What Is an Ephemeris? Position Data Explained

Quick answer

An ephemeris is a table or dataset giving a celestial object's position (and usually velocity) over time. Satellite operations run on them — from precise operator ephemerides used for collision screening to the broadcast ephemerides every GNSS receiver downloads to compute its fix.

📘 Full definition✓ Reviewed 2026-09-07

An ephemeris (plural: ephemerides) is a time-indexed record of where an object is: positions, and typically velocities, at a sequence of instants, either tabulated directly or encoded as parameters from which positions are computed. The concept spans astronomy and spaceflight. Planetary ephemerides from institutes like JPL underpin deep-space navigation; satellite operators generate precision ephemerides for their fleets — built from GPS receivers on board and ranging data — which feed conjunction screening far more accurately than public TLEs; and every GNSS satellite broadcasts its own ephemeris, the freshly uplinked orbital parameters a receiver needs to turn signal timing into position. The distinctions that matter are accuracy and shelf life: a broadcast GPS ephemeris is metre-accurate but valid for hours; a TLE is kilometre-class for days; an operator's ephemeris with planned manoeuvres folded in is the gold standard for avoiding collisions.

Low Precision

TLE + SGP4

High Precision

OEM (numerical)

Planetary Data

JPL Horizons

Accuracy

Metres (vs km for TLEs)

Understanding Ephemeris

A hierarchy of truth

Think of ephemerides as tiers. Tier one: precision orbit determination with onboard GNSS — centimetres to metres, the standard for science missions and conjunction work. Tier two: broadcast navigation ephemerides — metres, short-lived, continuously refreshed. Tier three: surveillance-derived catalogues and TLEs — kilometre-class, but available for every tracked object including debris that carries no transponder. Operations constantly blend tiers: screen everything with tier three, resolve events with tier one.

Ephemerides for the deep sky

For planets, moons and spacecraft beyond Earth, ephemeris services numerically integrate the solar system's dynamics, fitted to decades of radar, laser and tracking data. Mission navigation, occultation timing, telescope pointing and this site's deep-space and worlds tracking all consume such solutions — the quiet infrastructure beneath every "where is it now?" answer in astronomy.

Frequently Asked Questions

How does an ephemeris differ from a TLE? ▼

A TLE is one compact snapshot of mean orbital elements, meant for propagation with SGP4 at kilometre-class accuracy. An ephemeris is the richer product — explicit positions over time, often from precision orbit determination, including planned manoeuvres. Screening systems prefer operator ephemerides exactly because TLEs cannot know a burn is coming.

What is in a GNSS broadcast ephemeris? ▼

A set of quasi-Keplerian parameters and clock corrections, refreshed every few hours, from which the receiver computes the satellite's position to metre level. It is why a cold-starting receiver takes time to fix: it must first download ephemerides before it can solve.

Why do operators exchange ephemerides directly? ▼

Because in a close-approach event, both parties acting on the same precise, manoeuvre-aware data beats both guessing from public catalogues. Formats like the CCSDS OEM standardise the exchange, and screening services ingest them to refine conjunction probabilities.

Understanding Ephemeris

A hierarchy of truth

Ephemerides for the deep sky

Frequently Asked Questions

Sources & References