Best 3-D images

The TMC has been very useful in finding important details, says R.R. Navalgund.-V. GANESAN

THE Space Applications Centre (SAC), Ahmedabad, is one of the important centres of the Indian Space Research Organisation (ISRO). Dr R.R. Navalgund is its Director. The SAC has built scores of remote-sensing, meteorological and communication payloads for the satellites launched by the ISRO.

The centre played a key role in the Chandrayaan-1 project by building two of its 11 scientific instruments the Terrain Mapping Camera (TMC) and the Hyperspectral Imager (HySI). The TMC, an electronic camera, took black-and-white pictures of the moons surface. The pictures were taken in such a way that the height of the objects could be measured. In effect, it took three-dimensional pictures of the lunar features.

Even as Chandrayaan-1 was headed towards the moon, the TMC took superb pictures of the earth and the moon. It imaged objects, which just measured five metres, on the lunar surface.

As long as the Chandrayaan-1 spacecraft was alive, the TMC took thousands of breathtaking pictures of the moons hills, valleys, craters, and so on. The HySI, which is also a camera, took colour pictures of the moon. These pictures would help in understanding the way minerals are distributed on the lunar surface.

Before becoming the Director of the SAC, Navalgund had served as the Director of the National Remote Sensing Agency (now renamed National Remote Sensing Centre), Hyderabad. He took his postgraduate degree in physics from the Indian Institute of Technology (IIT), Mumbai, and earned his Ph.D in Physics from the Tata Institute of Fundamental Research, Mumbai. He played an important role in the formulation and execution of the remote-sensing application programmes of the ISRO, right from the Bhaskara mission in 1979 to the Indian Remote-Sensing Satellite (IRS) series. Navalgund has received several awards, including the Indian National Remote Sensing Award in 1992 instituted by the Indian Society for Remote Sensing, Dehra Dun. He represents India on the Group for Earth Observations, an international initiative.

Navalgund responded to an e-mail questionnaire from Frontline on the instruments of Chandrayaan-1 that helped find water on the moon. Excerpts:

How do you assess the importance of the Moon Impact Probe (MIP) of ISRO and the Moon Mineralogy Mapper (M3) of NASA finding water on the moon?

The Moon Mineralogy Mapper flown on Chandrayaan-1 has detected absorption features at 2,800 to 3,000 nm wavelengths in the polar regions of the moon. These features/chemical signatures are indicative of the presence of water molecules and the OH radical. This discovery is significant not only for understanding the origin of the moon but also for future missions and colonisation. The presence of water molecules in the polar regions is considered to be of lunar origin and believed to be related with the process of solar wind interactions with local rocks. It also gives an insight into the question relating to the transport of volatiles on the lunar surface.

Will the discovery lead to more lunar missions from other countries? Will it galvanise the revival of interest in the moon?

Surely, as such there is a revival of interest among many spacefaring nations towards the moon and this discovery will provide further impetus.

What were the challenges that the SAC faced in designing and building the TMC and the HySI?

The SAC was responsible for the design, development and realisation of two important payloads, the TMC and the HySI. The major challenges were to miniaturise the cameras in terms of weight, compactness and power consumption. In fact, the TMC weighed only about 6 kg and the HySI 2.5 kg. The TMC aimed at imaging the moons surface with a five-metre spatial resolution using a triplet, facilitating, therefore, the generation of 3-D images for the first time in this resolution.

The HySI instrument was intended to provide information on the lunar composition in 64 spectral bands covering 0.42 to 0.96 micrometre wavelengths. Imaging the moons surface in a large number of narrow spectral bands with adequate radiometric sensitivity was a challenging task and has been found quite useful in identifying different rock formations. In addition, the SAC team also developed the software for the TMC and the HySI data processing, which itself was a major challenge in view of the fact that we were working for the first time for a data format called Planetary Data System (PDS).

The HySI seems to be a relatively new concept. What are the advantages of this technique for lunar exploration?

Hyperspectral imaging from space platforms is a relatively new technology and it provides very precise information on the surface type, using what is known as imaging spectroscopy. Using this technology, one can find out the various rock types and their mineral composition. In fact, the discovery of water molecules on the lunar surface is also achieved by using the hyperspectral imaging method. The HySI instrument uses wedge-filter technology for light dispersion or to break white light into 64 different colours. The use of the wedge filter was very important to make the instrument compact and light-weight.

What are the most interesting lunar surface features seen by the TMC?

The TMC has been very useful in finding important details of the various morphological features on the lunar surface. We could detect the minutest details of the impact craters, the central ridges of large impact craters, Sinous Rilles (collapsed lava flow channels) and ray craters. Recently we picked interesting features of ejecta material (rich in iron-bearing minerals) on feldspatic hilltops around Apollo-17 landing sites. These findings give us an insight into the volcanic processes and their impact on the lunar surface.

How much of the lunar far side (the dark side of the moon that is permanently turned away from the earth) was photographed by the TMC?

About 50 per cent of the lunar surface, which includes both the near side and far side of the moon, including the polar regions.

How do you rate the performance of the TMC in providing 3-D images with high resolution?

The TMC sensor has three-dimensional imaging capability, using a stereo triplet. Using the principles of photogrammetry, one can generate height information using conjugate points among the triplets. At present, the TMC is providing height information on 20-metre spatial resolution, which is unprecedented. No one has imaged the moon in three dimensions at this resolution. When used with the TMC and HySI images, the 3-D information is most useful image interpretation.