[Elphel-support] 答复: 答复: sync stereo with lidar

[Tao Zhang]

Hi Andrey,
We have Garmin GPS 18x 5Hz GPS receiver bought from you. How should we connect GPS to stereo camera? Shall we have a 103696 board? But I can not find how to connect it to 10393 camera on 103696 introduction page (-https://wiki.elphel.com/wiki/103696).

Tao Zhang
章   涛

发件人: Andrey Filippov <andrey at elphel.com>
发送时间: 2018年2月1日 13:49
收件人: Tao Zhang <tao.zhang at blacksesame.com.cn>
抄送: Elphel Support <support-list at support.elphel.com>; Elphel Sales | Olga Filippova <sales at elphel.com>; Winston Zhang <winston.zhang at blacksesame.com.cn>; Qun Gu <qun.gu at blacksesame.com>; Pioneer Li <pioneer.li at blacksesame.com.cn>
主题: Re: 答复: [Elphel-support] sync stereo with lidar

Tao,

USB GPS does not have 1pps signal (only serial has), precise synchronization is not possible that way. GPS time is available in each frame Exif header data - recorded on internal SSD or sent over the network.

The high resolution time stamps (1 microsecond step) are also provided with each  image, but they use internal camera clock, not the GPS.

This is why for precise synchronization you need to use event logger- it records events and timestamps them using the same clock.

If you had serial GPS with https://wiki.elphel.com/wiki/103696 - you would still need to use event logger - with serial GPS there will be 1 PPS precise GPS events timestamped with camera internal clock, and the images timestamped wit the same clock - that provides absolute calibration for the camera internal clock.

Without serial GPS you may use pulses from the LIDAR (it is possible to provide LIDAR pulse number over the Ethernet in addition to the precise time over the sync cable)

Andrey

---- On Wed, 31 Jan 2018 18:12:00 -0800 Tao Zhang<tao.zhang at blacksesame.com.cn<mailto:tao.zhang at blacksesame.com.cn>> wrote ----
Hi Andrey,
We prefer to use GPS to synchronize stereo with lidar.
Where do those images that contain timestamp store in internal disk? Can we get it from ethernet continuously?

Tao Zhang
章   涛

发件人: Andrey Filippov [mailto:andrey at elphel.com<mailto:andrey at elphel.com>]
发送时间: 2018年2月1日 1:52
收件人: Tao Zhang <tao.zhang at blacksesame.com.cn<mailto:tao.zhang at blacksesame.com.cn>>
抄送: Elphel Support <support-list at support.elphel.com<mailto:support-list at support.elphel.com>>; Elphel Sales | Olga Filippova <sales at elphel.com<mailto:sales at elphel.com>>; Winston Zhang <winston.zhang at blacksesame.com.cn<mailto:winston.zhang at blacksesame.com.cn>>; Qun Gu <qun.gu at blacksesame.com<mailto:qun.gu at blacksesame.com>>; Pioneer Li <pioneer.li at blacksesame.com.cn<mailto:pioneer.li at blacksesame.com.cn>>
主题: Re: [Elphel-support] sync stereo with lidar

Hi Tao,

There are several ways how to do that:
One method would be be to get camera sync pulses for each frame and use external electronics to match with the LIDAR.
Another is to provide pulses from the LIDAR to the camera and record them with in-camera FPGA logger (https://wiki.elphel.com/wiki/Event_logger), together with the camera frames. In this case the camera will record 2 types of events:
 -  image frames with the frame number, frame timestamp that will be recorded in each frame header as Exif.
 - external pulse
Each of the two types of events will have a logger timestamp (with microsecond resolution). Logger can also record GPS timestamps (for serial GPS only and 103696 board -https://wiki.elphel.com/wiki/103696) and IMU (https://wiki.elphel.com/wiki/103695), in your case there are just 2 types listed above.

Both output (+5V pulses) and input (3.3V..5.0V) are available through the 2.5mm audio connector (marked "sync" https://wiki.elphel.com/images/0/0f/NC393-CS_marked_ports.jpeg ), here is the pinout and description: https://wiki.elphel.com/wiki/103891 .

Andrey

---- On Wed, 31 Jan 2018 04:19:04 -0800 Tao Zhang<tao.zhang at blacksesame.com.cn<mailto:tao.zhang at blacksesame.com.cn>> wrote ----
Hi Andrey,
We are going to use stereo with lidar. We need to sync stereo frames with lidar point cloud. Do you have any suggestions on how to do that?
Thanks.

Tao Zhang
章   涛

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