Matching sightings with ADS-B: closing gaps in your logbook cleanly

The Airbus came over the threshold in backlight, and the registration on the photo is just a gray shadow. Or in the rain you scribbled only “A320, Eurowings, approx. 3 p.m.” in your notebook. Anyone who keeps a logbook knows such gaps. The good news: aircraft leave digital traces. If you match your sightings with ADS-B data, you can complete many incomplete entries after the fact. This guide shows you which sources are suitable, how to proceed step by step and where the data can lead you astray.

The short answer

You match a sighting by comparing the time of your observation and your location with the position history of the aircraft in that time window. The aircraft whose track passes your spot at the right time and whose type fits your observation is your candidate. Via its transponder address (the hex code) you find the registration. If exactly one plausible candidate remains in the end, you can complete the sighting. If two remain, you leave it open.

That sounds simple, but it depends on three prerequisites: a correct time, a data source with history for the day in question and an aircraft that actually transmitted ADS-B. How ADS-B works in general is explained in the introduction Flight tracking with ADS-B for spotters.

When a match is worthwhile

Not every gap needs to be closed, and not every one can be. Typical cases in which ADS-B data really helps:

• Registration unreadable: Backlight, haze, heat shimmer or simply too much distance. You can still recognize type and airline, but not the registration.
• Note incomplete: Things went fast at the spot, and only the type is in the notebook. How to log faster at the fence is shown in the article Logging sightings quickly at the spot.
• Night sighting: At night you recognize lights and silhouette, but no registration.
• Doubt about your own note: You noted a registration but are unsure about one letter. The match confirms or corrects it.
• Old photos without notes: Archive pictures from earlier years can only be completed if history data still exists for that day.

The match makes less sense if you know neither the time nor the spot exactly. Then the candidate list is too long and the risk of a wrong match rises.

The most important step: the right time

Almost all failed matches fail because of the time. Your camera writes the time of capture into the EXIF data of every file, but only as precisely as its internal clock runs. Many cameras drift by a few seconds to minutes over months, and after the clock change it is easy to forget to reset them.

Then there is the time zone. ADS-B tools calculate in UTC. Your camera, by contrast, usually stores the time you set – in Germany that is CET or CEST. Between the two there is a difference of one or two hours. Anyone who forgets this searches in a completely wrong period.

How to store timestamps, registrations and other details cleanly in your images’ metadata is described in the guides Metadata for aircraft photos and Captioning photos properly. If your camera or smartphone stores GPS coordinates, the location is documented automatically as well – more on that in Geotagging aircraft photos.

Which data sources you can use

For the match you need a source that not only shows live data but can also look back. Three ways come into question.

Source	How far back	Strengths	Weaknesses
Own receiver with tar1090 and globe history	as long as you keep the files	full control, second-accurate tracks, lookup by hex or registration	only as far as your antenna reaches; setup required
Community tracker with history	depends on the service; some public archives	unfiltered, wide coverage, often free	more technical to use, availability not guaranteed
Commercial flight tracker services	limited for free, longer with a subscription	convenient, flight number and route directly visible	filtered aircraft, short free period

Your own receiver

Anyone who runs their own ADS-B receiver has the best source right at home. With the readsb variant maintained by wiedehopf, a history can be written to disk (option --write-globe-history). The tar1090 web interface can then display these tracks again. According to the documentation, URL parameters are enough for this: with icao= you select an aircraft by its hex code, with showTrace= a UTC date, and startTime= and endTime= limit the history to a UTC time window. With reg= you jump directly to a registration. If the heatmap function is also active, you can use replay to play back the traffic of a period like a recording – ideal for seeing which aircraft were over your runway at 15:12 UTC.

You should know two notes from the documentation: the history files are kept indefinitely, so you have to clean up yourself when storage runs low. And the data format can change with updates. How to set up the software is covered in the guide Installing readsb and tar1090; an overview of desktop solutions is given in Flight tracking on the PC. If you want to record right at the spot, you will find ideas in Mobile ADS-B reception at the spotting location.

Community trackers with history

Several community networks are based on the same software and therefore offer similar history functions. adsb.lol has even published the raw data of its history as daily archives on GitHub, under the Open Database License. For individual sightings that is usually too coarse – the archives are huge – but it shows that such data can remain available over the long term. Which trackers exist and how they differ is compared in the article Unfiltered community trackers compared; for ADS-B Exchange in particular there is the guide ADS-B Exchange explained.

Commercial flight tracker services

Flightradar24, FlightAware and others are the most convenient when you already know which flight number or registration you are looking for. The free look back is short, though. FlightAware’s FAQ gives the last seven days as the visible period, and according to the provider the premium subscriptions reach back several months depending on the tier – it is best to check the current periods directly with the provider. At Flightradar24 the length of the history also depends on the subscription – details are in Flightradar24 Silver, Gold and Business. How to retrieve the flight history of an individual aircraft is explained in Looking up an aircraft’s past flights. A simple rule follows from this: match on the same evening, not weeks later.

Step by step to a complete entry

The process is the same for every source. Take the incomplete entries of a session together – that is faster than researching each one individually.

How to narrow down an unreadable registration even without ADS-B is shown in Determining an unreadable registration. For step 5, take a look at Using Planespotters.net as a fleet database. How registrations are constructed in general and which country prefixes exist is explained in Understanding aircraft registrations. And if you want to search for a specific aircraft: Finding an aircraft by registration or hex code.

Hex code, callsign, registration: don’t mix them up

In the ADS-B history you come across three identifiers that are easy to confuse:

• Hex code (ICAO address): the 24-bit address of the Mode S transponder, displayed as six hexadecimal characters. It belongs to the aircraft and normally only changes with a re-registration, especially when moving to another country. Background in Transponder modes A, C and S explained.
• Callsign: the callsign of the particular flight, usually made up of airline code and flight number. It changes from flight to flight and tells you the airline, but not the aircraft.
• Registration: the registration on the fuselage. It is not transmitted directly via ADS-B but is matched to the hex code via databases.

This is exactly where a trap lies. When an aircraft changes owner and country, it gets a new registration and usually a new address. Not every database catches up immediately. With lease returns or ferry flights, a tracker can therefore briefly show an outdated registration. More on this in Re-registrations.

Where the data deceives you

A match seems objective because it is based on data. Nevertheless there are typical sources of error:

Missing signals: Not every aircraft transmits ADS-B. Older aircraft, parts of general aviation and many military aircraft show up at best via multilateration, and even then only with sufficient receiver density. More on this in MLAT explained and Why an aircraft doesn’t appear on Flightradar.

Filters: Commercial services hide certain aircraft at the request of operators. tar1090 also has its own database flags for military and privacy programs that you can filter by. If you can’t find an aircraft you saw in one source, try a second.

Ground movements: A distant antenna often no longer receives taxiing aircraft because buildings block the line of sight. For apron sightings, a receiver near the airport has a clear advantage.

Time windows that are too narrow: Start with plus/minus five minutes and only widen the window if you find nothing. If the list then gets long, only the photo can act as referee.

Labeling the source in your logbook

A completed entry is something different from a registration read off directly. To keep your collection meaningful, a simple label in every entry is worthwhile:

Status	Meaning	For statistics
Seen, registration legible	Registration read from photo or on site	count fully
Seen, confirmed via ADS-B	Type and airline observed, registration added via a unique candidate	count, note the source
Seen, not unambiguous	several candidates possible	leave open
Data only	Aircraft in the history, but not observed	not a sighting

How to build analyses from this is shown in Analyzing your own spotting statistics. And if you are still wondering where the logbook should live in the first place, you will find a suitable starting point in Documenting sightings: the spotting logbook and Notebook, spreadsheet or app.

Which source suits you?

What to watch out for

• Match promptly. The fresher the sighting, the more sources still have data and the better you remember details.
• Calculate consistently in UTC. Write down the conversion for summer and winter time until it sticks.
• Use the photo as proof. Even a half-legible registration, a special livery or an antenna can decide between two candidates.
• Distrust single hits in old databases. With re-registrations, a second source can uncover errors.
• Don’t invent sightings. What you only find in the data does not belong in the collection.

Conclusion

ADS-B data is a powerful tool for closing gaps in your logbook. What matters is not the data source but the preparation: a camera with a known time deviation, a cleanly noted spot and the match as soon as possible after the session. Your own receiver with saved history is the most flexible solution, commercial flight trackers are the most convenient for fresh sightings. And whichever source you use: what was added should remain recognizable as added.