Configuring an AIS Coverage Zone for Your Pilotage District (Any Bounding Box)

Most fleet-tracking tools start from a vessel: you register an MMSI, and the system follows that hull wherever it goes. That model is backwards for a pilots association. A dispatch desk does not care about a list of known ships — it cares about anything inbound to the pilot grounds, including the hull that filed late, the charter that swapped at the last minute, and the tug-and-tow that never makes a port call schedule. You serve a piece of water, not a roster.

That is why a dispatch board built for Binnacle Passage is organized around a coverage zone: a geographic box you draw once over your district, scanned continuously, with no pre-registration of vessels. This article walks through exactly how that works, why a bounding box is the right primitive, and how to configure one for your district — with three worked examples and an onboarding checklist.

Why a zone, not a vessel list

Under 46 USC 8501 and the state pilotage statutes built on it, foreign-trade and most coastwise vessels above a tonnage threshold take a pilot when they enter compulsory waters. The dispatcher's question is never "where is the Star Maru?" It is "what is going to need a pilot in the next twelve hours, and in what order?" A vessel-registration model cannot answer that, because the most operationally dangerous arrivals are exactly the ones nobody told you about.

A coverage zone inverts the logic. You define the water; the system reports every AIS target inside it. A vessel that enters your box appears on the board automatically — no MMSI added in advance, no manifest cross-reference required. When it leaves the box outbound, it falls off. The board is always a live picture of who is actually in your district right now, which is the only picture a dispatcher can launch a boat on.

This also means coverage degrades gracefully. If a hull's AIS is intermittent, you still see it whenever it transmits inside the zone; you are not depending on a registration that may be stale. (For the deeper question of which AIS feed sits underneath the zone — commercial aggregator versus the Coast Guard's NAIS — see NAIS vs. commercial AIS for pilot dispatch.)

How the zone actually works

A coverage zone is, mechanically, a latitude/longitude bounding box: a minimum and maximum latitude, a minimum and maximum longitude. Four numbers. Every target whose reported position falls inside those four numbers is in scope.

Scanned every two minutes. The board re-queries the feed on a two-minute cadence. That interval is a deliberate balance: fast enough that a vessel making 12 knots SOG moves only about 0.4 nautical miles between scans — well inside the resolution a dispatcher needs to sequence boardings — and slow enough that it does not hammer the upstream AIS source or burn through a metered feed. Each scan refreshes position, SOG, COG, and the derived ETA to the boarding point.

No MMSI pre-registration. You never maintain a vessel table. The zone is the only thing you configure. This is the single biggest operational difference from fleet-tracking software, and it is the reason a pilots association onboards in an afternoon rather than a quarter.

Multiple named zones per account. A district that spans more than one body of water — or an association that holds licenses across several grounds — defines several zones, each with its own name, box, and boarding point. The board can show them stacked or filtered. A Puget Sound operation, for instance, might run separate zones for the Strait of Juan de Fuca approaches and the Sound proper, because the transit times and boarding stations are entirely different even though it is one association.

The boarding point and the station-to-boarding transit time

A bounding box tells you what is inbound. Two more numbers tell you when to turn out the boat.

The boarding-point coordinate. This is the lat/lng of your pilot boarding area — the position where the pilot actually steps across. The board computes each inbound vessel's ETA to this exact point, not to a generic port centroid. The math is range-to-boarding-point divided by along-track speed, refreshed every scan. Where current matters (and in most districts it matters), the honest version folds predicted current into SOG so the ETA does not drift; you can sanity-check the tidal piece against a tide window calculator when you are sequencing a draft-constrained arrival.

The station-to-boarding transit time. This is how long it takes your pilot boat to run from its station or mooring out to the boarding point. It is a fixed input you set once per zone (you can override it for weather). The board subtracts it from the vessel's ETA to produce the number a dispatcher actually acts on: the launch-by time. If a bulker is 90 minutes from the boarding point and your boat needs 25 minutes to get there, the board flags a launch-by of 65 minutes out — and escalates as that clock runs down. (We go deep on this in pilot-boat departure alerts.)

Get those two inputs right and the board stops being a map and starts being a dispatch tool.

Worked example 1: Cook Inlet

Cook Inlet is the hard case, and it shows why the zone abstraction matters. A reasonable upper-inlet zone runs roughly:

• Lat: 60.5° N to 61.3° N
• Lng: −151.6° W to −150.0° W
• Boarding point: off the Nikiski/Anchorage approaches, set to the association's actual boarding coordinate
• Station-to-boarding transit: set from the pilot-boat base

The complication here is not the box — it is the feed and the tide. Commercial AIS aggregators have effectively no terrestrial coverage of the upper inlet, so the zone is configured to consume NAIS District 17 once the data-sharing agreement is in place. And because upper Cook Inlet runs one of the largest tidal ranges in North America with currents past six knots, the ETA to the boarding point must be current-aware or it is fiction. The zone, boarding point, and transit logic are identical to any other district; only the underlying feed and the current overlay change. Full treatment in Cook Inlet pilotage dispatch.

Worked example 2: Prince William Sound

Prince William Sound is the tanker district — Valdez crude out of the Alyeska Marine Terminal, escort-tug operations, and long approach distances. A workable zone:

• Lat: 60.0° N to 61.1° N
• Lng: −147.6° W to −146.0° W
• Boarding point: Hinchinbrook Entrance approaches (the association sets the exact coordinate)
• Station-to-boarding transit: longer than most districts — the boarding ground sits well offshore of the Valdez terminal

The teaching point here is the transit distance. Because the boarding point is far from the pilot station, the station-to-boarding transit time is large, which makes the launch-by lead time large. A district like this lives or dies on getting that one input correct; a 20-minute error in the transit assumption is a 20-minute error in every launch decision. PWS also runs the same NAIS coverage consideration as Cook Inlet — the commercial feeds thin out fast once you leave the terminal apron.

Worked example 3: a custom zone

Say you run a smaller, well-covered harbor — call it a regional bar or bay with good commercial AIS. The process is the same and takes five minutes:

1. Open a chart and read off the corners of the water you serve. Pad the box outbound enough to catch vessels before they reach the boarding point — you want lead time, so push the seaward edge of the box well out to sea.
2. Enter the four numbers as min/max lat and min/max lng.
3. Drop the boarding-point coordinate.
4. Set the station-to-boarding transit time from your boat's actual run.
5. Name the zone and save.

That is a live board. No MMSI list, no integration project, no manifest feed. If your district is commercially well-covered, you are running on a standard AIS aggregator from minute one; if it is dark, you slot NAIS underneath the same box later without changing anything else. Compare how this plays out in a high-traffic ship channel in Houston Ship Channel dispatch.

What to gather before onboarding

To stand up your district, have these ready:

• Bounding box corners — min/max latitude and longitude for each body of water you serve. A chart and five minutes is enough; we refine them with you.
• Boarding-point coordinate(s) — the actual lat/lng where pilots board, per zone.
• Station-to-boarding transit time(s) — how long your boat runs from station to boarding ground, plus any rough-weather override.
• Zone names — what your dispatchers call each piece of water, so the board reads in your language.
• Feed reality — whether your district has usable commercial AIS or needs NAIS. If it is dark, we help assemble the District data-sharing application; the package is pre-built.
• Tariff basis (optional, for billing) — your per-GT rate structure and any detention terms, if you want the transit record to feed invoicing. You can model the numbers against a pilotage tariff calculator first.

None of this requires a vessel roster, an IT project, or a feed integration on your side. You are describing a piece of water and a boat — things every association already knows cold.

The takeaway

A pilotage dispatch board should start where the work starts: with the water. A coverage zone — a simple lat/lng bounding box scanned every two minutes, with no vessel pre-registration — is the right primitive because it surfaces everything inbound to your grounds, including the arrivals nobody warned you about. Add a boarding-point coordinate and a station-to-boarding transit time, and the map becomes a launch-by clock your dispatchers can run a boat on. Multiple named zones cover districts that span regions, and the same configuration rides on top of either a commercial feed or NAIS without changing a thing above it.

If you want to see a coverage zone, a live AIS board, and the launch-by logic working together for a real district, Binnacle Passage is built for exactly this — see a live pilot board and bring your bounding box. We will have your district configured before the call is over.

This article is general information. Pilotage in your district is governed by the applicable state law and the licensed pilots association serving the grounds.