Positioning: NMEA and GNSS

Guidance, section control, and TC-GEO all need one thing the rest of ISOBUS does not provide: an accurate, fresh answer to “where am I, which way am I pointed, and how fast am I going?” That answer comes from a GNSS receiver, and on a modern machine it usually arrives over NMEA 2000 — a CAN-based standard that is a cousin, not a child, of ISOBUS. This chapter explains how the fix gets onto the bus and into a prescription.

Why this is a separate world

ISO 11783 is about machine control; positioning is a measurement domain with its own committee, its own message catalog, and its own history (the older serial NMEA 0183 sentences and the CAN-based NMEA 2000). Rather than reinvent it, ISOBUS machines simply carry NMEA 2000 traffic on the same wire and let agricultural software consume it. machbus treats positioning as a first-class but bounded subset: it decodes the PGNs guidance and TC-GEO actually need.

   GNSS receiver ──(NMEA 2000 PGNs)──► bus ──► guidance / TC-GEO / logging
        │                                            │
        position, COG/SOG, heading, attitude,        turns a fix + a map into
        dilution-of-precision, system time           per-section setpoints

What a fix actually contains

A position is more than a latitude/longitude pair. The signals that matter for agriculture, each its own NMEA 2000 PGN that machbus decodes:

Signal	Why a machine cares
Rapid position (lat/lon)	the basic fix, updated frequently for guidance
Detailed GNSS position	full fix with quality/satellite info
COG / SOG (course & speed over ground)	direction and speed for guidance and logging
Heading / track control	where the vehicle points (not always the same as COG)
Attitude (yaw/pitch/roll)	terrain compensation for accurate ground position
Rate of turn	smoothing and prediction
DOPs (dilution of precision)	how much to trust the fix
System time / date, local-time offset	timestamping the as-applied log

The recurring theme: a raw lat/lon is not enough. Accurate section control needs heading and attitude (the GNSS antenna is not at ground level, and a tilted machine projects its position sideways), and the DOP values tell the software when the fix is too poor to act on.

Fast Packet: the right-sized transport

A full GNSS position record does not fit in 8 bytes, but it is far smaller than an object pool. NMEA 2000 uses Fast Packet — a lightweight multi-frame scheme — for these modest records, rather than the heavier ISOBUS Transport Protocol. machbus’s network layer reassembles Fast Packet PGNs (the GNSS position-data message among them) so the decoder sees a complete record.

   Fast Packet:  frame 0 (header + first bytes) · frame 1 · frame 2 …
                 → reassembled into one GNSS position record → decoded

NMEA 0183, briefly

Some receivers still speak the older serial NMEA 0183 — comma-separated ASCII sentences (GGA, RMC, VTG, …) over a UART rather than CAN. machbus can parse these too, which is useful for bench setups and simpler receivers. The two worlds carry the same underlying information in very different envelopes; machbus normalizes both into the same position types.

From a fix to a setpoint

The payoff is the loop that closes guidance and variable-rate work:

   NMEA fix (lat/lon, heading, attitude, quality)
        │  normalize, check quality (DOP)
        ▼
   machine ground position (antenna offset + tilt corrected)
        │  +  DDOP geometry (section offsets, working width)
        │  +  prescription map
        ▼
   per-section coverage + setpoint rates          (TC-GEO)
        │
        ▼
   process data DOWN to the implement  ·  as-applied logged UP

This is why the positioning chapter sits next to the Task Controller: the fix is an input to TC-GEO, and the DDOP geometry is what turns “the machine is here” into “boom section 7 is over this square metre.”

Doing it with machbus

On the session facade, plug Gnss. It decodes the NMEA 2000 GNSS/navigation PGNs into cached state plus GnssEvents, and can broadcast a few of them:

   Session::builder(name, addr)
       .plug(Gnss::new(NMEAConfig::default().with_all(true)))
       .spawn(transport)
              │
              ▼  driver.poll()
       inbound NMEA PGNs → Event::Gnss(GnssEvent::Position / Cog / Sog / …)
       cached fix available via get::<Gnss>().latest_position()

The hands-on paths are NMEA 2000 and Serial GNSS.

From concept to code

You read about…	Build it with…	See…
Decoding NMEA 2000 GNSS PGNs	`session::plugins::Gnss`	NMEA 2000
Serial NMEA 0183 receivers	`nmea` parser	Serial GNSS
Steering on top of the fix	guidance helpers	Guidance
Position → setpoint	TC-GEO + DDOP geometry	TC-GEO prescription

Scope and honesty

machbus implements the GNSS/navigation PGNs that agricultural guidance and TC-GEO need, decoded and tested locally; it is deliberately a subset of the full NMEA 2000 catalog (it is not a marine chartplotter). It does not provide RTK correction, a positioning engine, or certified positioning accuracy — that is the receiver’s job. What machbus guarantees is faithful decoding of the records the receiver puts on the bus.

Failure modes worth knowing

Trusting a bad fix — acting on a position without checking DOP/quality leads to misapplied product; gate work on fix quality.
Ignoring attitude — on slopes, an uncorrected antenna position is metres off at ground level.
COG ≠ heading — at low speed, course-over-ground is noisy; heading is the reliable signal for orientation.
Stale fix — if the receiver drops out, the last position must be treated as stale, not current.

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