CARTOSAT-1 Carries Two State-Of-The-Artpanchromatic(PAN) Cameras That Take Black and White

Cartosat-1:

CARTOSAT 1is a stereoscopicEarth observation satellitein asun-synchronous orbit. The satellite was built, launched and maintained by theIndian Space Research Organization (IISRO). Weighing around 1560kg at launch, its applications will mainly be towardscartographyinIndia. It was launched by thePSLVon 5 May 2005 from the newly built second launch pad atSriharikota. Images from the satellite will be available fromGeoEyefor worldwide distribution.

CARTOSAT-1 carries two state-of-the-artpanchromatic(PAN) cameras that take black and white stereoscopic pictures of the earth in the visible region of theelectromagnetic spectrum. The swath covered by these high resolution PAN cameras is 30km and their spatial resolution is 2.5 metres. The cameras are mounted on the satellite in such a way that near simultaneous imaging of the same area from two different angles is possible. This facilitates the generation of accurate three-dimensional maps. The cameras are steerable across the direction of the satellite's movement to facilitate the imaging of an area more frequently. The images taken by CARTOSAT-1 cameras are compressed, encrypted, formatted and transmitted to the ground stations. The images are reconstructed from the data received at the ground stations.

CARTOSAT-1 also carries a solid state recorder with a capacity of 120 Giga Bits to store the images taken by its cameras. The stored images can be transmitted when the satellite comes within the visibility zone of a ground station.

The main constituents of this facility are, 1) Data Archival and Quick-look Browser (DAQLB) Systems, 2) Data Processing System (DPS) and 3) Cartosat Data Centre (CDC). The CDC interfaces with the Cartosat user community in getting the user requirements and processes the archived or acquired data, making use of the sub-modules like Stereo Strip Triangulation (SST), the Ground Control Point Library (GCPL) and the Data Products and Services modules. The stereo strip triangulation subsystem takes the primary GCPs and the DLI as input and generates (1) Triangulated Control Points (TCP), (2) Coarse DEM and (3) Updated orientation parameters. The TCPs and coarse DEMs and the IMS work order are the inputs for data products generation subsystem along with DLTs for generation of Data Products operationally. Various types of Data products planned using Cartosat images are (1) Image Data Products, (2) Image Map Data Products and (3) DEM Data products. Various aspects of the Data products (and various resource generations like coarse DEM generation and Triangulated Control Point (TCP) generations are briefly given below.

1. Image Data Products:

The levels of Image Data Products defined on the basis of their indented end use with attended impact on accuracy and turn-around time, covering both stereo and monomode of operations is given in table 3.1. Different types of image data products meeting the targeted user needs are generated based on the spacecraft operational modes like stereo mode or mono mode and the orbit and attitude determination modes. Different types of products meeting the station specific user needs over the entire globe coverage is also planned for different earth stations and onboard SSR modes of data acquisition.

Table 3.1 Levels of Data Products

Level of Product / RadiometricCorrection / Geometric Correction / Intended End-Use.
Level '0A'RAW / Not Applied / Not Applied / For internal use.
Level '0B'RAD / Applied / Not Applied / For stereo processing at user end
/ VAS providers / DQE
Level '1'SYS / Applied / System Knowledge / Quick turn-around-time digital products,
with acceptable quality for flat terrain or
mono mode data acquisition.
Level '2'GCP / Applied / System Knowledge + GCPs / For better location accuracy for
cases when DEM is not available,
for data acquired in mono mode
or for flat terrain.
Level '3A'DEMA / Applied / System Knowledge + GCPs + DEM
(external input) / Orthoimage products primarily for
IGS use, where GCPs and DEM are externally available.
Level '3B' DEMB / Applied / System knowledge+ GCPs + DEM (internal) / For Orthoimage product generation for data acquired in stereo mode.
Level '3C' DEMC / Applied / System Knowledge + GCPs + DEM
(external/internal, interactively edited
for density and surface discontinuities) / For precision Orthoimage / Orthoimage map generation/map updating.

1. Image Map Data Products (IMDP):

IMDP containing co-registered ortho corrected Cartosat-PAN raster images with one or more layers of cartographic vector information (available apriori or derived from Cartosat image) including a layer containing ASCII text strings as labels of vector elements, with necessary additional ancillary information shall be generated and supplied to the users. The following types of products are planned.

1. 2-D Satellite Image Map Products in conventional map projection or with user-defined projection parameters.
2. 2.5-D Satellite Image Map Products, representing the terrain elevation for one or more fixed, standard perspective view angles possibly with artificially exaggerated scaling effects incorporated to show the terrain relief.
3. 3-D Satellite Image Map Products, optionally including the 3-D viewing software as a part of the product.

All the types of image map products are corrected Cartosat-PAN images, either on standalone basis or desirably fused with available multi-spectral images with comparable spatial resolution as the base raster image.

1. DEM Data Products:
   The following types of DEM Data Products are planned to be generated.
   Type I: As computed originally:
2. As randomly distributed point heights as computed originally.
3. As triangulated Irregular Network (TIN) retaining all the originally computed points,as they are.
4. As progressively sampled rectangular grids, retaining all the originally computed points, as they are

Type II: As completed originally and incorporating break-lines:

• As a set of irregular point heights and break lines showing the surface discontinuities
• As a TIN model retaining all the originally computed points as they are; and in addition incorporating break lines, either as part of the TIN edges (referred as soft break lines) or as add-on specification (referred as hard break lines) indicating abrupt surface changes.
• As progressively sampled rectangular grids, retaining all the originally computed points, as they are plus the break lines manually identified.

Type III: As interpolated, mostly regular

• As a rectangular grid, generated by a suitable interpolation algorithm from the initial set of irregular points and break lines.
• As contours, connecting points of equal height at varying intervals.
• As a set of parallel vertical profiles, in any user desired direction.

1. Coarse DEM Generation (CDG):

It is necessary to carryout scene based processing for mono mode data acquisition for either fore or aft camera towards generating level 1 or level 2 products with minimum internal distortion as per the quality requirements. However, since the Cartosat cameras are mounted onboard the spacecraft in non-nadir viewing configuration, this can be achieved by carrying out terrain corrections apart from the system level corrections. Hence it is necessary to have DEM for all coverage for this purpose. Cartosat-1 being a stereo mission, allows for building such database on a pass or a pass segment basis.

1. Triangulated Control Points Library Generation (TCG):

To enable scene based precision processing for mono or stereo mode data acquisition, a second generation control points call TCP of approximately about 10 points in each standard scene (~30 x 30 KM) covering the entire region are generated and maintained as a data base and are used during products generation.

1. Data Product Accuracy:

The accuracy of various products planned to be generated depends upon the accuracies of the SST parameters, TCDS, Coarse DEM, Precision DEMs etc. It is planned to have SST parameters within an accuracy of 3.3 M (1) based on GCP coordinate of 3M in planimetry and height accuracies. The accuracy of TCP derived from the SST processing is about 4M (1 ) and based on conjugate point identification accuracy of about 2.5 m (3). The Coarse DEM will have planimetric error of about 3.1 (1 ) equivalent to the SST parameter error and the height is depending on the planimetric error and N/H ratio. The final DEM is generated after incorporating break lines / break points through manual / semi automatic methods with an RSS error of 2.5 m (1) in planimetry and 3.1 m in height.
The error budget calculated for scene level processing is for a scene size of about 30 KM x 30 KM and less and is of the order of 0.25 m (3).
The location accuracy of various data products are given below:

1. For system level correction (level 1) (3) : 220 m
2. With GCP (level 2) (3 ) : 18.7 m
3. With terrain (Coarse DEM) corrected (level 3A) (3): 21 m
4. With final DEM (LEVEL 3B) (3) : 18.7 m
5. With precision GCP & precision DEM (level 3C) : 6.4 m* (3)

*This product uses precise GCPS and precision DEM with incorporation of break lines and break points. Hence the internal distortion will be better than 1 pixel (<2.5 m). The level 2, 3A and 3B products are standard products whereas the level 3C products are always value added products whose turn-around time depends on the availability of the images and ancillary information (GCPs) etc.

1. Data Product Size, Scale and Datum:

Various Products size is (1) based on modes of operation vise mono, stereo or wide swath mode as defined by path/row or Lat./Long. referencing schemes, (2) User defined rectangular areas aligned to true North of minimum size 2.5 KM x 2.5 KM and area equivalent to the maximum scene size 30 KM x 30 KM, (3) User defined polygonal areas of minimum size 2.5 KM x 2.5 KM and (4) Standard Map sheet extents for 1:25,000 and 1:50,000 scales. All levels of products are oriented towards true North and standard map projection as applicable for different user needs across the globe. Also the horizontal and vertical datum of the geo corrected products is referenced to appropriate datum as applicable for different needs across the globe.

1. Data Products Format:

The digital products are generated and supplied in any conventional format like CEOS super structure and IRS fast formats with Cartosat specific changes, along with all necessary ancillary parameters addressing all possible data utilization needs of Cartosat-1 PAN data, for various levels of products for different application along with GIS and CAD and 3D software packages. The formats currently used in different GIS and CAD software packages such as Multi-Resolution Seamless Image Database (MRSID) universal data formats and Geotiff are suitably modified for Cartosat-I specific features.

Cartosat –1 (IRS –P5) Stereo Data Processing – A Case Study of Dehradun Area

Generation of Digital Elevation Models (DEMs) from satellite stereo data had been an important field from last few decades, started with the launch of the first civilian remote sensing satellite. Digital Elevation Models (DEMs) are a type of raster GIS layer. Raster GIS represents the world as a regular arrangement of locations. In a DEM, each cell has a value corresponding to its elevation. There is always a question mark how quickly and accurately the DEM’s can be generated especially using satellite stereo data.

In last two decades people had tried using across track satellite stereo data for generation of DEM’s using Satellite Photogrammetric approach. Some satellites are capable for acquiring stereo data using across track approach e.g., SPOT, IRS –1C/1D etc, and some can acquire stereo data using across as well as along track, from two different orbits e.g., IKONOS, QUIKBIRD etc. In both the cases two scenes of stereo pair were acquired from two different orbits, due to which there is always problems of time difference between two scenes of one stereo pair, Which can present difficulties while transferring Ground Control Points (GCP’s) in stereo model and during automatic image matching for DEM generation.

To overcome the time difference problem between two stereo scenes the vibrant Indian remote sensing satellite programme had launched advanced satellite, known as IRS-P5 (Cartosat-1) on May 5, 2005 aims to provide data with higher resolution for cartographic purpose. This satellite has two identical panchromatic sensors kept on the platform with fore, +26o and aft, -5o tilt respectively. This system is capable to provide stereo data without any time difference between two scenes. This paper presents one case study for processing stereo data acquired from Cartosat-1 satellite, of Dehradun area, for generation of DEM as well as Ortho-Image. A small evaluation study was also done, and it was observed that DEM generated using stereo data from Cartosat-1 satellite was having accuracy 2 to 12 m.

Data used:

For evaluation of Cartosat –1 data, data of Dehradun district covering west part of it was used. Fore and Aft scenes of west part of Dehradun town, dated 2nd October 2005 were used. The detailed information of fore and aft scenes are mentioned in table 1.

Table 1: Information of Fore and Aft scenes

In this paper the outputs (DEM, Contours and Ortho Image) generated from Cartosat –1 data were compared with outputs (DEM, Contours and Ortho Image) generated using IRS – 1C stereo data. The details of the IRS – 1C stereo data used in this work are mentioned in table 2.

Table 2: Parameters and Specifications of IRS-1C PAN stereo data

Data Processing:

The Fore and Aft scenes of Cartosat –1 data were provided with Rational functions (RFs) coefficients. Rational Functions (RFs) have been applied in photogrammetry and remote sensing to represent the transformation between the image space and object space whenever the rigorous model is made unavailable intentionally or unintentionally. It attracts more attention now because high-resolution images are being released to users with only RF coefficients.

For refining the orbital parameters of Fore and Aft scenes of Cartosat-1, few GCP’s (Ground Control Points), 11 in number were used for evaluating the cartosat-1 data of Dehradun area. Ground Control Points for this area were collected using Geodetic Single Frequency GPS in Relative mode. The Accuracy of GCP’s acquired from Geodetic Single Frequency GPS in Relative mode are shown in table 3. While generating the stereo model, eight ground points were used as GCP’s and other three known points on the ground were used as Tie Points as shown in following figure 1;

Table 3: Accuracy of GCP’s acquired from Geodetic Single Frequency GPS in Relative mode

Figure 1: Ground Control Points and tie points’ location on the stereo model

The exterior orientation parameters were refined while using eight GCP’s through triangulation. After triangulation Digital Elevation Model (DEM) was generated. Using the generated DEM Ortho image was generated from Aft PAN image.
For evaluation of the DEM generated using Cartosat-1 stereo data of Dehradun area, known points on the ground, used as GCP’s as well as Tie points used in the stereo model were studied. The comparison of the ground height generated by DEM as well as ground height information collected by Geodetic Single Frequency GPS in Relative mode is shown in table 4.

Table 4: For evaluating height from DEM generated from Cartosat-1 Stereo data.

Digital Elevation Model and Ortho Image generated from Cartosat-1 stereo data of Dehradun area shown in figure 2 and 3.

Figure 2: Digital Elevation Model generated from Cartosat-1 stereo data of Dehradun area

Figure 3: Ortho Image generated from Cartosat-1 stereo data of Dehradun area

Comparison Study:

A comparative study of outputs generated using Cartosat –1 stereo data of Dehradun area has been done in this section with IRS – 1C stereo data of same area. From stereo data of Cartosat –1 as well as IRS – 1C satellite three types of outputs were generated, e.g., DEM, Contours and Ortho Image. First a visual comparison for all the three outputs (DEM, Contours and Ortho Image) was done, using small common area lying on these stereo data sets. For this analysis, contours of 4 m (contour interval = 3*?, were ? is error in height) were generated using Cartosat –1 stereo data. Contour of 4 m interval as well as 10 m (contour interval = 3*?, were ? is error in height) interval was generated using IRS – 1C stereo data. It was found that DEM as well as contours generated using Cartosat –1 stereo data, were very much close to the ground information as shown in figure 4, 5, 6. Figure 7, 8 and 9 were used for evaluating the contours generated from both the data sets.

Figure 4: Contours generated from Cartosat-1 stereo data of Dehradun area at 4 m interval

Figure 5: Contours generated from IRS – 1C stereo data of Dehradun area at 10 m interval