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Does time of day make a difference?


nscaler

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I was wondering if the satellites orbiting above us follow the exact same path every day. They are supposed to take two orbits a day. If they are in the exact same position in the sky twice a day (i.e. 6:47 am and 6:47 pm), would your GPS be more accurate if you were to search for a cache at the same corresponding time of day as when the cache was hidden? Hmmm... Anybody know? This could help geocachers find a cache if they knew the time the cache was "marked" and looked at that time.

 

Just wondering.

nscale

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Wow. That's a good one. I'd say given the same exact conditions at the same time, probably not. If you watch a GPS screen when standing still, the unit will fluctuate quite a bit.

 

Maybe on the extreme end, if satellites were positioned more overhead than towards the horizon at the same time each day, it may make a difference, especially in a valley of some kind. But on the whole it probably doesn't.

 

Not much of an answer, eh? icon_eek.gif

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Wow. That's a good one. I'd say given the same exact conditions at the same time, probably not. If you watch a GPS screen when standing still, the unit will fluctuate quite a bit.

 

Maybe on the extreme end, if satellites were positioned more overhead than towards the horizon at the same time each day, it may make a difference, especially in a valley of some kind. But on the whole it probably doesn't.

 

Not much of an answer, eh? icon_eek.gif

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The satellites are in EXACTLY the same position at EXACTLY the same time each day ..... Well, that is, if you are observing them from space, not form Earth.

 

The satellites make exactly two ordits a day, but that is a sidereal day, not a solar day. You see, between this instant in time today and the same time tomorrow, we have moved through space 1/365 of an orbit around the sun. Our solar days are based on the time when the sun crosses our local meridian (the line of longitude that passes through you).

 

To get to that same position we were 24 hours ago, we need to rotate a bit more to compensate for the fact we are not where we were a day ago. We are actuall pointing a bit west. We need to rotate another 1/365 of a day. That's another 3 minutes and 56 seconds.

 

Therefore since our earth bound day (solar) is 4 minutes longer than a sidereal day, the satellites will arrive at their exact location 4 minutes earlier tomorrow than they did today.

 

(Note: I say "exactly" in the first paragraph above when in reality there are orbit errors that come into play, but think you get the idea)

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quote:
Originally posted by nscaler:

I was wondering if the satellites orbiting above us follow the exact same path every day. They are supposed to take two orbits a day. If they are in the exact same position in the sky twice a day (i.e. 6:47 am and 6:47 pm), would your GPS be more accurate if you were to search for a cache at the same corresponding time of day as when the cache was hidden? Hmmm... Anybody know? This could help geocachers find a cache if they knew the time the cache was "marked" and looked at that time.

 

Just wondering.

nscale


I hid a cache one day & got very good reception but went back 2 days later about the same time of day & couldn't hardly get a decent signal.

This doesn't help answer your question, just adds more "strangeness" to GPSing!

icon_smile.gif

 

When you come to a fork in the road, take it.

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quote:
Originally posted by nscaler:

I was wondering if the satellites orbiting above us follow the exact same path every day. They are supposed to take two orbits a day. If they are in the exact same position in the sky twice a day (i.e. 6:47 am and 6:47 pm), would your GPS be more accurate if you were to search for a cache at the same corresponding time of day as when the cache was hidden? Hmmm... Anybody know? This could help geocachers find a cache if they knew the time the cache was "marked" and looked at that time.

 

Just wondering.

nscale


I hid a cache one day & got very good reception but went back 2 days later about the same time of day & couldn't hardly get a decent signal.

This doesn't help answer your question, just adds more "strangeness" to GPSing!

icon_smile.gif

 

When you come to a fork in the road, take it.

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I was about to post the exact same question. I have been wondering this for a few days. I posted another thread regarding accuracy and had this as an afterthought. I would really like to hear from some of the folks out there...I know there are a few of you that have a lot of experience with satelites. My gut tells me that if you conect to the same satelites, and they are in the same position that they were when an initial reading was taken, the second reading will at least have fewer variables when considering the EPE. Hunting for a cache at the same time of the day that it was hidden will not alter other independant variables such as atmospheric noise or EMI...but it may help to at least keep some parts of the equation consistent.

 

I would like to hear from some of our scientist geocachers....I know you are out there. What can we do to get the best performance out of our GPSR's?

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MrGigabyte gives us a marvelous explanation of the orbits. Thanks! Now to answer this important part of the question:

 

"If they are in the exact same position in the sky twice a day (i.e. 6:47 am and 6:47 pm), would your GPS be more accurate if you were to search for a cache at the same corresponding time of day as when the cache was hidden? Hmmm... Anybody know? This could help geocachers find a cache if they knew the time the cache was "marked" and looked at that time."

 

The answer is no. Your GPSR would not be more accurate. However, what you are implying is that your GPSR would be subject to the same type of signals and therefore might arrive at similar readings. This makes sense, but I'm not sure that this will translate into readings that gibe. And, given the sidereal time rule, the relative positions will slowly be changing. If I place a geocache at noon and you hunt it tomorrow at noon, we'll have similar satellites overhead. But if you hunt it at noon six months later, it's different. Bottom line: I'd still take the encrypted hint. icon_wink.gif

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MrGigabyte gives us a marvelous explanation of the orbits. Thanks! Now to answer this important part of the question:

 

"If they are in the exact same position in the sky twice a day (i.e. 6:47 am and 6:47 pm), would your GPS be more accurate if you were to search for a cache at the same corresponding time of day as when the cache was hidden? Hmmm... Anybody know? This could help geocachers find a cache if they knew the time the cache was "marked" and looked at that time."

 

The answer is no. Your GPSR would not be more accurate. However, what you are implying is that your GPSR would be subject to the same type of signals and therefore might arrive at similar readings. This makes sense, but I'm not sure that this will translate into readings that gibe. And, given the sidereal time rule, the relative positions will slowly be changing. If I place a geocache at noon and you hunt it tomorrow at noon, we'll have similar satellites overhead. But if you hunt it at noon six months later, it's different. Bottom line: I'd still take the encrypted hint. icon_wink.gif

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quote:
Originally posted by Seth!:

"If they are in the exact same position in the sky twice a day (i.e. 6:47 am and 6:47 pm), would your GPS be more accurate if you were to search for a cache at the same corresponding time of day as when the cache was hidden? The answer is no.


 

Sorry, the answer is yes. GPS satellites broadcast simple radio signals. These signals are affected by many external forces before they hit your eTrex. Not the least of which is the refractive effect of the ionosphere.

 

Ionosphere effects on radio waves are significantly less when there is no solar effect on them. That is, any observations after sunset will always be better than those take 12 hours previous.

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quote:
Originally posted by MrGigabyte:

Ionosphere effects on radio waves are significantly less when there is no solar effect on them. That is, any observations after sunset will always be better than those take 12 hours previous.


 

This is correct. And in addition to this WAAS will have less of a correcting effect at night than in the day as there is less to correct. I guess the message here is "Nightcachers do it better"

 

2573_200.jpg

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Time of day certainly makes a difference (apart from the 4 odd minute rolling change each day) and if one could "exactly" replicate things from one day to the next then there really wouldn't be that much difference but unfortuneately what occured yesterday (as far as total circumstances/conditions) might never occur again.

 

So many variables to consider.

 

Cheers, Kerry.

 

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

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Time of day certainly makes a difference (apart from the 4 odd minute rolling change each day) and if one could "exactly" replicate things from one day to the next then there really wouldn't be that much difference but unfortuneately what occured yesterday (as far as total circumstances/conditions) might never occur again.

 

So many variables to consider.

 

Cheers, Kerry.

 

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

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I've been giving this and related questions alot of thought the last few weeks as I make the long drive to new caches.

 

I have some thoughts on why you would want a quad helix, why lots of satellites are better, why they need to be well distributed around the sky, why horizontal position is more precise / accurate than vertical, and how WAAS helps. And none of it is too difficult. Though it might be easier with pictures, especially of these "shells" intersecting.

 

Triangulation: The most accurate 2-point triangulations are when the 2 points form a right angle with respect to you. This is because the area formed by possible angular error is minimized in this case, approximately a square. If the points are close together this overlapping area is much larger, a long skinny diamond. If you add more and more widely separated points to your triangulation, you can reduce this error to roughly a circle the width of the angular error. This applies as much to GPS as it does to finding your location on a map by sighting with a compass.

 

In the case of GPS, you are finding the intersection of multiple hollow spheres. The GPS knows exactly where the satellites are and that the hollow spheres should all meet at a point in space. The thickness of the hollow spheres is the uncertainty caused by time errors inherent in the GPS system such as atmospheric effects, etc. Now imagine you are standing on the ground in a fairly flat place somewhere on the Earth. (This idea should work if you are in the air or at sea, but it's easier to visualize this on the ground.) The intersection of this hollow sphere with the earth is going to be an arc for satellites near the horizon. The "shell" will be perpendicular to the earth. In our small area of interest this arc will approximate a long, skinny rectangle stretching to the horizon. For satellites above our location (high in the sky), the "surface" of the hollow sphere will be parallel to the earth with a large area of intersection. The GPS will vary the time delay until all satellite signals intersect. The intersection of the near horizon satellites will be a fairly small area for the horizontal intersection, but large for vertical intersection. The high in the sky sats will have a small area of vertical intersection, but a large area of horizontal intersection. In addition, since there are no satellites below us to get a signal from, the vertical intersection is not as precise as the horizontal. The size of the area of intersection should be directly proportional to the stated GPS accuracy.

 

If I'm right (and it all makes sense to me), this explains why GPS is less accurate vertically than horizontally. All of the triangulation points are on one side. The best vertical measurements are from sats above you.

 

Sats at the horizon: The best horizontal measurements are from sats near the horizon. So geocachers should generally want their sats evenly spaced all the way around them on the horizon.

 

More sats better: The more satellites you have, the better the chances they will be in a good location.

 

Quad Helix: This also explains the quad helix antenna. The QH is best at picking up signals coming in fron the sides, right? So you hold your GPS antenna vertically, putting the horizon at the sides of the antenna. Thus maximizing the ability to pick up sats at the horizon, which are best for determining horizontal position. Which is usually what we care about. The patch antenna can usually pick these up, but in tough conditions the quad helix can pull these signals in more dependably.

 

WAAS: Reduces the thickness of these hollow shells by reducing the time uncertainty caused by atmospheric effects. Thinner shells, less intersection area.

 

icon_eek.gificon_eek.gif

Wow, that got long. That's about it though. This all makes sense to me, but if I'm wrong somewhere, please let me know.

 

rdw icon_confused.gif

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I've been giving this and related questions alot of thought the last few weeks as I make the long drive to new caches.

 

I have some thoughts on why you would want a quad helix, why lots of satellites are better, why they need to be well distributed around the sky, why horizontal position is more precise / accurate than vertical, and how WAAS helps. And none of it is too difficult. Though it might be easier with pictures, especially of these "shells" intersecting.

 

Triangulation: The most accurate 2-point triangulations are when the 2 points form a right angle with respect to you. This is because the area formed by possible angular error is minimized in this case, approximately a square. If the points are close together this overlapping area is much larger, a long skinny diamond. If you add more and more widely separated points to your triangulation, you can reduce this error to roughly a circle the width of the angular error. This applies as much to GPS as it does to finding your location on a map by sighting with a compass.

 

In the case of GPS, you are finding the intersection of multiple hollow spheres. The GPS knows exactly where the satellites are and that the hollow spheres should all meet at a point in space. The thickness of the hollow spheres is the uncertainty caused by time errors inherent in the GPS system such as atmospheric effects, etc. Now imagine you are standing on the ground in a fairly flat place somewhere on the Earth. (This idea should work if you are in the air or at sea, but it's easier to visualize this on the ground.) The intersection of this hollow sphere with the earth is going to be an arc for satellites near the horizon. The "shell" will be perpendicular to the earth. In our small area of interest this arc will approximate a long, skinny rectangle stretching to the horizon. For satellites above our location (high in the sky), the "surface" of the hollow sphere will be parallel to the earth with a large area of intersection. The GPS will vary the time delay until all satellite signals intersect. The intersection of the near horizon satellites will be a fairly small area for the horizontal intersection, but large for vertical intersection. The high in the sky sats will have a small area of vertical intersection, but a large area of horizontal intersection. In addition, since there are no satellites below us to get a signal from, the vertical intersection is not as precise as the horizontal. The size of the area of intersection should be directly proportional to the stated GPS accuracy.

 

If I'm right (and it all makes sense to me), this explains why GPS is less accurate vertically than horizontally. All of the triangulation points are on one side. The best vertical measurements are from sats above you.

 

Sats at the horizon: The best horizontal measurements are from sats near the horizon. So geocachers should generally want their sats evenly spaced all the way around them on the horizon.

 

More sats better: The more satellites you have, the better the chances they will be in a good location.

 

Quad Helix: This also explains the quad helix antenna. The QH is best at picking up signals coming in fron the sides, right? So you hold your GPS antenna vertically, putting the horizon at the sides of the antenna. Thus maximizing the ability to pick up sats at the horizon, which are best for determining horizontal position. Which is usually what we care about. The patch antenna can usually pick these up, but in tough conditions the quad helix can pull these signals in more dependably.

 

WAAS: Reduces the thickness of these hollow shells by reducing the time uncertainty caused by atmospheric effects. Thinner shells, less intersection area.

 

icon_eek.gificon_eek.gif

Wow, that got long. That's about it though. This all makes sense to me, but if I'm wrong somewhere, please let me know.

 

rdw icon_confused.gif

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So the moral of the huge post is that satellite geometry matters. Bad geometry will likely make your readings less accurate. I've seen same poor geometry lately. Only 6 or 7 satellites, most of them towards the south. Less than ideal geometry.

 

It would make sense that if the hider had good geometry, you stand a better chance of good coordinates. And if you have good geometry, you have a better chance of knowing where you are so you'll know where you are with respect to these coordinates. Of course, other thing influence you readings so you never really know. It would be nice for GPSs to display dilution of presence numbers so you wouldn't have to guess it. It would also be cool to know just how Garmin and Magellan compute their location. I'm sure that's a closely held trade secret though. icon_frown.gif

 

I'll shut up now. icon_rolleyes.gif

 

rdw

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So the moral of the huge post is that satellite geometry matters. Bad geometry will likely make your readings less accurate. I've seen same poor geometry lately. Only 6 or 7 satellites, most of them towards the south. Less than ideal geometry.

 

It would make sense that if the hider had good geometry, you stand a better chance of good coordinates. And if you have good geometry, you have a better chance of knowing where you are so you'll know where you are with respect to these coordinates. Of course, other thing influence you readings so you never really know. It would be nice for GPSs to display dilution of presence numbers so you wouldn't have to guess it. It would also be cool to know just how Garmin and Magellan compute their location. I'm sure that's a closely held trade secret though. icon_frown.gif

 

I'll shut up now. icon_rolleyes.gif

 

rdw

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quote:
Originally posted by rdw:

So the moral of the huge post is that satellite geometry matters. Bad geometry will likely make your readings less accurate. I've seen same poor geometry lately. Only 6 or 7 satellites, most of them towards the south. Less than ideal geometry.


 

Satellite geometry certainly matters but there's a trade off with those horizon sats as they have the most error in them due to more atmospheric influences as the signal scraps throught all that earth atmosphere.

 

Some of the better receivers will have a user definable mask (or maybe a fixed one which is better than nothing) which does exactly that, mask out the low horizon sats.

 

Also the scull cap effect where there are no sats anyway. From 45 deg either sides of the poles up to about 45 degrees. One reason why it's best to face a handheld towards the equator.

 

quote:

It would be nice for GPSs to display dilution of presence numbers so you wouldn't have to guess it. It would also be cool to know just how Garmin and Magellan compute their location. I'm sure that's a closely held trade secret though. icon_frown.gif


 

DOP's would be good but most manufacturers appear to think users won't understand them so they concock up their own version of accuracy and u betchya they aren't about to tell anybody.

 

BTW DOP is "Dilution Of Precision"

 

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 rdw:

So the moral of the huge post is that satellite geometry matters. Bad geometry will likely make your readings less accurate. I've seen same poor geometry lately. Only 6 or 7 satellites, most of them towards the south. Less than ideal geometry.


 

Satellite geometry certainly matters but there's a trade off with those horizon sats as they have the most error in them due to more atmospheric influences as the signal scraps throught all that earth atmosphere.

 

Some of the better receivers will have a user definable mask (or maybe a fixed one which is better than nothing) which does exactly that, mask out the low horizon sats.

 

Also the scull cap effect where there are no sats anyway. From 45 deg either sides of the poles up to about 45 degrees. One reason why it's best to face a handheld towards the equator.

 

quote:

It would be nice for GPSs to display dilution of presence numbers so you wouldn't have to guess it. It would also be cool to know just how Garmin and Magellan compute their location. I'm sure that's a closely held trade secret though. icon_frown.gif


 

DOP's would be good but most manufacturers appear to think users won't understand them so they concock up their own version of accuracy and u betchya they aren't about to tell anybody.

 

BTW DOP is "Dilution Of Precision"

 

Cheers, Kerry.

 

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

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For the near future when WAAS gets certified for one of its intended uses (precision aircraft approaches into airports with no navigational beacons/glideslope/ect) The FAA will actually have a GPS forcast available. one could see if there WILL BE sufficient gps sats visible at a particular location at a particular time to make a precision approach on GPS. I believe this forcast will take in account things like solar storms that might cause signal degredation.

 

This certification of WAAS is still a few years away, I dont know if you can actually get the forcasts yet....

 

[This message was edited by Gliderguy on March 11, 2002 at 08:31 PM.]

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Gliderguy, is that a forecast as in the future (like a day in advance) or past tense as per your wording ".. one could see if there was sufficient gps sats visible ..".

 

Basically standard GPS has that type of "forecast" now both what is planned to happen and what did actually happen (on a world wide basis) along with geomagnetic storm type activity but even now the storm stuff is only a prediction.

 

With the current information available and the right software one can "predict" when one should be at a certain place at the right time to "maximize" the benefit. Not that it's really all that critical to the type of GPS stuff that happens here but at times it would take the guess work out of things.

 

Cheers, Kerry.

 

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

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Gliderguy, is that a forecast as in the future (like a day in advance) or past tense as per your wording ".. one could see if there was sufficient gps sats visible ..".

 

Basically standard GPS has that type of "forecast" now both what is planned to happen and what did actually happen (on a world wide basis) along with geomagnetic storm type activity but even now the storm stuff is only a prediction.

 

With the current information available and the right software one can "predict" when one should be at a certain place at the right time to "maximize" the benefit. Not that it's really all that critical to the type of GPS stuff that happens here but at times it would take the guess work out of things.

 

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 MrGigabyte:

Originally posted by Seth!:

""If they are in the exact same position in the sky twice a day (i.e. 6:47 am and 6:47 pm), would your GPS be more accurate if you were to search for a cache at the same corresponding time of day as when the cache was hidden? The answer is no."

 

Reply by MrGigabyte:

 

Sorry, the answer is yes. GPS satellites broadcast simple radio signals. These signals are affected by many external forces before they hit your eTrex. Not the least of which is the refractive effect of the ionosphere.

 

Ionosphere effects on radio waves are significantly less when there is no solar effect on them. That is, any observations after sunset will always be better than those take 12 hours previous.


 

Sorry, but the answer is still NO. MrGigabyte may have misread my response. He rightly points out that accuracy is better some times than other, like after sunset. BUT that was not the question. The question was, is ACCURACY better if you hunt a geocache the SAME TIME OF DAY that it was placed. No, it isn't. If I place a geocache at noon and you hunt it after sunset, your accuracy may be better than mine was. Do you see what I mean?

 

[This message was edited by Seth! on March 11, 2002 at 02:23 PM.]

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Kerry, I edited my mangled verb tenses. It will be a future forcast. I can't remember exactly, but I think the forcast was going to be good for no more than 24 hours in the future.

 

The whole point of the forcast is for pilots to be able to see if a GPS precision approach will be possible to some airport out in the middle of nowhere before they depart for said airport. All pertinent factors will have to be considered: geometery, WAAS availability, potential solar storm interference, ect... I believe pilots will be able to get this info from Flight Service Stations here in the states. (the same place they would file their flight plans)

 

I was briefed on this the same time I was briefed on what WAAS was and how it impacted air traffic control. That has been over a year ago now and I dont remember many specifics. I suspect the FAA is just making the same information you refer to more accessible to pilots via their normal pre-flight briefing routine.

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