diff --git a/spatz/metrics/max_dev.py b/spatz/metrics/max_dev.py
new file mode 100644
index 0000000..2f465bd
--- /dev/null
+++ b/spatz/metrics/max_dev.py
@@ -0,0 +1,20 @@
+from numpy.typing import ArrayLike
+from spatz.metrics import Metric
+
+
+class MaxAbsDeviation(Metric):
+    def __init__(self) -> None:
+        """A metric tracking the maximum absolute deviation from the true value."""
+        super().__init__()
+
+    def _update(self, x: ArrayLike, y: ArrayLike):
+        self._score = max(self._score, abs(x - y))
+
+
+class MaxRelDeviation(Metric):
+    def __init__(self) -> None:
+        """A metric tracking the maximum deviation from the true value in percent."""
+        super().__init__()
+
+    def _update(self, x: ArrayLike, y: ArrayLike):
+        self._score = max(self._score, (x - y) / y)
diff --git a/spatz/metrics/metric.py b/spatz/metrics/metric.py
index 6ebf913..869e93f 100644
--- a/spatz/metrics/metric.py
+++ b/spatz/metrics/metric.py
@@ -1,14 +1,19 @@
+from abc import abstractmethod
 
-
+from numpy.typing import ArrayLike
 from typing import Any
 
 
 class Metric:
     def __init__(self) -> None:
-        self.__sum = 0
+        self._score = None
 
-    def get_score():
-        pass
+    def get_score(self):
+        return self._score
 
-    def __call__(self, *args) -> Any:
-        self.__sum += abs(x - y)
\ No newline at end of file
+    @abstractmethod
+    def _update(self, x: ArrayLike, y: ArrayLike):
+        raise NotImplementedError()
+
+    def __call__(self, x: ArrayLike, y: ArrayLike) -> Any:
+        self._update(x, y)
\ No newline at end of file
diff --git a/spatz/metrics/mse.py b/spatz/metrics/mse.py
new file mode 100644
index 0000000..4d43b5b
--- /dev/null
+++ b/spatz/metrics/mse.py
@@ -0,0 +1,15 @@
+from numpy.typing import ArrayLike
+from spatz.metrics import Metric
+
+
+class MSEMetric(Metric):
+    def __init__(self) -> None:
+        """Mean squared error
+        """
+        super().__init__()
+
+    def _update(self, x: ArrayLike, y: ArrayLike):
+        if self._score is None:
+            self._score = 0.5 * (x - y)**2
+        else:
+            self._score += 0.5 * (x - y)**2
diff --git a/spatz/sensors/gps/erinome1.py b/spatz/sensors/gps/erinome1.py
index 9a9483b..0b1d48a 100644
--- a/spatz/sensors/gps/erinome1.py
+++ b/spatz/sensors/gps/erinome1.py
@@ -14,4 +14,6 @@ class Erinome_I(GPS):
     def _sensor_specific_effects(self, x: ArrayLike) -> ArrayLike:
         # TODO: What's the GPS module's behavior?
 
+        # TODO: Only return measurements every second
+
         return x
\ No newline at end of file
diff --git a/spatz/sensors/gps/gps.py b/spatz/sensors/gps/gps.py
index 99b58e6..37b8c0b 100644
--- a/spatz/sensors/gps/gps.py
+++ b/spatz/sensors/gps/gps.py
@@ -16,7 +16,7 @@ class GPS(Sensor):
         """GPS Module which provides the following information:
         - Longitude (in °)
         - Latitiude (in °)
-        - Altitude (in °)
+        - Altitude (in m)
         """
         super().__init__(dataset, logger, transforms)