Langley ULDB Radiometery Home Page

Ultra-Long Duration Balloon (ULDB) Experiment at Langley Research Center

Project Overview

The Balloon

Instrumentation

Past Flights

Future Flights

Web Article

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Contact:
Dr. Tom Charlock

Contact:
Dr. Wenying Su

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Responsible NASA Official: Dr. Tom Charlock
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Last Updated: 2007/09/25
Project Overview: Our project goal is to develop a system to measure SW and LW radiant fluxes at the top of the atmosphere with calibrated hemispherical radiometers. Such measurements can then be used to validate satellite-based radiant fluxes which are derived from radiance measurements using directional (longwave LW) and bi-directional (shortwave SW) reflectance functions. The radiance-to-flux conversion process is the principal source of uncertainty in instantaneous (not time averaged) fluxes estimated from radiance measurements. For the Clouds & the Earth's Radiant Energy System (CERES) radiometers on board Terra and Aqua, it is estimated that this process introduces a 4% (1-sigma) uncertainty in SW flux, whereas calibration uncertainty of the instrumentation is less than 1%. The problem of inverting radiances from narrow field of view (NFOV pixels), Earth Radiation Budget (ERB) scanner measurements to broadband fluxes (irradiances) also confronted the earlier ERBE, ScaRaB, and Nimbus-7 teams.The spatial resolution of CERES measurements ranges from 15 to 60 km depending on the zenith angle of observation. Typical flux products are generated on 1-deg grids (about 100 km). Solar and terrestrial flux measurements from a stratospheric balloon at 35 km altitude can be made with a spatial resolution on the order of 70 km (half power circle diameter). The uncertainty of measurements made from 35 km is limited only by instrument accuracy. The goal is to characterize the hemisphericalradiometers at the 1% level. CERES presently has three operational scanners on the Terra(2) and Aqua(1) spacecraft. In standard cross-track scanning mode, CERES would typically view a scene observed by the ULDB radiometers about 28 times in daylight during a four week flight. Reprogramming of a CERES instrument to a special Programable Azimuth Plane Scan (PAPS), as during the July 2001 CLAMS field campaign, increases viewing of the selected target by a factor of 10.

Project Status: We have had a few (but unfortunately brief) flight tests. More importantly, our characterization of the radiometers in a test chamber simulating the environment at 35km (Contact W.Su for manuscript in press for J. Atmos. Oceanic Technol.) will enable accurate measurements on future flights. The project is on hold, simply waiting for an opportunity to fly.

The Balloon: The Ultra Long Duration Balloon (ULDB), developed by NASA Goddard's Wallops Flight Facility (essentially a floating sub-orbital UAV), is a balloon system capable of providing scientific measurements above 99% of the atmosphere. The balloons are designed for 100-day missions with floating altitude close to 35km. They are transported by stratospheric winds around the globe with an average speed of 30m/s and hence can circumvent the Earth in about 2 weeks. The balloons are 120 meters in diameter and can carry payloads up to 1500kg. A ULDB is a super-pressure balloon made of a composite fabric (polyester + polyethylene film and fabric) that is filled with Helium and hermetically sealed. Meridional tendons provide additional rigidity to the envelop. The pressure inside the envelop is maintained above the ambient pressure at all times to keep the balloon afloat at a constant altitude. During daytime the internal pressure increases due to solar heating but the volume remains constant due to the rigidity of the envelop. At night the pressure drops due to infrared radiative cooling to space, but as long as the internal pressure remains above the ambient pressure, the balloon stays at the same altitude. Launchs are critical and difficult.

Surface wind issues.

Additional information about the ULDB project can be found at NASA Wallops Isl. Flight Facility:

Instrumentation:

The thermal-vacuum chamber at Global Monitoring Division (GMD, formerly Climate Monitoring and Diagnostics Laboratory, CMDL) of NOAA was used to simulate atmospheric pressure down to 5 hPa and temperature down to -60 °C, close to conditions expected at an ULDB flight altitude of 35 km.

Figure 1 shows a picture and drawing of the pyranometer chamber test setup. The pyranometer was exposed to a 100 watt Halogen light source, which is constantly monitored by two LI-COR sensors. The reference light source and its monitors are kept outside the chamber and maintained at room temperature and pressure, with light entering the chamber through an optical window. We recorded relative light intensity measured by the external LI-COR sensors, chamber pressure and temperature, pyranometer body and cold junction temperatures, and pyranometer voltage output every minute. These data were used to derive the temperature dependency of sensitivity, as shown in Figure 2. A pressure adjustment factor was also derived, as shown in Figure 3.


Figure 1. Picture and drawing of pyranometer chamber. Picture shows frontal view and the drawing shows the side view.	Figure 2. Sensitivity of pyranometer CM22 (serial number 40100) as a function of body temperature: circles are sensitivity derived from chamber test data, solid line is a third-order polynomial fit to sensitivity as a function of body temperature, which also corresponds to the relative temperature dependency on the right axis. Triangles are relative temperature dependency provided by the manufacturer.	Figure 3. Box plot of pressure adjustment factor of CM22 pyranometer for different chamber pressures. Circles are median values of the adjustment factor. Top and bottom of the upper vertical lines correspond to tmaximum and third quartile, respectively. Top and bottom of lower vertical lines correspond to first quartile and minimum, respectively. Solid line is a linear fit of the median values of adjustment factor as a function of chamber pressure.

Figure 4 shows a picture and drawing of the pyrgeometer chamber test setup. We monitored output from the pyrgeometer which is viewing a blackbody calibration target while varying chamber pressure and temperature in stages. We used the GMD pyrgeometer calibration blackbody, which is a 20 Kg aluminum cylinder with a blackened cone-shaped cavity. This blackbody has been compared with national and international infrared calibration capabilities, and can be used in either a passive or an active mode. The latter was used here to keep target cavity temperature steady. For these experiments both the pyrgeometer and calibration blackbody were put inside the chamber. Temperature of the cavity is controlled by operating it with its exterior in the cold chamber and electrically heating it from within its casing to desired blackbody temperature. In this manner we can simulate a very cold instrument viewing a warmer target, not the norm for a surface deployed up-looking pyrgeometer. We recorded dome and body temperatures of the pyrgeometer, blackbody temperatures at five locations (we used the average in our analyses), chamber pressure and temperature, and the output of the pyrgeometer every minute for hours during each chamber run. The derived temperature dependency of the pyrgeometer sensitivity and its pressure adjustment factor are shown in Figures 5 and 6.


Figure 4. Picture and drawing of pyrgeometer chamber. The picture shows the blackbody and pyrgeometer and the drawing shows entire setup.	Figure 5. Pyrgeometer CG4 (serial number 40740) sensitivity as a function of absolute body temperature: circles are sensitivity derived from chamber test data, solid line is a linear-fit of the sensitivity as a function of absolute body temperature, which also corresponds to the relative temperature dependency on the right axis. Triangles are relative temperature dependency provided by the manufacturer.	Figure 6. Same as Figure 3, but for pyrgeometer CG4.

Past Flights:

1. ULDB test flight: Two ULDB were launched in February and March 2001 from Alice Springs, Australia. Flights were able to meet their main objective of launch and ascent to float altitude though both balloons developed leaks and were not able to remain at float altitude through more than one diurnal cycle. Each mission was terminated before the balloon reached the western coast of Australia. The balloon material must be modified to improve its elastic properties to prevent ruptures during the inflation process.

Balloon Inflation

2. ULDB Ballooncraft support systems test flight: A 28-hour test flight was performed on 26 May 2001. The ULDB ballooncraft attached to a zero-pressure balloon was launched from Ft Sumner, NM. A small platform carrying an Eppley pyrgeometer and the modified Eppley pyranometer was attached to the bottom of the ballooncraft. The instruments had an unobstructed hemispherical view of the upwelling radiative field. The ballooncraft was recovered in excellent condition but the radiometers were destroyed at impact.

3. ULDB Nightglow project: A radiometer payload was piggybacked on this ULDB flight from Alice Springs, Australia. The ULDB was launched on March 16, 2003. The flight was terminated after 11.5 hours due to pressure loss in the balloon. The domes of all radiometers were broken and no data were collected.

4. ULDB test flight: A tiltmeter and thermistor package was flown on Feb. 4, 2005 from Ft. Sumner. The ULDB was brought down right after it reached flight altitude. No useful data were collected.

5. ULDB Kiruna test flight: A tiltmeter and thermistor package was flown on June 10, 2006 from Kiruna, Sweden. The ULDB did not deploy as expected. No useful data were collected.

Ballooncraft

Future Flights:

There are no flights planned at this time.

Global Aerospace Corporation has been developing a trajectory control system based on a sail that provides enough pull on the balloon to control its latitude. More information can be found at http://www.gaerospace.com/publicPages/projectPages/StratoSail/index.html