A laser that works similar to your Lab 7 Ping sensor is used to measure the dist

Question

A laser that works similar to your Lab 7 Ping sensor is used to measure the distance to the Moon from Earth. The laser is fired at a mirror placed on the Moon, and a sensor attached to the laser detects when the reflection arrives back to the laser. As shown below. This is similar to an actual method used for measuring the Earth-Moon distance, see: http://en.wikipedia.org/wiki/Lunar_Laser_Ranging_experiment When the laser is activated, TIME s Input Capture pin is set to a 1. When the sensor detects the R1 reflection, this pin is set to 0. A program has been written that uses Input Capture to compute the distance between the Earth and the Moon. Given that Input Capture measures time in Timer ticks (i.e. clock cycles how different can the programs calculation of the Earth-Moon distance be from the actual distance? a) Explain what causes the error in the measured distance

Explanation / Answer

Input Capture computes time from the number of clocks between events. If an event does not occur exactly on a positive clock edge, then there will be error in the measured time. For example, if the “Sensor Detect" event happens right after a positive clock edge, then this time will be too long by nearly 1 clk cycle. And if the "Laser activated" event happens right after a positive edge, then this time will be too short by nearly 1 clk cycle. Thus a worst case time measurement error will occur when Laser activation occurs exactly on a positive edge of the Timers clock, and “Sensor Detect" occurs right after a positive edge. This gives a maximum error in measuring time of 1 clk tick.

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