MEMO: Use of DGPS

U.S. Department of Transportation
Federal Aviation Administration

INFORMATION: Use of Global Position Systems (GPS) Based Time Space Position Information (TSPI) System as a Truth Source for Certification Flight Testing

Date: Nov. 26, 1996
From: Manager, Aircraft Engineering Division, AIR-100
Reply to Attn. of: George Kaseote, 202-267-8541
To: Distribution

During the past two years, the Federal Aviation Administration (FAA) Technical Center Airborne Systems Technology Branch (ACD-330), conducted testing to determine if the Ashtec Z-12 TSPI system could be used as a truth source for flight testing. The TSPI system is a set of two GPS receivers that are used to calculate very accurate time history of position information. This is accomplished by placing one receiver at a surveyed location and the other on the test aircraft. Both positions are recorded, as well as, the GPS time reference. Post flight, the two positions are compared, and the GPS errors are removed to give a time history of the aircraft's position.

To determine the static accuracy of this system, ACD-330 conducted a test of the system that compared the TSPI calculated position to a first order National Geodetic Survey point. The system demonstrated a static accuracy of 2.2 centimeters in the worst case. To determine dynamic accuracy, ACD-330 conducted a total of 72 approaches using two different laser trackers for comparison. These tests revealed that the Ashtec Z-12 was more accurate than the laser tracker that was being used as a truth source. The complete results of these tests are contained in an ACD-330 report titled "Flight Test of Ashtec GPS Receiver for USE as Time Space Position Information System (TSPI) to Verify Specific Performance Standards." That report also contains more information on how the TSPI system works.

Concluding in September 1996, the FAA Technical Center Navigation Branch, ACT-360, performed comparison tests between the Ashtec Z-12 and the Trimble 4000SSI systems to determine if the Trimble system could also be used as a truth source for FAA flight testing. The conclusion is that the Trimble 4000SSI, used as a TSPI system, is equally suited for use as a truth source for all types of ground and flight testing. The report of the results of these tests is titled "Comparison of Ashtec Z-12 and Trimble 4000SSI used as Time Space Position Information (TSPI) Systems."

The results of testing allow endorsement of the Ashtec Z-12 and Trimble 4000SSI TSPI systems for use in the certification flight testing. This includes airspeed calibrations, runway and climb performance, approach testing, and any other tests that require a truth source to verify accuracy. This endorsement applies to the software supplied by Ashtec and Trimble required to post-process the position data. Applicants are not required to utilize either of these systems if other methods (such as low level overflights, comparison to other navigation signals, laser trackers, etc.) that have been previously allowed are available. However, these systems provide the best and least expensive truth source information for flight testing available at this time and applicants should be encouraged to take advantage of them.

At this time only the Ashtec Z-12 and Trimble 4000SSI systems have been tested by the FAA. Applicants may use other manufacturer's TSPI systems if they can be demonstrated to be equivalent. This can be done by comparison to a laser tracker system or to the Ashtec Z-12 or Trimble 4000SSI systems.

Please contact the Avionic Systems Branch, AIR-130, for assistance if an applicant proposes to use a different TSPI system than either of the above systems. AIR-130 will maintain a record of TSPI systems that have been approved for use in certification flight testing. AIR-130 will also supply a copy of each of the above reports upon request.

[signed] J. Williams

for John K. McGrath

Distribution

Los Angeles Aircraft Certification Office, ANM-100L

Anchorage Aircraft Certification Office, ACE-115N

Brussels Aircraft Certification Office, AEU-100

Atlanta Aircraft Certification Office, ACE-115A

Chicago Aircraft Certification Office, ACE-115C

Wichita Aircraft Certification Office, ACE-115W

Denver Aircraft Certification Office, ANM-100D

New York Aircraft Certification Office, ANE-170

Seattle Aircraft Certification Office, ANM-100S

Boston Aircraft Certification Office, ANE-150

Engine and Propeller Standards Staff, ANE-110

Transport Airplane Standards Staff, ANM-110

Small Airplane Standards Office, ACE-110

Airplane Certification Office, ASW-150

Rotorcraft Certification Office, ASW-170

Special Certification Office, ASW-190

Engine Certification Office, ANE-140

Rotorcraft Standards Staff, ASW-110