diff --git a/docs/api/openapi.yaml b/docs/api/openapi.yaml
index 511f7caf..375bd481 100644
--- a/docs/api/openapi.yaml
+++ b/docs/api/openapi.yaml
@@ -180,6 +180,36 @@ paths:
           description: No content
         "404": { $ref: "#/components/responses/NotFound" }
 
+  /beacon-monitor/measurements:
+    post:
+      tags: [monitors]
+      summary: Upload a beacon-signal-level measurement
+      description: |
+        Ingest endpoint for the `propmonitor` client. The Bearer token here
+        is the `BeaconMonitor` token (issued once by
+        `POST /me/beacon-monitors`), not a user API token. One POST per
+        integration window per monitor; payloads are never batched.
+
+        `signal_peak_dbfs`, `signal_avg_dbfs`, and the `snr_*` fields are
+        computed only over FFT frames whose in-band power exceeded
+        `noise_floor + 3 dB`. `signal_active_fraction` reports the duty
+        cycle of those frames over the window.
+      requestBody:
+        required: true
+        content:
+          application/json:
+            schema: { $ref: "#/components/schemas/BeaconMeasurementCreate" }
+      responses:
+        "204":
+          description: Accepted and recorded. The monitor's `last_seen_at` is updated.
+        "401": { $ref: "#/components/responses/Unauthorized" }
+        "404":
+          description: The supplied `beacon_id` is unknown, unapproved, or off-the-air.
+        "409":
+          description: A measurement for this monitor + beacon + measured_at already exists.
+        "422": { $ref: "#/components/responses/ValidationFailed" }
+        "429": { $ref: "#/components/responses/RateLimited" }
+
   /contacts:
     get:
       tags: [contacts]
@@ -661,6 +691,74 @@ components:
           type: array
           items: { $ref: "#/components/schemas/BeaconMonitor" }
 
+    BeaconMeasurementCreate:
+      type: object
+      required:
+        - beacon_id
+        - frequency_hz
+        - measured_at
+        - integration_s
+        - passband_hz
+        - gain_db
+        - noise_floor_dbfs
+        - signal_peak_dbfs
+        - signal_avg_dbfs
+        - snr_peak_db
+        - snr_avg_db
+        - signal_active_fraction
+        - propmonitor_version
+      properties:
+        beacon_id:
+          type: string
+          format: uuid
+          description: The beacon this measurement is for.
+        frequency_hz:
+          type: integer
+          minimum: 1
+          description: SDR tuned center frequency in Hz.
+        measured_at:
+          type: string
+          format: date-time
+          description: UTC ISO-8601 timestamp at the start of the integration window.
+        integration_s:
+          type: integer
+          minimum: 5
+          maximum: 3600
+          description: Integration window length in seconds.
+        passband_hz:
+          type: number
+          exclusiveMinimum: 0
+          description: Bandwidth of the in-band power window.
+        gain_db:
+          type: number
+          minimum: -10
+          maximum: 80
+          description: SDR-reported gain at upload time.
+        noise_floor_dbfs:
+          type: number
+          description: Median out-of-passband bin power, dBFS.
+        signal_peak_dbfs:
+          type: number
+          description: Peak in-passband power across detected-on-air frames, dBFS.
+        signal_avg_dbfs:
+          type: number
+          description: Mean in-passband power across detected-on-air frames, dBFS.
+        snr_peak_db:
+          type: number
+          description: signal_peak_dbfs minus noise_floor_dbfs.
+        snr_avg_db:
+          type: number
+          description: signal_avg_dbfs minus noise_floor_dbfs.
+        signal_active_fraction:
+          type: number
+          minimum: 0
+          maximum: 1
+          description: Fraction of FFT frames where in-band power exceeded noise_floor + 3 dB.
+        propmonitor_version:
+          type: string
+          maxLength: 32
+          description: Build version of the reporting client (diagnostic).
+
     ProfileResponse:
       type: object
       properties:
diff --git a/lib/microwaveprop/beacon_measurements.ex b/lib/microwaveprop/beacon_measurements.ex
new file mode 100644
index 00000000..20c89247
--- /dev/null
+++ b/lib/microwaveprop/beacon_measurements.ex
@@ -0,0 +1,109 @@
+defmodule Microwaveprop.BeaconMeasurements do
+  @moduledoc """
+  The BeaconMeasurements context: ingests beacon-signal-level
+  measurements uploaded by propmonitor clients.
+
+  See `Microwaveprop.BeaconMeasurements.BeaconMeasurement` for the
+  schema and field semantics.
+  """
+
+  import Ecto.Query
+
+  alias Microwaveprop.BeaconMeasurements.BeaconMeasurement
+  alias Microwaveprop.BeaconMonitors.BeaconMonitor
+  alias Microwaveprop.Repo
+
+  @doc """
+  Inserts a measurement for the given monitor. `attrs` is expected to
+  carry the wire fields from `POST /api/v1/beacon-monitor/measurements`
+  (including `beacon_id` as a UUID string).
+
+  Returns `{:error, :beacon_not_found}` if the beacon UUID does not
+  resolve to an approved on-the-air beacon — the propmonitor client
+  drops the measurement on this, so we keep the differentiation from
+  generic 422s.
+  """
+  @spec create_measurement(BeaconMonitor.t(), map()) ::
+          {:ok, BeaconMeasurement.t()}
+          | {:error, :beacon_not_found}
+          | {:error, Ecto.Changeset.t()}
+  def create_measurement(%BeaconMonitor{id: monitor_id}, attrs) when is_map(attrs) do
+    case lookup_beacon(attrs) do
+      :not_found ->
+        {:error, :beacon_not_found}
+
+      {:ok, beacon_id} ->
+        attrs =
+          attrs
+          |> normalize_attrs()
+          |> Map.put("monitor_id", monitor_id)
+          |> Map.put("beacon_id", beacon_id)
+
+        %BeaconMeasurement{}
+        |> BeaconMeasurement.changeset(attrs)
+        |> Repo.insert()
+    end
+  end
+
+  defp lookup_beacon(attrs) do
+    case Map.get(attrs, "beacon_id") || Map.get(attrs, :beacon_id) do
+      id when is_binary(id) and byte_size(id) > 0 ->
+        # Only let monitors report for approved on-the-air beacons. The
+        # client drops 404s without retry, so an admin un-approving a
+        # beacon cleanly stops the monitor pointing at it.
+        query =
+          from b in Microwaveprop.Beacons.Beacon,
+            where: b.id == ^id and b.approved == true and b.on_the_air == true,
+            select: b.id
+
+        case safe_one(query) do
+          nil -> :not_found
+          uuid -> {:ok, uuid}
+        end
+
+      _ ->
+        :not_found
+    end
+  end
+
+  defp safe_one(query) do
+    Repo.one(query)
+  rescue
+    Ecto.Query.CastError -> nil
+  end
+
+  # The wire payload uses string keys (Phoenix JSON params). Make sure
+  # we don't drop a value because some intermediate handed us atom keys.
+  defp normalize_attrs(attrs) do
+    Map.new(attrs, fn
+      {k, v} when is_atom(k) -> {Atom.to_string(k), v}
+      {k, v} -> {k, v}
+    end)
+  end
+
+  @doc """
+  Returns the most recent measurements for the given beacon, newest first.
+  """
+  @spec list_recent_for_beacon(Ecto.UUID.t(), pos_integer()) :: [BeaconMeasurement.t()]
+  def list_recent_for_beacon(beacon_id, limit \\ 100) do
+    Repo.all(
+      from m in BeaconMeasurement,
+        where: m.beacon_id == ^beacon_id,
+        order_by: [desc: m.measured_at],
+        limit: ^limit
+    )
+  end
+
+  @doc """
+  Returns the most recent measurements for the given monitor, newest first.
+  """
+  @spec list_recent_for_monitor(Ecto.UUID.t(), pos_integer()) :: [BeaconMeasurement.t()]
+  def list_recent_for_monitor(monitor_id, limit \\ 100) do
+    Repo.all(
+      from m in BeaconMeasurement,
+        where: m.monitor_id == ^monitor_id,
+        order_by: [desc: m.measured_at],
+        limit: ^limit
+    )
+  end
+end
diff --git a/lib/microwaveprop/beacon_measurements/beacon_measurement.ex b/lib/microwaveprop/beacon_measurements/beacon_measurement.ex
new file mode 100644
index 00000000..cdc36e72
--- /dev/null
+++ b/lib/microwaveprop/beacon_measurements/beacon_measurement.ex
@@ -0,0 +1,76 @@
+defmodule Microwaveprop.BeaconMeasurements.BeaconMeasurement do
+  @moduledoc """
+  A single beacon-signal-level measurement uploaded by a propmonitor
+  client. One row per integration window per (monitor, beacon).
+
+  All `*_dbfs` fields are uncalibrated and relative to the SDR ADC
+  full-scale; `gain_db` is the SDR's reported gain at upload time and
+  is needed to interpret dBFS trends across gain changes. SNR is
+  already gain-independent so it is the preferred calibration input.
+
+  `signal_*_dbfs` and `snr_*_db` are computed only over FFT frames
+  whose in-band power exceeded `noise_floor + 3 dB`; `signal_active_fraction`
+  reports the duty cycle of those frames. When `signal_active_fraction
+  == 0.0`, no signal was heard and the peak/avg fields sit near the
+  noise floor (SNR ≈ 0 dB).
+  """
+
+  use Ecto.Schema
+
+  import Ecto.Changeset
+
+  alias Microwaveprop.BeaconMonitors.BeaconMonitor
+  alias Microwaveprop.Beacons.Beacon
+
+  @primary_key {:id, :binary_id, autogenerate: true}
+  @foreign_key_type :binary_id
+  schema "beacon_measurements" do
+    belongs_to :beacon, Beacon
+    belongs_to :monitor, BeaconMonitor
+
+    field :measured_at, :utc_datetime
+    field :frequency_hz, :integer
+    field :integration_s, :integer
+    field :passband_hz, :float
+    field :gain_db, :float
+
+    field :noise_floor_dbfs, :float
+    field :signal_peak_dbfs, :float
+    field :signal_avg_dbfs, :float
+    field :snr_peak_db, :float
+    field :snr_avg_db, :float
+    field :signal_active_fraction, :float
+
+    field :propmonitor_version, :string
+
+    timestamps(type: :utc_datetime)
+  end
+
+  @type t :: %__MODULE__{}
+
+  @cast_fields ~w(beacon_id monitor_id measured_at frequency_hz integration_s
+                  passband_hz gain_db noise_floor_dbfs signal_peak_dbfs
+                  signal_avg_dbfs snr_peak_db snr_avg_db
+                  signal_active_fraction propmonitor_version)a
+
+  @spec changeset(t() | Ecto.Changeset.t(), map()) :: Ecto.Changeset.t()
+  def changeset(measurement, attrs) do
+    measurement
+    |> cast(attrs, @cast_fields)
+    |> validate_required(@cast_fields)
+    |> validate_number(:frequency_hz, greater_than: 0)
+    |> validate_number(:integration_s, greater_than_or_equal_to: 5, less_than_or_equal_to: 3600)
+    |> validate_number(:passband_hz, greater_than: 0.0)
+    |> validate_number(:gain_db, greater_than_or_equal_to: -10.0, less_than_or_equal_to: 80.0)
+    |> validate_number(:signal_active_fraction,
+      greater_than_or_equal_to: 0.0,
+      less_than_or_equal_to: 1.0
+    )
+    |> validate_length(:propmonitor_version, max: 32)
+    |> foreign_key_constraint(:beacon_id)
+    |> foreign_key_constraint(:monitor_id)
+    |> unique_constraint([:monitor_id, :beacon_id, :measured_at],
+      name: :beacon_measurements_monitor_beacon_measured_at_idx
+    )
+  end
+end
diff --git a/lib/microwaveprop/beacon_monitors.ex b/lib/microwaveprop/beacon_monitors.ex
index 77e2b29b..974c517c 100644
--- a/lib/microwaveprop/beacon_monitors.ex
+++ b/lib/microwaveprop/beacon_monitors.ex
@@ -64,6 +64,22 @@ defmodule Microwaveprop.BeaconMonitors do
     Repo.get_by(BeaconMonitor, token: token)
   end
 
+  @doc """
+  Stamps `last_seen_at` to the given timestamp (defaults to "now") for
+  the supplied monitor. Used by the measurement ingest endpoint as
+  effectively a heartbeat — every accepted upload nudges the monitor's
+  last-seen marker forward.
+  """
+  @spec touch_last_seen(BeaconMonitor.t(), DateTime.t() | nil) :: {non_neg_integer(), nil}
+  def touch_last_seen(%BeaconMonitor{id: id}, at \\ nil) do
+    at = DateTime.truncate(at || DateTime.utc_now(), :second)
+
+    Repo.update_all(
+      from(m in BeaconMonitor, where: m.id == ^id),
+      set: [last_seen_at: at]
+    )
+  end
+
   @doc """
   Returns a blank changeset for rendering the new-monitor form.
   """
diff --git a/lib/microwaveprop_web/api/monitor_auth.ex b/lib/microwaveprop_web/api/monitor_auth.ex
new file mode 100644
index 00000000..6dda99a6
--- /dev/null
+++ b/lib/microwaveprop_web/api/monitor_auth.ex
@@ -0,0 +1,71 @@
+defmodule MicrowavepropWeb.Api.MonitorAuth do
+  @moduledoc """
+  Bearer-token authentication for propmonitor uploads.
+
+  Resolves `Authorization: Bearer <token>` against
+  `Microwaveprop.BeaconMonitors.get_monitor_by_token/1`, separate from
+  the user-API-token plug at `MicrowavepropWeb.Api.Auth`. Monitor tokens
+  are issued via `POST /api/v1/me/beacon-monitors` and identify a
+  monitor station, not a user session.
+
+  On success, assigns:
+
+    * `:current_monitor` — the `BeaconMonitor` struct
+    * `:current_api_token` — `{:monitor, monitor_id}` so the
+      rate-limiter buckets the monitor by id rather than IP.
+
+  On failure, halts with a 401 `application/problem+json` response.
+  """
+
+  @behaviour Plug
+
+  import Plug.Conn
+
+  alias Microwaveprop.BeaconMonitors
+  alias MicrowavepropWeb.Api.ErrorJSON
+
+  @impl true
+  def init(opts), do: opts
+
+  @impl true
+  def call(conn, _opts) do
+    case extract_bearer(conn) do
+      {:ok, token} ->
+        case BeaconMonitors.get_monitor_by_token(token) do
+          nil -> halt_unauthorized(conn, "Bearer token is invalid, expired, or revoked.")
+          monitor -> assign_monitor(conn, monitor)
+        end
+
+      {:error, :missing_token} ->
+        halt_unauthorized(conn, "Missing bearer token in Authorization header.")
+
+      {:error, :invalid_authorization_header} ->
+        halt_unauthorized(conn, "Authorization header must be `Bearer <token>`.")
+    end
+  end
+
+  defp extract_bearer(conn) do
+    case get_req_header(conn, "authorization") do
+      ["Bearer " <> token] when byte_size(token) > 0 -> {:ok, token}
+      ["bearer " <> token] when byte_size(token) > 0 -> {:ok, token}
+      [] -> {:error, :missing_token}
+      _ -> {:error, :invalid_authorization_header}
+    end
+  end
+
+  defp assign_monitor(conn, monitor) do
+    # The rate-limiter buckets on `current_api_token.id`. Map the
+    # monitor onto that shape so retries from one monitor count
+    # against a per-monitor bucket, not the shared anon-IP bucket
+    # (which would punish co-located stations behind one NAT).
+    conn
+    |> assign(:current_monitor, monitor)
+    |> assign(:current_api_token, %{id: {:monitor, monitor.id}})
+  end
+
+  defp halt_unauthorized(conn, detail) do
+    conn
+    |> put_resp_header("www-authenticate", ~s(Bearer realm="api"))
+    |> ErrorJSON.send_problem(401, "unauthorized", detail)
+  end
+end
diff --git a/lib/microwaveprop_web/controllers/api/v1/beacon_monitor_measurement_controller.ex b/lib/microwaveprop_web/controllers/api/v1/beacon_monitor_measurement_controller.ex
new file mode 100644
index 00000000..e2630b63
--- /dev/null
+++ b/lib/microwaveprop_web/controllers/api/v1/beacon_monitor_measurement_controller.ex
@@ -0,0 +1,73 @@
+defmodule MicrowavepropWeb.Api.V1.BeaconMonitorMeasurementController do
+  @moduledoc """
+  Ingest endpoint for `propmonitor` clients.
+
+  Authenticated by `MicrowavepropWeb.Api.MonitorAuth`: the Bearer token
+  resolves to a `BeaconMonitor`, and the JSON body carries the
+  `beacon_id` plus signal/noise/SNR fields for one integration window.
+
+  Success returns `204 No Content`. Failures match the propmonitor
+  status-classification table in `output.md`:
+
+    * `401` — missing/invalid monitor token (auth plug handles this)
+    * `404` — unknown / unapproved / off-air `beacon_id`
+    * `422` — malformed body
+    * `409` — duplicate `(monitor, beacon, measured_at)` (retried POST)
+  """
+
+  use Phoenix.Controller, formats: [:json]
+
+  alias Microwaveprop.BeaconMeasurements
+  alias Microwaveprop.BeaconMonitors
+  alias MicrowavepropWeb.Api.ErrorJSON
+
+  plug :accepts, ["json"]
+
+  @wire_fields ~w(beacon_id frequency_hz measured_at integration_s passband_hz
+                  gain_db noise_floor_dbfs signal_peak_dbfs signal_avg_dbfs
+                  snr_peak_db snr_avg_db signal_active_fraction
+                  propmonitor_version)
+
+  def create(conn, params) do
+    monitor = conn.assigns.current_monitor
+    attrs = Map.take(params, @wire_fields)
+
+    case BeaconMeasurements.create_measurement(monitor, attrs) do
+      {:ok, _measurement} ->
+        # Heartbeat: every accepted POST nudges last_seen_at forward.
+        # propmonitor doesn't send a separate keepalive.
+        _ = BeaconMonitors.touch_last_seen(monitor)
+
+        conn
+        |> send_resp(204, "")
+        |> halt()
+
+      {:error, :beacon_not_found} ->
+        ErrorJSON.send_problem(
+          conn,
+          404,
+          "not_found",
+          "beacon_id is unknown, unapproved, or marked off-the-air."
+        )
+
+      {:error, %Ecto.Changeset{} = changeset} ->
+        if duplicate?(changeset) do
+          ErrorJSON.send_problem(
+            conn,
+            409,
+            "conflict",
+            "A measurement for this monitor, beacon, and measured_at already exists."
+          )
+        else
+          ErrorJSON.send_changeset(conn, changeset)
+        end
+    end
+  end
+
+  defp duplicate?(%Ecto.Changeset{errors: errors}) do
+    Enum.any?(errors, fn
+      {_field, {_msg, opts}} -> Keyword.get(opts, :constraint) == :unique
+      _ -> false
+    end)
+  end
+end
diff --git a/lib/microwaveprop_web/router.ex b/lib/microwaveprop_web/router.ex
index 2ac2f2dc..d57cf602 100644
--- a/lib/microwaveprop_web/router.ex
+++ b/lib/microwaveprop_web/router.ex
@@ -6,6 +6,7 @@ defmodule MicrowavepropWeb.Router do
   import Phoenix.LiveDashboard.Router
 
   alias MicrowavepropWeb.Api.Auth
+  alias MicrowavepropWeb.Api.MonitorAuth
   alias MicrowavepropWeb.Api.RateLimiter
   alias MicrowavepropWeb.Api.V1
 
@@ -161,6 +162,16 @@ defmodule MicrowavepropWeb.Router do
     plug RateLimiter, anon_limit: 30
   end
 
+  # propmonitor beacon-measurement uploads. Bearer token resolves to a
+  # `BeaconMonitor` rather than a user. One POST per integration window
+  # (default 60 s); allow a comfortable burst headroom for retry storms
+  # after a 5xx — well above the steady-state 1/min/monitor rate.
+  pipeline :api_v1_monitor do
+    plug :accepts, ["json"]
+    plug MonitorAuth
+    plug RateLimiter, auth_limit: 60
+  end
+
   # Health checks — no pipeline, minimal overhead.
   # /live = liveness, BEAM only. /health = readiness, also pings Repo.
   # Pods that serve a BEAM-alive /live but a failing /health get taken
@@ -297,6 +308,14 @@ defmodule MicrowavepropWeb.Router do
     post "/beacons", BeaconController, :create
   end
 
+  # propmonitor uploads. Separate pipeline because the Bearer here is a
+  # `BeaconMonitor` token, not a user API token.
+  scope "/api/v1", V1 do
+    pipe_through :api_v1_monitor
+
+    post "/beacon-monitor/measurements", BeaconMonitorMeasurementController, :create
+  end
+
   scope "/" do
     pipe_through [:browser, :require_authenticated_user]
 
diff --git a/priv/repo/migrations/20260513191924_create_beacon_measurements.exs b/priv/repo/migrations/20260513191924_create_beacon_measurements.exs
new file mode 100644
index 00000000..e85a8d28
--- /dev/null
+++ b/priv/repo/migrations/20260513191924_create_beacon_measurements.exs
@@ -0,0 +1,43 @@
+defmodule Microwaveprop.Repo.Migrations.CreateBeaconMeasurements do
+  use Ecto.Migration
+
+  def change do
+    create table(:beacon_measurements, primary_key: false) do
+      add :id, :binary_id, primary_key: true
+
+      add :beacon_id, references(:beacons, type: :binary_id, on_delete: :delete_all), null: false
+
+      add :monitor_id, references(:beacon_monitors, type: :binary_id, on_delete: :delete_all),
+        null: false
+
+      add :measured_at, :utc_datetime, null: false
+      add :frequency_hz, :bigint, null: false
+      add :integration_s, :integer, null: false
+      add :passband_hz, :float, null: false
+      add :gain_db, :float, null: false
+
+      add :noise_floor_dbfs, :float, null: false
+      add :signal_peak_dbfs, :float, null: false
+      add :signal_avg_dbfs, :float, null: false
+      add :snr_peak_db, :float, null: false
+      add :snr_avg_db, :float, null: false
+      add :signal_active_fraction, :float, null: false
+
+      add :propmonitor_version, :string, null: false
+
+      timestamps(type: :utc_datetime)
+    end
+
+    # Time-series query patterns: latest-N for one monitor, latest-N for one
+    # beacon, and joint monitor+beacon trends.
+    create index(:beacon_measurements, [:monitor_id, "measured_at DESC"])
+    create index(:beacon_measurements, [:beacon_id, "measured_at DESC"])
+
+    # Idempotency: the client retries on 5xx with the same `measured_at`. A
+    # given monitor cannot legitimately produce two distinct measurements for
+    # the same beacon at the same start-of-window timestamp.
+    create unique_index(:beacon_measurements, [:monitor_id, :beacon_id, :measured_at],
+             name: :beacon_measurements_monitor_beacon_measured_at_idx
+           )
+  end
+end
diff --git a/test/microwaveprop/beacon_measurements_test.exs b/test/microwaveprop/beacon_measurements_test.exs
new file mode 100644
index 00000000..c579ac42
--- /dev/null
+++ b/test/microwaveprop/beacon_measurements_test.exs
@@ -0,0 +1,139 @@
+defmodule Microwaveprop.BeaconMeasurementsTest do
+  use Microwaveprop.DataCase
+
+  import Microwaveprop.AccountsFixtures
+  import Microwaveprop.BeaconsFixtures
+
+  alias Microwaveprop.BeaconMeasurements
+  alias Microwaveprop.BeaconMeasurements.BeaconMeasurement
+  alias Microwaveprop.BeaconMonitors
+  alias Microwaveprop.Beacons
+
+  defp setup_actors(_) do
+    user = user_fixture()
+    {:ok, monitor} = BeaconMonitors.create_monitor(user, %{"name" => "Shack Pi"})
+
+    beacon = beacon_fixture(user)
+    {:ok, beacon} = Beacons.approve_beacon(beacon)
+
+    %{user: user, monitor: monitor, beacon: beacon}
+  end
+
+  defp wire_attrs(beacon, overrides \\ %{}) do
+    Map.merge(
+      %{
+        "beacon_id" => beacon.id,
+        "frequency_hz" => 28_330_000,
+        "measured_at" => "2026-05-13T15:30:00Z",
+        "integration_s" => 60,
+        "passband_hz" => 300.0,
+        "gain_db" => 10.0,
+        "noise_floor_dbfs" => -110.2,
+        "signal_peak_dbfs" => -88.4,
+        "signal_avg_dbfs" => -89.1,
+        "snr_peak_db" => 21.8,
+        "snr_avg_db" => 21.1,
+        "signal_active_fraction" => 0.48,
+        "propmonitor_version" => "0.1.0"
+      },
+      overrides
+    )
+  end
+
+  describe "create_measurement/2" do
+    setup :setup_actors
+
+    test "inserts a measurement for an approved on-air beacon", %{monitor: m, beacon: b} do
+      assert {:ok, %BeaconMeasurement{} = meas} =
+               BeaconMeasurements.create_measurement(m, wire_attrs(b))
+
+      assert meas.monitor_id == m.id
+      assert meas.beacon_id == b.id
+      assert meas.frequency_hz == 28_330_000
+      assert meas.signal_active_fraction == 0.48
+      assert meas.propmonitor_version == "0.1.0"
+    end
+
+    test "returns :beacon_not_found for an unknown beacon UUID", %{monitor: m} do
+      attrs = wire_attrs(%{id: "00000000-0000-0000-0000-000000000000"})
+
+      assert {:error, :beacon_not_found} =
+               BeaconMeasurements.create_measurement(m, attrs)
+    end
+
+    test "returns :beacon_not_found for a malformed beacon UUID", %{monitor: m, beacon: b} do
+      attrs = b |> wire_attrs() |> Map.put("beacon_id", "not-a-uuid")
+      assert {:error, :beacon_not_found} = BeaconMeasurements.create_measurement(m, attrs)
+    end
+
+    test "returns :beacon_not_found for an unapproved beacon", %{monitor: m, user: u} do
+      pending = beacon_fixture(u, callsign: "K5HOLD")
+      attrs = wire_attrs(pending)
+
+      assert {:error, :beacon_not_found} =
+               BeaconMeasurements.create_measurement(m, attrs)
+    end
+
+    test "returns :beacon_not_found for a beacon marked off-the-air", %{monitor: m, beacon: b} do
+      {:ok, _} = Beacons.update_beacon(b, %{"on_the_air" => false})
+      assert {:error, :beacon_not_found} = BeaconMeasurements.create_measurement(m, wire_attrs(b))
+    end
+
+    test "rejects out-of-range signal_active_fraction", %{monitor: m, beacon: b} do
+      attrs = wire_attrs(b, %{"signal_active_fraction" => 1.5})
+      assert {:error, changeset} = BeaconMeasurements.create_measurement(m, attrs)
+      assert %{signal_active_fraction: [_ | _]} = errors_on(changeset)
+    end
+
+    test "rejects integration_s below the floor", %{monitor: m, beacon: b} do
+      attrs = wire_attrs(b, %{"integration_s" => 1})
+      assert {:error, changeset} = BeaconMeasurements.create_measurement(m, attrs)
+      assert %{integration_s: [_ | _]} = errors_on(changeset)
+    end
+
+    test "rejects missing required fields", %{monitor: m, beacon: b} do
+      attrs = Map.delete(wire_attrs(b), "noise_floor_dbfs")
+      assert {:error, changeset} = BeaconMeasurements.create_measurement(m, attrs)
+      assert %{noise_floor_dbfs: ["can't be blank"]} = errors_on(changeset)
+    end
+
+    test "duplicate (monitor, beacon, measured_at) returns a unique-constraint error",
+         %{monitor: m, beacon: b} do
+      attrs = wire_attrs(b)
+      assert {:ok, _} = BeaconMeasurements.create_measurement(m, attrs)
+
+      assert {:error, %Ecto.Changeset{} = changeset} =
+               BeaconMeasurements.create_measurement(m, attrs)
+
+      assert Enum.any?(changeset.errors, fn {_, {_, opts}} ->
+               Keyword.get(opts, :constraint) == :unique
+             end)
+    end
+
+    test "two monitors can report for the same beacon at the same instant",
+         %{monitor: m, beacon: b, user: u} do
+      {:ok, m2} = BeaconMonitors.create_monitor(u, %{"name" => "Other Pi"})
+      attrs = wire_attrs(b)
+      assert {:ok, _} = BeaconMeasurements.create_measurement(m, attrs)
+      assert {:ok, _} = BeaconMeasurements.create_measurement(m2, attrs)
+    end
+  end
+
+  describe "list_recent_for_beacon/2 and list_recent_for_monitor/2" do
+    setup :setup_actors
+
+    test "returns measurements newest first", %{monitor: m, beacon: b} do
+      a1 = wire_attrs(b, %{"measured_at" => "2026-05-13T15:30:00Z"})
+      a2 = wire_attrs(b, %{"measured_at" => "2026-05-13T15:31:00Z"})
+
+      {:ok, _} = BeaconMeasurements.create_measurement(m, a1)
+      {:ok, _} = BeaconMeasurements.create_measurement(m, a2)
+
+      [latest, prev] = BeaconMeasurements.list_recent_for_beacon(b.id, 10)
+      assert latest.measured_at > prev.measured_at
+
+      [latest_m, _] = BeaconMeasurements.list_recent_for_monitor(m.id, 10)
+      assert latest_m.measured_at == latest.measured_at
+    end
+  end
+end
diff --git a/test/microwaveprop_web/controllers/api/v1/beacon_monitor_measurement_controller_test.exs b/test/microwaveprop_web/controllers/api/v1/beacon_monitor_measurement_controller_test.exs
new file mode 100644
index 00000000..96259c0e
--- /dev/null
+++ b/test/microwaveprop_web/controllers/api/v1/beacon_monitor_measurement_controller_test.exs
@@ -0,0 +1,127 @@
+defmodule MicrowavepropWeb.Api.V1.BeaconMonitorMeasurementControllerTest do
+  use MicrowavepropWeb.ConnCase, async: true
+
+  import Microwaveprop.AccountsFixtures
+  import Microwaveprop.BeaconsFixtures
+
+  alias Microwaveprop.BeaconMeasurements
+  alias Microwaveprop.BeaconMonitors
+  alias Microwaveprop.Beacons
+  alias Microwaveprop.Repo
+  alias MicrowavepropWeb.Api.RateLimiter
+
+  @path "/api/v1/beacon-monitor/measurements"
+
+  setup %{conn: conn} do
+    RateLimiter.reset()
+    user = user_fixture()
+    {:ok, monitor} = BeaconMonitors.create_monitor(user, %{"name" => "Shack Pi"})
+    beacon = beacon_fixture(user)
+    {:ok, beacon} = Beacons.approve_beacon(beacon)
+    %{conn: conn, user: user, monitor: monitor, beacon: beacon}
+  end
+
+  defp wire_body(beacon, overrides \\ %{}) do
+    Map.merge(
+      %{
+        "beacon_id" => beacon.id,
+        "frequency_hz" => 28_330_000,
+        "measured_at" => "2026-05-13T15:30:00Z",
+        "integration_s" => 60,
+        "passband_hz" => 300.0,
+        "gain_db" => 10.0,
+        "noise_floor_dbfs" => -110.2,
+        "signal_peak_dbfs" => -88.4,
+        "signal_avg_dbfs" => -89.1,
+        "snr_peak_db" => 21.8,
+        "snr_avg_db" => 21.1,
+        "signal_active_fraction" => 0.48,
+        "propmonitor_version" => "0.1.0"
+      },
+      overrides
+    )
+  end
+
+  describe "POST /api/v1/beacon-monitor/measurements" do
+    test "401 when no Authorization header is supplied", %{conn: conn, beacon: b} do
+      conn = post(conn, @path, wire_body(b))
+      assert response(conn, 401)
+    end
+
+    test "401 when bearer token is unknown", %{conn: conn, beacon: b} do
+      conn =
+        conn
+        |> put_req_header("authorization", "Bearer not-a-real-token")
+        |> post(@path, wire_body(b))
+
+      assert response(conn, 401)
+    end
+
+    test "204 on a valid upload, persists row, and stamps last_seen_at",
+         %{conn: conn, monitor: m, beacon: b} do
+      conn =
+        conn
+        |> put_req_header("authorization", "Bearer " <> m.token)
+        |> post(@path, wire_body(b))
+
+      assert response(conn, 204) == ""
+
+      [row] = BeaconMeasurements.list_recent_for_monitor(m.id, 10)
+      assert row.beacon_id == b.id
+      assert row.signal_active_fraction == 0.48
+
+      assert %{last_seen_at: %DateTime{}} = Repo.reload!(m)
+    end
+
+    test "404 when beacon_id is unknown", %{conn: conn, monitor: m} do
+      body = wire_body(%{id: "00000000-0000-0000-0000-000000000000"})
+
+      conn =
+        conn
+        |> put_req_header("authorization", "Bearer " <> m.token)
+        |> post(@path, body)
+
+      assert json_response(conn, 404)["title"] == "not_found"
+    end
+
+    test "404 when beacon is unapproved", %{conn: conn, monitor: m, user: u} do
+      pending = beacon_fixture(u, callsign: "K5HOLD")
+
+      conn =
+        conn
+        |> put_req_header("authorization", "Bearer " <> m.token)
+        |> post(@path, wire_body(pending))
+
+      assert json_response(conn, 404)
+    end
+
+    test "422 when body is missing required fields", %{conn: conn, monitor: m, beacon: b} do
+      body = Map.delete(wire_body(b), "noise_floor_dbfs")
+
+      conn =
+        conn
+        |> put_req_header("authorization", "Bearer " <> m.token)
+        |> post(@path, body)
+
+      body_json = json_response(conn, 422)
+      assert body_json["title"] == "validation_failed"
+      assert body_json["errors"]["noise_floor_dbfs"]
+    end
+
+    test "409 on duplicate (monitor, beacon, measured_at) retry", %{conn: conn, monitor: m, beacon: b} do
+      conn1 =
+        build_conn()
+        |> put_req_header("authorization", "Bearer " <> m.token)
+        |> post(@path, wire_body(b))
+
+      assert response(conn1, 204)
+
+      conn2 =
+        conn
+        |> put_req_header("authorization", "Bearer " <> m.token)
+        |> post(@path, wire_body(b))
+
+      assert json_response(conn2, 409)["title"] == "conflict"
+    end
+  end
+end