Module 4/5 · Weeks 10–12 · 27 h

C2 and DAA

DRT 342 Mission Planning, Autonomous Operations and Beyond Visual Line of Sight

About 90 minDraft, awaiting reviewLast updated 26 September 2026

By the end of this module you will be able to

Explain the C2 link as defined by ICAO and compare direct radio, cellular and satellite architectures
Calculate the first Fresnel zone radius and use it to assess antenna height
Plan a lost-link procedure as a timeline
Explain the functions of a DAA system and the well-clear concept
Compare the data that Remote ID must broadcast under US and EU rules

Prerequisites: DRT 342 module 3 · DRT 341 module 3 (FSPL and link budget)

Why this matters

In BVLOS flight, everything the pilot knows and every command sent travels over the C2 link. The job of “looking out for other aircraft”, once done by human eyes, moves into a DAA system. People on the ground who want to know whose drone is overhead rely on Remote ID. These three systems are the technical pillars of BVLOS from module 3.

The C2 link

ICAO Doc 10019 defines the C2 link as the data link between the remotely piloted aircraft and the remote pilot station for the purposes of managing the flight (review DRT 341 module 3). The C2 link is separate from the payload video link. Still seeing video does not prove that commands are reaching the aircraft, and frozen video does not prove that C2 is lost.

Three architectures

Three rows. Direct radio RLOS: ground station to aircraft via radio. Cellular: ground station via internet and base stations. Satellite BRLOS: ground station via ground earth station and satellite. All links are two-way — Figure 1 Three C2 link architectures

Type	Strengths	Limitations
Direct radio (Radio Line of Sight, RLOS)	Low latency; the operator controls the whole system	Range limited by radio horizon, obstacles and transmit power
Cellular network	Wide coverage where there is signal	Base stations are designed for ground users, so airborne coverage can be uneven; depends on the network operator
Satellite (Beyond Radio Line of Sight, BRLOS)	Covers remote areas	Higher latency; expensive antennas and service

Many BVLOS operations combine two types, for example direct radio as the primary link near the station and cellular as the backup. Using two dissimilar links reduces the chance that both fail together from a single cause (review parallel reliability in DRT 341 module 1).

The frequency bands allowed in Thailand are set by NBTC notifications, and users must check the current notification before choosing equipment. For larger aircraft, performance standards such as RTCA DO-362 (2016) set minimum performance for terrestrial C2 links.

Fresnel zones and antenna height

Even when the straight line between two antennas is unobstructed, the signal can still weaken if obstacles are close to that line, because radio waves occupy an ellipsoid around it called the first Fresnel zone. ITU-R P.530 gives its radius as:

F_{1} = 17.3 \frac{d _{1} d _{2}}{f _{GHz} d} m

where $d_{1}$ and $d_{2}$ are the distances from the point of interest to the two ends, and $d = d_{1} + d_{2}$ , all in kilometres. Designers commonly keep at least about 60% of $F_{1}$ clear of obstacles.

Example 1 A tree line midway on a 2.4 GHz link

The drone is 2 km from the station, with a tree line at the midpoint ( $d_{1} = d_{2} = 1$ km).

$F_{1} = 17.3 \frac{1 \times 1}{2.4 \times 2} = 17.3 \times 0.456 \approx 7.9$ m
Required clearance $0.6 \times 7.9 \approx 4.7$ m

The line between the antennas must pass at least about 4.7 m above the treetops. If it just grazes them, the signal drops even though the two ends “can see each other”. Raising the station antenna helps a great deal.

When the C2 link is lost

Link loss is an event to plan for, not a surprise. RTCA DO-400 (2023) provides guidance on lost C2 link procedures. The key principle is that the aircraft’s behaviour must be predictable: the crew and air traffic services must know in advance what it will do.

Timeline: link normal, degraded with warning, lost beyond timeout, failsafe RTL continue or land, then link regained or landed. Below, the ground crew notify air traffic services or stakeholders, track position via backup telemetry, and prepare the landing site — Figure 2 Lost C2 link timeline

The autopilot does not declare link loss the moment one packet is missed. It waits until a configured timeout expires, and the aircraft keeps flying in the meantime.

Example 2 Distance flown before the failsafe acts

PX4’s default COM_DL_LOSS_T is 10 s, and the drone flies at 12 m/s.

$d = 12 \times 10 = 120$ m

In the first 10 seconds after link loss the drone may fly up to 120 m further on its current heading. This distance must fit inside the contingency volume (module 1) and be included in the geofence margin (module 2). A shorter timeout makes the failsafe act sooner, but also more often on brief dropouts, so a balance is needed.

Detect and avoid (DAA)

ICAO Doc 10019 defines DAA as the capability to see, sense or detect conflicting traffic or other hazards and take the appropriate action.

The drone sits in two concentric rings: the outer ring is the detection range and the inner ring is the well-clear zone. A crewed intruder approaches from the right. Below is the sequence detect, track, evaluate, decide, avoid — Figure 3 Encounter geometry and DAA functions

A DAA system works in sequence: detect → track → evaluate whether the encounter will breach the threshold → decide → avoid. The aim is to stay well clear, not merely to avoid collision. For small UAS, ASTM F3442/F3442M-23 uses a well-clear volume of 2000 ft (about 610 m) horizontally and 250 ft (about 76 m) vertically. Larger UAS are covered by RTCA DO-365.

Sensor type	Examples	Notes
Cooperative	ADS-B, transponder	Detects only aircraft that carry the equipment and have it switched on
Non-cooperative	Radar, EO/IR cameras, acoustic sensors	Detects aircraft that do not transmit, but range and error rates depend on the environment
Ground-based	Radar along the route	Adds no weight to the drone but covers only where it is installed

Example 3 How much time is there to decide?

A helicopter at 100 kt ( $100 \times 0.5144 = 51.4$ m/s) flies head-on towards a drone at 15 m/s. The system detects it at 2000 m.

Closing speed $51.4 + 15 = 66.4$ m/s
Time to collision point $2000/66.4 \approx 30.1$ s
Time until the 610 m horizontal well-clear boundary is breached: $(2000 - 610) /66.4 \approx 20.9$ s

About 21 seconds must cover confirming the target, deciding, sending the command over the C2 link and the drone changing course. A system that detects at shorter range may not leave enough time. This is why detection range is a core DAA requirement.

Remote ID

Remote ID lets people on the ground, such as officials, identify which drone is flying, where it is and where its pilot is. There are two forms: broadcast, sent directly from the drone over short-range radio, and network, sent over the internet to a service provider. ASTM F3411-22a covers both.

Required data	FAA Part 89 standard	FAA Part 89 broadcast module	EU 2020/1058 (direct remote ID)
Aircraft identifier	Serial number or session ID	Module serial number	Operator registration number and aircraft serial number
Drone position and height	Yes	Yes	Yes
Velocity/course	Yes	Yes	Yes
Pilot location	Control station position	Take-off location instead	Pilot position (or take-off point if unavailable)
Time	Yes	Yes	Yes
Emergency status	Yes	No	Not in the Part 6 list

US Remote ID rules have applied since 16 September 2023. In Thailand, the 2026 CAAT notification does not set Remote ID requirements. Follow new CAAT and NBTC notifications.

Remote ID supports identification and enforcement and is a foundation of UAS traffic management (UTM), but it is not a collision-avoidance system. Most crewed aircraft do not receive Remote ID.

Class activity

Activity: C2 and abnormal events in a simulation

Use exercise B03 “C2 and abnormal events in simulation” from the drone knowledge hub.

Run the connected, warning and timeout state model with heartbeat ages of 0, 1, 3 and 7 seconds, comparing 5 s and 10 s timeouts.
Calculate how far the drone flies before the failsafe acts with each timeout at 12 m/s.
Calculate the Fresnel radius for your group mission link from module 3 and propose an antenna height.
Tabulate which information comes from C2, the payload link and DAA.

Common mistakes

Watch out

Calling every event “signal lost” without separating C2, video link and GNSS
Assuming the link is good because the ends can see each other, ignoring the Fresnel zone
Setting the link-loss timeout too short, so the failsafe triggers needlessly, or too long, so the aircraft leaves its area
Relying on ADS-B alone for DAA although some aircraft do not transmit it
Believing Remote ID helps avoid collisions

Summary

C2 links come in three main types: direct radio, cellular and satellite. Dissimilar primary and backup links improve reliability
Keep about 60% of the Fresnel radius $F_{1} = 17.3 d_{1} d_{2} / (f d)$ clear of obstacles
Lost-link procedures must make aircraft behaviour predictable; the distance flown before the failsafe acts is $v t_{timeout}$
DAA detects, tracks, evaluates, decides and avoids in order to stay well clear
Remote ID identifies and locates aircraft; it is not a collision-avoidance system

Check your understanding

The camera video freezes but telemetry values keep updating. Can you conclude that the C2 link is lost?
A 5.8 GHz link is 2 km long with an obstacle at the midpoint. What is $F_{1}$ there?
A drone flies at 15 m/s with a 5 s link-loss timeout. How far does it fly before the failsafe acts?
Two aircraft approach head-on with a closing speed of 60 m/s and are detected at 1800 m. How much time is there before collision?
Under FAA Part 89, what position does a broadcast module send instead of the control station location?

Answers

No. Frozen video may come from the video link or the display, while updating telemetry shows that C2 downlink data is still arriving. Check each source separately
$17.3 1 \times 1/ (5.8 \times 2) = 17.3 \times 0.294 \approx 5.1$ m
$15 \times 5 = 75$ m
$1800/60 = 30$ s
The take-off location

Key formulas

First Fresnel zone radius	$F_{1} = 17.3 \frac{d _{1} d _{2}}{f _{GHz} d} m$
Time to collision (head-on)	$t = \frac{r}{v _{1} + v _{2}}$
Distance flown before the failsafe acts	$d = v t_{timeout}$

Key references

International Civil Aviation Organization. (2015). Manual on remotely piloted aircraft systems (RPAS) (Doc 10019). ICAO. link
RTCA. (2016). Command and control (C2) data link minimum operational performance standards (MOPS) (terrestrial) (DO-362). link
RTCA. (2023). Guidance material: Lost C2 link procedures (DO-400). link
RTCA. (2022). Minimum operational performance standards (MOPS) for detect and avoid (DAA) systems (DO-365C). link
ASTM International. (2025). Standard specification for detect and avoid system performance requirements (ASTM F3442-25). link
Federal Aviation Administration. Remote identification of unmanned aircraft, 14 C.F.R. Part 89 (compliance date September 16, 2023). link
European Commission. (2020). Commission Delegated Regulation (EU) 2020/1058 amending Delegated Regulation (EU) 2019/945 (direct remote identification add-on). link
ASTM International. (2022). Standard specification for remote ID and tracking (ASTM F3411-22a). link
International Telecommunication Union. (2015). Propagation data and prediction methods required for the design of terrestrial line-of-sight systems (Recommendation ITU-R P.530-16). link
International Telecommunication Union. (2024). Calculation of free-space attenuation (Recommendation ITU-R P.525-5). link
PX4 Autopilot. PX4 user and developer guide. link

In class / field

Lecture, case discussion and in-class problem solving

Learning evidence: Quiz results and submitted exercises

Module quiz

This is a formative self-check, not a graded exam

Knowledge domain: Communications, networks and IoT · Law, safety and risk

C2 and DAA

By the end of this module you will be able to

Why this matters