The BRE-RAM (Bremen Radiometer for Atmospheric Measurments) is currently under construction at the Institute of Environmental Physics, University of Bremen. The goal of this projekt is to construct a radiometer at low cost and with the usibility at relatively humid sites. Therefore it is built for ground based measurments of the mainly pressure broadened thermally induced rotational emission line of ozone at 110.836 GHz. From the line-shape stratospheric ozone profiles over Bremen (53°04'N, 8°48'E) can be retrieved. The vertical resolution will be approximately 10 km for the altitude range of 20 - 55 km.
Figure 1. Schematics of the BRE-RAM: In the top part the quasi-optics and the calibration units, in the center the antenna, the Schottky diode mixer and the first local oscillator controlled by a phase-lock-loop (PLL) electronic are shown. The lower section contains the amplifier chain with the two intermediate frequency stages and the acusto-optical-spectrometer.
The detailed outline of the radiometer is shown in Figure 1. The beam guiding to the antenna is realized by optics based on Gaussian beam technics, using components like mirrows and grids. Special features of the optics are a single-sideband-filter (SSB) supressing the image sideband and a path length modulator (PLM) minimizing baseline effects in the spectra.
Figure 2. Total ozone columnes over Bremen based on TOMS-data. The solid line represents the average of ozone columnes in the eighties, the dotted lines the daily maxima and minima during the years.
Figure 3. Calculated atmospheric zenith-opacities at 110.8 GHz based on water vapor profiles available by the Data Assimilation Office of the NASA. At opacities less than 0.35 Neper good conditions of observations by BRE-RAM are expected. The dashed line represents monthly averages of water vapor columns. After passing the horn-antenna the incoming radiation is down converted to 8 GHz by an uncooled Schottky diode mixer. The first intermediate frequency (IF) stage consists of an isolator, two low noises amplifiers and a bandpass filter. A second down conversion stage to a frequency of 995 MHz including again two amplifiers, a bandpass filter and a isolator provides the input signal for the acusto-optical-spectrometer (AOS). The bandwith of the AOS is 600 MHz and the frequency resolution is approx. 1.5 MHz.
The whole radiometer is computer controlled and operates in the total power mode using a cold and a hot calibration load. The cold load is realized by a dewar filled with liquid nitrogen at 77 K, the hot load by a mm-wave-absorber at ambient temperature (295 K). The system-noise-temperatur of the radiometer is at approx. 2970 K in the single-sideband-mode.
Seasonal ozone changes over Bremen measured by the TOMS-instrument are shown in Figure 2. A typical annual variation with high ozone values in winter and low values in summer is evident. In wintertime changes of total ozone by approx. 200 DU in a few days are common and mainly caused by atmospheric dynamics. Ground based radiometry is limited by tropospheric water vapor. As shown in figure 3 weather conditions of Bremen are know to be rather humid. The average water vapor column in the atmosphere is approx. 27 mm. Globally gridded data of water vapor profiles abailable from the DAO GEOS1 Multiyear Assimilated Datasets of the Data Assimilation Office, NASA, which are based on measurements, were used for radiative transfer calculations at 110.8 GHz. The results of these calculations offer a typical seasonal cycle in zenith-opacities over Bremen, mainly induced and correlated by tropospheric water vapor conditions. We expect that measurements by BRE-RAM will be possible at opacities less then 0.35 Neper corresponding to a transmission of atmosphere of 50 % at a typical elevation angle of 30°. Therefore, measurements by the BRE-RAM may be limited in summer.
Figure 4. Figure 4. Radiative transfer calculations of the 110.836 GHz ozone emission line using a standard midlatitude ozone profile and tropospheric water vapor contents over Bremen based on global gridded data from the Data Assimilation Office, NASA. January 20, July 8 and August 29, 1992 are typical examples of low, middle and high water vapor content. The influence of nitrogen, oxygen and water vapor on the spectra is illustrated in the spectrum of July 8 (dotted lines). In order to illustrate the expected spectra measured by BRE-RAM we have carried out radiative transfer calculations with a N2-O2-H2O-O3 model atmosphere and measured profiles of water vapor, ozone, temperature and pressure. In figure 4 a wide range of calculated brightness temperature mainly due to variable influence of water vapor is shown. The 110.836 GHz emission line of ozone is situated on the wing of a very strong oxygen line at 118 GHz which causes an increased slope at higher frequency. The contribution of nitrgen is very low and nearly constant over the frequency range and the seasons.
Figure 5. Figure 5. A typical calibrated ozone measurement by the BRE-RAM on October 15, 1997.
In October 1997 first data were taken using a filterbank for the spectral analysis of data. To match the center frequency of 3.7 GHz of the filterbank a synthesizer of 11.7 GHz was used for the down conversion at the 8 GHz output signal of the first IF-stage. The filterbank with 28 single channels has a total bandwidth of 1.1 GHz and a frequency resolution of 8 MHz at the line center, 40 MHz at the line wings and 80 MHz at the edges of the spectral range. In Figure 5. a typical ozone spectrum measured at Bremen is presented. 120 files, i.e. 480 s integration time of the atmospheric signal, have been averaged under clear sky conditions and an elevation angle of 30°.
