Add slew constraints

* Add properties to the Mission class for overhead, slew
  velocity, and slew acceleration.
* Change the observation time from a binary decision variable to
  a continuous one.
* Implement the position-dependent slew constraints using a lazy
  constraint callback to keep the problem size manageable.

dorado/scheduling/mission.py

@@ -12,12 +12,14 @@
 
 import astroplan
 from astropy import units as u
+import numpy as np
 
 from . import data
 from .constraints import (BrightEarthLimbConstraint, EarthLimbConstraint,
                           TrappedParticleFluxConstraint, get_field_of_regard)
 from .fov import FOV
 from .orbit import Orbit
+from ._slew import slew_time
 
 __all__ = ('Mission', 'dorado', 'ultrasat', 'uvex')
 
@@ -32,13 +34,34 @@ class Mission:
     """Container for mission configuration."""
 
     constraints: Collection
+    """Observing constraint."""
+
     fov: FOV
+    """Field of view."""
+
     orbit: Orbit
+    """The orbit."""
+
+    min_overhead: u.Quantity
+    """Minimum overhead between observations (readout and settling time)."""
+
+    max_angular_velocity: u.Quantity
+    """Maximum angular velocity for slews."""
+
+    max_angular_acceleration: u.Quantity
+    """Maximum angular acceleration for slews."""
 
     def get_field_of_regard(self, *args, **kwargs):
         return get_field_of_regard(self.orbit, self.constraints,
                                    *args, **kwargs)
 
+    def overhead(self, coord1, coord2):
+        # FIXME: doesn't handle slews between different roll angles!
+        return np.maximum(self.min_overhead,
+                          slew_time(coord1.separation(coord2),
+                                    self.max_angular_velocity,
+                                    self.max_angular_acceleration))
+
 
 dorado = Mission(
     constraints=(
@@ -50,7 +73,10 @@ def get_field_of_regard(self, *args, **kwargs):
         astroplan.MoonSeparationConstraint(23 * u.deg),
         astroplan.GalacticLatitudeConstraint(10 * u.deg)),
     fov=FOV.from_rectangle(7.1 * u.deg),
-    orbit=_read_orbit('dorado-625km-sunsync.tle')
+    orbit=_read_orbit('dorado-625km-sunsync.tle'),
+    min_overhead=42 * u.s,
+    max_angular_velocity=0.872 * u.deg / u.s,
+    max_angular_acceleration=0.244 * u.deg / u.s**2
 )
 """Configuration for Dorado.
 
@@ -66,6 +92,10 @@ def get_field_of_regard(self, *args, **kwargs):
 * The orbit one of four synthetic plausible orbits generated by Craig
   Markwardt. It is a nearly circular sun-synchronous orbit at an altitude of
   625 km.
+
+* The maximum angular acceleration and angular velocity about its three
+  principal axes were given to us by the spacecraft vendor, but to be
+  conservative we are using the smallest values.
 """
 
 
@@ -76,7 +106,10 @@ def get_field_of_regard(self, *args, **kwargs):
         astroplan.MoonSeparationConstraint(23 * u.deg),
         astroplan.GalacticLatitudeConstraint(10 * u.deg)),
     fov=FOV.from_rectangle(14.1 * u.deg),
-    orbit=_read_orbit('goes17.tle')
+    orbit=_read_orbit('goes17.tle'),
+    min_overhead=42 * u.s,
+    max_angular_velocity=0.872 * u.deg / u.s,
+    max_angular_acceleration=0.244 * u.deg / u.s**2
 )
 """Configuration for ULTRASAT.
 
@@ -94,6 +127,9 @@ def get_field_of_regard(self, *args, **kwargs):
 * ULTRASAT will be in a geosynchronous orbit. Here, we are using the orbital
   elements of GOES-17, a real geosynchronous weather satellite that is
   currently on orbit.
+
+* Maximum angular acceleration, maximum angular velocity, and overhead time are
+  assumed to be the same as for Dorado.
 """
 
 
@@ -104,7 +140,10 @@ def get_field_of_regard(self, *args, **kwargs):
         astroplan.MoonSeparationConstraint(23 * u.deg)
     ),
     fov=FOV.from_rectangle(3.3 * u.deg),
-    orbit=_read_orbit('vela1.tle')
+    orbit=_read_orbit('vela1.tle'),
+    min_overhead=42 * u.s,
+    max_angular_velocity=0.872 * u.deg / u.s,
+    max_angular_acceleration=0.244 * u.deg / u.s**2
 )
 """Configuration for UVEX.
 
@@ -122,4 +161,7 @@ def get_field_of_regard(self, *args, **kwargs):
   with a perigee greater than 45,000 km. As a proxy, we use the actual two-line
   elements for VELA 1, a historically important gamma-ray satellite with a
   similar orbit.
+
+* Maximum angular acceleration, maximum angular velocity, and overhead time are
+  assumed to be the same as for Dorado.
 """