ISO's operational orbit had a period of just under 24 hours, an apogee height of 70600 km and a perigee height of 1000 km (see Figure 4.1). The lower parts of this orbit were inside the Earth's Van Allen belts of trapped electrons and protons. Inside these regions, ISO's scientific detectors were virtually unusable due to effects caused by radiation impacts.
The 24 hours satellite orbit can be broken down into six phases:
After perigee passage VILSPA re-acquired communications with ISO. There was an activation phase, not usable for scientific activities, when the spacecraft was re-configured, all instruments were switched on and any necessary detector curing took place. These activities continued until the VILSPA science window opened, approximately four hours after perigee. This occurred when ISO was outside the radiation belts and the instruments could operate normally without being affected by the particles in the radiation belts.
Normal operations, including scientific observations, then occurred for approximately the next nine hours.
Before ISO dropped below the VILSPA horizon, communications were handed over to Goldstone. During this time, lasting a minimum of 15 minutes, instrument re-curing took place, e.g. for ISOPHOT. Because observations could not be performed during this handover period the maximum length of an observation was limited to one of the two periods of normal science operations. While the length of these varied (such that the sum remained the same), generally the VILSPA science window was the longer. Normal observing was then continued from Goldstone.
Finally, as ISO neared the radiation belts approximately twenty hours after perigee, observations ended, the instruments were de-activated and the satellite was configured for perigee passage. Goldstone then lost the spacecraft's signal as it approached perigee.
At the beginning and end of revolutions, the spacecraft was outside the control of the normal pointing system even though scientific data were routinely still being collected. Instead, a number of procedures were used grouped under the name of Programmable Pointing Mode (PPM) intended to avoid violation of Sun and Earth constraints. The PPM ensured that the spacecraft's attitude was maintained with sufficient accuracy for observations to be resumed without need for further calibration.
Prior to launch, assumptions were made as to the detrimental effects on instrument sensitivity of impacts from high energy particles and how these effects would vary with position on the orbit. These models and calculations led to definition of a cut-off altitude of 43000 km, corresponding to a daily science window of 16 hours. During PV phase however, it became apparent that scientifically useful observations could continue longer than expected. Therefore the science window was extended by 40 minutes from revolution 66 onwards (i.e. approximately 3/4 of the way through PV). It was later found that LWS observations carried out late in the science window were affected by charged particles, the extent of the effect depending on the season. Therefore from revolution 204 onwards the LWS switch-off was rolled back 60 minutes.
For thermal (protecting the spacecraft from sunlight) and power
(illuminating the solar arrays) reasons and also to prevent straylight
from reaching the instruments, there were constraints on the allowed
pointing directions for the satellite. ISO always pointed only in a
direction between 60
and 120 away from the Sun.
Additionally, it was neither pointed closer than 77 to the
Earth limb, nor closer than 24 to the Moon. Jupiter was
usually kept away from the optical axis by at least 7 unless,
of course, Jupiter or one of its moons was the target of an
observation. The sum of these constraints meant that, typically, only
some 10-15% of the sky was available to ISO at any instant (see
Figure 4.2).
The orbit also precessed rather slowly. Thus, in the nominal 18-month long mission, there would have been an area of sky, centred on the Taurus-Orion region, inaccessible to ISO. Happily, the cryogen lasted longer than specified and almost all the sky was visible to ISO at some time during the mission.
