101 SYSTEMS: COMMAND AND DATA HANDLING—THE BRAINS OF THE INTERNATIONAL SPACE STATION CHAPTER 5 that are synchronized will eventually drift apart as the “pendulum swings” of their oscillator are different. For the computers used on the ISS, the difference is on the order of milliseconds. Still, information can get garbled as it passes back and forth due to these differences. For example, a command might not reach its correct destination because one computer is trying to pass it to a second computer that is not ready to receive it. If an MDM gets ahead of or behind the CCS, it will adjust its pendulum swing in the oscillator to drift back to the correct time before the difference becomes too large. Upon boot up, the default time in the C&C MDM is January 1, 1992 (the time when the GPSs were initialized), not unlike a digital clock that defaults to 12:00 when first plugged in. Thus, the time in the C&C MDM needs to be reset. This can be accomplished in multiple ways. Most modern spacecraft use the GPS time, due to its accuracy. The GNC system has multiple antennae to receive the GPS signals for this purpose. Unfortunately, these signals can become interrupted or confused when, for example, some of the signal is reflected off parts of the ISS to the antenna. This can create the undesirable effect of causing the time value to jump around. The lower- tier MDMs, designed to gradually drift their own clocks to keep up with the C&C, cannot respond fast enough. When this happens, lower-tier computers can become unsynchronized with the CCS. Therefore, the ODIN/CRONUS flight controller monitors the CCS time and manually adjusts its oscillator to maintain GPS time. Portable Computer System The PCS is the crew’s interface with the station’s computer system. With the PCS connected to the C&C MDM via a special cable to a 1553 bus, as shown in Figure 3, the crew can send commands to the vehicle and receive the status of most systems. As many as eight PCS laptops can be connected at any given time. These laptops are distributed around the ISS in areas where the crew will be working. DC Power Cable US DC Power and 1553 Cable (UOP to Power Supply) Figure 3. The PCS consists of a laptop with specially written software that plugs into a Utility Outlet Panel using a dedicated junction box that converts the station power to standard 120 V as well as a 1553 data cable. This panel provides electricity as well as a 1553 data connection to the CCS. The PCS is currently an IBM T61 laptop with a duo-core processor, which is in line with the goal of using as much commercial off-the-shelf equipment as possible. The PCS has the same hardware, although not the critical software, as the crew’s SSC so that spares can be swapped back and forth easily, as needed. Basic parameters of the PCS are listed below. The PCS platform is the Scientific Linux operating system, based on UNIX, which uses a graphical windowing environment based on X-Windows. Both the PCS and the SSC can talk to a printer. As the crew’s primary systems interface, the PCS needs to provide easy-to-use software that is intuitive to an astronaut of any nationality, especially during an emergency.