OPUS: Online Positioning User Service

What is OPUS?

This Online Positioning User Service (OPUS) provides simplified access to high-accuracy National Spatial Reference System (NSRS) coordinates. Upload a GPS data file collected with a survey-grade GPS receiver and obtain an NSRS position via email.

OPUS requires minimal user input and uses software which computes coordinates for NGS' Continuously Operating Reference Station (CORS) network. The resulting positions are accurate and consistent with other National Spatial Reference System users.

Your computed NSRS position is sent privately via email, and, if you choose, can also be shared publicly via the NGS website. To use properly, please familiarize yourself with the information below.

Handouts:
See also OPUS one-pager and observer field log.

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Uploading

Using OPUS requires just five simple steps:

Data File of dual-frequency GPS (L1/L2) full-wavelength carrier observables:
Antenna: Selecting your antenna will engage the appropriate antenna calibration model, to counter the unique measurement biases inherent in each antenna's design. Take care! Choosing an incorrect antenna may result in a height error as large as 80 cm vertical, 1 cm horizontal. Tip: Browse antenna calibrations to find an exact match.
Antenna Height: Enter the vertical height in meters of your Antenna Reference Point (ARP) above the mark you are positioning, as shown in the image at right. The ARP for your antenna, usually the center of the base or tripod mount, is illustrated within antenna calibration drawings, along with the North Reference Point (NRP), which should be oriented to true north during your observation.
Email: Enter the address to receive your solution.
Options: Click the Options button to customize the way your solution is performed or formatted. Your selections will override the optimized defaults and should therefore be employed with caution. Frequent users may enjoy the"My Profile" option, which preserves your email, antenna, height, and options used; subsequent uploads require only your email and a data file to engage your customized defaults.
Tip: Use email aliases to assign profiles for different equipment configurations.

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Processing

How does it work?

OPUS will use either a static or rapid-static process, depending on the duration of your data file.

Static: Files are processed using PAGES static software. Your coordinates are averaged from three independent, single-baseline solutions, each computed by double-differenced, carrier-phase measurements from one of three nearby CORS.

Rapid-static: Files are processed using RSGPS rapid-static software. Rapid-static processing employs more aggressive algorithms to resolve carrier phase ambiguities, but has more stringent data continuity and geometry requirements; therefore there are some remote areas of the country in which it will not work. See precision & availability map.

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Accuracy

How accurate is it?

Under normal conditions, most positions can be resolved to within a few centimeters. Estimating the accuracy for a specific solution is difficult, however, as formal error propagation is notoriously optimistic for GPS reductions. Systematic errors, such as misidentification of antenna or height, are not detected. Local multipath or adverse atmospheric conditions may also negatively impact your solution.

Static: Static processing provides peak-to-peak errors for each coordinate (X, Y, Z, Φ, λ, h, and H). These describe the error range, the disagreement between the 3 baseline solutions, as shown above.

One advantage of peak-to-peak errors is that they include any error from the CORS reference coordinates.

See precision & availability

Rapid static: Absent any warning messages, the best estimates of coordinate accuracy are the standard deviations reported by single baseline analysis. Our experiments indicate that the actual error is less than these estimated accuracies more than 95 percent of the time.

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How to improve your accuracy:

See Careful Field Procedures below, and follow these general rules-of-thumb:

Observe longer: A longer-duration session provides OPUS a better opportunity to accurately fix ambiguities and mitigate multipath error. The graph below shows the correlation between session duration and accuracy.

Observe again: A second, independent observation which yields a similar solution is an easy way to increase confidence in your results. Maximize independence by using a different observer, different equipment, on a different day, at a different time of day.

Wait a day: OPUS will use the best CORS and orbits available at the time you upload. While most CORS are archived within 30-minutes past the hour, some aren't available until the next day. Rapid orbits, available at 17:00 UTC that next day, may offer a slight improvement in your accuracy.

Process the data yourself: Manual data processing with suitable software will allow you to include custom cycle slip editing, outlier deletion, experimentation with tropospheric parameters, variable cut-off angle, and different constraints of the carrier phase ambiguities to integer values.

What to look for in a quality solution:

While there are no absolute rules, most accurate OPUS solutions contain the following:

orbit used = precise or rapid
> 90% observations used
> 50% ambiguities fixed
correct antenna and antenna height
Static: overall RMS < 3 cm, peak to peak errors < 5 cm.
Rapid Static: No warning messages.

Quality indicators that are suspiciously low
Normalized RMS that is suspiciously high.
Coordinate standard deviations that are suspiciously high.

Since RSGPS uses the double-differenced ionospheric delays at the CORS to interpolate to the delays at the rover, it may perform poorly or fail during periods of high ionospheric disturbance. In general, it is best to avoid performing any GPS survey during geomagnetic storms that cause large and variable ionospheric refraction. Geomagnetic storm alerts are issued by NOAA's Space Environment Center, so that the surveyor may avoid collecting data during these unusual events.

Similarly, RSGPS performs a simple geographic interpolation to predict the tropospheric delay at your GPS location. Under normal conditions this works well. However, it may not work well during the passage of a strong weather front, and these situations should be avoided.

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Solution Formats

Three solution formats are available; standard, extended, and shared. Samples are provided below:

`NGS OPUS SOLUTION REPORT ======================== 9999 OPUS DISCLAIMER OPUS DISCLAIMER OPUS DISCLAIMER OPUS DISCLAIMER` error and warning messages are appended here
`USER: Your.email@domain.com` Your email address	`DATE: October 27, 2004` The date and time you used OPUS
`RINEX FILE: 7615289n.04o` Your data file name	`TIME: 18:49:54 UTC` Coordinated Universal Time
`SOFTWARE: page5 0407.16 master7.pl` The software we used	`START: 2004/10/15 13:37:00` The first observation in your data file
`EPHEMERIS: igr12925.eph [rapid]` The orbit file we used	`STOP: 2004/10/15 18:10:00` The last observation in your data file
`NAV FILE: brdc2880.04n` The navigation file we used	`OBS USED: 8686 / 8804 : 99%` Usable / total observations in your data file
`ANT NAME: ASH700829.3 SNOW` Your selected antenna type	`# FIXED AMB: 41 / 42 : 98%` For static: Fixed / total ambiguities in your data file For rapid static: quality indicators from network and rover mode solutions (ambiguities are always 100% fixed)
`ARP HEIGHT: 1.295` Your selected antenna height	`OVERALL RMS: 0.020 (m)` For static: The formal statistical root mean square (RMS) error of your solution For rapid static: a unitless normalized RMS
Your position: earth-centered cartesian coordinates in the International GNSS Service (IGS) Reference Frame. The North American Datum of 1983 (NAD83) is also reported, if applicable. Accuracies below are reported as either peak-to-peak errors (static) or standard deviation estimates (rapid static) All initial computations are performed in IGS. Your NAD83 coordinates are derived by transforming IGS vectors into the NAD83 reference frame and recomputing the 3 independent and averaged positions (not a direct transformation of the IGS coordinates; a direct transformation could be considered more accurate, but wouldn't fit your surrounding NAD83 network as well.) For both IGS and NAD83, the reference coordinates for each CORS are derived from the NGS integrated database and are updated using crustal motion velocities from HTDP (Horizontal Time-Dependent Positioning software to your data file's epoch. Your final IGS reference frame coordinates retain this observed epoch, while your NAD83 coordinates are transformed again to the standard epoch date of January 1, 2010.
`REF FRAME: NAD_83(2011)(EPOCH:2010.0000)`	`IGS08 (EPOCH:2004.7887)`
`X: -552474.327(m) 0.015(m)`	`-552475.001(m) 0.015(m)`
`Y: -4664767.953(m) 0.021(m)`	`-4664766.631(m) 0.021(m)`
`Z: 4300548.721(m) 0.024(m)`	`300548.654(m) 0.024(m)`
ellipsoidal coordinates (latitude, longitude, ellipsoidal height) and accuracies
`LAT: 42 39 59.51026 0.007(m)`	`42 39 59.53576 0.008(m)`
`E LON: 263 14 44.18589 0.013(m)`	`263 14 44.14967 0.013(m)`
`W LON: 96 45 15.81411 0.013(m)`	`96 45 15.85033 0.013(m)`
`EL HGT: 314.705(m) 0.041(m)`	`313.753(m) 0.033(m)`
The North American Vertical Datum of 1988 (NAVD88) orthometric height, if applicable, along with the geoid model used
`ORTHO HGT: 340.240(m) 0.041(m)`	`[NAVD88 (Computed using GEOID12A)]`
Your position: Universal Transver Mercator (UTM) coordinates. State Plane Coordinates (SPC) are also reported, if applicable. Also reported are the associated zone IDs, meridian convergence, point scale, and combined factor
`UTM COORDINATES`	`STATE PLANE COORDINATES`
`UTM (Zone 14)`	`SPC (4002 SD S)`
`Northing (Y) [meters] 4726229.423`	`43336.983`
`Easting (X) [meters] 684026.367`	`893325.488`
`Convergence [degrees] 1.52234197`	`2.46893915`
`Point Scale 1.00001666`	`1.00004366`
`Combined Factor 0.99996731`	`0.99999430`
`US NATIONAL GRID DESIGNATOR: 14TPN8402626229(NAD 83)` The US National Grid coordinates and referenced datum are reported, if applicable
`BASE STATIONS USED` The CORS we used as reference stations and the nearest published mark are reported along with their positions and distances from your position.
`PID DESIGNATION`	`LATITUDE LONGITUDE DISTANCE(m)`
`AI1569 NLGN NELIGH CORS ARP`	`N421224.250 W0974743.043 99724.2`
`DF7469 SDSF EROS DATA CORS ARP`	`N434401.727 W0963718.541 119065.7`
`AH5054 OMH1 OMAHA 1 CORS ARP`	`N414641.765 W0955440.671 120751.8`
`NEAREST NGS PUBLISHED CONTROL POINT`
`NM0874 D 276`	`N423846. W0964505. 2286.4`
`The numerical values for this position solution have satisfied the quality control criteria of the National Geodetic Survey. The contributor has verified that the information submitted is accurate and complete.` Because OPUS is automated and assumes your entries are valid, we add this disclaimer to all solutions.

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Error Messages

I got a warning message that the IGS Precise Orbit was not available at the time of processing, but the "rapid" was used. What does that mean?: We will not have the "final" IGS Precise orbit until the International GPS Service (IGS) completes a full week (Sunday through Saturday). This final precise orbit is the combination of seven analysis centers worldwide. It can take these analysis centers several days to upload the orbit to the IGS so the availabilty of a Sunday orbit can be 19 days. The IGS rapid orbit is used in the absence of the IGS precise orbit. However, this is not cause for alarm since the IGS rapid is nearly as "good" as the IGS precise. How does this relate to positions on the ground?
Since most OPUS baselines are less than several hundred kilometers, the differences between using the IGR (rapid orbit) and the IGS (precise orbit) is barely detectable if at all. Because of this, OPUS has discontinued this warning message (1/1/2004). For more information see IGS.
Some of my attempts to submit data generates a return email saying, "The observations to slip ratio is too low. There were an unusually high number of cycle slips in the data set. Aborting ..." (Code 1012). What does this mean, and how can I correct it?: This error message primarily indicates that your carrier phase data set contains too many cycle slips to assure an automated hands-off processing to obtain accurate results. The data may still be useful, but will require human intervention to efficiently resolve the cycle slips. Perhaps nearby radio interference or obstructions have caused an unusual amount of cycle slips.
I am trying to upload a RINEX file and I am getting a message that there are illegal characters in the file name -- what am I doing wrong?: The problem is probably not with the file name, but with the path name. OPUS is run on a UNIX machine, and it can only read path names that contain numbers, letters, the period, dash, and the underscore. If you move your file to another directory, it should be able to be uploaded.

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FAQs - Static

I uploaded data. Why no response?: Solutions are usually sent within a few minutes, but it may take more than an hour to complete if traffic is heavy or your file is large. You will eventually receive either a solution or a failure message. Take care to enter your email address correctly and check your spam filters.
My nearest CORS weren't used. Why not?: OPUS tries to use your nearest CORS, but tests the integrity of each dataset, and will expand the search area until enough quality data are found. Also note some CORS data are not available until the following day. You may use OPUS options to force include or exclude specific CORS.
Is antenna required? If I select "NONE" will it use the L1 phase center?: While strongly recommended, if you leave the antenna as NONE your data will be processed with no offsets applied, resulting in a position a few centimeters above or below the L1 phase center. Ignoring your antenna's phase center variations will be interpreted as changes in tropo delays, causing errors in the tropo parameters which will degrade your solution.
What is the ARP height for Leica antenna model SR 399, with GRT44 tripod mount?: If you have this type of antenna mounting, the ARP height can be determined by using the following equation: Height of ARP (meters) = 0.350 + tape measurement (meters) "Tape Measurement" is the distance in meters from the bottom of the hook in the antenna mounting to the monument.
Why do so many antenna types include the term "NONE"?: NONE means no radome. We use antenna names as adopted by the International GPS Service (IGS). The field (columns 17-20) where NONE is located represents the radome used. The 4 character Id used can be: NONE (or blank), SPKE, SNOW, SCIS, SCIT, LEIC, SPKE, CONE, TCWD, UNAV, TZGD, etc. See the IGS antenna list for acceptable antenna codes.
My NAD83 coordinates are missing. Why?: Sometimes OPUS uses CORS from outside the NGS CORS network that only have IGS reference frame positions and no listed NAD83 positions. Your resulting positions will be just as good as if all CORS were used, except the NAD83 positions will not be listed. You may wish to go to the NGS home page, click on "Products and Services", and download the program HTDP. This program converts positions between different reference frames.
How do the NAVD88 heights from GEOID12B compare with heights from GEOID09, GEOID03, or GEOID99?: Newer geoid models provide improved orthometric height accuracy. Upgrade your older GPS heights as follows: NAVD88_{new geoid} = NAVD88_{old geoid} - new geoid ht + old geoid ht. Use the geoid toolkit to compute geoid heights for your project area.; contact OPUS