Microwave Anisotropy Probe

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:

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