Microwave-Radiometer Figure 1: History of cosmic background radiation measurements. Left: microwave instruments, right: background radiation as seen by the corresponding instrument. Picture: NASA/WMAP Science Team. 1 Introduction The term microwaves refers to alternating electromagnetic signals with frequencies between 300 MHz and 300 GHz (wavelengths 1 mm to 1 m). Because of the long wavelengths, compared to the visible and infrared, microwaves have special properties that can be used for many applications. Thanks to the long wavelength microwaves can penetrate through cloud cover, haze, dust, and even rain as they are less susceptible to atmospheric scattering. In addition, various molecules and atoms resonances occur in the microwave range. The majority of microwave applications can be found in communication systems, radar systems, environmental remote sensing, and medical systems. An incomplete list of applications: 1
Figure 2: Examples of microwave applications. Top left: A-Train, satellite instruments allow a comprehensive picture of Earth weather and climate (Picture: NASA). Top right: Cosmic background imager with its 13 radio antennas located in the Atacama dessert, Chile (Picture: CBI/Caltech/NSF). Middle left: Arctic ozone loss in 2011 as measured by Aura MLS (Picture NASA). Middle right: Antenna of. Bottom left: Stockhorn panorama seen in microwave range by SPIRA and in visible range. Bottom right: Example of an active millimeter wave scanner used to detect concealed objects. Mobile communication (WLAN...) GPS Remote sensing of the atmosphere, the earth, space (astronomy) 2
Radar systems for military, commercial and scientific use As an example of space applications the history of the measurement of the cosmic background radiation is illustrated in Fig. 1. Further examples are presented in Fig. 2. The main aims of this experiment are: Become familiar with the basics of radiative transfer, microwave radiometry and receiver techniques. Measure the brightness temperatures of the sky around the 22 GHz emission line of water vapor, the brightness temperature of a person and the transmission of different materials. The student should write a report answering the theoretical questions, describing the radiometer and the experiments as well as discussing the results. 2 Theory 2.1 Radiometry Describe the receiver setup and working principle of the prakti microwave radiometer (Fig. 3 and 6) and discuss with help of the advised literature (mainly [1] ch. 1, [2] ch. 13) the following terms and questions: The mixing principle. Why is mixing necessary? Square-law detector. Which physical quantity is actually measured? Describe the relationship between noise power and noise temperature. Explain system noise temperature, receiver noise temperature, antenna temperature. Antenna pattern. Definition, Illustration? Explain that the directivity D = 4πA e /λ 2, with A e the effective aperture area and λ the wavelength. Sensitivity. What determines the sensitivity of a radiometer (radiometer formula)? Calibration. How is a radiometer calibrated (hot-cold calibration)? 3
RF Mech. switch Low noise amplifier Mixer IF Filter Antenna Amplifier Attenuator Waveguide load, temp. sensor x2 Freq. doubler/ amplifier detector Noise diode LO synth A/D card USB Analog Figure 3: Blockdiagram of the radiometer. The dashed lines indicate that the synthesizer is external. Together with an active frequency doubler it provides the LO signal. 2.2 Radiative Transfer Shortly discuss the Radiative Transfer equation and its solution. The following terms should be explained: Emission and absorption. Brightness temperature. Physical temperature. Emissivity. Optical depth/opacity (t = e τ = e kαl ) Airmass factor. Assume a plane-parallel atmosphere with horizontally stratified temperature. Explain the relation between brightness temperature T B and cos(θ) 1 (see Fig. 4 for illustration). Radiative transfer determines the signal measured with the prakti microwave radiometer. Fig. 5 shows the frequency dependant brightness temperature in the microwave region for two different situations, a humid and a dry one. An other example showing the dependence of the brightness temperature on frequency as well as on the observation angle can be found in Fig. 13.6 of [2]. The prakti radiometer is operated in the vicinity of the 22.235 GHz water vapor line. 4
Figure 4: Geometry of sky observation. T 0 is the background temperature, T m is the mean temperature of the atmosphere and θ is the zenith angle. 3 Experiment The measurements are performed with the radiometer shown in Fig. 6. Section 3.1 presents practical hints on how to use the prakti radiometer and section 3.2 explains how to do a tipping curve measurement for the calibration of the radiometer. The measurements to be performed are given in sections 3.3 to 3.5. 3.1 Information and practical hints The measurements are made on the roof terrace of the building and should be performed under clear sky conditions since clouds affect the measurements. The noise power of the microwave components and especially the gain of the amplifier is strongly temperature dependent. To reduce large gaindrifts, the radiometer should be switched on at least 1 hour before the actual measurements. This gives enough time for the radiometer to reach thermal equilibrium. An external synthesizer in combination with a frequency doubler and amplifier is used as a local oscillator (LO). The LO frequency is twice the selected synthesizer frequency. The measurements can be performed around 18, 20, 22, 24 and 26 GHz. The mechanical waveguide switch allows to measure the signal from the antenna or to switch to an internal calibration load which acts as a blackbody source. Two calibration loads are coupled to it. A noise diode, followed by an attenuator [no. 3 and 4 of Fig. 6], and a matched waveguide load [no. 5 of Fig. 6]. The matched waveguide load is a blackbody at ambient 5
Figure 5: Intercomparison of brightness temperature spectra calculated from RS-92 radiosonde measurements at Thun and coincident brightness temperatures observed by ground-based microwave radiometers at Bern (black plus symbols at 21, 22 and 31 GHz and narrow O3 line profiles at 142 GHz). The red line denotes the brightness temperature spectrum of a humid summer day, while the blue line denotes the spectrum of a dry winter day. Values of integrated water vapor (IWV) and integrated liquid water (ILW) are given in the legend. Both figure and text taken from [3] temperature, which is measured by the temperature sensor [and defined in the measurement software as Tref]. An inclinometer is installed inside the receiver box, to determine the elevation of the radiometer. For observation angles close to zenith the inclinometer returns nan (not a number). The gain of the instrument changes over time. Therefore, it is important to assure that all measurements used for one calibration are performed consecutively. Especially at 18 GHz the instrument has limited stability. Measurement software: There is a Labview program on the laptop to facilitate the measurement and store the data. First, initialize the data acquisition card by double clicking on the icon RM Praktikum Init on the Desktop, and then Refresh. Then, open in the folder Praktikum/Software the Labview program Praktikum.vi. For the measurements: 1. Indicate the file path by pressing the yellow folder button and choosing the file. ASCII files are generated. 2. Start the software by pressing on the white arrow on the top left. 6
3. Start the measurement by pressing the START button. 4. To stop the measurement, press the STOP button long enough till the green Measuring led switches off. Alternatively you can perform measurements with fixed integration time by pressing the NEXT button. 5. To stop the software, press the End Program on the top right. 6. Do not save any changes. 7. To be able to see the elevation angle of the radiometer when not measuring you can run elevation continuous.vi. In the ASCII file, tab-separated variables are stored, such as the detector output, the elevation, temperature of the waveguide load and time. On the front panel you can choose antenna on/off (0/1 in the file), noise diode on/off (1/0 in the file) and the LO-frequency. These values are not used to control but only written into the file to make data analysis easier. Make sure that your choices are in agreement with your measurement set up (that means the real switches should agree with the ones in Labview)! 7
Figure 6: The frontend of the microwave radiometer. The numbers are explained in table 1. 8
Table 1: Numbers identifying the different components of the front end (Fig. 6). 1 : Antenna 2 : Mechanical switch 3 : Noise diode 4 : Attenuator (not used) 5 : Waveguide load + temperature sensor 6 : Amplifier at RF 7 : Mixer 8 : Filter 9 : Amplifier at IF 10 : Frequency doubler + amplifier 11 : Input from synthesizer 12 : Detector and amplifier 13 : A/D card 14 : Inclinometer 3.2 How to do a tipping curve You need to calibrate the output voltage to get the brightness temperature of the atmosphere. The knowledge gained in the theoretical part should help you. Some hints on how to calibrate the instrument using tipping curve measurements are given here. A tipping curve is a series of measurements of the sky U k under different zenith angles θ k. The aim of a tipping curve is to determine the opacity. With this information it is possible to determine the brightness temperature of the sky at zenith direction which can be used as a cold calibration target. Starting from the radiative transfer equation in the microwave region under the assumption of a one layered atmosphere with a mean temperature T m, the brightness temperature under an zenith angle θ is given by T b (θ) = T 0 e τ(θ) + (1 e τ(θ) )T m. (1) T 0 is the cosmic background radiation (= 2.7 K) and T m is the mean temperature of the atmosphere. T m can be estimated from the ground temperature T ground T m T ground 10K. (2) In addition to the sky measurement the hot calibration target is measured (U hot, switch to noise diode, turn noise diode off (measure T amb )). As cold calibration 9
target (U cold ) the sky at a high elevation angle is measured. The opacity can be found by iteration. The iteration starts with an initial estimation of the opacity τ i (e.g. τ i = 0.3). The iteration process works as follows: T b,cold is calculated using equation (1) and inserting τ i For zenith angles θ the tipping measurements are calibrated. T b,θ = U θ U hot U hot U cold (T hot T b,cold ) + T hot (3) With the calibrated measurements the opacities can be calculated solving equation (1): ( ) Tm T 0 τ θ = ln (4) T m T b,θ The slope of a linear regression of the opacities τ θ against the airmass factors is equal to the new zenith opacity τ since it airmass A(90 ) = 1. τ is taken as the new τ i. The iteration steps are repeated until the offset of the linear regression is smaller than 10 2. The linear fit has to pass through the origin because the opacity for zero airmass has to be zero. The zenith opacity found by iteration is used to calculate the brightness temperature of the cold calibration load T b,cold. With the brightness temperatures of the hot and the cold calibration load a hot-cold calibration can be performed. 3.3 Airmass and brightness temperature of the sky Verify the relation between brightness temperature of the atmosphere and cos(θ) 1 by performing an elevation scan (making measurements of the atmosphere under different zenith angles). Plot the brightness temperature versus airmass. Adjust the elevation angle step size in order to get a uniform spacing in airmass. Use tipping curve calibration. The antenna HPBW is 10. Determine the brightness temperatures of the sky at one elevation and 5 different frequencies (18, 20, 22, 24 and 26 GHz) and plot them against frequency. Discuss the relation of the measured T b with physical temperatures of the atmosphere (Hint: use RTE). Repeat the measurements with a perspex (plexiglas), a styrofoam and a glass plate in front of the antenna. Discuss the characteristics of the materials (transmittance, opacity). In addition, determine the brightness temperature of a person. 10
3.4 Calibration of the noise diode The noise diode in combination with the attenuator is an internal load with unknown noise temperature. Determine the brightness temperature of the noise diode by a tipping curve calibration (at 18, 20, 22, 24 and 26 GHz). 3.5 Receiver Noise Temperature and Radiometer noise formula Determine the receiver noise temperature of the radiometer (T rec ). Describe your calculations in detail. Prove the sensitivity relation by performing a series of measurements and compare the noise levels with different integration times t int by plotting T/T versus t int (T = T sys ). Hint: choose 1/16 s integration time. References [1] Michael A. Janssen, ed.: Atmospheric remote sensing by microwave radiometry, Wiley & Sons, New York, 1993, XKF 201 [2] David M. Pozar: Microwave Engineering, Wiley & Sons, 2005 [3] Klemens Hocke et al.: A complete long-term series of integrated water vapour from ground-based microwave radiometers, International Journal of Remote Sensing, Vol. 32, No. 3, 10 February 2011, 751-765 11