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 |
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 |
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)
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.
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.
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 |
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 |
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.
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).
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.
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.