Working with Mars Reconnaissance Orbiter CTX Data
From Isis Workshop
[edit] About MRO CTX
[edit] Launch and introductory mission info
NASA's Mars Reconnaissance Orbiter, launched in 2005, is searching for evidence that water persisted on the surface of Mars for a long period of time. While other Mars missions have shown that water flowed across the surface in Mars' history, it remains a mystery whether water was ever around long enough to provide a habitat for life.
[edit] Science goals of the mission
Mars Reconnaissance Orbiter seeks to obtain science data that will provide researchers with information for locating safe landing sites for future missions and tell researchers about the history of water on Mars. The spacecraft instruments will zoom in for extreme close-up images of the Martian surface in order to analyze minerals, look for subsurface water, trace how much dust and water are distributed in the atmosphere, and monitor daily global weather.
These studies will help determine if there are deposits of minerals that form in water over long periods of time, detect any shorelines of ancient seas and lakes, and analyze deposits placed in layers over time by flowing water. It will also be able to tell if the underground ice discovered by the Mars Odyssey orbiter is the top layer of a deep ice deposit, or if it is a shallow layer in equilibrium with the current atmosphere and its seasonal cycle of water vapor.
The orbiter's primary mission ends about three Earth years after launch, in November 2008. For details on all mission stages, see the Mission Timeline (NASA).
[edit] Science Instruments
During its two-year primary science mission, the Mars Reconnaissance Orbiter will conduct eight different science investigations at Mars. The investigations are functionally divided into three purposes: global mapping, regional surveying, and high-resolution targeting of specific spots on the surface.
- HiRISE (High Resolution Imaging Science Experiment) This high-resolution, visible-range camera can reveal small objects in the debris blankets of mysterious gullies and details of geologic structure of canyons, craters, and layered deposits.
- CTX (Context Camera) This camera will provide wide area views to help provide a context for high-resolution analysis of key spots on Mars provided by HiRISE and CRISM.
- MARCI (Mars Color Imager) This weather camera will monitor clouds and dust storms.
- CRISM (Compact Reconnaissance Imaging Spectrometer for Mars) This spectrometer covers the range of visible and near-infrared light, useful for identifying minerals, especially those likely formed in the presence of water.
- MCS (Mars Climate Sounder) This atmospheric profiler will detect vertical variations of temperature, dust, and water vapor concentrations in the Martian atmosphere.
- SHARAD (Shallow Radar) This sounding radar will probe beneath the Martian surface to see if water ice is present at depths greater than one meter.
[edit] Launch and introductory mission info
NASA's Mars Reconnaissance Orbiter, launched in 2005, is searching for evidence that water persisted on the surface of Mars for a long period of time. While other Mars missions have shown that water flowed across the surface in Mars' history, it remains a mystery whether water was ever around long enough to provide a habitat for life.
[edit] Science goals of the mission
Mars Reconnaissance Orbiter seeks to obtain science data that will provide researchers with information for locating safe landing sites for future missions and tell researchers about the history of water on Mars. The spacecraft instruments will zoom in for extreme close-up images of the Martian surface in order to analyze minerals, look for subsurface water, trace how much dust and water are distributed in the atmosphere, and monitor daily global weather.
These studies will help determine if there are deposits of minerals that form in water over long periods of time, detect any shorelines of ancient seas and lakes, and analyze deposits placed in layers over time by flowing water. It will also be able to tell if the underground ice discovered by the Mars Odyssey orbiter is the top layer of a deep ice deposit, or if it is a shallow layer in equilibrium with the current atmosphere and its seasonal cycle of water vapor.
The orbiter's primary mission ends about three Earth years after launch, in November 2008. For details on all mission stages, see the Mission Timeline (NASA).
[edit] Science Instruments
During its two-year primary science mission, the Mars Reconnaissance Orbiter will conduct eight different science investigations at Mars. The investigations are functionally divided into three purposes: global mapping, regional surveying, and high-resolution targeting of specific spots on the surface.
- HiRISE (High Resolution Imaging Science Experiment) This high-resolution, visible-range camera can reveal small objects in the debris blankets of mysterious gullies and details of geologic structure of canyons, craters, and layered deposits.
- CTX (Context Camera) This camera will provide wide area views to help provide a context for high-resolution analysis of key spots on Mars provided by HiRISE and CRISM.
- MARCI (Mars Color Imager) This weather camera will monitor clouds and dust storms.
- CRISM (Compact Reconnaissance Imaging Spectrometer for Mars) This spectrometer covers the range of visible and near-infrared light, useful for identifying minerals, especially those likely formed in the presence of water.
- MCS (Mars Climate Sounder) This atmospheric profiler will detect vertical variations of temperature, dust, and water vapor concentrations in the Martian atmosphere.
- SHARAD (Shallow Radar) This sounding radar will probe beneath the Martian surface to see if water ice is present at depths greater than one meter.
[edit] Instrument Overview
CTX will collect data simultaneously with the HiRISE camera and CRISM spectrometer. As the name suggests, CTX will provide the wider context for the data collect by the other two instuments. Scientists will use images from the other instrument to examne the details of mars, but CTX will allow a better understanding of the terrain that encompasses these details.
From an altitude of approximately 300 kilometers above Mars, CTX will return surface images that are 30 kilometers across with pixels representing 6 meters of the martian surface.
[edit] Technical Details
The CTX camera will obtain grayscale images of the martian surface. a typical CTX image 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 Malin Space Science Systems.
[edit] References & Related Resources
- CTX instrument description
- Malin Space Science Systems CTX web site
- Mars Reconnaissance Orbiter mission web site
[edit] Processing CTX Data
Because the CTX instrament 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 HiRISE image data files to the Isis image format.
- Add information to the Isis 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.
[edit] Level 0 Processing - Data Acquisition & Ingestion
There are three major components of level zero processing which are:
- 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).
- Ingestion of the PDS EDR formatted images. That is, the conversion from PDS EDR image format to the Isis image format.
- Initialization of each Isis 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.
[edit] 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
[edit] 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 Planetary Data System(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.
[edit] Search
The PDS Planetary Image Atlas provides a Product Search tool to interrogate the collection of HiRISE 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 images to the right show screenshots 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.
The PDS Planetary Image Atlas provides a Product Search tool to interrogate the collection of HiRISE 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 east for HiRISE) 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 HiRISE image collection. The images to the right show screenshots 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 Locataion |
| 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 logitude prameters in the "Adv/Geometry" tab to define you search to an area of interest. | Lat/Lon Locatiion |
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.
[edit] 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.
[edit] Downloading CTX images
When you know the CTX images that you would like to work with, two ways to download the data are:
[edit] 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.
[edit] Use Isis edrget Application
The Isis 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
[edit] Related Isis Applications
See the following Isis 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
[edit] Importing CTX Data
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 (click to view example label file) located at the beginning of the file. Only the information needed by other Isis programs is transferred from the PDS label to the Isis cube label Isis Cube label (click to view example label file).
[edit] Ingesting CTX EDR image into Isis
The program used to convert CTX EDR files to Isis cube files is mroctx2isis. The following example shows the command line usage. The resulting output file will be an Isis cube.
Example: ingesting a CTX EDR product into Isis:
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) and has other parameters.
[edit] Related Isis Applications
See the following Isis documentation for information about the applications you will need to use to perform this procedure:
- mroctx2isis: converts a CTX EDR to Isis cube format
[edit] Adding 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 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 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
[edit] Related Isis Applications
See the following Isis documentation for information about the applications you will need to use to perform this procedure:
- spiceinit: adds SPICE information to the input cube
[edit] 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.
[edit] Overview of Radiometric Calibration
[edit] Why perform radiometric calibration?
Both vidicon cameras (such as those carried on-board the Viking and Voyager missions), and charge coupled device (CCD) cameras (such as on the Clementine, Mars Reconnaissance Orbiter, and other contemporary missions) produce digital images with the inherent artifact known as camera shading. Camera shading results from the non-uniform sensitivity across the field-of-view of the imaging instrument.
Perhaps the best way to illustrate camera shading is to imagine acquiring a digital image of a target of uniform brightness, say a screen that has been painted a uniform shade of gray. If the camera sensitivity across the fields-of-view were ideal (and the flat-field target exactly the same brightness everywhere), then the acquired digital image would have the same DN value for all the pixels in the image. However, because of the non-uniform sensitivity of the camera, the DN values of the resulting image will vary throughout the image array (see the example to the right). A typical camera may have as much as 20% variation across the field-of-view. Camera shading corrections are applied to an image that correct for the non-uniform sensitivity so that, in our flat-field observation example, the radiometrically corrected image would contain pixels of identical value.
[edit] What is radiometric calibration?
Radiometric calibration recalculates the DNs in an image based on numerous factors, such as the exposure time, known values for the camera shading based on flat-field observations, dark current (output current of a detector when no energy is incident on the detector, such as when the shutter is closed), and other factors describing the unique electronics design and characteristics of an imaging system. Camera sensitivity may be time dependent because of the drift of the camera sensitivity throughout the course of the mission. The camera sensitivity is also dependent on the filter, operating modes of the instrument, and temperature of the cameras. Additionally, the camera response may be non-linear at various brightness levels.
[edit] How is the image changed by radiometric calibration?
A radiometrically calibrated image has DNs in radiometric units that are proportional to the brightness of a scene. Radiometric calibration applications in Isis produce output values that represent either:
- Radiance - The amount of electromagnetic energy emitted or reflected from an area of a planet, in units of µw/(cm2*sr)
- Reflectance - The ratio of reflected energy to incoming energy (i.e. irradiance/solar flux, often simply called I/F). A reflectance would be 1.0 for an ideal 100% reflector where the sun and camera orientations are perpendicular to the reflecting surface.
Generally, Isis radiometric calibration applications that offer both reflectance and radiance output will output reflectance by default. Due to the fact that radiometric calibration is mission dependent, each mission supported by Isis has its own radiometric calibration application. A few examples of radiometric calibration applications in Isis are:
[edit] Radiometric Calibration of CTX Data
The CTX detector has a total of 5000 pixels, devided among an A channal 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 spececraft 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 relativer refectance.
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
[edit] Related Isis Applications
See the following Isis documentation for information about the applications you will need to use to perform this procedure:
- ctxcal: radiometrically calibrates CTX images
[edit] Overview of Noise And Artifacts
Noise and artifact are terms used to describe speckles, spikes, reseaus, missing data, and other marks, blemishes, defects, and abnormalities in image data created during the acquisition, transmission, and processing of image data. The line between the definitions of artifact and noise is fuzzy (and often subject to opinion), and often the terms are used interchangeably. Some noise and artifacts are expected, even purposefully added, and can be removed during the radiometric calibration process.
[edit] What is noise?
In image processing, noise is a type of flaw or blemish in the image caused by:
- Telemetry data dropouts or transmission errors
- Malfunctioning or dead detectors
- Read noise native to the CCD system
- Coherent noise caused by spurious electronic signals from the operation of instruments onboard the spacecraft
Noise can take the appearance of speckling, missing data, random or orderly patterns, and other variations that cause the image to have a muddled appearance, or visually distracting blemishes or patterns. There are three categories of noise:
- Fixed-location noise always exist at the same location in the image array, with predictable positions. Fixed location noise can be cosmetically corrected by replacing the bad pixels with the weighted average of the unaffected neighborhood pixels. Fixed-location noise can result from malfunctioning or dead detectors.
- Randomly occurring noise results from data transmission errors causing data bits to be altered at random intervals in the image. The random noise produces discrete, isolated pixel variations or "spikes" and gives an image a "salt-and-pepper" appearance. Additionally, telemetry drop-outs can cause portions of an image to be completely missing. This type of noise is generally corrected using filtering techniques that recognize missing or anomalous data pixels and replaces these data points with a weighted average of the unaffected neighborhood pixels.
- Coherent noise can be introduced by spurious electronic signals produced by the operation of instruments onboard the spacecraft during image observations. The spurious signals interfere with the electronics of the imaging system causing coherent noise patterns to be electronically "added" to the images. For example, the shuttering electronics of the Viking cameras introduced a spurious "herring-bone" pattern at the top and bottom of the image. Noise-removal algorithms are designed to correct specific coherent noise problems such as this one.
[edit] What are artifacts?
Generally, image artifacts are a type of flaw or blemish in the image introduced during processing, intentionally introduced due to the design of system, or unintentional introduction of debris or energy external to the system. Examples of artifacts include:
- reseaus etched on the camera lens
- reseaus exposed on photographic film during pre-flight preparations for a mission
- minute dust specks located in the optical path or on the focal plane array
- cosmic rays and other charged particles impacting the sensor (particularly CCDs)
- fringe, ring, or visible patterns created during filtering, ratio analysis, and other enhancement processes
- quantization, checkerboarding, and other artifacts introduced by image compression algorithms during conversion from Isis cube format to a lossy image format or bit-type reductions that reduce the tonal resolution of the data
Most artifacts fit neatly into the categories of noise listed earlier and are corrected using many of the same processes. For example, dust specks create fixed-location blemishes, and cosmic rays cause random spikes. Reseaus are useful blemishes that are removed once they are analyzed for their locations within an image and the information saved for later processing.
 Reseaus, dropped data, and salt-and-pepper This example shows a number of noise and artifact types. The large, regularly spaced black dots across the image are caused by the reseaus on the camera lens. The pattern of vertical black lines across the bottom of the image was caused by transmission data loss. The black and white speckles, called salt-and-pepper, are random noise. |
 Instrument error The black streak running down the right side of this image may have been caused by an unusual error in the line-scanner camera that acquired this image. |
 Transmission error A glitch during the transmission of this image caused the data to become garbled (upper right) and some data was completely lost (the black area across the middle) |
 Image compression Exporting the image to a lossy, compressed image format (using very high compression), the compression algorithm introduced a tiled pattern across the image. |
[edit] Removing Vertical Striping Due to Even/Odd Detector Readout
If the an image of interest has a summing mode of 2 then you will not run ctxevenodd
[edit] Overview of Map Projecting Images
[edit] 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
[edit] 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 a Isis-supported mission -- cam2map depends on camera/instrument information in the Isis 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.
