SSEC Instruments Help Test Aircraft Weather Instruments
The Space Science and Engineering Center (SSEC) at the University of Wisconsin–Madison is participating in a NASA program to give pilots better weather information.
SSEC is helping to validate instruments that Airdat, L.L.C. and NASA’s Langley Research Center have designed and built to take weather measurements from commuter aircraft. The measurements, taken below 25,000 feet, will provide information on humidity, pressure, temperature, wind direction, wind speed, turbulence and icing conditions. That information will be shared with weather forecasters, pilots and airport officials to improve severe weather, icing, and visibility forecasts.
Since January, researchers in NASA and NOAA have been validating the Tropospheric Airborne Meteorological Data Reporting (TAMDAR) instrument flying on Mesaba Airlines Saab aircraft in the TAMDAR Great Lakes Fleet Experiment. In February, SSEC deployed a mobile suite of instruments at the Memphis Tennessee Air National Guard facility to validate TAMDAR measurements during Mesaba takeoff and landings at the Memphis, Tennessee Airport. Researchers from SSEC’s Cooperative Institute for Meteorological Satellite Studies (CIMSS) are conducting the TAMDAR AERIbago Validation Experiment (TAVE) from February 23 through March 11 to better understand the quality and reliability of the TAMDAR sensors, according to Wayne Feltz, the CIMSS scientist in charge of CIMSS’s TAVE operations.
The SSEC Mobile Weather Laboratory, also known as the AERIbago, is a converted Winnebago that carries a suite of instruments for characterizing atmospheric water vapor profiles. The AERIbago’s primary instrument is the Atmospheric Emitted Radiance Interferometer (AERI), which measures long-wave energy that arrives at the earth’s surface. According to CIMSS researcher Erik Olson, “The high spectral resolution nature of this data allows us to calculate the temperature and moisture profiles of the lower part of the atmosphere.” Because of its known accuracy, the AERI is routinely used to validate and calibrate NASA and NOAA satellites. Other available instrumentation includes surface meteorological sensors, radiosonde data receiving system, cloud base height sensor, and GPS receivers.
CIMSS researchers are launching five radiosondes per day from the ’Bago at the Air National Guard facilities. Radiosonde launches entail attaching small sensor packages to a large helium-filled weather balloon to directly measure the temperature, relative humidity, wind speed, and wind direction as the balloon rises through the atmosphere to a height of about 20 km. An accurate GPS instrument can also measure the variations in the GPS signal delay, which can be used to calculate the total amount of water vapor in a vertical column of the atmosphere. “By combining the radiosonde, AERI, and GPS measurements,” Olson said, “we can produce a highly accurate water vapor profile that can be compared to an independent water vapor measurement. In the case of the field experiment in Memphis we are using our water vapor profiles to determine the accuracy of the TAMDAR humidity measurements from the Mesaba Saab 340 aircraft.”
The TAMDAR instrument was developed by Georgia Tech Research Institute and AirDat, L.L.C. in Morrisville, NC for NASA’s Aviation Safety and Security Program. More information about TAVE and NASA’s TAMDAR program can be found on the Web: http://cimss.ssec.wisc.edu/tamdar/
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How does GPS give us water vapor measurements?
Contact:
Erik Olson (CIMSS)
608-262-9538
eriko@ssec.wisc.edu
GPS instruments are typically used to give us location in relation to a constellation of satellites by measuring the delay of time clock signals. An increase in atmospheric water vapor causes an increase in satellite time signal delay because it slows down radio communication signals. This normally manifests itself as small errors in the location derived from the GPS data. If we already have an accurate location measurement, a two week average of GPS location, for instance, then we can use the variations in the delay to calculate the total amount of water vapor in the atmosphere. It is rather complicated in practice since it involves modeling the other atmospheric factors that slow down the radio signal and remapping the results since the GPS satellites are not often directly overhead.
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