[fix](trx-wspr): reduce false positives with stricter validation

Restrict accepted power levels to the 19 valid WSPR values instead of
any 0-60. Require a digit at position 1 or 2 of the trimmed callsign
per the WSPR encoding rules. Skip candidates whose sync correlation
score falls below a minimum threshold before attempting Fano decode.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Signed-off-by: Stanislaw Grams <stanislawgrams@gmail.com>

src/decoders/trx-wspr/src/decoder.rs

@@ -25,6 +25,11 @@ const DT_SEARCH_STEP_SAMPLES: isize = (WSPR_SAMPLE_RATE as isize) / 2;
 // Number of top frequency candidates to try full decode on
 const MAX_FREQ_CANDIDATES: usize = 8;
 
+// Minimum sync correlation score to attempt a full decode.  Candidates below
+// this threshold are almost certainly noise and skipping them avoids expensive
+// Fano decode attempts that would produce false positives.
+const MIN_SYNC_SCORE: f32 = 10.0;
+
 /// WSPR sync vector (162 bits). symbol = sync[i] + 2*data[i].
 /// The LSB of each received symbol should match this pattern.
 #[rustfmt::skip]
@@ -123,7 +128,10 @@ impl WsprDecoder {
         let mut results = Vec::new();
         let mut seen_messages = std::collections::HashSet::new();
 
-        for &(freq, dt_samples, _) in candidates.iter().take(MAX_FREQ_CANDIDATES) {
+        for &(freq, dt_samples, score) in candidates.iter().take(MAX_FREQ_CANDIDATES) {
+            if score < MIN_SYNC_SCORE {
+                break; // candidates are sorted by score, no point continuing
+            }
             let start = (EXPECTED_SIGNAL_START_SAMPLES as isize + dt_samples) as usize;
             let signal = &samples[start..start + WSPR_SIGNAL_SAMPLES];
 

src/decoders/trx-wspr/src/protocol.rs

@@ -141,9 +141,12 @@ fn unpack_message(bits: &[u8; NBITS]) -> Option<String> {
         power_code = (power_code << 1) | b as u32;
     }
 
-    // power_code is the raw dBm value; valid WSPR levels are 0–60 dBm.
+    // WSPR only permits specific power levels (dBm).
+    const VALID_POWER: [i32; 19] = [
+        0, 3, 7, 10, 13, 17, 20, 23, 27, 30, 33, 37, 40, 43, 47, 50, 53, 57, 60,
+    ];
     let power_dbm = power_code as i32;
-    if !(0..=60).contains(&power_dbm) {
+    if !VALID_POWER.contains(&power_dbm) {
         return None;
     }
 
@@ -182,9 +185,23 @@ fn unpack_message(bits: &[u8; NBITS]) -> Option<String> {
     .trim()
     .to_string();
 
+    // WSPR callsigns: after trimming, the digit (from position 2 of the
+    // 6-char padded form) must appear at index 1 or 2.  The callsign must
+    // also contain at least one letter and be at least 3 characters long.
     if callsign.len() < 3 || !callsign.chars().any(|c| c.is_alphabetic()) {
         return None;
     }
+    let has_digit_at_1_or_2 = callsign
+        .chars()
+        .nth(1)
+        .is_some_and(|c| c.is_ascii_digit())
+        || callsign
+            .chars()
+            .nth(2)
+            .is_some_and(|c| c.is_ascii_digit());
+    if !has_digit_at_1_or_2 {
+        return None;
+    }
 
     // Decode Maidenhead grid from M1.
     // M1 = (179 - 10*loc1 - loc3)*180 + 10*loc2 + loc4