# What is DME Unlock?

Reading through a NOTAM I found something I don't understand,

A0311/17 NOTAMR A2635/16
Q) VTBB/QIDXX/I/NBO/A/000/999/1342N10046E025
A) VTBS  B) 1702080225 C) 1705081100
E) ILS DME I-SES CH38X RWY 01R UNUSABLE BEYOND 15DEG
LEFT OF LOC COURSE DUE TO DME UNLOCK


What is DME Unlock? How does it affect the flight?

• Could you post the NOTAM in full (or at least provide the location identifier)? – J. Hougaard Mar 22 '17 at 5:32
• @J.Hougaard added. Now it is very long. The part I am interested in is the last line. – vasin1987 Mar 22 '17 at 5:39

I think it just means a situation in which the DME is "no longer locked on". Sounds like the DME may not be seen properly from those areas mentioned in the NOTAM.

From the Naval Instrument Navigation Workbook

TACAN Characteristics

1.Bearing/Distance Unlock. TACAN bearing and distance signals are subject to line-of-sight restrictions because of their utilization of UHF frequencies. Because of the transmission/reception principles, unlock (indicated by rotating of bearing pointer and/or range indicator) will occur if these signals are obstructed.

...

Aircraft receiver memory circuits prevent unlock for short periods (approximately 10 seconds for DME and 2 seconds for azimuth.) But beyond this unlock occurs and will persist until the obstruction is removed and search cycles are completed. Unlock may occur during maneuvers, such as procedure turns, which cause the aircraft antenna to be obstructed for longer than 2 to 10 seconds