OPUS POLL

[Gromatici]

How many of you use OPUS for your GPS Static observations? What data does it tell you? Does it tell you what stations they held? The epoch?

I've never used it, but curious since it seems a lot of out-of-state surveyors use it.

Thanks.

[dmi]

Eric,

Yes....
The valves are returned in terms of an epoch. i believe it is 2007, but don't quote me on that. Three stations are used and these stations are selected automatically, based upon some NGS black box criteria,they tell you what is somewhere, or you can select 3 stations you want to use instead.

Try the website for the best info

http://www.ngs.noaa.gov/OPUS/

[Jim Frame]

OPUS-S is great for tying a locally-observed project to CCS83. OPUS-RS can be used for that purpose as well, though the tie will probably be a little looser.

I use OPUS-RS mostly to get seed positions for baseline processing. On big networks it saves a lot of time fiddling with position flowout. If you put an OPUS-RS position on every station in the network you can ignore the flowout entirely.

[Stan_K]

Jim,

You used "flowout" in your post. That is a foreign term to me. Please define.

Thanks,

[Jim Frame]

"You used "flowout" in your post. That is a foreign term to me. Please define."

When a baseline processor -- I'm most familiar with Trimble's WAVE processor, but I think this applies generally -- solves a vector between 2 receiver antennas, it needs a starting ("seed") position for one of the antennas to estimate the range errors to the satellites at the starting station. It then "flows" a position out to the other receiver antenna using the estimated range errors and the differenced corrections for clock and atmospheric delay errors. The more accurate the seed position, the more accurate the vector solution. The rule of thumb is that 1ppm error is introduced for every 10m of seed position error.

Since OPUS-RS positions from static sessions are likely to have errors denominated in centimeters rather than meters, they make great seed positions for processing. (Because the orbits are referenced to ITRF, you want to use ITRF seed positions if you're trying to minimize vector errors. If you use NAD83 seed positions, you'll introduce a small systematic error due to the offset from ITRF to NAD83.)

.

[Stan_K]

Thanks Jim. I am not that familiar with Trimble. The program I've used most handles that in a different manner and uses different terminology.

[Paul Goebel]

I've used OPUS to establish the position of a base station used for RTK surveys. Just start the survey with an autonomous "here" position, and collect static data at the base while you survey, then send it to OPUS.

[Dave Karoly, PLS]

OPUS returns two positions:
1) NAD83(CORS96) Epoch 2002.00 (on the left of the page)
2) ITRF00 Current Epoch (on the right of the page).

For all positions it reports peak-to-peak errors which I understand to be the spread of solutions it derives over the observation period. NGS says this is more realistic than standard GPS statitistics (such as RMS).

There's a lot of information on the NGS website about how it works and what it returns.

You can request extended output which will give a lot of information about exactly how the final answer was derived.

[Gromatici]

I'm just wondering why some choose to use it instead of doing their own processing? Generally we get rid of the "noise" and then process the baselines. I also don't have to sit on a point for 2 hours. Is it just the cost savings?

[Peter Ehlert]

see http://www.ngs.noaa.gov/OPUS/What_is_OPUS.html and read thru all the pages

you can use 15 minutes of data for OPUS-RS

when doing RTK just set the base to log data... simple and fast

you will get the results back before you finish cussing the crew chief for not taking good notes...