BDS60 is invalid if CAS & IAS are too far apart or mach is given when…

… status is not set

src/main/java/aero/t2s/modes/decoder/df/bds/Bds60.java

@@ -1,6 +1,7 @@
 package aero.t2s.modes.decoder.df.bds;
 
 import aero.t2s.modes.Track;
+import aero.t2s.modes.decoder.AltitudeEncoding;
 
 public class Bds60 extends Bds {
  private static final double MAG_ACCURACY = 90.0 / 512.0;
@@ -51,9 +52,16 @@ public Bds60(short[] data) {
 
  mach = ((data[7] << 2) | data[8] >> 6) * MACH_ACCURACY;
  mach /= 1000.0;
- if (statusMach && (mach <= 0 || mach > 1)) {
- invalidate();
- return;
+ if (statusMach) {
+ if (mach <= 0 || mach > 1) {
+ invalidate();
+ return;
+ }
+ } else {
+ if (mach != 0) {
+ invalidate();
+ return;
+ }
  }
 
  double baroSign = ((data[8] >>> 4) & 0x1) == 1 ? -512.0 : 0.0;
@@ -69,6 +77,44 @@ public Bds60(short[] data) {
  invalidate();
  return;
  }
+
+ if (statusMach && statusIas && data[0] >>> 3 == 20) {
+ double altitude = AltitudeEncoding.decode((data[2] & 0x1F) << 8 | data[3]).getAltitude();
+ if (altitude > 0) {
+ double cas = machToCas(altitude * 0.3048) / 0.514444;
+ if (Math.abs(cas - ias) > 20) {
+ invalidate();
+ return;
+ }
+ }
+ }
+ }
+
+ private double machToCas(double altitude) {
+ return tasToCas(machToTas(altitude), altitude);
+ }
+
+ private double tasToCas(double tas, double altitude) {
+ double rho0 = 1.225;
+ double T = Math.max(288.15 - 0.0065 * altitude, 216.65);
+ double rhotrop = 1.225 * Math.pow((T / 288.15), 4.256848030018761);
+ double dhstrat = Math.max(0.0, altitude - 11000.0);
+ double rho = rhotrop * Math.exp(-dhstrat / 6341.552161);
+ double R = 287.05287; // m2/(s2 x K), gas constant, sea level ISA
+ double p = rho * R * T;
+ double p0 = 101325.0; // Pa, air pressure, sea level ISA
+
+ double qdyn = p * (Math.pow((1.0 + rho * tas * tas / (7.0 * p)), 3.5) - 1.0);
+ return Math.sqrt(7.0 * p0 / rho0 * (Math.pow((qdyn / p0 + 1.0), (2.0 / 7.0)) - 1.0));
+ }
+
+ private double machToTas(double altitude) {
+ double T = Math.max(288.15 - 0.0065 * altitude, 216.65);
+ double R = 287.05287; // m2/(s2 x K), gas constant, sea level ISA
+ double gamma = 1.40; // cp/cv for air
+ double a = Math.sqrt(gamma * R * T);
+
+ return mach * a;
  }