# TeamIndus Lunar Descent Strategy

## The science and art of executing a Moon landing

Oct 6, 2017 · 5 min read

To date, only 3 countries have successfully landed on the moon — USA, USSR and China — proving that it’s one of the most difficult of missions. These were achieved by the respective governmental organizations with budgets in the billions of dollars and many years of prior space technology expertise & past space mission experience to rely on. This ought to give you an idea of the complexity involved in such an attempt.

Through this short piece, we aim to give you some information in the descent strategy being developed at TeamIndus.

# Understanding orbits before the descent

To land on the lunar surface in a fuel efficient manner, the spacecraft needs to start descending from the lowest point in the S4 orbit (periapsis), which is 12 km. The lowest point of the S4 orbit depends on the nature of the S3 orbit, which is where the physics advantage of the circular orbit comes in.

Had S3 been an elliptical orbit like S2, the closest point for starting the descent would lie where S4 is marked in the diagram. This would constrain the longitude for landing, leaving no flexibility in targeting various landing sites.

Instead, a circular orbit allows the engine burn to be executed anywhere in the circular S3 orbit and accordingly choose the lowest descent point in the S4 orbit. Circularizing the orbit thus allows flexibility in targeting various landing sites while requiring the least amount of energy expenditure.

# Hardware

`1. Laser Rangefinders (LRF)2. Laser Altimeters (LALT)3. Descent Cameras (LDS)4. Inertial Measurement Unit (IMU)`

# Parameters measured during descent

The key parameters that feed into the on-board computer (OBC) and used by the descent algorithm are:

`1. Altitude2. Velocity3. Position`

The Laser Altimeters (LALT) provides data used to derive altitude. The LRFs on the other hand are used only during terminal descent — from 100 meters above the lunar surface.

The IMU plays a key role as the central sensor for navigation of the spacecraft. An IMU comprises a highly precise accelerometer and gyroscope. For easier understanding, think of modern day smartphones that allow you to motion-control or play motion-based mobile games. The IMU measures even the minutest of changes allowing our algorithms to determine orientation. During orbit, the Mission Planning team can predict position and velocity with a required degree of accuracy. Hence at the time of descent sequence being initiated, with this information available to the on-board computer OBC, acceleration is tracked and fed into a mathematical model that can propagate position and determine velocity as the spacecraft decelerates and approaches the landing site.

The knowledge of the spacecraft’s position, velocity and attitude is fed to the guidance program which calculates the reference attitude and thrust. The control system program uses the guidance and navigation commands to orchestrate firing of the main engine and the attitude and reaction control thrusters in synchronized firing patterns to land the spacecraft gently.

# Descent Profile

## Terminal Descent

The LDS (cameras) begins imaging the lunar surface. Images are clicked in intervals and compared through image processing algorithms to determine the velocity with relation to the ground.

With Main engine throttling down and ACT keeping spacecraft orientated while minimizing lateral velocity, the spacecraft descends to the surface. This is where our spacecraft’s structure and landing gear will be put to the test.

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