Chandrayaan-2 Lunar Mission of ISRO
ISRO began India's planetary exploration program with the successful launch of Chandrayaan-1 orbiter mission to the Moon in 2008. The eleven remote-sensing scientific instruments from ISRO, NASA and ESA onboard Chandrayaan-1 (launched in October 2008) have made significant findings including discovery of water signature, spinal minerals, lunar lava tubes, evidences of recent volcanism, impact-triggered boulder movements and discovery of sputtered atomic oxygen and backscattered helium on the lunar surface. 1)
The Chandrayaan-2 spacecraft to the Moon is a composite module mission consisting of Orbiter, Lander and Rover. Chandrayaan-2 is planned to be launched onboard Geosynchronous Satellite Launch Vehicle (GSLV) in the summer of 2019. The Orbiter will carry the combined stack up to moon till the Lunar Orbit Insertion (LOI). The combined stack is then inserted into a lunar orbit of 100 km x 100 km. The Lander with the Rover is then planned to be separated from the Orbiter for soft-landing on a site near south polar lunar surface.
The overall objective of Chandrayaan-2 is to build on the successes of the Chandrayaan-1 mission, testing new technologies and conducting experiments on the moon. The rover will collect samples from the lunar surface and analyze them on site, relaying data to Earth via the orbiter. The orbiter will map the contents of the surface down to a depth of a few tens of meters and carry out a detailed study of the lunar exosphere.
India's maiden Moon trip was a significant achievement for its space program, but ended prematurely when ISRO lost contact with the orbiter ten months into the planned two-year mission. However, an instrument on a probe that reached the Moon's surface did gather enough data for scientists to confirm the presence of traces of water.
Chandrayaan-2 will attempt more ambitious technical maneuvers that will put Indian space technology to the test. For the first time, ISRO will attempt to give a craft a controlled, or soft, landing. The agency has had to develop advanced systems that can guide the lander to a touch down and successfully deploy the rover.
Chandrayaan–2 objectives: 2)
• Expand technologies from Chandrayaan-1 and demonstrate newer technologies for future planetary missions.
• Deploy a Lunar Lander-Rover capable of soft landing on a specified lunar site and deploy a Rover to carry out in-situ analysis of chemicals.
• Carry payloads in the Orbiter craft to enhance the scientific objectives of Chandrayaan-1 with improved resolution.
Chandrayaan-2 is an ISRO (Indian Space Research Organization) mission comprising the 'Orbiter Craft' and the 'Lander Craft'. The primary objective of Chandrayaan-2 is to demonstrate the ability to soft-land on the lunar surface and operate a robotic rover on the surface. The scientific goals include studies of lunar topography, mineralogy, elemental abundance, the lunar exosphere, and signatures of hydroxyl and water ice.
The Orbiter Craft Module structure is a three-metric ton category bus structure made of a central composite cylinder, shear webs and deck panels. It was developed by Hindustan Aeronautics Limited (HAL) and delivered in June 2015 to ISAC (ISRO Satellite Center) where the other spacecraft subsystems and payloads were built onto the structure. 3)
Chandrayaan-2, India's second lunar mission, has three modules namely Orbiter, Lander (Vikram) & Rover (Pragyan). The Orbiter and Lander modules will be interfaced mechanically and stacked together as an integrated module and accommodated inside the GSLV MK-III launch vehicle. The Rover is housed inside the Lander. After launch into earth bound orbit by GSLV MK-III, the integrated module will reach Moon orbit using Orbiter propulsion module. Subsequently, Lander will separate from the Orbiter and soft land at the predetermined site close to lunar South Pole. 4)
Figure 1: Photo of the 'Orbiter Craft Module Structure' of Chandrayaan-2 delivered by HAL to ISAC (ISRO Satellite Center), image credit: ISRO
Furthermore, the Rover will roll out for carrying out scientific experiments on the lunar surface. Instruments are also mounted on Lander and Orbiter for carrying out scientific experiments.
Chandrayaan-2 spacecraft architecture
Chandrayaan-2 Orbiter Craft is built around a cuboidal structure and houses the propulsion tanks and the separation mechanism of the launch vehicle at one end and lander at the other end. The Orbiter decks have the different housekeeping systems of the Spacecraft. The Solar array consists of two solar panels which are stowed in the launch configuration and deployed on separation to provide the power required for the Orbiter Craft during different phases around the earth and the moon. Lithium Ion battery provides the power support during eclipse and peak power requirements of the spacecraft. Orbiter is a three-axis body stabilized spacecraft with reaction wheels which provide a stable platform for imaging. Thrusters are present for momentum dumping and attitude corrections. A bipropellant liquid engine is used to raise the orbit of the composite from earth parking orbit to 100 km lunar orbit. The attitude and orbit control electronics receive the attitude data from the star sensors and the body rates from the Gyro's for S/C control. 5)
The other sensors used for spacecraft control are Sun sensors and accelerometers. The telemetry system provides health information of the spacecraft while the telecommand system handles the command execution and distribution. The different payloads on the Orbiter are interfaced to the base band data handling system for formatting and recording in solid state recorder for play back later. The RF system consists of a S-band TTC transponder and X-band transmitter for Payload data transmission to Indian Deep Space Network (IDSN) station. The payload data is transmitted through a X-band dual gimbal antenna which will be pointed to the ground station.
Chandrayaan-2 Lander structure is a truncated pyramid around a cylinder which houses the propellant tank and the interface for the separation mechanism of Orbiter. The vertical panels have solar cells while the stiffener panels house all the electronic systems. The lander leg mechanism (four nos.) provides stability upon landing on different terrains. The body mounted solar panels provide the power for the different systems during the mission in all phases. In addition, lithium ion battery supports the power requirements during eclipse and the lander descent. The Control electronics provide the interface to all the sensors and the actuator drives. The sensors are configured for inertial navigation from separation to the end of rough braking and the absolute sensors determine the position and velocity with respect to the landing site to guide the lander beyond the rough braking phase to the identified site.
The lander Navigation guidance and control will be autonomous from separation onwards and must ensure a precise, safe and soft landing on the lunar surface. The braking thrust for decelerating the lander is provided by four nos. of liquid engines. The attitude of the lander is maintained with eight nos. of thrusters. The lander leg mechanism ensures that the energy at touch down is absorbed and all the lander systems are integral and stable for further conduct of payload deployments and science on moon. Each leg consists of a telescopic leg assembly with crushable damper material in the leg and foot pad. Extensive analysis and tests are done for the lander leg mechanism to ensure stability under extreme terrain conditions and terminal velocity. The TTC communication between the Lander – IDSN is in S-band and the payload data is transmitted by a high torque dual gimbal antenna. The Lander has a TM-TC data handling system with inbuilt storage. The Chandrayaan-2 Rover is stowed in the lander during launch and upon landing the ramps are deployed and Rover starts its journey on the lunar surface. The Lander payloads will be deployed on landing.
Chandrayaan-2 Rover is a six-wheeled mobility system with the objective of performing mobility on the low gravity & vacuum of moon and in addition conduct science for understanding the lunar resources. The design of the Rover is based on the well-proven space rover "Sojourner" that was deployed by NASA for the exploration of Mars in July 1997. Rover chassis houses all the electronics and has two navigation cameras to generate stereo images for path planning. The deployed solar panel provides the power during the mission. The rocker bogie mechanism along with the six wheels ensure a rugged mobility system over obstacles and slopes along the identified path for exploration of the region. The Rover communicates to the IDSN via the Lander. The two Rover payloads conduct science on the lunar surface.
Figure 2: Orbiter and lander in stacked configuration (lander on top) with the rover inside the lander (image credit: ISRO)
Figure 3: Chandrayaan-2 mission elements (image credit: ISRO)
Launch: A launch of the Chandrayaan-2 mission is scheduled for 9 July 2019 (with an expected Moon landing on 6 September 2019) using a GLSV (Geosynchronous Satellite Launch Vehicle) Mark III launch vehicle from the SDSC (Satish Dhawan Space Center) on Sriharikota Island. 6)
Orbit: The lander-orbiter pair will go into an initial elliptical (180 x 24000 km altitude) Earth orbit, followed by a trans-lunar injection. Both craft go into an initial elliptical lunar orbit. After orbit insertion, the lander and orbiter separate. The orbiter evolves into a 100 km circular polar orbit and the lander brakes from orbit and lands on the surface in the high latitude areas near the south pole. The orbiter portion of the mission is planned to last 1 year. The rover will be deployed using a ramp shortly after landing and is planned for 14-15 days, one period of lunar daylight.
The Chandrayaan-2 mission profile starts with the GSLV MKII launch vehicle injecting the combined stack of lunar orbiter and lander modules (wet mass ~ 3320 kg) into a Transfer Orbit. The orbiter and lander are injected to a 170 x 18500 km transfer orbit by the launch vehicle. A series of mid-course orbit raising maneuvers and the final insertion maneuver are performed to place the spacecraft in a 100 x 100 km circular lunar orbit. Based on mission planning, after achieving the desired initial conditions, the lander is separated from orbiter and a short burn de-boost is carried out to reduce the perilune to 6 km. After a long coast phase, the lander will reach the perilune. Near the perilune, a second longer de-boost burn is carried out for horizontal braking. The objective of the braking phase is to efficiently kill the horizontal velocity to 0 at desired altitude. The lander will then follow a vertical descent, during which periodic firing shall be done to reduce the vertical velocity and achieve 0 m/s velocity, at 4 m height where the thrust will be cut off. The final phase is the free fall from 4 m to impact point with touch down velocity < 5 m/s (Ref. 1).
Figure 4: Chandrayaan-2 mission profile (image credit: ISRO)
The Lander module operations from separation to touch down shall be carried out by a closed loop NGC (Navigation, Guidance and Control) system. The INS (Inertial Navigation System) alone will not be able to meet the stringent touchdown requirement of < 5 m/s in vertical and horizontal velocity. The unbounded error growth in the INS with time is corrected with the help of other absolute external measurements. An integrated navigation system consisting of an INS, star tracker (2), altimeter (2), velocimeter (2) and image sensor (2) will be utilized. The initial attitude of IMU at de-boost is determined using star tracker. The accelerometer and gyro drifts are also updated before the first burn. The state vectors are established using Deep Space Network (DSN), Orbit Determination and ground uplink and transferred to the INS system.
The lander NGC will be active before the separation from orbiter itself. The INS after updating the state vector is used for the first burn. During the long coast phase also, the attitude and gyro drifts are updated using star tracker. The accelerometer bias also is updated during the long coast phase. The INS state vector is used during the second burn. During the vertical descent phase, radar altimeter is used for the height information. Doppler velocity sensor is primarily used to measure the horizontal velocity in the terminal landing phase to ensure safe landing with a touch down velocity of < 5 m/s. Vision aiding or terrain sensor using CCD camera is used to get the image of lunar surface to avoid the obstacles and re-targeting the landing surface. 7)
The Chandrayaan-2 Lander will employ a clustered configuration of four 800 N engines along with 50 N attitude control thrusters placed at the bottom of spacecraft, to decelerate the spacecraft for braking and soft landing on lunar surface. The lander-craft will be released from lunar orbit, which will further undergo various lunar bound phases like de-boosting, rough braking, precision braking, and vertical descent. The engines will be operated together in different phases to reduce spacecraft's velocity to move from 100 km North Pole to 6 km South Pole lunar altitude location. The Lander will have on board a radio altimeter, a pattern detection camera and a laser inertial reference and accelerometer package (LIRAP). The thermal protection system has been designed to maintain the temperature of lander-craft systems within the safe limits during this phase. 8)
The Proportional Flow Control Valve (PFCV) is the heart of the system which uses a movable pintle based design as a valving element, which moves in and out of the valve flow area thus closing and opening the valve in the process. This movement is controlled by a stepper motor based actuator which will provide stroke proportional to command and thereby provide smooth and continuous flow control. 9)
The Lander-Rover module with a mass of about 1250 kg will be soft landed on the specific lunar south polar site. The Lander will deploy a Lunar Rover (~ mass 20 kg) to carry out in-situ analysis. The Rover comprises of six independently driven wheels that are connected to the body of the rover using a rocker-bogie mechanism with 10 degrees of freedom (DOF). Rover chassis houses all electronics and has two cameras for generating stereo images for path planning. The deployed solar panel provides all the power during the mission. Rover is the integration of locomotion, navigation system, communication system, manipulator and science equipment. An onboard software will allow the Rover to roam the surface of the Moon in a semi-autonomous manner. ISRO will provide partial command and control instructions from the ground.
As per a memorandum of understanding signed with ISRO, IIT Kanpur has designed, developed and validated two software algorithms (a) kinematic control algorithm for the rover motion on an uneven terrain and (b) algorithms for computer vision based autonomous navigation system for mobile robots for the lunar rover mission. The vision based system would provide the 3D map of the terrain based on which the traction control algorithm would give the safest path for the rover. The path tracking control (PTC) is based on the kinematics and dynamics model of the Rover undergoing 3D motion with slips. A slip estimator for the rover would be used in the feedback for the path tracking controller. 10)
All the six wheels of the Rover are driven by DC brushless servo motors. The front and the rear wheels also have steering motors. The rover has two rocker arms connected to the rover body through a differential. Each rocker has a rear wheel connected to one end, and a bogie connected to the other end. The bogie is connected to the rocker with a free pivoting joint. The wheels are spherical in shape and this ensures that the normal force at the terrain contact passes through the wheel center to reduce the wheel torque requirement. The maximum allowable gradient that the rover can safely climb is 35', and it can move in a terrain with a maximum of 35' sidewise slope. 11)
The inertial navigation of the lander is carried out by LIRAP (Laser gyro based Inertial Reference Unit and Accelerometer Package). The LIRAP consists of four ILG (ISRO Laser Gyro) and four CSA (Ceramic Servo Accelerometer) sensors. This sensor provides attitude referencing for the lander after it separates from the orbiter till landing. The accelerometers provide velocity increment for the liquid engine cutoff during orbit maneuvers. It also provides inertial navigation information (position, velocity & quaternions) from lander separation to touchdown. One of the key elements essential for safe landing is the Hazard Detection and Avoidance (HDA) system. The HDA system comprises of several sensors like Orbiter High Resolution Camera (OHRC) for characterization of landing Site, Cameras for Horizontal velocity calculation, Camera for pattern matching and position estimation, Microwave and Laser altimeter, Laser Doppler velocimeter. All these sensors provide information like lander's horizontal velocity, vertical velocity, height above moon's surface, relative position of the lander w.r.t moon's surface, hazard/safe zone around the landing site. The HDA system onboard the lander processes the inputs from the various sensors, compares the data collected with the information already stored in the lander and provides the required inputs to the Navigation and Guidance system in real time to correct the trajectory at the end of rough braking to enable a safe and soft landing (Ref. 5).
Figure 5: Chandrayaan-2 Lunar Lander (with Rover) Soft Landing Sequence (image credit: ISRO)
Chandrayaan-2 Mission Operations
India's Chandrayaan-2 spacecraft consisting of an Orbiter module and a Lander (with Rover) module is expected to be launched in July 2019 . The GSLV MKII Rocket will place the Chandrayaan-2 spacecraft in a highly elliptical EPO (Earth Parking Orbit) of 170 km x 18,500 km. The Chandrayaan-2 spacecraft's Orbiter propulsion system raises the orbit around the Earth through a number of Earth burn maneuvers and propels the composite to a Lunar transfer trajectory. Further, the Orbiter gets captured into a Moon orbit through a precise maneuver by the propulsion system of the Orbiter. Further, maneuvers around the moon are planned such that the orbital path of the composite in the 100 km circular polar orbit will be over the landing site at the identified day (Ref. 5).
The day of launch / day and position of insertion into the Lunar Orbit will be timed so as to maximize the life of the Lander and Rover missions. This constraint will be met by proper planning of the Launch vehicle insertion parameters, orbit raising maneuvers and Lunar capture geometry with respect to Sun and Earth. The Orbital parameter of the Chandrayaan-2 spacecraft composite when around the moon will have to be precisely determined and corrections made so as to ensure that the composite is at the separation point at the pre-determined time. Once at this point, the Orbiter / Lander separation system will separate the two modules. The Chandrayaan-2 Orbiter will continue to orbit around the Moon and will perform the science over the moon.
On separation, a de-boost maneuver at 100 km altitude, causes a free fall of Lander to 18 km altitude. Powered descent to the designated landing site is initiated using a closed loop NGC (Navigation, Guidance and Control) system to ensure a precise soft landing at touchdown. The Lander, which will be travelling at 1.7 km/s at 100 km, on separation, will be de-orbited [by a Hohmann transfer] by firing its braking engines to reach a periapsis of 18 km. The Lander module will be precisely navigated as per plan with the onboard INS (Inertial Navigation System). Once at the periapsis the rough braking phase is initiated. During this powered descent phase the attitude of the Lander will be precisely controlled and the NGC system with the help of the inertial sensors will provide the closed loop feedback for the actuator systems. At the end of the rough braking phase [~7 km], the Hazard avoidance sensors will sense the position and velocity of the Lander with reference to the landing site. Based on the relative position and velocity with respect to the pre-determined landing site on the moon's surface, the further trajectory is planned on board and the sensors along with actuators will guide the Lander to a position over the landing site [~100 m]. At this point, the Lander hovers over the site and the hazard avoidance sensor will determine the safest landing point in the near vicinity and the Lander will be maneuvered to this point. At a height of 2 m, upon ensuring that the relative velocity with reference to moon surface is zero, the braking engines are cut off. The Lander freely falls to the surface and the landing leg mechanism will absorb the impact loads and ensure the integrity of the Lander for further operations. The entire operation from separation to touch down is fully autonomous and must be performed by the onboard computers in the Lander without any intervention from ground.
The onboard guidance algorithm takes current position and velocity from Navigation (at every guidance cycle) and generates steering profile in realtime by considering the end target states. The steering profile decides the magnitude of the thrust for each engine and the required attitude for the lander. The attitude controller tracks the reference attitude while ensuring closed loop stability. Inertial Navigation is prone to errors due to factors such as error in initial states, propagation errors and inherent inaccuracies. This needs to be corrected with updates from Absolute Navigation sensors. When the lander is at a height of 7 km from moon's surface, the absolute position of the lander with respect to the landing site is determined using the Lander position detection camera. In addition, at this instance the horizontal and vertical velocity, absolute height with respect to moon's surface are derived from the onboard instruments and provided to the closed loop NGC system for further refinement of the trajectory.
Given the absence of an atmosphere on the moon, active deceleration by thrusting utilizing a bipropellant system with four 800 N engines will be performed. Eight 50 N thrusters are used to ensure the required orientation during the entire phase of the descent. The error ellipse at separation (100 km) which is due to a composite state uncertainty, increases with time in view of the inertial navigation errors. To correct the same, at 7 km it is required to have controllability in the engine thrust and the same is obtained by providing throttlability in all the four engines. This variability in the engine thrust ensures a safe and soft landing at the identified site irrespective of the accumulated errors at the end of rough braking phase. The Lander follows the descent trajectory and after a short hovering phase at 100 m for reconfirmation of the safe landing site lands at the identified site. Once the Lander has landed on the surface, the Rover is deployed, and the Rover commences its journey on the moon surface.
Semi-autonomous navigation of the Rover is enabled by a pair of navigation cameras mounted on the Rover that are capable of taking images of the moon's surface in front of the Rover. These images are sent to the ground control center where the Digital Elevation Model of these images is created. Based on this data, the path in which the Rover can move is decided and the same is uplinked to the Rover (via Lander). The slope that the Rover can navigate, the size of the boulder that the Rover can climb, the sinkage/ slippage are the basic inputs that are considered while planning the path for Rover movement. An inclinometer mounted on the chassis of the Rover computes the slope being navigated on the moon's surface and the same is used for safety reasons to terminate the motion in case the safe limits are exceeded. Other similar autonomous safety parameters like motor wheel current, communication feasibility with Lander and power generation from solar panel in view of shadows are monitored to ensure safety of the Rover during mobility.
Orbiter Sensor Complement (CLASS, XSM, IIRS, SAR, CHACE-2, TCM-2)
The Chandrayaan-2 orbiter will orbit the Moon at an altitude of 100 km. The mission will carry five instruments on the orbiter. Three of them are new, while two others are improved versions of those flown on Chandrayaan-1. 12)
CLASS (Chandrayaan-2 Large Area Soft X-ray Spectrometer)
CLASS is provided by ISAC (ISRO Satellite Center), Bengaluru. The objective of CLASS is to map the abundance of the major rock forming elements on the lunar surface using the technique of X-ray fluorescence during solar flare events. CLASS is a continuation of the successful CIXS (Chandrayaan-1 Imaging X-ray Spectrometer) XRF experiment on Chandrayaan-1 (of RAL, UK). CLASS is designed to provide lunar mapping of elemental abundances with a nominal spatial resolution of 25 km (FWHM) from a 100 km polar, circular orbit of Chandrayaan-2. The science objectives of CLASS are to make global studies on the diversity and distribution of lunar lithologies, quantitative estimate of Mg abundance, essential for determining the distribution of Mg suite rocks, bulk composition of the crust, abundance patterns in the major crustal provinces and mare basalt diversity. CLASS is expected to provide global maps of major elements from Na to Fe at resolutions of a few tens of kilometers. Together with mineralogical data this would provide a comprehensive picture of lunar surface chemistry. 13)
Figure 6: CLASS instrument showing the four quadrants with four SCDs (Swept Charge Devices) each. The electronics is housed in the box behind the detector units. An aluminum door protects the detectors from radiation damage en-route to the Moon. Passive radiators connected to heat pipes provide the required low-temperature environment for the detectors (image credit: ISRO)
XSM (Solar X-ray Monitor)
XSM is provided by the PRL (Physical Research Laboratory) of Ahmedabad for mapping the major elements present on the lunar surface. XSM instrument will have two packages namely, the XSM sensor package and the XSM electronics package. XSM will accurately measure spectrum of Solar X-rays in the energy range of 1–15 keV with energy resolution ~200 eV @ 5.9 keV. This will be achieved by using state-of-the-art Silicon Drift Detector (SDD), which has a unique capability of maintaining high energy resolution at very high incident count rate expected from Solar X-rays. XSM onboard Chandrayaan-2 will be the first experiment to use such detector for Solar X-ray monitoring. 14)
IIRS (Imaging IR Spectrometer)
The IIRS instrument is provided by SAC of Ahmedabad. The goal is to map the lunar surface over a wide wavelength range for the study of minerals, water molecules and hydroxyl present. Coverage in the 0.8 -5 µm spectral range for lunar mineralogy and trace signatures of hydroxyl (OH) and water (H2O) molecules in polar regions. Study of mare volcanism, variations in basaltic compositions, mantle heterogeneity at basin and local scale.
SAR (Synthetic Aperture Radar in L- and S-band)
SAR was developed at SAC (Space Applications Center), Ahmedabad for probing the first few tens of meters of the lunar surface for the presence of different constituents including water ice. SAR is expected to provide further evidence confirming the presence of water ice below the shadowed regions of the moon.
The S-band SAR will provide continuity to the Chandrayaan-1MiniSAR data, whereas the L-band is expected to provide deeper penetration of the lunar regolith. The system will have a selectable slant-range resolution from 2 m to 75 m, along with standalone (L- or S-band) and simultaneous (L- and S-band)modes of imaging. Various features of the instrument like hybrid and full-polarimetry, a wide range of imaging incidence angles (~10º to ~35º) and the high spatial resolution will greatly enhance our understanding of surface properties especially in the polar regions of the Moon. The system will also help in resolving some of the ambiguities in interpreting high values of Circular Polarization Ratio (CPR) observed in MiniSAR data. The added information from full-polarimetric data will allow greater confidence in the results derived particularly in detecting the presence (and estimating the quantity) of water–ice in the polar craters. 15)
Being a planetary mission, the L&S-band SAR for Chandrayaan-2 faced stringent limits on mass, power and data rate (15 kg, 100 W and 160 Mbit/s, respectively), irrespective of any of the planned modes of operation. This necessitated large-scale miniaturization, extensive use of on-board processing, and devices and techniques to conserve power. This paper discusses the scientific objectives which drive the requirement of a lunar SAR mission and presents the configuration of the instrument, along with a description of a number of features of the system, designed to meet the science goals with optimum resources.
CHACE-2 (Neutral Mass Spectrometer)
CHACE-2, developed at SPL (Space Physics Laboratory), Thiruvananthapuram to carry out a detailed study of the lunar exosphere. The CHACE-2 mass spectrometer aboard the Chandrayaan-2 orbiter will study the lunar exosphere from 100 km polar orbit in the range of 1 to 300 amu with 1 amu mass resolution. CHACE- 2 will cover the polar regions of the moon including the permanently shadowed regions (PSR), which are believed to be pristine enough to retain the history of the inner solar system. Taking advantage of the axial rotation of the Moon and the polar orbit of Chandrayaan-2, the CHACE-2 will be useful to study the global distribution of the lunar exosphere. It will also study the day-night variation of the lunar neutral exosphere as well as the variation during the passage through the geomagnetic tail. 16)
TMC-2 (Terrain Mapping Camera-2)
TCM-2 is provided by SAC (Space Applications Center), Ahmedabad. The objective is to prepare a three-dimensional map essential for studying the lunar mineralogy and geology.
Vikram Lander Sensor Complement (RAMJBHA, ChaSTE, ILSA, LRA)
The Chandrayaan-2 Vikram Lander will detach from the orbiter and descend to a lunar orbit of 30 km x 100 km using its 800 N liquid main engines. It will then perform a comprehensive check of all its on-board systems before attempting to land on the lunar surface.
RAMBHA (Radio Anatomy of Moon Bound Hypersensitive ionosphere and Atmosphere)
The RAMBHA instrument is provided by the SPL (Space Physics Laboratory), Thiruvananthapuram. RAMBHA is unique payload package that would provide a comprehensive exploration of the lunar plasma environment. RAMBHA consists of a suite of three experiments, a LP (Langmuir Probe) and a DFRS (Dual Frequency Radio Science) experiment to measure the density of the lunar near-surface plasma and how it changes over time. DFRS will measure the total electron content of lunar ionosphere. 17)
ChaSTE (Chandra's Surface Thermo Physical Experiment)
ChaSTE is provided by the PRL (Physical Research Laboratory), Amehdabad. The goal of ChaSTE is to measure the vertical temperature gradient and thermal conductivity within the top 10 cm of the regolith. The experiment contains a thermal probe which will be deployed up to ~10 cm into the lunar regolith at the landing site. A harness, running from the probe, will connect the probe to the electronics placed inside the lander. An important aspect of the payload is the design of a precise and wide-range temperature measurement front-end (FE) and the selection of a custom developed Platinum RTD, PT1000 as a sensing element. 18)
ILSA (Instrument for Lunar Seismic Activity)
The ILSA instrument is provided by ISRO. The objective is to measure seismicity around the landing site.
LRA (Laser Reflector Array)
The LRA instrument is provided by NASA/GSFC for precise measurements of the Earth–Moon distance.
Rover Sensor Complement (APXS, LIBS)
APXS (Alpha Particle X-ray Spectroscope)
The APXS is provided by PRL (Physical Research Laboratory) of Ahmedabad. The objective is to study the elemental composition of Lunar rock and soil onboard Chandrayaan-2 rover by irradiation the lunar surface with alpha particles and X-rays using a radio-active alpha source. The working principle of APXS involves measuring the intensity of characteristic X-rays emitted from the sample due to Alpha Particle Induced X-ray Emission (PIXE) and X-ray florescence (XRF) processes using 241Am alpha source which allows the project to determine elements from Na to Br, spanning the energy range of 0.9 to 16 keV. The electronics design of the APXS experiment has been completed and shown that the developed system provides energy resolution of ~150 eV @ 5.9 keV which is comparable to an off-the-shelf SDD (Silicon Drift Detector) based X-ray spectrometers. 19)
The APXS instrument consists of two packages namely APXS sensor head and APXS backend electronics. APXS sensor head will be mounted on a robotic arm. On command, the robotic arm brings the sensor head close to the lunar surface (without touching it) and after the measurement, the sensor head is taken back to the parking position. APXS sensor head assembly contains SDD, six alpha sources and front end electronic circuits such as charge sensitive preamplifier (CSPA), shaper and filter circuits associated with the detector. Sensor head contains a circular disc which holds 6 alpha sources symmetrically around the disc and the detector at the center.
LIBS (Laser induced Breakdown Spectroscope)
LIBS was developed at LEOS (Laboratory for Electro Optic Systems), Bengaluru. The objective is to perform simultaneous multi-element determination of matter in any of its diverse forms, namely, solid, liquid or gas using an intense nanosecond pulse duration of laser beam of lunar regolith from an in-situ distance of 200 mm from the surface. The plasma emission emanating from the target surface is collected by a chromatic aberration corrected COU (Collection-Optics-Unit) and spectra are acquired using an aberration corrected concave holographic grating and linear-CCD based spectrograph. The spectrograph supports variable time delay in range of 1µs to 5 µs and integration time of 8 µs to 1ms. The LIBS instrument is realized with the mass of 1.2 kg, power consumption of <5 W and a footprint of 180 mm x 150 mm x 80 mm. 20)
Figure 7: Chandrayaan-2 rover in operational configuration (image credit: ISRO)
The Chandrayaan-2 Mission will utilize the ISRO ground segment consisting of the following four main entities: 21)
• The ISRO MOX (Mission Operations Complex) is located at Peenya campus of ISTRAC (ISRO Telemetry, Tracking and Command Network) near Bangalore in the state of Karnataka. MOX has facilities such as the Main Control Room, the Mission Analysis Room, Mission Planning and Flight Dynamics, the Mission Scheduling and Payload Scheduling Facility. Mission and spacecraft specialists along with the operations crew from ISTRAC carry out operations from the MOX.
• IDSN (Indian Deep Space Network) consisting of 11 m, 18 m and a 32 m antenna were established at the IDSN campus in Byalalu near Bangalore as part of the Chandrayaan-1 mission ground segment. The IDSN station will receive the Chandrayaan-2 spacecraft health data as well as the payload data. For the orbit raising phase, the TTC functions will be executed by ground stations of the ISTRAC network (Bangalore, Mauritius, Port Blair, Brunei, Biak, Trivandrum). The NASA/JPL DSN (Deep Space Network of Goldstone, Canberra, and Madrid) will provide deep space communication with the Chandrayaan-2 Orbiter as requisitioned.
• ISSDC (Indian Space Science Data Center) is a new facility established by ISRO for the Chandrayaan-1 and future deep space missions, as the primary data center for the payload data archives of Indian Space Science Missions. This data center, located at the IDSN (Indian Deep Space Network) campus in Bangalore, is responsible for the ingestion, archive, and dissemination of the payload data and related ancillary data for the Space Science missions. ISSDC interfaces with Mission Operations Complex (MOX) through dedicated communication links, Data reception centers, Payload designers, Payload operations centers, Principal investigators, Mission software developers and Science data users.
• POCs (Payload Operation Centers) focus on the higher levels of science data processing, planning of payload operations, performance assessment of the payload and payload calibration. These centers are co-located with the institutions/laboratories of the Instrument designers, Principal Investigators and will be processing and analyzing data from a specific payload. POCs will pull relevant payload (level 0 and level 1) and ancillary data sets from the ISSDC dissemination server and process the data to generate higher level products. These products will be archived in ISSDC after qualification.
Figure 8: Photo of the 18 m and 32 m antennas of the IDSN (Indian Deep Space Network), image credit: ISRO
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The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (email@example.com).