A laser that works similar to your Lab 7 Ping sensor is used to measure the dist
ID: 2081285 • Letter: A
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, TIMER1's Input Capture pin is set to a 1. When the sensor detects the 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 Compute the maximum error in distance for each the following speeds of the system clock used by TIMER1: 16MHz, 8MHz, 1MHz, 1KHzExplanation / Answer
The causes of error in the measured distance is due to the error in the measuring tme when calculatng
distance, since the equaton of distance is the multplicaton of velocity and measured tme. The error of
measured tme is caused by the input capture measures tme in Timer tcks between events, if the event
does not happen precisely at positve edge. Take an example of laser fred afer the positve edge, the
tme taken will be less by nearly 1 clock cycles. On the other hand, If the laser fred precisely at positve
edge, the maximum error in tme would be 1 clock cycle.
Velocity of light, c = 3e8
Max error = 1 period
T = 1/16MHz, Distance = v*T = 3e8/16M = 18.75m
T = 1/8MHz, Distance = v*T = 3e8/8M = 37.5m
T = 1/1MHz, Distance = v*T = 3e8/1M = 300m
T = 1/1kHz, Distance = v*T = 3e8/1k = 300,000m
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