diff --git a/src/backscattering_analyzer/modelisation/__init__.py b/src/backscattering_analyzer/modelisation/__init__.py
new file mode 100644
index 0000000..22fefdf
--- /dev/null
+++ b/src/backscattering_analyzer/modelisation/__init__.py
@@ -0,0 +1,13 @@
+from backscattering_analyzer.modelisation.bench import Bench
+
+
+class Modelisation:
+    def __init__(
+        self,
+        injection_power: float,
+        dark_fringe_power: float,
+        benches: list[Bench],
+    ):
+        self.injection_power = injection_power
+        self.dark_fringe_power = dark_fringe_power
+        self.benches = benches
diff --git a/src/backscattering_analyzer/modelisation/bench.py b/src/backscattering_analyzer/modelisation/bench.py
new file mode 100644
index 0000000..55067d2
--- /dev/null
+++ b/src/backscattering_analyzer/modelisation/bench.py
@@ -0,0 +1,10 @@
+from backscattering_analyzer.modelisation.transferfunction import (
+    TransferFunction,
+)
+
+
+class Bench:
+    def __init__(self, name: str, signal: TransferFunction, fsc: float):
+        self.name = name
+        self.signal = signal
+        self.fsc = fsc
diff --git a/src/backscattering_analyzer/modelisation/transferfunction.py b/src/backscattering_analyzer/modelisation/transferfunction.py
new file mode 100644
index 0000000..d603c37
--- /dev/null
+++ b/src/backscattering_analyzer/modelisation/transferfunction.py
@@ -0,0 +1,7 @@
+from gwpy.frequencyseries import FrequencySeries
+
+
+class TransferFunction:
+    def __init__(self, kn: FrequencySeries, kp: FrequencySeries):
+        self.kn = kn
+        self.kp = kp
diff --git a/src/backscattering_analyzer/projection.py b/src/backscattering_analyzer/projection.py
new file mode 100644
index 0000000..612236f
--- /dev/null
+++ b/src/backscattering_analyzer/projection.py
@@ -0,0 +1,41 @@
+from backscattering_analyzer.measurement import Measurement
+from backscattering_analyzer.modelisation import Modelisation
+from backscattering_analyzer.modelisation.bench import Bench
+from numpy import sin, cos, pi, sqrt
+from gwpy.frequencyseries import FrequencySeries
+
+
+class Projection:
+    def __init__(
+        self, measurement: Measurement, modelisation: Modelisation
+    ):
+        self.measurement = measurement
+        self.modelisation = modelisation
+        self.results: list[FrequencySeries] = []
+
+        for bench in self.measurement.benches:
+            modelisation_bench: None | Bench = None
+            for i in range(len(self.modelisation.benches)):
+                if bench.name == self.modelisation.benches[i].name:
+                    modelisation_bench = self.modelisation.benches[i]
+            if modelisation_bench is None:
+                raise Exception(
+                    "missing bench modelisation for {}".format(
+                        bench.name
+                    )
+                )
+            phase = 4 * pi / 1.064e-6
+            sin_side = sin(
+                bench.movement * phase
+            ).asd(
+                0.001
+            )  # FIXME: To change (0.001 s: what are 10 points for the movement rate ?)
+            cos_side = cos(bench.movement * phase).asd(0.001)
+            self.results.append(
+                sqrt(modelisation_bench.fsc)
+                / phase
+                * (
+                    modelisation_bench.signal.kn * sin_side
+                    + modelisation_bench.signal.kp * cos_side
+                )
+            )