Everything, right here.

RTK (Real-Time Kinematic) uses a nearby “base station” to sharpen GPS/GNSS accuracy from the usual few metres down to a few centimetres. The base station broadcasts its own satellite observations (and its precisely known position); your receiver then takes the difference between the base's data and its own. Because both experience almost the same errors, that difference cancels the common ionospheric and atmospheric errors (differential positioning). RTK measures the difference using the signal's carrier phase, which is what yields centimetre accuracy: older DGPS used the same idea but with the pseudorange (code).

GPS is the name of the US satellite positioning system. GNSS (Global Navigation Satellite System) is the umbrella term, covering GPS + GLONASS (Russia) + Galileo (Europe) + BeiDou (China) + QZSS/Michibiki (Japan). The more satellites you can use, the more stable the fix.

A base station is a fixed GNSS receiver whose coordinates are precisely known. It experiences the same errors as your receiver (the rover) at the same time, so taking the difference cancels the error and yields cm accuracy (differential positioning). The closer the base (the shorter the baseline), the better the accuracy.

NMEA ($GPGGA, etc.) is the “answer” the receiver has finished computing: position and time. Raw data (u-blox RXM-RAWX, RTCM3) is the “ingredients”: pseudorange and carrier phase. RINEX conversion and RTK post-processing need raw data; they cannot be made from NMEA (the ingredients have been thrown away).

RINEX (Receiver Independent Exchange Format) is the standard format for raw GNSS observations. Independent of the receiver brand, it is the precise data used as input to post-processing software like RTKLIB and to online PPP services.

NTRIP is the mechanism that delivers a base station's correction data (RTCM) over the internet. Connecting to a “mountpoint” on a caster lets you receive that station's corrections.

Just drop the file. You need a log that contains raw observations (RXM-RAWX for u-blox): NMEA cannot be converted.

The log most likely contains no raw ranging data (RXM-RAWX / RXM-SFRBX). Enable those two outputs in u-center and re-record the stream while positioning. Configuration debug logs and NMEA cannot be converted.

Open a RINEX observation (.obs) file to see how many satellites were visible over time, the epoch count and SNR: all in the browser, without installing RTKPLOT.

Drop a RINEX observation (.obs) or Hatanaka-compressed (.crx) file to get epoch count, completeness (gap %) and per-constellation observation counts. A .crx downloaded from IGS can be checked as-is.

Both compression and decompression are supported. Drop the file and it is auto-detected (RNXCMP, GSI).

You can convert between NMEA, .ubx, GPX, KML, GeoJSON and CSV. Export to GPX/KML to open directly in Google Earth or QGIS.

Drop a .log / .nmea / .ubx to plot the track at cm accuracy and get σ (scatter) and per-constellation statistics.

Everything is completely free. There are no ads.

No. Every tool and the map work right away, with no sign-up.

The conversion tools (RINEX conversion, Hatanaka, QC) process on a server, so files are uploaded temporarily but auto-deleted after 24 hours. The RINEX viewer, track converter and coordinate readout run entirely on your device and send nothing.

No ads. We use no third-party analytics (such as Google Analytics). We count visits ourselves and do not track individuals.

A 0–12 score computed from each station's RTCM message mix: MSM7 +3 / MSM4-5 +2, +1 per supported constellation, station coordinates (1005/1006) +2, high-grade receiver +1. It records the “richness of information”, not a judgement of quality.

We poll the public sourcetable every 10 minutes and record each station's up/down status (up to 144 times a day). Each permanent page shows a 60-day heatmap.

Please contact 369@kotaroasahina.com (we respond promptly). Listings are records based on the public NTRIP sourcetable.

The public casters RTK2GO, BizStation (Japan), GeoRTK (Japan) and Centipede-RTK (worldwide).

The height GNSS reports is “ellipsoidal height”. Everyday “elevation” is height above the geoid. Elevation = ellipsoidal height − geoid height. You can convert in KUON MAP's coordinate panel (Japan area, Geoid 2024).

We compute and display the declination at a point using the World Magnetic Model WMM2025 (worldwide, client-side). Useful for aligning map and compass, or calibrating a drone's heading.

KUON SKY gives the visible satellite count, DOP, a sky plot and the best 60-minute window over 24 hours, using SGP4 orbit prediction plus terrain masking.

A post-processing method that achieves cm–dm accuracy with a single receiver: without a base station: using precise satellite orbits and clocks. Feed the RINEX you make with KUON into RTKLIB or an online PPP service.

KUON GEO is the GPS / positioning division of Kuon R&D: a map and tool suite that puts GNSS/RTK within everyone's reach, free and account-free. It is run by Kotaro Asahina (an acoustic / web engineer in Obihiro, Hokkaido). We archive raw GNSS data daily, which also serves atmospheric research.

Professionals: surveyors, precision-agriculture, drone/robotics developers: of course, but our wish is that beginners wondering “how does GPS become so accurate?” pick it up too. That's why this FAQ answers in plain language first.