Senior Design 03-04: Team 6
Description
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.
Top
Objectives
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.
Top
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.
Top
Our team would like to thank both Professor Matt Heun and Global Aerospace Corporation for their help and for the valuable information they provided.
Top
