M1-T11: UAV communication and onboard architecture

Learning objectives

After this lesson, you should be able to:

• Distinguish the command and control, telemetry and payload links and their requirements.
• Compute a free-space link budget and the maximum range for a given margin.
• Choose an onboard bus (UART, CAN, Ethernet, USB, SPI, I²$C$) for each device.
• Specify the loss-of-link behaviour for each link.

Conceptual explanation

Off-vehicle: a command and control (C2) link (low rate, high reliability, long range), telemetry to a ground station (MAVLink) and a payload link (video, high rate, shorter range). On-vehicle: buses between flight controller, ESCs, sensors and the companion computer.

Engineering intuition

Match the bus to the requirement: control-critical, low-rate and long cable run suggests CAN; sensor on the same board suggests SPI; high-rate payload suggests Ethernet. Then ask of each link “what does the vehicle do if this goes silent?” and write the answer into the failsafe table (lesson M8-T14). The most dangerous links are the ones with no heartbeat, because silence looks like “nothing to report”.

Mathematical formulation

Free-space path loss and the link budget (established radio engineering):

Doubling distance costs 6 dB; doubling frequency costs 6 dB. The maximum range for a required margin $M$ follows by solving for $d$. Real links need extra margin for fading, antenna orientation, obstructions and interference; the free-space result is an upper bound.

$d$, $f$

distance and carrier frequency.

$P_{\text{tx}}$, $G_{\text{tx}}$, $G_{\text{rx}}$

transmit power and antenna gains (dBm, dBi).

sensitivity

minimum received power for the data rate (from the radio’s datasheet).

Architecture and data flow

Control path: ground station → C2 radio → flight controller. Autonomy path: companion → FC (UART/Ethernet). Logging path: flight controller → SD card; companion → disk. Time path: GNSS PPS or PTP to sensors. Document every link in the Interface Control Document (lesson M7-T17) with rate, latency, frame, and failure behaviour.

Real-world UAV applications

• Choosing a telemetry radio and antenna for a given range.
• Deciding the bus for a new payload.
• Defining link-loss actions and timeouts.
• Estimating whether live video is feasible at the mission range.

Implementation example

This runs as shown: free-space link budget and the maximum free-space range for a margin, with a check that doubling distance costs 6 dB.

import math

def fspl_db(d_km, f_mhz): return 20 * math.log10(d_km) + 20 * math.log10(f_mhz) + 32.44
def rx_dbm(ptx_dbm, gtx_dbi, grx_dbi, d_km, f_mhz, other_loss_db=0.0): return ptx_dbm + gtx_dbi + grx_dbi - fspl_db(d_km, f_mhz) - other_loss_db
def max_range_km(ptx_dbm, gtx, grx, f_mhz, sensitivity_dbm, margin_db):
    allowed_loss = ptx_dbm + gtx + grx - sensitivity_dbm - margin_db
    return 10 ** ((allowed_loss - 32.44 - 20 * math.log10(f_mhz)) / 20)

assert abs(fspl_db(2, 2400) - fspl_db(1, 2400) - 6.0206) < 1e-3                       # doubling the distance costs 6 dB
assert abs(fspl_db(1, 2400) - 100.04) < 0.05                                          # 1 km at 2.4 GHz is about 100 dB
d = max_range_km(20, 2, 2, 2400, -100, 20)                                            # 100 mW, 2 dBi antennas, -100 dBm sensitivity, 20 dB margin (illustrative)
print(f"free-space upper bound on range: {d:.1f} km (real links need extra margin)")
assert abs(rx_dbm(20, 2, 2, d, 2400) - (-100 + 20)) < 1e-9 and 1 < d < 3

Common engineering mistakes

Failure modes

Debugging approach

1. Log received signal strength and packet loss against distance and attitude.
2. Measure bus load and error counters.
3. Test link-loss behaviour by pulling the link in a tethered or simulated test.
4. Review the ICD against measured rates.

Practical exercises

1. Compute FSPL at 5.8 GHz and 3 km.
2. Find the maximum range if the margin requirement is 30 dB.
3. Choose buses for ESCs, GNSS, a LiDAR, a stereo camera and a companion computer and justify each.
4. Write the loss-of-link action for each link in your architecture.

Interview questions

Write an answer in your own words, then reveal the model answer. Nothing is sent anywhere.

References and further reading

Primary papers, official documentation and standards. Each claim above that is specific to a tool or version points here.

• PX4: DroneCAN
• MAVLink developer guide
• $T$. $S$. Rappaport, Wireless Communications: Principles and Practice, 2nd ed., Prentice Hall, 2002 (link budgets).
• PX4: peripherals (telemetry radios, GNSS, ESCs)

What’s next