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.
Learn more about SPORT:
SPOrt will perform direct measurements of the Q and U Stokes parameters. Latest design specifications are:
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).
|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.
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.
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.