Giving RTK a Whirl — GPS Aces Flight Tests

GPS World, April, 2000 by Mark Hardesty, Greg Ashe

Assembling the Parts. System integration proved relatively trouble free, with most difficulties involving simple cabling and power-supply problems. We created software to control data archiving and display using a graphical user interface programming language offering a multitude of analog and digital control and display options for the computer screen. As the software development progressed, we added a digital-to-analog output card to the aircraft computer to drive an analog cockpit indicator and guide the flight crew over a microphone array as required for FAA noise certification testing. Eventually, we also added the down-link capability, allowing real-time plotting of critical aircraft position and velocity data at the ground-based test director's station.

We currently operate the original 12-channel, L1-only code- and carrier-tracking receiver system as well as two other RTK packages, depending on the needs of the test in question. One of those systems offers 24-channel, parallel tracking of the L1, C/A-code and L2 P-code, with L1/L2 full-wave carrier measurements. The other is a 16-channel unit with two independent antennas tracking the code and carrier on eight L1 channels each. Each of these systems operate at a position update rate of 4 Hz, which is processed, archived, and decimated on board the aircraft and then down-linked to the ground station at a 2-Hz rate. This update rate has proven adequate and highly effective for flight crew guidance as well as for all certification and developmental testing.

Positioning the Antenna. We have used several GPS antenna locations on the helicopter with great success, the most desirable being centered on the rotor head. This position requires installing a special standpipe through the center of the main rotor drive shaft, something usually available only to helicopter manufacturers. When the instrumented rotor head hardware has not allowed for this installation, we have used a tail boom location. Both positions offer distinct advantages and disadvantages. The main rotor head most nearly approximates the aircraft center of gravity and is generally not influenced by yawing of the tail in gusty conditions or pitching motions during acceleration and deceleration maneuvers. It also allows for a completely unobstructed view of the sky, thus optimizing the reception of UPS satellite signals while minimizing multipath and signal blockage difficulties.

The tail boom, on the other hand, is subject to obstructions such as the upper forward fuselage and rotor head, as well as the tail empennage. Reception of signals passing through the rotor disk causes no particular problems for the receivers we use; however, some precision RTK surveying systems have demonstrated an inability to function under helicopter rotors. This appears to be a function of blade number, chord length, and rotor rotations per minute. Disadvantages of the tail boom location include artificially induced accelerations caused by the pitching and yawing motions of the aircraft that are not indicative of the aircraft's center of gravity. One particular advantage, however, is that when examining maneuvers such as low-speed controllability, this information can be related to pilot workload and ability to control the aircraft.