The COBE-DMR experiment was calibrated using the small dipole seasonal component due
to the Earth's revolution and the signal observed from the Moon. The absolute calibration
was fitted over the entire mission lifetime and checked on the short time scale using stable
noise sources (Bennett et al. 1992). In this work we have
shown that a second generation space mission for
high-resolution imaging of the CMB can rely for calibration on
the COBE measurement of the CMB dipole, making the non-trivial
implementation of an active calibration system not necessary.
We find that even
with a very conservative approach to remove the effects of
Galactic contamination, the COBRAS/SAMBA Low-Frequency Instrument
can be calibrated with 
accuracy (i.e. limited by the DMR measurement of the dipole) with a frequency of 10 to 20 days. In reality, more
efficient removal of the Galactic emission will be possible due to the multi-frequency
nature of the experiment, leading to more frequent and precise calibration.
Measuring the calibration over 1 year of observations, would
lead to extremely small statistical errors (
)
even with strict values for the
threshold 
, but the ultimate accuracy will be limited by
the 
a-priori uncertainty.
This uncertainty does not affect the precision of
the dipole subtraction procedure and leads to
an uncertainty of a fraction of 
K in the estimate of the primary anisotropy.
This is adequate for the scientific goals of the proposed COBRAS/SAMBA mission.
However, an even greater accuracy can be achieved
for the long-term calibration
by exploiting the spacecraft orbital motion. The seasonal
proper velocity
is known a-priori with extreme accuracy, allowing
absolute calibration
of the maps to better than 0.2%.
The observation of external planets will provide, a few times per year, independent absolute calibration with 3 to 5 percent accuracy. This will be a useful independent cross-check of the primary absolute calibration based on the CMB dipole anisotropy. The accuracy of planetary calibration is subject to improvement if progress is made in measurements and understanding of the planets' microwave emission. In addition, the signal from the planets can be used to accurately map the shape of the system main lobes.
Thermal variations or intrinsic instrumental effects can perturb the stability of the gain and
of the instrument baseline. Short time-scale effects are efficiently diluted by the redundant
observation of the same sky pixels for several scans as seen from different detectors. In this
work we have concentrated on analyzing drift effects over several hours time scale, which
may cause striping and systematic errors in the maps.
Our simulations show that the stability of the calibration constant can be monitored
with 
accuracy over time scales of 
hours and that thermal drifts of the
instrument baseline (occurring typically on 
days time scale) can be controlled to
within 1%. These results ensure that instrumental systematic effects, if significant, can be
readily recognized from the data, and that no artifacts will appear in the final maps.
The accuracy with which one can remove such effects from the data depends strongly on the instrument sensitivity as well as on the observing strategy. In addition, a very stable thermal environment and a large thermal constant at the focal plane instrument greatly simplify the analysis of thermal effects, by making them linear over all the time scales of interest. In the case of COBRAS/SAMBA, these conditions are achieved by a proper choice of the orbit (Sun-Earth L2 provides the best possible thermal conditions) and a careful design of the thermal architecture of the payload.