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


  1. Orbit Considerations:
    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.


    2. Table 2.2 Orbit Specifications
    S.No. Orbit Characteristic Specification
    1. Nominal Altitude (km) 617.99
    2. Number of orbits per day 15
    3. Orbital Repetivity Cycle (No. of days) 116
    4. Nominal Wait Time to Acquire Adj.Path 11 days
    5. Max. Wait Time for Revisit 5
    6. Node for P/L Operations Descending Node.
    7. Local Time for Equatorial Crossing 10:30 AM
    8. Orbital parameters
    a) Semi-major axis
    b) Eccentricity
    c) Inclination    		 6996.128 km.
    0.001
    97.87 deg.

  2. Spacecraft main frame systems:
    The spacecraft bus has to support the payload systems in terms of structure, thermal control, power supply, data compression, data formatting and encryption and transmissions, data storage, TTC, etc. The spacecraft will be equipped with precision Attitude and Orbit Control system along with attitude sensors and propulsion systems. A brief description of various main frame systems is given below.


    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.



      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.
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