[
  {
    "id": "S1",
    "observed_minute": 95,
    "freshness": "current"
  },
  {
    "id": "S2",
    "observed_minute": 70,
    "freshness": "stale"
  },
  {
    "id": "S3",
    "observed_minute": null,
    "freshness": "no_observation"
  }
]