How Chandrayaan-1 will help compile a 3D atlas

Published in The Hindu on November 6, 2008

Chandrayaan spacecraft
The Terrain Mapping Camera (TMC) on-board Chandrayaan-1 picks up data from three different angles. — Photo: ISRO

The Terrain Mapping Camera (TMC) on-board Chandrayaan-1 is a unique demonstration of space scientists’ ingenuity. It will be able to produce a 3D atlas of the moon using a single camera. The resolution will be 5 metres. This will help to prepare a 3D atlas with a unprecedented high-resolution.

Developed indigenously

Developed by the Ahmedabad based Space Applications Centre, the TMC will be able to image the moon’s surface from three directions — vertically down view, forward view and backward view along the path of the spacecraft’s orbit. The three view imaging feature of TMC is the first among ISRO’s remote sensing payloads.

“The three different views become possible as the camera picks up data from three different angles,” said Dr. Kiran Kumar A.S., Deputy Director, Sensor Development Area, Space Applications Centre, Ahmedabad. “The three images are picked up simultaneously from three different angles by the Terrain Mapping Camera.”

The data will enable the preparation of a three dimensional lunar atlas. For 3D information, at least 2 views of the region from different angles are required.

Overcoming occlusion

The three views of TMC will ensure that regions on slope where the viewing angle is smaller than the slope is not occluded, as the image of the slope will be available by the third view.

One would normally need three cameras to image a feature simultaneously from three different angles. So how does the Terrain Imaging Camera manage to do it with just one camera?

“It is due to the innovative design of the camera,” Dr. Kumar said. “A set of two mirrors in the camera are used to provide two angles apart from the nadir [view from the top] view.”

While a normal camera of four mega pixels would have 2,000 by 2,000 elements, the Terrain Mapping Camera does not capture data the same way. “We don’t get one frame at a time but one single line,” he said.

The 4,000 pixels (1 pixel covers an area of 5 metre x 5 metre from a height of 100 km from the moon) in the Terrain Mapping Camera are arranged in a linear manner. While the spacecraft moves in north-south polar orbit, the camera covers a width of 20 km in an east-west direction.

The swath

Hence the area covered in an instant is 5 m x 20 km (4,000 by 5 metres). This is called the swath. “We can map 4,000 elements by 5 metres (20 km swath) in one instant and the next moment we move to cover another 5 metres,” Dr. Kumar explained. An area of 1.5 km of the moon is imaged in one second.

All the three views generate a 2-D image, as each view covers north-south and east-west directions (X, Y directions). And a 3-D view of a point can be generated by combining the 2-D data by using data from any of the two views.

Since the three views of the camera are in the same direction of the spacecraft movement, a point lying in the path of the orbit is covered by all the three views. “Combining all the 3 views provides more details and takes care of the occlusion problem,” said A Roy Chowdhury, Head, Geo & Planetary Sensor Electronics Division and Instrument Scientist TMC & HySI, Chandrayaan-1 at Space Applications Centre.

The spacecraft will take nearly two hours to complete one north-south polar orbit. But the moon will not be imaged continuously for the full two hours of the orbit.

The solar illumination changes as the moon moves in its orbit. So the imaging time is limited to minimise the variation of illumination conditions.

Prime imaging period

Limiting the solar aspect angle to 30 degrees on either side of the equator will result in a prime imaging period of just 60 days in six months.

“We will get two slots of 60 days each in a year. We will pick up data during these two slots,” said Dr. Kumar.

So this results “in imaging for only 20 minutes per six visible orbits from the Indian ground station to cover the whole moon.”

The area covered during 20 minutes of imaging will be 1,800 km (1.5 km will be imaged in a second).

These are some of the reasons why the mission period is two years though imaging the moon can theoretically be completed in 28 days — the time taken by the moon to complete one rotation.

The camera has four exposure settings and this lets the camera record data from areas not well illuminated by the sun, particularly those lying in higher latitudes up to the poles.

While increasing the exposure time would allow imaging the less lit areas, the spacecraft will be moving during such long exposures. This will result in coarser resolution of the images.

The 3D atlas with a unprecedented high resolution will help in better understanding of the moon’s evolution process.

It will also help researchers to identify regions of the moon for detailed study. The images will also “be an important input for analysing data from other scientific instruments on Chandrayaan-1.