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Working with Mars Reconnaissance Orbiter CTX Data

CTX Observation (Orbit 1472)

CTX collects data simultaneously with the HiRISE camera and CRISM spectrometer. As the name suggests, CTX provides the wider context for the data collected by the other two instruments. Scientists use images from the other instruments to examine the details of Mars, but CTX allows a better understanding of the terrain that encompasses these details.

From an altitude of approximately 300 kilometers above Mars, CTX returns surface images that are 30 kilometers across, with pixels representing 6 meters of the Martian surface.

CTX Instrument Technical Details
  • Color: Grayscale
  • Typical Dimensions: 30km * 160km
  • Pixel Dimensions: 5064px wide, varying lengths
  • Focal Length: 350mm
  • FOV: 6 degrees
  • Image Sensor: 5064 pixels-wide charge coupled device (CCD) line array
  • Bands: 500-800nm

The team lead and supplier of CTX is Mike Malin from MSSS.

Reference & Related Resources

Processing CTX Data

CTX has one CCD, so processing its data is similar to other instruments. Making an uncontrolled CTX observation mosaic includes:

  • Level 0 - Data Ingestion
  • Level 1 – Radiometric Calibration and Noise Removal
  • Level 2 - Map Projection

Level 0 Processing - Data Acquisition & Ingestion

The steps of level 0 processing are:

  1. Obtaining Images - Finding and downloading CTX image data.
  2. Ingestion - Converting an EDR image to the ISIS cube format.
  3. Adding SPICE Data - Helps compute geometric properties.

Obtaining Images

CTX File Naming Conventions

Deciphering the filename can be helpful both when searching for data and also when managing files on your system. The PDS naming convention for EDRs is.

# A typical CTX image filename
P01_001472_1747_XI_05S146W.IMG

# We will break it down as follows...
PAA_BBBBBB_CCCC_XD_EEFGGGW.IMG

Where:

  • PAA: Mission phase. P01
  • BBBBBB: Orbit number. 001472
  • CCCC: Represests the center latitude in units of 0.1 degree. 0 degrees is the descending equator crossing; 90 is the south pole; 180 is ascending equator crossing; and 270 is the north pole. 1747
  • XD: Command mode, XI for ITL or XN for NIFL. XI
  • EE: Planned center latitude. 05
  • H: Latitude, N for north or S for south. S
  • BBBW Planned center longitude in positive west. 146W
Searching the PDS Image Atlas

The PDS Image Atlas IV can search for CTX images.

To find CTX images of Mars, filter by:

  • Instrument: CTX
  • Target: Mars

You may also want to add other filters, like Orbit Number, or use the map panel to draw a bounding box around an area of interest.

If you find an image you want, but have trouble downloading it, take note of the filename/product ID and try finding it manually on a data volume.

Browsing by Volume

Online Data Volumes offers manual access to the image data archive.

Ingesting CTX Data

Importing CTX EDR images into ISIS

About EDR IMG and ISIS cube formats

In order to work with CTX data in ISIS, the CTX EDR file must be converted to an ISIS cube file so ISIS programs can read and process the data.

EDR files should always have a file extension of .IMG. These files contain the image data as well as text describing the image data and the state of the instrument at the time the image was taken. The text is in the form of a standard PDS label located at the beginning of the file. Only the info needed by other ISIS programs is transferred from the PDS label to the ISIS cube label. See examples of each below.

Example PDS Label 
FILE_NAME = "P01_001472_1747_XI_05S146W.IMG"
RECORD_TYPE = FIXED_LENGTH
RECORD_BYTES = 5056
FILE_RECORDS = 7169
LABEL_RECORDS = 1
^IMAGE = 2
SPACECRAFT_NAME = MARS_RECONNAISSANCE_ORBITER
INSTRUMENT_NAME = "CONTEXT CAMERA"
INSTRUMENT_HOST_NAME = "MARS RECONNAISSANCE ORBITER"
MISSION_PHASE_NAME = "PSP"
TARGET_NAME = MARS
INSTRUMENT_ID = CTX
PRODUCER_ID = MRO_CTX_TEAM
DATA_SET_ID = "MRO-M-CTX-2-EDR-L0-V1.0"
PRODUCT_CREATION_TIME = 2007-05-18T22:41:04
SOFTWARE_NAME = "makepds05 $Revision: 1.7 $"
UPLOAD_ID = "UNK"
ORIGINAL_PRODUCT_ID = "4A_04_100500DD00"
PRODUCT_ID = "P01_001472_1747_XI_05S146W"
START_TIME = 2006-11-19T04:01:27.976
STOP_TIME = 2006-11-19T04:01:41.429
SPACECRAFT_CLOCK_START_COUNT = "0848376106:209"
SPACECRAFT_CLOCK_STOP_COUNT = "N/A"
FOCAL_PLANE_TEMPERATURE = 291.8 <K>
SAMPLE_BIT_MODE_ID = "SQROOT"
OFFSET_MODE_ID = "196/187/185"
LINE_EXPOSURE_DURATION = 1.877 <MSEC>
SAMPLING_FACTOR = 1
SAMPLE_FIRST_PIXEL = 0
RATIONALE_DESC = "Escarpment with slope streaks"
DATA_QUALITY_DESC = "OK"
ORBIT_NUMBER = 1472
OBJECT = IMAGE
LINES = 7168
LINE_SAMPLES = 5056
LINE_PREFIX_BYTES = 0
LINE_SUFFIX_BYTES = 0
SAMPLE_TYPE = UNSIGNED_INTEGER
SAMPLE_BITS = 8
SAMPLE_BIT_MASK = 2#11111111#
CHECKSUM = 16#D3B47773#
END_OBJECT = IMAGE
END
Example ISIS Cube Label 
Object = IsisCube
  Object = Core
    StartByte   = 65537
    Format      = Tile
    TileSamples = 128
    TileLines   = 128

    Group = Dimensions
      Samples = 5000
      Lines   = 7168
      Bands   = 1
    End_Group

    Group = Pixels
      Type       = SignedWord
      ByteOrder  = Lsb
      Base       = 0.0
      Multiplier = 1.0
    End_Group
  End_Object

  Group = Instrument
    SpacecraftName        = Mars_Reconnaissance_Orbiter
    InstrumentId          = CTX
    TargetName            = Mars
    MissionPhaseName      = PSP
    StartTime             = 2006-11-19T04:01:27.976
    SpacecraftClockCount  = 0848376106:209
    OffsetModeId          = 196/187/185
    LineExposureDuration  = 1.877 <MSEC>
    FocalPlaneTemperature = 291.8 <K>
    SampleBitModeId       = SQROOT
    SpatialSumming        = 1
    SampleFirstPixel      = 0
  End_Group

  Group = Archive
    DataSetId           = MRO-M-CTX-2-EDR-L0-V1.0
    ProductId           = P01_001472_1747_XI_05S146W
    ProducerId          = MRO_CTX_TEAM
    ProductCreationTime = 2007-05-18T22:41:04
    OrbitNumber         = 1472
  End_Group

  Group = BandBin
    FilterName = BroadBand
    Center     = 0.65 <micrometers>
    Width      = 0.15 <micrometers>
  End_Group

  Group = Kernels
    NaifFrameCode       = -74021
    LeapSecond          = $base/kernels/lsk/naif0008.tls
    TargetAttitudeShape = $base/kernels/pck/pck00008.tpc
    TargetPosition      = Table
    InstrumentPointing  = Table
    Instrument          = Null
    SpacecraftClock     = $mro/kernels/sclk/MRO_SCLKSCET.00022.65536.tsc
    InstrumentPosition  = Table
    InstrumentAddendum  = $mro/kernels/iak/mroctxAddendum001.ti
    ShapeModel          = $base/dems/molaMarsPlanetaryRadius0002.cub
  End_Group
End_Object

Converting a CTX EDR image to an ISIS cube with mroctx2isis

mroctx2isis from=P01_001472_1747_XI_05S146W.IMG to=P01_001472_1747_XI_05S146W.cub

The mroctx2isis program also converts the image header, prefix and suffix data to ISIS Binary Large OBject (BLOBs). See the mroctx2isis ISIS app docs for other related parameters.

Adding SPICE Data

About SPICE

An important capability of ISIS is the ability to geometrically and photometrically characterize pixels in raw planetary instrument images. Information such as latitude, longitude, phase angle, incidence angle, emission angle, local solar time, sun azimuth, and a many other pixel characteristics can be computed.

To compute this information, the SPICE (Spacecraft and Planetary ephemeredes, Instrument C-matrix and Event kernel) kernels must first be determined for the image. These kernels maintain the spacecraft position and orientation over time as well as the target position and instrument specific operating modes.

For more info, see Spice Overview

Adding SPICE info with spiceinit

spiceinit FROM=P01_001472_1747_XI_05S146W.cub

To add SPICE information to your cube, run spiceinit on the image so that camera/instrument-specific applications (e.g., cam2map , campt , qview ) will have the information they need to work properly. Generally, you can simply run spiceinit with your input filename and no other parameters.

Level 1 Processing - Radiometric Calibration and Noise Removal

Creating a level 1 image involves:

  • Radiometric calibration of the data so we have an image representative of an ideal image acquired by a camera system with perfect radiometric properties. Values in the resulting image represent the reflectance of the surface (I/F).
  • Removal of systematic noise from the image. For CTX, this noise appears as vertical striping, referred to as furrows, which occur under certain observing conditions, and tonal mismatches among the data sets collected by adjacent channels in a CCD.

Radiometric Calibration

About Radiometric Calibration of CTX Data

The CTX detector has a total of 5000 pixels, divided among an A channel and a B channel. The pixels alternate between the two channels: ABABABAB, etc. Images from CTX may or may not include all pixels in the acquired image.

There are special summing modes that are utilized on-board the spacecraft to average detector pixels to combine them into a single output pixel value. The value of the ISIS label keyword, SpatialSumming, indicated the number of samples that were summed and averaged to result in the pixel values stored in the file.

Dark current pixels are taken for each line and stored in the ISIS cube as a table, named "CTX Prefix Dark Pixels". During the calibration process, a dark current value is subtracted from the pixels in the image. The exact dark current value that is applied is dependent on the summing mode. You can also select the IOF parameter and output the pixel values as relative reflectance.

See Also: General Overview of Radiometric Calibration

Radiometrically calibrating an image with ctxcal

ctxcal from=P01_001472_1747_XI_05S146W.cub to=P01_001472_1747_XI_05S146W.cal.cub

Noise And Artifacts

Removing Vertical Striping Due to Even/Odd Detector Readout

If the image has a Summing Mode of 2, then do not run ctxevenodd.

Removing Vertical Striping with ctxevenodd

ctxevenodd from=P01_001472_1747_XI_05S146W.cal.cub to=P01_001472_1747_XI_05S146W.destripe.cub

Map Projecting Images

Map Templates

To project an image to a map projection, set up a list of parameters based on the projection you wish to use. Use maptemplate (or a text editor) to set up a map template defining the mapping parameters for the projection.

Example Map Template

The following is an example of a map template for defining the projection of an image of Mars to the sinusoidal projection.

Group = Mapping
    TargetName         = Mars
    EquatorialRadius   = 3396190.0 <meters>
    PolarRadius        = 3376200.0 <meters>
    LatitudeType       = Planetocentric
    LongitudeDirection = PositiveEast
    LongitudeDomain    = 360

    ProjectionName     = Sinusoidal
    CenterLongitude    = 227.95679808356

    MinimumLatitude    = 10.766902750622
    MaximumLatitude    = 34.44419678224
    MinimumLongitude   = 219.7240455337
    MaximumLongitude   = 236.18955063342

    PixelResolution    = 426.87763879023 <meters/pixel>
End_Group

Projecting an Image

cam2map converts a camera (instrument) image to a map projected image. cam2map will automatically compute defaults for most of the mapping parameters, so you only need to define a subset of the parameters in your map template (e.g. ProjectionName).

Projecting an image with cam2map

With the cube so far as *.lev1.cub and a map template (like above) saved to sinu.map.

cam2map from=P01_001472_1747_XI_05S146W.lev1.cub map=sinu.map to=P01_001472_1747_XI_05S146W.sinu.cub

Notes on cam2map

  • Your cube must be data from an ISIS-supported mission.

  • The same map template can be reused for multiple images by removing the lat/lon range parameters from your map template:

    • MinimumLatitude
    • MaximumLatitude
    • MinimumLongitude
    • MaximumLongitude

  • cam2map will automatically calculate parameter values, as long as the projection name is in the map template.

  • If planning to mosaick the projected images, make sure the PixelResolution is the same for all images. Some projections also require the CenterLongitude and CenterLatitude to be the same for a mosaic.

Further Resources