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Many commercial Lidar systems have GPS, PPS, and serial interfaces. What's the application and how's it being used?

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    \$\begingroup\$ I would expect it provides a very stable time base which is multiplied to pulse rate and rotation rate for accurate 3D echo resolution. \$\endgroup\$ – Sunnyskyguy EE75 Jul 6 '18 at 2:04
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Generally speaking, PPS input from a GPS has exceptional accuracy and very low jitter characteristics. So a microcontroller, for example, can use a timer peripheral to measure it's perceived duration of the time between pulses, and use that to calibrate it's measure of time.

For example if the pulse-to-pulse timing of a GPS PPS signal is measured by the microcontroller to be 999 milliseconds, then it can assume it's perception of time is (1000/999) times what it measures and adjust accordingly. By re-calibrating itself occasionally, this can account for drift / variation in cumulative timing errors attributable to, say, temperature, humidity, manufacturing tolerance, and so forth.

Light travels pretty fast, so small errors in timing reflections add up to big errors in distance calculations.

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  • \$\begingroup\$ MCU jitter depends on many things, including which part of the program space is executing, and which output pins are active. \$\endgroup\$ – analogsystemsrf Jul 6 '18 at 3:19
  • \$\begingroup\$ thanks. Any thought on why Lidar needs to receive GPS data via serial? \$\endgroup\$ – jsmith0910 Jul 6 '18 at 17:35
  • \$\begingroup\$ The theory here is mistaken. A GPS can't measure program jitter, nor would software be in the critical timing path of a LIDAR anyway. And as for clock crystal error, it doesn't actually matter if you are using light or sound; percentage (or PPM) error is still percentage error. \$\endgroup\$ – Chris Stratton May 6 at 18:03
  • \$\begingroup\$ @ChrisStratton I don't think anyone suggested that. \$\endgroup\$ – hobbs May 6 at 20:31
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Read the interface box manual to get a fuller description of the use of PPS and NMEA $GPRMC. Be aware you need both the PPS and NMEA sentence to work.

Once setup it allow you to understand the polar co-ordinates/time of Laser pulses.

The Velodyne Interface box manual is here.

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  • \$\begingroup\$ Thanks. But why does Lidar need to take GPS input and generate Ethernet packets? I suppose Lidar doesn't need GPS info to work. It seems that it's trying to synchronize the GPS info with the Lidar for MCU to process. Just wonder why it's designed this way. \$\endgroup\$ – jsmith0910 Jul 6 '18 at 17:33
  • \$\begingroup\$ A flood of Ethernet data without any timing and rotation information is harder to process. \$\endgroup\$ – Jack Creasey Jul 6 '18 at 19:13
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GPS makes LiDar more accurate through time synchronization of its clock with that of LiDar. GPS is a stratum0 clock which means it is pretty accurate, so the LiDar can be synced with it.

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  • \$\begingroup\$ You are mixing up one concept for a completely different one. LIDAR operates over extremely short periods of time, and only the time difference between the outgoing and return matters, it doesn't matter when that is done, so "syncing" to a GPS is meaningless for the accuracy of the actual measurement. \$\endgroup\$ – Chris Stratton May 6 at 18:06

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