Cutting edge
Cutting edge
R. RAMACHANDRAN
There were many technology firsts by ISRO in the ground segment, Chandrayaan’s design and in the experiments.
A picture of the moon's surface taken from the lunar orbit by Chandrayaan-1's Terrain Mapping Camera on November 15.
ON November 12, the scientists of the Indian Space Research Organisation (ISRO) successfully manoeuvred the Indian lunar orbiter, Chandrayaan-1, to its operational circumpolar 100 km x 100 km orbit around the moon. Further, with the deliberate crashing of the Moon Impact Probe (MIP) in the intended area on the moon’s surface on November 14, one of the five Indian experiments was also successfully completed (Frontline, December 5, 2008) although the media hype and hoopla surrounding the event was completely unwarranted and incommensurate with the scope of the experiment (see accompanying story). There were, however, many technology firsts and noteworthy innovations by ISRO in the ground segment, the spacecraft’s design and the other four Indian experiments.
The establishment of the Indian Deep Space Network (IDSN) to world standards around an indigenously built 32-metre diameter antenna, which can support the communication needs of not only lunar missions but also future deep-space probes, was a significant achievement in itself (Frontline, November 21, 2008). But significant technology developments in the ground segment have been not only in hardware but also in software.
According to M. Annadurai, Chandrayaan-1’s project director, improved data formatting and coding systems have been evolved for deep-space communication so as to minimise the amount of “link margins” – appropriate margins provided to account for randomly varying gains and losses in the received signal power – in the coding chain. Further, the technique of packetised telemetry for on-board instrument data transmission – where data are sent in packets – over a deep-space channel, as per the protocol defined by the National Aeronautics and Space Administration’s (NASA) Deep Space Network, has been implemented for the Chandrayaan-1 mission.
The overall spacecraft mass is at a premium in any deep-space mission. Given its limited capability, the PSLV-XL launcher could inject Chandrayaan-1 into an initial orbit (IO) with a perigee lower than the usual Geostationary Transfer Orbit (GTO). Miniaturisation, therefore, is a key feature of the satellite as well as its payloads to reduce the mass budget, said K. Thyagarajan, formerly of the ISRO Satellite Centre (ISAC), Bangalore. Also, according to Annadurai,
by using parts made of Composite Fibre Reinforced Plastic (CFRP) wherever possible, significant mass reduction was achieved.
Chandrayaan-1's orbit with respect to the sun.
“Unlike in all earlier satellite missions of ISRO, for Chandrayaan-1, the control gyros, which maintain the spacecraft’s attitude, had to be miniaturised,” said Annadurai. Similarly, the reaction wheels, which change the orientation of the spacecraft by a spin-up or spin-down operation (as was done during the lunar orbit injection (LOI) manoeuvre and will be done many times during the course of the mission) were not the ones that ISRO normally uses, he pointed out. In Chandrayaan-1, the wheels were spun at a much higher speed to achieve greater torques.
All the orbital manoeuvres to take the spacecraft from its earth-bound IO to the final operational lunar orbit have been performed with remarkable precision. Of these, the last of the earth-bound manoeuvres to reach the lunar transfer trajectory (LTT) and the LOI were particularly crucial. Each orbit manoeuvre corresponds to imparting a certain velocity change to the satellite, which is done by firing the on-board Liquid Apogee Motor (LAM). Precision manoeuvre requires that this velocity change is determined accurately.
In the usual earth-bound missions, on the basis of the specific impulse of the fuel as well as the engine parameters, the LAM is allowed to fire for a calculated amount of time through commands from the ground to change the velocity by a given amount. This is subsequently verified by determining the parameters of the orbit acquired.
“In deep-space missions, however, even a small error in this can result in a difference of several hundred kilometres in the apogee of the new orbit,” pointed out V. Adimurthy of the Vikram Sarabhai Space Centre (VSSC), Thiruvananthapuram. “The process of cut-off, the tailing off and the detailed behaviour of the propulsion system become important,” he added.
“We need autonomous determination of the velocity change realised and automatic cutting off of the firing,” said Annadurai. To realise this, ISRO has used ceramic servo-accelerometers for the first time in Chandrayaan-1 for precise orbit manoeuvres, and the success so far is a testimony to their performance.
Accelerometers are normally used as part of the on-board navigation, guidance and control system in launch vehicles, which experience large accelerations in a short time. In orbital manoeuvres of near-earth satellites, because of the overwhelming background of the earth’s gravitational acceleration, accelerometers are not very effective in determining the accelerations imparted to the satellite. In deep-space missions, beyond the earth’s gravity, where orbit changes across several tens of thousands of kilometres are realised, accelerometers are very sensitive. “This precision has helped us save a lot of on-board fuel,” said Annadurai.
In Chandrayaan-1, miniaturised accelerometers were used, and Micro Electromechanical System (MEMS)-based accelerometers may be used in future missions, according to Thyagarajan.
Thermal control of the spacecraft is very critical in deep-space missions like Chandrayaan-1, pointed out Annadurai. In a lunar mission, this becomes particularly complex because of the very different thermal environment that a lunar satellite encounters as compared with an earth-bound satellite. Maintaining the temperatures of the on-board instruments and detectors at the desired levels is a challenge as each device has its own appropriate operating temperature, which varies from a low –17oC to a high +40oC, while the general electronic components need to be maintained at about –10oC.
Solar panel orientation strategy: A 180o rotation of the satellite about its yaw axis in the first extreme configuration, performed twice a year (A and C), and a 180o flipping of the solar panel about the satellite's pitch axis in the other extreme configuration, performed after a three-month phase lag with the former (B and D).
In the case of earth-bound missions, while the thermal load on the sun-facing side of the satellite is about 1,300 W/m2, it is much less on the earth-facing side because of a low albedo (reflectance from a surface averaged over all frequencies). Also, since the earth has an atmosphere, only part of the heat absorbed by the earth is lost by radiation and the rest contributes to atmospheric convection. Temperature variations across the globe are, therefore, not drastic because of the atmosphere.
The solar load on the moon, too, is about the same as the earth and its albedo – in fact, it is about a third of the earth. However, since the moon has no atmosphere, the absorbed heat is almost fully radiated as heat. This causes the temperature of the moon to vary from a peak of about 125oC on the sunlit side to about –170oC on its dark side. Thus the spacecraft, which has an orbital period of only two hours, has to go through quick cycles of very cold and very hot lunar environment. Recent lunar missions such as the European SMART-1 (2003-06), the Japanese Kaguya/SELENE (launched in 2007) and the Chinese Chang’E-1 (also launched in 2007) have all had problems with the thermal management of their on-board instruments.
Generally,
thermal management of a satellite is based on two principles, explained D.R. Bhandari of the ISAC, who led the thermal modelling exercise for Chandrayaan-1.
One is the multilayer insulation (the golden metallic wrapping that one sees on the outside) that reduces the external thermal load on the satellite as a whole. But then there is also the heat generated internally within the satellite, which needs to be dissipated. So, the
second principle involves selecting some suitable radiating surfaces on the satellite that have very low solar absorption but high emissivity. But to meet the special requirements of designing for very low temperatures in some parts of the satellite, the heat is distributed with the aid of thick distributor plates or heat pipes and finally dissipated through special radiators incorporated in the satellite.
According to Bhandari, the actual layout of these is achieved through mathematical modelling in which the heat distribution is calculated by dividing the satellite into a number of parts. Further, to ensure that the critical components requiring low temperatures are exposed only to minimum solar and lunar load, they are mounted on non-sun/moon-facing sides of the satellite. Chandrayaan-1’s design is such that the solar panel is only on one side (the positive pitch face) and, as will be explained here, the satellite or the solar panel is suitably manoeuvred at the right time to ensure that this face is always sun-facing. Correspondingly, the negative pitch face, which carries all the critical instruments, will always be facing away from the sun.
An important feature of a circular polar orbit around the moon is that it is fixed in space (relative to the distant stars) unlike the sun-synchronous orbit of earth observation satellites. The latter precesses around the earth-axis because of the oblateness of the earth. The moon, on the other hand, has very small oblateness and the lunar orbit is always very nearly perpendicular to the earth-sun plane. This gives rise to two extreme situations as regards the solar illumination of the lunar orbit. One extreme is when the sun-pointing direction (with respect to the moon) is parallel to the lunar orbit plane and the other, which occurs after three months of the first extreme, is with the sun vector perpendicular to the orbital plane (see figure 1).
These orbit configurations have a direct bearing on the illumination on the satellite’s single-sided solar panel. If, as is usually done, the solar panel were mounted exactly perpendicular to the satellite face, it would face the sun directly in one extreme orbit configuration and generate 100 per cent power. In the other extreme configuration, it would be edge-wise and generate zero power. Hence, for optimum power generation throughout the mission, detailed orbit analysis shows that the solar panel is required to be canted, or offset with respect to the horizontal, by 30o.
In addition,
eight important manoeuvres – four on the satellite and four on the solar panel – need to be performed during the entire two-year period (see figure 2). As mentioned earlier, this complex strategy also automatically results in the negative pitch face (the aft side of the solar panel face) always being non-sun facing, an important requirement for thermal control.
One other important consideration is the occurrences of eclipse during the mission, when the solar panel does not generate any power. For the first time, ISRO has used compact (rechargeable) high-energy lithium-ion batteries to provide essential power during the eclipses, the maximum duration of which is about 48 minutes, according to Thyagarajan. Chandrayaan-1 will experience its first eclipse in February 2009.
Miniaturisation and mass reduction were key aspect of the payloads as well, Annadurai pointed out. For example, the
Terrain Mapping Camera (TMC), built by the Space Applications Centre (SAC), Ahmedabad, makes use of a very innovative camera design that is capable of acquiring stereoscopic images with a single lens camera. It has an unprecedented 5-metre resolution and is designed to prepare a three-dimensional atlas of the moon.
Two sets of mirrors help to obtain aft and fore views and this combined with the direct nadir view by the lens system, a field of view is imaged from three angles in a push-broom mode, which are combined to generate a 3D view (see figure 3).
The focal plane imaging is done by a 4,000-pixel linear array Active Pixel Sensor (APS)-based detector, an evolving technology in space applications. The device is a silicon-based CMOS (Complimentary Metal-Oxide Semiconductor) image digitiser, which has an in-built detector drive and on-board processing electronics. “This,” said A.S. Kiran Kumar of the SAC, “has helped to reduce additional hardware and minimise power, weight and size”.
Schematic of configuration and viewing mechanism of the Terrain Mapping Camera.
The Hyper Spectral Imager (HySI), intended to obtain mineralogical mapping of the lunar surface, also makes use of a focal plane APS detector – this time a 500 x 500 area array – with a digitiser to map the spectral bands.
The APS detectors, both for TMC and HySI, were designed in-house and fabricated by a Taiwanese foundry, according to Kiran Kumar. The uniqueness of HySI is in its capability to map in 64 contiguous bands in the spectral region of 0.4-0.95 micrometre (µ
wavelengths (visible and near-IR), with a spectral resolution better than 15 nanometre (nm) and a spatial resolution of 80 m.
The
dispersion into different spectral bands is achieved by using the new concept of a wedge filter, which is being used for the first time by ISRO. As against a prism or grating, the use of a wedge filter makes the instrument compact and reduces the weight. A wedge filter is basically an interference filter with varying thickness along one direction so that the transmitted spectral range varies in that direction. Pixels along the track direction will receive signals from different spatial regions in the same band while pixels in the perpendicular direction will receive signals in the different spectral bands.
The
High Energy X-ray Spectrometer (HEX) aboard Chandrayaan-1 is meant to detect naturally occurring emissions of Gamma-rays from the lunar surface owing to
radioactive decays of nuclides in the uranium-238 and thorium-232 series with energies in 20-250 kilo electron Volt (keV) range. Gamma-ray emissions in this range, however, are of low intensity. The HEX payload will use for the first time pixelated cadmium-zinc-telluride (CZT) array detectors that have high sensitivity and high energy resolution to pick up these weak signals. Regions of U/Th concentration can be mapped by detecting the 240 keV emissions from the decays of lead-212 and lead-214, nuclides of the series.
So far, no mission has detected Gamma-rays below 500 keV, according to J.N. Goswami, Director of the Physical Research Laboratory (PRL), Ahmedabad, and principal scientist, Chandrayaan-1 mission. CZT detectors have not been generally flown in space missions because of the high noise in high radiation and the need for on-board cooling, he pointed out. “Usually space missions have used cesium iodide scintillators or germanium detectors or proportional counters which do not have the required sensitivity,” said Goswami. Extensive thermal modelling has been done for these detectors in Chandrayaan-1 and these will be maintained below 0oC by passive cooling.
Specially designed CZT detectors were flown in the U.S. X-ray satellite SWIFT, launched in November 2004, which have been used so far for X-rays above 500 keV only. “This will also be the first experiment to detect volatile transport from the sunlit regions to the permanently shadowed cold regions in the moon,” said Goswami. “If we can pick up the 46.5 keV line characteristic of the decay of lead-210, a decay product of volatile radon-222, it will give us a handle to model volatile transport on the moon, which could in turn be used to study transport of water molecules to the poles,” Goswami said.
The Lunar Laser Ranging Instrument (LLRI), built by ISRO’s Laboratory for Electro-Optics Systems (LEOS), is aimed at studying the topography of the lunar surface and its gravitational field by measuring the altitude precisely from Chandrayaan’s orbit using a pulsed neodymium (Nd)-YAG laser (1,064 nm wavelength) and measuring the ‘time of flight’.
“While the laser has been bought off the shelf, they feel that in future they will be able to build it themselves,” said Goswami. “However, the entire optics has been designed and built by LEOS.” To handle the poor reflectivity of the lunar surface, a suitable silicon Avalanche Photo Detector (APD), with good resolution and a high-signal-to-noise ratio, and the associated electronics were built in house. This will provide altimetry data close to the poles for the first time.
In addition to the above indigenous instruments, ISRO’s contribution has been noteworthy in the collaborative payload called Chandrayaan-1 X-ray Spectrometer (C1XS). The mission objective of C1XS is to produce a high-quality X-ray spectroscopic map of the lunar surface and determine the abundance of elements such as magnesium, aluminium, silicon, calcium and titanium, which have a bearing on the origin and evolutionary history of the moon, using the X-ray fluorescence technique. The sun is the natural source of X-rays and the above elements absorb these primary X-rays and re-emit them as fluorescent X-rays with the energy characteristic of each element (1-10 keV). In normal solar conditions, C1XS can detect magnesium, aluminium and silicon and during solar flare time it would be able to detect elements such as iron, calcium and titanium.
C1XS will
use the recently developed Swept Charge Device (SCD) X-ray sensors, with 24 nadir pointing detectors. SCD was flown recently in the European SMART-1 lunar mission as part of the instrument called D-CIXS (Demonstration of Compact Imaging X-ray Spectrometer). It is similar to the conventional Charge Coupled Device (CCD) but allows sensitive spectroscopy to be done at low temperatures of –20oC to 0oC (achieved by on-board passive cooling with radiative plates and heat sinks). The technology of the device was successfully demonstrated in SMART-1 and, in fact, the mission detected calcium for the first time.
Graphic representation of Chandrayaan-1. The solar panel is canted by 300. C1XS (including XSM), RADOM, SIR-2, SARA (including CENA and SWIM), MiniSAR and M3 are the six foreign experiments.
However, an important consideration for its proper functioning is good thermal design and radiation shielding.
In fact, because of the long time of 15 months that it took to reach the moon and its highly elliptic 300 km x 3,000 km final orbit, SMART-1 had to pass through the near-earth radiation belt several times. This caused rapid degradation of the SCD and made its calibration and good energy resolution difficult.
But Chandrayaan-1, both because of its orbit and the timing of its launch, will be able to focus on science with potentially better results. From its low 100 x 100 km orbit,
Chandrayaan-1 is expected to provide much better spatial resolution of 25 km (as compared to over 100 km in SMART-1). But more importantly, while SMART-1 flew at the worst time in respect of solar flares, Chandrayaan-1 has been launched at the best time for solar flares, thus enabling much more robust data.
Further, the orbit is also relatively a low radiation environment region of space and therefore the device is not expected to deteriorate fast. In its journey to the lunar orbit, however, it did pass through the radiation belt once, but its switching-on on November 23 has indicated that the instrument is working fine.
The collaboration with ISAC has contributed substantially to its improved design, better thermal engineering and radiation shielding, which are expected to yield far better energy resolution than SMART-1’s 200 eV. To check its calibration in the lunar orbit, a radioactive iron-55 source was placed on a deployable door that the instrument carries. After the instrument was switched on, the spectrum of iron-55 obtained in the moon’s environment was found to be identical to the spectrum on the earth. This is indicative of the instrument’s efficient thermal control and robust calibration, and a testimony to the ISAC team’s successful effort headed by P. Sreekumar.
One of the hurdles during the preparation for the Chandrayaan-1 mission before 2004 was the export embargo placed on ISRO units by the U.S. An IR detector up to 3 µ frequency that ISRO wanted to include in HySI could not be procured. HySI now has a range of 0.4-0.9 µ only. Post-2004 a French company offered to supply it, but by then the instrument design had been frozen. The Japanese mission has included an IR detector that goes up to 2.2 µ. The U.S. payload Moon Mineralogy Mapper (M3) on Chandrayaan-1, however, has a range of 0.7-3 µ.
“By overlapping data from the three experiments we will have a complete mineral map of the lunar surface,” said Goswami.
