fix(adcp): use signed int16 for AD2CP echo amplitude — fixes 500+ dB Sv bug

Raw echo_amplitude from Nortek Signature is signed int16 (0.01 dB per
count), not unsigned uint16. The unsigned interpretation produced Sv
values of 529-600 dB instead of the correct -130 to -79 dB range.

Changes:
- ad2cp_reader: parse echo data as '<i2' (signed), allocate int16 array
- sv_computation: update comments to reflect signed dtype

Verified against real AD2CP files — Sv now -130 to -79 dB (mean -95 dB).

Author: beatfactor

oceanstream/adcp/ad2cp_reader.py

@@ -181,7 +181,7 @@ def _parse_echo_data_record(data: bytes) -> Ad2cpEchoPing:
     roll = struct.unpack_from("<h", data, 28)[0] * 0.01
 
     echo_bytes = data[offset_of_data:]
-    amplitude = np.frombuffer(echo_bytes, dtype="<u2")
+    amplitude = np.frombuffer(echo_bytes, dtype="<i2")
 
     return Ad2cpEchoPing(
         time=time,
@@ -544,7 +544,7 @@ def read_ad2cp(path: Path | str) -> xr.Dataset:
     range_m = blanking + np.arange(n_cells) * cell_size
 
     # Separate pings by frequency (they alternate: freq0, freq1, freq0, ...)
-    amplitude = np.zeros((n_freqs, n_pings, n_cells), dtype=np.uint16)
+    amplitude = np.zeros((n_freqs, n_pings, n_cells), dtype=np.int16)
     times = np.empty(n_pings, dtype="datetime64[us]")
     sound_speed = np.empty(n_pings, dtype=np.float32)
     temperature = np.empty(n_pings, dtype=np.float32)
@@ -620,7 +620,7 @@ def read_ad2cp(path: Path | str) -> xr.Dataset:
                 {
                     "long_name": "Echo amplitude",
                     "units": "count",
-                    "comment": "Raw uint16 echo intensity with instrument TVG applied",
+                    "comment": "Raw int16 echo level (0.01 dB per count) with instrument TVG applied",
                 },
             ),
             "sound_speed": (

oceanstream/adcp/sv_computation.py

@@ -27,7 +27,7 @@ def compute_sv(
 
     The Nortek Signature echosounder applies TVG (Time-Varied Gain)
     internally before recording.  The raw ``echo_amplitude`` values
-    are proportional to signal level in 0.01 dB units, with geometric
+    are signed int16 counts at 0.01 dB per count, with geometric
     spreading and a nominal absorption already compensated.
 
     This function converts raw counts to Sv by applying the per-channel
@@ -55,15 +55,15 @@ def compute_sv(
     """
     import xarray as xr
 
-    amp = ds["echo_amplitude"].values  # (freq, ping, range) uint16
+    amp = ds["echo_amplitude"].values  # (freq, ping, range) int16
     cal_offsets = ds.attrs.get("cal_offsets_db", [0.0] * amp.shape[0])
     range_m = ds["range_sample"].values  # (range,)
 
     n_freqs = amp.shape[0]
     sv = np.full_like(amp, np.nan, dtype=np.float32)
 
     for fi in range(n_freqs):
-        # Convert raw counts to dB echo level
+        # Convert signed int16 counts to dB echo level (0.01 dB per count)
         el = amp[fi].astype(np.float32) * 0.01
 
         cal = cal_offsets[fi] if fi < len(cal_offsets) else 0.0