Giving RTK a Whirl — GPS Aces Flight Tests

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

In the increasingly competitive aerospace business environment, more must be accomplished with less; fewer individuals must produce better results. Therefore, when we at The Boeing Company, formerly McDonnell Douglas Helicopter Systems, conduct tests to meet Federal Aviation Administration (FAA) helicopter regulations, we must operate efficiently and with a high degree of accuracy. Our test teams, working with reliable instrumentation and data systems, produce optimum results, but just a few minutes lost because of poor crew coordination, equipment malfunction, or air traffic interruptions can result in a costly additional day's work.

To minimize the chance of such an occurrence, we have been refining a real-time kinematic (RTK) GPS-based "Portable Test Range" since 1994 that has tremendously increased the efficiency and safety of experimental operations, in part because critical data are displayed to both the flight crew and the ground-based test director. The system -- a combination of hardware and software components, some in the helicopter and some on the ground -- also permits us to execute tests at a wider variety of facilities.

Flight-test programs requiring accurate three-dimensional (3D) position data referenced to ground objects, such as some of those to certify aircraft and their associated operational procedures, have historically employed such equipment as microwave trisponders, grid cameras, or encoding optical theodolites, These now antiquated systems require large open areas for proper system setup and operation, severely limiting the selection of test-range locations. Differential GPS (DGPS) operations are much less restrictive in that regard. Now, in addition to the improved helicopter flight-test procedures and results, our RTK GPS-enabled Portable Test Range allows us to prepare new test locations for operational use within a day of arrival on site, including locations selected for wind, temperature, terrain obstruction, or density altitude environmental considerations.

Our own location in particular, in Mesa, Arizona, influenced our decision to pursue RTK GPS technology. Precision helicopter flight tests involving control margins, performance, or airspeed system calibration require that winds be very light or calm and vertical air movement virtually nonexistent. In Arizona, conditions that will satisfy these requirements are typically found only during a small time frame each day, before solar heating causes convective turbulence or localized winds. It is therefore imperative that we efficiently use this critical atmospheric conditions window.

Because both of us are designated as FAA engineering representatives at The Boeing Company, with Mark Hardesty's focus on flight-test engineering and Greg Ashe's skills as a test pilot, we had a vested interest in improving test procedures and the equipment supporting those operations. We have both been involved with the Portable Test Range's development since its inception and will therefore present here a snapshot of the system's evolution. We will also discuss some of the tests already conducted with the equipment, comparing old and new procedures to illustrate the revolution RTK GPS has brought about in helicopter flight testing.

GETTING READY FOR RTK

A wide variety of helicopter flight tests -- for example, Category A, fly-over noise, height- velocity, and low-speed controllability -- benefit significantly from precise 3D position and velocity data. Handling qualities maneuvers described by Military Aeronautical Design Standard 33D can also be quickly and accurately scored using the same data. Although DGPS techniques had already existed for several years, we decided to venture into the field in 1994 because a GPS manufacturer had introduced a receiver capable of meeting our design goals. Employing an RTK OPS system, though, involves meeting an additional set of requirements.

First and foremost, because RTK GPS requires both a reference and roving receiver to achieve high accuracies, one must maintain a high-integrity data link between the two units during operation. This can easily become the biggest challenge in taking maximum advantage of the technology. In urban or heavy industrial environments, radio-frequency (RF) clutter can drastically affect the data link's reliability. Various links are available, but users should carefully consider their environment and operational constraints before investing in a specific system. (For additional details about selecting GPS and data link units, see the "Equipment to Enable RTK" sidebar.)

To set up an RTK system, one must place the GPS reference station antenna with an unobstructed view of the sky, as much as buildings or natural obstacles permit, and ensure a good line-of-sight between the air vehicle and the data link's RF antenna. When a locally established coordinate system is adequate for the test program, the reference station GPS antenna should be situated so that the installation can be precisely repeated. Once in place, the reference receiver should be allowed to acquire its position. Typically, latitude and longitude will be more accurate if one fixes the GPS antenna' s vertical position in the GPS receiver. This information can usually be adequately derived from local topographical maps or airport facilities directories.