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OrbitTrajectory

OrbitTrajectory¶

Specialized trajectory for orbital mechanics with multiple state representations.

Overview¶

OrbitTrajectory provides orbit-specific features beyond basic state storage:

Automatic Cartesian ↔ Keplerian conversions
Reference frame tracking (ECI/ECEF)
Orbit-aware interpolation
Built-in orbit analysis

Creating an OrbitTrajectory¶

import brahe as bh

traj = bh.OrbitTrajectory(
    frame=bh.OrbitFrame.ECI,
    representation=bh.OrbitRepresentation.CARTESIAN,
    interpolation=bh.InterpolationMethod.LINEAR,
    eviction=bh.TrajectoryEvictionPolicy.NONE
)

Reference Frames¶

Specify which reference frame states are stored in:

ECI: Earth-Centered Inertial (inertial frame)
ECEF: Earth-Centered Earth-Fixed (rotating with Earth)

import brahe as bh

# ECI frame (most common for orbit propagation)
traj_eci = bh.OrbitTrajectory(
    frame=bh.OrbitFrame.ECI,
    representation=bh.OrbitRepresentation.CARTESIAN
)

# ECEF frame (useful for ground tracking)
traj_ecef = bh.OrbitTrajectory(
    frame=bh.OrbitFrame.ECEF,
    representation=bh.OrbitRepresentation.CARTESIAN
)

State Representations¶

Choose how orbital states are stored:

Cartesian: [x, y, z, vx, vy, vz] in meters and m/s
Keplerian: [a, e, i, Ω, ω, ν] orbital elements

import brahe as bh

# Store as Cartesian states
traj_cart = bh.OrbitTrajectory(
    frame=bh.OrbitFrame.ECI,
    representation=bh.OrbitRepresentation.CARTESIAN
)

# Store as Keplerian elements
traj_kep = bh.OrbitTrajectory(
    frame=bh.OrbitFrame.ECI,
    representation=bh.OrbitRepresentation.KEPLERIAN
)

Adding Orbital States¶

import brahe as bh
import numpy as np

epoch = bh.Epoch.from_datetime(2024, 1, 1, 0, 0, 0.0, 0.0, bh.UTC)

# Cartesian state [x, y, z, vx, vy, vz]
state = np.array([7000e3, 0.0, 0.0, 0.0, 7.5e3, 0.0])
traj.add_state(epoch, state)

Querying in Different Representations¶

import brahe as bh

query_epoch = epoch + 600.0  # 10 minutes later

# Get Cartesian state
cart_state = traj.get_state_cartesian(query_epoch)

# Get Keplerian elements
kep_elements = traj.get_state_keplerian(query_epoch)
a, e, i, raan, argp, nu = kep_elements

Orbit-Specific Features¶

Ground Track¶

import brahe as bh

# Get ground track (lat, lon, alt) over time
ground_track = traj.compute_ground_track(
    start_epoch=epoch,
    end_epoch=epoch + 5400.0,  # 90 minutes
    step=10.0  # Every 10 seconds
)

Orbit Statistics¶

import brahe as bh

# Compute orbit statistics
stats = traj.get_orbit_statistics()
print(f"Mean altitude: {stats.mean_altitude / 1000:.1f} km")
print(f"Eccentricity: {stats.mean_eccentricity:.6f}")

Use Cases¶

OrbitTrajectory is ideal for:

Orbit visualization and analysis
Multi-representation workflows
Ground track computation
Orbit determination

See Also¶