Definition
Reference information transmitted by GPS satellites that describes the approximate orbits and status of every satellite in the GPS constellation. A receiver uses almanac data to predict which satellites will be in view at a given time and location, which speeds up satellite acquisition when the unit is turned on.
Plain English
A rough schedule of where every GPS satellite is and where it's heading. The receiver uses it as a head start so it knows which satellites to look for when you switch it on.
Context Anchor
Seen in GPS and instrument navigation discussions, especially when a receiver is starting up, searching for satellites, or updating stored satellite information.
Derivation
From the Arabic 'al-manakh,' meaning a calendar or table of astronomical events. Old paper almanacs listed where the sun, moon, and stars would be on any given day. GPS borrowed the same idea: a table predicting where the satellites will be.
Why Pilots Care
Fresh almanac data shortens the time required for a GPS receiver to acquire satellites and obtain a navigation solution after power-up or signal loss.
Analogy
It is like having a rough schedule that tells you which buses should be near your stop. It does not drive the bus for you, but it helps you know where to start looking.
Intuition Check
Almanac data is not a weather almanac or a pilot log. Here, it means stored GPS satellite information that helps the receiver find usable satellites.
Example Sentence 1
After the GPS sat unused for several months, it took a few extra minutes to download fresh almanac data before it could acquire a position.
Example Sentence 2
The receiver uses almanac data to predict which satellites will be above the horizon before it requests the more precise ephemeris data from each one.