An overall check of a computer can be done by the use of a maintenance program. The maintenance program provides a thorough and rapid method for the detection of failures in a specific portion of the computer. This type of overall maintenance check is flexible and efficient. The programs may use the same type of tape, memory, computing, and external storage media as operational programs. The maintenance program can be altered when the computer or auxiliary components are changed. The program can also be constantly improved. Generally, no extra test equipment is required since the computer circuits are used to perform the test. Testing by means of maintenance programs results in the computer circuits being used in a more comprehensive manner than during normal program execution. When a program has been checked and accepted as a good maintenance tool, it is not subject to deterioration. In contrast, test equipment may be checked and accepted only to become unreliable shortly after being placed in use. Maintenance programs are divided into two main classes: reliability and diagnostic. Maintenance programs that are used to detect the existence of errors are called reliability programs. Reliability programs should be arranged to check as many computer circuits as possible. Maintenance programs that are used to locate the circuits in which computer malfunctions originate are called diagnostic programs. An effective diagnostic program should locate the source of trouble as closely as possible. Actually, in many cases, reliability programs have some diagnostic features, just as diagnostic programs have some reliability features. For convenience, a program is called either a reliability or diagnostic program, depending on its intended emphasis.    In general, programs that check rather than diagnose are shorter and simpler. PERIPHERAL AVIONICS SYSTEMS Learning Objective: Identify peripheral avionics systems and describe their interaction with the computer. The aircraft computer is considered the most important avionics system in achieving the mission of the aircraft. However, the success of the computer depends upon its external sensors or other avionics systems. The quality of data fed to the computer determines the quality of data fed out of the computer. The following avionics systems provide inputs to and receive outputs from the computer: navigation, radar, ordnance/weapons, and data link. These are only a few of the major aircraft avionics systems that interface with the airborne computer. NAVIGATION Navigation systems are designed to tell pilots where they are, where they have been, and where they are going. The TACAN/DME system provides known station reference points, while an inertial navigation system provides continuous updating of such information as latitude and longitude. This information is fed to the computer where it is compared, updated, and sent out to other systems. SEARCH/TRACK RADAR A search radar system is designed to give visual indications of what is around the aircraft. Some of the present-day aircraft have a 150-mile or greater range. Depending upon the size and/or speed of the radar indications, a computer can determine whether the target is stationary or moving, a land mass or an aircraft, friendly or unfriendly, and many other items of information. If a target is determined to be unfriendly, a tracking radar can be used to tell the pilot what to do to eliminate the target. ORDNANCE/WEAPONS The design characteristics and ballistics of the many types of ordnance, weapons, and missiles require the use of a computer to store the information. The airborne computer aids the pilot by telling him/her when to release the weapons. The computer greatly increases the pilot’s chances of destroying designated targets. DATA LINK Combat aircraft have to have the most up-to-date information available to successfully complete combat missions. On an aircraft carrier, the combat information center, CIC, is normally in constant contact with an airborne CIC. The airborne CIC is usually an E-2 or P-3 aircraft. These two CICs will crosstalk by use of the data link system. Basically, data link involves a series of transmitted pulses that represent information. The pulsed information is sent to the computers of all combat aircraft to enhance their chances of success. 8-16