Navigation: From Dead Reckoning to Navstar GPS

Dec. 1, 1984

It was late in the day to be starting a race — but there was going to be one. It would be a race of fabric-covered, open-cockpit biplanes, with undertones of national pride: the British vs. the French. It would be another in a lengthening line of aviation firsts in the seven short years since Kitty Hawk. Man had flown the English Channel. There were heralded flights to Paris and other world-class cities. Now the world would see who would be the first person to fly the 185 miles between London and Manchester. A London newspaper, The Daily Mail, was offering £10,000 to the one who could do it in less than twenty-four hours.

For the Englishman, Claude Grahame-White, the challenge was great. The newspapers had made this flying daredevil a local hero. He had tried the flight before and failed, landing because of engine trouble sixty-eight miles short of Manchester. Now, in this race, there was a chance that the Frenchman, Louis Paulham, would win. The honor of England was at stake. Since the race started late, night closed in all too quickly, forcing both pilots to land rather than face one of the greatest terrors of aviation in those days: darkness. For Grahame-White it was particularly disturbing; he was fifty-seven miles behind the Frenchman. He had to do something, so he decided to try flying at night — something that had never been done before! He was hoping the moon would provide enough light to navigate by.

Cars with lights ablaze lined up to form a runway, an encouraging sight for Grahame-White. He started his engine and began to roll. As soon as he was off the ground, he was in serious trouble. A cloud passed in front of the moon, and Grahame-White found himself in total darkness. In his excitement, he caught his sleeve on the ignition switch, flipping it off. Groping in the silent darkness, he found the switch, turned it on, and the engine sprang back to life.

Grahame-White managed somehow to maintain altitude throughout the night and navigate to Manchester. It was a difficult task. In those days, there were few lights on the ground and only darkness in the cockpit. By the time he landed, at 5:30 in the morning, he was cold and visibly shaken. Climbing out of the cockpit, he said, “I would not do that again for ten times £10,000.” Grahame-White didn’t get lost, but he was probably one of the first aviators to use the term “temporarily disoriented.”

Days of Contact Flying

Early pilots depended on contact flying — the use of lighthouses, roads, and other landmarks as navigational aids (navaids). A cockpit full of instruments would be years in coming. So, for more than a decade, aviators relied on their compasses, crude maps (charts), and dead reckoning (the determining of position by using direction and speed data) for navigation. Dead reckoning in the air, however, was quite different from navigating on the earth’s surface.

Early airplanes reached speeds of forty to 100 knots, but could easily be blown off course when hit by crosswinds, which commonly peaked at forty knots. That situation led to the famous word that every pilot and navigator knows quite well: drift — the difference between the true heading of the aircraft and its ground track. Some early aircraft had drift sights, allowing the pilot or nav-pilot conveniently to align a grid with the ground moving below him.

Often, when early pilots did get “disoriented,” they would swoop down and read road signs. It also became common for people to write the names of towns on barn roofs, allowing pilots ease of navigation as well as the lucrative opportunity to give joyrides to adventurous passengers. Slowly, aviators were learning how to navigate a course through the air.

By World War I, navigation hadn’t really improved much. In fact, there weren’t many American military pilots, and even fewer aircraft — fewer than twenty. (Of the twenty-four machines originally purchased from Orville Wright beginning in 903, almost half had been destroyed by 1013.) Nevertheless, operating British and French-built aircraft, American pilots flew bombing missions and observation flights for distances of 150-200 miles, encountering major navigational problems. If they hoped ever to accomplish their combat missions and return safely from enemy territory, they had to be very familiar with the terrain and its landmarks. Flying at night still posed a monumental problem. As the saying of the time went, “Flying at night is no different from flying in the daytime, except that you can’t see.”

After the war, Lt. Col. William C. Sherman, the first Chief of Staff of the Army Air Service, published an aviation manual reflecting the lessons the Signal Corps had learned during World War I. According to Sherman, aviators of “ordinary intelligence” could in a short time learn to fly at night, helped greatly by “increasing the number of lighthouses, mortar signals, and cooperation between the units of the air service…” Pilots, he continued, must also be instructed in cloud flying but should be warned never to attempt it unless compelled to do so. “If overtaken by mist of clouds, [pilots] must never let the ground out of their sight. If necessary, [the pilot] should make a forced landing rather than attempt to get home at night by flying through the mist.”

Three months after the Armistice was signed, commercial aviation started in Germany when Deutsche Luftreederei began passenger service. That year also saw the first daily commercial air service between London and Paris. Obviously, with air traffic expanding rapidly, the “first-class” navigational aids of the World War I ear — searchlights, lighthouses, and mortars fired as signals — needed improving to constitute a worthwhile en route navigational system, especially for night flying.

The First Navaids

South of the United States Air Force Academy, there are the remnants of what used to be a fairly good-sized hill. Some old-timers call it Beacon Hill because of the rotating light beacon placed upon it in the 1920s. That beacon, which operated for several decades, was part of a cross-country navigation system of lighting aids and emergency fields. The lights were placed at short intervals so that pilots could fly from one to another at night. By 1926, these light beacons covered approximately 2,000 miles and, by 1929, 10,000 miles. Despite the relatively vast coverage of this navigational system, it was effective at night only when visibility was good.

As early as 1925, aviators could make use of the first electronic navigational aid — a simple radio beacon. The pilot could navigate between the two stations by listening to and interpreting radio signals. Two years later, the first US Army flight was made from San Francisco to Hawaii using a newly developed “radio range” system. A range was built at two installations, one on either end of the proposed flight. Although similar to the low-frequency radio beacons still used in many parts of the world, this system used a single pattern, producing four courses for navigation. Even through not completely successful (the transmitters proved reliable, but the aircraft’s receiver worked intermittently), this twenty-four-hour, fifty-minute flight showed the possibilities of radio navigation. By the end of the 1920s, technology had progressed, and reliable radios were introduced.

With the development of larger aircraft and more reliable air and ground navigational equipment, aviation expanded faster and further. When Charles Lindbergh made his solo Atlantic hop between New York and Paris on May 20, 1927, he was, in fact, the 114th person to fly across the Atlantic. Such commercial companies as Transcontinental and Western Air inaugurated coast-to-coast service between New York and Los Angeles in 1930. Boeing produced an aircraft called the 247, and Douglas, the DC-1. In 1934, Pan American Airways started survey flights across the Pacific with its flying boats. Two years later, the company started carrying passengers on its new transpacific route, Pan American’s experience in pioneering overseas navigation and in developing airline routes would end up serving America’s war effort well during World War II.

Up the Learning Curve

Despite all these advances, the largest factors limiting expansion of commercial and military aviation remained bad weather and dependable navigation through it. Aircrews had learned to navigate across extensive distances by using such “old-time” seafarer techniques as navigating celestially to one side of a destination on purpose. This technique compensated for the large dead-reckoning errors common on ocean crossings by allowing the crews to determine which way they needed to turn to find their destination.

For example, if a navigator were aiming directly for a small island, chances were he could miss seeing it if he were off course. But by deliberately setting a course to one side of the island, at a calculated point he knew which way to turn to find the island.

Bad weather also complicated and hindered navigation. A number of disastrous air crashes occurred during this period simply because pilots lacked adequate instruments and knowledge of all-weather navigation. Improvements were being made, however. It wasn’t long before the primary means for navigation, dead reckoning and celestial navigation (by hitchhiking on mariner experience), were joined by the direction finder and its associated radio beacon and the low-frequency radio range station. World War II promoted more advances in radio aids for navigation — and in instruction in the art.

Before 1933, navigational instruction was given only as part of pilot training, as the Army Air Corps had few long-range aircraft requiring specialized navigators. In 1939, when Germany invaded Poland, Army Air Corps plans still called for only about 500 officers to be trained in navigation. These plans also assumed all individual training of navigators would be conducted in a specialized Air Corps flying school, even though no such organization was in operation until 1941. In the meantime, besides continuing training in combat units, the Air Corps had some students receiving navigational instruction with Pan American Airways. After September 1944, the contract with Pan American was discontinued.

However, by 1943, an eighteen-week specialized Air Forces navigation course had been introduced. Navigators were no longer being drawn from those cadets eliminated from pilot training; rather, men were being assigned directly to navigator school. Their training qualified them in precision dead reckoning, with basic proficiency in pilotage (contact flying) and radio and celestial navigation. Theory was kept to a minimum. Every effort was made just to show cadets how to do their jobs. In all, 45,000 bombardiers and 50,000 navigators were graduated during the war from an aircrew program that started with a small number of qualified instructors.

The Demands of War

At the beginning of World War II, navigation posed great problems. British bombers in the early days of the war had great difficulty in finding and hitting their targets.

The bombers were attempting to fly long-range night missions using navigational techniques better suited for daylight. Directional finding, using low-frequency radio signals broadcast from England, provided positive navigational fixes only within the first 200 miles. After that, the navigator was forced to rely on his dead-reckoning skills, using chart and compass. If weather permitted, he could occasionally use celestial navigation (if he knew how to do it) — but the aircraft gave bumpy rides, often making the sextant shots inaccurate. Likewise, landmarks were virtually invisible in all but the brightest moonlight. Obviously, it was a difficult feat.

For the Americans, the problems were generally the same. After the United States entered the war, the Armey Air Forces bombing effort was based on pinpoint daylight bombing. But both the Americans and the British dearly needed a more accurate method of navigation to find their targets.

Despite all the advances made between the two world wars in flight instrumentation, radio aids, and other communications, many pilots were still reluctant to fly when they couldn’t see the ground. During the early 1930s, the Army developed a system of aircraft control called “needle, ball, and airspeed system.” By the mid-1930s, a flight instrument that was destined to revolutionize the art of instrument flying and navigation began to appear in the cockpit: the artificial horizon, which utilized the gyroscope. Although the instrument had several limitations, including an infamous tendency to tumble, it was very effective for controlling an aircraft during instrument flight conditions.

Another technological breakthrough — the gyrocompass — also resulted from the invention of the gyro. Even thought the two main drawbacks of the gyrocompass — rigidity and precession — have concerned most pilots and navigators during their careers, navigation indeed took a leap forward with its invention. The invention of radar during World War II also marked a turning point.

The Wartime Navigation Systems

Two famous navigational systems were developed during World War II: “Gee,” the first hyperbolic system (so called because the constant time difference between synchronized signals received from two ground stations appeared as hyperbolas when plotted on a map), and “Oboe,” a radar system. They were not self-contained systems; rather, they used beams transmitted from ground stations. This limited their range to 200 miles for aircraft traveling at about 20,000 feet. Gee proved to be an extremely valuable navigation aid, but it was not very good as a blind-bombing device. Oboe, which British had developed for short-range precision bombing, turned out to be more promising.

Frustrated by the inaccuracy of the Gee, the lack of Oboe equipment, and the serious limitations of this new equipment, the Army Forces — in particular, Lt. Gen. Ira Eaker’s Eighth Air Force — turned its attention to another British radar invention. Known as the H2S, it was an airborne, self-contained radar that scanned the ground below and produced a map-like picture of the territory on a cathode ray (television-like) tube. He American version of this system was called the H2SC.

On July 24, 1943, a British operation, code-named Gomorrah, successfully attacked Hamburg using an offensive strategy built around Gee and H2S. Navigators used Gee to maintain their route of flight, and later “bomb-aimers” (bombardiers) used their H2S to paint the target with radar and to reveal it with detailed clarity on their screens. In November 1943, the first twelve American navigators trained to use this system flew their first combat mission with it. Even though not completely successful, this initial use of radar bombing proved most encouraging. The circular error of the bombings was reasonably small — only one to two miles, a massive improvement over the error commonly accepted at the beginning of the war.

Loran and Decca

Long-range navigation (Loran) and Decca were also developed by the Americans and the British, respectively, during Would War II. Basically, Loran is also a hyperbolic system of electronic navigation, except that it uses longer wavelengths than its predecessor, Gee. Loran’s radio waves, unlike celestial lines of position, are stationary with respect to the earth’s surface. The radio pulses sent fro the widely separated stations are synchronized.

Loran offered many pluses to navigation: It was quicker; it required less skill than celestial; it worked in all weather, except during very severe electrical disturbances; and it was more accurate over greater distances. (The original Loran-A had an accuracy of a fraction of a mile to several miles, depending on ionospheric conditions. Modern-day Loran-C and —D have far-improved accuracies. Loran-D has a relatively short-range capability and is designed for tactical uses, such as close air support and interdiction, reconnaissance, and airdrops.)

The British-designed Decca navigation system, similar in principle to Loran, used one “master” and two “slave” ground stations to furnish continuous position information. The equipment in the cockpit gave the navigator pen-and-ink tracings across a terrain chart.

Decca equipment would stay around for a long time. Two decades after World War II, as the United States was entering the Vietnam War, eighteen C-123s, as well as a small number of C-47s, B-26s, and helicopters, were equipped with Decca equipment — but by then the results were disappointing. Aircrews had trouble receiving the signal, and malfunctions were frequent; readings were sometimes grossly erroneous.

Nevertheless, during World War II, these innovations — Loran, Decca, and, most importantly, radar — led to the defeated of Germany. They improved navigation, target location, and weapons delivery. This equipment, the men who invented it, and the navigators who operated it earned well their places in history.

New Ways to Navigate

By 1946, the AN/APS-10 radar, developed by the MIT Radiation Laboratory and manufactured by General Electric, was the radar whose time had come. At Morrison Field, Fla., a key staging point for aircraft belonging to the Air Transport Command of World War II, eighteen student navigators and an instructor boarded a C-54 Skymaster. A radome under it fuselage housed the AN/APS-10 radar. This was a first-of-its-kind training mission for navigators belonging to the Air Transport Command. Radar was now the new way to navigate — to be used with sextants, radio compasses, driftmeters, and the old standby, dead reckoning.

Two years later, the newly formed United States Force entered into a contract with Convair to begin the military conversion of the Convair Liner 240, designated the T-29 to serve as a navigator trainer. The T-29 first flew in 1949, and each nav student had his own map table, Loranscope, altimeter, and radio compass. The aircraft’s roof had four astrodomes, while later versions added a periscope sextant and other innovations in navigation. In 1973, the piston-engined T-29s would be replaced with Boeing T-43 jets.

As the flying classroom evolved, so did the navigator’s ground school. Back during World War II, various types of navigational trainers were tried. None proved completely successful. One, the G-2 dead-reckoning trainer, was mounted on rollers, and the student navigated it across the floor of the training room.

By the 1970s, computer-driven flight simulators came on the scene, allowing students to view a wide range of missions accurately: low-level flights over sea and land, night flights, airway navigation, high-altitude operations, and navigation at high speed. The greatest boon, however, was the instructor’s ability to “freeze” the action, talk out mistakes, back up the simulator, and do it again. Nav students of the 1940s — as well as those of the Korean era — didn’t have the benefit of such quiet controlled situations. They earned their wings shouting over the din of the props.

Bombs in the Barrel

As did pilots in previous wars, F-51 Mustang pilots of the Korean War attempted night harassing missions but met with almost no success because often they could not identify their targets. Some of the B-26s flown during the Korean War had radar altimeters and short-range navigation radar (Shoran), the AN/APQ-13 blind-bombing radar. But in the short interval between World War II and Korea, much of the technique learned about supporting ground forces under all weather conditions had been forgotten. Initial efforts to use the Shoran, which had demonstrated its effectiveness in World War II over Italy, were disappointing.

In 1951, Shoran finally became a success story with the B-26 crews. Shoran bombers frequently served as lead ships for daylight formation attacks and later were dispatched with B-29s for night raids. The bombing runs conducted under this navigation system gave a circular error of only about 485 feet, so its reliability and accuracy were good. The B-29s bettered the peak records of World War II for both sorties flown and tonnage of bombs delivered per plane on targets. The Strategic Air Command groups proved very skillful at radar bombing, despite weather, and it was a tradition they carried on to Vietnam.

In Southeast Asia, aircrews — as might be expected — had to cope with navigating through bad weather. This time, the monsoon weather provided the perfect setting for proving the B-52s’ all-weather radar-bombing capability. None of the radar navigators on the 740 could say he had seen his target firsthand. All bomb releases were a result of radar and computer-derived data. Subsequent bomb-damage assessment photography proved unquestionably the accuracy of this bombing system.

There were, in fact, many technologies used during the Vietnam War to improve navigation, weapons delivery, and recovery. For instance, Loran-equipped F-4s led other aircraft not so equipped to targets at night or day or in bad weather. In fact, in 1971, 196 tactical aircraft hit petroleum storage areas north of the demilitarized zone in the first all-instrument strike employing the Loran position-fixing bomb system exclusively. Also, the Tactical Air Navigation system (TACAN is an ultrahigh frequency electronic navaid giving 360 degrees of continuous bearing and distance to the station) was widely used by transport and strike aircraft. Unfortunately, the accuracy was inadequate for blind airdrops. However, the C-130, which proved to be an enormous asset in Southeast Asia, proved what navigational radar could do. The C-130 airdrops there demonstrated the considerable proficiency of the navigators and their equipment.

The Vietnam War also saw the deft use of another prime navaid, so to speak. Whoever first thought to use airborne tactical controllers to direct fighters and bombers to their targets is not known. But in World War II, I n the Korean War, and again in Vietnam, the airborne controller repeatedly demonstrated his great value. Often using nothing more than an aeronautical chart and a radio, these controllers located small, high-value targets and talked combat pilots to successful attacks.

Navigation Today

It’s surprising how many navigational devices used today are nothing more than spin-offs from the scientific achievements of World War II and, to a certain extent, the Korean era. Even before World War II had ended, the Army Air Forces had begun work on guided missiles. The unique feature of a missile is its guidance system. In order to navigate ballistic missiles to fixed targets, the Inertial Navigational System (INS) was developed and refined. INS uses an intricate arrangement of gyroscopes, built-in computers, and other complex equipment to guide the missile. Enemy electronic devices could not jam such a system since it did not depend on radar emissions. As early as 1953, researchers from MIT’s Instrumentation Laboratory had successfully tested and INS on a twelve-hour flight from Hanscom AFB, Mass., to Los Angeles, Calif. The INS weighted “2,800 pounds” and had a ten-mile error. In another test four years later, the prototype INS weighted 1,400 pounds and had an error of only half a mile!

Another type of system developed in conjunction with an INS was the automatic star tracker (called astro-tracker today). This used the stars to figure the position of the missile — much like the early aviators used celestial navigation. The technology that gave the Air Force potentially its most powerful weapon also gave aviation very accurate, reliable, and worldwide navigational tools.

The extreme accuracy and reliability of INS and Doppler navigation systems (a computer-driven system that provides latitude, longitude, ground speed, and drift) have changed the work load and have caused the role of the navigator to change in some ways. Pan Am, the first to school military navigators formally, was the last American carriers to use navigators. Today, modern commercial aircraft usually use triple INS mixing as their primary means of navigation. While highly accurate, INS is not infallible, probably more because of human factors than mechanical (INS is only as good as the information programmed into it). The ill-fated Korean Air Lines Flight 007 that wandered over Soviet territory last year was depending on its probably misporgrammed triple INS for precise navigation over the Pacific.

Navigation today is paramount to Air Force battle management. It involves more than accurate en route navigation and target location. It also involves rendezvous for such critical tasks as air-to-air refueling, it involves very accurate weapons delivery, and it involves recovery and redeployment. MAC, TAC, and SAC all depend on accurate navigation to accomplish their missions.

And despite the great advances that have been made in navigational equipment, there are some missions that require professionals who proudly wear navigator wings. B-52, KC-135, EC-135, FB-111, C-130, F-4, F-111, EF-111, EC-130, E-3, and E-4 aircraft all have such crewmembers. C-141s carry navigators on SOLL (Special Operations, Low Level) missions, and the new F-15E will carry a navigator. Obviously, the role of the navigator is evolving, and his responsibilities are expanding. Getting from point “A” to point “B” is now the easy part.

On the Deck with LANTIRN

The question is, then, how is navigation going to affect the Air Force of tomorrow

The Air Force is developing new navigation systems, and two innovations are especially exciting: the Low-Altitude Navigation and Targeting Infrared for Night (LANTIRN) pods and the receiver and data links for the Navstar Global Positioning System (GPS). Also being developed are new generations of radar altimeters, radar-warning receivers, and, perhaps not so far into the future, navigation systems employing laser gyroscopes.

LANTIRN is important because it will permit operations at night and in marginal weather, “down on the deck” where pilots can take advantage of terrain-masking to enter and leave the target area. LANTIRN’s navigational system includes a forward-looking infrared sensor, a terrain-avoidance radar, and a wide-angle, head-up display (HUD) enabling the fighter pilot to keep his eyes out of the cockpit, just as if he were flying in daylight. LANTIRN will, in fact, turn night into day by displaying clear images of the terrain in front of the displaying clear images of the terrain in front of the aircraft. By opening the tactical night window, LANTIRN will, unlike during World War II, give the combat commander the flexibility to use the same weapons and tactics as he would use in the daytime. The system will enable the pilot to deliver IR Mavericks, laser-guided bombs, or conventional weapons using proven standoff tactics.

LANTIRN is a two-pod system, with the navigational capability in one pod and the targeting capability in the second. When integrated with other aircraft avionics, LANTIRN will be a complete and effective strike system.

Throughout the decades, navigation affected the deployment of forces, location of targets, weapons delivery, and recovery and redeployment. Today’s navigational equipment is good, but it’s not perfect. Inertial navigation sets lose their accuracy over time, take time to warm up, and are very costly. Ground-based navigation aids, such as TACAN, Loran, and Omega (a very-low-frequency navigation system that improved on Loran, automatically giving latitude and longitude), are good but have limited accuracy. These navaids are also susceptible to enemy jamming and to meaconing (receiving and instantly rebroadcasting radio-beacon signals from false positions), and they can lose accuracy under adverse weather conditions. Radar is detectable by an enemy. Thus, if it is used in low-level navigation and target location, the mission may be endangered. In weapons delivery, tactics are often limited by the navigation equipment. For example, if pilots have to pop up sooner than they should simply to make sure they have acquired the target, the exposure could obviously be disastrous. Altogether, these problems limit the Air Force’s flexibility and effectiveness.

The Navstar Solution

Navstar GPS is a solution to all these serious combat concerns. It will be an integrated system, providing a significantly higher probability for mission success. A passive, three-dimensional all-weather global system, it will establish total time synchronization and a common grid for dependable navigation and weapons delivery. It will markedly improve the effectiveness of tactical and strategic bombing, land and sea rescue, and air refueling operations — where rendezvous and timing are critical. GPS is not just a dream; it is a reality. The system has already passed its field tests, and eight R&D Navstar satellites are now in orbit. Eventually, the GPS constellation will have eighteen operational satellites.

When completely operations, the satellites, which circle the globe every twelve hours, will beam continuous navigation signals to earth. With the proper equipment, a user can determine his position, his speed within a fraction of a mph, and the time of day to a billionth of a second. The Global Positioning System has the added advantage of being a passive system. There are no radio emissions to give away a user’s location and thus compromise a mission.

Unlike present en route navigation, which is limited by ground navaids and onboard navigation systems, GPS-equipped aircraft can fly any time of the day or night in any weather without the line-of-sight limitations of current ground-based system. GPS will let tactical aircraft take full advantage of terrain-masking in total radar silence. The accuracy of GPS velocity data results in weapons release with greater accuracy than any other current system. GPS could eventually be incorporated into a weapon’s guidance system, giving a true launch-and-leave capability with precise accuracy.

Navstar GPS equipment is being developed to perform many functions at a lower cost that are now collectively accomplished by a number of systems. In reality, however, the Global Positioning System is much more than just a new, accurate, low-cost navigation system. It is an added resource significantly enhancing the total military capability of this nation. It means greater survivability and higher probability of mission success.

If an Air Force Crew member were to have the chance to look into the cockpit of Claude Grahame-White’s 1910 biplane, he would probably be astonished by the sparseness and crudeness of it. Future aviators might react in the same way to cockpits we have today, since tomorrow’s aircraft will probably have data links, collision-avoidance systems, wind shear detectors, microwave landing systems, LANTIRN, Navstar GPS, and highly integrated, computer-driven displays that enlarge aircrew capabilities. The revolution in computers, semiconductors, and software is rapidly changing the nature of navigation, and, indeed, the days are gone when pilots swooped from the skies to read road signs.

Tomorrow’s flyers might wonder what all the navigation fuss was about. But we’ll probably never reach the day when you can’t hear the famous line echoing through the cockpit: “Nav, where are we?”

Lt. Col. Ralph R. Williams, USAF, is the Director of Public Affairs for AFSC’s Electronic Systems Division, Hanscom AFB, Mass. He has authored several books, including The Langue of Aviation.