Global Positioning System Reference
In-Depth Information
ers, and a thousand other devices. MEMS are inexpensive, small, and
robust; and they consume little power.
5
You can see that three mutually perpendicular gyrocompasses and three
mutually perpendicular accelerometers can monitor all six torques and
linear forces that act upon a moving object like an airplane, a ship, or an
automobile (see fig. 8.1). Such a collection of gyros and accelerometers
form the basis for an inertial navigation system. These systems have been a
mainstay of modern navigation for a few decades, and now it is time for us
to investigate them.
Inertial Navigation
Just as technology and an increased understanding of physics led to im-
proved compasses, so they also led to an improved version of the old dead
reckoning navigation technique. Inertial navigation (IN) is an automated
version of dead reckoning, with more accurate and much more frequent
updates to estimated platform speed and direction. Here is how it works.
In addition to accelerometer and gyro sensors, the third component of
an IN system is a computer. The computer frequently monitors the sensors'
output (many times per second in the case of an airplane IN system) and
uses these data to continuously calculate the position, speed, and heading
of an airplane (or missile or ship). Inertial navigation works without refer-
ence to the outside world; all the data that are gathered refer to the acceler-
ations and rotations of the platform itself. This feature is both an advantage
and a disadvantage. In the context of military platforms—be they nuclear
submarines, fighter planes, or guided missiles—the fact that an IN system
works independently of external data means that it cannot be electroni-
cally jammed. On the other hand, it means that the IN system can drift: as
with our dead reckoning example earlier, IN errors accumulate. Therefore,
eventually, it is necessary to update and calibrate the IN track with some
sort of external data. GPS is commonly used for this purpose these days.
Modern IN systems can maintain a platform bearing that is within 1\ of the
desired course for one hour in the absence of external position updates.
This is plenty of time for landing an aircraft in conditions of poor visibility,
or for the system to serve temporarily as an automatic pilot.
In general, IN systems are too expensive for most civilian applications.
5. To learn more about MEMS, see Acar and Shkel (2008, chap. 5).
