Glossary

Plain-language definitions of the terms used in Kshana. Each entry starts with a one-line "in plain terms" and then adds the precise meaning where it helps.

Navigation & timing

PNT — Positioning, Navigation, and Timing. Knowing where you are, which way you are going, and what time it is — precisely. Modern PNT mostly comes from satellite signals (GNSS); Kshana studies what happens to time and position when those signals are lost.

GNSS — Global Navigation Satellite System. The satellite constellations that provide PNT: GPS (USA), Galileo (EU), GLONASS (Russia), BeiDou (China). A receiver that can see ≥ 4 satellites can compute a full 3D position and time fix.

GNSS outage / denied / degraded / jammed. When the satellite signals are unavailable (blocked, jammed, or out of view). During an outage the system must "coast" on its own onboard sensors. Kshana's whole purpose is to measure how well it coasts.

Holdover. In plain terms: how long the clock can keep good time on its own after it loses GNSS. A better clock holds longer before its error crosses the allowed limit.

Dead-reckoning. In plain terms: estimating where you are by adding up your measured motion, with no outside reference. Errors accumulate, so a better inertial sensor drifts more slowly.

Spec / threshold. The maximum error you are allowed before the solution counts as "out of spec" — e.g. "timing must stay within 20 ns" or "position within 100 m".

The sensors Kshana compares

Clock (atomic clock). A device that keeps time by counting an atom's natural oscillation. Two examples here:

• CSAC — Chip-Scale Atomic Clock. A small, deployed, commercial clock (the "classical" reference). Good, but drifts noticeably over a long outage.
• Optical lattice clock (e.g. strontium). A far more stable "quantum" clock; the state of the art in laboratories, not yet flown in space.

Accelerometer. Measures acceleration (change in motion). Integrated twice, it gives position — so any error grows quickly. Cold-atom accelerometer is the quantum version with much better long-term stability; navigation-grade is the classical one.

Gyroscope. Measures rotation. A small rotation error tilts the platform, which leaks gravity into the horizontal direction and corrupts the position estimate.

Time transfer. Sending a precise time signal between two places (e.g. satellite to satellite). Optical links are far more precise than RF (radio) links.

How errors are modelled (clock noise)

Clock error is built from standard "noise types", each with a known signature:

White FM (white frequency modulation). In plain terms: fast, random jitter in the clock's rate. Averaging it down improves with time. Parameter q_wf.

Random-walk FM. In plain terms: the clock's rate slowly wanders. This dominates long outages. Parameter q_rw.

Flicker FM (1/f noise). In plain terms: a stubborn noise floor the clock can never average below. It is flat across averaging times. Parameter flicker_floor.

Aging / drift. A slow, predictable change in rate over time; because it is predictable, Kshana's estimator removes it. Parameter drift.

VRW — Velocity Random Walk. The accelerometer equivalent of white noise: random kicks to velocity that build up into position error. Parameter q_va.

ARW — Angular Random Walk. The gyroscope equivalent: random kicks to orientation. Parameter q_arw.

How stability is measured

Allan deviation (σ_y(τ)). In plain terms: the standard way to state how stable a clock is over a given averaging time τ. A clock datasheet quotes, e.g., $σ\text{\_y}(1 \text{s}) = 3 \times 10⁻¹⁰$. Kshana validates its clock model by computing the Allan deviation of its own output and checking it matches the published number. (Reference: Riley, NIST SP 1065.)

PSD — Power Spectral Density. How a noise's power is distributed across frequencies; the formal way to specify white / random-walk / flicker noise.

The figures of merit (how a run is scored)

The six operational PNT figures of merit Kshana reports (see the README "Output" table):

• Positioning / Timing performance — the size of the error (RMS and 95th percentile).
• Autonomy — the holdover duration (how long it stays in spec without GNSS).
• Resilience — how fast the error grows once GNSS is lost.
• Availability — the fraction of the run that has an in-spec solution.
• Integrity — can you trust the system's own estimate of its error? Kshana reports the fraction of outage samples whose true error stays inside the filter's protective bound.
• Security — robustness to spoofing: a clock-stability-based spoof-detectability score (since v0.3.0), meaningful only when a spoofing-attack scenario is configured. It is not aviation-grade integrity (no HPL/VPL/RAIM/ARAIM). Export-sensitive.

Estimation & geometry

Estimator. The algorithm that predicts the true position/time during an outage from the sensor model. Kshana has an analytic holdover predictor and a Kalman filter.

Kalman filter. In plain terms: an algorithm that tracks a quantity and also tracks how uncertain it is. Kshana uses its uncertainty bound to compute Integrity.

Walker constellation. A standard way to describe a satellite constellation by its number of orbital planes and satellites per plane (GPS is roughly a 24/6 Walker shell).

Elevation mask. The minimum angle above the local horizon at which a satellite is considered usable; signals too low are excluded.

Occultation. When the Earth physically blocks the line of sight to a satellite.

Reproducibility & licensing

Reproducible (deterministic). The same input always gives bit-for-bit identical output — scenario + seed + version → identical result. No hidden randomness.

Seed. The number that initialises the (deterministic) random generator, so runs are repeatable.

Open core. The business model: the engine is free and open source (AGPL-3.0, dual-licensed commercially); the sustaining business is support, integration, commercial licences, and proprietary add-ons — not seat fees on the open engine.

AGPL-3.0. The GNU Affero General Public License v3 — an OSI-approved, strong copyleft open-source licence. Like the GPL, but with an extra clause (§13) covering software offered to users over a network: a modified version reached over a network must offer those users its corresponding source. Kshana's open licence.

Dual-licensing. Offering the same code under two licences so users choose: here, the AGPL-3.0 (open, copyleft) or a commercial licence from Ashforde OÜ for proprietary/closed use the AGPL does not suit. See LICENSING.md.