Indian Remote Sensing Satellite Cartosat-1: Technical features and data products

M.Krishnaswamy* and S. Kalyanaraman**
* Project Director, Cartosat-1, **Programme Director, IRS

Introduction
In the area of Satellite based remote sensing in the past, the first generation satellite IRS-1A and 1B were designed, developed and launched successfully during 1988 and 1991 with multi-spectral cameras with spatial resolution of 72.5 m and 36 m. respectively. Subsequently, the second generation remote sensing satellites IRS-1C and 1D with improved spatial resolutions of 70 m in multi-spectral and 5.8 m. in Panchromatic bands and a wide field sensor with 188m resolution and 800 Km. swath, have been developed and successfully launched in 1995 and 1997 respectively. These satellites have become the principal components in the National Natural Resource Management System and the data was used in various applications, viz., agriculture and soil, land form and land use studies, water resource, forestry, drought and flood monitoring, cartography, town planning and coastal zone monitoring. Especially IRS-1C/D data has been used for cartographic and town planning application up to 1:10,000 scale. These satellites also provide stereo pairs of imageries to get height information to an accuracy of approximately 10 meters.

With the above scenario, India has a lead in the civilian remote sensing field in the world not only in terms of realisation and launching of complex satellites with high, medium and coarse resolution cameras, but also in the application areas as well. In order to maintain this lead and also provide continuity of data to global users, Cartosat-1 with two improved fore and aft PAN cameras with better than 2.5 m. spatial resolution is planned to be realised for launch by middle of 2003. This paper briefly presents the technical elements and the planned data products of the Cartosat-1 spacecraft.

Cartosat-1 Spacecraft Technical Elements:
The spacecraft is configured with the Panchromatic cameras which are mounted such that one camera is looking at +26 deg. w.r.t. nadir and the other at -5 deg. w.r.t. nadir along the track. These two cameras combinedly provide stereoscopic image pairs in the same pass. Also the whole spacecraft is steerable across track to provide wider coverage in a shorter period. A brief description of the payload and the other mainframe elements are given in the subsequent sections.

1. Remote Sensing Payloads:
   The payload performs the function of imaging an area along the track and transmits the data for ground processing. Each Panchromatic camera consists of three 3 mirror off-axis all reflective telescope with primary, secondary and tertiary mirrors. These mirrors are made from special zerodur glass blanks and are light weighted to about 60%. These mirrors are polished to an accuracy of l/80 and are coated with enhanced AlO2 coating. The mirrors are mounted to the Electro-optical module using iso-static mounts, so that the distortion on the light weighted mirrors are very minimum. The configuration of the electro-optical module of the camera is given in Fig.2.1. In order to meet the high resolution and the swath requirement 12K, 7 micron linear array CCD is planned to be used as a detector. The CCD processing electronics will be using high speed devices to meet the high data rate requirements. Some of the important specifications of the payload are given in Table 2.1.

Fig 2.1 Electro-optical module configuration of pan camera
Table 2.1: Payload Specifications

1. A polar sun synchronous orbit of altitude 618 Kms. with an inclination of 97.87 deg. and an equatorial cross-over local time of 10:30 hours and the descending node has been selected based on various considerations. The sun-synchronous orbit provides the imagery collection under near-constant illumination conditions throughout the life and repetitive coverage of the same area in a specified interval. In order to revisit the same place at a more frequent interval than the repetitive cycle, an off-nadir viewing capability is provided. Using this facility any area which could not be imaged on a given day due to cloud cover, etc. may be imaged on another day. The typical revisit cycle is 5 days with the off-nadir cross-track steering facility. Important orbital specifications are given in Table 2.2.

Table 2.1: Payload Specifications

1. Cartosat-1 Platform Configuration:
   The spacecraft will be 3-axis body stabilised by using 4 high torque Reaction Wheels mounted in a tetrahedral arrangement. The power generation capacity will be about 1100 watts at the end of life, to meet the global operation of the payloads. The overall spacecraft size will be about 2.4 m. x 2.7 m. and will weigh about 1450 Kg. The orbit configuration of the CARTOSAT-1 spacecraft is given in Fig.2.2.

Fig 2.2 On-orbit configuration of cartosat-1 spacecraft

2. Attitude and Orbit Control System (AOCS):
   In order to meet the stringent requirements of the high resolution payloads, it is necessary to have a precision Attitude Control System to provide a stable platform. Also in order to provide the required swath, overlap and to provide time invariant data and revisit requirements, the orbit control will be carried out periodically. Some of the important specifications of the AOCS are given below

   Attitude Pointing Accuracy (deg.) of all axes: 0.05
   Attitude drift (deg/sec) : 5 x 10 – 5
   Attitude determination accuracy (deg) : 0.01
   Ground location accuracy (m) : < 220

   The drift rate determines the image internal distortion figures, whereas the jitter affects the resolution parameters. The AOCS will meet the stringent attitude pointing accuracy and the stability using a wide area star sensor in Attitude Control loop and better control algorithms and using dynamic friction compensation technique for the ball bearing Reaction Wheels. AOCS will be configured with MIL-STD 31750 processor and with ASIC and HMCs. Various sensors like, earth sensors, star sensors, precision yaw sensors and precision digital sun-sensors will be used to control and determine the attitude of the spacecraft precisely. Hydrazine mono-propellant Reaction Control System with 4 Nos. of 11 Newton Thrusters and 8 Nos.of 1 Newton Thrusters will be used for backup control and for momentum dumping purposes. About 131 Kg. of RCS fuel will be planned to provide a minimum mission life of 5 years.

3. Earth Rotation Compensation:
   In the case of along track stereo data acquisition, same scene on the surface of earth is imaged with a time difference. The time difference is a function of the difference in forward and backward look angles chosen from other criteria and can be anywhere between 50 and 100 seconds. Major change in imaging conditions during this time period is due to rotation of earth. At the equator the effect of earth rotation is to shift the imaged point to the East by a distance of approximately 463.3 m for every one second. Thus during 50 seconds the shift is of the order of 23.2 Km. At 25 degrees latitude, the shift is 20.09 Km. If the separation in time between forward and backward imaging is more than 65 seconds then no overlap between them is present in case of zero yaw angle. In order to ensure stereo imaging it is necessary that the aft camera views the earth’s surface in such a way as to image the shifted point. This condition can be achieved by a continuous yaw manoeuvring. For any given latitude, it can also be achieved by mounting the payloads at appropriate yaw angle with respect to each other. A combination of fixed mounting, catering to stereo acquisition requirements for Indian latitudes and a yaw manoeuvring for other regions with minimum power consumption shall be adopted. Alternatively the spacecraft is manoeuvred such that the image strips will fall side by side so that wider swath images are obtained by the two cameras.
4. Data Handling System:
   The realisation of high precision cameras calls for the development of very high speed precision electronic systems, and requires gain bandwidth of low noise analog system in the range of a few GHz. Due to small IFOV, the signal amplitudes are also expected to be very low. The detectors also require ultra low noise, biases and high frequency read out clocks. The data rate requirement for 2.5 m. resolution system is about 340 MBPS for a typical 10 bit quanitisation. This high bit rate Data is compressed by 3.2 : 1 by JPEG Compression technique to bring down the data rate to 105 m compatible for X-Band Data transmission system. The payload data is transmitted in two X-band carriers one for each PAN camera, after QPSK modulation to the Data Reception Station (DRS). A spherical Phased Array Antenna with steerable beam to the required DRS is used to transmit the payload data. A solid state recorder with 120 GB capacity to store about 9.5 min. of payload data and playback to the required ground station is also planned for the global operation of the payloads.

Cartographic Data Products:
The overview of the Cartosat-1 Data Products generation facility is given in fig.3.1.

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 generation 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

1.  
   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.
   2. 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 (1s) based on GCP coordinate of 3M in planimetry and height accuracies. The accuracy of TCP derived from the SST processing is about 4M (1s ) and based on conjugate point identification accuracy of about 2.5 m (3s). The Coarse DEM will have planimetric error of about 3.1 (1s ) 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 (1s) 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 (3s).

      The location accuracy of various data products are given below:

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

      *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 (