400m on the track¶
One of the applications the MSX system is intended for is monitoring athletes on the track, in particular over the 400 metres, recording data at 100 Hz. It is a typical "special" use case where the high rate is not just useful but necessary.
Why 100 Hz is needed in sprinting¶
A 400 m runner moves at high speed (on average ~8–9 m/s, with peaks around 10 m/s on the fast sections). At these speeds a low rate "misses" the dynamics of the movement:
| Rate | Sample every | Distance between two samples at ~9 m/s |
|---|---|---|
| 1 Hz | 1000 ms | ~9 m → too sparse, "chopped" trajectory |
| 10 Hz | 100 ms | ~0.9 m → fine for the track, poor for the dynamics |
| 100 Hz | 10 ms | ~9 cm → high-resolution trajectory and speed profile |
At 100 Hz you get a sample every ~9 cm of running: dense enough to faithfully reconstruct the bend, the stride and the rapid speed variations.
What it lets you measure¶
With RTK fixed position (centimetre precision) sampled at 100 Hz you can derive, for each instant of the race:
- instantaneous speed (derivative of position) → speed curve over the lap;
- acceleration and deceleration → initial launch phase and the slowdown in the final 100 m (typical of the 400 m);
- actual trajectory versus the ideal lane line → metres actually run, "widening" on the bends;
- split times every 100 m / 200 m / 300 m derived from position, without photocells;
- bend handling → entry/exit speed, radius taken;
- effort distribution along the lap (pacing).
Why RTK + 100 Hz together¶
- RTK fixed gives the spatial precision (centimetres): without it you could not distinguish the lane or the small trajectory deviations;
- the 100 Hz gives the temporal resolution: without it you would lose the rapid speed and acceleration variations.
You need both: high precision and high rate.
Recommended setup¶
| Parameter | Recommended value |
|---|---|
| Mode | RTK (corrections over USB / NTRIP – see USB operation) |
| SBF logging rate | 100 Hz (msec10) – see Raw data |
| SBF blocks | MeasEpoch + PVTGeodetic (+ ephemerides) for position and PPK |
| Constellations | multi-constellation for robustness on bends and with the athlete's body |
| Antenna | lightweight active antenna, mounted high (e.g. on the back/helmet) under open sky |
Note
A 400 m race lasts ~45–50 s → at 100 Hz about 4500–5000 epochs: an SBF file of a few MB, perfectly manageable.
Monitoring with the LOG button¶
In this application the athletes are monitored using the LOG button on the
board (the tactile button P1, see Main). The workflow is
designed to be simple in the field and to move all the processing to a
later stage:
- one press of the LOG button starts recording the trajectory (SBF stream at 100 Hz);
- a second press stops the recording at the end of the run;
- each run is saved as a trajectory (a raw-data file);
- at the end of the session, all the trajectories collected are post-processed together (PPK) to extract the parameters of interest.
Note
The LOG button controls the start/stop of a mosaic-X5 logging stream. The stream (SBF blocks and rate) is set up in the configuration; pressing the button merely activates/deactivates it in the field, without a PC. (button mapping to be confirmed in the MSX firmware)
Typical workflow¶
- Setup: antenna on the athlete, receiver powered and ready.
- RTK fixed: wait for the RTK fix (LED – see Main).
- Start: press the LOG button just before the start → trajectory recording begins at 100 Hz.
- Race: the athlete runs the lap; the system records position and raw measurements.
- Stop: press the LOG button again at the finish → the trajectory is saved.
- Repeat: repeat for each athlete/run; each run is a distinct trajectory.
- Post-processing: at the end of the session, download the files, convert
them to RINEX (
sbf2rin) and run post-processing (PPK) on all the trajectories, computing speed, acceleration, splits and path.
Tip
The 100 Hz SBF log is "raw": it lets you reprocess each run afterwards (PPK), getting the best trajectory quality regardless of any brief coverage drops during the run. This is why you record in the field and process everything afterwards.
Practical considerations¶
- Open sky: the athletics track is an ideal environment (few obstructions); avoid stands/roofing that limit the sky view.
- Bends and the athlete's body: movement and the body can shadow part of the sky → multi-constellation and a well-placed antenna help keep the fix.
- Weight and size: the mini carrier + Main is compact; keeping the weight of the assembly (receiver + antenna + battery) down matters so as not to alter the athletic movement.
- Synchronisation: to align the GNSS data with other sensors (photocells, IMU) you can use the module's PPS signal as a common time reference (availability on the mini carrier to be confirmed).
Warning
100 Hz should be used for this class of applications. For "slow" training or coarser analysis, 10–20 Hz reduce the files and the load while keeping more than adequate information.