ARCADE II (Absolute Radiometer for Cosmology, Astrophysics, and Diffuse Emission) is a high altitude balloon-borne experiment which will measure the radiometric temperature of the sky at six frequencies, ranging from 3.3 GHz to 90 GHz. The radiometric temperature at a given frequency is a measure of the intensity of electromagnetic radiation at that frequency, being the temperature that the radiating body would be if it were a perfect blackbody emitter and emitting with the observed intensity at the frequency in question. After the contributions to the measured radiometric temperatures from the galactic foregrounds are subtracted out, the remainder will be the radiometric temperatures of the Cosmic Microwave Background Radiation (CMB).

The major science goal of the ARCADE II experiment is to achieve a measurement of the extent to which the CMB deviates from a blackbody spectrum at long wavelengths. The question of the extent to which the CMB deviates from a blackbody spectrum is as old as the study of the CMB itself. Measurements at frequencies higher than 60 GHz with the FIRAS (Far Infared Absolute Spectrometer) experiment have shown it to be very close to a blackbody spectrum in that regime (1). At longer wavelengths, published results are weaker, and the errors are larger than the theoretical deviations from blackbody predicted by common models (2).

The ARCADE I experiment, predecessor to ARCADE II, flew for data taking in June, 2003, and observed the sky at 10 GHz and 30 GHz. The results from that experiment are pending. With seven radiometers as opposed to two, and several at longer wavelength, ARCADE II is a much larger instrument. The dewar for ARCADE II is a cylinder 6 feet in diameter and seven feet tall.

The ARCADE project is a collaboration of NASA’s Goddard Space Flight Center and Jet Propulsion Laboratory, and the experimental cosmology group at the University of California, Santa Barbara physics department. Members of the science team are:

NASA Goddard Space Flight Center
Al Kogut (Principal Investigator)
Dale Fixsen
Michele Limon
Paul Mirel
Ed Wallack

NASA Jet Propulsion Laboratory
Steve Levin
Michael Seffert

Phil Lubin

The Instrument

The ARCADE II instrument, as well as the ARCADE I instrument before it, compares the radiometric temperature of the sky to that of a calibrator target, the temperature of which is adjustable and known, and which is designed to emit very close to a perfect blackbody spectrum. When the measured radiation intensity of the sky and target are equal, then the radiometric temperature of the sky is then known, as it will be the temperature of the target. The purpose of flying the experiment on a high altitude balloon is to get above most of the atmosphere, which is a not insignificant emitter in the microwave region of the radio spectrum.

There are, of course, challenges inherent in measuring radiation from weak emitters in the range of 3k. The noise in the radiometer signals must be kept to a minimum, which in ARCADE is achieved by using a cold HEMT amplifier and switching radiometer with an internal emitter. All cold components are kept cold by means of liquid helium boil off, as the entire cold part of the experiment will be within a giant dewar filled with liquid helium. All cryogenic temperatures will be measured by means of calibrated resistance thermometers, and the output of the radiometers, temperature sensors, and all other on board data is stored on the on board computer as well as transmitted to the ground.

The radiation is coupled to the radiometers by the means of scalar conical feed horns. The horns contain a regular pattern of grooves and teeth on the interior surface which serve to cancel the tangential component of the electric field of the incoming radiation at the surface, giving a beam pattern that is close to gaussian in the way in which the measured intensity falls off as a function of angle from the center. After the throat of the horns is a circular to rectangular waveguide transition, followed by waveguide bends which will direct the radiation toward the radiometers.

A switch will alternate at 100 Hz between allowing through the incoming radiation and radiation from an internal waveguide load. The radiation will then be launched from waveguide into a coaxial transmission line and amplified by a low-noise cold HEMT amplifier, before passing out of the dewar. A band pass filter will filter out any remnants of unwanted frequencies, and detector diodes will convert the radiation to current. A lockin amplifier in phase with the switch then multiplies the signal by 1 and –1, and the signal is integrated, with the maximum value of the integration signal held via a switch and hold and output for an entire period. Thus, the voltage of the signal output will be linearly related to the radiometric temperature being observed, and the constant of proportionality can be determined by calibrating the radiometer with a blackbody emitter of known temperature.


The first flight of the instrument, scheduled for June 2005, will be from NASA’s National Scientific Ballooning Facility in Palestine, Texas. In the year following the first launch, data analysis will take place, and also some redesigning and rebuilding selected instrument components to reduce sources of uncertainty. Tentatively, a second flight is scheduled for Spring 2006, which will be followed by more data analysis and possibly more rebuilding. A third flight may take place in 2007, possibly from Australia.

Node your advancement talk, or at least part thereof.

1. Fixsen, D.J. & Mather, J.C., 2002, ApJ, 581
2. Fixsen, D. J. “The Temperature of the CMB at 10 GHz” (publication pending)

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