Technology Explained – Computers

It is very fortunate that the Western Alliance has held a significant lead in computer technology, as the abilities of the onboard computers in a modern weapon system usually determine its capability to fight and survive in a modern war. The USSR is well aware of this and purchases quite substantial quantities of microprocessor and other chips directly or via black market sources in Western countries. One may assume the vast majority of these chips will eventually find their way into a Mikoyan, Sukhoi or neat little AA-? Guidance head, to the detriment of any Western pilot fighting against them.

In spite of their complexity, computers do operate upon some very simple principles; the following should serve to illustrate that.

The Binary Number System

We live in a decimal world. Or at least, most of us do. In the simplest of terms, we bunch numbers up by tens, or tens of tens, or tens of tens of tens, ad museum. In fact, most of us never even question it. In principle, we can bunch numbers up any way we want (at least the mathematicians say so), say in fours, counting 1,2,3, and what next? Well, since we are counting in fours, it will have to be one set of four and zero units, that is 10. So we have 1, 2,3,10. Now five is one set of four and one unit, which is 11,six is one set and two units, which is 12 and we can go on forever and ever like this, getting 1,2,3, 10,11,12,13, 20,21, 22,23, 30. . . We are dealing with a base four number system. We could, for instance, look at a base two system. Here two is our unit, so all we really have is zero, one and what comes next is a set of two. It may appear strange, but counting one to ten in binary (= base two) we have 0,1,10,11,100,101,110,111, 1000,1001,1010. If you look closely, you’ll see that 10,100, 1000 are 2,4,8 in decimal, which are powers of two. All the things we can do in decimal, we can do in binary, add subtract, multiply, divide. Of course, we’ll be working with slightly lengthier numbers, but on the other hand one time a number is the number itself and zero times it is zero, which does tend to simplify things. The importance of binary lies in the fact that it requires only two states, 1 and O to describe any number. This has important implications for the electronics used, as we will see later.

Combination Logic

When dealing with logic, we think in terms of things being true or false. A statement may be either true or false; the subject of combination logic deals with what happens when we have more than one statement. Consider the following:

Statement A – ‘the circle is white’, true if it is white, false otherwise.

Statement B – ‘The dog barks’, true if Rover barks, false if he doesn’t.

We can connect both of these statements into one statement, but whether the compound statement is true or false depends on both A and B and the connective we used to glue the two statements together. The two basic connectives we have are AND and OR. These connectives then define the truth value of the compound statement. The following tables illustrate this.

Say ‘the circle is white AND the dog barks’ – this will be true only if the circle is white and only if Rover does bark, if either or both statements are false, the compound statement must also be false.

These connectives can be represented by simple diagrams, logic gates (see diagram 1). Given that T and F are applied to the A, B inputs of the gate, the output Y will assume either T or F, depending on the inputs and the type of gate.

Combination logic is one of the most important aspects of digital electronics. In digital electronics, we employ circuits that can assume only two states – ON or OFF, equivalent to true or false and usually labelled as 1 and 0. Logic gates are,in fact, one of the most basic building blocks used, these are usually implemented with transistors and integrated onto a single chip, which will usually contain several gates.

The strength of using circuits that may assume only 1 or O states lies in the use of binary numbers, which by nature only assume values of 1 or 0. Adding, say, two binary numbers in an electronic circuit is then merely a matter of appropriately connecting a number of gates between the inputs and outputs. If we wish to multiply all we have to do is shift and add. In this fashion we may hardwire virtually any mathematical (within reason) function we wish, taking numbers from sensors or keyboards or whatever we wish, converting them to binary, feeding them into the appropriate network of gates, taking the output and converting it back to decimal (in fact earlier digital signal processors on air-air radar were of this sort). This way of doing things may be simple (the electronics may be very fast, also), but it lacks flexibility. A far more effective way of handling functions may be used – the digital computer.

Digital Computers

The block diagram of a computer is in diagram 2.

1/0 represents the input and output interface. This is a very general representation of the computer’s means of talking to the external world. The external world may be a keyboard, switch etc. on the input side, a printer, CRT (typ. TV) or missile launcher on the output side. What the 1/0 electronics do is convert whatever signal, instruction or information we are dealing with into a binary (or other) form understandable to the CPU, or central processor.

The 1/0, together with the CPU and memory all tie into the Bus. The bus is the channel via which all parts of the machine communicate with each other. The best analogy is to imagine a small round table, around which are seated Messrs 1/0, CPU and Mem. Mr CPU is the boss. He wishes to speak to Mem, so he places a card with the name Mem onto the table. Mem sees the card, so he places another card labelled OK onto the table. When CPU sees this card, he may lean over the table and communicate with Mem. The table is the bus. The bus may be used only by two systems at a time, the internal bus is controlled by the CPU, which uses it to talk to other parts of the machine.

The memory is another critical part of the machine. It serves to store both instructions (programs, in the general sense) and data, to be worked with or worked on. The memory is comprised of a large number of storage elements, these are grouped into fours, eights, sixteens, thirty-twos or other, depending on the number of binary digits (bits) the machine operates with. Each of these groups of elements has an assigned name, a location with a numerical address. Memories come in two basic types, Read Only (ROMs) and Random-Access (RAMs). ROMs may only be read from, they cannot be erased. RAM’s may be read from or written into, depending on what is required.

The CPU or Central Processing Unit is the critical component of the computer, the actual heart of the machine (a microprocessor is a single-chip CPU). The CPU is made up of various timing and control circuits, a set of several registers (a register is a memory with the size of one word) and an Arithmetic Logic Unit (ALU). The CPU will read instructions from the memory and execute them. The sequence of instructions is the program the machine is executing. The instructions the CPU handles are all very simple, like fetching the contents of a particular memory location, moving them to another location, moving them into a CPU register, adding the contents of a register to another, shifting left, shifting right, depending on the computer in question.

In operation the CPU looks like this: the CPU will fetch the contents of a particular memory location and look them up in its instruction set. It will then execute that instruction. After doing that, it wil add one to a special register, called the Program Counter (PC – this register held the address of the memory location with the just-completed instruction), after which it will fetch the instruction at the location whose address lies in the PC and subsequently execute it. So it will, step by step, execute instructions stored in the memory. Exactly what those instructions may be depends entirely on the person who wrote the program and loaded it into the computer’s memory (to quote one engineer, computers are dumb brutes that do only what you tell them to).

As one may observe, the computer is in principle a pile of electronics that carries out elementary arithmetic and swaps memory locations of binary numbers. Of course, the first thing an observant reader will point out is that writing programs in machine code (the term for the set of instructions in binary) must be pretty tedious, particularly as even simple multiplication will involve a pile of adding and shiftings. This observation is correct – in practical instances a high-level program language is used (e. g. Fortran, Ada, Pascal etc. ), resembling algebra (of sorts). This is then processed by another program, called a compiler, and converted into a neat pile of binary machine code, easily digestible by the CPU. (In many military operations, memory space and execution time are both in short supply in these instances assembly or machine code are used directly. )

Just as memory locations have addresses, so do all the external interfaces to the computer (via the above 1/0). This means the CPU may also accept information/commands (and issue information/commands) from (and to) the external world in much the same fashion as it deals with the memory.

The beauty of the concept lies in the fact that one may develop some elaborate mathematical model, write it down with an algebra-like language, feed it into the machine and have the computer then interact with the real physical world, according to the model used. An added bonus is the fact that several computers may be connected to an external bus and talk to each other, just as some computers may be slaved to others, to handle particular tasks.

The flexibility of the digital computer is unparalleled by that of any other system (one may object that man is more flexible, but then there do exist some people who will categorise the brain into the above); developments in electronics are constantly increasing machine speed and decreasing size, so we will see more and more capable and compact machines in the future. This will have a great effect on the design of weapon systems, as more intelligence becomes available at an autonomous level in the system. A look at the computer systems of a current fighter will demonstrate the potential available.

Integrated Display, Sensor, Weapon Management and BIT Systems – The F/A-18A

Australia’s future NTF is a weapon system built around its computers. The Hornet airframe is a scaled development of the YF-17A, that in turn is a development of the Northrop P-530. However, the F/A-18 was built for the USN, primarily as a replacement for the service’s F-4N/S Phantoms and A-7E Corsairs. The aircraft was to exceed both types in performance, mission effectiveness, reliability and maintainability, and also be a single-seater. If a conventional approach were to be used, unmanageable problems would have soon occurred. The radar would require a large number of controls to handle the multitude of air-air and air-ground modes, the wide array of weapons would require a lot of attention in arming and release. The laser/ Flir pods would require controls, just as the HUD. The Built-in test (BIT) system would also bite into cockpit space and pilot workload. Cockpit space was a particular problem, the airframe allowing for about 40% less area than current fighters.

The design approach adopted was the use of a highly automated display, weapon, sensor and test system, under the control of two digital mission computers. Virtually all the aircraft’s systems have their autonomous computer(s), which are, in turn, slaved to the mission computers. The system was designed with a lot of redundancy to increase immunity to failures in the computers. The aircraft operates in two combat master modes, air-to-air (A/A) and air-to-ground (A/G), the aircraft becomes a fighter or bomber at the depression of a single switch. Once in either mode weapons and radar modes may be changed with a set of controls on the throttle and stick, so the pilot need not release the controls during A/A or A/G engagements. The following details the avionic and computer systems of the aircraft.

1. Mission Computers and Buses

All primary avionic systems interface via dual redundant multiplex buses with the two AN/AYK-14 general-purpose mission computers. Each sixteen-bit processor has 64 k of RAM available. Mission Computer 1 (MC1), the Navigation Computer, performs processing for navigation, control/display management, status monitoring, and BIT and serves as a backup for MC2. MC2, the Weapon Delivery Computer, handles A/A and A/G processing, tactical control/display and is a backup for MC1. The backup software for either MC is simplified to essentials, in the event of a large failure, it will merely degrade the level of calculation that would have been otherwise available, still enabling the completion of the mission.

Information transfer between· subsystems and the MCs occurs via the three dual redundant channels of the MIL-STD- 1553A data bus (1 MHz serial, sixteen-bit words). Channel 1 links the MCs to the Flight Control Computers, Air Data Computer, Communication System Control, Stores Management Set, HARM CLC, left Digital Display Indicator (DOI) and other systems. Channel 2 links the MCs to the INS, radar, FUR, LST/SCAM, Data Link and right DOI. Channel 3 enables communication between MC1 and MC2.

2. Cockpit Controls and Displays

The principal elements of the control/display systems are four computer-controlled CRT displays, an Up-Front-Control (UFC) panel and Hands-On-Throttle-And-Stick (HOTAS) controls. The two primary elements of the display system are the left and right DDI, termed the Master Monitor Display (MMD) and Multi-Function Display (MFD) respectively. Each DDI contains an I/O processor, 4 K of RAM and a CPU, which controls two signal generators (SG). The VO processor communicates with the MCs, radar, and sensor pods and receives commands from the controls, all of this is then loaded into appropriate parts of the memory, where it is accessed by the CPU. The CPU will then generate commands to the two signal generators. Each SG then drives a single CRT display. The HUD and the third cockpit CAT display, the Horizontal Situation Display (although in some aircraft the HSD may be replaced with a simple CRT), are both ‘dumb’ CRT terminals, with no processing ability of their own they are slaved to the two DDIs, one each, driven by the second of the two SGs in the DDI. The HUD is the primary flight instrument and may be driven by either DDI. This arrangement provides maximum redundancy in the event that either a CRT or part of the electronics fails, as critical data may be channelled to any remaining operating CRT display.

The MMD usually serves to display store management, caution, advisory and BIT information, whereas the MFD is a radar and FLIR display. The HSD, where fitted, combines a moving map (35 mm colour film projector, with a specific ROM chip for each film reel, to provide reference to the MCs and DDis) display with a projected CRT image, via a HUD-like combiner system. This allows position and flight path images to be superimposed on a map in real-time (radar and FLIR images may also be superimposed on the map, to assist in target location), which takes the work out of navigation.

The Up Front Control panel, just below the HUD, controls the two ARC-182 radios, autopilot, IFF, TACAN, ILS, Data Link, Radar beacon and ADF, its keyboard also controls the HSD via the MCs. Further controls for the radar, sensor pods, IFF, INS and ILS are on the side consoles.

The HOTAS controls allow weapons and radar mode selection, and control of the sensor pods, aside from the usual controls one finds in these positions. The Throttle Designator Controller (TDC) enables the pilot to move a cursor on the MMD or MFD, to change modes or designate targets.

3. Navigation/Flight Aids

The F/A-18A is equipped with an AN/ASN-130 Inertial Navigation System (1. 5 nm/hr drift) with carrier/ground alignment and in-flight update capability (radar Doppler). The digital CP-1334 Air Data Computer processes analogue and digital outputs from a set of sensors (pistol-static pressure, angle-of-attack, temperature and magnetic azimuth) and provides calibrated data for the MCs (Mach number, true airspeed, AOA etc. ), engines, landing gear and environmental control, IFF.

4. Electronic Flight Control

The aircraft has a quadruple redundant digital fly-by-wire control system. The AN/ASW-44 Flight Control Set has two digital computers for pitch/roll/yaw processing and four sets of sensors and data channels. Two similar failures will be tolerated. after which direct electrical control is assumed, bypassing the computers. Direct mechanical backup in pitch/roll is also available.

5. Stores Management Set

The Armament Control Processor is a 32 K digital computer, that selects, conditions, releases, jettisons and monitors the status of all weapons and stores carried by the aircraft. The SMS interfaces with nine Command Decoders, each for one of the nine hardpoint/pylons under the wings and fuselage. The SMS also ties in with the M-61 cannon, the Command Launch Computer for the HARM anti-radiation missile and the AMAC system for nuclear stores.

6. AN/APG-65 Multimode Radar

The radar set is a design developed from experience with the F-15’s APG-63. It operates with low, medium, high and inter-leaved pulse repetition frequency modes in Doppler air-to-air operation and dual plane mono pulse for dogfight/single target track modes. The radar has a track-while-scan capability for 8 targets, has a raid assessment mode (examines fine Doppler shifts of a distant target return to determine the number of aircraft in the formation) and a proposed Non-Co-operative Target Recognition mode (NCTR- the radar will look at the doppler shifts and modulation of the radar return by the target’s engine and identify it from a computer file). A/G modes enable high-resolution ground mapping, terrain avoidance, moving target detection and tracking. The key to the radar’s capabilities lies in its programmable signal processor, most likely the fastest computer within the whole system.

Master Modes

The F/A-18A operates in three master modes, upon powering up the system enters the Navigation mode, with displays configured for navigation. The two combat modes are air-air (A/A) and air. Ground (A/G) can be entered by depressing either the A/A or A/G switches on the stores panel (which also carries a master arm and ordnance jettison switch), this is monitored by the SMS processor, which then notifies the mission computers. Once a master mode is entered, all displays are configured for that mode and the switches on the controls assume particular functions. In A/A the MMD carries a diagram of all missiles and the gun, including full status (as weapons are fired, they disappear off the screen) and ammunition count. The MFD becomes a radar display (track-while-scan PPI, velocity search bar graph). The HUD symbology depends on the weapon selected and the mode of the radar. Weapon selection by HOTAS is monitored by the SMS, which issues firing commands.

In A/G, the MMD once again monitors the status of all stores, typically the type of bomb, fusing, and guidance codes. The MFD will display radar imagery or FLIR imagery. HUD Modes for A/G include CCIP and manual release.

The HARM anti-radiation weapon may be released in both A/A and A/G modes, a factor that will be less than appreciated by SAM site crews.

The F/A-18A represents the first in a new generation of military aircraft, where computing power is as important as firepower. The extensive use of intelligent subsystems will increase effectiveness on all types of missions by reducing both pilot workload and the chances of pilot error. Many critical parts of the mission may be simplified for the pilot or taken over by the computers. The trend is likely to accelerate. Very Large Scale Integration chips in development promise to increase component density. The advancing technology of Gallium-Arsenide (electrons move around in GaAs several times faster than in silicon) will also offer much faster chips, which will increase processor speed. These developments may eventually allow the lightweight fighters of the late 1990s to carry systems such as conformal radar or adaptive camouflage.

As is apparent, the best is yet to come.