The Open Signal API

Live presence: every station currently on the air, the contacts in progress between them, and stations heard recently but now quiet. Cached 30 seconds.

{
  "generatedUtc": 1787856269752,
  "stations": [
    {
      "call": "G4RHD",
      "grid": "io93id",
      "lat": 53.146, "lon": -1.375,
      "freq": 144550000,
      "band": "2m",
      "mode": "hybrid",
      "with": "M0IZM",
      "act": "qso",
      "ageSeconds": 12
    }
  ],
  "links": [
    { "a": "G4RHD", "b": "M0IZM", "act": "qso", "km": 22 }
  ],
  "recent": [
    {
      "call": "M8IFY", "grid": "io93if",
      "lat": 53.19, "lon": -1.32,
      "band": "2m", "mode": "hybrid",
      "lastSeenUtc": 1787856100000
    }
  ]
}

Fields worth knowing

• act — "tx" transmitting, "qso" in contact, "" merely present.
• with — the callsign being worked. Empty when calling or idle.
• lat and lon can be null — a station that has never set a locator has no position. They are also Maidenhead square centres, not exact positions, so treat them as accurate to a few kilometres.

That null is the one that will catch you. List a station without a locator; do not try to map it.

The most recent pictures put on the air, newest first. Takes an optional ?limit=: the default is 24 and the maximum is 120 — ask for more and you get 120 rather than an error. Cached 60 seconds.

GET /api/v1/photos?limit=2

{
  "generatedUtc": 1787856269863,
  "condensed": true,
  "pictures": 2,
  "items": [
    {
      "serial": "A62D26",
      "url": "https://…/f/g/45b1ba54f38c.jpg",
      "thumbUrl": "https://…/f/g/45b1…jpg?w=480",
      "pageUrl": "https://…/p/A62D26",
      "originalUrl": "https://…?original=1",
      "blurDataUrl": "data:image/…",
      "name": "de 2E0MBH 07/23/2026",
      "sender": "M7GYN",
      "receivedBy": ["M0IZM", "M6YJB"],
      "copies": 2,
      "call": "M7GYN",
      "dir": "tx",
      "utc": 1787850000000,
      "ageSeconds": 6269,
      "freq": 144550000,
      "band": "2m",
      "mode": "hybrid"
    }
  ]
}

Condensing, and optional fields

One transmission is stored more than once — the sender’s copy and each receiver’s copy, under different serials. condensed: true means those have been merged, so items is a list of transmissions rather than of stored files. copies tells you how many were folded together.

• thumbUrl, pageUrl, originalUrl, blurDataUrl — present on every item at the time of writing.
• altText and imageUrl are frequently absent. They are generated at publish time, so older pictures have neither. Check for them; never assume them.
• Any picture URL takes ?w= and is resized on demand, snapping to the nearest of 160, 320, 480, 960 or 1440. It returns AVIF where the browser accepts it — a 480px thumbnail is roughly a tenth the bytes of the original.

One picture, with every station that reported copying it — the reception reports behind the signal figures on the live page.

{
  "serial": "1824AD",
  "url": "https://…/f/g/….jpg",
  "name": "DSC01118",
  "utc": 1787840000000,
  "band": "2m",
  "mode": "hybrid",
  "copies": 3,
  "sender": {
    "call": "M0IZM", "grid": "IO92KX",
    "lat": 52.94, "lon": -1.19
  },
  "receptions": [
    {
      "call": "M6YJB", "grid": "IO93IC",
      "lat": 53.16, "lon": -1.37,
      "snrDb": 17.2, "km": 18,
      "mode": "hybrid",
      "utc": 1787840012000
    }
  ]
}

The trap in this endpoint

sender is null for anything copied off the air rather than transmitted through Open Signal — which is every analogue SSTV picture. It is not an error and it is not rare: roughly half the pictures sampled while writing this page returned it.

This site read .lat off that null and a whole modal stopped rendering. Normalise it on the way in rather than discovering it in production.

• receptions may be an empty array — a picture nobody reported copying.
• A reception’s lat/lon are null when that operator has no locator. List them, do not map them.
• An unknown serial returns 400, not 404.

Aggregate figures, in blocks: live, today, week, allTime, records, topStations, activityByDay and service. Cached 60 seconds.

{
  "live":  { "stationsOnAir": 7, "qsosInProgress": 0,
             "bands": { "2m": 7 }, "modes": { … } },
  "today": { "activeStations": 4, "picturesShared": 26, … },
  "week":  { "meanSnrDb": 14.5, "newStations": 1, … },
  "allTime": { "stationsSeen": 13, "picturesShared": 1365, … },
  "records": {
    "longestReceptionEver": {
      "sender": "M0IZM", "receiver": "G0GOO", "km": 51
    }
  }
}

Two cautions

• Everything under records is optional. Only the records that exist are present, so read each one defensively rather than assuming the shape.
• service.imagesServedToday and service.imagesServedTotal are HTTP download counters, not radio activity. They count image files fetched over the web — the website loading its own thumbnails inflates them. Never present them as a count of pictures sent over the air; next to a figure like “on the air now” they read as exactly that.

• Check array lengths, not status codes. The service does not return 5xx — a backend fault degrades to an empty but valid document. A client watching for errors will see 200 and believe the band is dead.
• Respect the cache headers. 30 seconds on the map, 60 on the others, and the upstream data only changes on that cadence anyway. Polling faster buys no freshness.
• Range-check frequencies. The feed carries the occasional unit slip — 144550000000 Hz for 144.550 MHz. Anything outside roughly 100 kHz to 3 GHz is worth discarding rather than guessing at.

• Modes are missing on off-air copies. Analogue SSTV copied off the air arrives with an empty mode; the mode is often recoverable from the filename, which usually carries it.
• "partial NN" in a name means an incomplete decode — the picture arrived, but not all of it.
• Set a timeout. This site uses six seconds and falls back to an empty document, which is why a slow upstream never becomes a slow page.