The gap, not the map
On a long river transit, or on a coastal route away from towns, mobile coverage comes and goes. A tracker that sends its position over the mobile network either loses the positions it takes while out of range, or stores them and sends them in a burst when a tower is back in reach. Either way, for the hours in between, the office does not know where the vessel is. It only learns, later, where it was.
For the people deciding the transit, the difference matters. A position is useful while it is recent enough to act on: to warn a convoy about a reach that has changed, to confirm an arrival, to notice that a vessel has stopped where it should not.
What satellite reporting changes
A satellite tracking unit sends its position through a satellite network instead of a shore antenna, so it keeps reporting wherever it has a clear view of the sky and is inside the network’s coverage, which is worldwide for some constellations and regional for others. The units used on vessels and convoys are usually self-contained: a satellite positioning receiver, a transmitter and a battery, often charged by a small solar panel, in a sealed case fixed to the deck, the wheelhouse roof or a barge.
The report itself is short: the time of the fix, latitude and longitude, and in some units speed and course. It travels from the unit to the satellite network, from there to the provider’s ground stations, and then to the platform that displays it. The delay from fix to screen depends on the network; it is normally measured in minutes, not hours.
Satellite AIS reaches a similar result by another path: it picks up a vessel’s own AIS transmissions from orbit. It depends on the vessel carrying and running an AIS transponder, which many river craft and barges do not.
The interval decides what you can see
Satellite airtime is paid by the message or by the plan, and every transmission draws on the battery. So the unit reports at an interval — every few minutes, every quarter of an hour, every hour. The interval is the real design decision, because it sets the resolution of everything built on top of it.
A simple calculation shows why. A convoy making 8 knots covers about 2.5 kilometres in ten minutes and about 15 kilometres in an hour. At one report an hour, a stop, a turn back or a grounding can happen well between two positions. At one every ten minutes, the track follows the vessel closely enough to see a stop or a change of course within minutes, though anything shorter than the interval can still fall between two reports.
Schematic · no scale, no values. The same route and the same interval, reported two ways.
- Report over the mobile network
- Report over satellite
- Reach without mobile coverage
The interval does not have to be fixed. Many units can report more often while the vessel is moving and less often while it is moored, or change interval on command. Choosing those settings is part of choosing the equipment, not an afterthought.
Show the age, not only the position
A dot on a map looks equally current whether it arrived three minutes ago or three hours ago. That is the most common way tracking misleads: an old position read as a present one.
The rule is simple. Every position carries its time and its source, and the screen says how old it is — four minutes ago, by satellite. When a unit misses a report it was expected to send, that absence is information too, and it should be visible rather than hidden behind the last known dot.
What to check before relying on it
- Sky view. The unit needs open sky; a cabin roof, a crane or stacked cargo can block it.
- Power. A self-powered unit needs enough sun, or a battery that covers the season at the chosen interval.
- Interval and airtime. The plan should cover the interval the operation needs, including its busiest periods.
- Latency. Know how long a report takes to reach the screen, end to end.
- Access to the data. The operator should be able to receive its own positions, not only view them inside one application.
None of this depends on one river or one fleet. It is the same question wherever the water is the road: whether the position on the screen is recent enough to decide on. VigIA’s Tracking module is built for that case — position reported over satellite, independent of mobile coverage.
