NMEA 0183 Knowledge Patch

Claude knows basic NMEA 0183 sentence format and common sentences (GGA, RMC, GSV, GSA). This skill provides the full field value tables and multi-GNSS extensions through v4.30 (December 2023) that are needed for writing correct parsers and validators.

Index

Topic	Reference	Key features
Sentence Fields	references/sentence-fields.md	GGA quality 0-8, full FAA mode table, GGA-to-FAA mapping, satellite ID ranges
Multi-GNSS Extensions	references/multi-gnss-extensions.md	GSA System ID, GSV Signal ID, RMC Nav Status, v4.30 new sentences

---

Quick Reference

GGA Quality Indicator (field 6)

Values 0-2 are well-known. Values 3-8 are critical for RTK and precision applications:

Value	Meaning	Typical use
0	No fix	Receiver searching
1	GPS (SPS) fix	Standard autonomous positioning
2	DGPS fix	Differential corrections (SBAS/WAAS)
3	PPS fix	Precise Positioning Service (military)
4	Real Time Kinematic (fixed)	cm-level, integer ambiguity resolved
5	Float RTK	dm-level, ambiguity not fully resolved
6	Estimated (dead reckoning)	IMU/wheel sensor extrapolation
7	Manual input	Position entered manually
8	Simulation	Testing/demo mode

RTK fix progression: 0 → 1 → 5 (float) → 4 (fixed). Quality 4 is required for survey-grade work. Receivers may oscillate between 4 and 5 in challenging environments (urban canyons, tree canopy).

---

FAA Mode Indicator (NMEA 2.3+)

Added in NMEA 2.3 as the last field before checksum on RMC, VTG, GLL, BWC, XTE, and other sentences.

Value	Meaning	Since
A	Autonomous	NMEA 2.3
D	Differential (DGPS)	NMEA 2.3
E	Estimated (dead reckoning)	NMEA 2.3
F	RTK Float mode	NMEA 2.3
M	Manual input	NMEA 2.3
N	No fix / not valid	NMEA 2.3
P	Precise (no degradation)	NMEA 4.00
R	RTK Integer mode (fixed)	NMEA 2.3
S	Simulator	NMEA 2.3

GGA Quality to FAA Mode Mapping

When cross-referencing GGA and RMC from the same fix:

GGA quality	FAA mode
0 (no fix)	N
1 (GPS)	A
2 (DGPS)	D
4 (RTK fixed)	R
5 (RTK float)	F
6 (estimated)	E
7 (manual)	M
8 (simulation)	S

---

Satellite ID Ranges

Parsers handling multi-constellation GSV sentences need these boundaries to map satellite numbers to constellations:

Range	System	Notes
1-32	GPS	PRN number
33-54	SBAS	WAAS, EGNOS, MSAS, GAGAN
65-88	GLONASS	64 + slot number
173-182	IMES	Indoor Messaging System
193-197	QZSS	Quasi-Zenith Satellite System
201-235	BeiDou	u-blox non-standard mapping
301-336	Galileo	Standard NMEA mapping
401-437	BeiDou	NMEA standard mapping

GLONASS dual behavior: With $GL talker, IDs are 1-32 (slot number — add 64 for global). With $GN talker, IDs are already 65-96 (global numbering). Parsers must check the talker ID before mapping.

BeiDou dual range: 201-235 is used by u-blox receivers (non-standard but widely deployed). 401-437 is the NMEA standard range. Accept both ranges.

---

NMEA 4.1+ Multi-GNSS Additions

These changes are backward-incompatible for parsers that assume a fixed field count.

GSA — System ID (field 18)

Multi-constellation receivers emit one GSA sentence per constellation under a $GN talker:

$GNGSA,A,3,80,71,73,79,69,,,,,,,,1.83,1.09,1.47,1*17
                                                  ^-- System ID

System ID	Constellation
1	GPS
2	GLONASS
3	Galileo
4	BeiDou (BDS)
5	QZSS
6	NavIC (IRNSS)

Legacy parsers expect 17 fields. When $GP talker is used, System ID may be absent.

GSV — Signal ID (last field before checksum)

Signal ID values are per-system. GPS signal IDs:

ID	Signal
1	L1 C/A
5	L2C-M
6	L2C-L
7	L5-I
8	L5-Q

Receivers tracking multiple signals per constellation emit separate GSV groups per Signal ID. Total message count in field 1 is per-signal, not per-constellation — parsers that sum GSV messages without checking Signal ID will double-count satellites.

RMC — Nav Status (field 13)

Added after the FAA mode indicator (field 12):

$--RMC,hhmmss.ss,A,ddmm.mm,a,dddmm.mm,a,x.x,x.x,xxxx,x.x,a,m,s*hh
                                                                  ^-- Nav Status (4.1+)

Value	Meaning
A	Autonomous
D	Differential
E	Estimated (dead reckoning)
M	Manual input
N	Not valid
S	Simulator
V	Valid (older specs, equivalent to A)

The checksum position shifts by one field. Parsers must count fields dynamically or check NMEA version support. Field 12 (FAA mode) was added in NMEA 2.3; field 13 (Nav Status) in NMEA 4.1.

---

NMEA 0183 v4.30 (December 2023)

Latest version, replaces v4.11 (2018). New sentences for modern multi-GNSS use cases:

Sentence	Purpose
GIR	GNSS integrity information
GRP	High-accuracy positioning
GGC	Autonomous platform guidance (course)
GCF	Autonomous platform guidance (fix)
GSN	SBAS status and correction data
SMV	SafetyNet maritime distress messaging

The standard is proprietary ($1,150-$10,000) so field-level details for v4.30-specific sentences are not publicly available.

---

Reference Files

File	Contents
sentence-fields.md	Full GGA quality table with use cases, complete FAA mode indicator table with version history, GGA-to-FAA mapping, satellite ID ranges with GLONASS/BeiDou dual-numbering details, RTK quality progression
multi-gnss-extensions.md	GSA System ID field with parsing implications, GSV Signal ID field with GPS signal table, RMC Nav Status field with version history, v4.30 new sentence summary