Functions: Satellite

Functions for propagating TLEs, converting coordinate frames, computing ground tracks, and predicting satellite passes. These form the core satellite tracking pipeline in pg_orbit.

---

sgp4_propagate

Propagates a TLE to a given time using the SGP4 (near-earth) or SDP4 (deep-space) algorithm. The algorithm is selected automatically based on orbital period: elements with a period >= 225 minutes use SDP4.

Signature

sgp4_propagate(tle tle, t timestamptz) → eci_position

Parameters

Parameter	Type	Description
tle	tle	Two-Line Element set to propagate
t	timestamptz	Target epoch for propagation

Returns

An eci_position in the TEME reference frame. Position in km, velocity in km/s.

Errors

Raises an exception if SGP4/SDP4 returns a fatal error code (e.g., satellite decay, eccentricity out of range, mean motion near zero). Use sgp4_propagate_safe if you need NULL-on-error behavior.

Caution

SGP4 accuracy degrades as you propagate further from the TLE epoch. For LEO satellites, errors grow roughly 1-3 km per day. Keep TLEs fresh.

Example

WITH iss AS (
  SELECT '1 25544U 98067A   24001.50000000  .00016717  00000-0  10270-3 0  9025
2 25544  51.6400 208.9163 0006703  30.1694  61.7520 15.50100486 00001'::tle AS tle
)
SELECT eci_x(pos)     AS x_km,
       eci_y(pos)     AS y_km,
       eci_z(pos)     AS z_km,
       eci_speed(pos) AS speed_kms
FROM iss, sgp4_propagate(tle, '2024-01-02 12:00:00+00') AS pos;

---

sgp4_propagate_safe

Identical to sgp4_propagate, but returns NULL instead of raising an exception on propagation errors. This is the batch-safe variant for processing large TLE catalogs where some elements may be stale or invalid.

Signature

sgp4_propagate_safe(tle tle, t timestamptz) → eci_position

Parameters

Parameter	Type	Description
tle	tle	Two-Line Element set to propagate
t	timestamptz	Target epoch for propagation

Returns

An eci_position, or NULL if propagation fails.

Example

-- Propagate an entire catalog, skipping failed elements
SELECT norad_id,
       eci_x(pos) AS x_km,
       eci_y(pos) AS y_km,
       eci_z(pos) AS z_km
FROM satellite_catalog,
     sgp4_propagate_safe(tle, now()) AS pos
WHERE pos IS NOT NULL;

---

sgp4_propagate_series

Generates a time series of TEME ECI positions for a single TLE over a time range. Returns one row per time step. This is significantly faster than calling sgp4_propagate inside a generate_series because the SGP4 initializer runs once.

Signature

sgp4_propagate_series(
  tle tle,
  start_time timestamptz,
  end_time timestamptz,
  step interval
) → TABLE(t timestamptz, x float8, y float8, z float8, vx float8, vy float8, vz float8)

Parameters

Parameter	Type	Description
tle	tle	Two-Line Element set
start_time	timestamptz	Start of the time range
end_time	timestamptz	End of the time range (inclusive if aligned to step)
step	interval	Time between samples

Returns

A set of rows with timestamp and TEME position/velocity components. Position in km, velocity in km/s.

Example

WITH iss AS (
  SELECT '1 25544U 98067A   24001.50000000  .00016717  00000-0  10270-3 0  9025
2 25544  51.6400 208.9163 0006703  30.1694  61.7520 15.50100486 00001'::tle AS tle
)
SELECT t, x, y, z, vx, vy, vz
FROM iss,
     sgp4_propagate_series(tle,
       '2024-01-02 00:00:00+00',
       '2024-01-02 01:00:00+00',
       interval '1 minute');

---

tle_distance

Computes the Euclidean distance between two satellites at a given time. Both TLEs are propagated to the target time and the 3D distance between their TEME positions is returned.

Signature

tle_distance(a tle, b tle, t timestamptz) → float8

Parameters

Parameter	Type	Description
a	tle	First satellite
b	tle	Second satellite
t	timestamptz	Evaluation time

Returns

Distance in kilometers. Raises an exception if either TLE fails to propagate.

Example

-- Distance between two satellites at a specific time
SELECT tle_distance(sat_a.tle, sat_b.tle, '2024-06-15 12:00:00+00') AS dist_km
FROM satellite_catalog sat_a, satellite_catalog sat_b
WHERE sat_a.norad_id = 25544  -- ISS
  AND sat_b.norad_id = 48274; -- CSS (Tianhe)

---

eci_to_geodetic

Converts a TEME ECI position to WGS-84 geodetic coordinates. The timestamp is required to compute the Earth’s rotation angle (Greenwich Apparent Sidereal Time).

Signature

eci_to_geodetic(pos eci_position, t timestamptz) → geodetic

Parameters

Parameter	Type	Description
pos	eci_position	TEME ECI position
t	timestamptz	Time of the position (for sidereal time computation)

Returns

A geodetic with WGS-84 latitude, longitude, and altitude.

Example

WITH iss AS (
  SELECT '1 25544U 98067A   24001.50000000  .00016717  00000-0  10270-3 0  9025
2 25544  51.6400 208.9163 0006703  30.1694  61.7520 15.50100486 00001'::tle AS tle
)
SELECT geo_lat(g) AS lat,
       geo_lon(g) AS lon,
       geo_alt(g) AS alt_km
FROM iss,
     eci_to_geodetic(sgp4_propagate(tle, now()), now()) AS g;

---

eci_to_topocentric

Converts a TEME ECI position to topocentric (observer-relative) coordinates. Computes azimuth, elevation, slant range, and range rate.

Signature

eci_to_topocentric(pos eci_position, obs observer, t timestamptz) → topocentric

Parameters

Parameter	Type	Description
pos	eci_position	TEME ECI position and velocity
obs	observer	Observer location
t	timestamptz	Time of the position (for sidereal time computation)

Returns

A topocentric with azimuth, elevation, range, and range rate.

Example

WITH iss AS (
  SELECT '1 25544U 98067A   24001.50000000  .00016717  00000-0  10270-3 0  9025
2 25544  51.6400 208.9163 0006703  30.1694  61.7520 15.50100486 00001'::tle AS tle
)
SELECT topo_azimuth(tc)    AS az,
       topo_elevation(tc)  AS el,
       topo_range(tc)      AS range_km,
       topo_range_rate(tc) AS range_rate_kms
FROM iss,
     eci_to_topocentric(
       sgp4_propagate(tle, now()),
       '40.0N 105.3W 1655m'::observer,
       now()
     ) AS tc;

---

subsatellite_point

Returns the nadir (directly below the satellite) point on the WGS-84 ellipsoid for a given TLE at a given time. This is a convenience function equivalent to propagating and then converting to geodetic, but with altitude set to the satellite altitude.

Signature

subsatellite_point(tle tle, t timestamptz) → geodetic

Parameters

Parameter	Type	Description
tle	tle	Satellite TLE
t	timestamptz	Evaluation time

Returns

A geodetic with the latitude, longitude, and altitude of the satellite above the WGS-84 ellipsoid.

Example

WITH iss AS (
  SELECT '1 25544U 98067A   24001.50000000  .00016717  00000-0  10270-3 0  9025
2 25544  51.6400 208.9163 0006703  30.1694  61.7520 15.50100486 00001'::tle AS tle
)
SELECT geo_lat(sp) AS nadir_lat,
       geo_lon(sp) AS nadir_lon,
       geo_alt(sp) AS altitude_km
FROM iss, subsatellite_point(tle, now()) AS sp;

---

ground_track

Generates a time series of subsatellite points (nadir ground track) for a satellite over a time range. Each row contains the timestamp, latitude, longitude, and altitude.

Signature

ground_track(
  tle tle,
  start_time timestamptz,
  end_time timestamptz,
  step interval
) → TABLE(t timestamptz, lat float8, lon float8, alt float8)

Parameters

Parameter	Type	Description
tle	tle	Satellite TLE
start_time	timestamptz	Start of the time range
end_time	timestamptz	End of the time range
step	interval	Time between samples

Returns

A set of rows with timestamp, latitude (degrees), longitude (degrees), and altitude (km).

Example

-- ISS ground track for one orbit (~92 minutes) at 30-second resolution
WITH iss AS (
  SELECT '1 25544U 98067A   24001.50000000  .00016717  00000-0  10270-3 0  9025
2 25544  51.6400 208.9163 0006703  30.1694  61.7520 15.50100486 00001'::tle AS tle
)
SELECT t, lat, lon, alt
FROM iss, ground_track(tle, now(), now() + interval '92 minutes', interval '30 seconds');

---

observe

Propagates a TLE and computes topocentric look angles in a single call. Equivalent to eci_to_topocentric(sgp4_propagate(tle, t), obs, t), but avoids the intermediate allocation.

Signature

observe(tle tle, obs observer, t timestamptz) → topocentric

Parameters

Parameter	Type	Description
tle	tle	Satellite TLE
obs	observer	Observer location
t	timestamptz	Observation time

Returns

A topocentric with azimuth, elevation, range, and range rate.

Example

WITH iss AS (
  SELECT '1 25544U 98067A   24001.50000000  .00016717  00000-0  10270-3 0  9025
2 25544  51.6400 208.9163 0006703  30.1694  61.7520 15.50100486 00001'::tle AS tle
)
SELECT topo_azimuth(o)   AS az,
       topo_elevation(o) AS el,
       topo_range(o)     AS range_km
FROM iss, observe(tle, '40.0N 105.3W 1655m'::observer, now()) AS o;

---

observe_safe

Identical to observe, but returns NULL instead of raising an exception on propagation errors. Use this when processing large TLE catalogs in batch.

Signature

observe_safe(tle tle, obs observer, t timestamptz) → topocentric

Parameters

Parameter	Type	Description
tle	tle	Satellite TLE
obs	observer	Observer location
t	timestamptz	Observation time

Returns

A topocentric, or NULL if propagation fails.

Example

-- Find all satellites above the horizon right now, skipping stale TLEs
SELECT norad_id,
       topo_azimuth(o)   AS az,
       topo_elevation(o) AS el
FROM satellite_catalog,
     observe_safe(tle, '40.0N 105.3W 1655m'::observer, now()) AS o
WHERE o IS NOT NULL
  AND topo_elevation(o) > 0
ORDER BY topo_elevation(o) DESC;

---

next_pass

Finds the next satellite pass over an observer location. Searches forward from the given start time up to 7 days.

Signature

next_pass(tle tle, obs observer, start timestamptz) → pass_event

Parameters

Parameter	Type	Description
tle	tle	Satellite TLE
obs	observer	Observer location
start	timestamptz	Time to begin searching from

Returns

A pass_event with AOS, maximum elevation, LOS, azimuths, and duration. Returns NULL if no pass is found within 7 days (possible for equatorial observers looking for high-inclination satellites, or vice versa).

Tip

For finding multiple passes, use predict_passes instead. next_pass is optimized for the single-pass case (e.g., “when is the ISS visible next?”).

Example

WITH iss AS (
  SELECT '1 25544U 98067A   24001.50000000  .00016717  00000-0  10270-3 0  9025
2 25544  51.6400 208.9163 0006703  30.1694  61.7520 15.50100486 00001'::tle AS tle
)
SELECT pass_aos_time(p)      AS rise,
       pass_max_elevation(p) AS max_el,
       pass_los_time(p)      AS set,
       pass_duration(p)      AS duration
FROM iss, next_pass(tle, '40.0N 105.3W 1655m'::observer, now()) AS p;

---

predict_passes

Finds all satellite passes over an observer within a time window, optionally filtered by minimum elevation. Returns a set of pass_event records.

Signature

predict_passes(
  tle tle,
  obs observer,
  start_time timestamptz,
  end_time timestamptz,
  min_el float8 DEFAULT 0.0
) → SETOF pass_event

Parameters

Parameter	Type	Default	Description
tle	tle		Satellite TLE
obs	observer		Observer location
start_time	timestamptz		Start of the search window
end_time	timestamptz		End of the search window
min_el	float8	0.0	Minimum peak elevation in degrees. Passes whose maximum elevation is below this threshold are excluded.

Returns

A set of pass_event records, ordered by AOS time.

Example

-- All ISS passes above 20 degrees in the next 3 days
WITH iss AS (
  SELECT '1 25544U 98067A   24001.50000000  .00016717  00000-0  10270-3 0  9025
2 25544  51.6400 208.9163 0006703  30.1694  61.7520 15.50100486 00001'::tle AS tle
)
SELECT pass_aos_time(p)      AS rise,
       pass_max_elevation(p) AS max_el,
       pass_aos_azimuth(p)   AS rise_az,
       pass_los_azimuth(p)   AS set_az,
       pass_duration(p)      AS dur
FROM iss,
     predict_passes(tle, '40.0N 105.3W 1655m'::observer,
                    now(), now() + interval '3 days', 20.0) AS p;

---

pass_visible

Returns true if at least one satellite pass occurs over the observer during the given time window.

Signature

pass_visible(tle tle, obs observer, start_time timestamptz, end_time timestamptz) → boolean

Parameters

Parameter	Type	Description
tle	tle	Satellite TLE
obs	observer	Observer location
start_time	timestamptz	Start of the search window
end_time	timestamptz	End of the search window

Returns

true if any pass (elevation > 0) occurs in the window; false otherwise. This is faster than predict_passes when you only need a yes/no answer because it stops searching after the first pass is found.

Example

-- Which satellites from the catalog pass over Boulder tonight?
SELECT norad_id, name
FROM satellite_catalog
WHERE pass_visible(tle, '40.0N 105.3W 1655m'::observer,
                   '2024-06-15 02:00:00+00', '2024-06-15 10:00:00+00');