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Home arrow Archives   The American Surveyor     

OPUS-Projects—The Next Revolution in GPS Print E-mail
Written by Mark Silver   
Friday, 25 October 2013

A 2.462Mb PDF of this article as it appeared in the magazine—complete with images—is available by clicking HERE

Also, a 20-minute step-by-step video, available in two flavors, can be found here:

http://youtu.be/7tJ0Z7bqons

http://x90gps.com/video/OPUSProjects_Flash/OPUS_Projects.html

NGS's very popular `OPUS-Static' and `OPUS-Rapid Static' products have revolutionized easy accessibility to horizontal coordinates and ellipsoid elevations. In many areas, a short 15-minute static occupation with an inexpensive L1/L2 receiver will generate an accurate horizontal position.

A longer 8-hour static occupation will produce a centimeteraccurate horizontal position and ellipsoid elevation nearly anywhere in the world.

While OPUS-Static is great for grounding a single point, OPUS-Static does not handle multiple occupations on a site well. Each occupation is independently evaluated, while the shorter baselines between simultaneously occupied sites are left un-processed, un-evaluated and un-adjusted.

OPUS-Projects fixes this shortcoming by processing multi-observation sessions as a single group and provides an easy-to-use tool to rigorously adjust multiple sessions into a final observation set (with G-files, B-Files and SINEX sets that are nearly ready for Bluebooking.)

Before walking through a simple example, here are some initial observations:
• OPUS-Projects is not an incremental enhancement of OPUS-Static or OPUS-RS. OPUS-Projects allows for simultaneous observation `vector' processing of a virtually unlimited number of sites, spread through multiple sessions with a final adjustment tying sessions together generating coordinates for each unconstrained site. An unlimited number of CORS sites can be used in the process. This is very, very different than OPUS-Static which processes a single site against three CORS sites to generate a single site result.
• OPUS-Projects does not require you to download observation files for CORS sites. OPUS-Projects is in the cloud. All the CORS data is in the same cloud. It just works. Similarly, all of the processing results are automatically pushed into subsequent steps:
OPUS-Static » Session Processing » Final Adjustment
You don't need to manually transfer results which reduces the chance of error.
• OPUS-Projects automatically takes care of CORS antennas, CORS domes, time and seismic dependant CORS station positions, re-framing solutions to the current realization of NAD83 and applying the latest geoid model. This is a big deal, a real timesaver and eliminates the potential for common equipment and coordinate framing errors that plague commercial software.
• The requirement for clean observation data to get excellent results is loosened. If an observation file is clean enough to get an OPUS Static solution, when processed with nearby observations, the shorter effective baselines resolve more ambiguities. Sites that process with less than 70% `observations-used' in OPUS-Static, typically process with nearly 100% used in OPUS-Projects. For example: if you have to run a receiver in a canyon bottom, you can run a second `helper' receiver on a random point on the canyon rim in open conditions. The clean `helper' receiver will process against distant CORS and the shorter baseline to the troubled receiver will help the OPUS-Projects session processing engine resolve ambiguities to the receiver at canyon bottom.
• Unlike OPUS-Static and OPUS-RS, there are lots of settings to tweak under the OPUS-Projects hood. Thankfully, OPUS-Projects seams to generate similar results regardless of the processing path.
• OPUS-Projects is cloud based. It will probably fail or be unavailable every once in a while. But being cloud based means you will never have to personally install it on a computer, perform version updates, install new geoids, implement new reference frame realizations or backup your solutions. Someone else takes care of the housekeeping details for you.
• OPUS-Projects has been around for a long time. NGS has been working on it for nearly a decade. There will be enough users running big projects that any issues will be hammered out quickly and collectively.
• OPUS-Static solutions have a good reputation. Long occupations with low peak-to-peak errors have provenance. I believe that OPUS-Projects will enjoy a similar level of trust.
• OPUS-Projects like OPUS-Static and OPUS-RS is free.

Access to OPUS-Projects is only granted once you have taken a two day training course. From the initial response, these courses are going to be full for a long time. If you think you might want to use OPUS-Projects, put down this article, call your NGS State Advisor and ask to be added to `The OPUS-Projects Training List.'

The OPUS-Projects Process
OPUS-Projects is cloud based and includes a 3-tier project hierarchy that facilitates the delegation of project responsibilities.

Using the standard OPUS online submittal form with a project specific identifier code like "FSJS_J45" field crews can:
• Submit GPS observation data
• Enter stamping/setting/description information
• Upload close-up and horizon view pictures
• Tend the antenna/receiver model and serial numbers

OPUS-Projects processes observation files using OPUSStatic. When processing is complete, both the submitter and the Project Manager receive automatic notifications by email.

After uploading all concurrent observations for a session, the 2nd-tier Session Manager can process the session. As each session is successfully processed, email is sent to the Project Manager.

Finally, when all sessions are processed, the Project Manager can perform a final adjustment tying sessions together. Horizontal and Vertical control can be held as required with appropriate error allowances. The Project Manager can run multiple adjustments: one using CORS to find horizontal and ellipsoid elevations and 2nd which favors elevation benchmarks for orthometric height resolution.

Throughout the process, the quality and data spread are graphically shown using X Y Z plots.

A Simple OPUS-Projects Example
We have three points along a linear north-south bearing project for which accurate horizontal and vertical control is required. A single leveled benchmark (N 62) is available four miles from our project.

Create an OPUS Project
First, we make a new OPUS Project From the OPUS-Projects `Create Project' screen we enter our email address, a project title, approximate location, start date and field days.

Immediately after pushing the `Create' button we receive the credentials for our new project (a copy is also emailed to the administrator).
Figure 3: Project Credentials

With our project, session and manager keywords in hand, we are ready to go.

Collect GPS Observations
We have three dual frequency receivers available for simultaneous observations. We would like to spend the absolute minimum amount of time in the field.

A quick site recon (See Figures 4.1­4.4) establishes that VBM N62 has significant tree obstructions to the south, but it is the only remaining vertical control for miles and must be used. Q175 is difficult to occupy and has a telephone pole 1-foot to the north. V175 is wide open, however AJAX has a huge interstate transmission lines and a metal framed tower 100 feet to the west. We decide to occupy Q175 and N62 for as long as possible, moving the remaining receiver from V175 to AJAX after an initial 2-hour occupation. (2-hours is currently the minimum occupation time for an OPUS-Projects observation.)

The field work is completed without incident.

Upload Static Occupations
Uploading the four OBS files is nearly identical to submitting static observations file to OPUS, we just need to press the `Options' button and include the project identifier with each file submission:

After the occupation is uploaded, you are automatically prompted to supply a close up and horizon picture of the mark and enter the model / serial number of the receiver and antenna.

After submitting the four observation files, it only takes a few minutes and emails noting completed OPUS Static solutions are received by both the submitter and the Project Manager. These initial OPUS-Static solutions will be used as a priori solutions for the session processing.

Configure the Project
Now we can log into our project using either the session credentials or the Project Manager credentials for the project. Our points are shown, with the sessioning correctly shown in the lower left corner. Automatically suggested CORS are listed and displayed:

Click `Preferences' to change the project title, Project ID and keywords (Manger or Session):

You can specify additional email recipients for project activity notification, set the data and solution quality limits, set blunder detection limits and control the default processing and adjustment settings. For this project, we will leave everything at the default values.

One of the great features of OPUS-Projects is the automatic `Session Definition.' The `Minimum Data Duration' and `Minimum Session Overlap Multiplier' control the session assignments. Typically sessions will be defined and grouped correctly without the need to manually break occupations that extended through multiple sessions. This is a real timesaver.

Process Sessions
From the Project Manager's Overview Page, we can view all four of the OPUS-Static solutions by clicking on `Show File'. The `Add CORS' button has a slick interface for adding additional CORS sites.

For our example job, OPUS-Projects has correctly identified the two sessions and already chosen appropriate CORS sites.

If you click on the text above Session A (the first session on day 244) the session management screen is shown. At the bottom, a summary of the OPUS solutions that and the data overlap are shown. See Figure 8.

The `Session Quality Indicators' show the results of the initial OPUS-Static solution. N62 (the site with heavy tree canopy to the south) is highlighted as not meeting the Fixed Observation tolerance which was set on the Project Preferences page.

The `Data Availability' table summarizes the observation files, the SV count and file overlap for each site.

Pressing the `Set up Processing' button shows the processing options for the first session:

I chose to use N62 as a `Hub' for the network design. Since we will constrain the final adjustment there is no need to constrain heights and positions at this point, just click the `Perform Processing' button.

Next, repeat the session processing for session B with N62 set as a hub.

After a short wait, two emails are received indicating the session processing has been completed.

OPUS-Static vs. Session Processing
It is interesting to compare the original OPUS-Static with the session solutions. See Figure 10.

N62 has 95% OBS (vs. 91% in OPUS) and 93% Fixed (vs. 80% in OPUS.) The session quality estimates for LAT, LON and HGT are nearly 10 times better than the peak-to-peak OPUS-Static solutions.

Final Network Adjustment
The final step is to adjust the two sessions. Accept all of the defaults except constrain N62 as vertical control, entering the leveled orthometric height and un-constrain vertical for all CORS:

After you click `Perform Adjustment' (at the top of the form,) the adjustment is processed.

There are plenty of details available for each site:

Email is sent to the Project Manager with the final adjustment solution and several files:
• Final adjustment document
• XML file with the results
• Final adjustment summary
• Network SINEX file
• G-file, B-file, R80 and serfil files used for Bluebooking
• Final network adjustment vectors

Conclusion
OPUS-Projects takes care of all of the office details for GPS campaign surveys.

OPUS-Projects is free. It is available to anyone with dualfrequency GPS receivers who has taken the 2-day training course.

The `cloud' based implementation eliminates software installation and maintenance. CORS coordinates, hardware, output frame and geoid updates are updated automatically.

For GPS control campaigns, OPUS-Projects sets a high performance standard. It elegantly solves a very difficult problem. And the price can't be beat!

Mark Silver is an Electrical Engineer, a topographic map collector, and a long time vendor of GPS products.

A 2.462Mb PDF of this article as it appeared in the magazine—complete with images—is available by clicking HERE

Also, a 20-minute step-by-step video, available in two flavors, can be found here:

http://youtu.be/7tJ0Z7bqons 

http://x90gps.com/video/OPUSProjects_Flash/OPUS_Projects.html

 
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