use GAST for UVW calculations

Author: d3v-null

src/io/ms.rs

@@ -1780,7 +1780,13 @@ impl VisWrite for MeasurementSetWriter {
                     avg_centroid_timestamp,
                     self.dut1,
                 );
-                let hadec = self.phase_centre.to_hadec(lmst);
+                // We use the same time for TT as we did for UT1 (which is what
+                // get_lmst does).
+                let mjd = (avg_centroid_timestamp + self.dut1).to_mjd_utc_days();
+                let eqeq = crate::pal::palEqeq(mjd);
+                let last = lmst + eqeq;
+
+                let hadec = self.phase_centre.to_hadec(last);
                 (self.antenna_positions.as_slice().into(), hadec)
             };
 

src/pos/pal.rs

@@ -40,6 +40,23 @@ pub fn palGmst(ut1: f64) -> f64 {
     eraGmst06(ERFA_DJM0, ut1, ERFA_DJM0, ut1)
 }
 
+/// Equation of the equinoxes (IAU 2006)
+///
+/// # Arguments
+/// date = double (Given)
+///    TT as a modified Julian Date (JD-2400000.5)
+///
+/// # Returned Value
+/// Equation of the equinoxes
+///
+/// # Description
+/// The equation of the equinoxes is the difference between apparent and
+/// mean sidereal time (GAST - GMST).
+pub fn palEqeq(date: f64) -> f64 {
+    let rnpb = erfa::aliases::eraPnm06a(ERFA_DJM0, date);
+    erfa::aliases::eraGst06(ERFA_DJM0, date, ERFA_DJM0, date, rnpb) - erfa::aliases::eraGmst06(ERFA_DJM0, date, ERFA_DJM0, date)
+}
+
 /// Spherical coordinates to direction cosines
 ///
 /// Arguments: