Archive for December, 2008

How long can you use an almanac? (Part two)

December 05th, 2008 | Category: Navigation Accuracy

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As the saying goes, you don't need to beat a dead horse.  My covering almanacs again in The Nog seems like a long dead horse, but I'll make this one final entry - I promise.  I recently wrote a Nog entitled: How long can you use an almanac?  There, I outlined the timelines for almanac longevity and some general guidelines too.  Well, as luck would have it, the GNSS trade magazine InsideGNSS  liked that Nog and asked if I could do a little more research and publish the results in their GNSS Solutions column.  The column is finished and can be found here:, as well as in the November/December issue of the magazine.

In this Nog, I'll summarize the results of the analysis, since it differs a bit from the analysis in my previous post.  The following sections are covered:

Mission Planning

Almanacs are used in mission planning to predict dilution of precision (DOP) — a key component in navigation accuracy. DOP is not the only element of navigation accuracy; the other is the measurement accuracy, but DOP is a key indicator of mission success.

Receiver Operations

A critical receiver operation is signal acquisition, where the receiver scans both frequency and code phase to lock onto a GPS signal. When scanning the frequency, the amount to scan is determined by the predicted Doppler shift of the desired signal.

Satellite Maintenance

From the foregoing discussion, one may be tempted to use the almanac for long periods of time based on these results. However, the three PRNs that we examined did not undergo any maintenance during the 22-week period shown.

In my previous post, I covered the Mission Planning piece, but not Receiver Operations.  The reason behind the longevity numbers for an almanac, satellite maintenance, was not discussed previously either.  It turns out that required satellite maneuvers are the biggest reason that almanac usage time isn't longer.  (Not really surprising - right?)

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