# Why is 121.5 called “guard”?

What is a "guarded" frequency and how does it differ from a frequency that's unguarded?

It is called guard because everybody is supposed to listen/guard the frequency just in case someone has a problem.

Guard definitions:

• a state of caution, vigilance, or preparedness against adverse circumstances
• watch over in order to protect or control.

The AIM 6-3-1(h)(1) says (emphasis mine):

121.5 MHz and 243.0 MHz. Both have a range generally limited to line of sight. 121.5 MHz is guarded by direction finding stations and some military and civil aircraft. 243.0 MHz is guarded by military aircraft. Both 121.5 MHz and 243.0 MHz are guarded by military towers, most civil towers, FSSs, and radar facilities. Normally ARTCC emergency frequency capability does not extend to radar coverage limits. If an ARTCC does not respond when called on 121.5 MHz or 243.0 MHz, call the nearest tower or FSS.

• 121.5 is also guarded by that guy who never hesitates to radio "On guard!" when someone accidentally transmits on guard. – casey Jan 11 '14 at 15:38
• en guarde mom ami! – rbp Nov 27 '15 at 1:20
• @rbp: En garde mom! (but if you aren't talking to your mom, then it's: En garde mon ami !). – mins Jan 3 '16 at 14:24
• " India Golf niner-niner transmitting in the blind guard. Disengage! Repeat, disengage! " – Michael Jul 25 '17 at 2:17
• – President James K. Polk Jul 27 '19 at 15:47

Two reasons:

First, and most relevant today, all facilities are supposed to "guard" (monitor) the frequency if able. Most ATC facilities and Flight Service stations monitor 121.5MHz, as do many airliners or aircraft with two radios. Lnafziger already gave you the relevant AIM paragraphs that talk about this.

Second, "guard frequencies" in general radio terminology are frequencies that have extra protection in the band.
This is done for two reasons: to prevent use of the guard channel from interfering with other channels, and/or to ensure that the guard channel is always clear and that adjacent channels won't "bleed over" and render it unusable. This mattered more in the days of tube radios with poor frequency alignment & confinement - modern radios bleed much less, but some frequencies are still afforded extra band protection.

121.5MHz in the USA is guarded in both respects - monitored as Lnafziger describes, and afforded extra band protection (50KHz on either side, 121.425 - 121.475 MHz & 121.525 - 121.575 are unusable per AC 90-50).

• As long as you are talking about radio theory, a guard frequency also frequently can not be over-ridden. Ie, if ATC is transmitting on a secondary frequency, someone transmitting on the guard frequency can still be heard. – Lnafziger Jan 11 '14 at 7:36
• @Lnafziger Yup - my understanding is most ATC facilities are set up so 121.5 is always heard by someone - usually on a facility speaker and/or monitored in headsets either by everyone or at specific sectors. (Facilities that don't monitor 121.5 at all usually have a notice to that effect somewhere - like "Guard frequency 121.5 not available at tower" in the AFD remarks) – voretaq7 Jan 11 '14 at 8:21

I wonder whether it comes from the French, where the verb "guarder" means "to keep" or "to reserve" or "to keep in reserve". A "guarded" frequency would then mean "a reserved frequency" which makes perfect sense.

Note that Mayday comes from the French.

• Google says that the word guard (not just the aviation term) comes from Old French garde and garder. – Lnafziger Jan 11 '14 at 14:02
• I was surprised to find French in those other 'Mayday'-related words in Wikipedia: "pan pan", "sécurité", "silonce", etc. The term "guard band" is also used in radio (meaning, unused to prevent interference). – ChrisW Jan 11 '14 at 14:42
• France was pretty heavily involved in early aviation, so that's probably got something to do with it! – egid Jan 11 '14 at 19:27
• Fuselage empennage aileron etc – rbp Nov 27 '15 at 1:21

From my days as a Navy communications officer, if you guarded a frequency such as 121.5MHZ you also had a transmitter set for that frequency as well as a receiver. If you only monitored a frequency, you only had a receiver tuned to it. We at least monitored both 121.5 and 243.0 any time we put out to sea.