Institute of Applied Physics
University of Bern, Switzerland
The mm-wave technique for atmospheric composition sounding has already demonstrated its capabilities in ground-based, aircraft, balloon and spaceborne platforms which started about two decades ago. Technical advances are now providing access to the sub-mm wavelength range (300 – 3000 GHz), which has been the least explored spectral range. Difficulties in the sensor technology which lies between optical (free beam propagation, mirrors, lenses etc.) and microwave techniques (guided structures) as well as the attenuating nature of the earth atmosphere are the main reasons why this spectral range was only used by smaller research communities in radio astronomy and atmospheric sciences.
Due to the high tropospheric opacity of water vapor at frequencies above 300 GHz, observations are only possible for high altitude sites or from aircraft. Particularly the discovery of the ozone hole and the need for stratospheric trace gas measurements gave new impulses to this field since a lot of spectral lines from relevant atmospheric constituents are located in the sub-mm wave range, allowing the retrieval of altitude distributions of the corresponding molecules.
This report will mainly focus on upward looking heterodyning radiometers operated from aircraft, which have different technological requirements than spaceborne instruments (limb sounding geometry). In a limited time scale (e.g. during measurement campaigns) state of the art technology can be used in an aircraft, without severe limitations on power, weight, lifetime and reliability. Additionally, aircraft platforms serve as test facilities for later spaceborne instruments as the EOS MLS (Mm wave Limb Sounder), AMAS (Advanced Mm wave Atmospheric Sounder) who’s successors are currently on the UARS (Upper Atmospheric Research Satellite) and have flown on the ATLAS missions with the space shuttle, respectively. Sub-mm surveys of astronomical objects are also performed by NASA’s Kuiper Airborne Observatory.
The development of critical components in the heterodyning receivers as mixers and stable local oscillators is an active research area. The better the noise performance (and therefore the sensitivities at THz frequencies) is, the more applications will be possible. Promising improvements are possible in the SIS technology. Instruments covering the 650 GHz to 2.5 THz are currently under development and will show their capabilities in the near future.
II. State of the Art and Limitations
In the sub-mm range there are many rotational transitions available with low overlapping, where problems of band structures as in the optical regime are not apparent. Heterodyne spectroscopy gives simultaneously the best possible spectral resolution for thermal emission with better sensitivity than photometric techniques.
Up to about 700 GHz waveguide structures are in use. Current superconducting tunnel junction (SIS) technology which requires complex cooling systems are applied in the same frequency range. For airborne applications two stage cooling dewars with liquid nitrogen and liquid helium are used while for some ground-based applications closed cycle helium cooling systems are available. The most critical element concerning system noise and therefore sensitivity is the mixer. Particularly in airborne applications where integration time is limited, a good signal to noise ratio is of great importance. The double sideband system noise temperature increases with frequency and ranges between 50 K (300 GHz) and 300 K (700 GHz) for cooled SIS designs. Above still, Schottky diodes are used and double sideband system noise temperatures of 8000 K (1000 GHz) and 12000 K (2500 GHz) are reported. The SIS noise temperatures are 5 to 10 times the quantum limit and do not probably allow significant reduction due to losses in the mixer structures. However, there is a high potential in SIS applications at higher frequencies as well as in the Schottky technology. The THz technology is still emerging and further noise reduction can open a new field of spectroscopic measurements of various gases.
Another critical component is the local oscillator which is either an InP or GaAs based semiconductor Gunn oscillator with subsequent doublers and triplers or a laser for frequencies above 700 GHz. Phase lock systems are used for local oscillator stabilization. Coupling, guiding and focusing is mostly realized with optical techniques such as dielectric lenses, elliptical mirrors and grids for diplexers and filters. After the conversion to lower frequencies (heterodyne principle) further amplification can be achieved with cooled amplifiers using High Electron Mobility Transistors (HEMT).
Different possibilities do exist for the spectral analysis, which are all possible in aircraft but might be limited in space applications due to weight and power consumption. They range from discrete filterbanks, acousto optical spectrometers (AOS), Chirp spectrometer to autocorrelators. Some of the advantages of airborne radiometers with respect to spaceborne instruments are the quite stable environment, the possibilities of frequent calibration with liquid nitrogen loads and on-line tuning possibilities. Since SIS systems have to be cooled to 4 K, quite complex cooling systems are necessary for these applications. Schottky mixers need no or less cooling for its operation and are therefore more suitable for spaceborne radiometers due to their simpler and more reliable control, although they exhibit a lower noise performance.
Critical items are the limited observation time, the integration time of one spectrum and thus the latitude resolution, the bandwidth of the receiver and the tuning inside the possible frequency range for different spectral lines, the frequency stability of the local oscillator, the aircraft window transmission, the standing waves in the quasi-optics of the front-end due to reflections and the cooling system. Most of these are general limitations which apply also for spaceborne instruments.
The main interest in the sub-mm wave range lies in the spectral analysis of the rotational and fine structure transitions of trace gases in the earth atmosphere or in the interstellar matter and dust due to the more or less distinct emission lines. Additionally, sub-mm radiative transfer modeling is more straight forward than for IR emission or solar backscattered radiation in the UV and visible regions. Separation of the thermal emission lines means also that similar lines of various molecules in a typical receiver bandwidth require that the receiver must be tuned for the various molecules. Pressure broadening (linewidth proportional to pressure) allows, in the case of the earth atmosphere, the retrieval of altitude distributions of the corresponding molecules. For upward looking (airborne) radiometers the altitude resolution lies between 8 and 15 km in the altitude range from 15 to 70 km. The lower and upper limits are due to limited bandwidth, flat frequency response at the wings, and the dominance of Doppler over pressure broadening at low pressures.
Atmospheric species of interest at sub-mm frequencies are OH, HO2, H2O2, HOCl (2.5 THz), HF (1.2 THz), and chlorine and bromine compounds relevant for stratospheric chemistry (ozone depletion) such as (HCl, ClO and BrO) in the 650 GHz range. Molecules of mainly astrophysical interest are H2O and CO. The limb sounding geometry as often used in spaceborne platforms cannot be applied from aircraft since the flight level is usually to low. Typical elevation angles allowing sufficient stratospheric path length are on the order of 10 to 20 degrees. Nadir viewing (imaging) geometry is also not applicable due to the high tropospheric absorption in the sub-mm range. Since the main reason for the use of aircraft, beside latitudinal coverage, is to avoid the strong tropospheric attenuation, no specific requirements are necessary, i.e. there is no need to fly with high altitude aircraft such as the ER-2. Small jets as the German FALCON or the LEARJET of the Swiss Airforce which have already performed such flights, are sufficient for these atmospheric research applications. In the case of radio astronomy, where small angular resolution is needed, larger antennas and therefore larger aircraft have to be used, for example a Boeing 747 for the Stratospheric Observatory for Infrared Astronomy (SOFIA). Inside the 747, measurements of ClO and HCL distributions with a 650 GHz SIS receiver have been performed. Measurements with a 2.5 THz receiver are planned in the next few years.
IV. Recommendations for Future Activity
In this field of technology there is a clear need for higher frequencies with lower noise characteristics in order to explore all possible lines with low integration time. SIS is the leader at the lower frequencies, but this technology requires complex cooling to 4K. A substantial improvement towards less costly and complex operation would be the possibility of using SIS at temperatures of 20 K (typical 2 stage refrigerator) or 80 K (liquid nitrogen).
Another critical issue is the calibration standards and techniques which are essential in spectroscopic measurements of very weak signals. The same problem is also encountered in the current spectral analyzers which must be very stable and linear. Any non-linearity in the power response will result in spectral artifacts disturbing the retrieval of trace gas abundances. In the medium resolution range (about 1 MHz) AOS have been successfully used. For narrow band applications with frequency resolutions of 50 kHz, a gap still exists for satisfactory operation. Although there exist several solutions such as Chirp spectrometers, digital autocorrelators, and versions of narrow band AOS, they are either quite expensive or not very stable. Conventional filterbanks are readily available but have for each channel a different response. The higher demand on better spectral resolution will also stimulate this field.
In the same way the spectroscopic databases (e.g. JPL, HITRAN) should be updated with new laboratory measurements in order to improve the accuracy of the models used for the retrieval of molecular abundances. Efforts to improve the theoretical background of thermal sub-mm radiation, including the continuum, are on the way and will be very valuable for the future instrumentation either from aircraft or spacecraft.
Mees et al., "An airborne SIS-receiver for atmospheric measurements of trace gases at 625 to 760 GHz," Proc. 5th Symp. on Space TeraHertz Technology, pp. 142-156, Ann Arbor, 1994.
Crewell et al., "Comparison of ClO measurements by airborne and spaceborne microwave radiometers in the Arctic winter stratosphere 1993," J. Geophys. Res. Lett., 22, pp.1489-1492, 1995.
Febre P. et al., =91A 380 GHz SIS receiver using Nb/AlOx/Nb junctions for a radioastronomical ballon-borne experiment : PRONAOS," Proc. TeraHertz Tech., Univ. Michigan, April 1992.
The following review articles are located in the Nov. 1992 issue of Proc. of the IEEE:
Phillips T.G. and Keene J., "Submillimeter Astronomy."
Waters J., "Submillimeter-wavelength heterodyne spectroscopy and remote sensing of the upper atmosphere."
Erickson N.S., "Low noise submm receivers using single-diode harmonic mixers."
Crowe T.W. et al., "GaAs Schottky diodes for THz mixing applications."
Blundell R. and Tong C.-Y. E., "Submillimeter receivers for radio astronomy."