How Do I Know Almanac Has Loaded On 60csx?

[+TCOMGUY]

Hi guys! I keep reading posts that talk about loading the almanac. But...

 

1. What the Heck is it?

 

2. How do I know that it has actually loaded?

 

3. More specifically, what will my device say or show in the screen to let me know it is done.

 

4. Once it is loaded, I dont have to do it again every time I turn it on right?

 

5. Im gonna be travelling to S.F. this summer (over 1k miles). Do I need to reload the almanac when I get there?

 

Thanks!

[+Miragee]

I'm not up on all that technical jargon . . . I just know after my GPSr has a clear view of the sky the accuracy goes down to around 9 to 30 feet and at that I know it is good enough to find the caches.

 

With that GPSr, it shouldn't take more than 15 minutes with a clear view of the sky to get all the data it needs.

 

When you travel that far, from Puerto Rico to San Francisco, the GPSr will need a bit of time to figure out where it is, but that unit will figure it out very quickly.

 

Once the GPSr is in a location, it gets a lock very quickly . . . unless it gets confused as mine did last night when it thought I was out in the Pacific Ocean. I shut it off and less than a minute later it told me I was back on dry land right where I thought I was.

[+EraSeek]

Basically there is the GPS almanac which is broadcast from the GPS satellites every 12 minutes and tells your GPS important things such as where it should expect to see each of the satellites. Your GPS recieves this info automatically and if you can achieve a postion fix it is working.

 

Then there is the WAAS (wide area augmentation system) almanac. The WAAS almanac is a single one-second message (or two messages if there are more than 3 satellites) that's repeated at least every 5 minutes. It sends data to your WAAS enabled reciever which corrects for errors in the GPS signal such as position and timing errors and signal delay errors caused by an active ionosphere. To get this almanac to function you must enable WAAS on your reciever and expose the GPS to one of the WAAS sats in an open area long enough for it to cycle through the sats and download the WAAS almanac. The first exposure may take up to about 15 or even 20 minutes, or perhaps much shorter. When this process is complete you will see D's on the corrected satellite bars on your GPS. After the initial exposure, your unit will pick up WAAS much quicker every time you have a WAAS sat in sight.

[+apersson850]

The answers above are correct. I just want to add a bit more, even if you didn't ask, to aid in understanding this.

 

Think of the almanac as coarse orbital data. It will give the unit an idea of which satellites are overhead. On a Garmin unit, that's missing almanac data, all the satellites it searches for are shown due north, at the horizon. As soon as the unit gets almanac data for a satellite, it's moved to the proper place on the skyview, as it would look if you are on the same place as you were, last time the unit was used. You can check on the map to see where the unit assumes you to be located.

 

You have to do a master reset, or a specific almanac reset, of your unit to loose the almanac. Since it's coarse in its nature, it's valid for several months. Thus nothing special has to be done to maintain it.

 

Then there's precise orbital data, known as ephemeris data. To be able to compute your position, this data must be received too. Ephemeris data is sent in three blocks, each six seconds long, starting every tenth second. Hence it takes at least half a minute to get this data. Ephemeris data will be valid for an hour or so, which means that if you turn your unit off, then back on some hours later, then it's probably not valid any longer.

On Garmin units supporting the skyview, you can see when ephemeris data has been loaded for a satellite, since the signal strength bar gets filled/colored then.

 

Unlike almanac data, which is sent for all satellites by all satellites, ephemeris data is sent for each satellite by itself. This also explains why a unit, which has been allowed to load this data during good conditions, will perform better under foilage. If it doesn't have ephemeris data before going into difficult conditions, then the chance is small that it will be able to download it in there. But if the data is already present, then only a second of reception will allow a position update.

[+TCOMGUY]

Thank you all for your replies.

[+PMaholm]

Wow yeah. that definatly improved my understanding of the GPS system.

 

Edit: I am being serious. after posting I saw how some may read my post as sarcasam. This is not the case I did enjoy the replies and they did help.

Edited June 22, 2006 by PMaholm

[AdMan101]

I have a 60csx and this really helped me understand what's going on.

 

Thank you all.

[aranjit]

The answers above are correct. I just want to add a bit more, even if you didn't ask, to aid in understanding this.

 

Think of the almanac as coarse orbital data. It will give the unit an idea of which satellites are overhead. On a Garmin unit, that's missing almanac data, all the satellites it searches for are shown due north, at the horizon. As soon as the unit gets almanac data for a satellite, it's moved to the proper place on the skyview, as it would look if you are on the same place as you were, last time the unit was used. You can check on the map to see where the unit assumes you to be located.

 

You have to do a master reset, or a specific almanac reset, of your unit to loose the almanac. Since it's coarse in its nature, it's valid for several months. Thus nothing special has to be done to maintain it.

 

Then there's precise orbital data, known as ephemeris data. To be able to compute your position, this data must be received too. Ephemeris data is sent in three blocks, each six seconds long, starting every tenth second. Hence it takes at least half a minute to get this data. Ephemeris data will be valid for an hour or so, which means that if you turn your unit off, then back on some hours later, then it's probably not valid any longer.

On Garmin units supporting the skyview, you can see when ephemeris data has been loaded for a satellite, since the signal strength bar gets filled/colored then.

 

Unlike almanac data, which is sent for all satellites by all satellites, ephemeris data is sent for each satellite by itself. This also explains why a unit, which has been allowed to load this data during good conditions, will perform better under foilage. If it doesn't have ephemeris data before going into difficult conditions, then the chance is small that it will be able to download it in there. But if the data is already present, then only a second of reception will allow a position update.

 

I was just looking online as I was reading this thread and found this page which brought up a lot of questions.

 

I think from that page the 30s comes about because 5 frames (each 6s long) are repeatedly sent. The first 3 being the clock/ephemeris data. The page seems to show some fundamental limits on acquisition time which the sirfIII chips seemt to have gotten around, or perhaps I am misunderstanding whats going on.

 

From my understanding a cold start could take 30s no matter what gpsr is being used. If so how can sirfIII gpsr's talk of 1s ttff? I assume this means they already have the almanac and ephemeris data? Can a gpsr determine its location from the almanac data alone and thus determine position without needing to wait for the ephemeris data?

 

For the tree cover example, from my understanding, if you have the ephemeris and the almanac then you would just need to get the time from the satellite (frame 1) which would take 6s. Since this is only repeated every 30s, in theory you might need to wait 30s to get an accurate fix, is that right?

 

Thanks for the great info!