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Senior Design 03-04: Team 6



An Active Cavity Radiometer is a device that measures the amount of radiation given off by a surface. The concept behind the design is that the cavity is kept at a constant temperature higher than the ambient temperature. The mechanism of keeping this device at a constant temperature gives it the name active cavity. It is called a radiometer because it measures radiation.
The device measures the heat given off from a surface by determining the amount of power to be supplied to a heater to keep the cavity at a constant reference temperature when the temperature inside the cavity changes from absorbing heat from the surface.

Our team objective is to design and build a prototype active cavity radiometer. The device is used to measure the radiation being emitted from a surface. The intended use of our design is to measure the amount of radiation coming off of the earth. This information is relevant and applicable to climatology to improve the predictions of global warming and global weather patterns.
Although the manufacture of the actual instrument that will be used in the upper atmosphere would be fairly different and refined according to payload specifics for the balloon, we intend to build a prototype of the device and simulate the conditions of its intended use in a vacuum chamber.
Active cavity radiometers already exist and an example is the Active Cavity Radiometer Irradiance Monitor III (ACRIMSAT III), a satellite launched by NASA on December 20 1999 with the mission to study the total irradiance from the sun. ACRIMSAT, by measuring the amount of radiation received from the sun and correlating this data with measurements of ocean and atmosphere currents and temperatures, as well as surface temperatures makes estimates on the weather patterns and global warming. There is a lot of uncertainty on whether these measurements give a good indication as to whether global warming is taking place or not since the data is collected from a satellite at 800km above the earth and extrapolated to 35 km above the earth.
Furthermore, other causes of error are from the application of the device itself. Current applications utilize a radiometer mounted on a satellite in space about 800 km above the earth’s surface. A telescope is attached to the end of the cavity to enhance the cavity’s view of the earth since the satellite is at a considerably large altitude from the location it is reading (viewing) and this introduces errors into the data.

Design Approach
Our aim is to design a radiometer that would be attached to a balloon and fly over earth at about 35 kilometers. This height is used because it is at the very upper edge of the atmosphere, the stratosphere, so the radiation particles are not scattered by the air. This is also the region where the earth’s ozone layer is. The radiation data will be measured and collected by this device and then sent back to earth by an on board communication system. The power and communication system on the device will be completely modular to interface with the balloon’s onboard power and communication system.
Our part of designing the unit will be rather extensive. Although one already exists, the different elevation necessitates redesign and calibration to work properly. We have to calculate exactly how much of the earth the device will view and what percent of the actual radiation leaving the earth the device sees. We have to select a material that has a high enough absorptivity to satisfy our purpose of absorbing the radiation coming off the earth as well as radiation balance equations.
On the electronic side of the design, we have to design a controller for the heater. The controller will have the task of reading the input and modifying the output. This is will be challenging because the precision required is very large for the device to meet our requirements and goals. We also have to implement a communication protocol to transmit the measured data as well as communicate with other parts of the balloon such as GPS information for current location of reading. We have to research what the standard is for communicating among the different instruments on a balloon and then implement this so a separate transmitter is not required.
Although our device will have less error in the readings than the device currently in use, it will still exhibit some error and we will provide estimates of this error. Hence we intend on carrying out feasibility calculations for the functionality of the new device.

Our team would like to thank both Professor Matt Heun and Global Aerospace Corporation for their help and for the valuable information they provided.