Structure and Specifications of MAP
This page describes the physical structure of MAP, and the reasons
for this design.
The following is an exploded view of the spacecraft:

Differential
- MAP measures temperature differences on the sky using symmetric microwave
receivers coupled to back-to-back telescopes. By measuring temperature
differences, rather than absolute temperatures, most spurious signals will
cancel. This is analogous to measuring the relative height of bumps on a high
plateau rather than each bump's elevation above sea level.
Temperature Monitoring
- The Sun, Earth, and Moon all emit thermal radiation that could potentially
interfere with MAP's sensitive CMB anisotropy
data. (MAP's instruments are designed to measure temperature differences with an
accuracy of nearly a millionth of a degree Kelvin.)
Thus stray radiation from the Sun, Earth, and Moon must be minimized. This is
accomplished in the following ways:
- The Orbit of MAP
- MAP orbits the Earth and Sun from the second Lagrange point, which means
that the Earth, Sun and Moon are always in the same position relative to the
satellite. This means that all three stay on one side of the spacecraft so that
stray radiation can be kept to a minimum.
- Sun-Side Shielding
- MAP is equipped with a large Sun shield located on the "bottom" of the
spacecraft. This shield absorbs and deflects Solar radiation. Once heat is
absorbed by the shields, Multi-Layered Insulation (MLI) helps establish a
temperature gradient which allows the upper side of the spacecraft to remain
cool while the solar panels on the bottom are heated. This situation is
analogous to the roof of a car that sits outside overnight frosting over, even
if temperatures are above freezing. In the car's case, the wheels and base of
the car shield the roof of the car from the heat of the Earth, causing the
temperatures to drop and the roof to frost over. Cooling of the upper half of
the spacecraft is aided by two large passive radiators on the spacecraft.
Microwave System Subassembly
- MAP measures the Cosmic Microwave Background at five
different frequencies (from 22 to 90 GHz) in order to minimize error due to
emission from our Galaxy. The box sits directly on top of a thermally isolating
cylinder of MAP contains all the horns necessary to observe at these different
frequencies. The symmetry of the horns is due to the fact that MAP uses
differential data to analyze the
Cosmic Microwave Background in order to reduce
error. The following is a picture of the
design of this box:
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The Motion of MAP
- MAP follows several different motions in the sky during its
orbit. The star tracker and the gyroscope can
give engineers on Earth an extremely accurate picture of the direction in which
MAP's reflectors are pointing. In addition to this, MAP must adjust its orbit
from time to time, as even the slightest drift from the
L2 point subjects MAP to gravitational forces that
would pull it from that point entirely. To counteract this, MAP is equipped with
a propulsion tank containing a fuel called hydrazine, which it uses to adjust
its trajectory and orbit. The six thrusters equally spaced around the upper side
of MAP's shield correct the spin rate of MAP. In addition, the three reaction
wheels (also equally spaced along the base of the shield) can spin to slow down
the overall spinning of MAP. These wheels operate on the principle of
conservation of angular momentum.
Data Transmission
- Once MAP has collected its data, it returns it to Earth using its antennae.
During the phasing loops around the moon which
bring it into the right position to journey to the L2
Point, MAP communicates with the Earth via the two omni antennae attached
to the top and bottom of the spacecraft. After MAP has reached the
L2 Point, it transmits and receives data via the
Medium Gain Antenna attached to the "bottom" of the spacecraft, i.e., the side
that faces the Sun and Earth. The antennae are essential not only because they
transmit anisotropy measurements stored in MAP's computer, but also because they
provide the mechanism by which NASA engineers can correct MAP's position and
direction in the sky.

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Last updated:
Friday, 05-21-1999