III.C. Design

IRAS Explanatory Supplement
III. The IRAS Mission
C. Design

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  1. Basic Strategy
  2. The Second Six Months
  3. Scan Rate
  4. Strategy during South Atlantic Anomaly Passage
  5. Moon and Jupiter Avoidance Strategy
  6. Strategy of Attitude and Photometric Calibration
  7. Realization of Survey Strategy
  8. Half-Orbit Constraint
  9. Lune Constant
  10. Hole Recovery Strategy
  11. Pre-Survey Observations

C.1 Basic Strategy

The basic time interval of the operations and data acquisition was the time between station passes, varying between seven or eight orbits or 10 to 14 hours. During each such period, the survey strategy discussed below was implemented in a series of commands sent to the satellite called a Satellite Observation Plan, hereafter denoted as SOP. There were two SOPs per day and 600 SOPs in the entire mission.
Figure III.C.1 Lunes are sketched on the celestial sphere. Shaded zone is not allowed by Sun constraint.
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The strategy developed to achieve the goals of multiple survey coverage on various timescales divided the celestial sphere into units of half overlapping "lunes" in the ecliptic coordinate system (Lundy 1984). Lunes were defined as the area between two ecliptic meridians 30° apart (Fig. III.C.1). The lunes were "painted" by survey scans, one after the other, as they passed through the viewing window of the telescope. Figure III.C.2 illustrates the shape of a lune sketched onto a plane and, with a very exaggerated focal plane width, how it was covered by different survey scans each at a fixed cone angle from the Sun. The first scan in a lune was placed so that it crossed the ecliptic at the lower longitude boundary of the lune.
Figure III.C.2 Half-overlapping scan swaths (highly exaggerated) fill a lune with one hours-confirming layer.
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Successive scans were laid down at increasing ecliptic longitudes, each one shifted over by 14.23', that is by half the width of the focal plane minus a safety margin to account for the pointing limit cycle of the telescope. The overlap ensured that measurements of the same area of sky were repeated within a few orbits (for hours-confirmation) and that the "banana effect" (Section III.B.8) was not too severe. The criterion for hours-confirmation was that the hours-confirming scans had to be made within three SOPs (34-38 hours) of each other.
Figure III.C.3 A typical days survey coverage (clear regions at approximately ecliptic longitudes 167° and 347°) is plotted on a map of the sky in ecliptic coordinates. The two shaded regions centered on ecliptic longitudes 90° and 270° are forbidden by the Sun and Earth constraints.
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Two lunes in opposite hemispheres were observed simultaneously, one on the ascending side of the orbit and one on the descending side. Figure III.C.3 shows a typical day's survey coverage and the regions forbidden by the constraints. After a lune was filled, a second lune in the same hemisphere was started; it overlapped half of the first lune, ensuring that another hour's-confirming set of scans was repeated after about one to two weeks, thus providing the required repetition on the time scale of weeks (Fig. III.C.4).
Figure III.C.4 The lune coverage scheme is shown for the first two sets of hours-confirming scans. Each line represents a single hours-confirming coverage of an entire 30° lune (except short lines at beginning and end which represent half-lune coverage).
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The observing conditions were considerably worse during some orbits than during others. Orbits that crossed the SAA or contained a station prime pass were interrupted for significant amounts of time. When a long interruption occurred, the continuation of a survey scan was sometimes impossible and a small hole in the coverage resulted. To minimize such events only the nine (out of 14) orbits per day least affected by the SAA were used for the survey scans
Figure III.C.5 A zone of half-circle scans is drawn on the celestial sphere.
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The survey strategy aimed for four coverages (two sets of hours-confirming coverages) in the first six months and two coverages (a third set of hours-confirming coverages) in the second six months. Any time left over was used to recover survey observations lost because of the various constraints, to make the necessary calibration observations and to carry out additional, non-survey, pointed observations to attain higher sensitivity or spatial resolution. For the first two weeks of observations (February 10-23, 1983, SOPs 31-57) half circle scans, which give redundant coverage in the ecliptic polar areas (Fig. III.C.5), were used for increased initial coverage as insurance against an early failure of the satellite. Subsequently, the more efficient lune method was used for the rest of the first six months of the survey.

C.2 The Second Six Months

The first sky survey was completed on August 26, l983, with nearly a full second six months of operations expected. The survey was continued, but aimed at a coverage by only one set of hours-confirmed scans and using seven orbits per day. This plan covered many of the large number of small regions that the survey had missed for reasons explained elsewhere in this chapter, without the time consuming necessity of covering each hole separately, and also improved the survey's completeness for weaker sources (signal-to-noise ratio 5-l0). Another important goal was the coverage of a 5° gap left in the first six months' survey (Section III.D.5). This latter goal was never achieved because the helium supply was exhausted earlier than expected. Half circles rather than lunes were used in the second six months because the additional coverage in the ecliptic polar regions in August and September filled in regions that would be inaccessible later due to eclipses and a severe Earth infrared radiation constraint in December and January.

C.3 Scan Rate

Using two gyros, the spacecraft clock-angle (X)0 (Fig. III.B.7) was decreased at a rate of (3.85 / cos( 90 - ))' per second (which resulted in scanning the sky at a rate of 3.85' per second independent of ), i.e. 10% faster than the orbital rate of the satellite of 3.5' per second. This increased rate gave increased pointing flexibility with respect to the constraints and, in particular, helped to reduce the effects of the SAA.

C.4 Strategy during South Atlantic Anomaly Passage

Survey observations were interrupted for up to l4 minutes during SAA passages even after selection of the least affected orbits. When the telescope entered the SAA during a survey scan its scanning motion was halted and it remained pointing at the same point on the celestial sphere until it emerged from the SAA when the scan was restarted with an overlap. During the SAA passage, "bias boost" was applied to the 60 and 100  µm detectors until about three minutes before exit. The internal reference sources were flashed just before entry and just after exit at the end and beginning of survey scans (Section III.B.3). Some SAA passages required so much time that the telescope would have violated the Earth infrared constraint (Section III.B.4) after SAA passage. In such cases it was necessary to leave a hole in the sky coverage and try to recover it with specially prepared short scans in the preceding or succeeding pair of SOPs. The failure to recover all of these holes contributed to the variable depth of sky coverage.

C.5 Moon and Jupiter Avoidance Strategy

Figure III.C.6.1 The first hours-confirmed coverage is overlaid on a map of the sky in ecliptic coordinates. The scans converge at the ecliptic poles. Small, equally spaced holes in the ecliptic plane are due to the lunar avoidance stragegy and represent gaps in the sky coverage used to generate the extended emission images. The point source survey was not affected by these gaps.
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Jupiter was avoided by stopping a scan when it reached within 1° of the planet and side stepping by 1° for 2° of scan before coming back to resume the original scan; hence the Jupiter avoidance procedure left a 2° square hole on the sky which needed to be recovered later in the survey. Figure III.C.6.1-3 clearly show the three square holes on the ecliptic at longitude approximately 250° (RA about 17 hrs). Each was left by one of the three sets of hours-confirming coverages. There is no residual Jupiter hole when the effect of all the coverage is added.
Figure III.C.6.2 Same as Figure III.C.6.1, except for the second hours-confirming coverage.
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To avoid the moon in the same way as Jupiter would have left extremely large holes which would have been difficult to fill. The policy adopted, therefore, was to stop the survey on the moon's side of the orbit for the approximately three days needed for the moon to pass through this approximately 40° avoidance region.
Figure III.C.6.3 Same as Figure III.C.6.1, except for the third hours-confirming coverage.
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C.6 Strategy of Attitude and Photometric Calibration

To refine the pointing after the slew to the beginning point of a scan, attitude calibrations were made using visual stars at the beginning and end of each scan. Star sightings near the middle of the scan, if possible, allowed any pointing drifts to be detected and corrected (Section V.B). Long scans proved most suitable as they offered the greatest probability of finding these calibration stars.

To check the photometric calibration the internal reference sources were flashed at the beginning and end of every scan, including scans broken by the SAA. The internal reference sources were themselves regularly checked against an astronomical reference source (Section VI.A).

C.7 Realization of Survey Strategy

The survey scans resulting from this strategy and the constraints were generated by a computerized scheduling program (Oord et al. 1981). As well as generating the survey scans, the program reported which pieces of sky were not covered due to a combination of constraints, so that scans could be generated separately for inclusion elsewhere in the same or other SOPs (MacDougall 1984). It was not always possible to generate these scans within the constraints and so holes were left.

C.8 Half-Orbit Constraint

The scheduling program constrained survey scans to stop at the end of a half-orbit at one of the ecliptic poles. A side effect of this half-orbit design was that on occasions when the satellite had looked back in its orbit to cover a region of sky whose observation had been interrupted by a long SAA passage (Section III.C.4), the scan was cut short by reaching the end of its half-orbit, even though no other constraints had been violated.

C.9 Lune Constraint

Full coverage with the lune method required that while a lune was being painted it had to contain the meridian at 90° from the Sun. Otherwise the ecliptic poles could not be reached and holes would result. The scheduling program used to generate the survey scans also required that = 90° lie within the lune. This geometry became a hard constraint on the lune strategy, and became relevant to the decision that led to the 5° gap (III.D.5).

C.10 Hole Recovery Strategy

A record was maintained of those regions of the sky that were not covered by the automatically generated scans due to constraint violations and as suitable opportunities arose, attempts were made to fill the holes (Lau and Wolff 1984; Lundy 1984).

C. 11 Pre-Survey Observations

Before commencing survey operations numerous checks were required to verify the health and safety of the satellite and to determine the best modes of operation. The cooled aperture cover was kept on the telescope for the first six days to allow sufficient time for contaminants carried up with the satellite to outgas and disperse so that they would not freeze on the cold optics when the cover was ejected. The eight days after cover ejection were used to test those aspects of the instrument that could not be tested with the cover on. This period was followed by a period of repeated surveying on a limited region of sky to verify the survey strategy and the data processing facilities (Section VIII.D; Rowan-Robinson et al., 1984). The scans of this "minisurvey" were hand-tailored for maximum efficiency in coverage. After all these tests had been completed and the problems they revealed had been resolved (MacDougall et al. 1984), the all-sky survey was started. The dates of these and other important events are given in Table III.C.1.

Mission Chronology
Table III.C.1
(All dates are 1983 and given in GMT)
26 Jan 1 Launch 02h 17m
26-31 Jan 1-12 In Orbit Checkout (Cover on)
Outgassing of satellite
31 Jan 12 Cover ejection 19h 37m
31 Jan-8 Feb 12-28 In Orbit Checkout (Cover off)
9 Feb 29 SAA contour A (Fig. III.B.6) usage begins
9-10 Feb 29-30 Minisurvey layer 1 Hand made scans
10 Feb 31 Start first two hours-confirming
coverages using half circles.
11-12 Feb 33-34 Minisurvey layer 2 Hand made scans
13-14 Feb 37-38 Minisurvey layer 3 Hand made scans
15 Feb 41 Minisurvey layer 4a Hand made scans
16 Feb 43 Minisurvey layer 4b Hand made scans
23 Feb 57 Half circle method ended
23 Feb 58 Lune method started
3 Apr 135 Moon avoidance radius lowered
from 25° to 20°
9 May 207 SAA contour B (Fig. III.B.6) usage begins
26 Aug 425 End of first two hours-confirming coverages
26 Aug 426 Start third hours-confirming coverage
26 Aug 426 Moon avoidance radius lowered from 20 to 13°
9 Sep 454 Moon avoidance radius raised from 13 to 20°
18 Nov 593 First eclipse. Fallback to safety mode
21 Nov 600 Survey operations resumed 19h 40m
22 Nov 600 Liquid helium ran out 00h 16m
22 Nov 600 Last survey scan started 03h 34m
23 Nov 603 12  µm detector baselines saturated 09h 30m

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