GNSS Navigation Knowledge Patch

Claude's baseline covers basic GPS/GNSS concepts, WGS84, and satellite constellations (GPS/GLONASS/Galileo/BeiDou). This skill provides the practical implementation details needed to write GNSS positioning code: observation equations, correction protocols, receiver configuration, and platform APIs.

Index

Topic	Reference	Key content
Observation equations	references/observation-equations.md	Pseudorange/carrier phase models, ionosphere-free combination, error budget
RTK & RTCM corrections	references/rtk-rtcm.md	RTCM 3.x message types, MSM4/MSM7, base station output config
SPARTN & PPP corrections	references/spartn-ppp.md	SPARTN format v2.0.3, SSR corrections, atmosphere models, encryption
Galileo services	references/galileo-services.md	HAS (free PPP corrections), OSNMA (navigation authentication)
u-blox F9P & UBX protocol	references/ublox-f9p.md	CFG keys, NAV/RXM messages, RTK activation, SPARTN input
IGS products & formats	references/igs-products.md	SP3 orbits, Clock RINEX, IONEX, ANTEX, product tiers and latencies
Android raw GNSS	references/android-gnss.md	GnssMeasurement API, carrier phase, antenna info, dual-frequency

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GNSS Signal Frequencies — Quick Reference (inline)

Constellation	Signal	Frequency (MHz)	Wavelength (cm)	Notes
GPS	L1 C/A	1575.42	19.0	Legacy civil signal
GPS	L2C	1227.60	24.4	Modernized civil
GPS	L5	1176.45	25.5	Safety-of-life, highest precision
Galileo	E1	1575.42	19.0	Same center freq as L1
Galileo	E5a	1176.45	25.5	Same center freq as L5
Galileo	E5b	1207.14	24.8
Galileo	E6	1278.75	23.4	HAS corrections on E6-B
GLONASS	L1	~1602	~18.7	FDMA: per-satellite freq
GLONASS	L2	~1246	~24.1	FDMA: per-satellite freq
GLONASS	L3	1202.025	24.9	CDMA (modernized)
BeiDou	B1I	1561.098	19.2	Legacy
BeiDou	B1C	1575.42	19.0	Modernized, interoperable
BeiDou	B2a	1176.45	25.5	Interoperable with L5/E5a

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Observation Equations — Quick Reference (inline)

Pseudorange:

R = ρ + c(dt_r - dt^s) + T + (40.3e16/f²)·STEC + M + ε

Carrier phase:

Φ = ρ + c(dt_r - dt^s) + T - (40.3e16/f²)·STEC + λN + ε

Where: ρ = geometric range, c = speed of light, dt_r/dt^s = receiver/satellite clock errors, T = troposphere, STEC = slant total electron content, M = multipath, λN = integer ambiguity (carrier only), ε = noise.

Critical: Ionosphere sign flips — code delay (+), carrier advance (−). λN is an unknown integer number of cycles, lost on signal lock loss (cycle slip).

Ionosphere-free combination (eliminates ~99.9% first-order iono, standard for PPP):

P_IF  = (f1²·P1 - f2²·P2) / (f1² - f2²)
Φ_IF  = (f1²·Φ1 - f2²·Φ2) / (f1² - f2²)

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RTCM 3.x Message Types — Quick Reference (inline)

Message	Content	Interval
1005	Antenna Reference Point (ARP)	2 s
1074	GPS MSM4 (code + carrier)	1 s
1084	GLONASS MSM4	1 s
1094	Galileo MSM4	1 s
1124	BeiDou MSM4	1 s
1230	GLONASS code-phase biases	10 s

MSM7 variants (1077/1087/1097/1127) for higher precision. Typical throughput ~2 kB/s.

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u-blox F9P Key Config Items (inline)

CFG Key	Purpose
CFG-TMODE	Base mode: survey-in or fixed coordinates
CFG-MSGOUT-RTCM_*	Enable RTCM output per port
CFG-RATE	Navigation rate (1–10 Hz)
CFG-SPARTN	PPP corrections input (FW 1.51+)

Key output messages: NAV-PVT (position/velocity/time), NAV-RELPOSNED (RTK baseline), NAV-HPPOSLLH (high-precision LLH).

Raw logging: RXM-RAWX + RXM-SFRBX (PPK post-processing), RXM-SPARTNKEY (SPARTN decryption keys).

RTK auto-activates when RTCM data appears on any port. Default UART baud: 115200.

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IGS Product Tiers (inline)

Tier	Orbit	Clock	Latency	File pattern
Final	~2.5 cm	~20 ps	13 days	igsWWWWD.sp3
Rapid	~2.5 cm	~40 ps	17 hours	igrWWWWD.sp3
Ultra-Rapid	~3–5 cm	~30 ps	3 hours	iguWWWWD_HH.sp3

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PPP vs RTK — Decision Matrix (inline)

Aspect	RTK	PPP (SPARTN/HAS)	PPP (IGS post-process)
Accuracy	1–2 cm	3–6 cm (converged)	~2 cm
Convergence	Instant (with fix)	30–60 s (SPARTN), minutes (HAS)	N/A (post-process)
Coverage	~35 km from base	Global	Global
Infrastructure	Base station or NTRIP	Subscription (SPARTN) or free (HAS)	Free (IGS archives)
Latency	Real-time	Real-time	Hours to days
Frequency req.	Dual preferred	Dual required	Dual required

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Galileo Services Summary (inline)

HAS (High Accuracy Service): Free real-time PPP corrections (initial service Jan 2023). Orbit/clock/bias for Galileo E1/E5a/E5b/E6 + GPS L1/L2C. Delivery: E6-B signal (448 bps) or NTRIP IDD (requires GSC registration). Receiver must implement PPP algorithm. ~20 cm (95%) horizontal accuracy.

OSNMA (Navigation Message Authentication): TESLA-based broadcast authentication on E1-B (initial service Jul 2025). Protects against spoofing. Setup: install Merkle tree root key from GSC (one-time), maintain time sync within 30–300 s of GST. Key renewal: OTAR via SIS or manual download from GSC/EUSPA.

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SPARTN Corrections (inline)

SSR-based PPP corrections format (current v2.0.3, Nov 2025). Content: orbit/clock/bias + atmosphere (HPAC ionosphere, BPAC troposphere). Encrypted with dynamic key management. Supports GPS/GLO/GAL/BDS/QZSS. Primary service: u-blox PointPerfect (L-band or IP delivery).

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Android Raw GNSS (inline)

GnssMeasurement (API 24+): pseudorange, ADR (carrier phase), AGC, multi-frequency. GnssAntennaInfo (API 30+): PCO, PCV, signal gain. Mandatory raw measurements on API 29+. L1+L5 dual-frequency on most flagships since ~2020. Log via Google's GNSSLogger app.

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Error Budget — Single Point (inline)

Source	Pseudorange	Carrier phase
Ionosphere (uncorrected)	1–50 m	1–50 m
Troposphere	~2.3 m zenith	~2.3 m zenith
Satellite orbit (broadcast)	~2 m	~2 m
Satellite clock (broadcast)	~2 m	~2 m
Multipath	0.5–1 m	~mm
Receiver noise	~0.3 m	~2 mm

Iono-free combination eliminates ~99.9% of ionosphere but amplifies noise ~3×. Precise orbits/clocks (IGS) reduce orbit+clock to cm-level.