public void CompactPositionReportingCoordinate_Constructor_Initialises_To_Known_State()
{
var coordinate = new CompactPositionReportingCoordinate(1, 2, true, 4);
Assert.AreEqual(1, coordinate.Latitude);
Assert.AreEqual(2, coordinate.Longitude);
Assert.AreEqual(true, coordinate.OddFormat);
Assert.AreEqual((byte)4, coordinate.NumberOfBits);
}
public void CompactPositionReportingCoordinate_Default_Constructor_Initialises_To_Known_State_And_Properties_Work()
{
var coordinate = new CompactPositionReportingCoordinate();
TestUtilities.TestProperty(coordinate, r => r.Latitude, 0, 0x10000);
TestUtilities.TestProperty(coordinate, r => r.Longitude, 0, 0x10000);
TestUtilities.TestProperty(coordinate, r => r.OddFormat, false);
TestUtilities.TestProperty(coordinate, r => r.NumberOfBits, (byte)0, (byte)19);
}
/// <summary>
/// Extracts the fields describing a Compact Position Reporting coordinate in a message.
/// </summary>
/// <param name="message"></param>
/// <param name="encodingBits"></param>
/// <returns></returns>
private CompactPositionReportingCoordinate ExtractCprCoordinate(AdsbMessage message, byte encodingBits)
{
CompactPositionReportingCoordinate result = null;
if(message.Type != 0) {
bool isOddFormat = _BitStream.ReadBit();
int latitude = (int)_BitStream.ReadUInt32(17);
int longitude = (int)_BitStream.ReadUInt32(17);
result = new CompactPositionReportingCoordinate(latitude, longitude, isOddFormat, encodingBits);
}
return result;
}
/// <summary>
/// See interface docs.
/// </summary>
/// <param name="cprCoordinate"></param>
/// <param name="referenceCoordinate"></param>
/// <returns></returns>
/// <remarks><para>
/// It was found in testing that encoded latitudes of zero are incredibly sensitive to rounding errors introduced by the use of doubles. This
/// can be fixed by using decimals, which have much greater precision, and rounding to a very large number of decimal places - but using decimals
/// is bad news, they are a lot slower than doubles because all of the operations are done in software, there is no hardware FPU support for them.
/// 10 million divisions in LINQPad took 58 seconds on my development machine when using decimals and less than a second using doubles.
/// </para><para>
/// However it's only on the boundary between zones where rounding errors cause huge problems (e.g. returning a longitude of -87 instead of -90).
/// The problem revolves around the selection of the wrong value of m, rounding errors in doubles at boundaries between zones can push m out by
/// one, placing the longitude into the wrong zone and getting a longitude that it out by miles. Once the longitude is above zero rounding errors
/// can have an effect but the correct m is still selected and they only produce inaccuracies of a few feet. So the compromise was to use decimals
/// when accuracy is paramount - i.e. when the encoded longitude is 0 - and doubles for the rest of the time.
/// </para></remarks>
public GlobalCoordinate LocalDecode(CompactPositionReportingCoordinate cprCoordinate, GlobalCoordinate referenceCoordinate)
{
var result = new GlobalCoordinate();
var maxResult = MaxResultSize(cprCoordinate.NumberOfBits == 19 ? 17 : cprCoordinate.NumberOfBits);
var dlat = cprCoordinate.NumberOfBits == 19 ? cprCoordinate.OddFormat ? DLatOdd19Bit : DLatEven19Bit : cprCoordinate.OddFormat ? DLatOdd : DLatEven;
var dlonNumerator = cprCoordinate.NumberOfBits == 19 ? 90.0 : 360.0;
var refLat = referenceCoordinate.Latitude;
var encodedLatitudeSlice = (double)cprCoordinate.Latitude / maxResult;
var j = Math.Floor(refLat / dlat) + Math.Floor(0.5 + (CircleModulus(refLat, dlat) / dlat) - encodedLatitudeSlice);
result.Latitude = dlat * (j + encodedLatitudeSlice);
var oddEvenNL = NL(result.Latitude) - (cprCoordinate.OddFormat ? 1 : 0);
if(cprCoordinate.Longitude != 0) {
// The version that uses doubles for speed at the expense of potential rounding errors. Any changes here need duplicating below!
var refLon = referenceCoordinate.Longitude;
var dlon = oddEvenNL == 0 ? dlonNumerator : dlonNumerator / oddEvenNL;
var encodedLongitudeSlice = (double)cprCoordinate.Longitude / maxResult;
var m = Math.Floor(refLon / dlon) + Math.Floor(0.5 + (CircleModulus(refLon, dlon) / dlon) - encodedLongitudeSlice);
result.Longitude = dlon * (m + encodedLongitudeSlice);
} else {
// Same as the block above but uses decimals for accuracy at the expense of speed. It would be nice if I could use C# generic functions
// to remove this redundancy, but they're not very good for that. Sometimes I miss C++, I'd have 1000 ways to make the code common :)
// Anyway, any changes here need duplicating above!
var refLon = (decimal)referenceCoordinate.Longitude;
var dlon = oddEvenNL == 0 ? (decimal)dlonNumerator : (decimal)dlonNumerator / oddEvenNL;
var encodedLongitudeSlice = (decimal)cprCoordinate.Longitude / maxResult;
var m = Math.Floor(Math.Round(refLon / dlon, 20)) + Math.Floor(0.5M + (CircleModulus(refLon, dlon) / dlon) - encodedLongitudeSlice);
result.Longitude = (double)(dlon * (m + encodedLongitudeSlice));
}
return result;
}
public TrackedAircraftState State; // The current stage of position acquisition for the vehicle.
#endregion Fields
#region Methods
public void RecordMessage(DateTime time, CompactPositionReportingCoordinate cpr)
{
EarlierCpr = LaterCpr;
LaterCpr = new PositionMessage() { Time = time, Cpr = cpr };
}
public void CompactPositionReporting_LocalDecode_Produces_Correct_Results_For_CPR101_Tables()
{
// The values for this test are all taken directly from the transition latitude test tables in 1090-WP30-12 Proposed New Appendix CPR101
var worksheet = new ExcelWorksheetData(TestContext);
var numberOfBits = worksheet.Byte("Bits");
var oddFormat = worksheet.Bool("OddFormat");
var encodedLatitude = Convert.ToInt32(worksheet.String("ExpectedLatitude"), 16);
var encodedLongitude = Convert.ToInt32(worksheet.String("ExpectedLongitude"), 16);
var expectedLatitude = worksheet.Double("Latitude");
var expectedLongitude = worksheet.Double("Longitude");
var cprCoordinate = new CompactPositionReportingCoordinate(encodedLatitude, encodedLongitude, oddFormat, numberOfBits);
// The reference latitude and longitude is set to roughly 50km of the expected latitude and longitude
double? referenceLatitude, referenceLongitude;
GreatCircleMaths.Destination(expectedLatitude, expectedLongitude, 45, 50, out referenceLatitude, out referenceLongitude);
var referenceCoordinate = new GlobalCoordinate(referenceLatitude.Value, referenceLongitude.Value);
var dataRow = worksheet.Int("DataRow"); // helps set conditional breakpoints, VSTS doesn't always process rows in ascending order as they appear in the worksheet
var decodedCoordinate = _Cpr.LocalDecode(cprCoordinate, referenceCoordinate);
// We need to accept 180 and -180 as being the same longitude, taking into account rounding errors
if(expectedLongitude == -180.0 && decodedCoordinate.Longitude > 179.9999999999) expectedLongitude = 180.0;
else if(expectedLongitude == 180.0 && decodedCoordinate.Longitude < -179.9999999999) expectedLongitude = -180.0;
Assert.AreEqual(expectedLatitude, decodedCoordinate.Latitude, 0.0008); // The CPR tables cover all latitudes, sometimes the rounding introduced by selecting the midpoint of a zone can be quite large
Assert.AreEqual(expectedLongitude, decodedCoordinate.Longitude, 0.000000000001); // This can have a lower tolerance as the CPR101 tables aren't testing longitude zone boundaries so much
}
/// <summary>
/// See interface docs.
/// </summary>
/// <param name="earlyCpr"></param>
/// <param name="laterCpr"></param>
/// <param name="receiverLocation"></param>
/// <returns></returns>
public GlobalCoordinate GlobalDecode(CompactPositionReportingCoordinate earlyCpr, CompactPositionReportingCoordinate laterCpr, GlobalCoordinate receiverLocation)
{
GlobalCoordinate result = null;
if(earlyCpr != null && laterCpr != null && earlyCpr.OddFormat != laterCpr.OddFormat && earlyCpr.NumberOfBits == laterCpr.NumberOfBits && (laterCpr.NumberOfBits < 19 || receiverLocation != null)) {
var numberOfBits = earlyCpr.NumberOfBits;
var surfaceEncoding = numberOfBits == 19;
var dlatOdd = surfaceEncoding ? DLatOdd19Bit : DLatOdd;
var dlatEven = surfaceEncoding ? DLatEven19Bit : DLatEven;
var maxResult = MaxResultSize(surfaceEncoding ? 17 : numberOfBits);
var oddCoordinate = earlyCpr.OddFormat ? earlyCpr : laterCpr;
var evenCoordinate = oddCoordinate == earlyCpr ? laterCpr : earlyCpr;
var j = Math.Floor(((59 * (double)evenCoordinate.Latitude - 60 * (double)oddCoordinate.Latitude) / maxResult) + 0.5);
var rlatEven = dlatEven * (CircleModulus(j, 60) + ((double)evenCoordinate.Latitude / maxResult));
var rlatOdd = dlatOdd * (CircleModulus(j, 59) + ((double)oddCoordinate.Latitude / maxResult));
if(rlatEven >= 270.0) rlatEven -= 360.0;
if(rlatOdd >= 270.0) rlatOdd -= 360.0;
var rlat = laterCpr.OddFormat ? rlatOdd : rlatEven;
if(surfaceEncoding && AbsoluteDifference(rlat, receiverLocation.Latitude) > AbsoluteDifference(rlat - 90.0, receiverLocation.Latitude)) {
rlat -= 90.0;
rlatEven -= 90.0;
rlatOdd -= 90.0;
}
var nlEven = NL(rlatEven);
var nlOdd = NL(rlatOdd);
if(nlEven == nlOdd) {
var nl = nlEven;
var mNumerator = (double)evenCoordinate.Longitude * (nl - 1) - (double)oddCoordinate.Longitude * nl;
var m = Math.Floor((mNumerator / maxResult) + 0.5);
var ni = Math.Max(nl - (laterCpr.OddFormat ? 1 : 0), 1);
var dlon = (surfaceEncoding ? 90.0 : 360.0) / ni;
var rlon = dlon * (CircleModulus(m, ni) + ((double)laterCpr.Longitude / maxResult));
if(!surfaceEncoding) rlon = CircleModulus(rlon + 180.0, 360.0) - 180.0;
else {
var deltaDegrees = double.MaxValue;
var receiverBearing = LongitudeToBearing(receiverLocation.Longitude);
foreach(var possibleRlon in new double[] { rlon, rlon - 90, rlon - 180, rlon - 270 }) {
var adjustedRlon = CircleModulus(possibleRlon + 180.0, 360.0) - 180.0;
var rlonBearing = LongitudeToBearing(adjustedRlon);
var delta = SmallestDegreesBetweenBearings(rlonBearing, receiverBearing);
if(delta < 0.0) delta += 360.0;
if(delta < deltaDegrees) {
rlon = adjustedRlon;
deltaDegrees = delta;
}
}
}
result = new GlobalCoordinate(rlat, rlon);
}
}
return result;
}
public void CompactPositionReporting_GlobalDecode_Returns_Null_If_No_Receiver_Location_Is_Supplied_For_19Bit_Format()
{
var message1 = new CompactPositionReportingCoordinate(130929, 23302, false, 19); // {38.998363494873/-73.9999953560207}
var message2 = new CompactPositionReportingCoordinate(74133, 0, true, 19); // {38.998357, -74.0}
Assert.IsNull(_Cpr.GlobalDecode(message1, message2, null));
}
public void CompactPositionReporting_GlobalDecode_Calculates_1090_WP30_17_Test_Surface_Positions()
{
var message1 = new CompactPositionReportingCoordinate(130929, 23302, false, 19); // {38.998363494873/-73.9999953560207}
var message2 = new CompactPositionReportingCoordinate(74133, 0, true, 19); // {38.998357, -74.0}
var decoded = _Cpr.GlobalDecode(message1, message2, new GlobalCoordinate(38.99836, -74));
Assert.AreEqual(38.998357, decoded.Latitude, 0.000001);
Assert.AreEqual(-74.0, decoded.Longitude, 0.000001);
var message3 = new CompactPositionReportingCoordinate(0, 23302, false, 19);
decoded = _Cpr.GlobalDecode(message2, message3, new GlobalCoordinate(39.0, -74.0));
Assert.AreEqual(39.0, decoded.Latitude, 0.000001);
Assert.AreEqual(-73.999995, decoded.Longitude, 0.000001);
}
public void CompactPositionReporting_GlobalDecode_Calculates_Correct_Position_For_Internet_Example_1()
{
var earlyCpr = new CompactPositionReportingCoordinate(92095, 39846, false, 17);
var laterCpr = new CompactPositionReportingCoordinate(88385, 125818, true, 17);
var decoded = _Cpr.GlobalDecode(earlyCpr, laterCpr, null);
Assert.AreEqual(10.2162144547802, decoded.Latitude, 0.00001);
Assert.AreEqual(123.889128586342 , decoded.Longitude, 0.00001);
}
public void CompactPositionReporting_GlobalDecode_Returns_Null_If_Coordinate_Bits_Are_Different()
{
var earlier = new CompactPositionReportingCoordinate(1234, 0, true, 17);
var later = new CompactPositionReportingCoordinate(1235, 0, false, 19);
Assert.IsNull(_Cpr.GlobalDecode(earlier, later, null));
}
public void CompactPositionReporting_GlobalDecode_Returns_Null_If_Either_Coordinate_Is_Null()
{
var coordinate = new CompactPositionReportingCoordinate(0x10000, 0, true, 17);
Assert.IsNull(_Cpr.GlobalDecode(null, coordinate, null));
Assert.IsNull(_Cpr.GlobalDecode(coordinate, null, null));
}