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Keplerian Propagation

The KeplerianPropagator provides fast, analytical two-body orbital propagation using Kepler's equations. It assumes only gravitational attraction from a central body (Earth) with no perturbations, making it ideal for rapid trajectory generation, high-altitude orbits, or when perturbations are negligible.

For complete API documentation, see the KeplerianPropagator API Reference.

Initialization

The KeplerianPropagator can be initialized from several state representations.

From Keplerian Elements

The most direct initialization method uses classical Keplerian orbital elements.

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import brahe as bh
import numpy as np

bh.initialize_eop()

# Define initial epoch
epoch = bh.Epoch.from_datetime(2024, 1, 1, 12, 0, 0.0, 0.0, bh.TimeSystem.UTC)

# Define Keplerian elements [a, e, i, Ω, ω, M]
elements = np.array(
    [
        bh.R_EARTH + 500e3,  # Semi-major axis (m)
        0.001,  # Eccentricity
        97.8,  # Inclination (degrees)
        15.0,  # RAAN (degrees)
        30.0,  # Argument of perigee (degrees)
        45.0,  # Mean anomaly (degrees)
    ]
)

# Create propagator with 60-second step size
prop = bh.KeplerianPropagator.from_keplerian(
    epoch, elements, bh.AngleFormat.DEGREES, 60.0
)

print(f"Orbital period: {bh.orbital_period(elements[0]):.1f} seconds")

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use brahe as bh;
use nalgebra as na;

fn main() {
    bh::initialize_eop().unwrap();

    // Define initial epoch
    let epoch = bh::Epoch::from_datetime(2024, 1, 1, 12, 0, 0.0, 0.0, bh::TimeSystem::UTC);

    // Define Keplerian elements [a, e, i, Ω, ω, M]
    let elements = na::SVector::<f64, 6>::new(
        bh::R_EARTH + 500e3,  // Semi-major axis (m)
        0.001,                // Eccentricity
        97.8,                 // Inclination (degrees)
        15.0,                 // RAAN (degrees)
        30.0,                 // Argument of perigee (degrees)
        45.0                  // Mean anomaly (degrees)
    );

    // Create propagator with 60-second step size
    let _prop = bh::KeplerianPropagator::from_keplerian(
        epoch, elements, bh::AngleFormat::Degrees, 60.0
    );

    println!("Orbital period: {:.1} seconds", bh::orbital_period(elements[0]));
}
Output 
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Orbital period: 5677.0 seconds

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Orbital period: 5677.0 seconds

From ECI Cartesian State

Initialize from position and velocity vectors in the Earth-Centered Inertial (ECI) frame.

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import brahe as bh
import numpy as np

bh.initialize_eop()

epoch = bh.Epoch.from_datetime(2024, 1, 1, 12, 0, 0.0, 0.0, bh.TimeSystem.UTC)

# Define Cartesian state in ECI frame [x, y, z, vx, vy, vz]
# Convert from Keplerian elements for this example
elements = np.array([bh.R_EARTH + 500e3, 0.001, 97.8, 15.0, 30.0, 45.0])
state_eci = bh.state_koe_to_eci(elements, bh.AngleFormat.DEGREES)

# Create propagator from ECI state
prop = bh.KeplerianPropagator.from_eci(epoch, state_eci, 60.0)

print(f"Initial position magnitude: {np.linalg.norm(state_eci[:3]) / 1e3:.1f} km")

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use brahe as bh;
use nalgebra as na;

fn main() {
    bh::initialize_eop().unwrap();

    let epoch = bh::Epoch::from_datetime(2024, 1, 1, 12, 0, 0.0, 0.0, bh::TimeSystem::UTC);

    // Define Cartesian state in ECI frame [x, y, z, vx, vy, vz]
    // Convert from Keplerian elements for this example
    let elements = na::SVector::<f64, 6>::new(bh::R_EARTH + 500e3, 0.001, 97.8, 15.0, 30.0, 45.0);
    let state_eci = bh::state_koe_to_eci(elements, bh::AngleFormat::Degrees);

    // Create propagator from ECI state
    let _prop = bh::KeplerianPropagator::from_eci(epoch, state_eci, 60.0);

    println!("Initial position magnitude: {:.1} km",
             state_eci.fixed_rows::<3>(0).norm() / 1e3);
}
Output 
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Initial position magnitude: 6873.3 km

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Initial position magnitude: 6873.3 km

From ECEF Cartesian State

Initialize from position and velocity vectors in the Earth-Centered Earth-Fixed (ECEF) frame. The propagator will automatically convert to ECI internally.

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import brahe as bh
import numpy as np

bh.initialize_eop()  # Required for ECEF ↔ ECI transformations

epoch = bh.Epoch.from_datetime(2024, 1, 1, 12, 0, 0.0, 0.0, bh.TimeSystem.UTC)

# Get state in ECI, then convert to ECEF for demonstration
elements = np.array([bh.R_EARTH + 500e3, 0.001, 97.8, 15.0, 30.0, 45.0])
state_eci = bh.state_koe_to_eci(elements, bh.AngleFormat.DEGREES)
state_ecef = bh.state_eci_to_ecef(epoch, state_eci)

# Create propagator from ECEF state
prop = bh.KeplerianPropagator.from_ecef(epoch, state_ecef, 60.0)

print(f"ECEF position magnitude: {np.linalg.norm(state_ecef[:3]) / 1e3:.1f} km")

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use brahe as bh;
use nalgebra as na;

fn main() {
    bh::initialize_eop().unwrap();  // Required for ECEF ↔ ECI transformations

    let epoch = bh::Epoch::from_datetime(2024, 1, 1, 12, 0, 0.0, 0.0, bh::TimeSystem::UTC);

    // Get state in ECI, then convert to ECEF for demonstration
    let elements = na::SVector::<f64, 6>::new(bh::R_EARTH + 500e3, 0.001, 97.8, 15.0, 30.0, 45.0);
    let state_eci = bh::state_koe_to_eci(elements, bh::AngleFormat::Degrees);
    let state_ecef = bh::state_eci_to_ecef(epoch, state_eci);

    // Create propagator from ECEF state
    let _prop = bh::KeplerianPropagator::from_ecef(epoch, state_ecef, 60.0);

    println!("ECEF position magnitude: {:.1} km",
             state_ecef.fixed_rows::<3>(0).norm() / 1e3);
}
Output 
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ECEF position magnitude: 6873.3 km

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ECEF position magnitude: 6873.3 km

Stepping Through Time

One of the primary functions of propagators is to step forward in time, generating new states at regular intervals. There are several methods to advance the propagator's internal state. Each stepping operation adds new state(s) to the internal trajectory.

Single Steps

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import brahe as bh
import numpy as np

bh.initialize_eop()

# Create propagator
epoch = bh.Epoch.from_datetime(2024, 1, 1, 0, 0, 0.0, 0.0, bh.TimeSystem.UTC)
elements = np.array([bh.R_EARTH + 500e3, 0.001, 97.8, 15.0, 30.0, 45.0])
prop = bh.KeplerianPropagator.from_keplerian(
    epoch, elements, bh.AngleFormat.DEGREES, 60.0
)

# Take one step (60 seconds)
prop.step()
print(f"After 1 step: {prop.current_epoch()}")

# Step by custom duration (120 seconds)
prop.step_by(120.0)
print(f"After custom step: {prop.current_epoch()}")

# Trajectory now contains 3 states (initial + 2 steps)
print(f"Trajectory length: {len(prop.trajectory)}")

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use brahe as bh;
use bh::traits::{SStatePropagator, Trajectory};
use nalgebra as na;

fn main() {
    bh::initialize_eop().unwrap();

    // Create propagator
    let epoch = bh::Epoch::from_datetime(2024, 1, 1, 0, 0, 0.0, 0.0, bh::TimeSystem::UTC);
    let elements = na::SVector::<f64, 6>::new(bh::R_EARTH + 500e3, 0.001, 97.8, 15.0, 30.0, 45.0);
    let mut prop = bh::KeplerianPropagator::from_keplerian(
        epoch, elements, bh::AngleFormat::Degrees, 60.0
    );

    // Take one step (60 seconds)
    prop.step();
    println!("After 1 step: {}", prop.current_epoch());

    // Step by custom duration (120 seconds)
    prop.step_by(120.0);
    println!("After custom step: {}", prop.current_epoch());

    // Trajectory now contains 3 states (initial + 2 steps)
    println!("Trajectory length: {}", prop.trajectory.len());
}
Output 
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After 1 step: 2024-01-01 00:01:00.000 UTC
After custom step: 2024-01-01 00:03:00.000 UTC
Trajectory length: 3

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After 1 step: 2024-01-01 00:01:00.000 UTC
After custom step: 2024-01-01 00:03:00.000 UTC
Trajectory length: 3

Multiple Steps

The propagate_steps() method allows taking multiple fixed-size steps in one call.

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import brahe as bh
import numpy as np

bh.initialize_eop()

epoch = bh.Epoch.from_datetime(2024, 1, 1, 0, 0, 0.0, 0.0, bh.TimeSystem.UTC)
elements = np.array([bh.R_EARTH + 500e3, 0.001, 97.8, 15.0, 30.0, 45.0])
prop = bh.KeplerianPropagator.from_keplerian(
    epoch, elements, bh.AngleFormat.DEGREES, 60.0
)

# Take 10 steps (10 × 60 = 600 seconds)
prop.propagate_steps(10)
print(f"After 10 steps: {(prop.current_epoch() - epoch):.1f} seconds elapsed")
print(f"Trajectory length: {len(prop.trajectory)}")

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use brahe as bh;
use bh::traits::{SStatePropagator, Trajectory};
use nalgebra as na;

fn main() {
    bh::initialize_eop().unwrap();

    let epoch = bh::Epoch::from_datetime(2024, 1, 1, 0, 0, 0.0, 0.0, bh::TimeSystem::UTC);
    let elements = na::SVector::<f64, 6>::new(bh::R_EARTH + 500e3, 0.001, 97.8, 15.0, 30.0, 45.0);
    let mut prop = bh::KeplerianPropagator::from_keplerian(
        epoch, elements, bh::AngleFormat::Degrees, 60.0
    );

    // Take 10 steps (10 × 60 = 600 seconds)
    prop.propagate_steps(10);
    println!("After 10 steps: {:.1} seconds elapsed",
             prop.current_epoch() - epoch);
    println!("Trajectory length: {}", prop.trajectory.len());
}
Output 
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After 10 steps: 600.0 seconds elapsed
Trajectory length: 11

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After 10 steps: 600.0 seconds elapsed
Trajectory length: 11

Propagate to Target Epoch

For precise time targeting, use propagate_to() which adjusts the final step size to exactly reach the target epoch.

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import brahe as bh
import numpy as np

bh.initialize_eop()

epoch = bh.Epoch.from_datetime(2024, 1, 1, 0, 0, 0.0, 0.0, bh.TimeSystem.UTC)
elements = np.array([bh.R_EARTH + 500e3, 0.001, 97.8, 15.0, 30.0, 45.0])
prop = bh.KeplerianPropagator.from_keplerian(
    epoch, elements, bh.AngleFormat.DEGREES, 60.0
)

# Propagate exactly 500 seconds (not evenly divisible by step size)
target = epoch + 500.0
prop.propagate_to(target)

print(f"Target epoch: {target}")
print(f"Current epoch: {prop.current_epoch()}")
print(f"Difference: {abs(prop.current_epoch() - target):.10f} seconds")

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use brahe as bh;
use bh::traits::SStatePropagator;
use nalgebra as na;

fn main() {
    bh::initialize_eop().unwrap();

    let epoch = bh::Epoch::from_datetime(2024, 1, 1, 0, 0, 0.0, 0.0, bh::TimeSystem::UTC);
    let elements = na::SVector::<f64, 6>::new(bh::R_EARTH + 500e3, 0.001, 97.8, 15.0, 30.0, 45.0);
    let mut prop = bh::KeplerianPropagator::from_keplerian(
        epoch, elements, bh::AngleFormat::Degrees, 60.0
    );

    // Propagate exactly 500 seconds (not evenly divisible by step size)
    let target = epoch + 500.0;
    prop.propagate_to(target);

    println!("Target epoch: {}", target);
    println!("Current epoch: {}", prop.current_epoch());
    println!("Difference: {:.10} seconds",
             (prop.current_epoch() - target).abs());
}
Output 
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Target epoch: 2024-01-01 00:08:20.000 UTC
Current epoch: 2024-01-01 00:08:20.000 UTC
Difference: 0.0000000000 seconds

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Target epoch: 2024-01-01 00:08:20.000 UTC
Current epoch: 2024-01-01 00:08:20.000 UTC
Difference: 0.0000000000 seconds

Direct State Queries

The StateProvider trait allows computing states at arbitrary epochs without building a trajectory. This is useful for sparse sampling or parallel batch computation.

Single Epoch Queries

Single epoch queries like state(), state_eci(), and state_ecef() compute the state at a specific epoch on demand.

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import brahe as bh
import numpy as np

bh.initialize_eop()  # Required for frame transformations

epoch = bh.Epoch.from_datetime(2024, 1, 1, 0, 0, 0.0, 0.0, bh.TimeSystem.UTC)
elements = np.array([bh.R_EARTH + 500e3, 0.001, 97.8, 15.0, 30.0, 45.0])
prop = bh.KeplerianPropagator.from_keplerian(
    epoch, elements, bh.AngleFormat.DEGREES, 60.0
)

# Query state 1 hour later (doesn't add to trajectory)
query_epoch = epoch + 3600.0
state_native = prop.state(
    query_epoch
)  # Native format of propagator internal state (Keplerian)
state_eci = prop.state_eci(query_epoch)  # ECI Cartesian
state_ecef = prop.state_ecef(query_epoch)  # ECEF Cartesian
state_kep = prop.state_koe_osc(query_epoch, bh.AngleFormat.DEGREES)

print(f"Native state (Keplerian): a={state_native[0] / 1e3:.1f} km")
print(f"ECI position magnitude: {np.linalg.norm(state_eci[:3]) / 1e3:.1f} km")
print(f"ECEF position magnitude: {np.linalg.norm(state_ecef[:3]) / 1e3:.1f} km")

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use brahe as bh;
use bh::utils::{DStateProvider, DOrbitStateProvider};
use nalgebra as na;

fn main() {
    bh::initialize_eop().unwrap();  // Required for frame transformations

    let epoch = bh::Epoch::from_datetime(2024, 1, 1, 0, 0, 0.0, 0.0, bh::TimeSystem::UTC);
    let elements = na::SVector::<f64, 6>::new(bh::R_EARTH + 500e3, 0.001, 97.8, 15.0, 30.0, 45.0);
    let prop = bh::KeplerianPropagator::from_keplerian(
        epoch, elements, bh::AngleFormat::Degrees, 60.0
    );

    // Query state 1 hour later (doesn't add to trajectory)
    let query_epoch = epoch + 3600.0;
    let state_native = prop.state(query_epoch).unwrap();       // Native format of propagator internal state  (Keplerian)
    let state_eci = prop.state_eci(query_epoch).unwrap();      // ECI Cartesian
    let state_ecef = prop.state_ecef(query_epoch).unwrap();    // ECEF Cartesian
    let _state_kep = prop.state_koe_osc(
        query_epoch, bh::AngleFormat::Degrees
    ).unwrap();

    println!("Native state (Keplerian): a={:.1} km", state_native[0] / 1e3);
    println!("ECI position magnitude: {:.1} km",
             state_eci.fixed_rows::<3>(0).norm() / 1e3);
    println!("ECEF position magnitude: {:.1} km",
             state_ecef.fixed_rows::<3>(0).norm() / 1e3);
}
Output 
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Native state (Keplerian): a=6878.1 km
ECI position magnitude: 6877.7 km
ECEF position magnitude: 6877.7 km

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Native state (Keplerian): a=6878.1 km
ECI position magnitude: 6877.7 km
ECEF position magnitude: 6877.7 km

Batch Queries

Batch queries like states() and states_eci() compute states at each epoch in a provided list.

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import brahe as bh
import numpy as np

bh.initialize_eop()

epoch = bh.Epoch.from_datetime(2024, 1, 1, 0, 0, 0.0, 0.0, bh.TimeSystem.UTC)
elements = np.array([bh.R_EARTH + 500e3, 0.001, 97.8, 15.0, 30.0, 45.0])
prop = bh.KeplerianPropagator.from_keplerian(
    epoch, elements, bh.AngleFormat.DEGREES, 60.0
)

# Generate states at irregular intervals
query_epochs = [epoch + t for t in [0.0, 100.0, 500.0, 1000.0, 3600.0]]
states_eci = prop.states_eci(query_epochs)

print(f"Generated {len(states_eci)} states")
for i, state in enumerate(states_eci):
    print(f"  Epoch {i}: position magnitude = {np.linalg.norm(state[:3]) / 1e3:.1f} km")

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use brahe as bh;
use bh::utils::DOrbitStateProvider;
use nalgebra as na;

fn main() {
    bh::initialize_eop().unwrap();

    let epoch = bh::Epoch::from_datetime(2024, 1, 1, 0, 0, 0.0, 0.0, bh::TimeSystem::UTC);
    let elements = na::SVector::<f64, 6>::new(bh::R_EARTH + 500e3, 0.001, 97.8, 15.0, 30.0, 45.0);
    let prop = bh::KeplerianPropagator::from_keplerian(
        epoch, elements, bh::AngleFormat::Degrees, 60.0
    );

    // Generate states at irregular intervals
    let query_epochs = vec![
        epoch, epoch + 100.0, epoch + 500.0, epoch + 1000.0, epoch + 3600.0
    ];
    let states_eci = prop.states_eci(&query_epochs).unwrap();

    println!("Generated {} states", states_eci.len());
    for (i, state) in states_eci.iter().enumerate() {
        println!("  Epoch {}: position magnitude = {:.1} km",
                 i, state.fixed_rows::<3>(0).norm() / 1e3);
    }
}
Output 
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Generated 5 states
  Epoch 0: position magnitude = 6873.3 km
  Epoch 1: position magnitude = 6873.8 km
  Epoch 2: position magnitude = 6876.6 km
  Epoch 3: position magnitude = 6880.3 km
  Epoch 4: position magnitude = 6877.7 km

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Generated 5 states
  Epoch 0: position magnitude = 6873.3 km
  Epoch 1: position magnitude = 6873.8 km
  Epoch 2: position magnitude = 6876.6 km
  Epoch 3: position magnitude = 6880.3 km
  Epoch 4: position magnitude = 6877.7 km

Trajectory Management

The propagator stores all stepped states in an internal OrbitTrajectory. This trajectory can be accessed, converted, and managed.

Accessing the Trajectory

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import brahe as bh
import numpy as np

bh.initialize_eop()

epoch = bh.Epoch.from_datetime(2024, 1, 1, 0, 0, 0.0, 0.0, bh.TimeSystem.UTC)
elements = np.array([bh.R_EARTH + 500e3, 0.001, 97.8, 15.0, 30.0, 45.0])
prop = bh.KeplerianPropagator.from_keplerian(
    epoch, elements, bh.AngleFormat.DEGREES, 60.0
)

# Propagate for several steps
prop.propagate_steps(5)

# Access trajectory
traj = prop.trajectory
print(f"Trajectory contains {len(traj)} states")

# Iterate over epoch-state pairs
for epoch, state in traj:
    print(f"Epoch: {epoch}, semi-major axis: {state[0] / 1e3:.1f} km")

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use brahe as bh;
use bh::traits::{SStatePropagator, Trajectory};
use nalgebra as na;

fn main() {
    bh::initialize_eop().unwrap();

    let epoch = bh::Epoch::from_datetime(2024, 1, 1, 0, 0, 0.0, 0.0, bh::TimeSystem::UTC);
    let elements = na::SVector::<f64, 6>::new(bh::R_EARTH + 500e3, 0.001, 97.8, 15.0, 30.0, 45.0);
    let mut prop = bh::KeplerianPropagator::from_keplerian(
        epoch, elements, bh::AngleFormat::Degrees, 60.0
    );

    // Propagate for several steps
    prop.propagate_steps(5);

    // Access trajectory
    let traj = &prop.trajectory;
    println!("Trajectory contains {} states", traj.len());

    // Access by index
    for i in 0..traj.len() {
        let epoch = traj.epoch_at_idx(i).unwrap();
        let state = traj.state_at_idx(i).unwrap();
        println!("Epoch: {}, semi-major axis: {:.1} km", epoch, state[0] / 1e3);
    }
}
Output 
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Trajectory contains 6 states
Epoch: 2024-01-01 00:00:00.000 UTC, semi-major axis: 6878.1 km
Epoch: 2024-01-01 00:01:00.000 UTC, semi-major axis: 6878.1 km
Epoch: 2024-01-01 00:02:00.000 UTC, semi-major axis: 6878.1 km
Epoch: 2024-01-01 00:03:00.000 UTC, semi-major axis: 6878.1 km
Epoch: 2024-01-01 00:04:00.000 UTC, semi-major axis: 6878.1 km
Epoch: 2024-01-01 00:05:00.000 UTC, semi-major axis: 6878.1 km

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Trajectory contains 6 states
Epoch: 2024-01-01 00:00:00.000 UTC, semi-major axis: 6878.1 km
Epoch: 2024-01-01 00:01:00.000 UTC, semi-major axis: 6878.1 km
Epoch: 2024-01-01 00:02:00.000 UTC, semi-major axis: 6878.1 km
Epoch: 2024-01-01 00:03:00.000 UTC, semi-major axis: 6878.1 km
Epoch: 2024-01-01 00:04:00.000 UTC, semi-major axis: 6878.1 km
Epoch: 2024-01-01 00:05:00.000 UTC, semi-major axis: 6878.1 km

Frame Conversions

You can use the OrbitTrajectory's frame conversion methods to get the trajectory in different reference frames.

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import brahe as bh
import numpy as np

bh.initialize_eop()  # Required for ECEF conversions

epoch = bh.Epoch.from_datetime(2024, 1, 1, 0, 0, 0.0, 0.0, bh.TimeSystem.UTC)
elements = np.array([bh.R_EARTH + 500e3, 0.001, 97.8, 15.0, 30.0, 45.0])
prop = bh.KeplerianPropagator.from_keplerian(
    epoch, elements, bh.AngleFormat.DEGREES, 60.0
)
prop.propagate_steps(10)

# Convert entire trajectory to different frames
traj_eci = prop.trajectory.to_eci()  # ECI Cartesian
traj_ecef = prop.trajectory.to_ecef()  # ECEF Cartesian
traj_kep = prop.trajectory.to_keplerian(bh.AngleFormat.RADIANS)

print(f"ECI trajectory: {len(traj_eci)} states")
print(f"ECEF trajectory: {len(traj_ecef)} states")
print(f"Keplerian trajectory: {len(traj_kep)} states")

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use brahe as bh;
use bh::traits::{SStatePropagator, Trajectory};
use nalgebra as na;

fn main() {
    bh::initialize_eop().unwrap();  // Required for ECEF conversions

    let epoch = bh::Epoch::from_datetime(2024, 1, 1, 0, 0, 0.0, 0.0, bh::TimeSystem::UTC);
    let elements = na::SVector::<f64, 6>::new(bh::R_EARTH + 500e3, 0.001, 97.8, 15.0, 30.0, 45.0);
    let mut prop = bh::KeplerianPropagator::from_keplerian(
        epoch, elements, bh::AngleFormat::Degrees, 60.0
    );
    prop.propagate_steps(10);

    // Convert entire trajectory to different frames
    let traj_eci = prop.trajectory.to_eci();       // ECI Cartesian
    let traj_ecef = prop.trajectory.to_ecef();     // ECEF Cartesian
    let traj_kep = prop.trajectory.to_keplerian(bh::AngleFormat::Radians);

    println!("ECI trajectory: {} states", traj_eci.len());
    println!("ECEF trajectory: {} states", traj_ecef.len());
    println!("Keplerian trajectory: {} states", traj_kep.len());
}
Output 
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ECI trajectory: 11 states
ECEF trajectory: 11 states
Keplerian trajectory: 11 states

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ECI trajectory: 11 states
ECEF trajectory: 11 states
Keplerian trajectory: 11 states

Memory Management

Propagators support trajectory memory management via eviction policies to limit memory usage for long-running applications.

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import brahe as bh
import numpy as np

bh.initialize_eop()

epoch = bh.Epoch.from_datetime(2024, 1, 1, 0, 0, 0.0, 0.0, bh.TimeSystem.UTC)
elements = np.array([bh.R_EARTH + 500e3, 0.001, 97.8, 15.0, 30.0, 45.0])
prop = bh.KeplerianPropagator.from_keplerian(
    epoch, elements, bh.AngleFormat.DEGREES, 60.0
)

# Keep only 100 most recent states
prop.set_eviction_policy_max_size(100)

# Propagate many steps
prop.propagate_steps(500)
print(f"Trajectory length: {len(prop.trajectory)}")  # Will be 100

# Alternative: Keep only states within 1 hour of current time
prop.reset()
prop.set_eviction_policy_max_age(3600.0)  # 3600 seconds = 1 hour
prop.propagate_steps(500)
print(f"Trajectory length after age policy: {len(prop.trajectory)}")

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use brahe as bh;
use bh::traits::{SStatePropagator, Trajectory};
use nalgebra as na;

fn main() {
    bh::initialize_eop().unwrap();

    let epoch = bh::Epoch::from_datetime(2024, 1, 1, 0, 0, 0.0, 0.0, bh::TimeSystem::UTC);
    let elements = na::SVector::<f64, 6>::new(bh::R_EARTH + 500e3, 0.001, 97.8, 15.0, 30.0, 45.0);
    let mut prop = bh::KeplerianPropagator::from_keplerian(
        epoch, elements, bh::AngleFormat::Degrees, 60.0
    );

    // Keep only 100 most recent states
    prop.set_eviction_policy_max_size(100).unwrap();

    // Propagate many steps
    prop.propagate_steps(500);
    println!("Trajectory length: {}", prop.trajectory.len());  // Will be 100

    // Alternative: Keep only states within 1 hour of current time
    prop.reset();
    prop.set_eviction_policy_max_age(3600.0).unwrap();  // 3600 seconds = 1 hour
    prop.propagate_steps(500);
    println!("Trajectory length after age policy: {}", prop.trajectory.len());
}
Output 
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Trajectory length: 100
Trajectory length after age policy: 61

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Trajectory length: 100
Trajectory length after age policy: 61

Configuration and Control

There are several methods to manage and configure the propagator during its lifecycle.

Resetting the Propagator

You can reset the propagator to its initial conditions using the reset() method.

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import brahe as bh
import numpy as np

bh.initialize_eop()

epoch = bh.Epoch.from_datetime(2024, 1, 1, 0, 0, 0.0, 0.0, bh.TimeSystem.UTC)
elements = np.array([bh.R_EARTH + 500e3, 0.001, 97.8, 15.0, 30.0, 45.0])
prop = bh.KeplerianPropagator.from_keplerian(
    epoch, elements, bh.AngleFormat.DEGREES, 60.0
)

# Propagate forward
prop.propagate_steps(100)
print(f"After propagation: {len(prop.trajectory)} states")

# Reset to initial conditions
prop.reset()
print(f"After reset: {len(prop.trajectory)} states")
print(f"Current epoch: {prop.current_epoch()}")

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use brahe as bh;
use bh::traits::{SStatePropagator, Trajectory};
use nalgebra as na;

fn main() {
    bh::initialize_eop().unwrap();

    let epoch = bh::Epoch::from_datetime(2024, 1, 1, 0, 0, 0.0, 0.0, bh::TimeSystem::UTC);
    let elements = na::SVector::<f64, 6>::new(bh::R_EARTH + 500e3, 0.001, 97.8, 15.0, 30.0, 45.0);
    let mut prop = bh::KeplerianPropagator::from_keplerian(
        epoch, elements, bh::AngleFormat::Degrees, 60.0
    );

    // Propagate forward
    prop.propagate_steps(100);
    println!("After propagation: {} states", prop.trajectory.len());

    // Reset to initial conditions
    prop.reset();
    println!("After reset: {} states", prop.trajectory.len());
    println!("Current epoch: {}", prop.current_epoch());
}
Output 
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After propagation: 101 states
After reset: 1 states
Current epoch: 2024-01-01 00:00:00.000 UTC

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After propagation: 101 states
After reset: 1 states
Current epoch: 2024-01-01 00:00:00.000 UTC

Changing Step Size

If you need to adjust the default step size during propagation, use the set_step_size() method.

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import brahe as bh
import numpy as np

bh.initialize_eop()

epoch = bh.Epoch.from_datetime(2024, 1, 1, 0, 0, 0.0, 0.0, bh.TimeSystem.UTC)
elements = np.array([bh.R_EARTH + 500e3, 0.001, 97.8, 15.0, 30.0, 45.0])
prop = bh.KeplerianPropagator.from_keplerian(
    epoch, elements, bh.AngleFormat.DEGREES, 60.0
)

print(f"Initial step size: {prop.step_size} seconds")

# Change step size
prop.set_step_size(120.0)
print(f"New step size: {prop.step_size} seconds")

# Subsequent steps use new step size
prop.step()  # Advances 120 seconds
print(f"After step: {(prop.current_epoch() - epoch):.1f} seconds elapsed")

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use brahe as bh;
use bh::traits::SStatePropagator;
use nalgebra as na;

fn main() {
    bh::initialize_eop().unwrap();

    let epoch = bh::Epoch::from_datetime(2024, 1, 1, 0, 0, 0.0, 0.0, bh::TimeSystem::UTC);
    let elements = na::SVector::<f64, 6>::new(bh::R_EARTH + 500e3, 0.001, 97.8, 15.0, 30.0, 45.0);
    let mut prop = bh::KeplerianPropagator::from_keplerian(
        epoch, elements, bh::AngleFormat::Degrees, 60.0
    );

    println!("Initial step size: {} seconds", prop.step_size());

    // Change step size
    prop.set_step_size(120.0);
    println!("New step size: {} seconds", prop.step_size());

    // Subsequent steps use new step size
    prop.step();  // Advances 120 seconds
    println!("After step: {:.1} seconds elapsed", prop.current_epoch() - epoch);
}
Output 
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Initial step size: 60.0 seconds
New step size: 120.0 seconds
After step: 120.0 seconds elapsed

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Initial step size: 60 seconds
New step size: 120 seconds
After step: 120.0 seconds elapsed

Identity Tracking

Finally, the IdentifiableStateProvider trait allows you to set and get identity information for the propagator. This can be useful when managing multiple propagators in an application.

Track propagators with names, IDs, or UUIDs for multi-satellite scenarios.

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import brahe as bh
import numpy as np

bh.initialize_eop()

epoch = bh.Epoch.from_datetime(2024, 1, 1, 0, 0, 0.0, 0.0, bh.TimeSystem.UTC)
elements = np.array([bh.R_EARTH + 500e3, 0.001, 97.8, 15.0, 30.0, 45.0])

# Create propagator with identity (builder pattern)
prop = (
    bh.KeplerianPropagator.from_keplerian(epoch, elements, bh.AngleFormat.DEGREES, 60.0)
    .with_name("Satellite-A")
    .with_id(12345)
)

print(f"Name: {prop.get_name()}")
print(f"ID: {prop.get_id()}")
print(f"UUID: {prop.get_uuid()}")

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use brahe as bh;
use bh::utils::Identifiable;
use nalgebra as na;

fn main() {
    bh::initialize_eop().unwrap();

    let epoch = bh::Epoch::from_datetime(2024, 1, 1, 0, 0, 0.0, 0.0, bh::TimeSystem::UTC);
    let elements = na::SVector::<f64, 6>::new(bh::R_EARTH + 500e3, 0.001, 97.8, 15.0, 30.0, 45.0);

    // Create propagator with identity (builder pattern)
    let prop = bh::KeplerianPropagator::from_keplerian(
        epoch, elements, bh::AngleFormat::Degrees, 60.0
    ).with_name("Satellite-A").with_id(12345);

    println!("Name: {:?}", prop.get_name());
    println!("ID: {:?}", prop.get_id());
    println!("UUID: {:?}", prop.get_uuid());
}
Output 
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Name: Satellite-A
ID: 12345
UUID: 019d1881-4527-7ff0-b8a7-5736d85c1da2

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Name: Some("Satellite-A")
ID: Some(12345)
UUID: Some(019d187c-8021-7cc3-82f1-5c5ea4e45caa)

See Also