## 2 System Overview
### Electrical System Schematic without Fuel Cell Stack and Supercapacitor  
As shown in Figure 2, the heart of the quadrotor-based drone is the flight controller, i.e., an Ardupilot Mega (APM) [21]. The APM is loaded with ArduCopter firmware. This flight controller provides autopilot functions which enable the drone to perform missions, like GPS waypoint following and altitude holding. The flight controller also logs data in its flash memory, which is essential for analyzing flights. Also, as seen in Figure 2, the APM is connected to an external GPS unit, enabling positioning. There is also an output port which connects to the ESCs. The ESCs receive pulse width modulated (PWM) signals from the APM and drive the motors according to the received PWM commands, to achieve a desired motor speed. The power supply for the ESCs to drive the motors, is received from a power supply module. The power supply module has two inputs, an onboard DC-DC buck converter module, and several outputs. The inputs to the power supply module are from a battery with a rated voltage of 11.1 V. This approximately 11 V supply line is passed to the ESCs by the power module, (shown in Figure 2 by the thicker red and black lines. Also, the onboard DC-DC buck converter module provides a secondary output at 5 V and approximately 3 A (max). This second output (shown by the thin red and black lines) is used to power the APM and the other peripherals connected to it. The other thin wires from the power module to the APM represent the current and voltage sensing connections, which allow the current and voltage sensors embedded within the power module to measure and record the battery voltage and supplied current. The two other items connected to the APM are the telemetry radio module and the radio receiver for manual control. The 433 Mhz (100 mW) HopeRF HM-TRP telemetry radio module uses a UART communication interface and  
Figure 1: Overall system setup.  
transmits data at a rate of up to 250 kbps. Using this module, the drone's altitude, heading, pitch, battery voltage, current and a few other variables can be monitored via the ground station laptop. To enable manual control of the drone, a FlySky 6 channel 2.4 GHz radio receiver is used, and pairing a FlySky radio transmitter (remote) to it enables control of the thrust, yaw, pitch and roll of the drone manually. In addition, certain switches on the FlySky transmitter allow the drone to be switched into autopilot mode from manual control mode, mid-flight, by configuring the switches appropriately via the Mission Planner software [22].  
To provide the required thrust for the quadrotor-based drone to maintain flight, four 1000 kv outrunner brushless motors are used. The kv rating of the above mentioned motor is a number which provides an approximate idea about the no-load speed expected, when a unit voltage is applied to the motor. The propeller attached to each motor is a Hobbyking Slow Fly 10 \(\times\) 4.5, i.e., it has a 10 inch length and 4.5 inch pitch. The motors are powered by a 3 cell (11.1 V), 3000 mAh lithium polymer battery. Each of these motors can draw a maximum current of 12 A at the rated supply voltage of 11.1 V and produce a maximum thrust equivalent to a weight of around 0.7 kg. As mentioned above, the motors are powered by ESCs. The ESCs are rated at 11.1 V and 30 A. The current rating of the ESCs is selected to be much larger than the motor current rating to enable sudden spikes in the motor current that may occur due to in-flight turbulence, to be handled.