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Suitable For Satellite Observations


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I'd attempt to run GPS over that station.


But the problem is that it is useless to me to do so. Until some NGS project includes this Bench and improves it's quality, Using it for much is a waste of time.


If were were to GPS this I'd like to know, what direction are we facing in this photo? It could take a little longer to gather data with a standard RTK setup, Due to the proximity of the pole, but it's doable. You would just have to give the GPS Constellation some time over it. I could definitely do it with a rover and some DGPS. It wouldn't be geodetic quality, but good enough for construction staking and layout.


It is an interesting Bench Mark:



BL1861 ___________________________________________________________________

BL1861* NAD 83(1986)- 30 06 41. (N) 095 50 35. (W) SCALED

BL1861* NAVD 88 - 78.72 (+/-2cm) 258.3 (feet) VERTCON

BL1861 ___________________________________________________________________

BL1861 GEOID HEIGHT- -27.50 (meters) GEOID03




Not Adjusted from NGVD29, where it was adjusted and first order... Now in NAVD88 it is VERTCON which is not very high quality as compared to an adjustment at all... In many cases, I could not tie into it, due to the low quality. VERTCON is for ballpark estimating. It would be interesting as to why it was not included in adjustments.



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Likely because the adjustment program kicked it out as having large residual. In others words, it did fit and had to be omitted from the adjustment.


That pole so close would make it unusable for GPS IMO but the location (Lat & Lon) could render that umimportant (1. will explain below). With it being so close it becomes a larger obstruction to the GPS receviever view of the sky then if it would have been 50 ft away. I would put a note in the description to the effect. The pole 2 ft east is the only obstruction to this station be suitable for GPS.


To visualize this draw it out on paper, you will see a pole that close obstructs a lot more of the circle then a pole 50 ft away would.


The object with high accuracy GPS surveys (what NGS is concerned with and why they want the info in the report) is to be able to observe all the available SV's without disruption for up to 24hrs continuous or in 3 each 5-1/2 hr sessions. Everytime a SV's moves behind the pole is a time no data can be collected, the closer it is, the more time that lapses until the SV is picked up again. Just not the most desired situation, so they would likely consider it a waste of time to occupy that point and go look for something else to use.


A private surveyor may not be concerned as much about this, I have seen ones around here occupy marks we would never consider. But they were not setting control that others would rely on, it was just for thier work.


SV - Space Vehicle


1. The farther North you go (46N+), the SV's are more in the southern sky orbits so if the pole was on the north side of the mark, it may not obstuct any and or only a few SV's making it OK. Thats the kind of thing though only the project manager could decide on. By planning when the orbits would miss the pole and then occupy the mark during those times.

Edited by Z15
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I guess I will have to drive over and check out BL1869 A 1281 , another mark in this same series. If you look at the description, this mark was set only 2 feet from a power pole in 1978, but it is possible the pole has been moved since then.


BL1869  ___________________________________________________________________
BL1869* NAD 83(1993)-  30 11 49.36422(N)    095 45 10.68941(W)     ADJUSTED  
BL1869* NAVD 88     -        70.64   (meters)     231.8    (feet)  GPS OBS   
BL1869  ___________________________________________________________________
BL1869  X           -    -553,056.064 (meters)                     COMP
BL1869  Y           -  -5,489,550.695 (meters)                     COMP
BL1869  Z           -   3,189,293.472 (meters)                     COMP
BL1869  LAPLACE CORR-          -0.26  (seconds)                    DEFLEC99
BL1869  ELLIP HEIGHT-          43.12  (meters)          (12/03/01) GPS OBS
BL1869  GEOID HEIGHT-         -27.51  (meters)                     GEOID03
BL1869  VERT ORDER  -  FIRST     CLASS II (See Below)
BL1869.The horizontal coordinates were established by GPS observations
BL1869.and adjusted by the National Geodetic Survey in December 2001.
BL1869.The orthometric height was determined by GPS observations and a
BL1869.high-resolution geoid model using precise GPS observation and
BL1869.processing techniques.

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Between what Mike and I have Provided as thoughts on this, It is well covered. The only thing I think we left out was to say that these stations were originally placed and measured using Optical methods of survey, Not GPS, and so the pole's hinderance to Satellite tracking would not have been objectionable, nor perhaps even part of a criteria for selecting the location at the time.


We have also pointed out the differences in usability, As I pointed out, a private surveyor could find a way to use it for their needs, but a geodetic quality survey, which takes hours to perform could very will be hindered by this pole. The difference between Mike's answer and Mine is one of Usability for What. The what of it will dictate to the user which method they have to adhere to to get where they want to go.


I have observed during construction operations in and near foothill areas, that even with a local DGPS set up, often the GPS equipment operating in the Bulldozers, Excavators and Graders will "Go down" in the afternoon, sometimes after 3 PM due to the difficulty they are having in seeing an adequate GPS Constellation. This is made worse when the job sites have a poor southern exposure. We work around this by setting up some usable Survey control which can be used by optical methods for the rest of the day.



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evenfall Posted on Aug 26 2005, 11:10 AM

I have observed during construction operations in and near foothill areas, that even with a local DGPS set up, often the GPS equipment operating in the Bulldozers, Excavators and Graders will "Go down" in the afternoon, sometimes after 3 PM due to the difficulty they are having in seeing an adequate GPS Constellation. This is made worse when the job sites have a poor southern exposure. We work around this by setting up some usable Survey control which can be used by optical methods for the rest of the day.




At the risk of stepping on your toes (which is not intended) :


What you MIGHT be observing here, after 3pm, is something the RF (Radio Frequency) Industry terms "Sun Streaming". As the sun gets low in the west, photon streaming begins to play havoc with RF signals. This allows CB operators in New York to hear other operators in CA.


I do not pretend to be able to explain it in any terms, but it is well documented that such a situation exists. Think of it as the suns' photons 'blowing' the RF signals around and not allowing you to get an accurate fix. The streaming occurs from west to east.


Ham Radio operators often use this phenomenom to 'skip' their signals to places around the world that they would not ordinarily be able to hit. This is different from topospheric bounce.

Edited by Spoo
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I take no offense at all Spoo,


I have a Ham License. Advanced Class...


1559-1610 Mhz, Is the spectrum currently assigned for GPS uses. The Wavelength at these frequencies, is too short to be influenced by D layer Propagation during the Day, nor by E, F1 and F2 layer propagation at dusk and on into the night. These Layers are in fact a component of the way the Sun's radiation is hitting the Ionosphere, especially when the angle of those rays are not direct, but rather slipping over the edge of the globe as exposure to them wanes.


These frequencies are very line of sight, very direct, and because they are coming from space to earth at a high angle, almost a right angle to the Ionosphere, are not reflected, though there is some ionespheric refraction, and are not subject to the "Skip Propagation" You refer to. It is of note that they are also Circularly Polarized, Due to the spinning nature of Satellites. Even if we wanted, and no matter how we try, these frequencies at GPS spectrum allocations will not skip.


Skip Propagation is most common to frequencies below 54 Mhz, and is, depending on the time of day, a component of low angle RF energy traveling in a straight line off to the horizon, until it connects with Ionespheric conditions, dependent on factors including time of day and the relation to an 11 year sunspot cycle which is currently nearing it's null. The Ionesphere stratifies in changing ways during the day and RF energy at HF frequencies can be reflected off of it and back to earth. It can "skip" like this multiple times. It is the low angle RF that does this like a rock skipping off water. GPS is pointed straight at earth, and the so there is only a High incident angle in addition to the Super High Frequency that will not exhibit this skip.


The time of day is the factor for where the sun is and what effect it is having on the ionosphere at the time. Most of the activity is along the "Greyline", which is twilight, either morning or night. This is why here in the States we can have HF radio contact with Europe and Africa in the morning as it is their evening, and during our evenings we can hear Asia and Australia. CB Radios are at 27mhz, and limited to just 4 watts. Low power but skip just fine as you say, and this is also known as the 11 meter band. Just above it from 28-30 Mhz is the 10 meter band. One of the best places on the frequency spectrum for this kind of propagation. Hams can use up to 1500 watts and High Gain antennas to enhance communications in this part of the spectrum.


Now The GPS constellation, and where we are surveying, or using this constellation is a separate problem. There are not many Birds to the North and we get our highest quality from the birds that are low on the horizon, because the "spread has the largest sized triangles for radio triangulation solutions. In the foothills and near woods, which is what we have plenty of, the forest trees and foothills can and do climb up and cover that low horizon view of the GPS constellation... Them Dern Satellites are always moving! By afternoon, too many are too far south and too low in the sky to be seeable by our equipment. A look at the Ephemeris for the GPS Constellation which was kept by the USCG and now by NGS Here: http://www.ngs.noaa.gov/GPS/GPS.html will give you a look at where the constellation is at any time... And when you are working in 47 degrees north, you'll have these things.


Thanks for raising a good point Spoo, It was something we have not covered before.



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First off let me backtrack a little bit. My primary reason for posing these questions was to learn to provide accurate recoveries to the NGS. I am working under the following primary assumptions. IF THESE ASSUMPTIONS ARE IN ERROR PLEASE LET ME KNOW. I will stick to entering DON'T KNOW when the satellite question comes up on the recovery form.


FIRST ASSUMPTION: The NGS would like to receive, from Amateurs like myself, information on whether a site is suitable for satellite observations.


SECOND ASSUMPTION: This applies to most PIDs in the database, not just the high accuracy HARN types,etc., although particular attention should be given to the better sites. I would automatically exclude intersection stationsand the like.


THIRD ASSUMPTION: When the screen on the recovery submission asks if a site is suitable for satellite observations, it refers ONLY to the reception of the satellite signal, and NOT to ANY OTHER FACTOR such as difficulty of mounting a tripod, security of the site, etc., for which I am totally unqualified to express an opinion.


FOURTH ASSUMPTION: The NGS or a local surveyor could query the Database to see if the _SATELLITE: flag was set to quickly pull up a list of only those sites with good sat reception, then look at the individual Datasheets of the sites to determine if that site meets their needs.


Rob and Mike, I apppreciate your responses in helping me to understand this. I thought I had it all figured out with your help... but...Before I had even gotten your replies, I wanted to see what I could find out on my own. I wanted to see if there were any pre-GPS benchmarks that the NGS had put to use as a GPS site. The Database showed me one nearby BL1869 A 1281.


I rode my bike over there today to take a look, and found this.






Here is a benchmark, only 18 inches from the edge of a wooden power pole. Across the street, there is a line of tall trees up and down the road. The nearest tree goes up to about 25 - 30 degrees in elevation. I would estimate that, east of the station, the sky is partially obscured to an elevation of 15 degrees or more for 45 degrees in azimuth! (As a side note, people driving by while I was taking the picture of my outstretched hand thought I was nuts.) Yet, the NGS used this site in 2001 for Height Modernization, and DEWBERRY DAVIS said one year ago the site was suitable for satellite observation.


What is an amatuer to think?


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As an amateur, I limit myself to "don't know" unless the site is obviously NOT suitable, for example under heavy tree cover or near large obstructions, in which case I figure I might be doing someone a service by reporting that it is not suitable.


Like you, I've seen stations that have had GPS observations taken on them that don't seem to meet the general criteria.

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Well, you can see this subject is open to interpertation. But generally the requirments are that the mark be clear for 360° from 15° above the horizon. An occasional tree or pole may not interfer with GPS observations. When push comes to shove, you have to use whats out there. If the next perfect mark is 10 miles away, the one with the pole next to it looks good.


So if you think its Ok, its your call. If you find marks like this, at least mention in the description of the object that may interfer so the person who might use it can make an informed decision. Thats all we can ask for.


keep up the good work.

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TerraVador -


Assuming that your arm length/hand spread size is within a few standard deviations of the mean, that's an excellent technique for estimating 15 degrees above the horizon. It seems I usually leave my inclinometer in the truck, especially when the mark is a ten-minute hike in. But I've yet to leave my hand in the truck.


BTW, nice bike.



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Using the info you have, I personally would log a Not Sure, and mention the pole directly to the south 1.5 feet away. It covers 18 degrees or so of the southern half of the sky (10%), which may be important. The only time I log a Not Good for GPS is when a station is on or right next to a building, or something massive. Otherwise, I let the professionals make the decision.


Sorry, TerraVador, have to divert the conversation again. Ham radioists, bear with my little rant. As to the radio coverage, from a radio astronomy point of view everything's interference. The ionosphere's reflective below 40 MHz or so (depends on latitude, time of year and so forth) so AM radio is 0.5-1.5 MHz, which is highly reflected. TV channels 2-6 are 54-88 MHz, not reflected, which is why TV requires line of sight. FM radio is higher frequency at 88-108 MHz and also not reflected.


This is why satellite radio works, and solves reception problems due to mountains and so forth. This is also why it stinks for astronomy, with a big power source broadcasting. The XM system uses geostationary satellites; the Sirius system uses Low-Earth Orbit (LEO) satellites, like the GPS sats.


Typically the atmosphere is reflective even to near-zenith (directly overhead) sources below 15 MHz. This varies over the course of the day, with the reflectance barrier lowering to 8-10 MHZ around 2-3 AM.


I feel a little guilty supporting the GPS satellite network by using its signal, but...the benefits of GPS are so ubiquitiously good that it's one source of interference I'm willing to work with. Besides, it's in the sub-centimeter bands, where there's a lot of atmospheric absorption, where observing is tough anyways.

Edited by BuckBrooke
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I feel a little guilty supporting the GPS satellite network by using its signal, but...the benefits of GPS are so ubiquitiously good that it's one source of interference I'm willing to work with.



Since the GPS birds will be in TX mode regardless of your usage, I'm not sure there is any reason for you to feel guilty.


On the other hand, if you're using your headlights while driving at night, the optical astronomy folks might have something to complain about. ;-)



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I haven't read enough about how the sats work to be directly critical about them. I need to read more, and should squeeze the time in to my schedule in the near future.


As for interference near observatories, that's a different matter. I was highly anxious driving across the South African Astronomical Observatory's site at night, 15 mph, on the wrong side of the road, with my lights off and springbok all about. My emergency flashers gave enough light to see the stripes on the road, but still...


One of the biggest (though temporary) sources of interference around radio astronomy sights is the radio pulses made by spark plugs in gasoline engines. At least at the National Radio Astronomy Observatory (NRAO) sites, people drive diesel vehicles (even diesel tour buses). Ham radio operators are banned from nearby operating, as are unshielded microwave ovens on site, and so forth.

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Yes, the suitable for GPS question is vague, and being yes/no, unfortunately inconducive to shades of gray. For me, the answer is geographically dependent; how does the sky visibility compare with other similar-accuracy stations in the area (7 km or so.) Use the text notes and (someday soon) the photo to augment your report.


Our CORS guidelines are the most stringent; they require NO obstructions above 10 degrees. In the real world, GPSable means a better-than-average view of the sky. A few technical notes:

-- obstructions to the north are less important (in N.America) as there are no GPS satellites over the pole.

-- flat/metallic obstructions could create multipath (aka "ghosting") and throw off the GPS.

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Yes, the suitable for GPS question is vague, and being yes/no, unfortunately inconducive to shades of gray.

I guess I'm lucky. For me, the question is seldom vague. It is almost universally "no". If it's not buildings, it's trees or mountains. I'd be hard pressed to find a 360 degree view anywhere near me. :rolleyes: So, on the very rare occasion that I do, I log a "Don't Know". Mountain summits are about the only exception. Those often are good. But I don't get to many mountain summits. :blink:



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