Key takeaway
Two received positions do not establish the route, behavior or reason for missing broadcasts between them.
Start with the receiver’s evidence
An AIS archive records messages that a receiving system acquired. An empty interval can reflect several unresolved conditions; it is not automatically evidence that a vessel stopped transmitting, left an area or changed behavior. A buyer considering route or port-visit analysis needs to know the source network’s observation boundary before interpreting a smooth line on a map.
The US Coast Guard explains that AIS uses VHF transmitters and receivers with information from shipboard systems. Its message reference distinguishes position fields, unavailable values and time semantics. These technical passages were read 11 October 2026. They establish that the observation has a transmission and reception path; they do not establish the coverage, completeness or accuracy of a particular commercial archive.
Keep a gap instead of inventing a voyage
This hypothetical receiving log has valid positions at 12:00 and 12:45 UTC. An intervening Class A message at 12:03 has unavailable position fields. The operator’s own hypothetical health record says receiver R1 was offline from 12:04 until 12:44. The table uses symbolic locations P1 and P2, no real vessel identifier or coordinates. It does not establish where the vessel was between those times.
A display can connect P1 and P2 for convenience, but the 45-minute valid-position gap remains. A calculated straight-line segment is an interpolation, not a received route. Label it separately if the proposed task permits such a derivation, preserve its method and avoid inferring a port visit or deliberate shutdown from it. The invalid-position message is still reception evidence even though it cannot support a position claim.
| UTC event | Held evidence | Supported statement | Hold |
|---|---|---|---|
| 12:00 | Valid message at P1 | Position received | No later route yet |
| 12:03 | Message; unavailable position | Broadcast received; no usable point | Do not plot sentinel |
| 12:04–12:44 | R1 health log says offline | Receiver coverage unavailable | Vessel behavior unresolved |
| 12:45 | Valid message at P2 | Position received | Between-point route unknown |
Preserve message-specific time and position meaning
In the Coast Guard’s Class A reference, longitude 181 and latitude 91 indicate unavailable values. They must not become ordinary coordinates. The same reference notes that long-range Message 27 has no timestamp field and expects the receiving system to provide reception time. A parser that labels every timestamp as the instant the vessel occupied the point can therefore give different message families an unjustified common meaning.
Retain message identifier, source feed, reception timestamp, available message time, position-validity state, accuracy indicators and original encoded values where permitted. Document conversions and precision without upgrading a transmitted quality flag into independent verification. Review a sentinel-value example and a mixed-message example before calculating transit time. If the archive has already discarded message family or original timing, declare that interpretive limit explicitly.
Investigate jumps before identifying another voyage
The Coast Guard FAQ warns that vessels using the same MMSI can appear as one jumping track on a display. An apparent leap can consequently be an identity problem rather than extraordinary movement. Ask whether the source retains identity conflicts, static-message versions and the time each identity field was observed. A present-day vessel-name lookup cannot prove the historical identity of every old message.
For a bounded buyer test, include one received point during healthy coverage, one unavailable position, one receiver outage and one intentionally synthetic shared-identifier conflict. Define the expected interpretation before running a route reconstruction. Do not present the test’s invented conflicts as actual Coast Guard findings or commercial fraud. The evaluation should distinguish observed, derived and unresolved states, rather than rewarding a system for producing a continuous track every time.
Make the scope useful for a first discussion
The fictional review supports a description of two valid position observations separated by a receiver outage, with a separate invalid-position message. It withholds the route, shutdown cause and port-visit conclusions. That is a defensible evidence boundary even when a buyer hoped for a richer history. The next task is to recover documented network health and message semantics, not to infer missing movement from marketing coverage claims.
Use Due diligence to specify the geography, period, observation source and desired inference; Inventory records which fields and coverage logs actually exist. Real vessel tracks may expose commercial activity or relate to identifiable people, and source-feed permissions can restrict reuse. Approval for a named metadata introduction does not grant a sample or license. VOID’s initial referral has no upfront seller fee and may receive disclosed conditional receiving-program compensation; it does not certify AIS completeness or navigation safety.