  We present detailed predictions for the confusion noise due to extragalactic
sources in the far-IR/(sub)-millimeter channels of ESA/ISO, NASA/Spitzer,
ESA/Herschel and ESA/Planck satellites, including the contribution from
clustering of unresolved SCUBA galaxies. Clustering is found to increase the
confusion noise, compared to the case of purely Poisson fluctuations, by 10-15%
for the lowest frequency (i.e. lowest angular resolution) Spitzer and Herschel
channels, by 25-35% for the 175 micron ISOPHOT channel, and to dominate in the
case of Planck/HFI channels at nu>143GHz. Although our calculations make use of
a specific evolutionary model (Granato et al. 2004), the results are strongly
constrained by the observed counts and by data on the redshift distribution of
SCUBA sources, and therefore are not expected to be heavily model dependent.
The main uncertainty arises from the poor observational definition of the
source clustering properties. Two models have been used for the latter: a
power-law with constant slope and a redshift-independent comoving correlation
length,r_0, and the standard theoretical model for clustering evolution in a
LambdaCDM universe, with a redshift-dependent bias factor. In both cases, the
clustering amplitude has been normalized to yield a unit angular correlation
function at theta_0=1-2 arcsec for 850 micron sources fainter than 2 mJy,
consistent with the results by Peacock et al. (2000). This normalization
yields, for the first model, r_0=8.3$ Mpc/h, and, for the second model, an
effective mass of dark matter haloes in which these sources reside of
M_halo=1.8*10^{13} M_sun/h. These results are consistent with independent
estimates for SCUBA galaxies and for other, likely related, sources.
