🚤Vessel Speed

In AISdb, the speed of a vessel is calculated using the aisdb.gis.delta_knots function, which computes the speed over ground (SOG) in knots between consecutive positions within a given track. This calculation is important for the denoising encoder, as it compares the vessel's speed against a set threshold to aid in the data cleaning process.

Vessel speed calculation requires the distance the vessel has traveled between two consecutive positions and the time interval. This distance is computed using the haversine distance function, and the time interval is simply the difference in timestamps between the two consecutive AIS position reports. The speed is then computed using the formula:

$$Speed(knot) = \frac{Haversine Distance}{Time} \times 1.9438445$$

The factor 1.9438445 converts the speed from meters per second to knots, the standard speed unit used in maritime contexts.

With the example track we created in Haversine Distance, we can calculate the vessel speed between each two consecutive positions:

import aisdb
import numpy as np
from datetime import datetime
from aisdb.gis import dt_2_epoch

# Generate example track
y1, x1 = 44.57039426840729, -63.52931373766157
y2, x2 = 44.51304767533133, -63.494075674952555
y3, x3 = 44.458038982492134, -63.535634138077945
y4, x4 = 44.393941339104074, -63.53826396955358
y5, x5 = 44.14245580737021, -64.16608964280064

t1 = dt_2_epoch( datetime(2021, 1, 1, 1) )
t2 = dt_2_epoch( datetime(2021, 1, 1, 2) )
t3 = dt_2_epoch( datetime(2021, 1, 1, 3) )
t4 = dt_2_epoch( datetime(2021, 1, 1, 4) )
t5 = dt_2_epoch( datetime(2021, 1, 1, 7) )

# Create a sample track
tracks_short = [
    dict(
        mmsi=123456789,
        lon=np.array([x1, x2, x3, x4, x5]),
        lat=np.array([y1, y2, y3, y4, y5]),
        time=np.array([t1, t2, t3, t4, t5]),
        dynamic=set(['lon', 'lat', 'time']),
        static=set(['mmsi'])
    )
]

# Calculate the vessel speed in knots
for track in tracks_short:
    print(aisdb.gis.delta_knots(track))

[3.7588560005768947 3.7519408684140214 3.8501088005116215 10.309565520121597]