Satellite Almanac Life Expectancy

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Q: For how long can an almanac be used?

A: The designers of the Global Positioning System, in figuring out how they could help GPS receivers determine their position more quickly, designed the almanac in order to identify GPS satellites’ orbital positions or ephemerides (singular: ephemeris). They knew that each GPS satellite would have to broadcast its own precise, meter-level ephemeris to the receiver, but because they were constrained by the navigation message data size, each satellite could not deliver the precise ephemerides for all other satellites.

The almanac was developed as a coarse equivalent to the precise ephemeris, reducing the number of bits required to transmit the necessary data at the cost of reduced accuracy of orbital propagation. The propagated orbits are, however, still accurate enough for the intended purpose of determining which satellites are above the horizon and computing a rough estimate of its Doppler shift (for a given receiver position).

When downloaded by the receiver, the almanac data is stored in no particular format, although the 2nd Space Operations Squadron makes the almanacs available via their web site in the SEM and YUMA formats. The almanac file format descriptions as well as the almanacs themselves can be downloaded from the GPS section of the Celestrak website: <http://celestrak.com/GPS>.

Because almanacs provide a reduced accuracy ephemeris and are used in the receiver for operations as well as in many planning tools for analysis, a look at how long these almanacs can be used is warranted. The GPS Control Segment generates a new almanac every day and sends it to each satellite with the next scheduled navigation data upload.

Just because a new almanac is available, does that always mean it must be downloaded and used? For continuously operating receivers, there is little additional cost to extracting and using the latest almanac data. However, for power-limited receivers, downloading an almanac may take more time and power than is deemed acceptable.

Furthermore, for mission analysts, retrieving a new almanac every day may require additional tasks that may not be logistically easy or worthwhile. We’ll look at these two areas and determine how long an almanac can be used for predicting PDOP and Doppler shift.

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.

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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.

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Summary
Almanacs provide a good ephemeris for mission planning and receiver operations with the computed orbits providing a good basis for predictions well over two weeks into the future. For mission planning and PDOP prediction, an almanac of up to four weeks old would still work well. For Doppler shift prediction, the almanac could be used for an even longer period of time.

Both types of predictions will suffer, however, when satellite additions, removals, or maneuvers take place, drastically reducing the predictive ability of an almanac. One recommendation for optimizing the almanac download is to retrieve a new almanac only when the health indicator of any satellite changes.

Through the predictive analysis of PDOP and Doppler, we can conclude that even though almanacs are uploaded to GPS satellites each day, GPS receivers do not necessarily need to download and use it at this frequency. Given the error analysis described in this article, one can now determine the best schedule for downloading and using an almanac to suit a particular application.

(For the rest of Ted Driver's answer to this question, please download the complete article using the pdf link above.)

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