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Satellite Usage

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When reporting to the NGS, the last page asks, "Is this station suitable for satellite observations?". I was looking for benchmark discs along Rt 50 today and many of them were covered with dirt from erosion. The discs are visible now that I uncovered them but it's only a matter of time before they are covered again. How do I answer the satellite question?

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Any use of the disk will require that covering dirt, etc. be moved, so nothing special there regarding satellites. The satellites aren't going to see the disk but equipment set up over the disk needs to receive the satellites.

 

To be usable for satellite observation, equipment at about head height needs to have a clear view of most of the sky from 15 degrees above the horizon all the way up and all the way around. You can estimate 15 degrees by stretching out your arm and spreading your fingers. Put the tip of your little finger about level with your eye (where the horizon would be if unobstructed) and look for anything higher than your outstretched thumb.

 

If there is no more than a pole or small distant tree blocking the view then it is suitable. If there is serious blockage, then it is not suitable. In between is a gray area that depends on how bad someone needs to use it. If it isn't obviously good or bad, I just mark "Don't Know".

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Nice explanation, thanks ;) I live in the Mountain State and there is very little level land here. You can't see very far, just hilltops. Benchmark disks are usually placed in cuts along the highways like this:

 

benchmark-1.jpg

 

The disk in the picture is on the north side of the hill. Would that be suitable for satellite?

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No way. Too much hill. 15 degrees above the horizon needs to be clear, except for a few skinny poles or trees. In my mind, above is a definite "not suitable". I'm not a surveyor, but I understand the way GPS works for them, and that looks pretty bad.

--Klemmer

Edited by Klemmer & TeddyBearMama

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That's what I thought. So far, I've only checked yes for disks on bridges and other open places. We don't have much of a horizon around here.

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FWIW, I stay out of the whole fray by saying that I don't know whether a site is suitable for satellite observations.

 

Patty

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Here is a recent bench mark tie on a project on Mount Hood and it is also a good photo to show the discussions we have in this arena as to GPSable. The mark may be difficult to occupy and there was a lot of trees surrounding it, but with good timing and enough time I was able to occupy and use this mark with good results.

 

954a1bb8-31a7-489b-ae45-04cfc607f183.jpg

 

CallawayMT

 

I am using a post that I made back in October about this subject. For you guys the answer is definitely Maybe. If you look at my photo above, this is a spot that I would not normally use, but in Oregon you don't have a lot of choices. Most of the time if the horizon on the west and south are clear you will get enough satellites.

 

So ideally what Klemmer & Bill have stated is true, but surveyors do not get to work in an ideal world and we will use less than ideal setups. We will plot out the obstructions based on azimuth and elevation to look at a site and then run a satellite prediction based on the latitude, longitude and the obstructions to see when the optimum times are to run GPS observations on more difficult sites.

 

When in doubt, don't mark it yes or no.

 

CallawayMT

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Here are a couple of perfect "Suitable for Satellite Observations" marks

 

HARN Station (2001 REO)

MVC-004F.jpg

 

Same Harn from farther away (2001)

MVC-012X.jpg

 

1st Order Tri Station (1992) Notice-Tree trimming to increase visibility, known in the profession as wind damage LOL.

3.jpg

 

Route Location Control Marks (2000)

B-xing.jpg

MVC-001F.jpg

MVC-003F.jpg

MVC-002F.jpg

Edited by Z15

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CallawayMT - good post and picture.

 

I do have a question/point. Is the question “How do I answer the question when logging something with the NGS”? or is the question “Is it true that…”? In other words, the NGS seems to have a “standard” way to answer their question (they have a short write-up where you answer the question). It looks from your picture the “standard” NGS answer would be “No”. However, as you indicate using your experience/good timing/enough time/etc. you can take a NGS “No” and make it a “Yes”.

 

If database says “Yes” then you should expect that you can get satellite observations without doing anything special, but if it says ”No” then you should expect that if you can use satellite observations it will take some extra effort on your part.

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I think it's probably best for us to err on the conservative side.

I'm sure I have made my share of faulty observations just the same.

As CallawayMT demonstrates, a determined professional can turn a 'probably not' into a 'definite yes'.

I would rather lead someone into a GPS observation with a skeptical eye, rather than show them a primrose path.

I do wonder how much credence any professional would put on a report by a hobbyist with a hand-held GPSr anyway.

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CallawayMT - good post and picture.

 

I do have a question/point. Is the question “How do I answer the question when logging something with the NGS”? or is the question “Is it true that…”? In other words, the NGS seems to have a “standard” way to answer their question (they have a short write-up where you answer the question). It looks from your picture the “standard” NGS answer would be “No”. However, as you indicate using your experience/good timing/enough time/etc. you can take a NGS “No” and make it a “Yes”.

 

If database says “Yes” then you should expect that you can get satellite observations without doing anything special, but if it says ”No” then you should expect that if you can use satellite observations it will take some extra effort on your part.

 

Lost and AZ,

 

Personally, I look at both the NGS data sheets as well as the Geocaching benchmark logs when I am planning; more than once I have sent a crew out with the photos that were posted by a geocacher which would help them find a mark, as well as any extra help that they may have added. One reason why I really like to see at least one or more vicinity photos, I also like a close up of the disk to see if they have the actual mark or an accessory.

 

Whether I am going to a mark or sending a crew I generally use the geocaching and NGS logs as a guideline, but what we find on the ground is what makes the final determination of whether or not we use a station. If you are unsure on how to state the GPSability, then put items in your description such as trees to the west or steep slope to the north approximately 50' away or power pole 4' to the north. I generally do not like to set up on marks with a power pole that is closer than about 6' away, it can knock out too many satellites any closer.

 

My best advice is to be descriptive and keep finding those marks, I appreciate the work that everybody does around here. Keep searching and keep taking those photos, they help out and they also are great to see in Blackdog's photo posts.

 

CallawayMT

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CallawayMT -

 

A lot of the stations out here in the valley/desert are easily identifiable as usable for satellite observations, there are some that easily don’t pass the NGS guidelines, and there are some that we’re not sure about.

 

The biggest questionable offenders are the ones near high tension lines. I know that sometimes even our handheld unit can be affected by them, so we just mark them as unknown.

 

Another thing we look for is who and how that last reported satellite observation was made. We typically don’t mess with an observation by the NGS (we may even mark it as unknown so the NGS observation remains unchanged), though if it’s an obvious one we will update the observation. We have seen some rather odd observations from some other groups.

 

Another area of potential question is for stations that don’t appear to be easy to occupy. If you can’t occupy it then how can you observe satellites from it? I’ve seen the link for the Washington Monument, but I’m sure that’s the exception, not the rule. If we think it’s hard/impossible to occupy then we mark it as unknown.

 

Glad you like to see the photos. It does take some extra time to take them, record the direction, then resize and publish them. It would be a lot easier to skip taking them, but it does seem like they have some value - plus it give us a reason to use our digital cameras. :)

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Also, in the northern hemisphere, the GPS satellites are mostly south of one's position, so an obstruction to the north of a mark is less significant than an obstruction to the south.

 

GeorgeL

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Also, in the northern hemisphere, the GPS satellites are mostly south of one's position, so an obstruction to the north of a mark is less significant than an obstruction to the south.

 

GeorgeL

 

You beat me to it george. Up here in washington with tree cover etc... if it's a good satellite day, low PDOP and an open sky to the south it's usually no problem. i love the shortleg setup btw callaway great picture

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What NGS Surveyor and nick107 say is true for very accurate survey purposes (I presume, I'm not a surveyor, only a knowlegable GPS user). But I would not like to leave the impression to the average forum reader that their handheld GPS receiver will have a serious problem in the Northern US or Canada. Sure, there will be a slightly lower probability of very good GPS satellite reception up there. If you are north of about 55° latitude, and are totally blocked from directly overhead and down to the south, you could have a problem at times. HERE is a good explanation.

 

WAAS satellite reception is a quote different subject, as those satellites orbits are over the equator (geostationary orbits). Do dual frequency survey grade GPS receivers even use WAAS? I wouldn't think so with post-processing (such as OPUS).

Edited by Klemmer & TeddyBearMama

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WAAS

 

The worst-case accuracy is within 7.6 meters of the true position 95% of the time, and it provides integrity information equivalent to or better than Receiver Autonomous Integrity Monitoring (RAIM).

 

This is achieved via a network of ground stations located throughout North America which monitor and measure the GPS signal.

Measurements from the reference stations are routed to master stations, which generate and send the correction messages to geostationary satellites.

Those satellites broadcast the correction messages back to Earth, where WAAS-enabled GPS receivers apply the corrections while computing their position.

 

I just thought I would add that ground stations were also involved.

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What NGS Surveyor and nick107 say is true for very accurate survey purposes (I presume, I'm not a surveyor, only a knowlegable GPS user). But I would not like to leave the impression to the average forum reader that their handheld GPS receiver will have a serious problem in the Northern US or Canada. Sure, there will be a slightly lower probability of very good GPS satellite reception up there. If you are north of about 55° latitude, and are totally blocked from directly overhead and down to the south, you could have a problem at times. HERE is a good explanation.

 

WAAS satellite reception is a quote different subject, as those satellites orbits are over the equator (geostationary orbits). Do dual frequency survey grade GPS receivers even use WAAS? I wouldn't think so with post-processing (such as OPUS).

 

Klemmer,

 

The survey grade GPS units can use WAAS, this option can be turned on and off by the operator to alleviate problems with data that appears to be better than it really is. A WAAS position could theoretically appear to have a real time fixed status and be stored as high quality survey grade position.

 

CallawayMT

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