Is it even possible to drive a car without some disembodied voice telling you to turn left, right and make a U-turn when possible? For a generation raised on sat navs and smartphones, this is a serious question—how did people actually work out where they were? The truth is, before GPS, if the average person wanted to know where they were, they had to resort to using maps, observations and wildly inaccurate assumptions. And, if the situation got really bad, they asked for directions.
However, for individuals and industries with lots of money, plenty of space and a genuine need to know where they were, their salvation frequently came in the form of an inertial navigation system INS. Inertial navigation systems come in all shapes and sizes. One thing they have in common though is their use of multiple inertial sensors, and some form of central processing unit to keep track of the measurements coming from those sensors.
The sensors an INS uses are typically gyros and accelerometers—and there are normally several of each inside. Switch on a GPS receiver and, assuming everything works correctly, after a short time it will generate a position measurement. Ignoring the inaccuracies GPS has, the position measurement the receiver generates is quite specific. In their case, the measurement they generate is relative to their last known position.
So why do people use inertial navigation systems at all? Thankfully the answer to these question is simple. An accelerometer measures proper acceleration. This is the acceleration it experiences relative to free-fall, and the acceleration felt by people and objects.
Put another way, at any point in space-time the equivalence principle guarantees the existence of a local inertial frame, and an accelerometer measures the acceleration relative to that frame. A Gyroscope is a physical sensor that detects and measures the angular motion of an object relative to an inertial reference frame.
It measures the absolute motion of an object without any external infrastructure or reference signal. A magnetometer is a measuring sensor used to measure the strength or direction of magnetic fields in order to provide bearing. The typical applications do not differ much from the AHRS market but cover much more application fields.
INS can be used for any type of market, from surveying to unmanned vehicles. INS are also often installed on general aviation aircraft. For the unmanned ground vehicles market, GPS-aided inertial navigation systems integrating dead reckoning would be the best fit, especially when the application needs high accuracy.
This also applies to autonomous vehicles such as driverless cars or shuttles, for example. Stabilised platforms have some disadvantages. Gyros are usually mounted in three gimbals on bearings; this allows the aircraft and gimbals to rotate around the gyros without moving the platform. However, when two of the three gimbals align, and are effectively operating around the same axis, they can become locked together and be directly affected by movement around the remaining third axis.
The solution is to complicate the system further by adding a fourth motorised gimbal, which is continuously driven to avoid alignment with the other three. Compensation for drift requires complex bearing assemblies and special lubricants. Maintenance of stabilised platform INS is complex, costly and time-consuming. Three gyroscopes sense the rate of roll, pitch, and yaw; and three accelerometers detect accelerations along each aircraft axis. With few moving parts strap-down systems are easier to maintain and more reliable over time.
They do require more accurate gyroscopes and greater computing power. However, the advantages of reduced cost, size, weight and reliability make these systems the preferred choice of INS for aircraft. INS accuracy will drift with time; this can occur for many different reasons. An INS is, generally, not the sole means of navigation on commercial airliners.
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