Monitoring the Aircraft Band

The band

118.000 – 136.975 MHz, AM voice, 25 kHz channels (8.33 kHz in much of Europe). Immediately above the 2 metre amateur band, and receivable on almost any scanner or SDR — including the RTL-SDR from the lab track, which needs no modification to hear it.

The assignments come from the §87.173(b) frequency table, which is worth reading once rather than trusting a forum post:

Everything is on 25 kHz channel spacing (8.33 kHz in much of Europe).

Two easy mistakes the table settles. 128.825–132.000 is not centre — it is company and dispatch traffic, and tuning there expecting ATC will confuse you. And UNICOM splits on whether the field has a tower: 122.950 is the towered one, the rest of 122.700–123.075 is not.

The frequency you care about most as a drone operator is the CTAF — Common Traffic Advisory Frequency — at any non-towered airport near your site.

Why aviation is still AM

Every other VHF service moved to FM decades ago. Aviation did not, and the reason is directly relevant to the signals module.

The capture effect. An FM receiver locks onto the strongest signal and suppresses everything else entirely. Two aircraft transmitting at once on FM would mean the weaker one vanishes — nobody hears it, and nobody knows it happened.

On AM, two simultaneous transmissions produce a heterodyne — an audible squeal — and both voices remain partly intelligible. Everyone immediately knows two people transmitted. That failure mode is loud rather than silent, and in aviation a loud failure is much better than a silent one.

Two consequences follow:

• AM is less efficient and noisier than FM. Aviation accepted that trade deliberately.
• You must set your receiver to AM. An NFM-demodulated aircraft transmission is a distorted mess, which is the usual reason a new scanner user concludes the band is dead.

What you may legally receive

Receiving is not regulated the way transmitting is. You may listen to the aircraft band with any receiver.

Two limits worth knowing, and neither is an FAA rule:

• The Communications Act (47 U.S.C. 605) prohibits divulging or benefiting from the content of certain intercepted communications. Listening is fine; using what you hear against someone’s interest is not.
• A few US states restrict scanners in motor vehicles. Aircraft-band monitoring is rarely the target, but the statutes are worded broadly.

You may not transmit on the aircraft band without an FCC aircraft station licence and the appropriate operator authorisation. An amateur licence does not convey this, and a Part 107 certificate certainly does not. This band is receive-only for you.

Using it as a drone operator

At a non-towered field, monitoring CTAF is the single highest-value radio thing you can do. You hear aircraft announcing position and intentions — “five miles north, inbound, runway two seven” — which tells you where manned traffic will be several minutes before you could see it.

Add ATIS or AWOS for the field: continuously broadcast weather, including the ceiling and visibility figures that decide whether your flight meets §107.51’s 3 statute miles and 500 below / 2,000 horizontal cloud clearances. That is an authoritative source rather than a phone app’s estimate.

This is situational awareness, not a substitute for anything. §107.43 requires you not to interfere with airport operations, and §107.37 gives manned aircraft right of way. Monitoring helps you comply with both; it does not replace the visual line-of-sight obligation, and you cannot answer.

The two transmitters you may not carry

§107.52 — Unless otherwise authorized by the Administrator, no person may operate a small unmanned aircraft system under this part with a transponder on.

§107.53 — Unless otherwise authorized by the Administrator, no person may operate a small unmanned aircraft system under this part with ADS-B Out equipment in transmit mode.

These surprise people, because both sound like safety equipment. The reasoning is system capacity: ATC surveillance and the ADS-B network were sized for manned aircraft. Tens of thousands of drones squawking would saturate the infrastructure and degrade it for the aircraft that depend on it — the classic case of individually sensible behaviour that fails collectively.

Note the asymmetry, which is the point:

• ADS-B In (receiving) is fine, and useful — it shows you nearby traffic.
• ADS-B Out (transmitting) is prohibited.
• Remote ID is the drone-scale answer, broadcasting locally over Bluetooth or Wi-Fi instead of into the aviation surveillance network. That is the next lesson but one.

Check yourself

1. You tune 122.8 MHz and hear only noise, though the waterfall shows a signal. What is wrong?
2. Why does aviation use AM when FM has better noise performance?
3. Your aircraft has an ADS-B module. May you use it?