Short answer
Two related types of orbital data broadcast by GNSS satellites. Ephemeris gives precise (cm-class) satellite positions, validity 4 hours. Almanac gives coarse (km-class) positions for the entire constellation, validity weeks. Receivers need both for cold start.
Detailed explanation
Ephemeris is the precise orbit model for ONE satellite, valid for a short window (4 hours for GPS, similar for other GNSS). It contains the satellite's Keplerian orbital elements plus correction terms that account for non-Keplerian perturbations. A receiver needs current ephemeris for every satellite it wants to range to in order to compute satellite position with cm-level accuracy.
Almanac is a coarse orbit model for ALL satellites in the constellation, valid for weeks (typically refreshed daily). The almanac lets a receiver know which satellites should be visible at any given time and approximate location, which is essential for warm-start acquisition. Almanac accuracy is km-class — too coarse for position fixing, just for visibility prediction.
Both are broadcast in the navigation message at low data rates (50 bit/s on GPS legacy). A complete ephemeris frame takes 30 seconds per satellite; a complete almanac for all 32 GPS satellites takes 12.5 minutes. This is why cold start (no almanac) is slow — and why A-GNSS (cellular-delivered almanac + ephemeris) dramatically accelerates first fix.
Broadcast ephemeris is fine for navigation but not for high-precision processing. PPP services use precise ephemeris computed post-fact from a global tracking network — IGS Ultra-Rapid (3–9 hour latency, 5 cm accuracy), Rapid (17 hour latency, 2.5 cm), Final (12 day latency, 2.5 cm or better). The GPS Time Converter tool helps verify ephemeris validity windows in week + TOW.
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GPS Time Converter
Open the toolRelated terms
TTFF
The elapsed time from receiver power-on (or warm/cold start) to the first valid position fix. Cold start (no almanac, no recent position, no time) is typically 30–60 seconds; warm start (with valid almanac and approximate position) is 5–15 seconds; hot start (with current ephemeris) is <1 second.
RINEX
The standard text-based file format for storing raw GNSS observations (.YYo files), navigation messages (.YYn), and meteorological data (.YYm). All survey-grade receivers can write RINEX, and all post-processing software can read it. Versions 2.11 and 3.05 are the most common.
PPP
A precise GNSS positioning technique using globally-broadcast precise orbit, clock, and atmospheric corrections to deliver decimetre-to-centimetre accuracy with a single receiver — no local base station required. Convergence times of 10–60 minutes for cm-level; instant for decimetre via PPP-RTK.
CORS
A permanently-installed GNSS receiver streaming raw measurements 24/7. CORS networks are the backbone of RTK and network-RTK services, geodetic monitoring, and post-processed precise positioning. Typical inter-station spacing is 50–100 km for regional RTK, 200–300 km for national geodetic networks.