Satellite Observation - Ngs Report

[ArtMan]

When reporting benchmarks to NGS, one is asked to assess suitability of the station for satellite observation:

 

Satellite Usage

 

Generally, a station is suitable for satellite observations

if there is a clear and unobstructed view of the sky from

approximately 15 degrees above the horizon at the location

of the station. Small objects such as a light pole or

small tree are excepted.

 

Is this station suitable for satellite observations?

 

_ Yes _ No _ Don't know

 

My question is, is this question relevant for observed monuments such as church spires, broadcast towers, obelisks, etc.?

 

I've been generally answering 'don't know,' even if - in theory - one could mount an antenna at the location in question. Satellite observations were done a few years ago at station WASHINGTON MONUMENT 1913 when the monument was surrounded in scaffolding and was accessible to GPS observations (as our friend DaveD can attest). But clearly, this is not a routine practice.

 

Any thoughts on this? Thanks!

[Z15]

It only pertains to mark that can be occupied for surveying i.e. a tripod with a GPS antenna like on this HARN point ; Landmarks would not be occupied by a surveyor. The Washington Monument is a rare exception.

 

Edited December 4, 2003 by elcamino

[+kd7rvh]

Pardon my ignorance on the subject, but what exactly is the purpose of doing GPS observations from a horizontal control point, or other mark?

 

My guess is that it is to determine the current local GPS 'error' in the area, which would produce an offset value for all other GPS units being used to survey the area. Am I at least slightly correct about this?

[+GeckoGeek]

Pardon my ignorance on the subject, but what exactly is the purpose of doing GPS observations from a horizontal control point, or other mark? 

I'd think GPS errors are transitory. The type of observations are carefully constructed to average out such errors.

 

a) Since marks are measured from other marks, I'd image at some point the accumulation of errors will add up. It's inevitable no matter how carefully you make the measurements.

 

Although the GPS is set up with a simplified earth model - that's not perfect, so it might be so as to figure out the GPS "model" errors at that point.

 

c) Both of the above?

 

Bottom line they are two separate independent measuring systems and are bound to have some degree of mismatch between them. If you're going to use GPS to do some surveying, you'll need to know what the local error is if you wish to correlate your GPS readings against the NGS network.

Edited December 4, 2003 by GeckoGeek

[Z15]

You use an existing mark to position new marks you may want in your survey.

That is a general statement, and its a lot more complicated than that. Kind of hard to understand unless you get involved in a survey operation.

 

It has nothing to do with determining errors.

 

The tripod in the photo was 1 of 4 other units in operation that day, 2 were on known points in both horizontal and vertical, and the rest were on unknown new points along our highway survey. This one was about 25-50 miles from all the others. So with GPS you can cover large areas in days that would take weeks with convetional surveys. You take several sets of observations (redundancy) and we set 8 new stations in about 50 sq miles using 3 existing NGS horizontal marks and 4 NGS vertical marks. Some of the marks were both horz and vert as this one was. All the computations are handled by sophisticated software programs.

 

There is also of method of surveying called RTK (real time kinematic) that is very popular today. You would place a GPS receiver on a known control point equipped with a radio to broadcast the corrections to the roamers. That receiver would be programed with the lat, lon and elevation. One or more GPS receivers would be out roaming on all other points (could be 1-1000's) in your survey (mapping for instance), your roamer would receive GPS signals and corrected radio signals (DGPS) from the base station(s) and you would get real time position on the points in your survey. We used that to map out the roadway for the designers to develop improvements etc. We can get to 2cm accuracy with RTK. The training for this work was 1 week so its hard to give a clear picture in this post although computer savvy people can pickup up on the methods fairly quick. The equipment and the associated software gives you the answers in seconds but knowing what to do with it takes the skill and training. You can have two base stations and get an instance check on your positions for more accurate survey work like in property determination etc. You are looking at equipment that would cost up-wards of $50,000 for a GPS base station and 2 roamers + the software to process that data.

 

Here is the equipment I had to work with

And the software

Edited December 5, 2003 by elcamino