GPS World, June 2017
JUNE 2017 WWW GPSWORLD COM GPS WORLD 59 OVER THE YEARS many philosophers tried to describe the phenomenon of inertia but it was Newton in his Philosophiæ Naturalis Principia Mathematica who unified the states of rest and movement in his First Law of Motion One rendering of this law states Every body continues in its state of rest or of uniform motion in a straight line unless it is compelled to change that state by forces impressed upon it Newton didnt actually use the word inertia in describing the phenomenon but that is how we now refer to it In his other two laws of motion Newton describes how a force including that of gravity can accelerate a body And as we all know acceleration is the rate of change of velocity and velocity is the rate of change of position So if the acceleration vector of a body can be precisely measured then a double integration of it can provide an estimate of the bodys position That sounds quite straightforward but the devil is in the details Not only do we have to worry about the constants of integration or the initial conditions of velocity and position but also the direction of the acceleration vector and its orthogonal components Nevertheless the first attempts at mechanizing the equations of motion to produce what we call an inertial measurement unit or IMU were made before and during World War II to guide rockets Nowadays IMUs typically consist of three orthogonal accelerometers and three orthogonal rate gyroscopes to provide the position and orientation of the body to which it is attached And ever since the first units were developed scientists and engineers have worked to miniaturize them We now have micro electro mechanical systems or MEMS versions of them so small that they can be housed in small packages with dimensions of a few centimeters or embedded in other devices One problem with IMUs and with the less costly MEMS IMUs in particular is that they have biases that grow with time One way to limit these biases is to periodically use another technique such as GNSS to ameliorate their effects But what if GNSS is unavailable Well in this months column we take a look at an ingenious technique that makes use of how the human body works to develop an accurate pedestrian navigation system one whose accuracy has been checked using drone imagery As they might say in New York Hey Im walking with accuracy here INNOVATION INSIGHTS BY RICHARD B LANGLEY
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