Generate a TLE for an object above the horizon for a given observer and a given time

faketle.py

#!/usr/bin/env python3
import ephem
import datetime
import math

# Compute checksum
def set_checksum(line):
    s = 0
    for c in line[:-1]:
        if c.isdigit():
            s += int(c)
        if c == "-":
            s += 1
    return line[:-1]+"%d"%(s%10)        

def datetime_to_epoch(t):
    epochyr = t.timetuple().tm_year - 2000
    fday = t.hour / 24 + t.minute / 1440 + t.second / 86400
    epochdoy = t.timetuple().tm_yday + fday

    return epochyr, epochdoy

def generate_fake_tle(t, observer):
    # Set epoch 10 minutes later
    tepoch = t + datetime.timedelta(minutes=10)
    observer.date = ephem.date(tepoch)

    # Compute epoch
    epochyr, epochdoy = datetime_to_epoch(tepoch)

    # RA of ascending node
    raan = (observer.sidereal_time() * 180 / math.pi) % 360

    # Mean anomaly
    m = (observer.lat * 180 / math.pi) % 360 

    # Format TLE strings
    line0 = "Test satellite"
    line1 = set_checksum(f"1 70000U 20600A   {epochyr:02d}{epochdoy:012.8f}  " \
                         f".00000000  00000-0  50000-4 0  0000")
    line2 = set_checksum(f"2 70000  90.0000 {raan:8.4f} 0001000 000.0000 " \
            f"{m:8.4f} 15.50000000     0")

    return line0, line1, line2

def generate_fake_tle_at_rise(t, observer):
    # Intial guess
    dt = datetime.timedelta(seconds=600)

    # Interate for 5 steps
    for i in range(5):
        # Set epoch
        tepoch = t + dt
        observer.date = ephem.date(tepoch)

        # Compute epoch
        epochyr, epochdoy = datetime_to_epoch(tepoch)

        # RA of ascending node
        raan = (observer.sidereal_time() * 180 / math.pi) % 360

        # Mean anomaly
        m = (observer.lat * 180 / math.pi) % 360 

        # Format TLE strings
        line0 = "Test satellite"
        line1 = set_checksum(f"1 70000U 20600A   {epochyr:02d}{epochdoy:012.8f}  " \
                             f".00000000  00000-0  50000-4 0  0000")
        line2 = set_checksum(f"2 70000  90.0000 {raan:8.4f} 0001000 000.0000 " \
                             f"{m:8.4f} 15.50000000     0")

        # Compute next pass
        observer.date = ephem.date(t)
        sat = ephem.readtle(line0, line1, line2)
        sat.compute(observer)
        tr, azr, tt, altt, ts, azs = observer.next_pass(sat)

        # Time offset
        dt -= tr.datetime() - t - datetime.timedelta(seconds=1)

    return line0, line1, line2
  
if __name__ == "__main__":
    # Set observer
    observer = ephem.Observer()
    observer.lon = "10.00"
    observer.lat = "45.00"
    observer.elevation = 0.0

    # Set time
    t = datetime.datetime(2020, 5, 19, 6, 55, 0, 0)

    # Generate TLE
    line0, line1, line2 = generate_fake_tle_at_rise(t, observer)

    # Set date
    observer.date = ephem.date(t)
    
    # Define satellite
    sat = ephem.readtle(line0, line1, line2)
    sat.compute(observer)

    # Compute next pass
    tr, azr, tt, altt, ts, azs = observer.next_pass(sat)
    print(f"Rise at:    {tr}, azimuth:  {azr}")
    print(f"Transit at: {tt}, altitude: {altt}")
    print(f"Set at :    {ts}, azimuth:  {azs}")