If OPUS comes back with a weak solution, or rejects the file, the first place to look is usually not your receiver settings. It is which CORS station you leaned on, and how far that station sits from your base.
NGS OPUS is free. You upload a static GNSS observation file and get a coordinate report back. FixFinder already walks through that upload flow on OPUS: get a precise position for your base. This page is the step before that: picking a sane station using the live base station distance map.
Why distance matters (imperial first)
OPUS quality falls off as the baseline grows. On FixFinder we use the same field rule of thumb as elsewhere on the site: stay inside about 12 miles of the station you care about when you can. Longer baselines can still process, but peak-to-peak errors grow and the report gets harder to trust for a site base.
Rough feel for planning:
| Straight-line distance | What you should expect |
|---|---|
| Under ~12 mi (~19 km) | Preferred for a jobsite base |
| ~12 to 25 mi (~19 to 40 km) | Often still usable; read the report carefully |
| Beyond ~25 mi (~40 km) | Higher chance of a soft or rejected solution |
Always read the OPUS report numbers. Distance is a screening tool, not a guarantee.
Step 1. Mark the base you will occupy
Open the nearest base station map.
Type the address, city, or ZIP where the base will sit, or use your phone location if you are already on site. You want the point on the map to match the tripod, not the office.
Step 2. Read the NOAA / NGS CORS candidates
The map lists public stations by network, including NOAA CORS. For OPUS you care about a CORS station that:
- Has a published position you can trust
- Is close enough in miles for your session length
- Was actually healthy for the hours you logged (OPUS and NGS docs cover data availability; confirm on the NGS side before you blame the rover)
Write down the 4-character site ID and the distance in miles. If two stations are similar in distance, prefer the one with a cleaner sky view and a longer tracking history when NGS shows that information.
Step 3. Do not mix this up with live NTRIP
OPUS is post-processed. You are not logging into a caster here. Live RTK or NTRIP coverage for the same spot is a different question. If you also need a live fix on the job, run the coverage check separately. Keep the two workflows straight so you do not chase the wrong error.
Step 4. Upload only after the distance check
Once the station on the map looks sane:
- Log enough static time on the base (see the OPUS how-to for session length guidance from NGS).
- Upload through NGS OPUS.
- Read peak-to-peak and RMS on the report before you adopt the coordinate as gospel.
If the solution is still soft, try the next-nearest healthy CORS on the map and process again. Do not keep hammering the same far station.
Step 5. Use the coordinate on the job
When the report looks solid, set that coordinate as your base known point in the collector or drone workflow. Then check a known control point or nail if you have one on site. A good OPUS report plus a bad typed digit is still a bad day.
Quick checklist
- Base point entered on /nearest
- Closest usable NOAA CORS noted with distance in miles
- Distance inside your comfort band (aim under ~12 mi when you can)
- Static log long enough for OPUS-S or OPUS-RS (see /learn/opus)
- Report peak-to-peak / RMS reviewed before field use
- Live NTRIP needs checked separately on /check if the crew needs a rover fix
Related FixFinder pages
- Tool: How far is the nearest base station?
- How-to: OPUS: get a precise position for your base
- Related: PPK with free NOAA CORS data
- Live coverage (different job): Is my spot covered?