OPUS variation

[MightyMoe]

No, that’s not normal, expect to see differences under 1cm. And from your description of what you did with the two sessions, that would mostly be rotation of the base receiver. I assume you meant you rotated the tribrach. It’s always best to rotate the tribrach, the receiver, and “break” the legs and raise or lower the tripod. When I measure the receiver, I will use meters and feet. You can write down the foot measurement in the book, then take the meter measurement, input the number in the DC with a m behind and it will automatically convert it to feet so you can compare the two.

The difference you describe does relate to the distance I would see with a “bad fix” back in the early days of GPS. I had a Trimble rep tell me to expect one wavelength error with a “bad fix”. I recall that one wavelength is 21cm. Someone here will probably correct that, it’s been years since I cared to look it up.

So that does look like a bad fix, I can’t imagine it persisting for 3 hours using R8-3s which aren’t even close to the old 4400s of the 1990 where I would see them happen and stick for 20 minutes or so before failing. R8-3s aren’t really old tech, they are great for a base receiver.

[BStrand]

I think 8-10 satellites is pretty bad. I’m assuming that’s an instrument or software limitation and not the result of having the base in a bad spot.

Also, I thought OPUS required a minimum of 4 hours of data in order to generate a solution. Did you upload your files separately or did you find a way to combine them first?

Lastly, I wouldn’t spin the base like that.

[dave-o]

I think 8-10 satellites is pretty bad. I’m assuming that’s an instrument or software limitation and not the result of having the base in a bad spot.
Also, I thought OPUS required a minimum of 4 hours of data in order to generate a solution. Did you upload your files separately or did you find a way to combine them first?
Lastly, I wouldn’t spin the base like that.

10 probably is low. They do only get GPS and GLONASS but still thought it should be better than that.

2 hours min

?? Not sure how spinning the base (the tribrach) could make any difference in ortho height. If it does, we’ve got serious industry issues. I did it to get a second collection under ‘different’ conditions, without changing the height.

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dd

• This reply was modified 2 weeks, 1 day ago by  dave-o.

[dave-o]

<i style=”background-color: transparent; font-family: inherit; font-size: inherit; color: var(–bb-body-text-color);”>No, that’s not normal, expect to see differences under 1cm. And from your description of what you did with the two sessions, that would mostly be rotation of the base receiver. I assume you meant you rotated the tribrach. It’s always best to rotate the tribrach, the receiver, and “break” the legs and raise or lower the tripod. When I measure the receiver, I will use meters and feet. You can write down the foot measurement in the book, then take the meter measurement, input the number in the DC with a m behind and it will automatically convert it to feet so you can compare the two.
<i style=”background-color: transparent; font-family: inherit; font-size: inherit; color: var(–bb-body-text-color);”>The difference you describe does relate to the distance I would see with a “bad fix” back in the early days of GPS. I had a Trimble rep tell me to expect one wavelength error with a “bad fix”. I recall that one wavelength is 21cm. Someone here will probably correct that, it’s been years since I cared to look it up.
<i style=”background-color: transparent; font-family: inherit; font-size: inherit; color: var(–bb-body-text-color);”>So that does look like a bad fix, I can’t imagine it persisting for 3 hours using R8-3s which aren’t even close to the old 4400s of the 1990 where I would see them happen and stick for 20 minutes or so before failing. R8-3s aren’t really old tech, they are great for a base receiver.

Thanks. That’s helpful info. I’ll probably look into it with my service guy. The radio link drops made me uncomfortable.

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dd

• This reply was modified 2 weeks, 1 day ago by  dave-o.

[BStrand]

10 probably is low. They do only get GPS and GLONASS but still thought it should be better than that.

Yeah, you should get around 8 just from GPS. OPUS only looks at GPS data still as far as I know so if you’re showing 10 and half of those are GLONASS then OPUS is processing 5 which might account for the poor results?

?? Not sure how spinning the base (the tribrach) could make any difference in ortho height. If it does, we’ve got serious industry issues. I did it to get a second collection under ‘different’ conditions, without changing the height.

My mistake, I misread and thought you spun the base while it was collecting static.

I did lose radio link several times with it coming back online on it’s own within a couple minutes.

Lost radio at the rover? That shouldn’t affect the static collection at all.

[dave-o]

Good points. Thanks. I also just noticed that in the 2 OPUS reports, where they show the Base Stations used, they share 2 of 3 and the third base station listed is different between them, I’m assuming that should also never cause a discrepancy like this but I wonder why they changed between back to back readings.

In the 2nd static reading that differing base station is more than 30 times further away than the one in the first reading.

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dd

[OleManRiver]

Are your opus results different or the RTK points.

If it’s OPUS results differing then when you state you rotated the head that could cause an issue. Opus now uses the absolute antenna calibration file instead of the relative. So you need to make sure your antenna receiver is facing north according to the NRP in NGS guidance on that particular model.

[lurker]

It is not unheard of for the “base stations” that OPUS uses to have bad information. Antennas have been changed without their positions being updated resulting in faulty reference data. You could select your own CORS (base) stations to be used for the processing. You might find by using other stations with your resubmitted data that you get consistent results by eliminating one of the stations that was initially used in your prior OPUS reports.

[Jon Payne]

How far away are the CORS stations used to process the OPUS results?

I’m in a pretty sparse area as far as national CORS nearby, so OPUS pulls stations from 50 to 70+ miles away. In those areas, 4, or preferably more, hours really was needed to get good results from OPUS. Less than 4 hours often returned results that would be in line with what you are discussing.

If I didn’t have time for 4 hours of data, I would download the static from the my home state’s CORS stations (20 to 40 miles away) and process at my office. This would usually provide very good results.

[MightyMoe]

I just looked at the CORS positions and there are only 4 now scattered across the islands. Most of the CORS are out of commission. There is only one on the big island, one on Maui, and two near Honolulu.

I’m guessing it’s a way more challenging environment than what the CONUS crowd is used to. You might try resubmitting after a few days.

• This reply was modified 2 weeks ago by  MightyMoe.

[dave-o]

Good idea. Yeah, we’re pretty sparse. I give it another try in a few.

Looking at the distances from my base:

each had one at 163.26 miles and 37.57 miles

Then one has one at 71.85 miles and the other, 2302.2 (!) That’s friggin’ northern CA

And the discrepancy seems to only be in the OPUS. The RTK shots had some that were close to duplicate from pre battery swap to post and they are very close H&V

Thanks.

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dd

[MightyMoe]

Do you have TBC to roll your own, the CORS on Mauna Kea isn’t very far away, the one on Haleakala should also work well, after that it’s getting out there for control.

If you can single process to those two and they match, I would consider that a win.

[dave-o]

Great idea. I don’t have TBC yet – it’s actually something I’m saving for and have been watching tutorials and such to help prepare for the day I can get it installed. Kinda pathetic 🙂

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dd

[OleManRiver]

This is a very true statement. I have gone through the process to check the stations themselves against each other often. Especially when I am above to do a very large static network. I have found a few antennas that were replaced and had not had the time to be updated at the OPUS level.

[OleManRiver]

You can download a 30 day free trial of TBC. If you are in a pickle. I have done this before just to help a friend. Or use opus projects instead of the generic OPUS so you can get more data and solve the network solution to check for outliers etc.

[OleManRiver]

lol on 10 being bad. I remember planning and seeing 4 to 5 after planning sessions. We have become spoiled with multi constellations for sure. I remember the first time I had a r12 put in my hands and I could see 32 satellites and it was using over 20 of them. I started laughing and the guy I was with said what’s so funny. I said well I see 30 plus satellites. The gps constellation itself is only close to that number which you would never see that many gps ones at a single moment. Now with all the other constellations if less than 20 we start worrying. Man how things have changed.

[Jon Payne]

Looking at the distances from my base:
each had one at 163.26 miles and 37.57 miles
Then one has one at 71.85 miles and the other, 2302.2 (!) That’s friggin’ northern CA

I thought you might be looking at some long distances. Working back from the spec sheet of the unit a rough calculation (easing math by assuming all error was in the GPS unit) of the ppm error I had you at least 100 miles from the CORS stations. If that is the closest CORS, you might be seeing the best results you will get out of OPUS processing.

Given the distances you posted, I would probably download the data from the CORS and process each 3 hour dataset with the 37.57 mile and the 71.85 (still a pretty good ways away) mile station and see how they worked. If the answers weren’t good, I would do a single baseline from the 37.57 mile CORS for each 3 hour dataset and compare them to each other.

[RobertUSA]

The R8-3 brought Galileo tracking but you mentioned only GPS (US) and glonsss.

And static observations do not give an absolute elevation determination without long observations. So even in the US you can log a file for 4 hours in the morning and 4 hours in the afternoon and they can return elevations 0.1 feet difference. NGS guidelines would be I think something like 3 separate 4 hour observations reach 12 hours apart , or a 12 hour observation. I’m probably off a bit. But you get the idea that you’ll have variability with individual 3 hour static files.

[MightyMoe]

OPUS isn’t the tool for dave-o’s enviroment. I would hold the island’s one CORS point on Mauna Kea and compare the resulting value to a tie from the CORS on Haleakala. Once you have a good number (if that’s possible) I would show enough site control to recreate it on site and also show the CORS values used with the date of the survey.

Where dave-o is ground is moving fast, moving probably up and moving westerly.

Local site control is critical.