PRISM Flight Modes

This section deals with the primary functions of the flight control system.

Flight Control System

The open source ARDUCOPTER firmware provides a fully-customizable, feature-rich autopilot system for PRISM. The flight controller has been pre-configured by WATTS INNOVATIONS to enable the operator to complete most tasks in a safe and efficient manner. A more in-depth study of the autopilot system and features may be conducted using the online resources at https://ardupilot.org/copter.

It is important to note however, that PRISM utilizes a custom Arducopter firmware. This means that it should not be updated to a standard Arducopter firmware, as this will break all of the intuitive features found within PRISM. Updating to a stock Arducopter firmware will void your warranty.

Guidance, navigation, and control are provided by the Pixhawk CUBE flight controller system. Sensors in the CUBE include a triple-redundant, 6-axis inertial measurement system; GPS/ GNSS receiver and magnetometer (compass); and dual barometric pressure sensors for detecting aircraft altitude. This system allows for operation in 5 primary flight modes, and multiple failsafe modes. The pilot can change flight modes with the three-position FLIGHT MODE switch on the remote control unit, and the mode of each position may be user-customized in the WattsQGC app. When operating through the mobile device interface, the pilot will have the ability to select from a variety of other flight modes as required. This manual deals with the 5 primary modes that can be selected using the handheld radio control unit only.

It should also be noted that the user may choose to use STABILIZE flight mode, which is a self-leveling mode (for both the roll and pitch axis), where the throttle is a direct correlation to the % of throttle given by the user. Pilots must be extremely careful when arming and taking off in STABILIZE mode. In addition, the RPA may either “jump” or “drop” up to 3m when transitioning from STABILIZE to ALTHOLD mode. Flight in STABILIZE mode is not recommended but can be useful in the hands of a skilled pilot.

Altitude Hold (ALT HOLD)

ALT HOLD mode is the base flight mode of this PRA, and provides the most manual control over the aircraft. In this flight mode, the aircraft uses the onboard sensors to only stabilize its attitude and altitude. Pitch and roll are commanded by manipulating the respective control sticks.

At neutral control input (center pitch and roll stick position) the RPA will remain level and not increase or decrease altitude (See Caution below). Altitude rate-of-change is controlled by the amount of deviation from center point of the throttle stick. The higher the throttle stick position, the faster the climb. Conversely, the lower the throttle stick position, the faster the RPA will descend.

CAUTION: This flight mode does not provide any lateral positional control of the RPA. The PIC must be aware that the RPA will drift in any wind.

Position Hold (POS HOLD)

This flight mode uses GPS inputs to maintain position during flight. Pilot inputs via the pitch/roll sticks command changes in ground speed. Pitch and roll stick deflection will command fore/aft and left/right ground speeds respectively. Controlling altitude in POS HOLD mode is the same as in ALT HOLD. With pitch and roll controls centered, the RPA will attempt to hold its current position and altitude. The aircraft will self-correct for disturbances that may force it from the desired position, including lateral forces from wind.

CAUTION: This mode requires a strong GPS signal and communication with a minimum of 6 satellites. Although the GPS system functions without issue most of the time, it is still vulnerable to interference from outside elements. This interference may be caused by multipath error, solar storms, operation in close proximity to powerful radio transmitters, or operations within urban canyons. If the GPS reception is weak, the RPA will not enter POS HOLD Mode. If the GPS signal degrades while in POS HOLD Mode, the aircraft will automatically revert to ALT HOLD Mode.

Auto Mode

Auto mode instructs the RPA to follow the waypoint mission plan uploaded to the autopilot system. The pilot will only be able to control the RPA's heading while the mission is being carried out; they will not be able to deviate horizontally or vertically from the waypoint path. The autonomous mission can be paused at any time by toggling the flight mode switch on the RPS. To exit AUTO mode, the user can simply flip the switch into either ALT HOLD or POS HOLD modes.

CAUTION: Although the mission can initiate an automatic take off and landing, it is highly recommended that the PIC perform a manual take off and landing.

Return-To-Land Mode (RTL MODE)

This mode can be selected in an emergency situation, and will command the aircraft to automatically return to home and land. The home position is recorded automatically when the aircraft is armed and there is sufficient GPS signal strength. The aircraft will return to these GPS coordinates during an automated Return-to-Land event. For this reason, ensure that the drone may safely be landed in extreme circumstances, especially operating from a moving platform (such as a truck, boat, etc).

As part of the preflight planning, the pilot should identify the height of all tall objects in the flight operations area. The RTL ALTITUDE should be set to a height that is greater than that of the tallest object with a comfortable margin of error (10-20%). This is done to ensure that the aircraft can always maintain separation from all objects while autonomously returning to land. The RTL ALTITUDE can be set in the SAFETY menu of WattsQGC.

Once the RPA has arrived above the home position, it will automatically descend and perform a landing. This situation is described in detail in the Emergency Procedures section. The RTL altitude and behavior should be set by the PIC via the GCS prior to flight.

CAUTION: When operating from a moving vehicle, the pilot must be aware that “Home” position is recorded where the RPA first took off. This may be distant from the position of the pilot when the RTL is commanded. The aircraft will not return to the pilot.

Failsafe Modes

This Autopilot system makes provision for a number of failsafe conditions and corresponding actions. These failsafe triggers and responses are fully customizable, although it recommended not to alter the factory settings unless absolutely necessary.

Radio Control Link Loss (RADIO FAILSAFE)

In the event of a sustained loss of the C2 link between the RPS and the aircraft, the RPA will enter Radio Failsafe Mode. This mode enacts a preconfigured action in the emergency situation. Prior to flight operations, the PIC should determine the most suitable failsafe action, depending on the operating environment and nature of flight.

The failsafe settings are configured in the WattsQGC application.

The default configuration is to automatically enter RTL MODE.

Ground Control Station Link Loss (GCS FAILSAFE)

This mode is relevant during missions in AUTO mode, where the Pilot is not controlling the RPA manually via the radio control. It may be the case that the RPA is operating beyond the range of the Radio Control link.

In the event of a sustained loss of the Ground Control Station link between the RPS and the aircraft, the RPA can be set up to perform two actions: The aircraft can be set to RTL regardless of flight mode (the default setting); or it can continue the mission in AUTO mode even while the C2 link is lost.

The failsafe settings are configured in the WattsQGC application. By default, the GCS Failsafe is not enabled, as the RPA has dual communication with the GCS (either by Radio or by Datalink). Only the loss of RC Control will initiate a RTL. For example, if the pilot exits WattsQGC during flight, the aircraft will not initiate a RTL. However, if the pilot turns off the EchoSky Mobile handset while in flight, this will initiate a RTL, as the RC radio itself has been turned off.

CAUTION: It may be the case that while executing an AUTO mission, the RPA is beyond the range of both video and control communication links. The Pilot will not receive telemetry information or be able to control the RPA. This situation should be avoided by comprehensive mission planning, taking into account control link range and LOS conditions.

Battery Failsafe

If the battery voltage drops below the Low Voltage Threshold, the RPA will enter low battery Failsafe mode. This is indicated by a Yellow Flashing Status LED and on the GCS monitor. The RPA will enter RTL mode and begin to return back home. The pilot is to terminate the flight and land as soon as possible, as the battery failsafe does not factor in travel time to the home location.

The second level of battery failsafe is invoked when the voltage drops below the CRITICAL VOLTAGE threshold. In this case, the RPA will enter LAND mode and begin an automated descent in place. This mode is a last resort to prevent total battery depletion while in the air. The LED will flash red and "Critical Battery Voltage" will be displayed on the GCS.

Low battery failsafe procedures are described in more detail in the section on emergency procedures.

WARNING: Low Battery is the leading cause of RPAS incidents. The Pilot should never let the battery voltage drop to either of the battery failsafe thresholds during normal operations. Flights should be planned to land before Battery Failsafe is invoked.