gps-details

COROS - Alignment Issues

Author: Michael George

Created: 6 Apr 2025

Overview

The following image shows part of a session from a VERTIX 2 with firmware 3.0808.0.

The solid blue line is the recorded speed, and the dotted blue line is the position-derived speed.

trix-surf

Two significant issues are apparent in the data:

  1. The speed data and positional data are significantly offset, and the difference is more than 40 seconds in this track.
  2. No speeds are being recorded whilst the position is not changing, so significant chunks of speed data is being discarded.

Issues

Alpha 500

The calculation of alpha 500 results on GP3S relies upon positional data and speed data. The alignment issue / temporal offset causes alpha 500 results on GP3S to be completely wrong.

Similar alignment / temporal offset issues have been present on the APEX 2 / APEX 2 Pro / VERTIX 2 for several years. The original document describing the issue referred to these bogus results as fantasy alphas.

Distance

The speed surfing world integrates the speed data to calculate the total distance travelled. Since lots of speed data is not being recorded by this firmware (due to the alignment issue), distances are coming up short on GP3S; 38 km instead of 50 km for this particular session.

Testing

Since this issue is intermittent it is very difficult to test and diagnose. It’s not happened to me personally, but the issue affects many others at random.

Remedy

It is conceivable that the underlying issue relates to the stitching together of NMEA sentences such as GGA and VTG, or some other combination. It is unique to APEX 2 / APEX 2 Pro / VERTIX 2 and has never been seen on Garmin watches using the same Airoha chipsets. I’m also conscious that the firmware issues during the latter half of 2024 were after an attempt to fix the original alignment issue, so due care is required.

Reference