Inertial Navigation

Gyroscopes: Great Travelers

Historically, guiding aerial navigation relied partly on science and partly on art: dead reckoning. Solid pre-flight calculations and flying strictly by compass heading, then applying en-route corrections using the ocular system v1.0, were essential. This system still works with very satisfactory efficiency. Things became complicated when weather deteriorated and there was no option but to continue. Just think of the great flights of the era, when the goal was to prove the reliability of airmail service, passenger transport, or to complete military missions.

Great Flights with a White Cane

In the early 1920s, Americans established the Transcontinental Airway System. This “system” consisted of a massive series of large yellow arrows cast in concrete on the ground, along with rotating beacons marking turning points along the transcontinental route.

In the 1930s, public funds were invested to establish a radio-navigation system. However, the complexity of long-range navigation was not truly solved. Pilots still had to cross oceans or slip through enemy territory with precision, relying entirely on dead reckoning. This necessity demanded constant in-flight work from navigators and pilots equipped with a wide array of instruments: chronometers, compasses, sextants, and astro-compasses. A jumble of scales, slide rules, almanacs, pencils (with erasers and sharpeners), protractors, and dividers covered the large charts at the navigator’s station. The profession was at its peak.

“First Steps”

Navigation had to function without radio aids. Inevitably, the avant-garde would come from military research. The Luftwaffe’s V1 was assembled with one of the earliest autonomous systems. Primitive as it was, its operation was nevertheless ingenious. The launch base was oriented toward the desired target based on wind conditions. The nose of the V1 was fitted with a propeller connected to a worm screw, itself linked to a mechanical counter. Each propeller rotation corresponded to a distance traveled. The total distance was known in advance. Once the required number of rotations was reached, the mechanical counter commanded the elevator to pitch down. Control—an integral part of space navigation—was provided by stabilizing gyroscopes. System accuracy was on the order of 12 km over 250 km. Many may smile at these figures, but from that moment on, precision would only continue to improve.

Soon thereafter, inertial navigation systems appeared: INS (Inertial Navigation Systems). These consisted of a mobile platform equipped with three gyroscopes and three accelerometers aligned along the three axes. Motion was calculated by electromechanical “computers,” and information was displayed on mechanical indicators.

By the late 1960s, the accuracy achieved was remarkable. Commercial systems were being produced that drifted only 2 nautical miles (3,704 m) per hour of flight. To visualize gyroscope precision, just consider that your good old directional gyro—“DG” to its friends—detects, through spatial inertia, the rotation of the Earth itself. This is partly why it must be realigned by about 3.75 degrees every 15 minutes, whether on the ground or in flight.

The constraints limiting INS accuracy were:

• Coriolis acceleration (Earth’s rotation)
• Effects of vertical motion
• The Earth being an ellipsoid rather than a perfect sphere, adding computational complexity
• Gimbal lock (when gyro supports reach their mechanical limits), a serious issue in air combat maneuvering or orbital maneuvers

Navigating the World

These systems were marvels of mechanical packaging, yet bulky and heavy (34 kg) in a field where weight savings are critical. Combined space and military research enabled engineers to develop strapdown INS without a moving platform. Inertial platforms with fixed components became much lighter, using gyroscopes without moving gimbals. The internal computer—now digital—became fundamental to INS operation. Apollo missions used such systems, equipped with integrated-circuit computers (the very first!). For quite some time, these INS units were operated via advanced LED interfaces with a memory bank of nine waypoints. Luckier aviators could operate INS units coupled to the autopilot.

Virtually free of moving parts, the first laser gyroscope systems (RLG) appeared in 1982. An IRS (Inertial Reference System) built with RLGs was lighter (7.7 kg), more compact (13 × 20 × 38 cm), and required only 50 watts from the generators. Accuracy was on the order of 0.1 nm/hr (0.002 deg/hr), as guaranteed by the manufacturer. In practice, drift was nearly zero. IRS alignment required only 15 minutes instead of the 40 minutes needed for early INS systems. Another advantage of IRS was that traditional navigation instruments (artificial horizon, DG, and turn indicator) became obsolete, as their gyroscopic functions were now provided by the RLGs. Additionally, a new navigation dimension was introduced: geometric flight paths computed as a function of wind.

RLG operation is based on two beams projected simultaneously in opposite directions inside a prism that oscillates very slightly. When the aircraft rotates about the prism axis, a delay is created in the arrival of one of the beams. No more spatial inertia or precession is involved. Despite its excellent accuracy, the main drawback is production cost, making the technology accessible only to high-end users: airlines, military, and executive aviation.

While A320 and B737 crews navigated effortlessly using IRS, researchers were already designing the next generation of gyroscopes. Several concepts emerged.

The Coriolis vibratory gyroscope relies on a vibrating structure that tends to remain in the same plane. When the oscillation plane is displaced, a sensor transfers the information to a computer. With no true moving parts, it was less accurate than the best INS gyros but ideal for replacing traditional artificial horizons. Engineers even experimented with a brandy-balloon gyro!

Miniaturization and Added Precision

The true breakthrough of the decade is undoubtedly the quartz rate sensor. A masterpiece of miniaturization, it is currently the state of the art for non-military applications. Highly accurate and inexpensive to manufacture, the system is integrated onto a silicon chip. Two tuning forks are connected by a stem. Aluminum electrodes are vapor-deposited on both sides of the forks, which are then mounted on the chip. These electrodes both excite and sense motion. The forks vibrate and tend to remain fixed in their plane. This motion is counteracted by an electrostatic force from the electrodes, creating a measurable capacitance difference used to determine angular motion. The MEMS (Micro-Electro-Mechanical System) weighs only 50 g and measures a delicate 25 mm in diameter by 26 mm in height. Power consumption is about 4 W. These sensors are already found behind emergency instruments on modern transport aircraft and in AHRS systems installed in general aviation. AHRS units automatically correct for gyroscopic drift errors.

We have now entered the era of reliable, compact, and affordable inertial measurement units. Integrated attitude information is a reality. Even gliders are being equipped with platforms linked to handheld computers offering EFIS interfaces. Synthetic vision systems will soon be widely available.

GPS: Yes, but…

The arrival of GPS technology has certainly revolutionized ultra-precise navigation. Can one imagine a better system, guaranteeing accuracy within less than the span of the tailplane after an 11,300 km flight? As celestial as it may be, GPS remains a form of radio navigation subject to criminal jamming or spoofing. Moreover, it is practically incapable of determining an aircraft’s spatial attitude. In reality, GPS is not the successor to IRS but a necessary complement to modern integrated navigation systems. It is essential for operations in increasingly crowded airspace. However, the global geopolitical situation suggests that we are far from finished installing gyroscopes aboard civil aircraft.

Acknowledgments to Messrs. Paul Loughran, Roger Dykmann, and Tom Ryno of Honeywell.