Read an ADS-B vertical rate from one shared field reader

src/sources/adsbFields.js

@@ -0,0 +1,16 @@
+/** A readsb rate field: a finite JSON number, else null. */
+function rateField(value) {
+  return typeof value === 'number' && Number.isFinite(value) ? value : null;
+}
+
+/**
+ * Vertical rate in ft/min from a readsb-style aircraft row (readsb, tar1090,
+ * adsb.lol): the barometric `baro_rate`, else the GNSS-sourced `geom_rate`
+ * readsb sends instead when it has no barometric rate. A barometric 0 is kept.
+ * Only a finite number counts; null when neither field has one.
+ * @param {object|null|undefined} row
+ * @returns {number|null}
+ */
+export function readAdsbVerticalRateFpm(row) {
+  return rateField(row?.baro_rate) ?? rateField(row?.geom_rate);
+}

src/sources/adsbFields.test.mjs

@@ -0,0 +1,33 @@
+import assert from 'node:assert/strict';
+import test from 'node:test';
+import { readAdsbVerticalRateFpm } from './adsbFields.js';
+
+test('the barometric rate wins, a barometric 0 included', () => {
+  assert.equal(
+    readAdsbVerticalRateFpm({ baro_rate: -640, geom_rate: -832 }),
+    -640,
+  );
+  assert.equal(readAdsbVerticalRateFpm({ baro_rate: 0, geom_rate: 600 }), 0);
+});
+
+test('a GNSS rate stands in when no barometric rate is usable', () => {
+  for (const baro_rate of [
+    undefined,
+    null,
+    Number.NaN,
+    Infinity,
+    '300',
+    false,
+  ]) {
+    assert.equal(readAdsbVerticalRateFpm({ baro_rate, geom_rate: -832 }), -832);
+  }
+});
+
+test('no usable rate reads as null', () => {
+  assert.equal(readAdsbVerticalRateFpm({}), null);
+  assert.equal(readAdsbVerticalRateFpm(null), null);
+  assert.equal(
+    readAdsbVerticalRateFpm({ baro_rate: '300', geom_rate: true }),
+    null,
+  );
+});

src/sources/adsbRecords.js

@@ -30,6 +30,8 @@
  * This module is portable: no Cesium, DOM, network or storage access.
  */
 
+import { readAdsbVerticalRateFpm } from './adsbFields.js';
+
 /** A marker is dropped once its newest position is this old. */
 export const LOCAL_ADSB_POSITION_STALE_MS = 60_000;
 /** An aircraft is forgotten once no message has arrived for this long. */
@@ -247,9 +249,7 @@ export function normalizeDump1090Aircraft(json, nowMs, { band = '1090' } = {}) {
       altitudeFt: altitude,
       groundSpeedKt: finiteOrNull(entry.gs),
       trackDeg: normalizeTrack(entry.track),
-      // A GNSS-sourced rate is sent as geom_rate instead of baro_rate.
-      verticalRateFpm:
-        finiteOrNull(entry.baro_rate) ?? finiteOrNull(entry.geom_rate),
+      verticalRateFpm: readAdsbVerticalRateFpm(entry),
       lastPositionAt: hasPosition
         ? receivedAt - Math.max(0, seenPosS) * 1000
         : null,

src/sources/live/aircraft.js

@@ -6,6 +6,7 @@ import {
   epoch,
   finite,
 } from './contract.js';
+import { readAdsbVerticalRateFpm } from '../adsbFields.js';
 
 const openSkyId = (row) =>
   Array.isArray(row) ? cleanText(row[0]).toLowerCase() : '';
@@ -48,8 +49,7 @@ export function normalizeReadsbAircraft(row, snapshotTimeMs) {
   const baroFt = finite(row.alt_baro),
     geoFt = finite(row.alt_geom);
   const speedKts = finite(row.gs),
-    // readsb sends a GNSS-sourced rate as geom_rate instead of baro_rate.
-    rateFtMin = finite(row.baro_rate) ?? finite(row.geom_rate);
+    rateFtMin = readAdsbVerticalRateFpm(row);
   const seen = finite(row.seen),
     seenPos = finite(row.seen_pos);
   return {