Question about velocity of ADC when stopped

[tuanngokien]

System information

• OS Platform and Distribution: Ubuntu 18.04
• Apollo installed from (source or binary): source
• Apollo version (3.5, 5.0, 5.5, 6.0): 7.0 (master branch - commit #14556)

Context:

When analyzing CyberRT record of a specific scenario, sometimes even when seeing ADC completely stopped in stop line (with STOP fence) does not mean that the corresponding linear_velocity extracted from messages in channel /apollo/localization/pose would be 0.0 (the linear_velocity values are decreased really small but never be absolute 0.0).

Meanwhile, Apollo source code has several code snippets to check if ADC is completely stopped. Actually, they do not compare the linear_velocity with 0.0, instead comparing it with a configured threshold max_adc_stop_speed (as 0.2 for the car model Lincoln MKZ).

https://github.com/ApolloAuto/apollo/blob/b09259f8be3ae85b3b3471d45d301175d46c2a8f/modules/planning/scenarios/stop_sign/unprotected/stage_pre_stop.cc#L233-L240

Questions

1. Why is it sometimes ADC's linear_velocity cannot reach absolute 0.0 at a stop sign, as it is supposed to be?
2. What is max_adc_stop_speed and how is it considered for checking if ADC is stopped or not?
3. Is that somehow related to the rolling stop mechanism? Does Apollo decide in each specific scenario to have a complete stop or a rolling stop?

Supporting materials (screenshots, command lines, code/script snippets):

Demo CyberRT Record: https://drive.google.com/file/d/1noE5XO8GPisnOMoaYH_820OGLckKwajC/view