diff --git a/gps_sync/tests/test_core.py b/gps_sync/tests/test_core.py
new file mode 100644
index 0000000..b7e7543
--- /dev/null
+++ b/gps_sync/tests/test_core.py
@@ -0,0 +1,65 @@
+import pytest
+from gps_sync import core
+
+@pytest.fixture
+def sample_gps_data():
+    return [
+        {"timestamp": "2024-05-01T12:00:00", "latitude": 48.5701, "longitude": 13.4551},
+        {"timestamp": "2024-05-01T12:00:05", "latitude": 48.5702, "longitude": 13.4552},
+        {"timestamp": "2024-05-01T12:00:10", "latitude": 48.5703, "longitude": 13.4553},
+    ]
+
+
+@pytest.fixture
+def sample_wifi_data():
+    return [
+        {"timestamp": "2024-05-01T12:00:01", "signal_strength": -55.0},
+        {"timestamp": "2024-05-01T12:00:06", "signal_strength": -60.0},
+        {"timestamp": "2024-05-01T12:00:11", "signal_strength": -65.0},
+    ]
+
+
+def test_sync_with_gps_nominal(sample_gps_data, sample_wifi_data):
+    result = core.sync_with_gps(sample_gps_data, sample_wifi_data)
+    assert isinstance(result, list)
+    assert len(result) == 3
+
+    first = result[0]
+    assert set(first.keys()) == {"timestamp", "latitude", "longitude", "signal_strength"}
+    assert isinstance(first["latitude"], float)
+    assert isinstance(first["longitude"], float)
+    assert isinstance(first["signal_strength"], float)
+
+
+def test_sync_with_gps_empty_inputs():
+    result = core.sync_with_gps([], [])
+    assert result == []
+
+
+def test_sync_with_gps_missing_fields():
+    gps_data = [{"timestamp": "2024-05-01T12:00:00", "latitude": 48.5}]  # missing longitude
+    wifi_data = [{"timestamp": "2024-05-01T12:00:00", "signal_strength": -50.0}]
+
+    with pytest.raises((KeyError, ValueError, TypeError)):
+        core.sync_with_gps(gps_data, wifi_data)
+
+
+def test_sync_with_gps_timestamps_mismatch(sample_gps_data):
+    wifi_data = [
+        {"timestamp": "2024-05-01T13:00:00", "signal_strength": -40.0},
+        {"timestamp": "2024-05-01T14:00:00", "signal_strength": -42.0},
+    ]
+    result = core.sync_with_gps(sample_gps_data, wifi_data)
+    # Depending on implementation, may return nearest or limited results
+    assert isinstance(result, list)
+    for entry in result:
+        assert 'signal_strength' in entry
+        assert isinstance(entry['signal_strength'], float)
+
+
+def test_sync_with_gps_data_integrity(sample_gps_data, sample_wifi_data):
+    result = core.sync_with_gps(sample_gps_data, sample_wifi_data)
+    for entry in result:
+        assert -100.0 <= entry['signal_strength'] <= 0.0
+        assert -90.0 <= entry['latitude'] <= 90.0
+        assert -180.0 <= entry['longitude'] <= 180.0
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