thistle 0.2.0b1

A wrapper for sgp4 that handles TLE switching

Thistle

A wrapper for sgp4 and skyfield that handles automatic TLE switching for accurate long-duration satellite orbit propagation.

The Problem

A single Two-Line Element (TLE) set degrades in accuracy as you propagate further from its epoch. For tracking a satellite over days or weeks you need multiple TLEs, and you need to decide when to switch between them.

The Solution

Thistle manages multiple TLEs for a satellite and automatically selects the most appropriate one based on the propagation time.

Installation

pip install thistle

Usage

from thistle import Propagator, read_tle

# Load TLEs (multiple epochs for one satellite)
tles = read_tle("satellite_data.tle")

# Create a propagator with a switching strategy
prop = Propagator(tles, method="midpoint")

# Propagate — Thistle picks the best TLE for each time
geo = prop.at(times)

Switching Strategies

Strategy	Description
"epoch"	Uses the most recent TLE at or before the target time (conservative)
"midpoint"	Uses the nearest TLE by epoch; transitions at midpoints between consecutive epochs
"tca"	Transitions at the time of closest approach between neighboring TLEs

Lookup Methods

You can look up the active TLE for any point in time:

import numpy as np

time = np.datetime64("2024-01-15T12:00:00")

# Get the EarthSatellite (skyfield)
satellite = prop.find_satellite(time)

# Get the Satrec (sgp4)
satrec = prop.find_satrec(time)

# Get the TLE lines as a (line1, line2) tuple
tle = prop.find_tle(time)

Requirements

Python >= 3.9