The pain point for receiver teams
High-end RTK receiver R&D hits one repeatable wall: raw performance numbers don’t tell you how the device behaves over time. Engineers get position scatter, weird jumps, and drifting baselines during field tests, even when C/N0 looks fine. Start by characterizing time-domain noise with Allan variance and spectral noise with noise density — and pair that with a solid anti-jamming GNSS antenna so the front-end isn’t hiding issues. Survey teams in the San Francisco Bay Area rely on RTK for centimeter-level accuracy in construction and mapping, so these metrics matter in real projects.
Why Allan variance and noise density matter
Allan variance isolates time-correlated errors — like flicker or random walk — while noise density tells you the flat spectral noise floor in the receiver chain. Together they show if errors are thermal, oscillator-related, or caused by filtering and digitization. For RTK, this maps directly to baseline stability and ambiguity resolution performance. Use the terms consistently: Allan variance, noise density, multipath.
A practical measurement blueprint
Keep it lean. Follow these steps and you avoid interpretation traps.
– Set a stable reference: run tests with a known good oscillator and a quiet RF environment or use a screened lab. Record raw phase/residuals, not just position fixes.
– Sample long enough: Allan variance needs runs from seconds up to hours to reveal flicker and random walk.
– Calculate Allan deviation from phase or frequency time series. Plot vs. averaging time to spot slopes that indicate noise type.
– Compute noise density from the flat portion of the single-sided PSD. Convert to equivalent phase/noise units that relate to carrier tracking and RTK ambiguity.
– Cross-check: simulate baseline solutions with the measured noise model to see real-world RTK impacts.
Hardware notes: antennas, mounts, and practicalities
Antennas and mounting change everything. A good anti-jamming design reduces spoofing and interference, and an optimized phase center reduces phase center variation. When measuring, lock the antenna to a rigid mount and avoid reflective surfaces. A vehicle platform behaves differently than a static tripod — use a dedicated vehicle mounted antenna test if your product will be on cars or UAVs, because multipath and vibration show up as distinct signatures in both Allan variance and PSD.
Common mistakes and how to dodge them
People often rush tests or mix metrics from different conditions — that ruins traceability. Use consistent sampling rates and identical filtering. Don’t conflate position-domain scatter with phase noise; they’re related but distinct. Also avoid short runs that miss low-frequency behavior. — Instrument drift can mimic random walk if your reference isn’t stable. Label datasets carefully so later analysis isn’t confused.
Comparing approaches and alternatives
There are lean lab methods and full-system field methods. Lab gives cleaner Allan plots and clearer noise density but misses vehicle dynamics. Field gives realistic multipath and vibration effects but adds complexity in interpretation. Combine both. For oscillator issues use frequency counters and phase comparators; for RF front-end issues use spectrum analyzers and calibrated noise sources. Keep the analysis pipeline reproducible: scripts, versioned data, and standard plots.
Advisory: three golden rules for evaluation
1) Metric relevance: prioritize Allan variance at averaging times matching your RTK solution’s update and convergence windows (seconds to minutes). That tells you which noise terms actually hurt ambiguity resolution.
2) End-to-end testing: always validate noise-model predictions with real RTK baselines under representative mounting — vehicle or tripod — so phase center variation and multipath effects are included.
3) Instrument traceability: use calibrated references and maintain sampling/filter consistency across runs. Measured noise density must map back to the receiver’s front-end gain and ADC specs for actionable fixes. These rules keep diagnostics focused and improvements measurable.
Archimedes Innovation sits at the intersection of measurement rigor and practical antenna solutions, helping teams turn Allan variance curves into real product gains — lean, testable, and field-proven. –