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Time of Day Affects GPS Accuracy?


AlphaOp
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I was out on a little adventure today and I took my GPS. For the first hour or so of my hike, I turned the unit on and off, marking a few waypoints to save battery power. Then, later, for some odd reason, the unit had a ton of trouble getting satellite locks. It would get one satellite, or two, but the others it would have problems with. I wasn't patient enough to let it run for more then three minutes.

 

Has anyone else ever noticed this? It might have been that the satellites were in just the right position, combined with the terrain, that caused a "black-out" for a time period.

 

AOsig.JPG

 

http://thealphaoperator.tripod.com

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I had problems with accuracy today even though I left the unit on continuously. While looking for a cache (and eating lunch) I left the unit sitting on the curb next to the suspected cache location for fifteen minutes to average out. This is in a dead-flat parking lot, in the middle of five miles of dead-flat farmland, with 4 to 7 satellites showing on the status screen. I got to within one foot of the cache location, but couldn't find it. Two hours later, I put the unit back on the same spot on the curb, left it there for another fifteen minutes with 5 to 9 satellites showing, and found the curb had moved 34 feet! What's going on here? I had wide-open sky, 15 minutes of averaging, lots of satellites, and the curb moves? Any tips on how to get consistent readings would be appreciated.

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For sure time of day matters and what one is possibly experiencing is exactly what the system can/might/does do depending on the overall dynamics of the system relative to the location and the receiever being used.

 

More satellites DOES NOT necessarily equate to better accuracy but with an over populated constellation compared to what the spec's are based on some peoples expectation will be really dashed if ever the number gets to what we are supposed to have.

 

Consistency really isn't possible as the overall system is dynamic (satellites and users) but one can maximize to make the best possible use of what the system provides by being at where one wants to be at the best possible time, it's called planning.

 

Cheers, Kerry.

 

I never get lost icon_smile.gif everybody keeps telling me where to go icon_wink.gif

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quote:
Originally posted by Hopscotch:

I got to within one foot of the cache location, but couldn't find it.


 

You got to within one foot of the posted coordinates which may only be within 50' of the actual cache location. Depends on how well the cache hider came up with that location. Also, your EPE is just an estimate based mostly on satellite geometry which is constantly changing.

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The time of day should not be an issue. Holding the GPS close to your body can be a problem. Using way point averaging can also be a problem if the person who hid the cache did not use averaging. Tree cover can be a problem. Time of day, no problem. I have used my GPS at most any time of the day and have never had a problem going to and finding waypoints. Geocahing related or not. Keep in mind, the best you can get even with WASS is about 20 feet. The claim is the WASS can get you within 9 feet, but in what direction, figure about 20 feet. When I am looking for a cache, when my GPS says 20 feet, it goes into my pocket.

 

[This message was edited by Johnnyvegas on April 20, 2003 at 07:20 AM.]

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quote:
Originally posted by Johnnyvegas:

The time of day should not be an issue. Holding the GPS close to your body can be a problem. Using way point averaging can also be a problem if the person who hid the cache did not use averaging. Tree cover can be a problem. Time of day, no problem.


 

Is that saying there's no difference between say 4 Sats, PDOP 28 and 6 Sats PDOP 2 and 4 Sats PDOP 2.3 and 8 Sats PDOP 6 etc etc as all those situations/events are possible given the right circumstances on any day.

 

I'd be interested in your reasons why you don't believe Time of day can not be a problem.

 

Cheers, Kerry.

 

I never get lost icon_smile.gif everybody keeps telling me where to go icon_wink.gif

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Almost every day, there are times when DOP is extremely high and accuracy can be way off. These periods are usually very brief but they do exist. Accuracy is highly dependent on satellite geometry which is constantly changing. There is software available that many professionals use to determine the best times of a day to get the most accurate fix. Also, when used properly WAAS can improve your accuracy to less than 9 feet.

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quote:
Originally posted by Kerry:

quote:
Originally posted by Johnnyvegas:

The time of day should not be an issue. Holding the GPS close to your body can be a problem. Using way point averaging can also be a problem if the person who hid the cache did not use averaging. Tree cover can be a problem. Time of day, no problem.


 

Is that saying there's no difference between say 4 Sats, PDOP 28 and 6 Sats PDOP 2 and 4 Sats PDOP 2.3 and 8 Sats PDOP 6 etc etc as all those situations/events are possible given the right circumstances on any day.

 

I'd be interested in your reasons why you don't believe Time of day can not be a problem.

 

Cheers, Kerry.

 

I never get lost icon_smile.gif everybody keeps telling me where to go icon_wink.gif


 

It real simple.

 

I have used my GPSR at all times of the day and night with way points I have saved in it or load from map programs I have and use, and I have never had a problem with accuracy. Of course I am only speaking from experiance in the field. Let face it, what happens in the feild under real conditions icon_biggrin.gifis what really counts.

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quote:
Originally posted by Johnnyvegas:

It real simple.

 

I have used my GPSR at all times of the day and night with way points I have saved in it or load from map programs I have and use, and I have never had a problem with accuracy. Of course I am only speaking from experiance in the field. Let face it, what happens in the feild under real conditions icon_biggrin.gifis what really counts.


 

Well it ain't that simple and obviously you don't use GPS for any thing that critical as if you did then you'd understand more about the possible problems and issues.

 

The next time you fly at least hope the pilot understands why they do pre flight checks for possible outages and what RAIM predictions are all about. In certain areas NOT knowing what is probably going to happen "in the field" (in advance) comes down to incompetence.

 

Cheers, Kerry.

 

I never get lost icon_smile.gif everybody keeps telling me where to go icon_wink.gif

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quote:
Originally posted by Kerry:

quote:
Originally posted by Johnnyvegas:

It real simple.

 

I have used my GPSR at all times of the day and night with way points I have saved in it or load from map programs I have and use, and I have never had a problem with accuracy. Of course I am only speaking from experiance in the field. Let face it, what happens in the feild under real conditions icon_biggrin.gifis what really counts.


 

Well it ain't that simple and obviously you don't use GPS for any thing that critical as if you did then you'd understand more about the possible problems and issues.

 

The next time you fly at least hope the pilot understands why they do pre flight checks for possible outages and what RAIM predictions are all about. In certain areas NOT knowing what is probably going to happen "in the field" (in advance) comes down to incompetence.

 

Cheers, Kerry.

 

I never get lost icon_smile.gif everybody keeps telling me where to go icon_wink.gif


 

I think you need to keep thing in perspective, This is a Goecaching forum, Most people are not concerned with using a GPS to fly a plane, Map out property lines, Drill for blasting a borax mine, etc. For geocahing and recreational use, a std GPS is all they will ever need., I have customers form time to time that insist they need a GPS that will get them to within 1 foot of their camp site when they are camping, To my way of thinking, if a person cann't find a camp site when you are 20 or 30 even 100 feet from it

they have other issues to deal with. As far as Geocaching, when my GPS diplays reads that I am 20 or 30 feet away, my GPS goes into my pocket.

If you are concerned about getting accuracy any better than you get form Garmin or Magellan, go spend a fortune for a profesional system. For geocahing, it is not needed. icon_razz.gif

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Johnnyvegas

 

Now your changing the subject. In your original post you stated that "The time of day should not be an issue." In fact, in "can" be. Usually, it doesn't make much difference when geocaching. As you say, when your within around 20-30 feet, it's time to put the receiver away and start searching. BUT, for some locations because of the terrain, planning your search for the best time of day can be a great help. If you want to see for yourself how the time of day can affect accuracy, check out this free software. You'll be amazed at how the time of day can affect accuracy and it shows why you sometimes get some weird readings. There are times when in just a 10 minute period your accuracy can go "off the charts"!

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Time of day DOES affect GPS accuracy!

 

The satellite signals propagate through atmospheric layers as they travel from the satellite to the receiver. Two layers are generally considered when dealing with GPS: the ionosphere which extends from a height of 70 to 1000 km above the Earth, and the troposphere which from the ground level to 70 km (Gu et al, 1993).

 

As the signal propagates through the ionosphere, the carrier experiences a phase advance and the codes experience a group delay. In other words, the GPS code information is delayed resulting in the pseudoranges being measured too long as compared to the geometric distance to the satellite (Hofmann-Wellenhof et al, 1992). . This is dependent on three further factors: the geomagnetic latitude of the receiver, the time of day and the elevation of the satellite. Significantly larger delays occur for signals emitted from low elevation satellites (since they travel through a greater section of the ionosphere), peaking during the daytime and subsiding during the night (due to solar radiation). In regions near the geomagnetic equator or near the poles, the delays are also larger (Spilker, 1980).

 

The ionospheric delay is frequency dependent and can therefore be eliminated using dual frequency GPS observations, hence the two carrier frequencies in the GPS design. Single frequency users, however, can partially model the effect of the ionosphere using the Klobuchar model (see Hofmann-Wellenhof et al 1992). Eight parameters for this model are transmitted with the broadcast data for the satellites, and are used as the coefficients for two third order polynomial expansions which are also dependent on the time of day and the geomagnetic latitude of the receiver. These polynomials result in an estimate of the vertical ionospheric delay, which is then combined with an obliquity factor, dependent on satellite elevation, producing a delay for the receiver-satellite line of sight. The final value provides an estimate within 50% of the true delay (Cohen et al, 1992) and produces delays ranging from 5m (night) to 30m (day) for low elevation satellites and 3-5m for high elevation satellites at mid latitudes (Gu et al,1993).

 

The troposphere causes a delay in both the code and carrier observations. Since it is not frequency dependent (within the GPS L band range) it cannot be canceled out by using dual frequency measurements but it can, however, be successfully modeled. The troposphere is split into two parts: the dry component which constitutes about 90% of the total refraction, and the wet part which constitutes the remaining 10%. Values for temperature, pressure and relative humidity are required to model the vertical delay due to the wet and dry part, along with the satellite elevation angle which is used with an obliquity/mapping function. Models put forward by Hopfield, Black and Saastamoninen are all successful in predicting the dry part delay to approximately 1 cm and the wet part to 5 cm.

 

Simply stated GPS receivers are slightly more accurate at night than in the daytime.

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We use Trimbles at work, where precision is a requirement, and as long as the PDOP is under 4 and we've got >1 hour of averaging, we're satisfied. Using the Trimble Planning Software (as mentioned by Johnnyvegas) and some back-calculating, I learned the PDOP never got above 6 when my Meridian reported the curb moved 34' in 2 hours. I'm disappointed in that magnitude of error, but I guess I'll learn to live with it - and keep the Trimble's batteries charged, just in case. I've learned that satellite configuration (which is independant of the local time of day) can produce an error of six or so inches for the Trimble, so I guess I'll have to do some side-by-side comparisons to find how bad the error can get for the Meridian.

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Johnnyvegas

 

You sound like your trying to say the dynamics of the system don't affect Goecaching and all Geocachers simply have to be oblivious to these dynamics.

 

Maybe there wouldn't be as many queries similar to this if there was more understanding of these dynamics even if these dynamics might not play a major role in some applications all the time (such as caching) they certainly will play a role at some time, that's almost guaranteed.

 

It's really an absurd statement saying "Time of day, no problem" as put simply it does and if you'd like to put forward some facts that dispute that "Time of Day" is no problem then please feel free.

 

Cheers, Kerry.

 

I never get lost icon_smile.gif everybody keeps telling me where to go icon_wink.gif

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While JV is organising some facts that TOD isn't an issue maybe others might be interested in these FACTS and PERSPECTIVE compared to some of the myth and miss-information.

 

icon_biggrin.gif.

 

Would probably also be a similar cry at around 2.30pm (+/- 20 minutes), 6.15am, 6.45pm and possibly those times with only 4 sats? (especially in conjunction with high PDOP's)

 

And it basically would have made no difference regardless of the receiver type being used, no signals, obstructed signals is basically no signals, obstructed signals to all/any receivers.

 

I'm not sure about some but most should be able to see there's certain times of the day when trying to fix a position in my backyard today is totally a waste of time. Not being able to shift/move the obstructions (as in the real world) then the other options are pack up and shift somewhere else (with possible better visibility etc) or observe at a "Time of Day" when conditions are better (best).

 

That time is when the grey area (number of satellites are the highest and the red line (PDOP) is the lowest.

 

Then on top of all this more location/physical stuff, which is real and can actually be analyized there's other issues/unknowns, which can be rather difficult (in practice) to take into account.

 

Cheers, Kerry.

 

Maleki you might recognize the software these plots come from icon_smile.gif

 

I never get lost icon_smile.gif everybody keeps telling me where to go icon_wink.gif

 

[This message was edited by Kerry on April 21, 2003 at 05:56 PM.]

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quote:
Originally posted by Johnnyvegas:

As I have said. For geocacing and recreational use, Time of day in not an issue.


 

Then if it's not an issue you still haven't explained/outlined WHY it's not an issue apart from "As I have said", which says absolutely nothing apart from an obvious lack of understanding.

 

Cheers, Kerry.

 

I never get lost icon_smile.gif everybody keeps telling me where to go icon_wink.gif

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quote:
Originally posted by Johnnyvegas:

As I have said. For geocacing and recreational use, Time of day in not an issue.


 

And your wrong, plain and simple. I't an issue, regardless of what you are using the GPS for. Your just one of those guys that can NEVER admit when they are wrong about something, but that's OK, your in good company icon_rolleyes.gif

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quote:
Originally posted by Kerry:

quote:
Originally posted by Johnnyvegas:

As I have said. For geocacing and recreational use, Time of day in not an issue.


 

Then if it's not an issue you still haven't explained/outlined WHY it's not an issue apart from "As I have said", which says absolutely nothing apart from an obvious lack of understanding.

 

Cheers, Kerry.

 

I never get lost icon_smile.gif everybody keeps telling me where to go icon_wink.gif


 

SUre I have. Like I have said, I have never had a problem with accuracy with my GPS regardless of time of day. Or maybe that is just to easy for some people to understand.

What type of accuracy do you expect from a GPS?

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quote:
Originally posted by Poindexter:

quote:
Originally posted by Johnnyvegas:

As I have said. For geocacing and recreational use, Time of day in not an issue.


 

And your wrong, plain and simple. I't an issue, regardless of what you are using the GPS for. Your just one of those guys that can NEVER admit when they are wrong about something, but that's OK, your in good company icon_rolleyes.gif


 

No. I just speak from experiance in the field. Or maybe you just don't like it when simple facts clouds your version of reality.

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Kerry, Maybe you need more experiance in Geocaching

 

Profile for Kerry (Kerry.)

Member since March, 2002

Email this user

Last visit: 4/22/2003

 

Caches Found/Hidden: 0/0

Travel Bugs Owned/Found: 0/0

Benchmarks Found: 0

 

[This message was edited by Johnnyvegas on April 22, 2003 at 06:49 AM.]

 

[This message was edited by Johnnyvegas on April 22, 2003 at 07:00 AM.]

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quote:
Originally posted by Johnnyvegas:

No. I just speak from experiance in the field. Or maybe you just don't like it when simple facts clouds your version of reality.


Oh, that's right, I forgot. You've used your GPS at ALL times of the day and night and have NEVER had a problem with accuracy, so this issue of time of day affecting accuracy can't possibly be true. I guess all those guys who wrote the software for predicting the best times of day for getting an accurate fix wasted their time. As for Kerry, he is very knowledgable about GPS. Geocaching isn't the only way to gain experience with a GPS receiver. At least not in the REAL world.

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Again this is his cache record.

He also reg. with Geocachin.com in March of 2006.You would think in 13 months he would have found at least one cache. icon_razz.gificon_razz.gificon_razz.gificon_razz.gif

 

Profile for Kerry (Kerry.)

Member since March, 2002

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Last visit: 4/22/2003

 

Caches Found/Hidden: 0/0

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Benchmarks Found: 0

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maybe im nitpicking, but the accuracy of the gps never changes, rather the quality of information it recieves is greatly affected by satellite geometry.

 

yesterday morning for example, i had 6 sats but all were in the 3-6 pm portion of my status screen. even though i had 6 sats their geometry wasent very good and this was reflected in the accuracy of the track log recorded by my gps when uploaded to my pc and inlaid on the map.

 

having said that however, i hardly consider geocaching am important use of the gps and big deal if i had gone geocaching and found nothing because my gps was being feed less than acceptable data. problem is, when our 'consumer grade' gps unit is being feed substandard data it doesnt know, and therefore neither do you as the user.

 

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

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quote:
Originally posted by Johnnyvegas:

As far as I am concerned he knows nothing about geocaching. Look up his profile yourself, He has never found a single cache, If has has never found one how can he know about what is involved if has has never looked for or found a cache.

 

What type af accuracy do you expect from a GPSR?


 

What does that have to do with the question? The question is, does the time of day affect GPS accuracy?. The answer is YES. I knew that long before I got into Geocaching. Iv'e been working with GPS receivers for 14 years. Does all this mean anything to the typical cacher? NO, but the question was asked, and it deserves a CORRECT answer.

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JohnnyVeges

 

What an utter load of waffle and totally irrelevent credibility attempt. But this is generally the avenue some will take when their own credibility is on the line, don't have any answers and couldn't argue their own way out of the corner in a blue fit.

 

Some advice Johnny, try and support your own credibility first before you attempt to attack other's about who you know absolutely nothing about. If I was you, I'd first get my facts right before making stupid statements.

 

Actually I didn't know it is a prerequisite to hide and find things before one knows what they are on about icon_razz.gif. Really not sure what your thinking is behind that one but your starting to sound like your beginning to know more and more about less and less.

 

It's been obvious almost from the start that you wouldn't know reality if you fell over it, as for experience well say on more. So far your comments and lack of ability to backup any of your "experience" apart from just because "YOU" haven't seen things means absolute ziltch in the real world.

 

Oh and Johnny what has knowing/not knowing anything specifically about geocaching (as you ignorantly put it) got to do with what the subject of this post is all about.

 

Get back on the subject, support your comments or simply sit down, shutup, listen and you might actually gain something new.

 

At least if your going to make a comment then support that comment as otherwise simply because "YOU" haven't experienced or don't understand it or didn't know it was an issue has no relevence to what actually happens.

 

As for "Time Of Day" NOT being an issue (in your own "experience") then your comments are absolute rubbish lacking any credibility or understanding of the system what so ever.

 

And it doesn't require one to be a "geocacher" to know, understand or have some background on that one, it's simple fundamental basics that most who use the system should at least understand or in your case try to understand.

 

Accuracy icon_biggrin.gif well that's another subject but that's also another favourite of mine and also averaging so feel free to bring anything up anytime you like. Just come prepared with the facts.

 

You had a typical "real world" example put before you, now if you refute that then lets hear the reasoning and justification. Oh because "you" haven't seen or experienced that is irrelevent.

 

Cheers, Kerry.

 

I never get lost icon_smile.gif everybody keeps telling me where to go icon_wink.gif

 

[This message was edited by Kerry on April 22, 2003 at 07:17 PM.]

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I also believe that there is very little dependence on "time of day" for geocaching. This is not to say that there won't be variations over time, just that they are not well correlated with any specific 'time of day'.

In the example plots that Kerry showed there was relatively poor performance around 7 a.m. on that particular day at that place. But on other days and/or places the reception is just fine at 7 a.m. So there's lots of variation over time, but it isn't tied to specific times of the day.

 

Similarly, if someone says that some phenomenom is "seasonal" I would expect that he could point out specific seasons when it occurs and other seasons when it doesn't. If it happens some months and not others on a rather random variation I wouldn't agree with the characterization of 'seasonal.'

 

So the only effect I know of that is really tied to specific "times of day" are the variations in the ionosphere where the solar radiation during the day induces greater disturbances than are generally seen at night. This effect does have an effect on GPS accuracy but generally not at a level that will make it significant for geocaching.

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quote:
Originally posted by Johnnyvegas:

This is a geocaching NG. Not one dedicated to GPS issues. It would seem you seem to think you know about goecaching, though you have never treid it. Would you go to a doctor that has only read some books, with no real experiance. In my opinion you are a troll, and I have to time for you.


 

Your a laugh frog.gif

 

Cheers, Kerry.

 

I never get lost icon_smile.gif everybody keeps telling me where to go icon_wink.gif

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Wow, this is the most rediculous discussion I have seen on the forums to date. I would like to point out that I have used both mapping grade GPS (Trimble) and recreational grade GPS for Geocaching. I have 97 finds, which at last count was 4 more than Johnnyvegas has found. So maybe he will find some credibility in what I have to say.

 

Basically, Johnnyvegas is wrong, Kerry is right. You may think that Kerry knows nothing about geocaching since he has zero finds. However, the discussion on effect time of day has on GPS reception has more to do with the theory of GPS - which is something Kerry obviously knows a great deal about. (Kerry, I'd be interested in hearing what you do for a living and why you know so much about GPS).

 

By the way, thanks to Poindexter for the link to the free Planning Software. I have been wondering if there was something out there that I could use at home.

 

Time of day definitely has an effect. The satellites are in 11 hour 58 minute orbits, which means that their position in the sky changes every day. The system is designed to provide global GPS coverage of at least 4 satellites at any given time, 24 hours a day. The planning software used by professional grade GPS users, applies an elevation mask (usually 15 degrees from the horizon) which basically means it only takes into account those satellites which are above that. There are short time periods when only 3 satellites are visible - and this is under optimal conditions. If you have buildings, or trees, or cliffs or anything else blocking the satellites, you will not be able to lock on to all available satellites.

 

These block out times are short though - only about 20 minutes or so. It will vary by location and as I have said before, it will vary by day.

 

Now even in geocaching, you have probably found that there are times when you have a hard time getting a signal - even when you have an unobstructed view of the sky, but it doesn't last long. It is not as big a deal as it is for the mapping grade GPS when you want to make sure that the data you are collecting are within a certain tolerance of error. But to make a blanket statement that time of day has no effect for recreational grade GPS, is simply wrong.

 

Johnnyvegas, you mentioned that you are giving advise to customers on GPS. If that is true, you should invest some time learning how the system works!

 

-Junglehair

 

There are 10 kinds of people in this world - those who understand binary and those who don't.

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Peter, this "Time of Day" issue (certainly wouldn't call it "specific" but) is a generic one and for sure there many variables, which go into affecting this. The atmospherics and much of that similar type of stuff is generally untouchable, nobody has all that much control if sun spot activity will be a problem today, tomorrow or whenever.

 

This type of issue is dependent on many things like location, obstructions and apart from other things just what the system is actually doing at the time. All things aren't the same for all locations all of the time but depending on circumstances there can be issues some of the time and these times (all things being equal) are basically predictable.

 

This "Time of Day" issue doesn't target or favour geocaching or any other use of the system as there is basically only one system.

 

One has to understand the whole system, including the users are dynamic and for sure what occured here this morning at what ever time isn't "exactly" occuring anywhere else either but that's simply a factor of a specific user's environmenet in relation to the constellation at a particular time. Come back to the same point at some other time and obviously things have changed as if they didn't then GPS wouldn't work.

 

Basically if users are unable to grasp this principle that the whole system and the relationship of that system to the user is constantly changing then there is some definite miss-understanding.

 

First there has to be this understanding that if at a particular time at a certain location due to several variables a user doesn't have enough satellites to get a position fix but can come back after a short period and there is enough satellites then if time of day doesn't matter then I'd like to know what tag some call it.

 

Gee, there's so many variables involved here, apart from the atmospherics issues, different satellites are on maintainence almost everyday, users are another, different locations. Even though there is some "basic" consistency in the constellation, really NOTHING stays the same, especially the user.

 

That also one reason why the current accuracy specifications (oh by the way these accuracy specs apply to ALL users, geocaching isn't singled out as being special either, geocachers are simply just "another" user) are based on Signal-In-Space specifications as that's what's the owners/operators of the system have most (some) control over. They don't have any control over how/when/where or why a user wants/expects their equipment to work and under what conditions.

 

Cheers, Kerry.

 

I never get lost icon_smile.gif everybody keeps telling me where to go icon_wink.gif

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quote:
Originally posted by junglehair:

Wow, this is the most rediculous discussion I have seen on the forums to date ....

 

(Kerry, I'd be interested in hearing what you do for a living and why you know so much about GPS).


 

Lets just say GPS and what I do for a living has a fairly close connection (and that's any time of day icon_wink.gif). All types for all purposes a GPS is basically a GPS and there's some fairly fundamental principles that apply regardless of the type or use.

 

And for sure one thing that stands out from time to time is some of the absolute miss-informed myth's that get propagated by the "applicance" sales types.

 

Realize this is getting off the topic but it might also shock some to find out that in straight SPS mode there's practically bugga all difference (in practical SPS accuracy) between the normal run of the mill $400 handheld and a $40,000 "survey grade" receiver.

 

For your info those plots above are based on a 5 degree mask angle as ideally NO receiver should really use low horizon satellites. Some manufacturers run a 0 (zero) mask, other a few degrees, some even 10 degrees. Most who use recreational units wouldn't have a clue what mask angle their receiver is anyway.

 

That 15 degree for geodetic stuff cut-off basically ensures the best possible signal data is used and not corrupted with suspect obs from the lower horizon, more atmospheric influenced signal.

 

But these days with sky clear above 15 degrees things aren't all that bad but of course there's certainly been times (even lately) when one may as well pack up and go home.

 

Cheers, Kerry.

 

I never get lost icon_smile.gif everybody keeps telling me where to go icon_wink.gif

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Not finished yet icon_biggrin.gif

 

The following is the same backyard (today) and just for the fun of it SVN25 (PRN25) has been made unavailable (like if it was on maintainence, which can/could/might easily happen). The difference 25 makes is totally around that 6-7am time frame. But also some might note things have changed (for the worse) around 2pm, which is totally and solely due to being a couple of days apart (all based on the same current almanac).

 

I'll pose the question this way.

 

If "Time of Day" doesn't matter then why would it be so that's there absolutely no chance of getting a position at this particular location at ANY time today. In other words if time of day doesn't matter then one would "expect" to get a position fix at this location, under these circumstances at ANY time over ANY 24 hour period.

 

icon_smile.gif everybody keeps telling me where to go icon_wink.gif

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quote:
Originally posted by Johnnyvegas:

This is a geocaching NG. Not one dedicated to GPS issues. It would seem you seem to think you know about goecaching, though you have never treid it. Would you go to a doctor that has only read some books, with no real experiance. In my opinion you are a troll, and I have to time for you.


 

And this forum is dedicated to issues concerning "GPS Units and Software". That includes GPS issues.

 

quote:
I have used my GPS at most any time of the day and have never had a problem going to and finding waypoints.

 

I think most of us would admit that we hardly ever have problems finding waypoints. But is the waypoint we find the actual location we seek? I expect not always. Would you mind sharing what GPSr you are using? Because I want one.

 

quote:
Keep in mind, the best you can get even with WASS is about 20 feet.

 

Could you please provide a reference to that info?

 

 

The FAA says...

 

quote:
WAAS testing in September 2002 confirmed accuracy performance of 1 – 2 meters horizontal and 2 –3 meters vertical throughout the majority of the continental U.S. and portions of Alaska.

 

Just remember. Getting there is half the fun...

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quote:
Originally posted by junglehair:

Wow, this is the most rediculous discussion I have seen on the forums to date. I would like to point out that I have used both mapping grade GPS (Trimble) and recreational grade GPS for Geocaching. I have 97 finds, which at last count was 4 more than Johnnyvegas has found. So maybe he will find some credibility in what I have to say.

 

Basically, Johnnyvegas is wrong, Kerry is right. You may think that Kerry knows nothing about geocaching since he has zero finds. However, the discussion on effect time of day has on GPS reception has more to do with the theory of GPS - which is something Kerry obviously knows a great deal about. (Kerry, I'd be interested in hearing what you do for a living and why you know so much about GPS).

 

By the way, thanks to Poindexter for the link to the free Planning Software. I have been wondering if there was something out there that I could use at home.

 

Time of day definitely has an effect. The satellites are in 11 hour 58 minute orbits, which means that their position in the sky changes every day. The system is designed to provide global GPS coverage of at least 4 satellites at any given time, 24 hours a day. The planning software used by professional grade GPS users, applies an elevation mask (usually 15 degrees from the horizon) which basically means it only takes into account those satellites which are above that. There are short time periods when only 3 satellites are visible - and this is under optimal conditions. If you have buildings, or trees, or cliffs or anything else blocking the satellites, you will not be able to lock on to all available satellites.

 

These block out times are short though - only about 20 minutes or so. It will vary by location and as I have said before, it will vary by day.

 

Now even in geocaching, you have probably found that there are times when you have a hard time getting a signal - even when you have an unobstructed view of the sky, but it doesn't last long. It is not as big a deal as it is for the mapping grade GPS when you want to make sure that the data you are collecting are within a certain tolerance of error. But to make a blanket statement that time of day has no effect for recreational grade GPS, is simply wrong.

 

Johnnyvegas, you mentioned that you are giving advise to customers on GPS. If that is true, you should invest some time learning how the system works!

 

-Junglehair

 

There are 10 kinds of people in this world - those who understand binary and those who don't.


 

97 in 13 months

vs

94 in 4 months

I can live with that

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quote:
Originally posted by Kerry:

First there has to be this understanding that if at a particular time at a certain location due to several variables a user doesn't have enough satellites to get a position fix but can come back after a short period and there is enough satellites then if time of day doesn't matter then I'd like to know what tag some call it.


 

I would call it what it is - i.e. that due to a number of factors, i.e. satellite positions, placement of obstructions, satellite availability, etc. the expected accuracy of a GPS measurement will vary over time.

 

When you say that this is a "time of day" effect then I would ask at what times of day you'd expect the best result and the answer would be that there is no one good "time of day" because a good time one day will be a bad time on another day. You can work out the satellite positions for a specific location and predict relatively good times to do GPS survey work there, but they won't be at specific "times of day" - they'll just be a set of times for that location.

 

Let's say you told me that when planting XYZ the "time of the year" was very important. I then ask you which are the best times of the year. If it were like GPS your response would be something like "Well in Orlando last year the best times were Jan 15th, May5th, June 20th and Oct. 19th; but next year the dates will be completely different." Looks like maybe what's important in planting XYX is not actually the time of the year, but rather that it be done right after a heavy rain or some such environmental event. I'd say it's misleading in that case to say that "time of year" is the important factor in planting XYZ.

 

Similar with GPS reception, it's not the "time of day" that's important, but rather the satellite availability, positioning, and obstructions. Certainly these will vary with time, but they aren't tied to the "time of day" except to the extent that due to the synchronization of the orbits to sidereal time the satellite positions will repeat at about the same time for a number of days (with a slow drift due to solar vs. sidereal timing).

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quote:
Originally posted by Anders:

Just in case you want a real-life example of when "time of day" matters, check ...


 

Anders,

Note that when we say that the "time of year" matters when planting crops, we mean that they should be planted at a specific season each year. When we say that the quality of cars may depend on the "time of the week" when they were built, we mean that certain specific times that repeat each week, like Monday mornings and Friday afternoons, may have a workforce that's less attentive than usual. In neither case is the meaning just that things vary with time, but rather that they are related in some way to the stated period (year and week in these examples). Similarly, the phrase "time of day" carries the same meaning.

 

Your example shows that the chance of success varies with time - some times will be better than others for reasons that have been discussed above. It does not show that there is a "time of day" that has the best chance of success.

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quote:
Originally posted by peter:

I would call it what it is - i.e. that due to a number of factors, i.e. satellite positions, placement of obstructions, satellite availability, etc. the expected accuracy of a GPS measurement will vary over time.


 

I'm not sure I'd call it "vary over time" but maybe a better term is "flucuate over time" (sometimes can fluctuate from good to completely bad). But what your saying in that is, there's times thoughout the day that are better than others? right. Doesn't that translate into the "Time of Day" does/can matter?

 

quote:
When you say that this is a "time of day" effect then I would ask at what times of day you'd expect the best result and the answer would be that there is no one good "time of day" because a good time one day will be a bad time on another day. You can work out the satellite positions for a specific location and predict relatively good times to do GPS survey work there, but they won't be at specific "times of day" - they'll just be a set of times for that location.

 

I don't believe the word "specific" (that's your terms) has anything to do with the comment that "Time of day" matters.

 

Again what you've just said in that is that certain times of the day will/can be better than other periods/times throughout the day, correct?. Doesn't that translate into that "Time of Day" does/can matter?

 

quote:
Similar with GPS reception, it's not the "time of day" that's important, but rather the satellite availability, positioning, and obstructions. Certainly these will vary with time, but they aren't tied to the "time of day" except to the extent that due to the synchronization of the orbits to sidereal time the satellite positions will repeat at about the same time for a number of days (with a slow drift due to solar vs. sidereal timing).

 

Gee, I wouldn't compare GPS availability and all that stuff with planting crops, really can't see any comparision in that what so ever. Leave that to the long range weather forcasters.

 

And isn't it all that satellite availability, obstructions, positioning etc that will dictate exactly when the "best" times will be on a particular day. Doesn't that translate into "Time of Day" does/can matter if one wants/expects the best results.

 

My initial comment was that "Time of Day" can matter, and it does, but no where have I got into this "specific Time of Day" as specifics times have nothing to do with anything.

 

As for the orbit precession timing difference (~4 minutes per day) that's not really a major influencing factor when compared to the real issues.

 

All that you've mentioned has basically said that reception under certain conditions will be better at certain "Times of day" than other times. I don't have a problem with that as that's exactly what can occur.

 

Cheers, Kerry.

 

I never get lost icon_smile.gif everybody keeps telling me where to go icon_wink.gif

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quote:
Originally posted by Kerry:

All that you've mentioned has basically said that reception under certain conditions will be better at certain "Times of day" than other times.


 

No, I agree that due to changing conditions reception will be better at certain times. *Not* certain "times of day". The two phrases have different meanings in the English language as I've tried to make clear with examples from the similar phrases "times of the year" and "times of the week", but you dismissed those sarcastically without addressing the point. If to you the additional words "of day" don't change the meaning in any way, then leave them off - adding superfluous words is never good form. To me the added words do distinctly change the meaning - and in a way that makes the statement incorrect or at least misleading.

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Peter, I'll leave you explain the english language if it makes things sound correct.

 

So is "Time of day" and "Times of day" different.

 

Today might be April 23, 2003 but in GPS terms it is Day of Year (DOY) 113. Is "Day Of Year" as used in all almanac/rinex references incorrect?

 

Also GPS time for specific purposes is referred to as "Time Of Week", which is in seconds starting midnight, Saturday. This TOW resets every week, is "Time Of Week" incorrect do you think?

 

These TOW (Time Of Week) and DOY (Day Of Year) abbrev's are all standard GPS terms just like TOD (Time Of Day).

 

Does appear we're talking about the same thing thou, even if the english might not be what some expect, it's all GPS language.

 

Cheers, Kerry.

 

I never get lost icon_smile.gif everybody keeps telling me where to go icon_wink.gif

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quote:
Originally posted by Anders:

Just in case you want a real-life example of when "time of day" matters, check http://www.geocaching.com/seek/cache_details.aspx?ID=41685

Look at the (at this moment) last log, when I visited the place.


 

Anders, was just reading a request from a GIS user locating "street furniture" in a city/high rise area. Not only was he wanting to be in the right place at the best time (or is that best time for a specific location) but on a real-time/position in the street basis.

 

In other words apart from the dynamics of the system there is also the requirement of position in the street and one might imagine just stepping sideways could/might be enough to change the whole scenario.

 

Mission planning on the time-line in real time, it has some interesting "problems" to say the least.

 

Cheers, Kerry.

 

I never get lost icon_smile.gif everybody keeps telling me where to go icon_wink.gif

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Kerry, I agree with you, as you've understood, that going to the same place, with the same receiver, will not give the same result, depending upon when you go there.

 

I've also learned that there is special software available, for mission planning, which let you compute the better time for critical tasks, or the times when you could just as well pack your things and go back home.

 

Peter, since I'm not sure that I can express exactly what I mean in English, considering that it's not my native language, this is for you: icon_wink.gif

 

Eftersom satelliterna far omkring runt jorden med hög hastighet (de hinner med två varv om dygnet, ungefär), ändrar sig det antal satelliter som finns synliga över horisonten hela tiden. Speciellt om man ska försöka bestämma en position i en ravin, eller bland höga träd, är det en fördel om man kan välja ett tillfälle, när det finns många synliga satelliter, gärna högt över horisonten. Risken minskar då att alltför många satelliter skyms, så att positioneringen blir osäker eller helt omöjlig.

 

That was in a language I understand. icon_smile.gif

 

Anders

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WOW, what a stack of juvenile spew! In the midst of all this useless crap, I'd like to thank Hiemdahl for discussing actual time-of-day factors that affect GPS accuracy. I had assumed that the only time-of-day factors were heat-driven humidity changes and normal daily atmospheric dilation, but this is obviously incorrect. Everybody else (except Peter) has confused satellite geometry with unrelated factors, and thus provided no useful info.

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