UAS Communication Systems and Data Links 1 — Questions and Answers
Question 1: What frequency band is most commonly used for UAS command and control (C2) links in consumer and commercial operations?
- AM broadcast band (540–1700 kHz)
- 2.4 GHz ISM band (Correct answer)
- FM broadcast band (88–108 MHz)
- HF shortwave band (3–30 MHz)
Correct answer: 2.4 GHz ISM band
The 2.4 GHz ISM band is widely used for UAS C2 links because it is license-free, provides adequate range, and is supported by most RC and commercial UAS control systems.
Question 2: What does the term 'C2 link' refer to in UAS operations?
- Camera-to-camera communication between multiple drones
- Command and Control link between the ground control station and the aircraft (Correct answer)
- Cloud-to-cloud data synchronization for mission logs
- Cellular-to-controller backup communication channel
Correct answer: Command and Control link between the ground control station and the aircraft
The C2 (Command and Control) link is the bidirectional radio communication link that allows the remote pilot to send flight commands to the UAS and receive status information back.
Question 3: What is the primary function of a telemetry downlink in a UAS system?
- Transmitting live video from the camera payload to the ground
- Sending navigation waypoints from the ground station to the aircraft
- Transmitting real-time aircraft status data such as altitude, speed, and battery level to the operator (Correct answer)
- Establishing an internet connection for in-flight software updates
Correct answer: Transmitting real-time aircraft status data such as altitude, speed, and battery level to the operator
The telemetry downlink sends real-time aircraft health and position data — including altitude, airspeed, GPS coordinates, and battery voltage — to the ground control station for operator awareness.
Question 4: Which phenomenon describes the reduction in radio signal strength as the distance between transmitter and receiver increases?
- Doppler shift
- Free-space path loss (Correct answer)
- Multipath interference
- Antenna polarization mismatch
Correct answer: Free-space path loss
Free-space path loss describes how radio signal power decreases with the square of distance, which is the primary factor limiting the range of UAS control links in open environments.
Question 5: What does RSSI measure in a UAS communication system?
- Remote Sensor Signal Interface value
- Received Signal Strength Indicator — the power level of the received radio signal (Correct answer)
- Radio Spectrum Safety Index for regulatory compliance
- Real-time Status Signal Input from onboard sensors
Correct answer: Received Signal Strength Indicator — the power level of the received radio signal
RSSI (Received Signal Strength Indicator) is a measurement of the power level of the radio signal being received, allowing operators to assess link quality and determine how close they are to the edge of reliable range.
Question 6: Why is Frequency Hopping Spread Spectrum (FHSS) used in some UAS control systems?
- To increase maximum data throughput for high-definition video
- To reduce aircraft power consumption during transmission
- To improve resistance to radio frequency interference and unauthorized signal interception (Correct answer)
- To extend control range beyond the visual line of sight
Correct answer: To improve resistance to radio frequency interference and unauthorized signal interception
FHSS rapidly switches the transmission across many frequencies in a pseudorandom sequence, making the control signal harder to jam, intercept, or disrupt by interference sources on a fixed frequency.
Question 7: What primarily limits the effective range of a standard UAS radio control link?
- The battery capacity of the ground controller transmitter
- Line-of-sight conditions and antenna gain characteristics (Correct answer)
- FAA regulatory distance limits imposed by Part 107
- Operating system software range caps built into the flight controller
Correct answer: Line-of-sight conditions and antenna gain characteristics
Radio control link range is primarily limited by line-of-sight propagation conditions and antenna gain, since terrain, obstacles, and antenna directivity directly affect how well the signal travels between ground station and aircraft.
UAS Communication Systems and Data Links 1
What frequency band is most commonly used for UAS command and control (C2) links in consumer and commercial operations?