QuickGPSfix

[crash893]

Im not sure i fully understand what the quickgpsfix is for. I know its to help you locate sats faster but how

i thought the positions were locked so why would a list of them help me find them faster.

and why can i only download them for 7 days. The file is only .1 mb so why not 30 days or 90?


anyone have any thoughts on the topic?

 

[a.b.normal]

and why can i only download them for 7 days.

because it contains where the satellites are going to be for seven days.

 

[Tim25]

The satellites are not in the same place. Communications satellites, such as the ones which bring tv to your house, appear pretty well fixed, but GPS satellites are at roughly half the height of geostationary comm satellites. Rather than appearing at a fixed point in the sky, the GPS satellites orbit moves around the earth roughly twice a day.

The orbits change slightly and are adjusted over time. The current orbits are mathematically modelled with parameters called ephemeris data. The ephemeris data actually used by the satellites is only good for about 4 hours. So, having current ephemeris data provides a good model of the satellites current orbit, but this changes and is adjusted over time.

Your TT can still lock onto satellites without current ephemeris data, but it can take longer and require a stronger signal. With current ephemeris, the TT can better predict current locations of satellites and lock on quicker...and be up and running even with a weaker signal.

I like KICHU1979's description at:
http://tomtomforums.com/showthread.php?t=7322

Also, if you look at the graphic on the right side of the technical description at http://en.wikipedia.org/wiki/GPS, you will get a sense for the number of satellites and how they are visible from a location over time....each of those orbits is modelled with ephemeris data...and varies over time.

 

[crash893]

i think you all missed my point

if it can be mathmaticly calculated where the sats are going to be why cant i get a update for 30 or 90 days

rather then just 7 days

 

[Smokiewolf]

i think you all missed my point

if it can be mathmaticly calculated where the sats are going to be why cant i get a update for 30 or 90 days

rather then just 7 days

Maybe the size of that file would be wayyyyy too big

 

[crash893]

I doubt it

the 7 days is .1mb

so using that as an example 70 day would be 1.0mb

 

[Tim25]

i think you all missed my point

if it can be mathmaticly calculated where the sats are going to be why cant i get a update for 30 or 90 days

rather then just 7 days

The current ephemeris data provides coefficients for a mathematical model of the orbit based on CURRENT tracking information of the satellite. The important thing is that their is some measure of VARIATION which must be updated by continually tracking and calculating new ephemeris. If the parameters used in the model are way out of date, they will not predict the correct satellite location. New ephemeris for the GPS satellites is updated every 4 hours...Will running without ephemeris updates for several days or more really throw things off? To an extent, but it will probably not be all that noticeable in the short to medium term.

At work, we work w/ geostationary comm satellites. We update the ephemeris info for the operational system very frequently. For our test system, we generally update the ephemeris at the beginning of each major test activity. If the ephemeris is very old, strange things can happen....I have seen where the ephemeris is about 60 to 90 days old...satellites which are over the Atlantic calculate out as if they were over the Pacific. There is a limited lifetime to how long those parameters are good for (and that is why they are updated on a regular basis).

 

[crash893]

yes but you are dealing with a much higher end system.

I guess what i dont understand is there some level of unpredictablty in the sats? if so that would explain the need for updates


also the tomtom is basicly a computer as it is why cant it calculate its new tables unless there is something that cant be modeled mathematically?

 

[Tim25]

In my last reply, I indicated that using very old ephemeris could even make geoSTATIONARY satellites calculate as if they had moved to a completely different location...let alone trying to model a GPS satellite which is actually moving around the earth at thousands of miles per hour. Since my other explanations didn't seem to help, I can only try an analogy at this point:

Suppose you had a sophisticated program to see how on-track you were for retirement savings...You entered all the parameters your first year out of school: your salary, your expenses, inflation rates, etc....If you plugged all this in you would expect it to yield good results...certainly for the first year or 2...or even a little longer, even if you didn't update the numbers. However, as the years went by, you would certainly get increasingly erroneous projections if you never updated the numbers.

With ephemeris that is updated every 4 hours....it's not too far off track if you don't update it immediately...maybe not even that bad after a week, but you are going to have too many accumulated errors if you don't update to current numbers for 90 days....That is like expecting to enter your first year salary and expense info into the retirement estimator and expecting it to yield accurate results at the end of your life without ever adjusting the numbers along the way.

If that still doesn't help explain it at some level....well...that's as good as I can do.

 

[crash893]

so the sats have some level of unpredictability in there orbit?

 

[Tim25]

so the sats have some level of unpredictability in there orbit?

Yes...due to the effects of drag and the firing of thrusters to correct the orbit.