Radiometer for Atmospheric Measurements (RAM)
Instrument and Retrieval

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Contact: Dr Mathias Palm, mathias <at> iup <dot> physik <dot> uni-bremen <dot> de


General | Instrument | Spectra | Retrieval

Picture of the RAM instrumentation. Click to see a larger picture

Figure 1: Inside the Observatory in Ny-Ålesund. The leftmost rack is the water-vapour radiometer, in the center is the ozone radiometer, on the right is the chlorine-monoxide radiometer. On the top part of the racks, the quasi-optics, the mixer and the calibration facilities are mounted. The lower part of the central rack holds the amplifier chain, the spectrometer and control electronics.

General

The Radiometer for Atmospheric Measurements (RAM) is operated by the Institute of Environmental Physics at the University of Bremen in cooperation with the Alfred-Wegener-Institute for Polar and Marine Research. It is part of the Network for the Detection of Atmospheric Composition Change (NDACC). The Instrument is located at the Koldewey-Station in Ny-Ålesund, Spitsbergen, at 78°55' N, 11°55' E. This observation site was chosen for the special climatic conditions. The polar vortex is mostly located above Ny-Ålesund, which is essential for observing disturbed stratospheric chemistry. The RAM is operational since November 1994. At present, it consists of three different radiometer frontends for measuring the ozone emission line at 142 GHz. From the measured spectra, altitude profiles of the trace gas mixing-ratios are retrieved.

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Parameter ozone chlorine-monoxide4 water-vapour5 carbon monoxide6
Operational since 1993 1994 - 2003 1999 - 2008 2017
Physical receiver temp. 70 K 1 13 K ambient 70
Receiver noise temperature 1000 K 2 1150 K 200 K 300 K
Typical signal strength 15 K 0.05 K 0.2 K 4 K
Observation scheme total power reference total power total power
Typical integration time 15 min 24 h tbd 15 min
Single sideband filter Martin Puplett Martin Puplett SMA Wave guide band pass
Observation frequency 142.2 GHz 204.4 GHz 22.2 GHz 230.15 GHz
Spectral bandwith 1.65 GHz (100 kHz since 2006) 0.96 GHz 60kHz
Altitude range 20...70 km3 10...35 km tbd 60-80 km

Table 1. Technical data for the RAM instruments.
1 before Oct 2004, the ozone frontend was operated at ambient temperature
2 before Oct 2004, the receiver noise temperature was on the order of 3400 K
3 before 2006 : 15 - 55 km
4 permanently removed
5 permanently removed
6 since 10.2017

Instrument

Frontend

The idea behind microwave radiometry is the passive measurement of the rotational transition of thermal excited atmospheric trace gas molecules. The weak intensities of these signals require a special technique for amplification and detection. The instrument consists of three frontends sharing one backend. The basic design of the frontends is similar. The atmospheric signal is received through a styrofoam window which is transparent for microwaves. The beam is guided to a horn antenna by a system of mirrors and wire grids. Herein the beam is mixed with a strong signal to convert it to a lower frequency that can be amplified easily. The mixing process produces two frequency bands (side bands). The RAM is operated in the single-sideband mode by suppressing one of the sidebands with a filter. A path length modulator is used for the extinction of standing waves.

Spectrometer AOS CTS FFTS
Time of operation 1993 - 2007 2006-2007 from 2007
Instantaneous bandwidth 1.15 GHz 44.40 MHz 1 GHz
Number of channels 1728 4096 16384
Channel spacing 0.67 MHz 11 kHz 61 kHz

Table 2. Technical data for the RAM spectrometer backends.

Backend

The backend contains the intermediate frequency chain and an acousto optical spectrometer (AOS). The whole system is computer controlled. The purpose of the intermediate frequency chain is the down conversion from the first intermediate frequency of 8 GHz to the spectrometer input frequency of 2.1 GHz and the adjustment of the input power levels.

In 2006 the backend has been appended with a fine resolution spectrometer, a so called CTS. It features a bandwith of 40 MHz and a resolution of 12.5 kHz. The CTS spectrometer is run parallel to the AOS. The CTS measuring range is positioned at the respective line center and resolves the line center of the species of question. The line center contains high altitude information of the species under consideration. From autum 2007, the AOS/CTS combination has been replaced by a FFTS spectrometer, which features a broad bandwith and a sufficiently high resolution.

Observation scheme

Cold and hot loads are available for calibration purposes. We use two different observation schemes. The total power method (ozone) compares the atmospheric signal to the cold and hot loads.


Radiative transfer

The linear relationship is caused by the pressure broadening of the line. The higher the pressure, the broader is the line. This is due to collisions shortening the lifetime of the excited states. The final spectrum is composed of two parts:

Line shape - high altitude Line shape - middle altitude Line shape - low altitude

Figure 9-11: Theoretical lines from different altitude levels


Inversion

It is not possible to invert the equation above directly, for it is in fact underdeterminated. The inversion has to be made more stable by introducing a priori data that comes from independent measurements. The following data is needed:

Temperature profile Spectrum Presure profile

Figure 12-14: Data needed for optimal estimation

Retrieved profile

There are about 20 resulting profiles per day. Due to a highly optimized algorithm, the inversion takes only about a few seconds per profile. The altitude range is limited by the frequency resolution (high altitudes) and the flat line shape for high pressures (low altitudes). The mixing ratios obtained are weighted means of the real mixing ratios over a certain altitude span.

Ozone profile

Figure 15: Retrieved ozone profile

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Last changed: 29-05-2018, Mathias Palm