FLIGHT TEST WEEK 'OPEN CONFERENCE' - hosted by Society of Flight Test Engineers (SFTE)

Running in Auditorium 3 on the Exhibits floor) on 18, 19, 20 Nov 2008


Using a NOAA Database, GPS, and/or Pressure Altitude Measurements to Determine Height above Terrain
Wednesday 19th November 2008
14.00 - 14.30
Auditorium 3



  For operational loads monitoring efforts to be successful, gathered data (normal acceleration, strains, etc.) must be correlated to flight condition. One important flight parameter is height above terrain (HAT). HAT data is routinely collected via the radar altimeter output. The problem arises when either (1) the subject aircraft has no radar altimeter or (2) the data collection suite cannot access data from the on-board radar altimeter.
  The objective of this effort was to collect HAT data in the absence of radar altimeter information for an operational usage data collection effort on a U.S. Coast Guard HC-130H aircraft. The approach used a NOAA 30-arc-second gridded database of worldwide terrain heights in conjunction with on-board Global Positioning System (GPS) and pressure altitude measurements. During post-processing of data, the NOAA database was interrogated with the latitude/longitude information from the GPS to determine the elevation of the terrain beneath the aircraft. Knowing the GPS altitude at that point allowed the calculation of an acceptable estimate of HAT. These calculations were supplemented with similarly derived HAT calculations from aircraft pressure altitude, corrected for the last known field elevation where the weight-on-wheels sensor determined the aircraft was on the ground. GPS data were collected as an RS-232 stream by a parser module in the ACRA Control, Inc. model KAM-500 data acquisition unit. Data included position (latitude and longitude), altitude, ground speed, true course, and magnetic course. Pressure was sampled using a Honeywell Precision Pressure Transducer connected to a static pressure line on the left fuselage wall. Post-processing software was written by the author, and NOAA subroutines were integrated into the post-processing software for the interrogation of the terrain database.
The approach usually allowed an acceptably accurate calculation for HAT for the purpose of segregating the flight data by altitude bands. The accuracy of GPS in the vertical coordinate (as opposed to lat/long accuracy) was occasionally unacceptably poor, at which times the supplemental information from pressure altitude data became important. Moreover, over very rugged terrain, the interpolation algorithm used to interrogate the database “smooths” terrain heights to give incorrect HAT values. A flight path of the subject aircraft departing Colorado Springs airport and the associated calculated terrain heights and HAT data are shown in the paper.