Chandrayaan-1 Lunar Mission
Chandrayaan-1 is India's first mission to the moon, designed and developed by ISRO (Indian Space Research Organization), Bangalore, India. This science mission represents a new dimension and a true challenge in the Indian Space Program. In Hindi, the term chandra = moon, yaan= ship (a litteral translation is MoonCraft). Major science mission objectives are: 1) 2)
• To provide high-resolution mineralogical and chemical imaging of the global surface of the moon, with stereographic coverage of most of the moon's surface with 5 m resolution, to provide new insights in understanding the moon's origin and evolution
• To search for surface or sub-surface water-ice, especially at the lunar poles and study of permanently shadowed north and south polar regions
• To identify chemical end members of lunar highland rocks
• To observe the X-ray (and γ-ray) spectrum in the energy region of 10-200 keV to provide information on lunar volatiles.
Figure 1: Artist's rendition of Chandrayaan-1 in lunar orbit (image credit: ISRO)
Background: A lunar program initiative at ISRO was started by K. Kasturirangan, Chairman of ISRO in 1999. Initial planning of the moon mission at ISRO started in 2000. The Chandrayaan mission was announced by the Prime Minister of India, Mr. Atal Bihari Vajpayee, on August 15, 2003 (Indian Independence Day) as a goal of India's Space Program.
In January 2004, an AO (Announcement of Opportunity), released by ISRO, invited proposals for additional international instruments. A total payload mass of 10 kg, out of the planned 55 kg payload, were offered to the international scientific community. Of the 16 international proposals received, 5 were selected for flight. In September 2004, ISRO announced a possible launch date of the Chandrayaan-1 mission for the fall 2007 or early 2008. As of May 2006, the ISRO sensor complement onboard Chandrayaan-1 consists of the following instruments/payloads: TMC, HySI, LLRI and HEX, as well as MIP (Moon Impact Probe).
• The heads of ESA and ISRO signed a cooperative agreement on June 27, 2005 to include European instruments on board the Chandrayaan-1 spacecraft. ESA provides three instruments of SMART-1 heritage: CIXS, SARA, and SIR-2; in addition, ESA contributes to the hardware of HEX, an ISRO instrument. ESA and ISRO will also share the data resulting from their respective experiments. - A series of discussions between ESA and ISRO concerning the Indian lunar mission were already held at the occasion of lunar and planetary conferences in the timeframe 2002-2004.
• Another agreement was signed between ISRO and the Bulgarian Academy of Sciences. The instrument selected is RADOM.
• The Chandrayaan-1 mission represents also an important step forward in U.S.-India space ties. On May 9, 2006, a MOU (Memorandum of Understanding) was signed between ISRO (G. Madhavan Nair) and NASA (Michael D. Griffin)) in Bangalore, India, to include two NASA instruments (Mini-SAR and M3) on the Chandrayaan-1 spacecraft.
The visit of the NASA Administrator Michael Griffin was predated by an India-US “Conference on Space Science, Applications and Commerce” in Bangalore during June 21-25, 2004. An outcome of this conference was the establishment of a Joint Working Group to enhance the cooperation in civil space projects between India and USA. The Joint Working Group met for the first time in June 2005 in Bangalore.
These instrument commitments along with other operational services and collaborations make Chandrayaan-1 a truly international cooperative moon mission.
The spacecraft design makes use of the flight-proven IRS series bus (and of Kalpana-1/MetSat-1 heritage). The spacecraft bus is a cube of 1.5 m side length with a dry mass of 550 kg. Electric power of 750 W is provided by a canted solar array which charges a Li-ion battery.
A bipropellant propulsion system is used to transfer Chandrayaan-1 into lunar orbit and to maintain attitude. The spacecraft is 3-axis stabilized using attitude control thrusters and reaction wheels. Attitude knowledge is provided by star sensors, accelerometers, and an inertial reference unit.
Figure 2: Schematic view of the Chandrayaan spacecraft (image credit: ISRO)
RF communications: The S-band is used for TT&C operations while the X-band provides the downlink for all science data. The onboard SSR-1 (Solid State Recorder-1) is downlinked in X-band at a data rate of 8.4 Mbit/s. The SSR-1 capacity is sized for a 20 minute source data input at 50 Mbit/s (the readout/downlink period is 50 minutes). A data compression scheme is used (both lossy and lossless) to reduce the downlink volume. 5)
The initial GTO and cruise phases of the spacecraft are being tracked by ISRO's existing S-band network of ISTRAC (ISRO Telemetry and Command Center) stations, up to a slant range of 100, 000 km. Furthermore, a DSN station is being proposed to be installed at Bangalore, and a limited worldwide DSN network of NASA in the early mission phases. 6)
Figure 3: Overview of the communication link architecture for the Chandrayaan-1 mission (image credit: ISRO)
Figure 4: Photo of the Chandrayaan-1 spacecraft during integration (image credit: ISRO)
Launch and launch sequence: A launch of Chandrayaan-1 spacecraft took place on October 22, 2008 on a PSLV (Polar Satellite Launch Vehicle, PSLV-C11) from the Satish Dhawan Space Center (SDSC) in Sriharikota, India. 7) 8)
• The launch mass is 1050 kg and the initial orbit is a quasi-GTO (Geosynchronous Transfer Orbit) of 240 km x 24,000 km and an inclination of 18º. 9)
• Subsequently, the spacecraft's own propulsion system will be used intermittently for the lunar transfer trajectory (LTT) in a period of about 5.5 days.
• After the cruse phase to the moon, the spacecraft will be captured into an initial 1000 km lunar insertion orbit which will be lowered to a 200 km (checkout orbit), and finally into a 100 km altitude circular polar orbit (science orbit of the moon).
• The lunar orbital mass of the spacecraft is 550 kg, of which the payload mass is 55 kg (nominal mission design life of 2 years).
Lunar orbit: Once Chandrayaan-1 is in its final circular polar orbit at a 100 km altitude, a MIP (Moon Impact Probe) of 25 kg mass will be released and descend to the lunar surface in a hard landing mode. - MIP itself carries 3 more instruments: a high-resolution mass spectrometer, an S-band altimeter, and a video camera that can study the lunar surface as it crashes onto the moon. The effect of the device's impact will be used to analyze the chemical composition of the moon's dust.
Figure 5: Artist's view of the S/C mission profile toward the moon (image credit: ISRO)
Figure 6: LEOP (Launch and Early Orbit Phase) orbit determination of Chandrayaan-1 (image credit: ISRO Ref. 8)
Findings of water on the moon:
• August 2013: Scientists have detected magmatic water — water that originates from deep within the Moon's interior — on the surface of the Moon. These findings represent the first such remote detection of this type of lunar water, and were arrived at using data from NASA's M3 (Moon Mineralogy Mapper) on Chandrayaan-1 of ISRO.10) 11)
Spectroscopic data from the M3 instrument were analyzed which showed that the central peak of Bullialdus Crater is significantly enhanced in hydroxyl relative to its surroundings. It is concluded that the strong and localized hydroxyl absorption features are inconsistent with a surficial origin. Instead, they are consistent with hydroxyl bound to magmatic minerals that were excavated from depth by the impact that formed Bullialdus Crater. Furthermore, estimates of thorium concentration in the central peak using data from the Lunar Prospector orbiter indicate an enhancement in incompatible elements, in contrast to the compositions of water-bearing lunar samples. 12)
Figure 7: The 1km high central peak of Bullialdus crater (image credit: NASA/GSFC/Arizona State University)
M3 fully imaged the large impact crater Bullialdus in 2009. It’s within 25º latitude of the equator and so not in a favorable location for the solar wind to produce significant surface water. The rocks in the central peak of the crater are of a type called norite that usually crystallizes when magma ascends but gets trapped underground instead of erupting at the surface as lava. Bullialdus crater is not the only location where this rock type is found, but the exposure of these rocks combined with a generally low regional water abundance enabled us to quantify the amount of internal water in these rocks. 13) 14)
The detection of internal water from orbit means scientists can begin to test some of the findings from sample studies in a broader context, including in regions that are far from where the Apollo sites are clustered on the near side of the moon. For many years, researchers believed that the rocks from the moon were bone-dry and any water detected in the Apollo samples had to be contamination from Earth (Ref. #.
• In early March 2010, NASA and ISRO are reporting, that Mini-SAR (also spelled as “MiniSAR”) data analysis has revealed ice deposits near the moon's north pole. Mini-SAR had imaged many of the permanently shadowed regions that exist at both poles of the moon. These dark areas are extremely cold and it has been hypothesized that volatile material, including water ice, could be present in quantity here. The main science object of the Mini-SAR experiment was to map and characterize any deposits that exist. 15) 16)
The Mini-SAR instrument found more than 40 small craters (2-15 km in diameter) with sub-surface water ice located at their base. The interior of these craters is in permanent sun shadow. Paul Spudis, the PI of Mini-SAR said: “The new discoveries by Chandrayaan-1 and other lunar missions show that the moon is an even more interesting and attractive scientific, exploration and operational destination than people had previously thought."
Legend to Figure 8: Mini-SAR map of the Circular Polarization Ratio (CPR) of the north pole of the Moon. Fresh, “normal” craters (red circles) show high values of CPR inside and outside their rims. This is consistent with the distribution of rocks and ejected blocks around fresh impact features, indicating that the high CPR here is surface scattering. The “anomalous” craters (green circles) have high CPR within, but not outside their rims. Their interiors are also in permanent sun shadow. These relations are consistent with the high CPR in this case being caused by water ice, which is only stable in the polar dark cold traps. The project estimates over 600 million cubic meters (1 cubic meter = 1 metric ton) of water in these features.
• In late September 2009, a team of scientists announced finding water molecule signatures across much of the moon's surface with measurements taken by the M3 (Moon Mineralogy Mapper) of NASA. 17)
• In October 2009, a second instrument on board India's Chandrayaan-1’s lunar orbiter confirmed how the water is being produced. The SARA (Sub keV Atom reflecting Analyzer) instrument of ESA and ISRO confirms the findings of the M3 instrument that solar hydrogen nuclei are indeed being absorbed by the lunar regolith (a loose collection of irregular dust grains on the lunar surface known as regolith). However, SARA data show that not every proton is absorbed. One out of every five rebounds into space. In the process, the proton joins with an electron to become an atom of hydrogen. - The moon acts like a big sponge that absorbs electrically charged particles given out by the sun. These particles interact with the oxygen present in some dust grains on the lunar surface, producing water. 18)
Figure 9: SARA measurements of hydrogen flux on the moon (image credit: ESA)
• On Aug. 29, 2009, ISRO lost abruptly contact with the Chandrayaan-1 spacecraft. All attempts failed to recover the mission. As a consequence, ISRO declared the mission to be terminated. The spacecraft has completed 312 days in orbit making more than 3400 orbits around the moon. The planned mission of two years was cut in half. ISRO said the mission was a great success, about 95% of the mission objectives were completed including the collection of over 70,000 images. 19) 20)
• A planned bi-static radar experiment, involving the Mini-SAR instrument on Chandrayaan-1 and on the LRO spacecraft of NASA, was attempted on Aug. 20, 2009. The objective was to look for possible water ice hiding in polar craters on the moon from locations in nearby orbits. However, while the Mini-SAR on LRO performed the observation, the Mini-SAR on the Chandrayaan-1 spacecraft wasn't able to point into the common target direction to conduct the joint observations in stereo. 21)
• On July 17, 2009, the Chandrayaan-1 spacecraft completed 3000 orbits around the moon. The onboard star sensor used for determining the orientation of the spacecraft started malfunctioning on April 26, 2009. To overcome this anomaly, ISRO devised an innovative technique of using redundant sensors (gyroscopes) along with antenna pointing information and images of specific location on the surface of the moon, for determining the orientation of the spacecraft.
• Orbit raising maneuver on May 19, 2009. After the successful completion of all the major mission objectives, the orbit of Chandrayaan-1 spacecraft, which was at an altitude of 100 km from the lunar surface since November 2008, has now been raised to 200 km. The spacecraft in this higher altitude will enable further studies on orbit perturbations, gravitational field variation of the Moon and also enable imaging lunar surface with a wider swath. 22) 23)
• In April 2009, the Mini-SAR instrument neared completion of the first mapping cycle of the lunar surface. 24)
">• In early 2009, the star tracker of the AOCS (Attitude and Orbit Control Subsystem) malfunctioned requiring backup solutions with a gyroscope to maintain the attitude of the spacecraft. Prior to the start of lunar operations in Nov. 2008, Chandrayaan-1 suffered already from overheating. But ISRO was able to change the spacecraft's orientation and cut down on the amount of time the instruments were used to compensate.25)
• On Dec. 12, 2008, the CIXS (Chandrayaan-1 Imaging X-ray Spectrometer) instrument of the UK and ESA detected the first X-ray signal from the moon. The solar flare that caused the X-ray fluorescence was exceedingly weak, approximately 20 times smaller than the minimum C1XS was designed to detect, which is good news for the sensitivity of the instrument. The detection is a key step in mapping the mineralogical composition of the moon’s surface to study its origin and evolution. 26) 27)
• The spacecraft has about 183 kg fuel onboard. Orbital maneuvers need to be carried out on the spacecraft once every 28 days to ensure that it stays in the designated 100 km circular orbit and does not go astray. About three kg fuel is used when onboard motors are fired for carrying out the orbital maneuver.
• The MIP (Moon Impactor Probe) was released on Nov. 14, 2008 from the Chandrayaan-1 spacecraft and touched down on the lunar surface 25 minutes after its ejection (the impact was close to the Shackleton crater a place close lunar south pole). MIP carried three instruments: a video imaging system, a radar altimeter and a mass spectrometer. The video camera took pictures of the Moon as it approached the surface, the radar was used to determine the altitude, and the mass spectrometer was used to study the thin lunar atmosphere. 28)
• Chandrayaan-1 spacecraft successfully reached its final operational orbit around the moon on November 12, 2008. The spacecraft is now circling the moon at an altitude of about 100 km. - After being captured into lunar orbit on November 8, the spacecraft performed three orbit reduction maneuvers. 29)
• Chandrayaan-1 successfully entered lunar orbit on November 8, 2008. The spacecraft fired its engines to reduce velocity and enable the moon's gravity to capture it; the engines were fired for 817 seconds when Chandrayaan-1 was about 500 km away from the moon (504 km x 7502 km current orbit which takes about 11 hours).
• On November 4, 2008, Chandrayaan-1 entered the lunar transfer trajectory and is heading to an apogee of 3,80,000 km. In the fifth and final orbit-raising maneuver, the spacecraft’s 440 N liquid-fuel propelled engine was fired for about two and a half minutes. The spacecraft, which is being tracked from ISTRAC (ISRO Telemetry, Tracking and Command Network) in Bangalore, India, is working nominally.
Sensor complement: (TMC, HySI, LLRI, HEX, MIP, CIXS, SARA, SIR-2, Mini-SAR, M3, RADOM)
The payload consists of 5 devices provided by ISRO plus devices solicited by the international community. India received 16 proposals from around the world to be a part of the moon mission. NASA is providing two instruments while ESA developed three devices, one instrument comes from the Bulgarian Space Laboratory. 30) 31) 32)
Figure 10: Overview of the sensor complement location of the Chandrayaan-1 spacecraft (image credit: ISRO)
TMC (Terrain Mapping stereo Camera):
TMC is an ISRO instrument, designed and developed at SAC (Space Application Center), Ahmedabad, India. TMC is observing in the panchromatic band (400-900 nm) with a spatial resolution of 5 m on a swath of 20 km. The objective is to create a high-resolution atlas in 3-D of the moon's surface. TMC features along-track three-line stereo pushbroom imaging with three linear array detectors for nadir, fore and aft viewing (the 3 linear arrays are in the focal plane of a single lens). The fore and aft view angle is ±25º with respect to nadir. Each of the 3 cameras has a source data rate of 12.7 Mbit/s. 33)
Figure 11: Conceptual view of the TMC (image credit: ISRO)
Table 1: Some parameters of the TMC instrument
Figure 12: Stereoscopic imaging concept of the 3-line pushbroom camera (image credit: ISRO)
HySI (Hyperspectral Imager):
HySI is an ISRO instrument, designed at developed at SAC, Ahmedabad, India. The instrument is observing in the band 400-930 nm with a spectral resolution of 15 nm and a spatial resolution of 80 m on a swath of 20 km. The objective is mineralogical mapping. HySI features a wedge filter coupled to an area array detector. The wedge filter is an interference filter of varying thickness along one dimension. There are 32 continuous channels available (spectral resolution ≤ 15 nm). The detector array type (256 x 512 pixels) is of APS (Active Pixel Sensor) design. The source data rate of HySI is 3.8 Mbit/s.
Figure 13: Observation scheme of HySI (image credit: ISRO)
Figure 14: Conceptual view of HySI EO module (image credit: ISRO)
Table 2: Parameters of the HySI instrument
Figure 15: Photo of the HySI instrument (image credit: ISRO)
LLRI (Lunar Laser Ranging Instrument):
LLRI is an ISRO device, developed at the Laboratory for Electro-Optics Systems, Bangalore, India. The objective is to provide a means for lunar orbit determination of the spacecraft in support of topological mapping (gravity model). LLRI is a pulsed Nd:YAG solid-state laser (1064 nm laser beam, energy of 20-50 mJ), using a 17 cm optics receiver coupled to a Si-APD (Silicon-Avalanche Photodiode) detector. LLRI operates at 10Hz (5ns pulse) providing a vertical resolution of ≤ 5 m. LLRI and TMC will provide complementary data for generating a topographic map of the moon. The LLRI, in particular, will provide the first such data set for the polar region.
Table 3: Parameters of the LLRI
Figure 16: Illustration of LLRI and functional overview (image credit: ISRO)
HEX (High Energy X-ray/γ-ray Spectrometer):
HEX is an ISRO instrument (SAC, Ahmedabad) with an ESA contribution. HEX measures in the energy range of 20-250 keV with a ground spatial resolution of about 20-40 km. The objective is to detect such items as: 219Pb, 222Rn degassing, U, Th, etc. to investigate the phenomenon of transport of volatiles on the lunar surface.
Figure 17: Illustration of the HEX instrument (image credit: ISRO)
HEX employs CdZnTe solid-state detectors (also referred to as CZT detectors) arrays, each 4 cm x 4 cm (5 mm thick), composed of 256 (16 x 16) pixels (size: 2.5 mm x 2.5 mm). Each CZT array is readout using two closely mounted ASICs, which provide a self-triggering capability. A specially designed collimator provides a FOV (Field of View) of 40 km x 40 km. The mass of the HEX device is about 16 kg, a power consumption of 24 W, and a size of 180 mm x 145 mm x 194 mm.
HEX uses a CsI anticoincidence system for reducing background noise. The instrument observations are primarily intended for the study of volatile transport on the moon using the 46.5 keV γ-ray line from 210Pb decay as tracer. An attempt will be made to infer compositional characteristics of the lunar terrain from a study of the continuum background in this energy range as well as low resolution Th and U mapping of terrains enriched in these elements.
Figure 18: Schematic of CdZnTe detector array with collimator (image credit: ISRO)
Figure 19: Photo of the HEX instrument (image credit: ISRO)
MIP (Moon Impactor Probe):
MIP is an ISRO instrument, developed by the Vikram Sarabhai Space Center (VSSC), Trivandrum, India. The primary objective is landing the probe at the desired location and to qualify some technologies for a soft landing mission.
MIP is carrying a highly sensitive mass spectrometer, a video camera, and a radar altimeter. The impactor will be released at the beginning of the mission and an attempt will be made to land the probe in a predetermined location on the lunar surface. Apart from the video imaging of the landing site, the onboard mass spectrometer will try to detect a possible presence of trace gases in the lunar exosphere.
Figure 20: Illustration of the MIP structure with all subsystems (image credit: ISRO)
MIP has a mass of 29 kg and rides on the top deck of the main orbiter. After release and during the descent phase, the payload is spin stabilized. The descent phase to the surface of the moon is estimated to be about 20 minutes.
CIXS (Chandrayaan-1 Imaging X-ray Spectrometer):
CIXS is an ESA funded instrument, designed and developed at RAL (Rutherford Appleton Laboratory), UK (PI: M. Grande, University of Wales, Aberystwyth, UK). CIXS is an upgraded version of D-CIXS (Demonstration of a Compact Imaging X-ray Spectrometer) heritage, flown on ESA's SMART-1 mission; it replaces the LEX (Low Energy X-ray) instrument of ISRO, originally planned for Chandrayaan-1. 34) 35)
The primary goal of the CIXS instrument, also referred to as C1XS (Chandrayaan-1 X-ray Spectrometer), is to carry out high quality X-ray spectroscopic mapping of the moon. CIXS employs the X-ray fluorescence technique observing in the energy range of 1-10 keV by measuring elemental abundance of Mg, Al, Si, Ca, Fe, Ti distributed over the surface of the moon with a nominal spatial resolution of 25 km. CIXS has been designed as a thin, low profile detector.
The CIXS instrument hardware is built by an international team led by RAL. There is also a major science and design contribution from ISRO/ISAC at Bangalore, India. The CESR (Centre d'Etude Spatiale des Rayonnements) of CNRS in Toulouse, France is providing the 3-D Plus amplifier assemblies, and there is an important contribution to the detector development from Brunel University, UK.
Figure 21: Schematic view of the CIXS instrument (image credit: ESA)
The instrument uses the recently developed technology of the Swept Charge Device (SCD) X-ray detectors, mounted behind low profile gold/copper collimators and aluminium /polycarbonate thin film filters. The SCD is a CCD-like device which achieves near Fano-limited spectroscopy below -10ºC. It's read out is similar to a conventional CCD, requiring 575 clock triplets to read out the 1.1cm2 detector area. Micro-machined collimators provide a FOV of 14º FWHM (Full Width Half Maximum), equivalent to 25 km from an altitude of 100 km. A deployable door protects the instrument during launch and cruise, and also provides a Fe55 calibration X-ray sources for each of the detectors.
The SCD system has the virtue of providing superior X-ray detection, spectroscopic and spatial measurement capabilities, while also operating at near room temperature. A deployable shield protects the SCDs during passages through the Earth' s radiation belts, and from major particle events when at the moon. In order to record the incident solar X-ray flux at the moon, which is needed to derive absolute lunar elemental surface abundances, CIXS carries also an X-ray Solar Monitor (XSM), provided by the University of Helsinki, Finland. which is also used as a calibrator.
XSM is a complementary experiment alongside CIXS providing simultaneous observations of solar X-rays using a solid-state detector. The XSM consists of a separate Si detector unit on the spacecraft. The non-imaging HPSi PIN sensor has a wide FOV to enable sun visibility during a significant fraction of the mission lifetime, which is essential for obtaining calibration spectra for the X-ray fluorescence measurements by the imaging C1XS spectrometer. The energy range (1-20 keV), spectral resolution (~ 250 eV at 6 keV), and sensitivity (~ 7000 cps at flux level of 10-4 W m-2 in the range 1-8 Å) are tuned to provide optimal knowledge about the solar X-ray flux on the lunar surface, matching well with the activating energy range for the fluorescence measured by C1XS.
Calibration measurements: In comparison to D-CIXS, CIXS and XSM will be far better calibrated. The RESIK X-ray beam facility at RAL, UK, is designed to provide a controlled X-ray beam (continuum + lines) using a variety of targets. The 24 SCD detectors for CIXS underwent calibration measurements to address individual spectral response, absolute detection efficiency and angular response of the collimator. In addition, the dependence of these parameters on temperature were also studied. A Si-PIN detector calibrated at the Synchrotron facility, PTB, BESSY II was used as the reference for absolute calibration. The extensive calibration measurements and generation of a more detailed instrument response should provide the most accurate results on the major fluorescent X-ray line flux from the lunar surface. 36)
Figure 22: Photo of the CIXS flight instrument during integration (image credit: RAL)
The CIXS instrument has a mass of 5.2 kg, a power consumption of 28 W, and a size of 185 mm x 112 mm x 140 mm. In normal solar conditions, CIXS will be able to detect elemental Mg, Al and Si on the lunar surface. During solar flare events, it might be possible to detect other elements such as Ca, Ti and Fe.
SARA (Sub keV Atom Reflecting Analyzer):
SARA was designed and developed jointly for ESA by IRF (Institute of Space Physics), Kiruna, Sweden (PI: S. Barabash of IRF), ISRO's Vikram Sarabhai Space Center at Trivandrum (Kerela State of India), JAXA of Tokyo, and the University of Bern, Switzerland. The objective is to map the composition of LENAs (Low Energy Neutral Atoms) kicked-off from the outermost layer of the moon's surface by the solar wind. LENA imaging is an important observation technique in planetary environments. - Note: Since the moon does not possess a global magnetosphere or an extended atmosphere, the solar wind ions can directly precipitate onto the moon's surface resulting in the production of LENAs through sputtering and backscattering processes. LENAs in this context are defined as atoms in the energy range of 10 eV to a about 2 keV. 37) 38) 39)
The SARA instrument consists of of three major subsystems: LENA sensor [which is also referred to as CENA (Chandrayaan-1 Energetic Neutral Analyzer)], SWIM (Solar Wind Monitor), and DPU (Data Processing Unit), and it is being built in collaboration between the participating institutes. At the CENA sensor, the neutral atoms are converted to positive ions on an ionization surface, after being swept away from the ambient flux by an electrostatic deflector, and then enter the sensor. The particle velocity is taken by a time-of-flight measurement, and the energy and mass are deduced by the electrostatic analyzer. The mass resolution is such that H, O, Na-Mg, K-Ca and Fe group elements can be distinguished. Since the moon does not have a magnetosphere or atmosphere, neutral atom density in the moon's environment is extremely small, produced mainly by sputtering due to solar wind ions. The contribution due to micrometeorite vaporization and solar photon simulated desorption is estimated to be small in this low energy region of interest. CENA imaging of the neutral atoms will thus provide maps of the sputtered elements which can be converted into surface composition maps, making suitable corrections for the sputtering yield and the solar wind flux, which depends on the cosine of the solar zenith angle.
Figure 23: View of SARA with CENA (left) and SWIM (right), image credit: ISRO, ESA
Table 4: Parameters of SARA
Figure 24: Main components of CENA (image credit: ISRO)
SIR-2 (Near Infrared Spectrometer):
SIR-2 is an ESA instrument of SIR (SMART-1 Infrared Spectrometer) heritage. SIR-2 is being developed for ESA by MPS (Max Planck Institut für Sonnensystemforschung), Katlenburg-Lindau, Germany, in collaboration with the University of Bergen (UiB), Norway. UiB is designing the ICU (Instrument Control Unit) which is based on the fault-tolerant and radiation-tolerant LEON3-FT-RTAX processor of Aeroflex Gaisler, Sweden. The PSU (Power Supply Unit) of SIR-2 is built by the Space Research Center, Polish Academy of Science (SRC/PAS), Warsaw.
SIR-2 is an upgraded, compact grating, near-infrared spectrometer, which covers the wavelength range between 0.93 and 2.45 µm, with a spectral resolution of Δλ pixel = 6 nm and an angular resolution of 2.2 mrad. The instrument collects reflected sunlight off the moon's surface with the help of a main and secondary mirror. This light is led through an optical fiber to the instrument's sensor head where it hits a grating. The light dispersed by the grating ultimately reaches a detector which consists of a row of photosensitive pixels which measure the intensity of the dispersed light at the different wavelengths and produces an electronic signal which is read out and processed by the experiment's electronics.
The objective of SIR-2 is to address the surface-related aspects of lunar science in the following broad categories:
• Analyze in unprecedented detail the lunar surface in various geological/mineralogical and topographical units
• Study the vertical distribution of crustal material
• Investigate the process of basin, maria and crater formation on the moon
• Explore the “space weathering” processes of the lunar surface
• Survey mineral lunar resources for future landing sites and exploration.
The SIR-2 NIR data, combined with the hyperspectral data from the HySI instrument on Chandrayaan-1, will provide, for the first time, a full spectral coverage of the olivine and large part of the pyroxene bands, thus allowing to extract from the data the necessary input parameters for the mineralogical mixing models.
The SIR-2 instrument consists of three main parts, sensor unit, power unit, and the the ICU. The main electronics consists of the InGaAs photodiode detector array, the readout electronics, a 16 bit analog-to-digital converter (ADC), a microprocessor/controller for data and command handling and a custom designed DC/DC converter which powers the various parts of the electronics. The instrument has a mass of 2.3 kg, a power consumption of 2.2 W, and a size of 260 mm x 171 mm x 143 mm (EO module), 146 mm x 125 mm x 33.5 mm (electronics box). 40) 41)
Figure 25: Illustration of the SIR-2 instrument units (image credit: ISRO,ESA)
Figure 26: Photo of the optics unit of SIR-2 covered in thermal insulation (image credit: ESA, MPS)
Figure 27: Photo of the ICU (image credit: University of Bergen)
Figure 28: Schematic view of the SIR-2 optics (image credit: UiB)
Figure 29: Block diagram of the SIR-2 ICU (image credit: UiB)
Figure 30: Photo of the SIR-2 PSU device (image credit: SRC/PAS)
Mini-SAR (Miniature Synthetic Aperture Radar):
Mini-SAR is a NASA and DoD (USA) instrument package, designed by JHU/APL (Johns Hopkins University/Applied Physics Laboratory) of Laurel, MD, and NAWC (Naval Air Warfare Center), Patuxent River, MD, USA. Mini-SAR is being built by Raytheon with BAE & Surrey Satellites. The Naval Air Warfare Center is the executing agent, with JHU/APL providing the instrument SOC (Science Operations Center), backup ground station and science and programmatic support. 42) 43) 44)
The main objective is to chart the moon's poles (radar-scanning for water ice thought to be present in permanently shadowed lunar craters). These ice deposits would represent a significant potential resource for the manned human base that is to be set up at one of the moon's poles late in the next decade.
Measurement concept: Mini-SAR will transmit RCP (Right Circular Polarization) and receive both LCP (Left Circular Polarization) and RCP. In scatterometer mode, the system will measure the RCP and LCP response in the altimetry footprint along the nadir groundtrack. The system will measure the surface RF emissivity, allowing a determination of the near normal incidence Fresnel reflectivity. Meter-scale surface roughness and circular polarization ratio (CPR) will also be determined for this footprint. This allows the characterization of the radar and physical properties of the lunar surface (e.g., dielectric constant, porosity) for a network of points. When directed off nadir the radar system will image a swath parallel to the orbital track by delay/Doppler methods (SAR mode) in both RCP and LCP.
Since ice in concentration exhibits the Coherent Backscatter Opposition Effect (CBOE), which causes an increase in radar echo reflectivity and CPR enhancement along the backscatter direction, Mini-SAR will allow extensive data to be collected on the location and distribution of lunar ice deposits. At S-band, CBOE is sensitive to 1-10 meter-scale ice deposits covered by up to 40 cm of dry lunar regolith.
The Mini-SAR instrument features a new hybrid-polarity architecture, a dual-polarized system with a linearly-polarized antenna - leading to a simpler yet more capable radar. The essential feature of the hybrid-polarity architecture is: transmit circular polarization (by driving the orthogonal linear feeds simultaneously by two identical waveforms, 90º out of phase), and receive H and V linear polarizations, coherently. Once calibrated, the H and V single-look complex amplitude data are sufficient to form all four Stokes parameters, from which the circular-polarization ratio may be found, along with several other quantitative characterizations in the image domain. 45) 46) 47) 48) 49)
Figure 31: Schematic diagram of the generic hybrid-polarity SAR instrument architecture (image credit: JHU/APL)
The Mini-SAR system consists of an electronics box and antenna (Figure 32); data handling and storage are handled by the Chandrayaan-1 spacecraft data subsystem. The Mini-SAR electronics box contains the waveform generator, digital circuits, receiver and transmitter (Figure 31). The electronics box is mounted on the back of the moon-facing panel on the Chandrayaan-1 spacecraft.
A form-core, cross linear array antenna allows a broadband approach with a single antenna panel, without any deployable mechanisms (e.g. feeds) while meeting stringent weight and volume constraints. The thermal design, materials selection, manufacturing, and test qualification heritage of Chandrayaan-1 Mini-SAR antenna were applied also to the LRO Mini-RF unit. The combined mass of the antenna and electronics is less than 9 kg.
A processor module based on a heritage OBC 695 and associated firmware and software, developed by SSTL (Surrey Satellite Technology Ltd) controls the Mini-SAR system. Data handling and processing is done via the Chandrayaan-1 spacecraft solid state memory and data handling systems.
Figure 32: Photo of the Mini-SAR antenna during testing (image credit: Raytheon, NASA)
Table 5: Mini-SAR instrument parameters
Imaging of the lunar surface by the Mini-SAR mapper precludes the simultaneous imaging of the moon by the other Chandrayaan-1 sensors due to power limitations. The main goal of Mini-SAR on Chandrayaan-1 is to conduct systematic SAR mapping polewards of 80º for both poles.
Data products from Mini-SAR include maps of lunar surface scattering properties, including maps of CPR, indicative of ice. The polar backscatter maps will have a typical resolution of 1-2 km/pixel. In addition, complete SAR mosaics are being obtained in both RCP and LCP of the polar regions at about 150 m/pixel. These images will display the locations of polar ice and the topography and morphology of the permanently dark regions around both lunar poles.
If the Chandrayaan spacecraft and its instruments are still operational when the NASA mission LRO (Lunar Reconnaissance Orbiter) arrives in lunar orbit (June of 2009), then an attempt is being made to conduct a bistatic imaging experiment with the Mini-RF imager of LRO. By transmitting RCP from the Chandrayaan-1 Mini-SAR and receiving RCP and LCP on the LRO, it is possible to image the polar deposits through the beta (phase) angle, providing definitive evidence for the presence of water ice at the poles. Monostatic radar can only image the deposits at zero phase (β = 0) and thus, there is always an ambiguity as to the high back scattering, being caused by roughness (surface) or ice (volume) scattering. Bistatic imaging can eliminate this ambiguity. Coordinated radar observations from Chandrayaan-1 and LRO spacecraft should be a high priority for these mission operations.
Figure 33: Mini-SAR equipment box (image credit: ISRO, NASA)
Figure 34: Bistatic imaging experiment of Mini-SAR flown on the Chandrayaan-1 and LRO spacecraft (image credit: NASA, ISRO)
Table 6: Overview of some Mini-SAR and Mini-RF parameters
Mini-SAR calibration: Laboratory calibration data was acquired before launch during spacecraft integration and test. The overarching goal of these activities was to ensure production of a calibrated instrument. All waveforms in the waveform table were tested on brass board hardware while selected waveforms were tested on flight hardware. Additional waveform testing was done on the flight instrument during thermal vacuum temperature ramp cycles. Internal calibration data are acquired every time Mini-SAR takes a science data collect; a chirp, noise, and tone calibration is done both immediately before, and immediately after a data collect.
External calibration is planned en route to the moon by taking data using ground-based assets. During the translunar cruise phase, the project will conduct calibration measurements with the Greenbank Radio Telescope in West Virginia, USA. A transmitted signal from Mini-SAR is received by Greenbank while the antenna boresight is scanned via spacecraft attitude changes over a range of angles.
The Mini-SAR instrument was activated on November 17, 2008 and acquired SAR images of both poles during a commissioning test. The instrument performed nominally. 50)
M3 (Moon Mineralogy Mapper):
M3 is a NASA funded instrument, a state-of-the-art high spectral resolution imaging spectrometer (VNIR, SWIR) jointly designed and developed at JPL and at Brown University, Providence, RI (PI: C. M. Pieters). The objective is to characterize and map the mineral composition of the moon to improve our understanding about the early evolution of the terrestrial planet. 51) 52) 53)
M3 uses a compact system of optics (the mirrors that collect and direct the light) known as an ”Offner” design, which produces little or no distortion, either spatially or spectrally. The instrument is a pushbroom imaging spectrometer providing two spatial and one spectral dimension (use of a HgCdTe detector array. The instrument provides solar reflectance spectra of the moon's surface:
- 0.70 to 3.0 µm [0.43 to 3.0 µm projected]
- 40 km FOV, contiguous orbits
- high SNR
- 1 Gbyte/orbit of data
The spectral range is being dispersed into 261 discrete bands of 10 nm width. The high spectral resolution data enables to detect the fine detail required for mineral identification.
Figure 35: Optical configuration of the M3 instrument (image credit: JPL)
Operational modes of M3: Since the M3 instrument observations yield very high data rates, mapping the entire surface of the moon at both high spatial and spectral resolution would exceed the nominal operational lifetime of the mission. Therefore, the M3 instrument has been designed to operate in two distinct modes: target mode and global mode. 54)
• The target mode captures the high spectral and spatial resolution data desired for science applications (zeroing in on features of special interest).
• Global (mapping the entire surface). In global mode, the instrument merges groups of pixels to reduce the data rate and accommodate the limited available downlink time. This will result in an effective spatial resolution of 140m and spectral resolution of 40 nm.
All data will be acquired from the nominal 100 km polar orbit of Chandrayaan-1 yielding a 40 km wide field of view.
- Global-mode data will be acquired in 145º latitudinal swaths at a spatial resolution of ~140 m/pixel and will measure spectra from 86 of the available 260 spectral channels.
- The target-mode data will be acquired at ~70 m/pixel using the full spectral range of the instrument. Targeted data can be acquired in ~12º swaths per orbit and the coverage need not be continuous. The target mode thus requires the definition of science regions along each operational orbit.
Table 7: Overview of M3 instrument characteristics
Figure 36: Characteristics of the M3 instrument (image credit: JPL)
Background: M3 was selected in early 2005. The first spectrum was acquired in the laboratory on December 15, 2006. Calibration took place during the month of April 2007. A complete set of spectral, radiometric, spatial and uniformity calibration measurements was acquired. Following laboratory calibration M3 completed a pre-ship review on May 3, 2007. Initial integration of M3 at ISRO was completed on August 10, 2007 with M3 successfully commanded from the Chandrayaan-1 spacecraft system.
Figure 37: View of the M3 instrument prior to shipment (image credit: JPL)
RADOM (Radiation Dose Monitor):
RADOM was developed by the Solar Terrestrial Influences Laboratory of the Bulgarian Academy of Sciences (BAS), Sofia, Bulgaria (PI: T. P. Dachev). The objective is to monitor qualitatively and quantitatively the radiation environment (particle flux) around the moon. The specific goals are:
• Measure the particle flux, deposited energy spectrum, accumulated absorbed dose rates in lunar orbit
• Provide an estimate of the dose map around the moon at different altitudes and latitudes
• Evaluate the shielding characteristics (if any) of the moon near environment towards galactic and solar cosmic radiation and solar particle events
• Study the radiation hazards during the moon exploration through the Chandrayaan-1 mission. Data obtained will be used for the evaluation of radiation environment and radiation shielding requirements on future manned moon missions.
RADOM is a miniature spectrometer-dosimeter containing one semiconductor detector of 0.3 mm thickness, one charge-sensitive preamplifier and two micro controllers. The Si detector with an area of 2 cm2 has a mass of 0.139 gram. Its threshold level is 8 keV. The pulse analysis technique is used for obtaining the deposited energy spectrum, which is further converted to the deposited dose and flux in the silicon detector. The exposure time for one spectrum is fixed at 30 seconds. RADOM measures the spectrum of the deposited energy from primary and secondary particles in 256 channels. The instrument will make its measurements in the lunar environment as a function of altitude as the spacecraft descends from the lunar capture orbit to its final altitude of 100 km. 55)
Figure 38: Illustration of RADOM (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.