Follow this link to read a great article on seven other free online GPS post-processing services that are comparable to OPUS written by Eric Gakstatter of GPSWorld.com 

NGSShutdownScreenOn October 1, 2013, the U.S. Federal Government shut down and furloughed 800,000 non-essential workers. While services considered essential remained active, those considered non-essential services, like the National Geodetic Survey’s Online Positioning User Service (OPUS) were shutdown. OPUS is a free, online GPS post-processing service. If you try to access www.ngs.noaa.gov, the following screen will be displayed:

For those of you who rely on OPUS for GPS post-processing, now is a great time to try one of the other seven free online post-processing services that are available and comparable to OPUS.

Here is a list of the seven free online GPS post-processing services:

CSRS-PPP: Canadian Spatial Reference System, Natural Resources Canada

AUSPOS: Geoscience Australia

GAPS: University of New Brunswick

APPS: Jet Propulsion Laboratory

SCOUT: Scripps Orbit and Permanent Array Center (SOPAC). University of California, San Diego

magicGNSS: GMV

CenterPoint RTX: Trimble Navigation

My colleague Mark Silver, creator of the X90-OPUS receiver I wrote about a few months ago, embarked on an effort to run test data through each of the online post-processing services to demonstrate that there are free, online GPS post-processing services available world-wide that produce results comparable to OPUS. The following report is the result of his efforts:


 

A Comparison of Free GPS Online Post-Processing Services

By Mark Silver

You are probably familiar with the National Geodetic Survey’s OPUS suite of online post processing tools (OPUS-Static, OPUS-Rapid Static and OPUS-Projects.) These services are capable of producing centimeter-level positioning from static GPS observations. What you may not realize is there are at least six viable alternatives to OPUS.

All are free, easy to use, provide world-wide coverage, and generate surprisingly similar results.

Since each uses a unique baseline tool and processing strategies they form an excellent reality check against each other.

IGS orbits and the IGS permanent CORS arrays are used by many of the services, however some use proprietary equipment arrays and orbit products that provide additional redundancy.

How comparable are these services? Which one is the best?

Criteria for Comparing

Comparing results is a difficult proposition:

  • The true/correct answer for any site is unknown.
  • What grading scale should be used? Should elevation differences be weighted differently than horizontal differences?
  • Should the peak-to-peak range or the standard-deviation be prized?
  • Should comparisons be made on long 24-hour data sets or short 2-hour occupations?
  • Is a single data set sufficient for a meaningful comparison or are multiple data sets preferable?
  • Should a service be ‘thrown out’ of consideration because the solutions are substantially different from the mean?

The answer to all of these questions is “it depends.” Your evaluation will depend on your specific application.

For this evaluation, the following rules governed the data set selection:

  • Choose a site known to be stable with a clean EMI environment.
  • Use 24-hour observation sets to enable ‘best case’ processing.
  • Use a sufficiently large data set, 32-consecutive days, to expose trends.
  • Choose a time period, 90-days in the past, so precise orbits are available to reduce ephemeris effects.
  • Only consider GPS data.
  • Use default settings for every option on each processing service.

Scoring

This would not be as interesting without a little competition.

To keep the evaluation simple, the sum of the X, Y and Height range will be the score and the services will be ranked from lowest score to highest score, with the low score being the ‘best.’

Range was chosen as an indicator of the expected maximum error that might be encountered if only a single 24-hour file was observed.

The combined range rewards a processing scheme that best estimates delays, interference, clock errors and other sources of change that occurred during the 32-day trial.

Remember that the every aspect of this ‘competition’ is arbitrary: from the selection of observation sets, to the final scoring system.

The real take-away from this evaluation is not that one service is better, but how close all of the services are to each other.

Two services (JPS’s APPS, magicGNSS) won’t be acceptable to the average user and a third (RTX Centerpoint) may not work for some users based on receiver and antenna support. Details of these problems are presented with the service descriptions below.

The Test Data

SGU1 in St. George, UT USA was chosen as the observation base. The observations consist of 32 consecutive days (May 3, 2013 through June 3, 2013), 24-hour observation files, 30-second interval, GPS only data. The data files were downloaded from the NGS CORS archive.

Each of the 32 files were submitted to each of the processing services and the results have been tabulated for X, Y and Ellipsoid Height. All data is presented in IGS08 current epoch framed coordinates. All data has been projected to UTM Meters for these comparisons.

The Average Values

Remember, the real story is how close each of these services produce results to one another. Let’s look at the average positions from each service and the difference from OPUS:

Fig 1: Average Solution Difference from OPUS

Fig 1: Average Solution Difference from OPUS

As you can see in Figure 1 above, the services were generally within 5mm of OPUS in X, Y and Height.

Position Tracking vs. Time

Fig 2: Service Results X vs. Time

Fig 2: Service Results X vs. Time

 

Fig 3: Service Results X Range, Average

Fig 3: Service Results X Range, Average

 

Fig 4: Service Results vs. Time

Fig 4: Service Results vs. Time

 

5_YGrid

Fig 5: Service Results Y Range, Average

 

Fig 6: Service Results Z vs. Time

Fig 6: Service Results Z vs. Time

 

Fig 7: Service Results Z vs. Time

Fig 7: Service Results Z vs. Time

 

And the Winner Is…

Following are the scores, based on the combination of X, Y and Height range:

Fig 8: The Scores

Fig 8: The Scores

 

Score ranking (remember this is just for fun as the services provided remarkably similar results):

  1. AUSPOS
  2. CenterPointRTX
  3. GAPS
  4. APPS
  5. OPUS
  6. CSRS-PPP
  7. magicGNSS

There is a significant issue in the JPL APPS’s reported output positions, which will keep it from being of any use to most users. magicGNSS’s results are significantly different than the other services. User’s should independently evaluate magicGNSS’s suitability for their purpose. SOPAC’s SCOUT could not be evaluated because it patently does not support either the receiver or antenna that was used at the test site.


AUSPOS: Geoscience Australia

Score: 0.023

Submittal Page: http://www.ga.gov.au/bin/gps.pl

AUSPOS is a free service from Geoscience Australia. Access is via a simple web interface, the antenna height and type are entered along with a email address for the returned report set. File submission is via FTP or directly from the web interface.

The returned PDF report is the best looking of the reviewed services and includes a Processing Summary showing a map of the CORS sites that were used in the solution. SINEX files are also available.

AUSPOS uses the Bernese GNSS Software for processing baselines, IGS orbits and IGS network stations. Solutions are available for anywhere on the earth.

RINEX files need to be at least 1-hour in length, 6-hour files are recommended. Compact RINEX files are also accepted. Files may be compressed with UNIX, Hatanaka, ZIP, gzip or bzip compression.


Centerpoint RTX Post Processing: Trimble Navigation Limited

Score: 0.030

Submittal Page: http://www.trimblertx.com/UploadForm.aspx

CenterPoint RTX Post Processing is a free service offered by Trimble.
It works anywhere in the world and is based on a proprietary Trimble 100+ worldwide CORS network. Accuracy is 2 cm with 1-hour of observation data; 1 cm with 24-hours. Files longer than 24-hours are not accepted.

RTX uses GPS, GLONASS and QZSS tracked SV’s.

The reported output frames include ITRF2008 at current epoch and a user selectable frame like NAD83/2011 2010.0. RTX is one of the few services that will directly export NAD83 framed results.
A single page PDF and a XML result file are returned by RTX. Unfortunately, it is not possible to copy numerical results from the read-only PDF result file to the clipboard.

RTX supports a limited number of receivers (Trimble) and a relatively small subset of IGS modeled antennas. For this test, TEQC was used to stuff the RINEX headers with a comparable Trimble receiver to the actual Ashtech ProFlex 500 receiver that is in use at SGU1. This was all that was required to spoof an accepted device. If the antenna had not been listed, it would have been necessary to spoof the antenna and adjust the height to reflect the difference in L1 phase center offset.


GAPS: University of New Brunswick

Score: 0.032

Submittal Page: http://gaps.gge.unb.ca/indexv520a.php

GAPS is an ongoing project at the University of New Brunswick and was developed by the Department of Geodesy and Geomatics Engineering.

File submission is by a web page and GAPS provides a large number of user inputs and potentially allows the highest level of customization of any of the reviewed services:

  • You may enter a priori coordinates, and a priori constraints
  • GAPS accepts static or kinematic files
  • You can set the elevation mask
  • The Neutral Atmosphere Delay model is selectable
  • Earth Body Tides and Ocean Tidal Loading can be applied or disabled

GAPS only processes GPS data (no GLONASS.)

Submitted filenames must adhere to the SSSSDDDh.YYt file format. GAPS accepts RINEX and compact RINEX files, they may optionally be gzip, unix compressed or ZIP compressed.


APPS: Jet Propulsion Laboratory

Score: 0.033

Submittal Page: http://apps.gdgps.net/apps_file_upload.php

WARNING! APPS only reports the derived position to the nearest decimeter-meter in geographic (lat/lon) coordinates, while reporting ECEF coordinates to a fraction of a millimeter. If you choose to use APPS, you will need to manually convert the ECEF XYZ to geographic coordinates.

JPL’s APPS is based on GIPSY-OASIS (currently version 5). APPS uses NASA’s 70+ Global GPS Network plus densification from other systems (100+ total receivers distributed globally.) Solutions are typically available with 5 seconds delay from observation.

APPS is easy to use, you just specify a file to upload and then click on ‘Upload’ it takes only 15 seconds to get a result after the file upload is complete. You can optionally register for a free account and use email or FTP for bulk uploads.

APPS also has receiver Live Performance Monitoring: (http://www.gdgps.net/monitoring/index.html) which generates a real time graph of three receivers spread through the world.


OPUS: U.S. National Geodetic Survey

Score: 0.035

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

OPUS solutions are the most common PPP Post-Processed solution in the United States. Two flavors of OPUS are available for single points:

  1. OPUS-Static: Available worldwide, requires 2-hours of data
  2. OPUS-Rapid Static: Available with sufficient nearby CORS stations, requires 15-minutes of data

Long occupations (6+ hours) result in excellent horizontal and GPS-derived ellipsoid heights.

The new OPUS-Projects service processes multiple receivers through multiple sessions to a final processed network adjustment.


CSRS-PPP: Natural Resources Canada

Score: 0.039

Submittal Page: http://webapp.geod.nrcan.gc.ca/geod/tools-outils/ppp.php

Before using CSRS-PPP, you will need to register for a free user account.

CSRS has a fantastic desktop application named PPP-Direct that you can just drag and drop files onto. PPP-Direct automatically submits the file and saves all typing, greatly reducing the chance of error.

CSRS-PPP uses both GPS and GLONASS (if available) observables. Ocean Title Loading corrections can be overridden.

CSRS-PPP will accept single frequency files for processing. CSRS will accept RINEX and Compact RINEX, and will decode ZIP, GZIP and unix compression formats.

CSRS-PPP has a fantastic PDF report, a .csv file detailing results epoch by epoch and a great machine readable summary file.

The desktop submission tool, coupled with the great output reports made CSRS-PPP my favorite tool.


magicGNSS: GMV

Score: 0.081

Submittal Instructions: http://magicgnss.gmv.com/ppp/

magicGNSS Blog: http://magicgnss.gmv.com/wordpress/

magicGNSS accepts emailed files and returns solutions by email. Turnaround time is fast and features a nice PDF report plus SINEX, receiver clock bias files, tropospheric delay, KML trajectory and RINEX CLK clock bias files.

Static and kinematic files with observations from GPS, GLONASS are processed by magicGNSS and the service reportedly Galileo-ready.

magicGNSS uses a subset of IGS stations to provide core coverage.


SCOUT: Scripps Orbit and Permanent Array Center (SOPAC). University of California, San Diego

Scout accepts RINEX and compact RINEX files, compressed (Z, gz, ZIP) submitted from an FTP site or pushed onto a provided FTP server.

Files must be generated on a limited subset of receivers and antennas. While the IGS antenna and receiver files are the basis for acceptable devices, not all IGS-listed devices are on the allowable device list. SCOUT documentation specifically warns against spoofing devices and antennas.

SCOUT uses the GAMIT processing engine.

Because the test data for this article is from a unsupported receiver and the submittal process requires a FTP host server with anonymous access which most users will not bother with, the output from SCOUT was not evaluated.


Conclusion

The similarity of results between all of the services I processed is amazing. That they differ only by millimeters demonstrates the robustness of the algorithms and processes they use.

The difference between AUSPOS, RTX, GAPS, OPUS and CSRS-PPP solutions are negligible. For important positioning projects, it undoubtedly makes sense to use them all.

For locations in the United States, OPUS and RTX return NAD83-2011 framed results. Only OPUS returns derived orthometric heights using GEOID12A. While OPUS has more provenance than the other services, it is easy enough to submit important observations to multiple services as a reality check for important positions.

###

As you read from Mark’s report above, even though OPUS is shut down until the U.S. Congress can resolve its differences, don’t let that stop you from processing your GPS static sessions. However, some level of due diligence on your part is needed as requirements vary for each service. For example, static sessions for the OPUS-RS service can be as short as 15 minutes while other services require two hour GPS static sessions. Furthermore, some services process GPS L1 data while others require both GPS L1 and GPS L2 observations.

See you next month.

Follow me on Twitter at https://twitter.com/GPSGIS_Eric

 

About the Author: 


Eric Gakstatter has been involved in the GPS/GNSS industry for more than 20 years. For 10 years, he held several product management positions in the GPS/GNSS industry, managing the development of several medium- and high-precision GNSS products along with associated data-collection and post-processing software. Since 2000, he's been a power user of GPS/GNSS technology as well as consulted with capital management companies; federal, state and local government agencies; and private companies on the application and/or development of GPS technology. Since 2006, he's been a contributor to GPS Worldmagazine, serving as editor of the monthly Survey Scene newsletter until 2015, and as editor of Geospatial Solutions monthly newsletter for GPS World's sister site Geospatial Solutions, which focuses on GIS and geospatial technologies.

 The Center for GeoInformatics (C4G) at Louisiana State University (LSU) is the appointed Louisiana Spatial Reference Center (LSRC) and as such developed a network of Continually Operating Reference Stations (CORS) aligned with the National Spatial Reference System (NSRS) defined by the United States National Geodetic Survey (NGS).

GULFnet & C4Gnet CORS
LSU C4G's GULFnet & C4Gnet CORS (Yellow indicates NGS CORS)

The mission of NGS is to define, maintain, and provide access to the NSRS, which is the official reference system for latitude, longitude, height, scale, gravity, and orientation throughout the United States and its territories. On June 30, 2012, NGS completed a nationwide adjustment of NGS "passive" control (physical marks, such as brass disk benchmarks) positioned using Global Navigation Satellite System (GNSS) technology. The adjustment was constrained to current North American Datum of 1983 (NAD 83) of NGS Continuously Operating Reference Stations (CORS), an "active" control system consisting of permanently mounted GNSS antennas that are the geometric foundation of the NSRS.

Current NAD 83 CORS coordinates were determined by re-processing all CORS data collected in the NGS initial Multi-Year CORS Solution (MYCS1) project. The resulting CORS coordinates were published by NGS in September 2011, and constitute a new realization referred to as NAD 83(2011), NAD 83(PA11), and NAD 83(MA11) Epoch 2010.00.

Read more about The National Adjustment of 2011 Project

The Louisiana Spatial Reference Center (LSRC) was established in 2002 at Louisiana State University in response to users’ and public safety needs. The LSRC operates in conjunction with NOAA to develop and provide height modernization procedures in Louisiana as well as to share technology development with others.

Read more about NOAA05-R499-03 - IMPORTANCE OF USING NEW ELEVATIONS IN LOUISIANA

NGS performs a daily coordinate quality check for each National CORS site. If the results of these daily solutions indicate a change in the current position of more than 2 cm horizontally, or 4 cm vertically, the posted coordinates for the site in error will be revised.

Read more about the National CORS system

However, there has been great difficulty in keeping the revisions of published coordinates, particularly heights, up-to-date in areas of rapid subsidence, e.g., South Louisiana and the northern coast of the Gulf of Mexico. Datasheets for these stations will lack a published elevation and contain this warning.

 ** This station is in an area of known vertical motion. If an
** orthometric height was ever established but is not available
** in the current survey control section, the orthometric height
** is considered suspect.  Suspect heights are available in the
** superseded section only if requested.

In the standard publication model CORS’ published coordinates are updated if they have changed by greater than 2 cm horizontally or greater than 4 cm vertically. Therefore, the initial coordinates used for reference in the real-time-network (RTN) may differ from the more precisely resolved coordinates fitted by the active software. In the subsidence area that is South Louisiana, the RTN struggled with using some stations’ published coordinates. Some were seeing a difference of as much as 13 cm! The staff of LSU C4G performed a careful network adjustment for those errant stations to determine the most probably correct values for them in the NSRS. Some advanced users of the C4Gnet may study the vectors in their RTN solutions and notice a difference from the coordinate of a CORS from its published coordinates. This is to be expected if the station is one that was brought more closely into line with the NSRS by the adjustment.

The issue may be restated this way. LSU C4G constrains C4Gnet to the NSRS by holding the stations to the published values of the thirty-one National CORS within the network. When the published coordinates are within the expected error budget allowed by NGS the network performs as expected. If a station is determined to have a position outside the expected error budget, the network software then struggles as it tries to make that errant position fit, and will discontinue its use until the source of the difference is resolved. Six stations of the thirty-one exhibited excessive deviation and updated positions were determined for them as follows.

 

  SITE Latitude Longitude Elipsoid Height
1 AMER 29°26’58.49765” N 91°20’17.21198” W -14.403 m
2 CALC 29°46’05.28102” N 93°20’34.37130” W -13.860 m
3 GRIS 29°15’55.88294” N 89°57’26.26208” W -15.688 m
4 LMCN 29°15’17.90439” N 90°39’40.65134” W -14.791 m
5 MCNE 30°10’50.02279” N 93°13’03.84340” W   -8.769 m
6 SBCH 29°52’05.20564” N 89°40’23.63833” W -14.860 m
These coordinate adjustments were made in December of 2017 

 

If a user of the C4Gnet is working near one of these stations and takes the trouble to determine the position of the station on the reference side of a vector, he will see these as the coordinates in lieu of the out-of-spec published coordinates

The whole of the LSU C4G RTN, called C4Gnet, is carefully aligned with the NSRS. NGS specifies that to make that claim 10% of a network’s CORS be National CORS.  Thirty-one of the fifty-seven GULFnet CORS in Louisiana are National CORS, 54%. Subscribers to C4Gnet RTN may have very high confidence that it is well aligned to and represents the NSRS, and results from using the C4Gnet compliant with Louisiana Revised Statute 50:173.1.

Louisiana Revised Statute 50:173.1.
Vertical controls for all surveys shall be determined in the North American Vertical Datum of 1988 (NAVD88). All measurements shall be referenced to local control stations of the National Spatial Reference System, specifically the public domain Louisiana State University Continuously Operating Reference Stations network or other currently National Oceanographic and Atmospheric Administration National Geodetic Survey approved reference stations, such as benchmarks, monuments, or continually operating reference stations.

- JAC 2018

The NGS has issued a notice on their website regarding observed discrepancies between the 2009 and 2012 GEOID models. The notice expansion on August 8, 2012, came in the wake of analysis performed by the LSU Center 4 GeoInformatics back in the week of July 15th where we identified issues in Louisiana, Mississippi, Texas, and other states within the CONUS. C4G notified the NGS of the issues found during the ESRI conference in San Diego, CA during the week of July 25th and we are working with them to resolve the problems as soon as possible.

You may find it useful to do your own testing of GEOID12 in your project area but C4G is recommending that everyone in Louisiana stick with GEOID09 until we learn more about what caused the issues we are seeing in GEOID12 and have confidence that NGS has resolved or justified the differences we are seeing to our satisfaction.

You can keep up with this and other C4G related topics by following us on TwitterFacebookYouTubeC4Gnet RSS feed and the C4G website. The following is a copy of the NGS Notice Update.

**NOTICE UPDATE**

GEOID12 errors found in northern Gulf Coast region
 
The National Geodetic Survey (NGS) has determined that the new hybrid geoid model, GEOID12, contains localized errors in Louisiana, Mississippi, Texas, and Alabama. Some of these errors are in addition to those identified in the previous notice. Click here for more information.

In New Orleans, During Its Tricentennial
September 12-15, 2018

The Surveyors Historical Society is dedicated to exploring, preserving and teaching the accomplishments of surveyors. Their annual Rendezvous has grown to become a premier event in the United States, educational, affordable and fun and anyone may attend! The Louisiana Society of professional Surveyors is hosting the event with Ralph Gipson’s leadership as chairman.

The Surveyors Historical Society erects a monument or researches some important survey corner as part of each year’s Rendezvous. The New Orleans Rendezvous in the fall of 2018, will see the preservation of the first Initial Point in the Public Land Survey System. The Government Land Office surveys started in the Mississippi Territory in 1803 with a 40-mile experimental line run from Natchez Mississippi to the 31st Parallel set by Andrew Ellicott known as the “Line of Demarcation”, or “Ellicott’s Line”. 

Deputy Surveyor Charles Defrance ran East 6 miles and 12 perches Commencing at a mound (# 18) set by Ellicott, where on November 27th, 2018 he set the Initial Point of the Washington Meridian. Thomas Freeman returned to this Initial Point in 1819 and ran a Meridian South that became the St. Helena’s Meridian, which controlled GLO surveys in Louisiana.

An iron pipe as shown in Albert White’s book on Initial Points of the United States currently marks the point (30-59-56.0 N, 91-09-36.8 W). The line of demarcation has been researched by Milton Denny, PLS and Larry Crowley PE, PhD. Additional work needed to prove the correct location including looking for original witness tree ties will be performed in the field research in May.

Once the correct location is proven, the plan is to have a stone monument cut about 4 feet in height and 12 inches square with Washington Meridian inscribed on the North side, the South side will say St. Helena Meridian, the East side will say Line of Demarcation and the West will say Ellicott Line.

The LSU Center for GeoInformatics (C4G) supports efforts to preserve survey controls and their histories. Mr. J. Anthony Cavell, resident surveyor for the LSU C4G will join in the proving survey May 11 & 12. Mr. Cavell, is also one of the featured speakers at the Rendezvous in September. Watch this space for updates.

Click here to visit the Surveyors’ Historical Society website. Click here to download the event brochure.

Vertical controls for all surveys shall be determined in the North American Vertical Datum of 1988 (NAVD88). All measurements shall be referenced to local control stations of the National Spatial Reference System, specifically the public domain Louisiana State University Continuously Operating Reference Stations network or other currently National Oceanographic and Atmospheric Administration National Geodetic Survey approved reference stations, such as benchmarks, monuments, or continually operating reference stations.