Controlling Spacecraft

Shuttle 



300 major electronic black boxes, 300 miles of electrical wiring, 120,400 wire segments, 6,491 connectors, wiring and connectors weigh approximately 7,000 pounds, wiring alone weighing approximately 4,600 pounds. Seems daunting? In fact, they are all a part of the Space Shuttle’s avionics systems, the lifeline of the shuttle way back in 1988. Other data processing systems consist of five general-purpose computers (GPC), mass storage, a time-shared computer data bus network for communication between the computers 

As of 1991, all five GPCs were IBM AP-101S computers. Before that they were the IBM AP-101Bs. The older GPCs had non-volatile memory, but with the AP-101S the shuttle computers moved to volatile memory with battery back up. Each of the GPCs consists of a central processor unit and an input/output processor. The first four systems comprise the PASS (Primary Avionics Software System). It may seem surprising that NASA uses ancient computers, but they do so because of a very good reason: these computers have been thoroughly tested. Such reliability is extremely necessary when dealing in mission-critical operations.

The fifth GPC is loaded with different software created by a different company but performing the same functions and operations. This software is the Backup Flight System (BFS). The BFS monitors the other GPCs to keep track of the current state of the vehicle. If required, the BFS can take over control of the vehicle upon the press of a button. The BFS also performs the systems management functions during ascent and entry. BFS software is always loaded into the fifth before flight, but any of the five GPCs could be made the BFS GPC if necessary. NASA is developing a Shuttle Upgrade project to install a Vehicle Health Monitor (VHM) system. The VHM system will be used to detect, troubleshoot and resolve hardware problems during and after flight. To match the hardware, the software is also written and verified meticulously. 

The Shuttle control software has some 420,000 lines of code. The software is subject to countless hours of verification and testing. Yet, there are flaws. There are still bugs in the code, but when compared to other software, they are very less .The two companies involved in the shuttle software creation were IBM Federal systems and Lockheed Martin.

The shuttle’s original CRT- based displays were replaced with LCD, saving weight, power consumption and fuel

When information and voice is sent to the Shuttle, it is scrambled for security, so there is a computer onboard to decrypt this data so that the astronauts can read it or hear the voice from Mission Control in Houston, Texas. Once the data is decrypted, there are other computers onboard that control the attitude, velocity, temperature, and toilet/bathroom system, just to name a few. A computer called the Ground Communication Logic Controller (GCIL) receives this data and then routes it to the appropriate computer system to perform one of these functions. Houston, Texas controls most of the computers onboard from the ground itself because the crew has experiments and science data to collect and sometimes cannot be responsible for all functions.

Path Finder 



The Path Finder mission that put a rover, Sojourner on Mars was conceptualized in 1992. Traditionally, spacecraft landed on alien territory by the means of rockets (The Viking mission to Mars). The Path Finder mission was a technological first, in demonstrating a low-cost landing mechanism by the means of parachutes and airbags as suspension. Given the distance of Mars from the earth, the time taken by the signals from Mars to reach the earth is about 10 mins. It was launched on December 4, 1997 and, after nearly seven months of space travel, reached Mars. The Path Finder mission also carried a rover, Sojourner to Mars. The payload also included an alpha proton X ray spectrometer to analyze the elemental composition of the soil on Mars, to act like an on-site geologist. As mentioned, the time lag for signals to reach the earth was around 10 mins. So, some amount of autonomous control had to be built in to the system. After landing it took another 2 Martian days (1 Martian day, the Sol, is 40 mins longer than an earth day) to get the rover functional, and it remained so for the next three months. During these three months Sojourner sent back over 2.6 billion bits of data. It also did 20 soil analysis tests during this time. The computing power behind the rover is a radiation protected IBM RISC 6000 single chip CPU. The complete Mars Path Finder Flight Computer had 128 MB DRAM. The code was developed using VxWorks as the real time OS and the good old C and assembly for coding. 

The communication between the Lander and the rover is not exactly rocket science. The protocol uses a simple acknowledge/negative acknowledge algorithm (whether the data was received or not). A CRC check is done on the receiving side to validate the transmission .The maximum frame size used was 256 bytes. Such methodology is nothing but the more commonly used protocols stripped to bare minimum. 

Ankit Khare