In the mid-1960s, work began on a series of radiometers to be flown in satellites with the aim of measuring the distribution of temperature in the upper atmosphere. This program was highly successful. The first of these radiometers was developed by a joint team from the University of Oxford and the University of Reading in the United Kingdom, with the Reading team later relocating to Heriot-Watt University during the project’s final stages. The radiometer was flown on the Nimbus IV spacecraft and was named the Selective Chopper Radiometer (SCR) due to its design principles, which made extensive use of filters.
The SCR provided a height resolution of 10 km for temperature measurements within the atmospheric region spanning 15 to 50 km, encompassing the troposphere and part of the stratosphere. The methodology for temperature sounding employed by the SCR and subsequent radiometers was based on detecting and measuring radiation from atmospheric carbon dioxide (CO₂).
Atmospheric Temperature and Wavelength Selection
The temperature structure of the atmosphere, as understood at the time, suggested typical temperatures of about 200 K at a height of 10 km, rising to 240–280 K at 50 km. The black-body radiation peaks for these temperatures occur at wavelengths of 15 μm (200 K) and 11 μm (280 K), respectively. Consequently, the ideal wavelength band for atmospheric temperature measurements was identified as 11–15 μm.
Radiation emitted from the atmosphere is governed by the equivalence of absorptance and emittance. This principle, combined with the attenuation of radiation as it traverses the atmospheric layers, enables the deduction of temperature profiles. The methodology was first proposed by Kaplan.
CO₂, evenly distributed in the atmosphere, has extensive absorption bands around 15 μm, making it a reliable temperature indicator. Water vapor and ozone, which could interfere with measurements, are significant only near the ground and at specific altitudes, respectively, and can be accounted for in calculations.
Mathematical Model of Radiance and Emission
The emittance of any atmospheric layer can be calculated using its absorptance. Let us consider a layer at depth \( z \) below the spacecraft. If the transmittance of the atmosphere at frequency \( \nu \) above this layer is \( T_z \), the fractional loss in irradiance as it passes through a thickness \( dz \) is the absorptance \( A_{dz} \).
Radiation Equation
For radiation of initial irradiance \( I \) at frequency \( \nu \), the change in irradiance can be expressed as:
\[
dI = I_z \, T_z – (I – dI)T_{z – dz}
\]
This leads to:
\[
A_{dz} = \frac{dI}{I T_z} = -\frac{dT_z}{T_z}
\]
If \( T \) is the mean temperature of the layer, the black-body emission per unit frequency interval is given by \( B(T) \) at frequency \( \nu \). The irradiance emanating from the layer is:
\[
dI_z = K (1 – T_z) B(T) \, dz
\]
where \( K \) is a constant.
For a radiometer with bandwidth \( \Delta \nu \), the detected power over this band becomes:
\[
dI_\nu = K \int_{\nu – \Delta \nu / 2}^{\nu + \Delta \nu / 2} \left[ (1 – T_z) B(T) \right] d\nu
\]
The output of the radiometer is expressed as:
\[
D = \int R_\nu \left[ \frac{dT_z}{dz} B(T) \right] dz
\]
where \( R_\nu \) is the response function of the radiometer.
Weighting Function
The weighting function represents the response of the radiometer to radiation from depth \( z \):
\[
W(z) = \int_{\nu – \Delta \nu / 2}^{\nu + \Delta \nu / 2} R_\nu \frac{dT_z}{dz} \, d\nu
\]
Assuming a single absorption line, the absorption coefficient \( k_\nu \) near the line center is given by the Lorentz formula for pressure broadening:
\[
k_\nu = \frac{S}{\pi} \frac{\gamma}{(\nu – \nu_0)^2 + \gamma^2}
\]
where:
– \( S \) is the line strength,
– \( \gamma \) is the half-width of the line, proportional to pressure.
Selective Chopper Radiometer Design
To address challenges in achieving narrow bandwidths, the SCR utilized a selective chopper filled with CO₂. Key features of the radiometer include:
- Division of the entrance aperture: One half views deep space, while the other half views the Earth’s atmosphere.
- Vibrating chopper mirror: Alternates between viewing deep space and the atmosphere, effectively chopping the incoming signal.
- Narrowband filters: Achieved bandwidths as small as 1 cm⁻¹ using interference filters and CO₂-filled cells.
The radiometer employed six separate channels, each calibrated using variable mirrors that could alternately view the atmosphere, deep space, or a calibration black body.
Results and Impact
The radiometer, launched in April 1970 aboard the Nimbus IV spacecraft, provided groundbreaking data on the upper atmosphere’s temperature structure, uncovering novel and unexpected findings. Figures illustrating the radiometer’s design and performance, along with measured transmittance curves, are shown in Figures 15.6, 15.7, and 15.8.
