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<br />Calibration <br />Manufacturer calibrated. However, occasional checks are made by the flight crew and a <br />specific test was conducted of the system over the Boulder Atmospheric Observatory (BAO) <br />on 13 April 1990. The system reliably determines ground positions and accurately derived <br />the position of the tower. <br /> <br />Operational procedures <br />The flight crew controlled the Loran-C as well as the radios and other navigation aids. The <br />output data from the Loran-C and the heading data were recorded by the onboard data <br />acquisition system and used to compute in-flight wind speed and direction. <br /> <br />4.9 Global Positioning System (GPS) <br /> <br />A Trimble Navigation Model 2000 airborne GPS receiver was added (late 1992) to the suite <br />of instruments onboard the aircraft for use by the flight crew and as a scientific research tool. The <br />Global Positioning System, developed by the Department of Defense, is a satellite-based triangulation <br />method of determining a position (ground or airborne). This triangulation technique measures <br />distance by comparing the travel time of pseudo-random coded radio transmissions from precisely <br />positioned reference satellites. The receiver processor and the satellites contain very accurate <br />synchronized clocks and generate exactly the same codes at exactly the same time. The timing <br />difference between the received satellite code and the receiver internal code is processed as a <br />distance. Several such satellite range determinations processed with precise satellite position <br />information yield a specific receiver position. The sophisticated receiver in this application provides <br />three-dimensional position (latitude, longitude, and altitude) as well as velocity information, high- <br />resolution display and much more for the user. <br /> <br />Calibration <br />Manufacturer calibrated. An internal comprehensive diagnostics routine assures proper <br />performance at all times. <br /> <br />Operational Procedures <br />The flight crew controlled the GPS as one of the onboard communications and navigation <br />aids. The output data from the GPS were recorded by the onboard research data acquisition <br />system installed in the cabin. These data were used post-flight in computations of wind and <br />position information for comparison to the Loran-C generated information. <br /> <br />4.10 Sulfur Hexafluoride Analyzer <br /> <br />The ScienTech Model TGA-4000 trace gas analyzer was designed for continuous, real-time <br />measurement of sulfur hexafluoride (SF 6) ( Benner and Lamb, 1985). Equipped with a custom <br />pulsed mode electron capture detector, it is capable of detecting SF 6 in air at mixing ratios as low as <br />5 parts-per-trillion by volume (pptv). This type of detector is particularly sensitive for compounds <br />containing halogens (such as fluorine) as well as compounds containing sulfur, nitrogen, and oxygen. <br />Hydrocarbons and oxygen in the sample are continuously removed by catalyzed reaction with a <br /> <br />14 <br />