1. VIRTIS Operations at Lutetia
VIRTIS Team
VIRTIS Science Team Meeting
Paris May 2010

2. VIRTIS Observations Summary
Summary of the required observations:
Light Curve; this is performed during the approach phase up to about 1hour before closest approach. During this phase the angular size of the object varies from about 75rad up to 2000 rad. Only VIRTIS-M operates.
Scanning Imaging; From CA -1800s to CA-500s relative speed between S/C and asteroid is low enough to allow hyperspectral imaging with VIRTIS-M with its internal scan mirror..
Pushbroom Imaging; at closer range the speed is too large to allow scanning; both V-M and V-H will be operated in pushbroom mode. A strategy similar to the Steins cooperative pointing (with ALICE and MIRO) can be adopted.
After CA due to large phase and low night temperature, observations shall be limited to about 20min.

3. Timeline

4. General
First Light curve: a S/C offset in the Y direction as been introduced to avoid the damaged region of the filter
Second Light curve: during this LC we must consider the S/C flip performed between 11:34 and 12:14; this could imply some data loss.
Additionally, due to the instability of the Reaction Wheel B of the S/C, there could be the possibility that the fly-by is carried out on 3 RW. If this is the case and if the S/C incur in an emergency safe condition then there will be a general restart of operations just after the S/C flip.

5. Extract from Mission Operation Report #134 april 2010

6. Pixel Size Vs time during the approach phase
Stares 16 mrad off Nadir looking
16 mrad
The Second Light Curve will continue until 14:42 (start of) Asteroid Fly By Mode

7. Composite spectrum of Lutetia
As an input to calculate the expected signal from VIRTIS we have used a compilation of spectra taken from several authors: Rivkin et al., 2000 and Birlan et al, 2004 (other contribution from Lazzarin et al, 2002; Bell et al, 1998; Nedelcu et al 2008 are consistent with these ones)
the observed reflectances have been scaled to an albedo of and resampled on the VIRTIS bands.
In the region 3.5 to 5 micron a constant value has been adopted

8. Lutetia Radiance VIS Channel
From the composite spectral reflectance shown before and the following parameters:
Phase 10°
Distance from Sun 2.71AU
Target Diameter 100km
We have calculated the radiance at the instrument in the VIS and IR channels
Two extreme surface temperatures were taken into account
IR Channel
T = 170K
IR Channel
T = 220K

9. First Light Curve VIRTIS-M VIS Signal evaluation
Signal at start of First LC
(Lutetia sub-pixel (0.36))
Signal at end of First LC
(Lutetia sub-pixel (0.58))

10. First Light Curve VIRTIS-M IR Signal evaluation
T=200K
Signal at start of First LC
(Lutetia sub-pixel (0.36))
T=200K
Signal at end of First LC
(Lutetia sub-pixel (0.58))

11. Light Curve VIRTIS-M IR Signal evaluation as a function of T
T=170K
T=220K

12. First Light Curve: Operative Parameters
15 lines for each acquisition (+2dark)
5 s repetition >> 85s >> punti
10 s repetition >> 170s >> punti
20 s repetition >> 340s >> punti Selected
Lutetia rotates with angular speed of deg/s
With 20s repetition we get one LC point every 4.25deg
Compression wavelet 2; factor 8
Binning Mode
Modo reduced slit: 3x1
Modo all pixel : 1x1 Selected
Integration Times 4-5s VIS e 2s IR
Verify Temperature increase of CCD due to the selected integration time

13. Parameters for ITL First Light Curve
20s repetition
15lines (start angle and stop angle need offset)
Dark frequency one every 15 acquisitions
Wavelet 2 compression
Mode full window (1x1)
8 hours operations start 09 July 19:00 stop 10 July 03:00
DATA VOLUME  660Mbit = 82.5MBytes
Integration times 4s for VIS and 2s for IR
Every hour stop acquisition to limit size of cube: we shall get 8 cubes in total

14. Second Light Curve VIRTIS-M VIS Signal evaluation
Signal at start of Second LC
(Lutetia sub-pixel (0.75))
Signal at end of Second LC
(Lutetia 4pixels)

15. Second Light Curve VIRTIS-M IR Signal evaluation
T=220K
Signal at start of Second LC
(Lutetia sub-pixel (0.75))
T=220K
Signal at end of Second LC
(Lutetia 4pixel )

16. Second Light Curve: Operative Parameters
15 lines for each acquisition (+2dark)
5 s repetition >> 85s >> punti
10 s repetition >> 170s >> punti
20 s repetition >> 340s >> punti Selected
Compression wavelet 2; factor 8
Binning Mode
Modo reduced slit: 3x1
Modo all pixel : 1x1 Selected
Integration times 1.5s VIS e 1s IR

17. Parameters for ITL Second Light Curve
20s repetition
15lines (Alice Narrow Slit Boresight)
Dark frequency one every 15 acquisitions
Wavelet 2 compression
Mode full window (1x1)
8h 33min operations: start 10 July 06:09 stop 10 July 14:42
DATA VOLUME  700Mbit = 87.5MBytes
Integration times 1.5s for VIS and 1s for IR
Every hour stop acquisition to limit size of cube: we shall get 8 cubes in total

18. Internal Scan – First part
The reason for the scan rely on the absence of longitudal motion between 15:00 and 15:40, this suggest to use an internal scan to cover the full area of the asteroid.
On the other hand as we approach the asteroid the pixel resolution increases; this imply that the maximum number of acquisition per scan shall be limited by the change in resolution. This should be less than about 15%.
The S/C will point with the Cooperative Boresight (a Z offset that satisfy VIRTIS-H, MIRO and Alice requirements).
VIRTIS-H will start operating from this point.
In the first part from 15:15 to 15:30 we shall perform full surface scans.

19. Internal Scan – Second Part
After 15:30 we shall not be able to image the full asteroid without introducing distortion in the resulting image, change of resolution from the first to the last acquisition of each scan.
But the Longitude variation shall still be minimal until about 300s before CA.
We have then introduced a second scan phase, limited to a portion of the asteroid surface, limited to a scan of 10lines
This will have the additional advantage that we could properly sample the zero phase passage.
This phase will last from 15:32 to about 15:43 (about 450s before CA)

20. Lutetia S/C footprint over surface

21. The sub S/C longitude doesn’t vary considerably until 300s before CA;

22. To keep the resolution variation across one image scan image less than 15% we split the scans in two parts
From 2250 to 1250 seconds before CA: full surface scans
From 1250 to 450 seconds before CA: limited surface scans

23. Parameters for ITL Internal Scan Phase First Part
First 7 scans (continuous scan; produces a single cube)
5s repetition
Integration times 1.5s for VIS and 1s for IR
20lines
Dark frequency one every 20 acquisitions
Wavelet 2 compression
Total duration 750s
Mode full window (1x1)
DATA VOLUME each scan 70Mbit = 8.75MBytes
Scan 8 and 9
30 lines
Total duration 315s
DATA VOLUME each scan 16Mbit = 2MBytes

24. Parameters for ITL Internal Scan Phase Second Part
Scans 10 to 18 (continuous scan; produces a single cube)
5s repetition
Integration times 1.5s for VIS and 1s for IR
10lines
Dark frequency one every 20 acquisitions
Wavelet 2 compression
Total duration 475s
Mode full window (1x1)
DATA VOLUME each scan 45Mbit = 5.5MBytes
VIRTIS-H will perform backup and nominal mode acquisitions during this time

25. Pushbroom Scan
During this part of the sequence, VIRTIS-M will operate in pushbroom mode, relying only on the S/C motion over the surface.
Both VIRTIS-M and H will operate.
Between 450s and 150s there will be oversampling the instrument ground resolution being larger than the relative shift of the sub S/C point on ground.
During the 300s around CA on the contrary the S/C motion shall be faster than the instrument resolution giving undersampling conditions (See next slides)
This phase shall last from 15:45 until 16:00

26. CA Operations Ground Resolution and Surface track
The S/C is at distance D from the asteroid, and its track moves of angles  and  in one repetition time over the asteriod surface;
We assume Virtis Slit perpendicular to S/C ground track;
Ground Resolution = D * IFOV
S/C at t=0
S/C at t=5s
S/C track surface projection

27. OVERSAMPLING
Ground track shift over 5s
Pixel Resolution
At ±2min around CA there will be coverage gaps between subsequent acquisition, due to the combined S/C motion and repetition time

28. Phase angle variation during pushbroom phase

29. High Resolution VIRTIS-M VIS Signal evaluation
VIS Channel

30. High Resolution VIRTIS-M IR Signal evaluation
IR Channel
T=170K
IR Channel
T = 220K

31. Thermal emission signal evaluation

32. Parameters for ITL Pushbroom Phase
Continuous scan from 380s before CA until 520s after CA
5s repetition
Integration times 1.5s for VIS and 1s for IR
No scan mirror motion
Dark frequency one every 20 acquisitions
LossLess compression
Total duration 900s
Mode full window (1x1)
DATA VOLUME each scan 330Mbit = 41MBytes