GEOG 892
Geospatial Applications of Unmanned Aerial Systems (UAS)

The Command and Control Element


The command and control element is the nerve center for the UAS operation. It controls the following tasks:

  • launching the vehicle,
  • flying the vehicle,
  • recovery of the vehicle,
  • receiving and processing data from internal sensors of the flight system,
  • receiving and processing data from external sensors of the payload,
  • controlling the operations of the payload, and
  • providing the interfaces between the UAS and the outside world.

The command and control element utilizes several subsystems to accomplish its missions. They are:

  • UAV status and controls,
  • payload data display and control,
  • map displays for mission planning and for monitoring the flight path,
  • autopilot to provide the ability for the UAV to execute its mission based on preprogrammed instructions without operator intervention,
  • ground terminal for two-way communication with the UAV and the payload,
  • computer(s) to:
    • provide an interface between the UAV and operator,
    • control the data link and the data flow between the UAV and the command and control station,
    • perform the navigation functions for the system,
    • perform necessary computations for the autopilot and the payload control functions,    
  • communication links to other organizations for command and control and for dissemination of information gathered by the UAV.

The most important parts of the command and control element are the Autopilot and the ground control station, as described in the following subsections:


Autopilot is the sub-system that enables partial or fully autonomous flight. A UAV can be operated completely by a remote control, where an operator steers the air vehicle all the time, or a UAV can be flown autonomously, where a pre-programmed path is fully executed from takeoff to landing by the autopilot sub-system without any pilot intervention. Small, light-weight autopilots are readily available and are made by a few manufacturers. Besides guiding the air vehicle throughout the pre-set flight path, the autopilot also executes a “lost link” routine if the UAV loses contact with the ground control station. The lost link procedure guides the UAV to a known waypoint, where contact with the ground control station can again be established. The following scenario was developed for a typical emergency procedure based on loss of link between the Yamaha RMAX UAS and the ground Control Station:

Emergency Procedures for Yamaha RMAX UAS

The RMAX utilizes a redundant communication system to ensure constant contact between the aircraft and the remote pilot. The ground control station provides real-time data regarding aircraft location, altitude and flight characteristics. The pilot constantly monitors the flight information provided to the ground control station, and through the assistance of a trained observer, maintains a visual line of sight to the aircraft. In the event of a loss of link between the aircraft and the ground control station, the subsequent procedures are followed: 

  • Preflight Actions - Prior to any flight, and as part of the mission preparation, the mission operator will insert appropriate lost link settings to allow the RMAX to safely return to the predetermined landing location. The settings are stored on the aircraft so that in the event of a lost link, the RMAX is able to continue operations under autonomous control.

    The mission operator will identify a safe altitude and a location for the aircraft to fly to once the RMAX detects a lost link. Once the aircraft reaches the specific GPS location, it will begin at auto descent and shut off the rotors upon landing.
  • In the Air - The RMAX continuously monitors the status of communication with the ground station. When the RMAX detects a loss of link with the ground station, it starts a timer. This timer value (typically 5 seconds) is set by the operator in the mission settings page. When this timer expires, the RMAX goes into lost communication mode and will command the vehicle to an operator-indicated lost communication waypoint at a predetermined altitude. The aircraft then commands a 20-second descent until touchdown. Once the aircraft lands, the aircraft automatically turns the rotors off. 
Problem: Sign of Problem: Monitored throughout: Solution:
Table 1 Emergency procedure for Yamaha RMAX UAS
Low Signal Vehicle is slow to respond to manual commands or PCC commands. Autopilot terminates steering mode. Audible and warning light alarms. Yes, signal strength displayed in percentage and packet update rate. Turn Autopilot on and abandon manual flight. Initiate auto-land.
Loss of Communication Autopilot terminates manual control or fails to respond to PCC commands. Audible communication alarm and warning light. Yes. The vehicle returns to loss communication waypoint, hovers until elapse of flight timer, then commences auto-land procedure.
Loss of GPS First indication is poor altitude hold performance, also poor position hold during hover. Yes, indicated by the number of satellites tracked and GPS Quality PDOP. Assume manual control of aircraft and land.
Low Power Avionics Lower than nominal voltage displayed. Yes. Land Immediately.
Engine Failure Noise level or RPM changes, engine loses power. Yes, monitored by rotor RPM through the RPM sensor. Return and land immediately. If the engine dies, initiate autorotation procedure.
Tail Rotor Failure Loss of tail control. No. Switch to manual control and initiate autorotation procedure.

Ground Control Station

The ground command station (GCS) is the site where the pilot controls the UAV during the flight. The GCS size and sophistication depends on the category of the UAS/UAV. Some large UASs require a formal facility with multiple workstations and personnel, while a GCS for small UAS can be a handheld transmitter. Most UASs used by the geospatial community are small UASs that do not require a dedicated GCS.

To Read

  1. Section 3.3 of chapter 3  of Introduction to the Unmanned Aircraft Systems
  2. Chapter 9 of the textbook Introduction to UAV Systems (Aerospace Series)
  3. The paper "Towards Low-cost Cooperative Multispectral Remote Sensing Using Small Unmanned Aircraft Systems"