diff --git a/__tests__/sgp4.js b/__tests__/sgp4.js
index 122a72d..d6f4ff6 100644
--- a/__tests__/sgp4.js
+++ b/__tests__/sgp4.js
@@ -473,3 +473,17 @@ describe('getVisibleSatellites', () => {
         expect(allVisible.length).toEqual(1);
     });
 });
+
+describe('getSatelliteInfo observer defaults', () => {
+    test('respects an explicit observer at (0, 0, 0)', () => {
+        // A falsy check (`observerLat || default`) previously replaced an
+        // explicit 0 (equator / prime meridian / sea level) with the default
+        // observer, so an explicit origin observer must match a near-origin one.
+        const timestamp = 1501039265000;
+        const atOrigin = getSatelliteInfo(tleStr, timestamp, 0, 0, 0);
+        const nearOrigin = getSatelliteInfo(tleStr, timestamp, 1e-7, 1e-7, 1e-7);
+        expect(atOrigin.azimuth).toBeCloseTo(nearOrigin.azimuth, 4);
+        expect(atOrigin.elevation).toBeCloseTo(nearOrigin.elevation, 4);
+        expect(atOrigin.range).toBeCloseTo(nearOrigin.range, 2);
+    });
+});
diff --git a/src/sgp4.js b/src/sgp4.js
index 12587e9..daac5db 100644
--- a/src/sgp4.js
+++ b/src/sgp4.js
@@ -84,7 +84,7 @@ export function clearCache() {
  * TODO: return error instead of throwing?
  */
 export function getSatelliteInfo(rawTLE, rawTimestamp, observerLat, observerLng, observerHeight) {
-    const timestamp = rawTimestamp || Date.now();
+    const timestamp = rawTimestamp ?? Date.now();
 
     const { tle, error: parseError } = parseTLE(rawTLE);
 
@@ -98,9 +98,9 @@ export function getSatelliteInfo(rawTLE, rawTimestamp, observerLat, observerLng,
         height: 0.37,
     };
 
-    const obsLat = observerLat || defaultObserverPosition.lat;
-    const obsLng = observerLng || defaultObserverPosition.lng;
-    const obsHeight = observerHeight || defaultObserverPosition.height;
+    const obsLat = observerLat ?? defaultObserverPosition.lat;
+    const obsLng = observerLng ?? defaultObserverPosition.lng;
+    const obsHeight = observerHeight ?? defaultObserverPosition.height;
 
     // Memoization
     const cacheKey = `${tle[0]}-${timestamp}-${observerLat}-${observerLng}