Inertial Navigation

Accelerometers with an incredible sense of direction

February 1, 1997

Although not as accurate as GPS, an inertial navigation system has the unique advantage of being the only totally self-contained method of long-range navigation. INS does not receive or transmit radio signals and is oblivious to the earth's magnetic field; it does not require sighting celestial bodies or terrestrial landmarks. It does, however, require a hefty investment and a steady 400-watt diet of AC and DC electrical power.

Because INS is impervious to exterior influence, it found early military acceptance. An enemy cannot jam, intercept, or detect signals to or from an inertially guided device, because there are none.

During World War II, Germany became the first country to use inertial navigation. When a V-2 missile had traveled a preset distance (as measured by the crude inertial computer), a cutout signal was sent to the rocket engine. Without power, the missile then fell toward its prey. The system admittedly was primitive. Had it been more accurate, its effects on the British would have been considerably more devastating.

In theory, inertial navigation is quite simple; in practice, it is simpler yet. Pilots use it routinely to fly the world's oceans without needing a computer, a plotter, a pencil, or even a chart. A reasonably bright person could just as competently use INS to go anywhere in the world. All he needs to know is where the flight is to begin and where he wants to go.

Inertial navigation operates on the principle of inertia, a property possessed by every object in the universe. Sir Isaac Newton probably best defined inertia with his first law of motion: "An object at rest will remain at rest, and an object in motion will continue in motion with a constant velocity, unless it experiences a net external force."

Because every human being has mass — some more than others — and a brain, each of us is a crude inertial navigator who can operate, in principle, the way INS does.

Assume, for example, that a man sitting blindfolded in the passenger seat of a stationary sports car is told his exact location and the direction in which the car is pointing. In effect, his brain has been "loaded" or "initialized" with the necessary "preflight" information.

The driver starts the engine and accelerates the sports car to 60 mph. The blindfolded man's body has mass (inertia), so the acceleration presses him against the back of the seat. Now assume that he can approximate the amount of this force and conclude that he has been accelerated to about 60 mph. In the meantime, his internal clock counts the minutes and seconds, during which time the driver maintains a constant speed. After 30 minutes, the driver slows to 30 mph, a deceleration detected by the passenger as his body pitches forward slightly. "Aha," the passenger thinks. "I have traveled 60 mph for 30 minutes and, therefore, have gone 30 miles. I am now traveling at only 30 mph."

The car stops half an hour later. The passenger concludes that while traveling at 30 mph for 30 minutes, he has traveled an additional 15 miles, or a total of 45 miles. By carefully measuring acceleration and deceleration, the passenger has determined his speed at any given time and converted this to distance traveled (a mathematical process known as integration).

Had the driver changed direction at any time, the passenger would have detected the centrifugal force resulting from the turn and could have computed the amount of turn and the new direction of travel.

The human brain, of course, cannot accurately measure acceleration, keep track of time, and account for the myriad velocity changes that occur during even a short ride. But the computer of an inertial navigation system can.

The heart of INS consists of three extremely sensitive accelerometers that measure the rate of change of speed (acceleration) in three planes or axes: longitudinal (fore and aft), lateral (right and left), and vertical. Any aircraft motion acts in one or more of these axes.

The accelerometers are so sensitive that they can measure any acceleration between 0.0008 and 10 Gs. This means that INS can measure acceleration, whether it takes an object one hour to increase speed from 2 to 4 knots or — at the other extreme — a missile rockets from a standstill to 600 knots in a few seconds.

All aircraft speed and attitude changes, no matter how slight, are detected by the accelerometers. The INS computer continuously converts this information into track and distance data. The computer knows at all times where it has been, where it is, where it is going, and at what speed.

Because the accelerometers measure speed changes with respect to the Earth, they must be kept aligned with the Earth's surface. This is accomplished by placing them on a small platform gyroscopically held parallel to the Earth's surface in the same manner as an artificial horizon. The result is called an inertial platform (or a stable table).

These gyros keep the platform so perfectly aligned with the Earth that they can also serve as reference sensors for the pilot's attitude and heading indicators and as stabilizers for a radar antenna. Compensation circuits within the INS prevent gyroscopic precession from affecting either the inertial platform or the attitude display.

Operating an INS is only slightly more complicated than using a push-button telephone and more intuitive than many GPS receivers. Before departure, the pilot enters the exact latitude and longitude coordinates defining the aircraft's position and then the coordinates of the first nine waypoints along his route of flight. That's all there is to it.

Because INS senses the Earth's rotation, during warm-up it determines the direction of true north to the nearest tenth of a degree. This information can be fed to directional gyros so that a pilot can navigate with respect to true north (if and when he chooses to do so). This is particularly useful when flying within a thousand miles or so of the north magnetic pole, where conventional compasses are useless. No longer does a pilot have to consider magnetic variation and deviation. Gone will be those memory joggers about true virgins and ducks that make vertical turns.

At flight's end, the INS computer may disagree with the actual aircraft position by a few miles. It makes you wonder whether the airport is really where everyone thinks it is. When the actual position of the aircraft is inserted into the computer prior to the next flight, the INS will digest the error, cogitate the new data, and then take action to remove a portion of the error during the subsequent flight. This has the effect of increasing INS accuracy with usage.

The pilot also can update the INS en route by inserting into the computer actual aircraft position data obtained from other navigational fixes.

Considering that INS receives no en route updating from ground- or satellite-based stations (unless supplied by the pilot), it is very accurate. Tests conducted during 3,000 flights and 14,000 hours of flying reveal that 50 percent of the time, INS errs less than 0.41 nm per hour of flight. Ninety-five percent of the time, the error is less than 1.4 nm per hour. This means that at the end of a 10-hour polar flight, for example, an inertially guided aircraft has a 95-percent chance of being within 14 nm of its destination. Now INS is being integrated with GPS so that the INS solution can be continually updated with highly accurate GPS satellite inputs.

First- and second-generation inertial navigation systems utilize accelerometers and spinning-mass gyroscopes that represent 1960s technology. These are being supplanted in air carrier, military, and business jets by inertial reference systems (IRSs), which use smaller, simpler, and more reliable ring laser gyros (RLGs). RLGs actually are not gyros at all, in that they do not utilize a rotating mass and have no moving parts whatsoever.

Instead, they measure the time required for two beams of light to travel in opposite directions around a triangular path. Any component of aircraft motion, acceleration, or deceleration that acts along the plane in which the light beams travel makes one beam complete the circuit slightly faster, the other slower. From the difference, the IRS computer derives rates, speeds, and headings. An RLG (or two, for redundancy) is oriented to each of the three flight axes along which it senses aircraft motion. An IRS based on RLGs — or the even newer fiber optic gyro technology (using the same principles) — does not require a separate stabilized platform to isolate it from the aircraft structure and is referred to as a strapdown inertial system.

Although IRS and INS systems are identical in principle, IRS is designed to interface with flight management systems (FMSs) when other navigational sensors may not be available. An INS can be thought of as an IRS with its own built-in and extremely simplified FMS.

Considering the accuracy, reliability, and increasing popularity of GPS, it would be logical to conclude that INS's days are numbered. But according to Airbus Industries, Boeing, and McDonnell Douglas, this is not the case. None of these manufacturers plans to eliminate INS or IRS from future aircraft.

The weakest link in the INS chain is the flight crew, who might program the computer with the wrong coordinates. Evidence suggests that such an error led Korean Air Lines Flight 007, a Boeing 747, into Soviet airspace, where it was shot down on September 1, 1983, resulting in the loss of 269 lives.

Without the human factor, you could probably insert the coordinates of heaven and have every reason to believe that INS would guide you there (even though GPS will do it more accurately).