Guard SBAS Viterbi startup traceback

Wait until the continuous decoder has accumulated the requested traceback depth before advancing trellis iterators. Add a regression test for the 30-bit SBAS input block versus its 35-bit traceback depth.

Signed-off-by: miguekf <miguel.gomezlopez@uclm.es>

src/algorithms/telemetry_decoder/libs/viterbi_decoder_sbas.cc

@@ -101,6 +101,15 @@ float Viterbi_Decoder_Sbas::decode_continuous(const double sym[],
 
     // do add compare select
     do_acs(sym, nbits_requested);
+    // Keep accumulating trellis history until a complete traceback is
+    // available. do_traceback() can clamp the requested depth, but
+    // do_tb_and_decode() expects that same depth to be present and advances
+    // its deque iterators by traceback_depth.
+    if (traceback_depth > static_cast<int>(d_trellis_paths.size()))
+        {
+            nbits_decoded = 0;
+            return d_indicator_metric;
+        }
     // the ML sequence in the newest part of the trellis can not be decoded
     // since it depends on the future values -> traceback, but don't decode
     const int state = do_traceback(traceback_depth);

tests/unit-tests/signal-processing-blocks/telemetry_decoder/sbas_l1_telemetry_decoder_test.cc

@@ -31,7 +31,6 @@ TEST(SbasL1TelemetryDecoderTest, TimestampMatchesFirstBitWhenFullyAligned)
     EXPECT_DOUBLE_EQ(first_bit_stamp_s, message_stamp_s);
 }
 
-
 TEST(SbasL1TelemetryDecoderTest, TimestampCorrectionIsBoundedBySymbolPeriod)
 {
     // The correction is always a whole number of code periods, and spans at