Time of Day Affects GPS Accuracy?

[Kerry.]

Hopscotch, I don't have any real issues with Hiemdahl either but you appear another lost soul in a field that I'm not sure you understand either as you've also lost complete track of the original query.

 

Try going back to the beginning and read the original query again (maybe twice).

 

quote:

---

Any tips on how to get consistent readings would be appreciated.

---

 

To answer your query you basically can't (averaging now just can't wait for that as an answer), that's simply (sort of) the way it is with a SPS receiver BUT you can make the most of what there is with some planning AND understanding BUT you've chose to dismiss those comments.

 

Cheers, Kerry.

 

I never get lost everybody keeps telling me where to go

 

[This message was edited by Kerry on April 23, 2003 at 03:00 PM.]

[dave and jaime]

maybe if the question was worded:

how can i get consistantly reliable readings?

 

the response would be to monitor the sat geometry and signal strength?

 

i realize that there are windows of errors but i think it would be possible to trust your readings if you use good judgement when taking them and ensure that there is a well spread out distribution of sats.

 

am i totaly of base here?

 

'Get to the point---speak English!!!!'

[+RangerRick]

Lots of good information in this section even though tempers got a litte heated. Well the attitude adjuster just arrived. First of all I think you are all correct in what you say. Personally, for geocaching purposes, rarely will the time of day make a difference. I found well over a hundred caches and some were at 5 in the morning, some were in the afternoon and some were midnight caches. If I was concerned about finding the cache as easy as possible, then I certainly would not attempt a midnight cache. So being off 5 feet at 3 pm instead of of 1 foot at 10 am really doesn't matter to me.

On the other hand, sometimes I use my GPS to locate property boundries. In that case, every inch or centimeter counts.

After reading the second post in this thread, I think we should be more concerned about why that curb is moving. Believe it or not, I have a seven hundred foot long driveway and it moves one eighth of an inch to the west every year. Actually, it is because I live on the side of a hill. LOL Hope this lessens the tension here a litte.

Anyway, thanks to all above for the info.

[Kerry.]

RRick, not going to be of concern to Geocaching (directly) but this "movement" thing might become more of an issue than most people think.

 

Your driveway might be moving 1/8" per year but some countries are moving 3" (or more per year) and based on fixed time frame references in a few more years when things have moved forward a bit more and technology has moved back a bit, basically the accuracy of the (then) normal common garden variety receiver will/should be able be detect this overall movement (since fixed).

 

It's a daunting issue for some applications.

 

Cheers, Kerry.

 

I never get lost everybody keeps telling me where to go

[cptwwwest]

The quality of the GPS unit is the greatest thing to think about if you want accuracy. I do not believe that time of dat has any effect. Cloud coverage does have an effect though. I use a Magellan Color Trak that has never failed me yet. I have had it for three years now, sailing, hiking, Kayaking in the Bayous, and driving. It will bring me to within five feet of the Geocache that I am looking for. The olnly thing the has any effect is how many clouds are in the sky. The sats are geosynchronious. (Stationary above the earth) If you have a satalite television you will see how when a cloud flies by, you loose reception while it covers the path of the tv signal to the dish.

 

http://www.mindspring.com/~dickandellen/sunktreasure.jpg

[+RangerRick]

Originally posted by Kerry:

RRick, not going to be of concern to Geocaching (directly) but this "movement" thing might become more of an issue than most people think.

 

Your driveway might be moving 1/8" per year but some countries are moving 3" (or more per year) and based on fixed time frame references in a few more years when things have moved forward a bit more and technology has moved back a bit, basically the accuracy of the (then) normal common garden variety receiver will/should be able be detect this overall movement (since fixed).

 

It's a daunting issue for some applications.

 

Cheers, Kerry.

 

Kerry, I see how this could be a daunting issue for your line of work especially when these countries are moving at different rates. Seems like ground based transmitters may be needed for corrections, similar to waas.

[+Poindexter]

cptwwwest

 

Thanks for adding a little humor to this neverending topic, I needed that!

[+apersson850]

So, does everybody agree now upon a few things, at least:

 

• Since satellite geometry change over time, the propability of getting an accurately determined position varies over time.

• Due to the impact of the sunshine, the properties of the atmosphere change over time, which also affects the probability to get an accurately determined position.

• Since the satellites aren't synchronized in their movement, relative to any particular spot on the earth, the "best" time of the day varies from day to day. (Or hawever we now should express that in English... )

• Sometimes a satellite geometry which in itself could provide an accurate position (sats spread all over the sky) isn't useful at all at a certain location. That location may benefit from more sats directly overhead, to allow any reception at all, assuming sats at lower elevations are blocked by various obstructions.

• Finally, it's easier to navigate with a GPS if you already know where you are.

 

Anders

[+RangerRick]

Perfect summary Anders!

[+Hiemdahl]

Gee, we haven't even discussed multipath and masking yet. Also when do we get to go into more detailed discussions of satellite geometery?

[Kerry.]

.... and solar flux (Anders I'll let you slide with sunshine, for now ) interactions with the earth's magnetic field and electron count (gradient density) contributing to errors in the single-frequency model .... nobody has much control over these.

 

With the worst 2 satellites removed the "conservative" SPS accuracy "on the ground" using a single freq receiver under standard conditions (no obstructions etc) is 33 metres @ 95%.

 

With the mixture of satellite technology (and ages) now up there and the differing User Range Errors that some of these Sats exhibit some of the accuracy "expectations" and claims by some are rather interesting (to say the least).

 

Cheers, Kerry.

 

I never get lost everybody keeps telling me where to go

[+apersson850]

Well, Hiemdahl, this forum doesn't run by itself, so I guess if you want something discussed, an excellent method is to start discussing it.

 

To meet you a little, as far as I've understood, the DOP values are some direct measure of the quality of the geometry, aren't they?

 

Kerry, I just want to tell you that I do know the difference between moonlight and moonshine, although there is none, if you translate any of these words into Swedish. We do have other words for "moonshine", though...

 

Anders

[Kerry.]

Anders, I knew you would, when we're finished with the sun today we'll let you have it for the next few hours, as long as you promise to send it back tomorrow.

 

But really day v night doesn't have a great concernable effect on accuracy, accuracy that is going to be a real issue anyway in these circles, unless of course you're using differential then there can be some issues just after the sun goes down.

 

The solar flux is the main villan and it's on about a 11 year cyle between peaks/max. Peaked back in about 2000 so things can only get better. At least for the next 3 or 4 so years

 

Catch ya again when the sun comes back .

 

Cheers, Kerry.

 

I never get lost everybody keeps telling me where to go

[+apersson850]

Well, the sun is here now, but I've put a wedge into the wheel which makes the world go around, so chances are you'll not see it again for a while.

 

If we look at the geocaching application, I would say that my experience implies that the current satellite geometry is most important for the accuracy. Not so much for the DOP value it represents, but for the ability to get any signal at all, in shaded locations.

 

Now I do have one of these infamous little patch antennas, but one have to learn to live with what one have. Besides, I could imagine that a quad antenna may give you a position which isn't too much worth, if the signals are subject to extensive fading and multipath?

 

Anders

[+Hiemdahl]

Unfortunately work will have me out in the field for the next week. I will be at Yuma Proving Grounds in Arizona for the next week or so (doing some GPS testing). In my line of work (precision targeting for Close Air Support), actually repeatability is more important than accuracy.

 

I have found that loading a map and a sattelite image of the area of interest is a definite help in correcting any position errors that creep in regardless of the source. Given the number of error sources, I am continually amazed that we can achieve the accuracies that we do.

 

Regards,

Hiemdahl