High and Dry: Trading Water Vapor, Fuel and Observing Time for Airborne Infrared Astronomy

Scheduling astronomy observations for the Stratospheric Observatory for Infrared Astronomy (SOFIA) requires assessing tradeoffs between the percentage of scheduled observations, the Line Of Sight Water Vapor (LOS-WV) achieved on those observations, and fuel consumption. This trade space is complex, depending on time of year and specific mixes of observations, and cannot be effectively analyzed by hand. We demonstrate the complexity of these tradeoffs and show that an Automated Flight Planner (AFP) is a crucial part of trade space analysis during flight planning. The Stratospheric Observatory for Infrared Astronomy (SOFIA) is NASA’s next generation airborne astronomical observatory. The facility consists of a 747-SP modified to accommodate a 2.5 meter telescope. SOFIA is expected to fly an average of 140 science flights per year over its 20 year lifetime, and will commence operations in early 2005. The SOFIA telescope is mounted aft of the wings on the port side of the aircraft and is articulated through a range of 20◦ to 60◦ of elevation. A significant problem in future SOFIA operations is that of scheduling Facility Instrument (FI) flights in support of the SOFIA General Investigator (GI) program. GIs are expected to propose small numbers of observations, and many observations must be grouped together to make up single flights. Approximately 70 GI flight per year are expected, with 5-15 observations per flight. Fig. 1. Variation in csc(h) in the interval h = [20◦, 60◦]. An important goal of flight planning for SOFIA is to ensure that line-of-sight water vapor (LOS WV) is minimized during observing [1]. This can be accomplished in one of three ways. It has long been known that water vapor decreases with altitude, thus observing at higher altitude reduces LOS WV. If we analyze csc(h) where h is the telescope elevation in the range 20◦ ≤ h ≤ 60◦ (shown in Figure 1), we see that it takes on values in the range [2, 5.7]; thus, choosing the position and time for observing at high telescope elevations reduces LOS WV. Finally, Becklin and Horn [1] showed that, in general, atmospheric water vapor is lower near the poles (as shown in Figure 2); thus, LOS WV can be reduced by repositioning the aircraft. Fig. 2. 5 year average of Water Vapor Overburden at 216.626 hPa (approximately FL 370) over the entire Earth for 31 December. Data provided courtesy of the Wind and temperature data from European Center for Medium Range Weather Forecasting, file provided courtesy of Michael A. K. Gross. For comparison, at 147.474 hPa (approximately FL 450) WV overburden is uniformly below 10μ. As an airborne observatory, SOFIA allows great flexibility in optimizing any single observation. Judicious choice of takeoff time, altitude selection and observatory position can ensure that LOS WV is minimized for one observation. Unfortunately, there are complex tradeoffs between the takeoff fuel weight, flight duration, percentage of requested observations that can be performed, and average LOS WV for a flight. During flight, aircraft altitude is limited by aircraft weight. The more fuel carried, the longer the flight, but also the more limited the aircraft’s initial operating altitude. Fuel is costly (JPA costing $3.00 a gallon in April of 2005) so using it wisely is important. Furthermore, repositioning the aircraft to seek drier air or maximize telescope elevation will generally require preparatory ”dead-legs” (during which no observations are performed); this will reduce time for observing. The constraints governing legal flights are complex enough US Government work not protected by US Copyright Report Documentation Page Form Approved