How will Chandrayaan-3 soft-land on Moon? Here's the tech simplified
Sensor instruments will generate crucial data like speed, orientation of the ship, altitude above the lunar surface and hazards for a successful landing
Several novel sensor instruments will guide and enable the Chandrayaan-3craft to soft-land on the lunar surface. These sensors will generate crucial data such as the speed, direction of the motion, orientation of the ship, altitude above the lunar surface, distance to the landing site, hazardous spots at the landing site and so on. These sensors are the eyes and ears of the AI-enabled navigational system of the craft. Using these data, the navigational and descent system of the craft will steer the vessel and safely soft land it in a spot avoiding hazardous regions.
Where is the spacecraft, which direction is it moving?
Laser Inertial Referencing and Accelerometer Package (LIRAP) is the sensor that provides vital data on the craft's orientation and acceleration. Inertial Referencing and Accelerometer Package (LIRAP) consists of gyroscopes and accelerometers.
When a stationary bus starts moving, we fall backwards, and when the moving bus stops, we tilt forward. This is due to inertia. Using this principle, the inertial accelerometer works. Laser helps the on-board AI-enabled computer to compute the orientation of the craft with respect to the lunar surface and how fast it is accelerating or decelerating at every instance.
Remember playing with a fidget spinner? When it is in rapid spinning motion, we can easily balance it on our fingers. When we hold one spinning rapidly between our fingers, it will try to resist attempts to tilt it or change direction. We can feel the resistance force on our fingers as we turn the spinner from side to side. Gyroscopes work on this angular inertia principle.
How high is it?
The Ka-Band Altimeter (KaRA) measures the craft's distance from the Moon's surface. The instrument sends a pulse of Ka-band radio waves toward the lunar surface. It measures the exact time the pulse returns after bouncing back from the lunar surface. From the time taken for the pulse to return after the reflection instrument computes the distance, that is, the height of the craft at that instance.
Laser Altimeter (LASA) is another instrument that helps compute the height of the craft above the lunar surface. It works very much like the Ka-band altimeter. In this instrument, a laser pulse is used to detect the height of the craft. While the KaRA is mainly used during the rough breaking phase, this instrument is crucial during the final descent phase to estimate the rate of fall of the craft and adjust the thrusters to slow the ship to safe levels.
Which way should the spacecraft navigate to reach its landing site?
Lander Position Detection Camera (LPDC) is the GPS for Chandrayaan-3 lander craft. It tells where the craft is with respect to the landing site and provides the direction and distance that the vessel needs to move to reach the landing site.
The technology works by what is known as ‘feature recognition’ software. Some of our mobile phones can recognise our face and open. When we buy the phone and register our face, the software takes an image and analyses it. Features on our face, like the distance between the nose and the eye or the thickness of lips, are extracted. The AI software creates a database of these features. Next time we show the face to the mobile, the facial recognition system compares the features extracted at that instance with those in the database. If the components match, then the phone is unlocked.
Lander Position Detection Camera (LPDC) works on a similar principle. The Chandrayaan orbiter has captured a high-resolution image of the landing site. The AI-powered onboard computer has stored the extracted features from this image. While landing, the LPDC will take pictures, and the AI software will compare and locate where the craft is in relation to the landing site. This will help determine the navigational path.
The Micro-Star sensor is another instrument that provides location.
The star sensor is a tiny telescopic camera. It takes an image of the sky, and the software identifies the stars in them by comparing the onboard database. The AI software will compute the orientation and location by determining the relative position of the stars with respect to the craft.
Is this spot safe to land?
LHDAC (Lander Hazard Detection & Avoidance Camera) helps identify if a spot is safe and free of dangers like giant boulders, large craters and steep slopes.
The surface of the Moon is studded with big and small boulders and enormous and tiny crates. If one of the legs is placed on top of a rock or inside a crater, the whole craft will tilt and wobble, making it difficult for the rover to roll out. The lander has to avoid giant boulders and large craters. LHDAC will help the craft to identify a safe landing spot inside the landing site.
At the time of the landing, the Sun will be at an angle to the lunar surface. Hence both a boulder and crater walls will cast shadows. By analysing the shade, the AI software will be able to compute the crater's depth, the size of the boulder and the estimate of the slope of the lunar terrain.
Slopes greater than 10 degrees, craters larger than 1.2 metres in diameter and boulders bigger than 28 cm are classified as hazards. During the terminal descent phase, the AI software will analyse the image and choose a safe location within the landing site. The craft will be moved towards this spot before finally descending towards the lunar surface.
Should it slow down?
If the craft moves above the prescribed speed, it will crash. The speed of the ship has to be reduced to safe levels. The laser Doppler Velocimeter (LDV) is a new instrument augmented in Chandrayaan-3. This works with the Doppler principle.
When an ambulance speeds past us with its siren blaring, we can notice that the pitch of the sound is higher than when it is moving away from us. The siren is consistently producing a constant frequency. However, the observed frequency and pitch increase when the source moves towards us. When the source moves away, the observed frequency and pitch decrease.This is called the Doppler principle.
The laser Doppler velocity meter sends a pulse to the ground and observes the change in the frequency of the reflected pulse. From the pulse difference, the craft's velocity can be precisely measured. This sensor helps the navigational software control the thrusters' throttle and reduce the craft's speed to the desired level.
When the craft is near the surface, during the terminal descent phase, like 60 meters up close to the lunar surface, the LDV may not be effective. The Chandrayaan-3 craft is fitted with the Lander Horizontal Velocity Camera (LHVC) to estimate the speed of the fall during the terminal phase.
Imagine a bus rushing at you. The bus's image size would have been small when it was at a distance. However, as it approaches you, the size will enlarge. The rate of the size expansion is related to the velocity at which the bus is moving towards you. Likewise, this front-facing camera will take images of the surface and compare their features. The software will compute the rate of the zooming of the image and estimate the velocity at which the craft is falling towards the Moon. This data will be used by the onboard navigation system to control the speed of the ship, in particular during the terminal descent phase.
Is it straight?
When the craft de-orbits before descent, its legs are front facing. Slowly, the ship's orientation is turned to make the legs meet the lunar ground. The vessel carries an inclinometer that measures the tilt of the craft with respect to the lunar surface. This also provides a clue as to the ship's orientation for the navigational software during the terminal descent phase. After touchdown, this instrument will measure the inclination of the craft, and using this information, the sliding door for the rover will be deployed.
Has it landed?
All four legs of the lander have a touchdown sensor, which becomes active once it feels the pressure. On a successful soft landing, the touchdown sensor of the corresponding lander leg will be first triggered. This data will be transmitted to the ISRO ground station, confirming the safe and sound soft landing.