TACAN is a polar-coordinate type radio air navigation system that provided military aircrafts with distance information, from distance measuring equipment (DME), and bearing (azimuth) information from a ground or ship-borne station. TACAN was the military version of the VOR/DME system.

=> Distance Measuring Equipment (DME) => is defined as a combination of ground and airborne equipment which gives a continuous slant range distance-from-station readout by measuring time-lapse of a signal transmitted by the aircraft to the station and responded back.

• Therefore, the distance indication (slant range) obtained is only valid to the indicated position (distance from the station).

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Let's Decode the following info:

Q) EDGG/QNNCS/IV/BO/AE/000/999/5049N00638E025 A) ETNN B) FROM: 15/11/11 09:00 C) TO: 16/02/10 21:50 EST E) AD ETNN: MOBILE TACAN NOE CH79Y IN USE. PSN 504947N0063847E 259DEG 859M FROM ARP.

Here is the decoded information:

Q: Qualifier - the ICAO code of the FIR where the affected place, equipment or service is located (EDGG) Langen Radar

QNNCS: the subject of the NOTAM and its status. "N" Com and nav systems and en-route navigation facilities. "C" Changes

IV: Traffic affected, in this case, IFR and VFR.

BO: Purpose or significance of the NOTAM - "B" of operational significance selected for pre-flight information bulletin entry. "O" concerning flight operations.

AE: Flight phase affected - "A" aerodrome. "E" En-route.

000/999: The vertical limit

5049N00638E025: Coordinates and radius, indicates the coodinates and radius of the centre and size of the lateral limits of the place affectes by the NOTAM.

Item A) affected location: ETNN (four-letter ICAO aerodrome or FIR) - Norvenich airbase

Item B) valid from: this shows the date (year YY, month MM, day DD, hours HH, minutes MM) 11 Nov 2015 (15/11/11) 09:00

Item C) valid to: 10 feb 2016 (16/02/10) 21:50

Item E) NOTAM message (AD ETNN: MOBILE TACAN NOE CH79Y IN USE. PSN 504947N0063847E 259DEG 859M FROM ARP): the TACAN ‘NOE’ on the channel 79Y is in use. If the TACAN is in use, then you will have valid slant range indications since this is how a DME measures the distance from a given position.