Japan GNSS · 2 of 6 · specification values, not marketing

CLAS: centimetres from the sky, inside Japan, with no base station

CLAS is the Centimeter Level Augmentation Service that QZSS broadcasts on its L6D signal. A compatible receiver reads corrections for satellite orbits, clocks, biases and the atmosphere straight from the satellite and reaches centimetre-level positions with no ground base station and no mobile data. It is free, it is Japan-only, and it is the single thing in Japanese positioning that visiting engineers most often have not used before.

What CLAS actually is

Technically CLAS is PPP-RTK: the corrections are state-space (per-satellite orbit, clock, code and phase bias, plus ionosphere and troposphere for a grid over Japan), compressed into a format called Compact SSR and broadcast at 2,000 bits per second on L6D. Your receiver applies them to its own observations and resolves carrier-phase ambiguities, which is why it converges in about a minute rather than the twenty to thirty minutes of classic PPP. The nationwide reference data behind it come from the GEONET electronic reference stations of the Geospatial Information Authority of Japan.

The numbers that are actually promised

Those are the performance-standard values. Measured results under open sky are usually better; the 2020 evaluations quoted by Navipedia put static horizontal error between 1.3 and 2.7 cm. Read both numbers the right way: the spec is what you can rely on, the measurement is what you will often get. If your work needs 1 cm repeatable, CLAS is not what you sign your name to; RTK with a short baseline is.

The current message specification, IS-QZSS-L6-003, raises the number of augmented satellites to a maximum of 17 across GPS, QZSS and Galileo. Receivers built for the earlier specification needed a firmware update to keep working after the transition, so if you bring an older L6 unit, check its firmware before relying on it.

What hardware you need

An ordinary RTK receiver does not see L6. You need either an integrated receiver that lists CLAS support, or a separate L6 decoder feeding corrections into your RTK engine. The most common low-cost arrangement is the u-blox NEO-D9C, a small module that decodes L6D and L6E and outputs SSR data, paired with a ZED-F9P that does the positioning. Development boards that combine the two are sold in Japan; one of them, the D9CX1 from Geosense in Obihiro, is the board on which this site's own experiments run. The Cabinet Office keeps a list of Michibiki-compatible products; that list, not memory, is where to check whether a given receiver decodes CLAS.

Antenna matters more than with L1-only work. L6 sits at 1278.75 MHz, and many compact antennas are not rated for it. A study in 2023 measured L6 reception characteristics of compact antennas precisely because so many failed to pull the signal in reliably. Use an antenna whose datasheet covers L6 (or the L-band in general).

CLAS against the other ways to get centimetres in Japan

For moving vehicles, machinery and fieldwork with no mobile coverage, CLAS is the obvious answer. For a survey pillar you will return to, RTK to a nearby base or a VRS subscription still wins on repeatability. Many engineers run both: CLAS as the fallback that never loses signal, NTRIP as the primary when the network is there. The list of public bases on this site is free and carries no key, so the fallback costs nothing to configure; see letting the receiver pick the base.

Where CLAS does not help

Under trees, beside tall buildings, in narrow valleys: the same places RTK fails, because the limit is the carrier phase, not the correction. It is also Japan-only by design; take the receiver to Korea and the L6D stream means nothing there. And the L6 receiver chain is a second piece of hardware that draws power and needs its own antenna path; on a small drone or a handheld logger that is a real cost.

Sources

Cabinet Office, Centimeter Level Augmentation Service (CLAS); IS-QZSS-L6 (message specification; L6-003 is the current revision). ESA Navipedia, QZSS (2020 performance evaluation). Cabinet Office product page, u-blox NEO-D9C. PMC, Evaluation of L6 augmentation signal reception characteristics of compact antennas (2023).