SPOrt (Sky Polarization Observatory)


SPOrt is an Astrophysical Project aimed at observing the polarization of the sky in the microwave range 20-100 GHz, with angular resolution of 7°. Primary goals are:

SPOrt is carried on under the scientific responsibility of an International collaboration of Institutes headed by the IASF-CNR in Bologna and is fully funded by the Italian Space Agency (ASI).

It has been selected by ESA to be flown on board the International Space Station (ISS) for a minimum lifetime of 18 months starting in 2005.

The payload
Learn more about SPORT:

Instrument Design

SPOrt will perform direct measurements of the Q and U Stokes parameters. Latest design specifications are:

SPOrt main features (sensitivities, in TA, refer to 2 years of observation)
Freq
(Hz)
Channels
 
BW
(%)
FWHM
(°)
Sky Coverage
(%)
Npix
 
sigma1s
(mKs½)
sigmapix
(µK)
22 1 10 7 80 660 0.5 1.6
32 1 10 7 80 660 0.5 1.6
90 2 10 7 80 660 0.57 1.8
Sensitivity to CMBP in termodinamic temperature
sigma1s (mKs½) sigmapix (µK) sigmaprms (µK)
0.53 1.7 0.15

The low level of the expected CMBP signal requires optimization of the instrument design with respect to systematics generation, long term stability and observing time efficiency. The following recipe was thus followed while designing SPOrt:

Schematic block diagram of the SPOrt radiometers. The antenna collects the signal, then the polarizer and the OMT extract the two circularly polarized components of the incoming radiation. After amplification, the signals are correlated by the correlation unit providing direct values of Q and U

The instability of a radiometer can be measured in terms of the knee frequency (fknee). This provides the time scale at which the 1/f component of the noise power spectrum prevails on the white noise. Destriping techniques can remove most effects of the 1/f noise, provided the knee frequency is lower than the signal modulation frequency. For SPOrt this corresponds to the ISS orbit frequency forbit=1.8·10-4 Hz. Thanks to the hardware development done by the SPOrt team an offset value as low as Toffset~50 mK is guaranteed, which translates into a knee frequency fkneelna~2.5·10-5 (given our Tsys~100 K).

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Scientific goals

The CMB is a powerful tool to understand origin and evolution of the Universe. It looks like a Black Body at 2.725 K, almost isotropic and unpolarized. Deviations from this ideal behaviour are expected from cosmological models, their measurement allowing to estimate cosmological parameters.
Very small temperature anisotropies have already been detected at both large and small angular scales (DT/T in the order of 30µK). The first detection of CMB Polarization (CMBP) at degree angular scales has recently been claimed (Kovac et al. 2002), though contamination from synchrotron radiation has not been excluded with high confidence. The CMBP predicted level is very low (few µK on sub-degree scales and <1 µK at large scales).

Temperature anysotropy and E-mode power spectra for two cosmological models (Omegalambda = 0.7) differing only in the re-ionization optical depth tau. The information carried by CMBP can solve degeneracies among cosmological parameters that CMB anisotropy alone is not able to remove. In particular, as shown in the picture on the left, the polarization E-mode power spectrum is much more sensitive to the optical depth of the re-ionized medium than the T spectrum, this new information being found on large angular scales (multipole l<20, or theta>10°).

At subdegree angular scales CMBP measurements allow tests of the inflationary model. In fact, this predicts a well defined Doppler peak pattern in the T and E power spectra. Precise measurements of CMBP can also allow the separation between scalar and tensorial components in the primordial fluctuations, providing a way to disentangle among different inflationary models.

Our Galaxy is featured by a smooth linearly polarized background emission, carrying information on the Galactic structure. Besides its intrinsic interest, the Galaxy acts as a foreground for CMB experiments: its accurate study is necessary for clean measurements of CMB features.

Up to now polarized surveys have been carried out only at frequencies up to 2.7 GHz, where the Galactic emission appeares to be dominated by synchrotron. However, these surveys are either widely undersampled or restricted to narrow stripes around the Galactic Plane. Sky coverages of existing continuous polarization surveys are shown on the left in Galactic coordinates.

The picture on the left shows the expected polarized brightness temperature of the Galactic foregrounds relevant for CMBP studies, togheter with the CMBP behaviour predicted by a model with Omegalambda=0.7 and tau=0.1, evaluated on 7° angular scale. The synchrotron normalization has been obtained by extrapolating the Galactic background emission from the Duncan et al. data. Assuming a polarization fraction of 30%, the COBE-DMR unpolarized data would provide similar values.

SPOrt will be able to produce Galaxy maps at both 22 and 30 GHz. However, its pixel sensitivity does not allow the building of CMBP maps. According to our studies, full-sky statistical analyses (e.g. Maximum Likelihood flat spectrum) including foreground subtraction should allow a measurement of the mean polarized signal Prms = (<Q²+U²>)½ with expected error sigma(Prms)=0.15 µK (1 sigma C.L) after two years of operation.

SPOrt capabilities in determining the optical depth of the re-ionized medium (tau) have been evaluated by a Fisher matrix analysis providing a sensitivity of 0.05 for models with tau in the range 0.05-02 (two years of operation).
The mean polarized signal on 7° scale versus the optical depht tau is shown in the picture on the left for various cosmological models.

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Observing Strategy

Differently from balloon and ground-based experiments, which are limited to observations of small sky patches, SPOrt is a space mission covering about 80% of the sky and thus sensitive to the large angular scales where CMBP carries new information with respect to CMB anysotropy (multipoles l<10, or theta>20°).

The SPOrt scanning strategy is bounded by the motion of the International Space Station (ISS), whose orbit is tilted by 51.6° with respect to the Celestial equator and is characterized by an orbit period of 5400 s.
The SPOrt antennae, looking at the zenit, will cover the sky under precessing circles intersecting each other and will produce a map of the accessible sky every 70 days.

A few sky scans are shown on the left (top) together with the pixel observing time (bottom), in seconds, corresponding to two years of data taking (the pixel size is about 7°, e.g. HEALPix parameter Nside=8).

The SPOrt mission is scheduled to last at least 18 months.

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Technological developments

Coming soon...

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Last updated: 18 May 2004, nicastro at pa.iasf.cnr.it