diff --git a/api/lib/extrapolation.js b/api/lib/extrapolation.js
index 2a55993..f8411b8 100644
--- a/api/lib/extrapolation.js
+++ b/api/lib/extrapolation.js
@@ -16,7 +16,8 @@ function extrapolatePosition(aircraft, targetTimestamp) {
   
   const dx = velocityMs * Math.sin(trackRad) * dt;
   const dy = velocityMs * Math.cos(trackRad) * dt;
-  const dz = (aircraft.geom_rate || 0) * 0.00508 * dt;
+  // geom_rate is ft/min; convert to ft/s to keep altitude in feet
+  const dz = (aircraft.geom_rate || 0) / 60 * dt;
   
   const latRad = aircraft.lat * Math.PI / 180;
   const newLat = aircraft.lat + (dy / 111320);
diff --git a/api/lib/geometry.js b/api/lib/geometry.js
index 6cb7033..8aea293 100644
--- a/api/lib/geometry.js
+++ b/api/lib/geometry.js
@@ -22,10 +22,10 @@ function calculateBistaticDelay(aircraftPos, rxPos, txPos) {
   let dRxAircraft = haversineDistance(rxPos, aircraftPos);
   let dTxAircraft = haversineDistance(txPos, aircraftPos);
   let dRxTx = haversineDistance(rxPos, txPos);
-  
-  const altRx = rxPos.alt || 0;
-  const altTx = txPos.alt || 0;
-  const altAircraft = aircraftPos.alt || 0;
+
+  const altRx = rxPos.alt || 0;       // meters (from config)
+  const altTx = txPos.alt || 0;       // meters (from config)
+  const altAircraft = (aircraftPos.alt || 0) * 0.3048; // feet -> meters
   
   dRxAircraft = Math.sqrt(dRxAircraft**2 + (altAircraft - altRx)**2);
   dTxAircraft = Math.sqrt(dTxAircraft**2 + (altAircraft - altTx)**2);