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Navigation Camera for the Rosetta Orbiter
The ROSETTA Navigation Camera (NAVCAM) is a part of the ROSETTA Attitude and Orbital Control System (AOCS). There are two identical cameras - a nominal and a redundant one. The cameras' field of view is 5 degrees. They are equipped with CCD detectors comprising 1024x1024 pixels.
Objectives of the NAVCAM
In general, the Rosetta NAVCAM camera has three major functions:
- Track point-like targets (in Point Target Tracking Mode).
- Track one extended object, during asteroid fly-by and comet approach phases (in Asteroid Tracking Mode).
- Acquire navigation images of asteroids and the comet nucleus (Imaging mode, also performed in Asteroid Tracking Mode).
The ROSETTA NAVCAM Instrument
Each NAVCAM consists of a Camera Optical Head (CAM-OH), a Camera Electronic Unit (CAM-EU) and a Camera Baffle (CAM-BAF). The NAVCAM optical heads are located on the +Z S/C axis with each protruding from the main body of the Spacecraft.
Camera Optical Head (CAM-OH)
The Camera Optical Head for the Rosetta NAVCAM contains the optical system, the CCD detector and the electronics required to operate the CCD. It also supports the dust cover and attenuation mechanism in front of the optics. Only the optical system & CCD detector are described below.
There are three possible configurations of the optical system with an exchangeable first element:
- DNA mode. Defocused not attenuated. The first element is a
plano-parallel window. The aperture is 70mm. This mode is used for
determining the position of point sources with improved centroiding.
During the comet escort phase this mode is also used by default as a
dust cover when the camera is not operated.
Metadata keyword : ROSETTA:CAM COVER POSITION = DEFOC NATT
- FA mode. Focused attenuated. The first lens is a quasi-plano
parallel window, i.e. a lens with a very low converging power, in
order to focus the image on the CCD. One face has an attenuation
coating in order to decrease the transmission. The aperture is 30mm.
This mode is used for imaging and navigation close to the extended
Metadata keyword : ROSETTA:CAM COVER POSITION = FOC ATT
- FNA mode. Focused not attenuated. The first lens is a
quasi-plano parallel window with an aperture of 70mm. The image is
focused on the CCD and there is no attenuation coating.
Metadata keyword : ROSETTA:CAM COVER POSITION = FOC NATT.
For each acquired image the mission specific keyword ROSETTA:CAM COVER POSITION records the applicable cover position in the meta-information of the data product files. The effects of the reduced aperture and the attenuation coating combined result in a reduction of the transmission by a factor of 580 for the attenuated mode. For the defocused mode the point spread function is broader. For a point source located in the centre of a pixel approximately 50-55% of the signal are counted in the respective pixel, whereas this fraction ranges between 65 and 70% for the focused modes.
The CCD detector is a front-illuminated frame transfer device with a broad spectral sensitivity in the visible range. Two different values of the gain can be selected when commanding the camera.
- High Gain. Increases the grey signal level resolution when
faint targets are imaged. In this case the A/D Converter saturates
(at 12 bits) before saturation of the CCD is reached.
Metadata keyword : ROSETTA:CAM GAIN = HIGH
- Low Gain. The A/D Converter saturates at the same time as the
CCD and so the full dynamic range can be used.
Metadata keyword : ROSETTA:CAM GAIN = LOW
The chosen gain value is documented in the mission specific keyword ROSETTA:CAM GAIN in the product files.
Camera Baffle (CAM-BAF)
The Camera Baffle provides protection against stray light produced by the Sun and reflected from planetary bodies and the satellite. This level of protection allows the tracking of faint objects. The baffle is mechanically supported by the S/C structure so as to avoid mechanical stress of the optical head. This is done owing to the required high pointing stability of the camera boresight in order to achieve the desired accuracy.
Understanding the NAVCAM image Metadata fields provided with each image
IMAGE_FILENAME : <MISSION>_<CAM#>_<YYYYMMDDThhmmss>_P.<EXT>
<MISSION> = ROS
<CAM#> = CAM1 or CAM2
<YYYYMMDDThhmmss> = Date/Time
P = Processed
MISSION_NAME : INTERNATIONAL ROSETTA MISSION
INSTRUMENT_NAME : NAVIGATION CAMERA
DATA_SET_ID: <INST HOST>-<TARGET
ID>-<INST>-<CODMAC LEVEL>- <MISSION PHASE
<INST HOST> : RO for ROSETTA Orbiter
<TARGET ID> : A, C, E, M (Asteroid, Comet, Earth, Mars)
<INST> : NAVCAM
<CODMAC LEVEL> : Processing Level (2, 3, 5)
<MISSION PHASE ABBREVIATION> Mission Phase e.g. Prelanding (PRL)
<VERSION> : Version Number
DATA_SET_NAME : <INSTRUMENT HOST NAME> <TARGET>(
<OPTIONAL>) <INST> <CODMAC LEVEL> <MISSION PHASE
<INSTRUMENT HOST NAME> = ROSETTA-ORBITER
<TARGET> = 67P
<INST> = NAVCAM
<CODMAC LEVEL> : Data Processing Level
<MISSION PHASE ABBREVIATION> = Mission phase e.g. PRL-MTP003 (Prelanding & Medium Term Plan 3)
<VERSION> = Version number V1.0
PRODUCT_ID : <MISSION> <CAM#>
<MISSION> = ROS for ROSETTA
<CAM#> = CAM1 or CAM2
<YYYYMMDDThhmmss> = Date/time
IMAGE_TIME : Time in UTC that image was taken
OBSERVATION_TYPE : NAVIGATION IMAGE
MISSION_PHASE_NAME : Mission Phases & planning stage e.g. PRELANDING MTP003
TARGET_NAME : 67P/CHURYUMOV-GERASIMENKO 1 (1969 R1)
TARGET_CENTER_DISTANCE : Distance from target centre in km
CHANNEL_ID : CAM1
EXPOSURE_DURATION : Duration of exposure in seconds
PRODUCER_INSTITUTION_NAME : EUROPEAN SPACE AGENCY-ESAC
ROSETTA:CAM_COVER_POSITION : DEFOC_NATT
ROSETTA:CAM_GAIN : LOW