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Rebecca Leggett edited this page Aug 30, 2019 · 1 revision

CTX

Working with Mars Reconnaissance Orbiter CTX Data

About MRO CTX

The Mars Reconnaissance Orbiter Mission

Instrument Overview

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.

Technical Details

The CTX camera obtains grayscale images of the martian surface. A typical CTX image is maybe as wide as 30 kilometers and as long as 160 kilometers, or more. The instrument has a 350 mm focal length and 6 degree field of view that images onto a 5064 pixels-wide charge coupled device line array. The CCD detects a broad band of visible light from 500 to 800 nanometers in wavelength.

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

CTX Observation: This is the CTX image 
that will be used in this lesson. 
The observation was taken on orbit 
1472 and the image has been reduced 
to 1/5th of its original resolution.

Reference & Related Resources

Processing CTX Data

Because the CTX instrument has only one CCD, the processing of the data is similar to many other instruments.

The process of constructing an uncontrolled CTX observation mosaic includes the following steps:

Level 0 - Data Ingestion

  • Acquire and convert CTX image data files to the ISIS3 image format.
  • Add information to the ISIS3 image in order to compute geometric properties such as latitude/longitude range and illumination angles of the image.

Level 1 – Radiometric Calibration and Noise Removal

  • Convert raw pixel numbers to reflectance (irradiance/solar flux or I/F).
  • Remove noise.

Level 2 - Projection

  • Geometrically rectify to a map projection.

Level 0 Processing - Data Acquisition & Ingestion

There are three major components of level zero processing which are:

  1. Acquisition of raw CTX image data for an observation. The Planetary Data System ( PDS ) archives these raw images in a standard format called the Experiment Data Record (EDR).
  2. Ingestion of the PDS EDR formatted images. That is, the conversion from PDS EDR image format to the ISIS3 image format.
  3. Initialization of each ISIS3 image with SPICE (Spacecraft and Planetary ephemeredes, Instrument C-matrix and Event) kernel data. Recall this information is used to compute geometric properties about the CTX observation, such as the latitude/longitude range or illumination angles.

File Naming Convention

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.

PPP_XXXXXX_YYYY_"X"X_AAHBBB"W".IMG

Where:

  • PPP is the mission phase
  • XXXXXX is the orbit number
  • YYYY is a representation of center latitude in units of 0.1 degree. 0 degrees is the decending equator crossing; 90 is the south pole; 180 is ascending equator crossing; and 270 is the north pole.
  • X is the command mode
    • I is ITL
    • N is NIFL
  • AA is the planned center latitude of the image
  • H designate is latitude is north or south
    • N north
    • S south
  • B is the planned center longitude of the image in posative west values

Planetary Image Atlas

The PDS Planetary Image Atlas , managed by the Jet Propulsion Laboratory (JPL) and USGS Astrogeology Research Program, allows the user to search through the CTX database and identify images based on parameters entered by the user. This tool includes both basic and advanced search capabilities.

The PDS Imaging Node houses data from several planetary missions, and offers a variety of methods for accessing their holdings. Within this lesson, we will focus on CTX data.

Screenshot of the Image Atlas 
'Quick Search' options for 
CTX. The yellow highlights are 
areas of the page that are 
useful when executing a quick 
search of CTX data

Search

The PDS Planetary Image Atlas provides a Product Search tool to interrogate the collection of CTX images. This tool lets us query information about each image and ignores the data that we have no interest in. A good way to reduce the number of images to look at is by defining an area of interest with latitude and longitude ranges. You can also restrict the search by choosing a minimum and maximum resolution. Remember to keep in mind the coordinate system ( areocentric west for CTX) and units (generally meters for distances) required by the search tool. Launch the Mars Reconnaissance Orbiter Product Search to give it a try.

The table below lists the primary search parameters that can help you narrow down the number of images that are returned by a search of the PDS CTX image collection. The image above shows a screenshot of the MRO Product Search . Note there are two categories (the tabs above the search form) where these search parameters are found: Quick Search and Instrument.

Parameter Notes Page Location
General: Instrument Select CTX from the selection choices on the left side of the window. Select Instrument Location
Quick Search: Target Name Since we're interested in working with images of Mars, choose Mars so you don't have to wade through listings for other bodies. Target Location
Quick Search: Orbit Number Enter minimum and maximum orbit number range. Orbit Location
Geometry: Latitude and Longitude You can be helpful to use the latitude and longitude parameters in the "Adv/Geometry" tab to define you search to an area of interest. Lat/Lon Location

Once you've made your search parameter selections, click the Get Count to see how many results your search will return, or Get Results to perform the search and access the results.

Browsing by Volume

You can also go to the online data and Browse Online Data Volumes, which offers FTP access to the image data archive. This allows you to look at the image and text files in the archive, where you can find more helpful information. To give it a try, launch the Planetary Image Atlas in a new browser window. Now click the Mars Reconnaissance Orbiter Browse Online Data Volumes and choose a volume to look at.

Downloading CTX images

When you know the CTX images that you would like to work with, two ways to download the data are:

From the PDS website

Step One : Right click on the image of interest.

Step Two : Select "Save Link As..." from the pop-up box.

Step Three : Choose a location on you system to save to
file to, then click the Save button in the Lower right corner
of the Save As box.

Use ISIS3 edrget Application

The ISIS3 application edrget can also be used to download a CTX file of interest.

Example: downloading a CTX EDR prodct:

edrget \
url=http://pdsimg.jpl.nasa.gov/data/mro/mars_reconnaissance_orbiter/ctx/mrox_0011/data/P01_001472_1747_XI_05S146W.IMG

Related ISIS3 Applications

See the following ISIS3 documentation for information about the applications you will need to use to perform this procedure:

  • edrget : downloads a CTX file when given the full URL

Importing CTX Data

In order to work with CTX data in ISIS3, the CTX EDR file must be converted to an ISIS3 cube file so ISIS3 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 (click to view example label file) located at the beginning of the file. Only the information needed by other ISIS3 programs is transferred from the PDS label to the ISIS3 cube label (click to view example label file) .

Ingesting CTX EDR image into ISIS3

The program used to convert CTX EDR files to ISIS3 cube files is mroctx2isis . The following example shows the command line usage. The resulting output file will be an ISIS3 cube.

Example: ingesting a CTX EDR product into ISIS3:

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 ISIS3 Binary Large OBject (BLOBs) and has other parameters.

Related ISIS3 Applications

See the following ISIS3 documentation for information about the applications you will need to use to perform this procedure:

  • mroctx2isis : converts a CTX EDR to ISIS3 cube format

Adding SPICE

An important capability of ISIS3 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 ( S pacecraft and P lanetary ephemeredes, I nstrument C -matrix and E vent kernel) kernels must first be determined for the particular raw instrument image. These kernels maintain the spacecraft position and orientation over time as well as the target position and instrument specific operating modes.

To add SPICE information to your cube, run the spiceinit application 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:

spiceinit FROM=P01_001472_1747_XI_05S146W.cub

Related ISIS3 Applications

See the following ISIS3 documentation for information about the applications you will need to use to perform this procedure:

  • spiceinit : adds SPICE information to the input cube

Level 1 Processing

Radiometric Calibration and Noise Removal

In this section we discuss how to create a level 1 CTX image. The process of generating 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.

Overview of Radiometric Calibration

Overview_of_Radiometric_Calibration

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 ISIS3 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 ISIS3 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.

The following example shows the command line for calibrating a CTX image.

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

Related ISIS3 Applications

See the following ISIS3 documentation for information about the applications you will need to use to perform this procedure:

  • ctxcal : radiometrically calibrates CTX images

Overview of Noise And Artifacts

Overview of Noise And Artifacts

Removing Vertical Striping Due to Even/Odd Dectector Readout

If the image of interest has a summing mode of 2 then you will not run ctxevenodd.

Overview of Map Projecting Images

The Map Template

In order to project an image to a specific map projection, you'll need to set up a list of parameters based on the projection you wish to use. Use the maptemplate application program (or your favorite text editor)to set up a map template defining the mapping parameters for the projection. The following is a 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 the Image

The cam2map application 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).

  • Your cube must be data from an ISIS3-supported mission -- cam2map depends on camera/instrument information in the ISIS3 system to perform map projections.
  • If you are projecting several images with the same projection parameters, you can reuse the same map template for all your images simply by removing the latitude longitude range parameters (MinimumLatitude, MaximumLatitude, MinimumLongitude, and MaximumLongitude) from your map template.
  • cam2map will automatically calculate parameter values for you -- all you really need is the projection name in your map template.
  • If you're planning on mosaicking your projected images, make sure the PixelResolution is the same for all images. Some projections also require the CenterLongitude and CenterLatitude to be the same when creating a mosaic.

P01_001472_1747_XI_05S146W_small.png View (1.37 MB) Ian Humphrey, 2016-05-31 05:51 PM

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