Mapping to the Moon
How Apollo Found its Way
The moon landings are undeniably an orgasmic piece of history to any space geek, and objectively one of the most remarkable achievements of mankind. The legacy of the likes of Armstrong and Buzz Aldrin will forever live on, but the groundbreaking engineering innovations behind the Apollo missions are often overseen. Some entire fields of science and engineering came into being to make this program a reality, and made way for generations of invention.
One particular aspect of Apollo that I’ve always wondered about is the part where they actually get to the moon, and some critical components in this process are the navigation instruments. In space, there’s no magnetic field to use a compass. No gravity to tell which way is up, even. This was just about the time when a technique called inertial guidance was gaining traction.
Inertial Guidance
Inertial guidance is a wonderful tool which is found everywhere today as a navigational aid — in ships, planes, spacecraft and more — but the system wasn’t always designed with such intentions in mind. It was invented by American pioneer Robert Goddard as a way to guide missiles with never-before seen levels of accuracy. Eventually however, it was recognized as a powerful instrument for driving the future of space travel. The guidance system on Apollo (called the Inertial Measurement Unit) was developed completely in the MIT Instrumentation Lab, along with the flight computer.
Inertial guidance typically uses a system of gyroscopes and accelerometers. A gyroscope is just a wheel or a disk that spins, and is suspended freely. Hence, due to conservation of angular momentum, the orientation of the disk is not affected by a change in the surrounding environment! This makes it vary suitable as a means to maintain a static reference point as your own orientation changes.
The gyroscope is usually suspended inside concentric circular rings, each of which track a different axis of rotation of the spacecraft. The rings, which are called gimbals, can then be used to tell attitude, or direction the spacecraft is pointing.
Accelerometers, also called motion sensors, were also mounted to the inner gyroscope core, so that the acceleration or movement in any direction could also be detected.
With knowledge of where the mission started, and how the spacecraft moves at every instant, spacecraft’s flight computer could rather accurately determine where it was a given time. This is called dead reckoning.
The Problem
There’s one big problem when it comes to using inertial guidance and gimbals, something that may not initially be apparent. Since the gimbals are free to spin about the each axes, what happens when two axes come into alignment? If they do, they can no longer independently maintain rotational freedom. They’ll always rotate together, meaning you will now lose information about one entire axis! This unfortunate situation is called gimbal lock. Astronauts had to be very careful not to put themselves in the circumstance, and the flight computer would screech alarms when they were getting close to losing a gimbal.
Now when you hear the words “Gimbal lock!” being shouted on that scene in Apollo 13, you know what it means!
One possible way to solve gimbal lock is to include a fourth gimbal, one that is actively driven to be aligned 90 degrees from the direction of the first. Now, even when two axes come into alignment, you have an extra axis to continue giving you a reference point to automatically correct the misbehaving gimbal.
At this juncture, we must keep in mind the conditions on-board the Apollo spacecraft. Minimizing the overall weight was of utmost importance, and every kilogram was expensive. Hence, the project leads made a very controversial decision — to include only three gimbals instead of four.
The Solution
The problem of gimbal lock made it impossible to rely only on dead reckoning to determine the spacecraft’s state. NASA needed a more reliable way of updating the position, and hence came the Deep Space Network. The DSN is a number of large antennas spread over different countries across the planet, such that there’s always at least three of them on any one side of the Earth. It’s large, 70-meter antennas were developed for unmanned deep-space missions. Together, they can effectively triangulate the position of the spacecraft and transmit that to the flight computer! Eventually, NASA decided to use the DSN as the primary means of determining the spacecraft’s position. The on-board inertial guidance system was still important, as it gave the astronauts a real time indication of their attitude.
However, a mission of such magnitude required a lot of redundancy, and there were still some problems with using just the DSN. For one, the system wouldn’t work on the dark side of the moon, where radio communication wouldn’t be possible with the Moon in the way. Hence, yet another system was added to aid navigation, a system used by ancient voyagers to calculate their heading — the stars! The spacecraft was equipped with a sextant, which can accurately measure angles, and the astronauts were trained to identify specific stars, using which they could re-calibrate the gimbals if all else failed.
In fact, astronauts regularly used the star alignment method to re-calibrate the gimbals, and also simply as an exercise to check if the flight computer was giving them the correct readings. Such an alignment typically took about an hour, and was a pretty tedious process for the spacemen. The calibration program on the flight computer would show the error between the coordinates that the astronaut got and the actual values it measured, and displayed them on the screen. If the astronaut got it perfectly, the system would show 0 0 0 0 0, indicating no error, and was also called "All Balls"!
The Test
The Deep Space Network performed excellently, and would become the standard for NASA missions for generations to come. Even other countries’ missions (Like India’s Mangalyaan for example) use the DSN.
Gimbal lock still caused problems, and occurred thrice over the 10 missions that went out to the moon. Despite that, the inclusion of redundant systems and the ingenuity of the Apollo Guidance Computer meant that it never posed a serious threat to mission safety. The stars would be there to save the day!
A Way for the Future
Over the years, guidance systems have improved immensely in capability and yet, even modern spacecraft still conceptually the same inertial guidance units for navigation. The Apollo missions pioneered several technologies that were still in their infancy, and truly inspired a leap for the world technology and engineering.
If you’d like to read more, I wrote an article about the Apollo Guidance Computer over on the Delta Force blog. Do check it out!
