History

WMAP mission was proposed to NASA in 1995. The WMAP spacecraft arrived at the Kennedy Space Center on 20 April 2001. The Wilkinson Microwave Anisotropy Probe (WMAP) mission reveals conditions as they existed in the early universe by measuring the properties of the cosmic microwave background radiation over the full sky. This microwave radiation was released approximately 380,000 years after the birth of the universe. WMAP creates a picture of the microwave radiation using differences in temperature measured from opposite directions (anisotropy). The WMAP satellite ended science observations on 20 August 2010. On 8 September 2010, WMAP departed L2 for a heliocentric orbit approximately 7% larger the Earth's orbit. The complete nine-year data set is now being processed and the final legacy data products will be released by 2012.

Launch

After being tested for two months, it was launched via a Med-Lite Delta II 7425 rocket on 30 June 2001 into a lunar assisted trajectory to the Sun-Earth L2 libration point for a nominal 27 month mission (3 months transit to L2, 24 months observing). On 2 July, it began working, first with in-flight testing (from launching until 17 August), then began constant, formal work. Afterwards, it effected three Earth-Moon phase loops, measuring its sidelobes, then flew by the Moon on 30 July, enroute to the Sun-Earth L2 Lagrangian point, arriving there on 1 October 2001, becoming, thereby, the first CMB observation mission permanently posted there.The mission has now been extended for several years to collect additional high quality data. The total payload mass is ~830 kg. The WMAP instrument is continuously shaded from the Sun, Earth, and Moon by the spacecraft to allow lower thermal disturbances.

Orbit
Summary

The WMAP (Wilkinson Microwave Anisotropy Probe) mission is designed to determine the geometry, content, and evolution of the universe via a 13 arcminute FWHM resolution full sky map of the temperature anisotropy of the cosmic microwave background radiation. The choice of orbit, sky-scanning strategy and instrument/spacecraft design were driven by the goals of uncorrelated pixel noise, minimal systematic errors, multifrequency observations, and accurate calibration. The skymap data products derived from the WMAP observations have 45 times the sensitivity and 33 times the angular resolution of the COBE DMR mission. The WMAP mission characteristics are summarized in the table found in the Spacecraft section below.

Overview

Intro

The WMAP instrument consists of a set of passively cooled microwave radiometers (connected to radiator panels with metal straps) with 1.4 x 1.6 meter diameter primary reflectors to provide the desired angular resolution. Measuring the temperature of the microwave sky to an accuracy of one millionth of a degree requires careful attention to possible sources of systematic errors. The avoidance of systematic measurement errors drove the design of WMAP. The instrument has five frequency bands from 22 to 90 GHz to facilitate separation of galactic foreground signals from the cosmic background radiation.

WMAP is a differential experiment: if you would like to know whether one piece of wood is longer than another, it is better to put the pieces directly next to each other than to measure them separately with a ruler. WMAP measures the temperature difference between two points in the sky rather than measuring absolute temperatures.

An orbit about the Sun-Earth L2 libration point that provides for a very stable thermal environment and near 100% observing efficiency since the Sun, Earth, and Moon are always behind the instrument's field of view; a scan strategy that rapidly covers the sky and allows for a comparison of many sky pixels on many time scales.

The following tables give the high-level technical specifications for the WMAP instrument and observatory.

WMAP Mission Characteristics: Instruments
Bands
KKaQVW
Wavelength (mm)139.17.34.93.2
Frequency (GHz)2333416194
Bandwidth (GHz)5.57.08.314.020.5
Number of Differencing
Assemblies
11224
Number of Radiometers22448
Number of Channels448816
Beam Size (deg)0.880.660.510.350.22
System Temperature,
Tsys(K)
2939591.21.6
Sensitivity (mK Sec½)0.80.81.04.93.2
WMAP Mission Characteristics: Observatory
Sky CoverageFull sky
Optical SystemBack-to-Back Gregorian, 1.4 x 1.6 m primaries
Radiometric SystemDifferential polarization sensitive receivers
Detection HEMT amplifiers
Radiometer Modulation2.5 kHz phase switch
Spin Modulation0.464 rpm = ~ 7.57 mHz spacecraft spin
Precession Modulation 1 rev hr -1 = ~ 0.3 mHz spacecraft precession
Calibration In-flight: amplitude from dipole modulation, beam from Jupiter
Cooling System Passively cooled to ~ 90 K
Attitude Control3-axis controlled, 3 wheels, gyros, star trackers, sun sensors
Propulsion Blow-down hydrazine with 8 thrusters
RF Communication 2 GHz transponders, 667 kbps down-link to 70 m DSN
Power 419 Watts
Mass840 kg
LaunchDelta II 7425-10 on June 30, 2001 at 3:46:46.183 EDT
Orbit1° - 10° Lissajous orbit about second Lagrange point, L2
Trajectory3 Earth-Moon phasing loops, lunar gravity assist to L2
Design Lifetime27 months = 3 month trajectory + 2 yrs at L2
Details

WMAP Optics

The WMAP instrument consists of two back to back, symmetric telescopes that produce two focal planes, A and B, on opposite sides of the spacecraft symmetry axis. A set of ten corrugated feeds lie in each focal plane and transport power to the amplification electronics.

Reflectors

The reflector design incorporates two back-to-back off-axis Gregorian telescopes with 1.4 m x 1.6 m primary reflectors and 0.9 m x 1.0 m secondary reflectors. The off-axis Gregorian optical design produces sufficient focal plane area, while satisfying the constraints of fitting the optical system in the Delta-rocket fairing envelope, placing the two focal planes in close proximity to one another, and minimizing sidelobe response with no obstructions in the beam. The principal focus is between the primary and the secondary. The reflector surfaces are shaped (i.e. designed with deliberate departures from conic sections) for optimal performance. Each primary is a (shaped) elliptical section of a paraboloid with a 1.4 m semi-minor axis and a 1.6 m semi-major axis. When viewed along the optic axis, the primary has a circular cross-section with a diameter of 1.4 m. The two focal planes produced by this arrangement are slightly convex with plate scales of ~15'/cm. The 99.5% encircled energy spot size diameter is less than 1 cm over a 15 x 15 cm region of the focal plane, and less than 0.33 cm over the central 8 x 8 cm region.

The reflectors are constructed of a Korex carbon-composite material, which have approximately 2.5 µm of vapor deposited aluminum and 2.2 µm of vapor deposited silicon oxide. The silicon oxide achieves the required thermal properties (A/E ~ 0.8, E ~ 0.5) while negligibly affecting the microwave signals. The reflectors are fixed-mounted onto a carbon-composite (XN-70 and M46-J) truss structure. The reflectors and their supporting truss structure were manufactured by Programmed Composites Inc. with the coating applied by Surface Optics Inc. Use of the composite materials minimizes both mass and on-orbit cool-down shrinkage. The reflectors are rigidly mounted to be in focus when cool so ambient preflight measurements are slightly out of focus.

The microwave emissivity of the reflectors appears to be that of ideal shiny bulk aluminum. In order to limit diffracted signals to less than 0.5 µK, diffraction shields are employed above, below, and to the sides of each secondary. In addition, the deployable solar panels and multi-layer insulation act as a Sun shield to guarantee that the secondaries remain at least 6 degrees into shadow during observing. In order to limit diffracted signals to less than 0.5 µK, diffraction shields are employed above, below, and to the sides of each secondary. In addition, the deployable solar panels and multi-layer insulation act as a Sun shield to guarantee that the secondaries remain at least 6 degrees into shadow during observing.

WMAP Frequency Coverage

Galactic foreground signals are distinguishable from CMB anisotropy by their differing spectra and spatial distributions. Multiple frequency coverage is needed to reliably separate Galactic foreground signals from CMB anisotropy. WMAP observes with five frequency bands between 22 and 90 GHz.

The figure above shows the frequency dependence of the expected CMB anisotropy (red band) and of three known sources of foreground emission from our Galaxy, in units of antenna temperature. The five WMAP frequency bands are indicated in tan vertical bars on the plot. There are at least three physical mechanisms that contribute to the Galactic microwave emission: synchrotron radiation, free-free radiation, and thermal radiation from interstellar dust. There are hints of a possible fourth mechanism that might be due to rapidly spinning dust grains emitting microwave radiation, though the evidence for this is still quite tentative and so has been omitted from this plot. Results show that at high Galactic latitudes (way from galactic plane) CMB anisotropy dominates in the range ~30-150 GHz. However, the Galactic foreground is measured and removed from some of the WMAP data (Sky cut Kp1 and Kp2). The two lowest frequency WMAP bands (K and Ka) are especially valuable for characterizing galactic emissions.

Five frequency bands with comparable sensitivity are necessary to solve for the five possible signals: synchrotron, free-free, dust, CMB anisotropy, and perhaps spinning dust. The range of frequency coverage is more important than the specific choice of frequencies within the range. The lowest frequency to survey from space should be at the 22 GHz atmospheric water line since frequencies below this can (with some difficulty) be accurately measured from the ground. The highest frequency to survey should be ~100 GHz to reduce the dust contribution and minimize the number of competing foreground signals. The choice of frequencies between 22 and 100 GHz were dictated by the practical consideration of standard waveguide bands. Based on these considerations, WMAP observes in the following five frequency bands, which are indicated on the above plot:

WMAP Frequency Bands
Microwave BandKKaQVW
Frequency (GHz)2333416194
Wavelength (mm)139.17.34.93.2

WMAP Angular Resolution

CMB anisotropy information from current and proposed high resolution (< 0.3°) measurements over limited sky regions will likely succeed from ground and balloon-based platforms. The priority for the WMAP space mission is to map the entire sky with (< 0.3°) angular resolution where the cosmological return is high, and the data cannot be readily obtained in any other way. The WMAP optics feature back to back 1.4 m x 1.6 m primary reflectors which lead to an angular resolution of (< 0.25°) in the highest frequency (90 GHz) channel. The following table gives the angular resolution to be obtained from each of the five WMAP frequency bands. The value quoted is the full width at half maximum (FWHM) of the approximately Gaussian central beam lobe, in degrees. The definitive measurements of the main beam properties will be obtained from observations of the planet Jupiter in flight. This information will be used to compute the effective beam response on the sky after accounting for the azimuthal averaging that occurs from observing a given sky pixel with a range of beam orientations. This complete information will be made available with the sky map data when it is processed.

WMAP Angular Resolution
Frequency22 GHz30 GHz40 GHz60 GHz90 GHz
FWHM, (deg)0.930.680.530.2394

WMAP Sensitivity

The WMAP specification calls for an equal noise sensitivity per frequency band of ~35 µK per 0.3° x 0.3° square pixel. The mission duration required to meet this specification is two years of continuous observation. If Galactic emission is negligible at high latitudes above 40 GHz, as was the case for COBE, the sensitivity achievable by combining the three highest frequency channels is ~20 µK per 0.3° x 0.3° pixel. The following table gives the specified sensitivity for each of the five WMAP frequency bands.

WMAP Sensitivity (μK, 0.3˚ x 0.3˚ pixel)
Frequency22 GHz30 GHz40 GHz60 GHz90 GHz
Specification~35~35~35~35~35

The corresponding sensitivity to the angular power spectrum is illustrated in the following plot which shows the anisotropy signal amplitude as a function of angular scale, which is characterized either by the “multipole moment” or by the angular scale in degrees. Three popular cosmological model curves are shown: the orange curve is the “Standard Cold Dark Matter (CDM)” cosmological model, the purple curve is a high baryon density CDM model, and the red curve is a cosmological constant model. The purple data points on the left are from COBE, the green data are from TOCO, the blue data are from BOOMERanG, the red data are from MAXIMA and the orange data are from CBI. The outer and inner error bars on the data represent the uncertainties with and without calibration uncertainties included, respectively. The gray band superposed on the “lambda CDM” model shows the expected WMAP 1-sigma error band. This band corresponds to two years of observations based on currently measured instrument performance, with a band width of delta l = 50, and a 10° Galaxy contamination cut.

Introduction

The Wilkinson Microwave Anisotropy Probe (WMAP) is named after Dr. David Wilkinson, a member of the science team and pioneer in the study of cosmic background radiation. The science goals of the WMAP broadly dictate that the relative Cosmic Microwave Background (CMB) temperature be measured accurately over the full sky with high angular resolution and sensitivity. The overriding priority in the design was the need to control systematic errors in the final maps. The specific goal of WMAP is to map the relative CMB temperature over the full sky with an angular resolution of at least 0.3°, a sensitivity of 20 µK per 0.3° square pixel, with systematic artifacts limited to 5 µK per pixel.

To achieve these goals, WMAP uses differential microwave radiometers that measure temperature differences between two points on the sky. WMAP observes the sky from an orbit about the L2 Sun-Earth Lagrange point, 1.5 million km from Earth. This vantage point offers an exceptionally stable environment for observing since the observatory can always point away from the Sun, Earth and Moon while maintaining an unobstructed view to deep space. WMAP scans the sky in such a way as to cover ~30% of the sky each day and as the L2 point follows the Earth around the Sun WMAP observes the full sky every six months. To facilitate rejection of foreground signals from our own Galaxy, WMAP uses five separate frequency bands from 22 to 90 GHz.

Mission Objectives

General Objectives

Scan Strategy

Sky Coverage

A primary requirement of the WMAP mission is to observe the full sky. Since a major goal of cosmology is to determine the statistical properties of the universe, it is clear that the largest possible number of sky samples improves constraints on cosmological models. The measurement of each individual position on the sky is an independent sample of the physics of the universe. Moreover, full sky coverage is absolutely required to accurately determine the low-order spherical harmonic moments. While the largest angular scales were observed by COBE, WMAP re-measures the full sky with higher resolution to:
• Avoid relative calibration errors when two or more experimental results area combined (e.g.., COBE and WMAP).
• Provide greater sensitivity to the angular power spectrum.
• Independently verify the COBE results.
• As shown in the diagram below WMAP observes the full sky every six months to provide fourfold redundancy in the data collected over a period of two years.

Strategy

• The WMAP scan strategy plays an important role in systematic error rejection. It was designed with the following goals in mind:
• Scan a large fraction of the sky as rapidly as possible, consistent with reasonable requirements on the controlling hardware and the telemetry data rate.
• Scan each sky pixel through as many azimuth angles as possible for the reasons listed below.
• Observe a given pixel on as many different time scales as possible.
• Maintain the instrument in continuous shadow for optimal passive cooling and avoidance of stray signals from the Sun, Earth, and Moon.
• Maintain a constant angle between the Sun and the plane of the solar panels for thermal and power stability.

Since WMAP is a differential experiment - it measures the difference in temperature between two points a fixed distance apart on the sky - it is also desirable that the angular separation between the two observing beams should be “large” in order to maintain sensitivity to signal at large angular scales. This is important for comparing the WMAP results to COBE, for properly normalizing the angular power spectrum, and for retaining sensitivity to the dipole which will serve as WMAP's primary calibration source. The separation between WMAP's two lines of sight is roughly 141° (smaller for some channels, larger for others).

The scan strategy that was ultimately adopted combines a “fast” spin about the spacecraft symmetry axis with a slow precession 22.5 ° about the Sun-WMAP line (which is always within 0.1° of the Sun-Earth line at L2). Since each telescope line of sight is ~70° off the symmetry axis, the path swept out on the sky by a given line of sight resembles a Spirograph® pattern that reaches from the north to south ecliptic poles. Since the spin and precession periods are incommensurate, the combined motion will cause the observing beams to fill an annulus centered on the local solar vector with inner and outer radii of ~48° and ~93° respectively. Thus WMAP will observe more than 30% of the sky each day and will observe the ecliptic poles every day. The spin period will be 2.2 minutes while the precession period will be 1 hour. The image below depicts the WMAP scan pattern after one complete spacecraft precession (1 hour); the bold circle shows the path for a single spin (2.2 minutes).

WMAP Scaning Geometry

Note that because of the large size of the annulus, the beams will always see a substantial modulation due to the CMB dipole. Since the dipole anisotropy was precisely measured by COBE, this known modulation pattern serves as an ideal continuous calibration source. The WMAP scan strategy achieves a reasonable level of azimuth coverage in each sky pixel. For example, a pixel in the ecliptic equator is observed over ~30% of the possible angles of attack; a pixel at the cusp of the annular coverage at ~45° ecliptic latitude is observed over about 70% of possible angles of attack; and a pixel near the ecliptic poles is observed from 100% of the possible azimuth orientations. Large azimuth coverage provides numerous desirable features in the data:
• Helps to produce a stable sky map solution.
• Produces small pixel-pixel covariance at the beam separation scale.
• Minimizes striping due to any residual 1/f noise in the differential data.
• Maximizes polarization sensitivity.
• Maximizes azimuth symmetry of the beam response on the sky.

From the standpoint of azimuth coverage, the WMAP strategy is not as complete as COBE's, which achieved nearly 100% azimuth coverage in all pixels. However, in order to achieve such completeness, the spacecraft spin axis must ultimately point to every pixel on the sky which is less desirable from the standpoint of systematic error avoidance. The WMAP strategy achieves reasonable azimuth coverage consistent with strong systematic error constraints. Extensive simulations of the map making and power spectrum estimation procedures have shown that the strategy is more than adequate to meet WMAP's scientific goals. As WMAP orbits the Sun, the annular scan pattern continuously revolves around the sky so that full sky coverage is first achieved after 6 months of observing at L2, as depicted below. This coverage is repeated every six months for the duration of the mission. The redundancy of this coverage provides an important stability check as several independent full sky maps based on independent six month intervals can be compared for consistency.

Operational Orbit

The Italian-French mathematician Joseph-Louis Lagrange discovered five special points in the vicinity of two orbiting masses where a third, smaller mass can orbit at a fixed distance from the larger masses. More precisely, the Lagrange Points mark positions where the gravitational pull of the two large masses precisely equals the centripetal force required to rotate with them. Of the five Lagrange points, three are unstable and two are stable. The unstable Lagrange points - labeled L1, L2 and L3 - lie along the line connecting the two large masses. The stable Lagrange points - labeled L4 and L5 - form the apex of two equilateral triangles that have the large masses at their vertices.

The L1 point of the Earth-Sun system affords an uninterrupted view of the sun and is currently home to the Solar and Heliospheric Observatory Satellite SOHO. The L2 point of the Earth-Sun system is home to the WMAP spacecraft and (perhaps by the year 2014) the James Webb Space Telescope. The L1 and L2 points are unstable on a time scale of approximately 23 days, which requires satellites parked at these positions to undergo regular course and attitude corrections.

Orbital Insertion

To minimize environmental disturbances and maximize observing efficiency, WMAP observes from a Lissajous orbit about the L2 Sun-Earth Lagrange point 1.5 million km from Earth. The trajectory to reach the observing station consisted of 3 lunar phasing loops followed by a ~100 day cruise to L2.

The Lagrange points mark positions where the combined gravitational pull of two large masses precisely equals the centripetal force required to rotate with them. The L2 Lagrange point offers a virtually ideal location from which to carry out CMB observations. Because of its distance, 1.5 million km from Earth, it affords great protection from the Earth's microwave emission, magnetic fields, and other disturbances. It also provides for a very stable thermal environment and near 100% observing efficiency since the Sun, Earth, and Moon are always behind the instrument's field of view.

The Need to go to L2

The Lagrange points mark positions where the combined gravitational pull of two large masses precisely equals the centripetal force required to rotate with them. The L2 Lagrange point offers a virtually ideal location from which to carry out CMB observations. Because of its distance, 1.5 million km from Earth, it affords great protection from the Earth's microwave emission, magnetic fields, and other disturbances. It also provides for a very stable thermal environment and near 100% observing efficiency since the Sun, Earth, and Moon are always behind the instrument's field of view.

WMAP Trajectory to L2

XThe following sketch indicates the path to L2. The trajectory features 3 or 5 lunar phasing loops depending on when in the lunar cycle WMAP launches (WMAP used only 3) and a lunar flyby to assist the spacecraft in reaching L2. The cruise time to L2 is approximately 100 days after the lunar phasing loops are completed. The launch window for this trajectory is ~20 minutes/day for 7 consecutive days twice each month. Once in orbit about L2, the satellite maintains a Lissajous orbit such that the WMAP-Earth vector remains between 1 and 10 degrees off the Sun-Earth vector to satisfy communications requirements while avoiding eclipses. Station-keeping maneuvers are required about 4 times per year to maintain the orbit. WMAP Trajectory to L2:

Science

Comming Soon

Introduction:

The most prominent feature of the WMAP spacecraft is a pair of back-to-back telescopes that focus the microwave radiation from two spots on the sky roughly 140° apart and feed it to 10 separate differential receivers that sit in an assembly directly underneath the optics. Large ”elephant ear“ radiators provide cooling for the sensitive amplifiers in the receiver assembly. The bottom half of the spacecraft provides the services necessary to carry out the mission including command and data collection electronics, attitude (pointing) control and determination, power services and a hydrazine propulsion system. The entire observatory is kept in continuous shade by a large deployable sun shield that also supports the solar panels.

The WMAP instrument consists of a set of passively cooled microwave radiometers (connected to radiator panels with metal straps) with 1.4 x 1.6 meter diameter primary reflectors to provide the desired angular resolution. Measuring the temperature of the microwave sky to an accuracy of one millionth of a degree requires careful attention to possible sources of systematic errors. The avoidance of systematic measurement errors drove the design of WMAP:

• The instrument has five frequency bands from 22 to 90 GHz to facilitate separation of galactic foreground signals from the cosmic background radiation.
• WMAP is a differential experiment: if you would like to know whether one piece of wood is longer than another, it is better to put the pieces directly next to each other than to measure them separately with a ruler. WMAP measures the temperature difference between two points in the sky rather than measuring absolute temperatures.
• An orbit about the Sun-Earth L2 libration point that provides for a very stable thermal environment and near 100% observing efficiency since the Sun, Earth, and Moon are always behind the instrument's field of view.
• A scan strategy that rapidly covers the sky and allows for a comparison of many sky pixels on many time scales.

The WMAP spacecraft
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