CartoSat-2 is a direct follow-on satellite to CartoSat-1 (IRS-P5) of ISRO, Bangalore, India. The objective is to provide high-resolution imagery (≤ 1 m, with an event monitoring capability) from a highly agile spacecraft. ^{1) 2) 3) 4) 5) 6)}

Note: The IRS (Indian Remote Sensing Satellite) series experienced a transition from the earlier generic purpose satellites (labeled as the P-series) to theme-oriented satellites like CartoSat, EduSat, OceanSat, Metsat etc. Hence, ISRO management decided not to affix the 'P' in the IRS series anymore, though they do constitute part of the IRS constellation of satellites.

Figure 1: Photo of the deployed CartoSat-2 spacecraft (image credit: ISRO)

Spacecraft:

Several new technology elements have been introduced into the design of the CartoSat-2. spacecraft. These are:

• Hexagonal-shaped, light weight and compact spacecraft structure

• CFRP (Carbon Fabric Reinforced Plastic) based electro-optic payload structure with a two mirror on-axis single camera

• Introduction of BMU (Bus Management Unit) for integrated main spacecraft bus functions. The BMU integrates the TT&C, AOCS, and the thermal control functions, dual gimbal antenna (for downloading the payload data), the high torque reaction wheels, the highly stable IRU (Inertial Measurement Unit), and advanced control algorithms. BMU is realized using dual microprocessor configuration, each one acting as redundant processor for all the above functions.

• High bit-rate telemetry, telecommand and data-handling systems

• Solid state onboard data recorder of 64 Gbit capacity

• Dual-gimballed antenna ⁷⁾

• Advanced triple junction solar cells. Note: GSAT-2, INSAT-4A and CartoSat-2 are the first ISRO missions to have these multijunction solar cell arrays as the prime power generators. ⁸⁾

• Improved IRU (Inertial Reference Unit)

• High performance star sensor.

• Also use of the SPS (Satellite Positioning System), an 8-channel GPS receiver (C/A code) on-board for the provision of instantaneous state vectors (state vector using pseudo range and range rate measurements) for the spacecraft. The first SPS system was introduced on IRS-P5 / CartoSat-1. ⁹⁾

The spacecraft platform is a generic IRS bus. The spacecraft is 3-axis stabilized using high-torque reaction wheels, magnetic torquers and hydrazine thrusters as actuators. The spacecraft yaw axis is pointing into the nadir direction which is also the optical axis of the spacecraft. The pointing accuracies are ±0.05º in all axes, attitude knowledge = 0.01º, the stability (attitude drift) is 0.0003º/s, and the ground location accuracy is 100 m. The satellite is very agile providing a body-pointing capability in along-track and cross-track of up to ±45º (providing a revisit capability of certain target regions within 4 days).

The AOCS (Attitude and Orbit Control Subsystem) is also capable to orient the dual-gimballed data transmission antenna to point towards the required data reception station during the spacecraft overpass. Attitude sensing is provided by the star sensors (EADS Sodern SED26) and IRU with three 2-axis DTGs (Dry Tuned Gyros) as the prime sensors, along with sun sensors and magnetic sensors. - The actuators consist of a four reaction wheel system with momentum and reaction torque capability of 15 Nms and 0.3 Nm, respectively. There are eight 1N thrusters and 3 magnetic torquers.

The sun sensor, with a 4π FOV, is used for initial sun pointing and in the safe mode attitude holding operations of the spacecraft. The magnetometers are used for updating the magnetic torquers, which in turn dump the wheel momentum. The star sensor is used as the prime attitude sensor for roll, pitch, and yaw attitude measurements. It interfaces with other systems through the MIL-STD-1553 bus. The spacecraft is normally kept in sun-pointing mode and is brought to Earth pointing only during payload imaging periods. Upon imaging completion, the spacecraft is slewed back to the sun-pointing attitude.

Figure 2: Tetrahedral reaction wheel configuration of CartoSat-2 (image credit: ISRO)

The AOCS employs many innovative concepts like generalized reference quaternion generation, parallel computation of quaternion w.r.t various frames of reference like sun pointing frame, earth pointing frame, ECI (Earth Centered Inertial) frame for usage to switchover between various modes with ease at any instant; rate-to-rate maneuver algorithms to achieve prescribed attitude and attitude rate at precise prescribed times; usage of gyro stellar reference in various star sensor updating profiles; provisions for correcting various sensor and actuator misalignments; many safety features during imaging maneuvers; momentum dumping using three torquers; automated back and forth rotations for orbit maneuvers and various sensor calibration maneuvers; capability for autonomous reconfiguration of star sensors and reaction wheels; various types of safe modes; autonomous attitude acquisition modes etc. ¹⁰⁾

IAC (Inertial Attitude Control) mode: This mode provides spacecraft control using propagated inertial quaternion. IAC Mode is used in support of the following functions:

• To hold the spacecraft inertially to given reference quaternion

• To maintain the spacecraft using thrusters in the sun acquired geometry with negative pitch axis pointing towards the sun and with latitude dependent roll bias to meet thermal requirements for payload and for providing FOV clearance for the star sensor

• For rotating the spacecraft using thrusters, through the automated OM (Orbit Maneuver) sequencer, from the sun-pointing geometry to the required Earth-pointing attitude before the OM operation, and the reverse rotation to the instantaneous sun-pointing geometry after OM operation.

The DHS (Data Handling System) consists of the baseband data handling (DH) and SSR (Solid State Recorder). The DH receives the 10 bit video-digital data from the CCD video processor electronics of the payload through the 8 ports at the rate of 4.2 Msample/s. The source data rate of the payload being 336 Mbit/s.

Data compression: The data compression system compresses the video data with a compression ratio of 1:3.2 (JPEG algorithm). The compressed data are encrypted, RS (Reed Solomon) encoded and formatted, and appended with necessary auxiliary data from the 4 odd ports (I channel with 52.5 Mbit/s) and even ports (Q channel with 52.5 Mbit/s). The formatted data is transmitted to the ground through the QPSK modulator/transmitter at 105 Mbit/s. Additionally, the formatted data may also be recorded onto a 2-channel SSR with a storage capacity of 64 Gbit.

The fixed solar arrays provide a power of 900 W when pointed toward the sun; two NiCd batteries of 18 Ah capacity are being used for ecliptic phase bridging. CartoSat-2 has a launch mass of 680 kg and a design life of 5 years.

Table 1: Overview of spacecraft parameters

Figure 3: View of the CartoSat-2 bus and telescope aperture (image credit: ISRO)

Figure 4: Illustration of the deployed CartoSat-2 spacecraft (image credit: ISRO)

Launch: A launch of CartoSat-2 took place on January 10, 2007 on the PSLV-C7 launcher from SDSC (Satish Dhawan Space Center), Sriharikota Range, India (680 kg). - For the first time, a DLA (Dual Launch Adopter) was used in a PSLV vehicle to accommodate the payloads. CartoSat-2 was mounted on top of DLA. The three secondary payloads on this flight are:

• SRE (Space capsule Recovery Experiment) of ISRO (550 kg). The structure of the capsule is sphere-cone-flare configuration with a nose radius of 0.5 m, a base diameter of 2 m, and a height of 1.6 m. The objective of SRE-1 is to demonstrate microgravity experiments. SRE-1 carries two experiments, an Isothermal Heating Furnace [IHF] and a Biomimetic (Biomineralization of inorganic materials) experiment. The capsule, equipped with an aero-thermo structure as well as deceleration and floatation systems, will be de-orbited and recovered. The SRE-1 reentry test is meant for future manned missions. ^{11) 12) 13) 14)}

• LAPAN-TUBSAT - a cooperative microsatellite project between TUB (Technical University of Berlin) and LAPAN (Lembaga Penerbangan dan Antariksa Nasional), or the Indonesian National Institute of Aeronautics and Space, Jakarta (56 kg)

• PehuenSat-1, an educational nanosatellite (6 kg) of Argentina, developed by the National University of Comahue (Universidad Nacional del Comahue), AATE (Argentine Association for Space Technology), and AMSAT Argentina. ¹⁵⁾

Orbit: Sun-synchronous near-circular orbit, altitude = 635 km, inclination = 97.87º, period of 97.4 min, nodal equatorial crossing time on descending node (LTDN) at 9:30 hours.

RF communications: The imagery is downlinked in X-band (8125 MHz) at a data rate of 105 Mbit/s. The TT&C data link is in S-band (2067 MHz). Both links operate in parallel.

Figure 5: Phased array antenna of CartoSat-2 (image credit: ISRO/ISAC)

Figure 6: Alternate view of the deployed CartoSat-2 spacecraft (image credit: ISRO)

Mission status:

• CartoSat-2 and its payload are operating nominally in 2014.

• CartoSat-2 and its payload are operating nominally in 2013. ¹⁶⁾

• CartoSat-2 and its payload are operating nominally in 2012 (in its 6^th year on orbit). ¹⁷⁾

• CartoSat-2 and its payload are operating nominally in 2011.

• CartoSat-2 and its payload are operating nominally in 2010. ¹⁸⁾

Figure 7: A 1m panchromatic CARTOSAT-2 image of Bangalore, India (image credit: NRSC)

• The commissioning phase of CartoSat-2 ended successfully at the end of July 2007 when the spacecraft was declared as operational.

• The first imagery was received at NRSA (National Remote Sensing Agency), Hyderabad, on Jan. 12. 2007.

• ISRO recovered the space capsule, SRE-1 (Space capsule Recovery Experiment), on Jan. 22, 2007, after a 12 day stay in orbit, from the Bay of Bengal, about 140 km east of the Sriharikota launch site. The de-orbit operations were conducted from the ISRO control center in Bangalore. All systems, including the heat shield, performed to satisfaction. ¹⁹⁾

• Soon after its separation from the DLA, the two solar arrays of CartoSat-2 were automatically deployed to generate the electrical power for the satellite. The satellite health is being continuously monitored from the Spacecraft Control Center of ISTRAC at Bangalore with the help of its network of stations at Bangalore, Lucknow, Mauritius, Bearslake in Russia, Biak in Indonesia, as well as support from Svalbard ground station of Norway for the initial phase of the CartoSat-2 mission.

Sensor compliment:

PAN Camera (Panchromatic Camera):

The objective is to provide imagery for cartographic applications. The optical system is designed with two mirror Ritchey-Chretien on-axis obscured reflective telescope system with a concave hyperboloidal primary mirror and convex hyperboloid secondary mirrors and the field correcting relay optics. The mirrors are made of special Zerodur glass and are light-weighted to about 60% as in CartoSat-1 series. The mirrors are mounted inside the telescope cylinder made of CFRP with special MFDs (Mirror Fixation Devices) and the whole telescope assembly is mounted to the spacecraft structure through a special suspension arrangement. The optical system is designed to provide < 1 m resolution across track. The along track GSD of 0.8 m is achieved by apparent velocity reduction by a factor of 2.5.

The spacecraft can be suitably biased to provide various modes of imaging:

1) Continuous strip monoscopic mode

2) Spot scene imaging (strips on either side of the ground track can be imaged)

3) Paint brush mode of imaging. This mode is used to increase the total swath. Both roll tilt and pitch tilt is employed.

The PAN Camera is a nadir-pointing pushbroom CCD instrument (detector line array of 12, 288 pixels), observing in the visible spectral range of 0.5-0.85 µm with a GSD (Ground Sample Distance) of < 1 m, and a swath width of 9.6 km at nadir.

Table 2: Summary of PAN Camera parameters

Figure 8: Focal plane alignment of the panchromatic camera (image credit: ISRO)

Figure 9: Configuration of the electrooptical module assembly (image credit: ISRO)

Figure 10: FOR (Field of Regard) of CartoSat-2 for various spacecraft pointing angles (image credit: ISRO) ²⁰⁾

Figure 11: Hyderabad (India) as seen by the Pan Camera of CartoSat-2 (image credit: ISRO)

Figure 12: Artist's rendition of the PSLV-7 flight spacecraft orbiting Earth (image credit: ISRO)

Figure 13: Schematic view of SRE-1 mission sequence (image credit: ISRO)

Figure 14: Illustration of the SRE-1 capsule (image credit: ISRO)

ISRO perspective of high-resolution imaging:

• With IRS-1C and IRS-1D (launch 1995 and 1997 respectively), stereo imaging was realized by obtaining the data of same area at different view angles in different orbits. While this provided ~5 m stereo images it had the stereo pair corresponding to different days. ²¹⁾

• With CartoSat-1 (launch May 5, 2005) the national requirements for high-resolution digital elevation information was met by using a dedicated same-orbit stereo imaging system, namely the two-line Pan camera, making use of an optics telescope with 0.5 m in aperture. ²²⁾

• The need for higher spatial resolution data for various applications were met by progressively improving the Earth observation system capability by continuously adopting newer technologies and using innovative techniques. The image resolution was improved from 72 m in 1988 to about 0.8 m in 2007 (Figure 15). The Pan camera of CartoSat-2 (launch Jan. 10. 2007) features a telescope of 0.7 m in aperture.

Figure 15: a) Improvement in angular resolution; b) Growing aperture of optical systems developed by ISRO (image credit: ISRO)