Scenario 5 - The Noise Bump

Session: Monday, 16:00 UTC

Station: BKYD-GPS — patch antenna on a paint-stick mast

Operator: You

Riley: Here, in experiment mode

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Riley’s Note

New toy. Ten-dollar GPS patch antenna on a paint stick, pointed straight up, because tonight is not about catching a pass — the GPS constellation is always up there. NAVSTAR-77 is nearly at zenith right now and stays high all evening.

Tonight is about a signal you will not see at all. A GPS satellite puts about 27 dBW toward the ground, and from twenty thousand kilometers that arrives at your antenna around −128 to −130 dBm — spread across two megahertz, that puts it below your own noise floor. Below. It is quieter than the static. Point the patch at it and the waterfall shows noise. Just noise.

And yet every phone on earth uses it. Work out how before you look at the waterfall — I’ll quiz you.

— Riley

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Tonight’s Sky

Satellite	Type	Elevation
NAVSTAR-77	GPS Block III, MEO	89° at start, still 62° an hour later

MEO birds don’t rise and set on your timescale. There is no pass to catch and no rotator to fly. The signal is simply there, all evening.

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Link Reference

Signal	Frequency	Bandwidth	Modulation
NAVSTAR-77 L1	1575.42 MHz	2.046 MHz	Spread spectrum

The Rig, Configured Wide

Setting	Value	Why
Spectrum span	8 MHz	All two megahertz of L1 in view, with floor on both sides
IF filter	5 MHz	Wide enough to pass a 2 MHz signal
Modem channel	2.5 MHz, BPSK 1/2	Deliberately mismatched — see below

Caution

This receiver will never show GPS and will never say LOCKED on it, and that is not a fault. A spread-spectrum signal is smeared across two megahertz by a code your demodulator does not have. Without the code, the signal genuinely is noise: its power sits several decibels under the floor, so the trace stays flat — no stripe, no bump. A GPS receiver correlates against the code and gets back about 43 dB of processing gain; that is the only reason it works.

The flip side is a rule worth keeping: on L1, real satellites never stand above the floor. Anything you can see there did not come from orbit.

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Timing on This Station

The rig’s reference is GPS-disciplined. The console’s SOURCE section reports it:

Readout	Meaning
REF	Clock source — GPS · n SATS, HOLDOVER, or ACQUIRING
CLK ΔT	Accumulated timing offset, microseconds
HOLD / GPS button	Switches the reference between disciplined and free-running

Holdover means the oven-controlled oscillator free-runs on its own stability and ignores GPS entirely. It drifts slowly — but it drifts honestly, which tonight may be the more valuable property.

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Failure Signatures Worth Knowing

Symptom	Satellite count	Timing offset	Diagnosis
Antenna disconnected	Drops to 0	Freezes, then drifts	Outage
Sky obstructed	Drops	Degrades	Obstruction
Healthy count, offset walking steadily	Unchanged	Growing	Something is lying

A real GPS failure takes satellites away from you. It is loud and obvious. The dangerous case is the one where every indicator stays green and the numbers quietly stop being true.

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Tonight’s Work

• Look for L1 on the waterfall with NAVSTAR-77 overhead — and find only noise
• Understand why a signal you cannot see still runs every GPS fix around you
• Watch the SOURCE panel through the evening
• If the clock starts walking while the satellite count stays healthy, decide what to do about it — and do it
• Come back to GPS only when you can point at evidence that it’s safe

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End of Briefing

Everything with a clock in it trusts GPS: the power grid, financial timestamps, cell networks, and the little oscillator in this radio. It’s a timing signal broadcast from twenty thousand kilometers up, at less power than a lightbulb, with no authentication of any kind.

Nothing in the signal proves who sent it. The only things a liar can’t fake are the physics: how strong it should be, how wide, and how it should move.

Watch the numbers tonight.