gps-details

Garmin - GPS Metadata

Author: Michael George

Created: 17 Jan 2024

Introduction

This is a very brief document to report some observations relating the “GPS Metadata” in Garmin FIT files.

The test data was recorded on a Fenix 7 Pro using APPro with multi-band enabled.

Observations

Speed

A quick comparison of the speed data in FIT records and GPS metadata.

• Speed in the FIT record is believed to be the raw Doppler-derived speed.
• Speed in the GPS metadata is believed to be a smoothed version of the Doppler-derived speed.
• Position-derived speed is calculated by GPS Speedreader using changes in latitude + longitude.
  • The positional accuracy wasn’t great, due to the watch being worn under winter clothing.

Altitude

A quick visual comparison of the altitude / elevation data in FIT records and GPS metadata.

• Altitude in the FIT record is derived from the barometer in the watch.
• Altitude in the GPS metadata is probably the altitude / elevation from the GNSS receiver.
  • The accuracy wasn’t great, probably due to the watch being worn under winter clothing.

Timestamps

It is worth noting that the timestamps in FIT records and GPS metadata differ. This appears to be the difference between the Garmin system time (timestamp in FIT records) vs GPS metadata (NMEA timestamp).

This is most obvious when examining a test file that has intentionally had the GNSS signals blocked using aluminium foil. The occasions when GNSS data is available (or not) make it very easy to determine the time offset.

It is important to bear the timestamp differences in mind in the following sections of this document.

Loss of Lock

It can be seen that GPS metadata is not saved when there is a loss of lock.

This is particular evident in a short test file when the GNSS signals were deliberately blocked using aluminium foil.

Fix Quality

In addition to complete loss of lock there is a more subtle factor, demonstrating how GPS metadata is only saved when the fix quality is 4 (QUALITY_GOOD).

Take the following example where there is some missing GPS metadata:

• 13:57:12 to 13:57:14
• 13:57:21
• 13:57:24 to 14:16:08

The corresponding FIT records have a fix quality of 3 (QUALITY_USABLE) or suggest a loss of lock on those occasions:

• 13:57:13 to 13:57:15
• 13:57:22
• 13:57:25 to 14:16:09

Note: The FIT records have a timestamp which is 1 second later than GPS metadata. This probably corresponds to the 1 second delay that is seen in the speed data from Garmin FIT watches, when compared against something like a Motion GPS.

Frozen Records

A short test walk was seen to have one missing entry in GPS metadata at 17:09:17.

This occurred when a frozen latitude + longitude + speed was also present in the FIT records.

Bearing in mind that the timestamps in FIT records and GPS metadata do not necessarily match, it may be that the NMEA record at 17:09:17 was somehow lost. Either it was never output by the GNSS chipset, or it was lost due to unpredictable timings within the Garmin event handling.

Further testing is planned to try to ascertain why NMEA records appear to be lost on some Garmin watches.

Summary

• The speed in GPS metadata is a smoothed version of the Doppler-derived speed and useless to speed sailors.
• The altitude in GPS metadata may well be from the GNSS chipset and relative to MSL. Technically the “orthometric height”.
• Timestamps in the FIT records (Garmin system time) and GPS metadata (NMEA timestamp) often differ by a second.
• Unlike FIT records, GPS metadata is only saved to the file when the fix quality is 4 (QUALITY_GOOD).
  • GPS metadata is not saved when there is a loss of lock, because the fix quality is 0 (QUALITY_NOT_AVAILABLE).
• Frozen FIT records are sometimes (maybe often or always) accompanied by missing records in GPS metadata.
  • This could be due to imprecise timings within the Garmin event handling, or related to the NMEA data.
• Further investigations are planned in relation to NMEA records appearing to be lost on some Garmin watches.