If you set up your own base, it needs to know exactly where it is. Otherwise every point you measure is off by the same amount. OPUS solves this for free.
OPUS stands for Online Positioning User Service. It's run by NGS. You leave your receiver still and recording for a while, then upload the file. OPUS emails back your official coordinates. It uses the same software NGS uses for the national survey marks and the NOAANational Oceanic and Atmospheric Administration. The US agency that includes NGS and runs the CORS network.More CORSContinuously Operating Reference Station. A permanent GPS receiver that records satellite data all day, every day.More stations. It works by comparing your file to nearby CORS stations, kind of like getting directions from three neighbors who all know their own addresses.
OPUS-S vs OPUS-RS (according to NGS)
- What it accepts
- Static data only, meaning the antenna did not move. Anywhere from 15 minutes to 48 hours, crossing midnight UTC no more than once.
- OPUS-SThe standard version of OPUS for 2 to 48 hours of data.More (Static surveyRecording GPS data with the antenna sitting perfectly still for a while, which gives very precise results.More)
- For 2 to 48 hours of data. It averages three separate answers, each from one nearby CORS station.
- OPUS-RSThe rapid version of OPUS for 15 minutes to 2 hours of data.More (rapid-static)
- For 15 minutes to 2 hours of data. It is pickier about gaps in the data and satellite geometry, and may not work in some remote areas.
- Files under 2 hours
- Must include the P2 signal plus P1 or C1. (Your receiver's export settings control this.)
Get ready and upload
- Set the base over a Survey markA metal disk or rod set in the ground at a spot with a known, published position.More or another solid spot and record static data. Longer is better. OPUS-S needs at least 2 hours.
- Export or convert the file to RINEXA standard file format for raw GPS data. Any brand of receiver or software can read it.More. Most receivers, and Emlid Studio, can do this.
- On the OPUS page, upload the file and type your email.
- Antenna type: pick your exact antenna model from the list. NGS has tested each model.
- Antenna heightThe straight-up distance from the ground mark to the antenna's reference point. Getting it wrong shifts every height you measure.More: the straight-up height, in meters, from the mark to the ARPAntenna Reference Point. A specific spot on the antenna, usually the bottom of the mount, that heights are measured to.More (ARP). The ARP is a specific spot on the antenna, usually the bottom of the mount. NGS warns that OPUS cannot catch a wrong antenna type or height, so double-check them.
Read the report
NGS gives a checklist for a good result:
- Precise or rapid OrbitsThe paths satellites follow. Precise orbit files tell software exactly where each satellite was.More were used.
- More than 90% of observations were used.
- More than 50% of Integer ambiguityThe unknown whole number of signal waves between a satellite and your receiver. Solving it is what turns FLOAT into FIX.More were fixed (the "FixThe best RTK status. The receiver has finished its math and your position is good to a couple of centimeters.More AMB" line).
- The antenna and ARP height are correct.
- For OPUS-S: overall RMSRoot mean square. A single number that sums up how much results scatter. Smaller is better.More under 3 cm, and Peak-to-peakIn OPUS, how far apart the separate answers landed. A small number means they agree.More under 5 cm. Peak-to-peak is how far apart the three answers landed.
- For OPUS-RS: no warning messages.
The report lists positions in NAD 83(2011)The official US datum for horizontal position right now. Most state networks give coordinates in it.More and ITRFInternational Terrestrial Reference Frame. A worldwide datum. In the US it's about a meter different from NAD 83.More, the Ellipsoid heightHeight above the smooth math shape (the ellipsoid). It's what GPS measures, and it can be very different from elevation.More, and the NAVD 88The official US datum for elevations. Heights in it are roughly "height above sea level."More elevation from the GEOID18NOAA's model that converts GPS heights to official NAVD 88 elevations in the US.More model (datums).
OPUS Projects and OPUS Share
OPUS Projects handles many sessions and marks together and adjusts them as a group. It can also be used to send surveys to NGS. NGS says OPUS Projects submissions for the current national system must reach NGS by January 13, 2027. OPUS Share publishes your result for a mark so other people can use it.
Use it for your base
- Type the OPUS coordinate (in your project's DatumThe starting point and ruler that coordinates are measured from. The same spot has different numbers in different datums.More) into your base as its known position, for RTK or PPK.
- Come back to the same mark with the same antenna height, and the whole project stays lined up.
- Check against a known mark or CheckpointA surveyed point you leave out of processing, then use to test how accurate your finished map really is.More before you trust a full deliverable.
Related: PPK with NOAA CORS · Nearest base station tool