Overview

The prototype aircraft is designed to have a useful mission range of 10-20 miles with a wide flight envelope and the capability of flight durations greater than 20 minutes. It will also be made more autonomous in the future, incorporating feedback control and waypoint tracking software. These design criteria had an impact with the type of electrical system selected for the aircraft. For example, long-range RF modems are used instead of traditional Bluetooth or 802.11 communication protocols for the extended range. Also, the flight computer has been assembled using the existing PC/104 standard, a proven, low-power, robust, and modular industrial computing standard. A dedicated long-range camera system was also used to avoid bandwidth issues in the RF modems and increase the redundancy in the system. The power system has been designed to be modular, allowing easy addition of another battery and switching supply for greater flight durations.

Ground Systems

The ground computer is a Dell notebook running LabView 7.0. This program manages all communication between the ground and flight electronics. The user interface resembles that of a normal personal aircraft (such as a Cessna) and the virtual cockpit has readouts that include heading, airspeed, altitude (pressure as well as physical), climb rate, groundspeed, angle of attack, pitch, yaw, and an artificial horizon. The ground computer will also be using a neural network to interpret raw pressure transducer data sent from the flight computer.

The control interface will be easy for potential UAV pilots to learn, as it will have a distinct “Flight Simulator” feel to it. The main control input will be a USB joystick purchased over the shelf at Best Buy. During piloted flight the aircraft will be flown much like a video game. A TV screen will display the video image to show where the aircraft is flying and what is in front of it. The computer display will display all of the flight data needed to fly the aircraft with instruments. During unmanned flight the computer will be available for the operator to monitor the situation and will also allow the operator to take immediate control if the need arose. The operator will also relay information from the officers on the ground to the UAV in the sky.

Flight Computer

The flight computer is built on the existing PC/104+ standard. This standard allows for future integration of stability feedback software and waypoint tracking. Since the PC/104 standard is modular, different add-on cards can be purchased and integrated for different tasks. For example, GPS, Digital I/O, and PWM (pulse width modulation) boards have been added for compatibility with existing aircraft servos, control of external circuitry, and future waypoint tracking capability. The low-power operation of some PC/104 boards (<10W) allows for extended operation on the battery.

Power System

The power source of the prototype electrical system is a 3.6 pound, NiMH, 12V battery. This source will have an initial capability of running the electronics for about 20 minutes. For future, long duration missions, another 12V, NiMH batter will be added. Because RC aircraft chargers are widely available for these cell types and sizes, mission durations of 1-2 hours should be easily attainable using this modular system.

To get longer flight durations an alternator will have to be adapted to the micro turbine engine. This will allow for sufficient power for all of the onboard electronics for as long as the aircraft’s engine is running. This will also decrease the weight of the aircraft as adding 3.6 pound batteries will quickly increase the overall weight. An alternator will require slightly different power circuitry, though this will likely cost little more to the end consumer than the standard NiMH battery setup.

Because the Flight Computer and other electronics don’t operate at 12V, there are Texas Instruments switching supplies onboard with high energy conversion efficiencies (>80%) that convert to the required voltages (regulated 12V, 9V, and 5V). Because of their efficiency, switching regulators are widely used to convert 24V to 12V. This can then be used in the modular approach the electrical system for long-duration flights using multiple batteries.

RF Modems

The RF Modems are long-range communication modems with their own communication protocol. They operate at a frequency of 900MHz and are Frequency Hopping Spread Spectrum modems, allowing an operational output of 1 Watt. Because the modems are FHSS, no additional FCC license is needed. One drawback of using the ISM bands is that they are heavily dependent on LOS, or line of sight, between the transmitter and receiver. The production version of the aircraft will remedy this problem 1 of two ways: either the consumer can select the RF modem option and set up different broadcast antennas around the city (which would not be very difficult if they are allowed to mount the network on several rooftops in the area) or a new satellite communication system will be developed for the aircraft. The cost of a satellite system will most likely be much more expensive to operate though, and could be cost-prohibitive to smaller communities.

Figure 1 outlines the RF interface between the flight and ground computers. This protocol ensures that, in the event of a temporary signal disconnect, the ground computer will continue to send data and the flight computer will not be unnecessarily tying up the air-side modem by attempting to send air-to-ground (A2G) packets. This safety measure is in place because the ground to air (G2A) packets are much more essential to maintaining flight than the A2G packets, especially if the camera system is still transmitting. This is a likely scenario since the RF modems have a slightly shorter and range and hence will lose a connection sooner than the independent camera system.

Figure 1

Camera

The camera used for the cockpit view is a standard RC aircraft camera. It operates at a frequency of 1.5GHz and is a standard analog FM video transmitter operating at 1 Watt of RF power. This corresponds to an approximate range of 10-15 miles and therefore would not be much use to a satellite operational vehicle. For such a vehicle, the uplink to the satellite will contain the video feed. For production models using the regular RF camera feed, the site operating the UAV will need a general Amateur Radio License to comply with FCC licensing regulations.