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ACOUSTIC POSITION AND VELOCITY MEASUREMENT

A Polaroid acoustic transducer, mounted on the helicopter, continuously broadcasts a tex2html_wrap_inline293 kHz. tone downward onto a cone-shaped reflector that spreads the signal in the horizontal plane. Three Polaroid microphones at each end of the field receive the signal and electronics amplify it so that the resulting saturated waveform is a square-wave. Each sensor counts the number of pulses that it receives. The ground computer reads the pulse counts through an interface board over a serial line at tex2html_wrap_inline257 Hz.

The tex2html_wrap_inline297 wave is emitted from the helicopter at time tex2html_wrap_inline299. The tex2html_wrap_inline297 wave crest is received at sensor 1 at time tex2html_wrap_inline305 where tex2html_wrap_inline307 is the distance from the helicopter to sensor 1 when the wave is emitted, and a is the speed of sound.
displaymath313
If receiver 1 starts listening at time tex2html_wrap_inline317, the number of pulses tex2html_wrap_inline319 counted at time t equals the number of pulses emitted since time tex2html_wrap_inline323 minus the number of pulses between the vehicle and the sensor. Therefore, each sensor measures the distance to the helicopter at some previous time, depending on the relative locations of the vehicle and sensor.

The sensor measurements drive a Kalman filter which estimates the vehicle's 2-D position and velocity. The state of the system is propagated to the time of the sensor measurement and the measurement is corrected for the vehicle's velocity. For small velocities, a correction for the velocity was not considered necessary. Therefore, (1) reduces to:
displaymath325
Each sensor may be initialized by knowing the initial distance to the vehicle and solving (2) for the value of tex2html_wrap_inline323 for that sensor. By counting pulses, at any future time, the current distance may be determined by solving (2) using this value of tex2html_wrap_inline317. To protect against sensor dropout, measurements are checked for reasonability before being incorporated into the state estimate. If a sensor drops out, the position estimate from the other sensors is sufficient to re-initialize the failed sensor.


next up previous
Next: GUIDANCE Up: NAVIGATION Previous: LASER POSITION MEASUREMENT

Bill Hall
Fri Jan 31 14:15:17 EST 1997