GPS Almanac — what is this?

These 3 questions will be answered in this Blog Posting:

  1. What is a GPS Almanac and what is its purpose?
  2. How do I receive a more current GPS Almanac?
  3. What other GPS resources are available?

Almanac: approximate orbit information and clock information for all satellites in the constellation.

  • Almanac data is included in the “Nav – Navigation” message broadcast by each satellite.
  • The receiver uses this data to determine which satellites it should track (which ones are “in view” and offer the best satellite geometry for the most accurate position fix).
  • The almanac data is retained in your receiver’s memory to enable rapid satellite signal acquisition and to calculate satellites’ azimuths and elevations.
  • If your GPS unit has not been turned on for several weeks, month’s or has been transported to a new region it may take longer to acquire satellites and update the GPS Almanac file.

If you start a data file in TerraSync or ArcPad with Trimble GPS Correct you should receive an updated GPS Almanac within 12.5 minutes but sometimes it may take up to 15-20 minutes.

You can refer to your TerraSync, GPS Correct or GPS Controller “Getting Started/User Guides” for more information about GPS Almanac.  You can download them at the Trimble website underneath your software category and under Documentation link.    Trimble Support and Training/ Support/“Support A-Z List”

  • Click on the first letter of the name of the product and then search for your software:
  • GPS Controller, GPS Correct and or TerraSync.  Once you click on the software name then click on “Documentation” to find the “Getting Stared/User Guides”.

If your GPS receiver is not working properly and you are in an open area and still are not receiving 4 or more satellites within 5 minutes you can try this. 

  • Go to the Setup page and click on Options and click on Reset GPS Receiver.   This will only work if your GPS is on and receiving satellites.  This refreshes the receiver and cleans out any old GPS Almanac file.  Within 15-20 minutes while a data file is open you should receive an Almanac. 
  • To check the Almanac date in Trimble TerraSync, Trimble GPS Correct or Trimble GPS Controller software click on Satellite Info and it should be in the lower left of the screen.  It may not always be today’s date but it will be within 3 days.  The Department of Defense doesn’t upload a new GPS Almanac everyday – only when needed to update information about a GPS satellite i.e. health changed, position change, decommissioned or a new satellite is now active and broadcasting.

Other GPS resources:

GPS 101 Tutorial on Trimble’s website.

GPS – The First Global Satellite Navigation System   This is a PDF free version of the soft cover 151 page book published by Trimble in 2007.   Just some light reading material.  It’s one of the better GPS resources in a small book format that you can read for few minutes, put it down and come back when you need to look up another term or topic.  The book can also be found in paperback format found online. 

TKN:  Trimble Knowledge Network  You can create your own username and password and check out the Learning Center and watch Trimble Mapping, Survey and Construction videos at this website.

From the Archives from 2002:  Trimble Mapping Systems – General Reference   This document explains basic GPS and has a good glossary.

*If you know of other GPS resources that you would like for me to share per this post please e-mail me at:  mapsupport@seilerinst.com

**Check back for Future posts that relate to the GPS Almanac topic that will cover using the GNSS Planning” software on your Mobile device, desktop computer and online resources.

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