, ¶126.96.36.199] Code 7600 shall be reserved internationally for use by pilots encountering a state of radio communication failure.
[ICAO Doc 8168, Vol I, §III-3-1-2, ¶1.5] The pilot of an aircraft losing two-way communications shall set the transponder to Mode A Code 7600. Note.— A controller who observes an SSR response indicating selection of the communications failure code will determine the extent of the failure by instructing the pilot to SQUAWK IDENT or to change code. If it is determined that the aircraft receiver is functioning, further control of the aircraft will be continued using code changes or IDENT transmission to acknowledge receipt of clearances. Different procedures may be applied to Mode S equipped aircraft in areas of Mode S coverage.
[ICAO Doc 4444, ¶188.8.131.52] Code 7700 shall be reserved internationally for use by pilots encountering a state of emergency.
[ICAO Doc 8168, Vol I, §III-3-1-2, ¶1.4] The pilot of an aircraft in a state of emergency shall set the transponder to Mode A Code 7700 unless ATC has previously directed the pilot to operate the transponder on a specified code. In the latter case, the pilot shall continue to use the specified code unless otherwise advised by ATC. However, a pilot may select Mode A Code 7700 whenever there is a specific reason to believe that this would be the best course of action.
Portions of this page can be found in the book International Flight Operations, Part V, Chapter 4.
Advisory Circular 91-70A, Oceanic and International Operations, 8/12/10, U.S. Department of Transportation
Air Traffic Organization Policy Order JO 7110.65U, February 9, 2012, U.S. Department of Transportation
Department of Defense Dictionary of Military and Associated Terms, Joint Publication 1-02, 15 November 2013
ICAO Doc 4444 - Air Traffic Management, Fourteenth Edition, Procedures for Air Navigation Services, International Civil Aviation Organization, 2001 *
ICAO Doc 4444 - Air Traffic Management, Fifteenth Edition, Procedures for Air Navigation Services, International Civil Aviation Organization, 2007 *
* Not all of Doc 4444 seems to have been reproduced in the 15th edition, so you might need to look at the 15th edition and then then 14th edition for some sections.
ICAO Doc 4444 - Amendment No. 1, Procedures for Air Navigation Services, International Civil Aviation Organization, Amendment No. 1, 2007
ICAO Doc 4444 - Amendment No. 2, Procedures for Air Navigation Services, International Civil Aviation Organization, Amendment No. 2, 19/11/09
ICAO Doc 7030 - Regional Supplementary Procedures, International Civil Aviation Organization, 2 2008
ICAO Doc 8168 - Aircraft Operations - Vol I - Flight Procedures, Procedures for Air Navigation Services, International Civil Aviation Organization, 2006
NAT Doc 007, North Atlantic Operations and Airspace Manual Doc 007, Edition 2013