THE SOFIA WATER VAPOR MONITOR

Thomas L. Roellig, Robert Cooper, Brian Shiroyama, Regina Flores, Lunming Yuen, and Allan Meyer

The Stratospheric Observatory for Infrared Astronomy (SOFIA), a 3-meter class telescope mounted in a Boeing 747 aircraft, is being developed for NASA by a consortium consisting of the University Space Research Association, Raytheon E-Systems, and United Airlines. This new facility will be a replacement for the retired Kuiper Airborne Observatory that used to fly out of Moffett Field. As part of this development, NASA Ames Research Center is providing an instrument that will measure the integrated amount of water vapor seen along the telescope line-of-sight. Since the presence of water vapor strongly affects the astronomical infrared signals detected, such a water vapor monitor (WVM) is critical for proper calibration of the observed emission. The design and engineering model development of the water vapor monitor is now complete and the hardware to be used in the flight unit is being fabricated and tested.

The SOFIA water vapor monitor measures the water vapor content of the atmosphere integrated along the line-of-sight at a 40° elevation angle by making radiometric measurements of the center and wings of the 183.3 GHz rotational line of water. These measurements are then converted to the integrated water vapor along the telescope line-of-sight. The monitor hardware consists of three physically distinct sub-systems:

1) The Radiometer Head Assembly, which contains an antenna that views the sky, a calibrated reference target, a radio-frequency (RF) switch, a mixer, a local oscillator, and an intermediate-frequency (IF) amplifier. All of these components are mounted together and are attached to the inner surface of the aircraft fuselage, so that the antenna can observe the sky through a microwave-transparent window. The radiometer and antenna were ordered from a commercial vendor and have been modified at Ames to include an internal reference calibrator. Laboratory tests of this sub-assembly have indicated a signal-to-noise performance over a factor of two better than required.

2) The IF Converter Box Assembly, which consist of IF filters, IF power splitters, RF amplifiers, RF power meters, analog amplifiers, A/D converters, and an RS-232 serial interface driver. These electronics are mounted in a cabinet just under the radiometer head and are connected to both the radiometer head and a dedicated WVM computer (CPU). All of the flight electronics boards have been fabricated and have been shown through testing to meet their requirements. A small micro-processor that handles the lowest level data collection and timing has been programmed in assembly language to collect and co-add the radiometer data and communicate with the software residing in the WVM CPU.

3) A dedicated WVM CPU that converts the radiometer measurements to measured microns of precipitable water and communicates with the rest of the SOFIA Mission and Communications Control System (MCCS). A non-flight version of this computer hardware has been procured for laboratory testing and the flight software is under development, with approximately 60% of the software coded and unit-tested. Proper command and data communications between the Water Vapor Monitor and the SOFIA MCCS have been demonstrated using an MCCS simulator located on-site at Ames that has been developed by the SOFIA Project.

Figure 1. The SOFIA Water Vapor Monitor 183 GHz Radiometer Assembly