  In this paper we discuss the issue of loosing telemetry (TM) data due to
different reasons (e.g. spacecraft-ground transmissions) while performing a
full-sky survey with space-borne instrumentation. This is a particularly
important issue considering the current and future space missions (like Planck
from ESA and WMAP from NASA) operating from an orbit far from Earth with short
periods of visibility from ground stations. We consider, as a working case, the
Low Frequency Instrument (LFI) on-board the Planck satellite albeit the
approach developed here can be easily applied to any kind of experiment that
makes use of an observing (scanning) strategy which assumes repeated pointings
of the same region of the sky on different time scales. The issue is addressed
by means of a Monte Carlo approach. Our analysis clearly shows that, under
quite general conditions, it is better to cover the sky more times with a lower
fraction of TM retained than less times with a higher guaranteed TM fraction.
In the case of Planck, an extension of mission time to allow a third sky
coverage with 95% of the total TM guaranteed provides a significant reduction
of the probability to loose scientific information with respect to an increase
of the total guaranteed TM to 98% with the two nominal sky coverages.
