# Presentation: 111081

## Utilization of the Aura HIRDLS Spacecraft Pitch Manoeuvres

**Utilization of the Aura HIRDLS Spacecraft Pitch Manoeuvres**
**J. Barnett*, J. Craft**, J. ****Gille**, H. Lee**, C. Halvorson**, B. Nardi**, T. Eden**, C. Hepplewhite***
-           *Atmospheric, Oceanic, and Planetary Physics, Oxford University
-         **Center for Limb Atmospheric Sounding (CLAS), University of Colorado at Boulder

**Abstract**
**During the Aura mission to-date there have been seven Aura spacecraft pitch manoeuvres and an eigth is due in early November 2006. These manoeuvres pitch up the HIRDLS telescope boresight so that the background signal of space is seen over the complete scan range. The spatial and temporal dependent radiance signal from the 21 HIRDLS detectors during the manoeuvere is used to characterize the emission from the obstruction in the telescope and validate the correction method adopted. In the past this has helped in the selection of optimum scan patterns used for the normal science acquisition mode. The salient attributes of the pitch are presented and the main aspects of the data and how they are used is described.**

**Pitch Manoeuvres to date**
**1     2004/344  16:41:02 to 20:07:00   to -5.25 deg**
**2     2005/040  17:45:32 to 21:45:00   to -5.25 deg**
**3     2005/194  14:30:32 to 22:27:00   to -5.25 deg**
**4a   2005/306  12:38:55 to 14:25:00   to -2.62 deg**
**4b   2005/306  15:55:32 to 21:50:00   to -5.25 deg**
**5a   2006/053  12:21:55 to 14:07:00   to -2.62 deg**
**5b   2006/053  17:20:32 to 22:53:00   to -5.25 deg**
**6     2006/143  13:27:32 to 15:14:00   to -5.25 deg**
**7a   2006/191  13:23:55 to 15:05:00   to -2.62 deg**
**7b   2006/191  16:42:32 to 21:42:00   to -5.25 deg**
**(times are for when the spacecraft is fully pitched)**

**Constraints**
-  Sunlight should not expose dark (+Y) or Earth facing (+Z) sides of satellite to sunlight
-      - no problem.
-  Attitude rates and accelerations must not be excessive, to maintain pointing knowledge
-      - leads to slew times of approx 4 and 5.5 minutes to -2.62 and -5.25 deg   respectively.
-  Real time data available for monitoring slews  -  Tracking and Data Relay Satellites
-      enable this at any point in the orbit, but competition for time can be a problem.
-  Moon must not be in HIRDLS atmospheric view at any point during the orbit
-      - a few orbits on 2 days per month excluded
-  Moon must not be in HIRDLS space reference port - a few days per year excluded.
-  The slews out and back give useful data so preferably located at orbit positions of slow
-      temperature change -  slew out is preferred a few minutes after sun set.

- Various temperatures on Day 143 2006. The arrows show the duration of the pitch manoeuvre which was to -5.25 deg. and lasted for 1.1 orbits. The six large peaks occur at sunset on successive orbits. The orbit cycles repeat accurately but small differences can be seen for the pitched orbit.

**Pitch manoeuvres involve tilting the spacecraft in pitch by -2.62 or -5.25 deg so that the HIRDLS tangent heights at the end of the scan where there is a partial view of the atmosphere are well above the levels for which there is detectable atmospheric emission. This is used to perform a zero radiance calibration at angles which normally never receive zero radiance and is also used to calculate the fraction of beam area obstructed.**
**The pitch angle is a compromise –**
***Too small***** a tilt and some channels will receive atmospheric emission at the bottom of the scan  (for the HIRDLS view angle a  -5.25 deg pitch rotation moves the scan up by approximately 190 km).**
***Too big***** a tilt and the instrument thermal balance will change enough to affect the behaviour.**
**A smaller tilt of -2.62 deg is also employed as a check on the change on thermal balance (some channels see the atmosphere for part of the time) because a small change attributed to thermal balance is found.**
**The instrument thermal behaviour varies:**
**i)   strongly around the orbit – necessitates at least one orbit of data for a given scan sequence**
**ii)  moderately with beta angle (angle between the orbit plane and the vector to the sun)  – need measurements for different times of the year.**
**iii)  weakly from orbit to orbit due to variations in reflected sunlight and emissions from the earth  and clouds – can be corrected for using spaceview radiances (from high altitudes).**

- Example Pitch Manoeuvre scan containing an azimuth raster plus atmospheric elevation scans. Several different scan types are normally used for a manoeuvre including the standard sequence used for taking standard science data. Science scans are made at -23.5 deg azimuth shaft angle, which gives -47 deg view azimuth view direction.

**Above**: Channel 18 standard science scan data (repeated vertical scans through the partially clear aperture at -47 deg azimuth view direction). The data have been high pass filtered to show oscillations caused by the obstruction.  1.8 orbits are plotted, time up the plot, but spacecraft pitched at -5.25 deg for ~49000-55000 sec., i.e. approximately the central 1 orbit. Time relative to bottom of scan is plotted horizontally. The time between about -4 and +4 secs corresponds to channel 18 viewing the atmosphere for normal orientation. During this time the atmospheric radiances mask the oscillations, but the correction process needs to function accurately. While pitched, the oscillations can be tracked all the way across (they undergo various amplitude changes), so correction methods can be devised and checked.
**Right:** As above but for channel 5. The atmosphere is much more opaque for this channel, but the high opacity leads to smooth radiances at low altitudes and the oscillations again become visible but noisy in normal orientation. Where visible, the oscillations closely match those for channel 18.
- Data is for day 143 2006. Scan pattern is 50601, standard geophysical scan.

**Uses of pitch manoeuvre data:**
- Pitch manoeuvre data are used for three distinct purposes:
**1) Development and validation of deoscillation algorithms**
- As the scan mirror moves it triggers oscillations in the optical blockage which cause oscillating radiance offsets. These are in-phase on all channels because they share a common input beam.  Certain channels (notably 18) provide a means to observe these, measure their behaviour in real time, and provide a basis for extrapolating below the level where the channel receives radiation from the atmospheric scene. However these algorithms need to be developed and validated using data with zero scene radiance, which is only obtained for all necessary elevation angles if the spacecraft is pitched.
**2) Modelling Radiometric zero**
- The blockage adds radiance to the signal to be measured from the atmospheric or space scene in proportion to the obstructed area and the Planck function of the material, both of which vary with mirror angles. The gross effect of Planck function variations can be allowed for by subtracting the signal when viewing space at the top of the scan, but vertical variation must be modelled and predicted, partly based on gradients over the space part of the scan, and also using data obtained during pitch manoeuvres.
**3) Derivation of the unobstructed area fraction.**
- Once the blockage emission has been subtracted, the signal still needs to be increased (divided by the unobscured fraction) to allow for the diminished beam area which views the scene. Various methods have been used to derive this area (which varies with mirror position and channel), but these rely upon using radiance signals which include known scene emissions (e.g. zero when pitched). As with (2) the most important mirror angles are those where the atmosphere is in the view, so only a pitch manoeuvre will cause those angles to view space.