Fixed-site GNSS integrity monitoring

Atri Sangam — the assembly that notices when the sky goes dark

यं वै सूर्यं स्वर्भानुस्तमसाविध्यदासुरः ।

अत्रयस्तमन्वविन्दन्नह्यन्ये अशक्नुवन् ॥

The sun, whom the Asura Svarbhānu had enveloped with darkness, the Atris recovered; no others were able.

Rig Veda 5.40.9

A GPS receiver cannot audit itself. This monitor cross-checks it against references that do not come from the sky — network time, cryptographically signed time, radio time, the drift of the local clock — and watches for the moment they stop agreeing.

Four replays

A healthy site, and the three shapes an attack takes.

Clean

A healthy site. The consensus band is a hairline and GPS sits inside it — every reference telling the same story.

Time Walk

A slow spoof. GPS drifts away from the band while the independent references hold flat. Too gradual for a threshold alone; CUSUM catches the accumulation.

Position Jump

A crude spoof. The reported position jumps away from the surveyed antenna, and the step detector fires at once.

Dropout

An outage. The channels go silent, and the shaded gaps are the absence itself — jamming looks exactly like this.

Terms used on these pages

Term	Stands for	What it means here
GNSS	Global Navigation Satellite System	The family of constellations — GPS, GLONASS, Galileo, BeiDou. This monitor watches whichever ones the receiver reports.
PNT	Positioning, Navigation and Timing	What a GNSS receiver provides, and what this system checks for integrity. Most infrastructure depends on the T more than the PN.
NMEA 0183	National Marine Electronics Association standard 0183	The sentence format a receiver speaks. Each line is one record with a checksum, capped at 82 characters.
RMC / GGA / GSV	Recommended Minimum / Global positioning fix / Satellites in View	The three sentence types read here: fix and time, fix quality, and per-satellite signal strength.
C/N₀	Carrier-to-Noise density ratio	Per-satellite signal strength in dB-Hz. Its spread across satellites is a spoofing tell.
CUSUM	Cumulative Sum control chart	A detector that accumulates small deviations, so a drift too gradual to trip a threshold still gets caught.
SNTP / NTP	(Simple) Network Time Protocol	Time over the network, RFC 4330. Independent of the sky, but not of the network.
Roughtime	—	Cryptographically signed time: an Ed25519 signature over a response that includes a Merkle proof for the nonce we sent.
WWVB	NIST radio station WWVB, 60 kHz	A terrestrial time broadcast. Needs no network, so it survives outages that remove every other reference.
Holdover	—	The local oscillator's own drift model, used to predict what the clock should be doing without any external reference.
Marzullo	Marzullo's algorithm	Interval intersection: the region the largest number of sources agree on. Retained in code as a public export, but no longer decides the consensus offset — a lying server could shape which honest servers were heard by choosing its own interval. The offset is now the median of every source past a width bound, corroborated by a quorum.