Distance Channel Calculation Error

JMcDonough · Post by **JMcDonough** » Thu Apr 16, 2015 1:15 am

Comparing some of my RCP data with a friend's DL1 at the same autocross revealed very different distances between the two systems. The RCP was distance appears about 1.4x the DL1's data (this is after converting the RCP's miles to match the DL1's km convention). If I take the speed channel (in mph) and integrate it over the course of the run, I get the distance to match the DL1 data. Hence the post in the "Bug Reporting" sub forum

While I'm on the topic, I'd like to see the resolution in the distance channel improved. With the current resolution of 0.001 mile the distance channel increments in 5 ft steps, which for auto-x is kind of a bid deal. As a comparison, if I updated it a 50hz by integrating the speed, I get a more reasonable ~1-1.5ft resolution at auto-x speeds.

Can provide data file if needed. This was running firmware version 2.7.9 on a mk2.

Hope this helps!

Post by **brentp** » Tue Apr 28, 2015 10:21 pm

Hi, thanks for the report.

Currently we calculating distance from GPS point to GPS point, and had good results (so we thought). but we had some discussions about integrating with speed and time. Sounds like from your testing is showing otherwise. We'll get it on the research list.

Also will look at updating the resolution of the GPS distance channel.

JMcDonough · Post by **JMcDonough** » Thu Apr 30, 2015 12:47 pm

Thanks, Brent & co.

If I look closely at the GPS data, it is pretty noisy and occasionally will jump around mid run. That would explain the erroneous distance values. I've been running at 50Hz. Per the GPS How-To, http://www.autosportlabs.net/RaceCapturePro2_GPS, I'll drop it down to 10Hz and see if that helps with the jumping around. Would that have any impact on the distance calc or will it continue doing distance calculating at 50Hz?

Post by **brentp** » Thu Apr 30, 2015 2:53 pm

Josh,

Yes, we've found 50Hz GPS requires impeccable signal quality - not only clear sky above, but an unobstructed view to the horizon. As in a desert setting or similar. Such is the nature of GPS.

The 2.8.0 firmware has an change that sets the GPS module to internal update rate that matches the selected sample rate. Currently with 2.7.9 and below the sample rate internally runs at 50, but simply reports at the rate selected by the user. We made this change to help select the right rate for the terrain / conditions.

2.8.0 is scheduled to go out by this weekend - we're adding some last minute bug fixes and testing.