Garmin Oregon - Almanac storage?

[+somegeek]

Curious if the Oregon 300 stores an individual almanac for normal and WAAS modes?

 

Does this require a good 15-20 minute soak to build a solid almanac for each mode?

 

Is it necessary to reset this almanac periodically to get a fresh almanac build?

 

Appreciate any input.

 

somegeek

[+splashy]

Normally you do it the first time after you bought it.

If you move the gps while it's off out of your timezone you might need a longer time to a sat fix.

The waas, depends where you are, it's not always a blessing and most keep it off.

[+twolpert]

The almanac contains gross satellite position information for the satellites which provide position information. This is necessary because those satellites are not in geosynchronous orbit. The almanac is good for a couple of months from the time it is acquired. All units update the almanac continuously when they have a fix. So the 20 minute soak is necessary only the first time you power up after the unit has been off for a long time (more than a couple of months).

 

There is no separate almanac for WAAS mode. In fact, the satellites which provide WAAS correction information are in geosynchronous orbit -- so the almanac is not required to locate them. However, you may need to have a fix before the unit will start looking for the WAAS satellite. Many units (can't say about the Oregon specifically) have a geographic bias as to which WAAS satellite they try first.

 

Note that there is no WAAS-related information that can be stored for more than a few hours. The whole point of WAAS is to get corrections that correspond to current conditions (especially ionospheric conditions).

 

There are three things that the unit needs to know to get a quick initial fix. First, it has to have a valid almanac. Second, it has to know approximately where it is. Normally, this is based on the last known position (when the unit was last powered down with a fix). Third, it has to know the approximate date and time. Normally, the unit keeps a clock running even when powered off specifically for this purpose. Accurate time is acquired from the satellites once at least one satellite has been located. (Note that timezone has no bearing here.) The unit uses the almanac with the location and date/time to decide what satellites should be overhead. Those are the ones it tries first. If that fails, it eventually degenerates into a full sky search, which can be time-consuming.

 

Typically, the only things that will interfere with a fairly quick initial fix are leaving the unit off for a long time (almanac becomes invalid), moving the unit more than a few hundred miles with it off (last known location is no longer "close enough"), and leaving the unit off long enough to run the batteries dead (clock stops so date/time is not known).

[+somegeek]

Excellent - thanks for the replies. All set here.