Using GroundEphemeris

This example shows how to compute ephemeris for a ground-based observatory at a fixed location on Earth’s surface.

import datetime as dt
import numpy as np
import rust_ephem as re

# Define observatory location (geodetic coordinates)
# Example: Apache Point Observatory, New Mexico
latitude = 32.7797   # degrees
longitude = -105.8204  # degrees
height = 2788.0      # meters above WGS84 ellipsoid

# Define time range
begin = dt.datetime(2024, 1, 1, 0, 0, 0, tzinfo=dt.timezone.utc)
end = dt.datetime(2024, 1, 1, 1, 0, 0, tzinfo=dt.timezone.utc)
step_size = 60  # seconds

# Create ground ephemeris
# All frames are pre-computed during initialization
observatory = re.GroundEphemeris(
    latitude=latitude,
    longitude=longitude,
    height=height,
    begin=begin,
    end=end,
    step_size=step_size,
    polar_motion=True  # Enable polar motion corrections
)

# Access observatory location properties
# `latitude_deg` and `longitude_deg` return numpy arrays (one per timestamp)
# For the stationary observatory these are constant arrays; index [0] gets the scalar value
print("Latitude:", observatory.latitude_deg[0], "degrees")
print("Longitude:", observatory.longitude_deg[0], "degrees")
print("Height:", observatory.height_m[0], "meters")

# Access pre-computed frames (PositionVelocityData objects)
pv_itrs = observatory.itrs_pv  # Earth-fixed frame
pv_gcrs = observatory.gcrs_pv       # Celestial frame

# Access Sun and Moon positions/velocities in GCRS PositionVelocityData
sun = observatory.sun_pv
moon = observatory.moon_pv
# Access timestamps
times = observatory.timestamp

# Observatory position and velocity
print("ITRS position (km):", pv_itrs.position[0])  # First timestep
print("ITRS velocity (km/s):", pv_itrs.velocity[0])  # Zero for fixed site
print("GCRS position (km):", pv_gcrs.position[0])

# Sun and Moon positions
print("Sun position (km):", sun.position[0])
print("Moon position (km):", moon.position[0])

# Access astropy SkyCoord objects (requires astropy)
gcrs_skycoord = observatory.gcrs
itrs_skycoord = observatory.itrs
sun_skycoord = observatory.sun
moon_skycoord = observatory.moon

# Access obsgeoloc/obsgeovel for astropy GCRS frames
# These are compatible with astropy.coordinates.GCRS frame parameters
obsgeoloc = observatory.obsgeoloc  # Observatory location in GCRS
obsgeovel = observatory.obsgeovel  # Observatory velocity in GCRS

GroundEphemeris Use Cases

  • Computing visibility windows from ground stations

  • Calculating topocentric coordinates for satellite tracking

  • Generating observatory-centric Sun/Moon ephemerides

  • Providing location data for astropy coordinate transformations

GroundEphemeris Notes

  • The observatory position is fixed in ITRS (Earth-fixed frame)

  • Velocity in ITRS is zero for fixed sites; GCRS velocity reflects Earth’s rotation

  • Position in GCRS changes over time due to Earth’s rotation and precession

  • The polar_motion parameter enables polar motion corrections (requires EOP data)

  • Use re.init_eop_provider() to load EOP data for polar motion corrections

  • See tests under tests/test_ground_ephemeris.py for more examples