Add 'orbit' unit for orbital period

doc/reference/index.rst

@@ -12,3 +12,4 @@ Modules
     orbit
     fov
     skygrid
+    units

doc/reference/units.rst

@@ -0,0 +1,21 @@
+Units (`dorado.scheduling.units`)
+=================================
+.. automodule:: dorado.scheduling.units
+
+Examples
+--------
+
+First, some imports:
+
+>>> from astropy import units as u
+>>> from dorado.scheduling import data
+>>> from dorado.scheduling import Orbit
+>>> from dorado.scheduling.units import equivalencies
+>>> from importlib import resources
+
+Load an example two-line element:
+
+>>> with resources.path(data, 'dorado-625km-sunsync.tle') as p:
+...     orbit = Orbit(p)
+
+>>> u.Quantity('2 orbit').to(u.s, equivalencies=equivalencies.orbital(orbit))

dorado/scheduling/units/__init__.py

@@ -0,0 +1,11 @@
+#
+# Copyright © 2020 United States Government as represented by the Administrator
+# of the National Aeronautics and Space Administration. No copyright is claimed
+# in the United States under Title 17, U.S. Code. All Other Rights Reserved.
+#
+# SPDX-License-Identifier: NASA-1.3
+#
+from .orbital import orbit
+from . import equivalencies
+
+__all__ = ('equivalencies', 'orbit')

dorado/scheduling/units/equivalencies.py

@@ -0,0 +1,17 @@
+#
+# Copyright © 2020 United States Government as represented by the Administrator
+# of the National Aeronautics and Space Administration. No copyright is claimed
+# in the United States under Title 17, U.S. Code. All Other Rights Reserved.
+#
+# SPDX-License-Identifier: NASA-1.3
+#
+from astropy.units.equivalencies import Equivalency
+
+from .orbital import orbit as _orbit
+
+
+def orbital(orbit):
+    return Equivalency([(_orbit, orbit.period.unit,
+                         lambda x: x * orbit.period.value,
+                         lambda x: x / orbit.period.value)],
+                       name='orbital')

dorado/scheduling/units/orbital.py

@@ -0,0 +1,24 @@
+#
+# Copyright © 2020 United States Government as represented by the Administrator
+# of the National Aeronautics and Space Administration. No copyright is claimed
+# in the United States under Title 17, U.S. Code. All Other Rights Reserved.
+#
+# SPDX-License-Identifier: NASA-1.3
+#
+from astropy.units import def_unit
+
+_ns = globals()
+
+def_unit('orbit', namespace=_ns,
+         doc='a unit of time equal to one orbital period')
+
+del _ns, def_unit
+
+
+def _enable():
+    import inspect
+    from astropy.units import add_enabled_units
+    add_enabled_units(inspect.getmodule(_enable))
+
+
+_enable()