first commit

api/app/core/diffraction.py

@@ -0,0 +1,71 @@
+from __future__ import annotations
+
+from math import log10, sqrt
+
+from app.core.fresnel import wavelength
+from app.core.surface import SurfaceProfile
+
+
+def knife_edge_loss(v: float) -> float:
+    if v <= -0.78:
+        return 0.0
+    return 6.9 + 20 * log10(sqrt((v - 0.1) ** 2 + 1) + v - 0.1)
+
+
+def knife_edge_v(h: float, d1: float, d2: float, freq_hz: float) -> float:
+    if d1 <= 0 or d2 <= 0:
+        return float("-inf")
+    lmbda = wavelength(freq_hz)
+    return h * sqrt(2 * (d1 + d2) / (lmbda * d1 * d2))
+
+
+def deygout(
+    profile: SurfaceProfile,
+    tx_height_agl: float,
+    rx_height_agl: float,
+    freq_hz: float,
+) -> float:
+    """Recursive Deygout diffraction loss using the dominant edge and subprofiles."""
+    if len(profile.samples) < 3:
+        return 0.0
+
+    endpoint_heights = {
+        0: profile.samples[0].ground_m + tx_height_agl,
+        len(profile.samples) - 1: profile.samples[-1].ground_m + rx_height_agl,
+    }
+
+    def sample_height(index: int) -> float:
+        return endpoint_heights.get(index, profile.samples[index].surface_m)
+
+    def solve(left: int, right: int) -> float:
+        if right - left < 2:
+            return 0.0
+
+        left_sample = profile.samples[left]
+        right_sample = profile.samples[right]
+        span_m = right_sample.distance_m - left_sample.distance_m
+        left_height = sample_height(left)
+        right_height = sample_height(right)
+        max_v = float("-inf")
+        max_index: int | None = None
+
+        for index in range(left + 1, right):
+            sample = profile.samples[index]
+            d1 = sample.distance_m - left_sample.distance_m
+            d2 = right_sample.distance_m - sample.distance_m
+            path_height = left_height + (right_height - left_height) * (d1 / span_m)
+            h = sample.surface_m - path_height
+            v = knife_edge_v(h, d1, d2, freq_hz)
+            if v > max_v:
+                max_v = v
+                max_index = index
+
+        if max_index is None:
+            return 0.0
+
+        main_loss = knife_edge_loss(max_v)
+        if main_loss == 0.0:
+            return 0.0
+        return main_loss + solve(left, max_index) + solve(max_index, right)
+
+    return solve(0, len(profile.samples) - 1)