docs(bom): select the GNSS antenna and record the receiver alternatives

[PurpleSentinel]

Fills the last unspecified item in the reception chain, and records the receiver question so it
is not reopened from scratch.

H3: Taoglas Magma X AA.170.301111, RS Components 2857133, £26.58 inc VAT

Band coverage decides this, and it is easy to get wrong. The M9N tracks four constellations
concurrently and they do not share a frequency:

Constellation	Frequency
GPS and Galileo	1575.42 MHz
BeiDou B1	1561.098 MHz
GLONASS L1	1602 ± 8 MHz

Many antennas sold as "GPS/GNSS" are tuned 1575–1610 MHz — including SparkFun's own £18
part — which sits above BeiDou. Fitting one costs a quarter of the receiver's constellations
and does so silently: no error, just fewer satellites and worse geometry, which is precisely
what hurts at a circuit ringed by grandstands and pit buildings.

Electrically it fits without adaptors: 1.8–5.5 V against the breakout's 3.3 V bias,
26–32 dB LNA covering its own cable loss without over-driving the M9N front end, SMA(M)
direct to the GPS-17285, IP67, IATF 16949.

Rejected the multi-band ANN-MB-00 at £98.84 — its L2 band is unusable on an L1 receiver.

Subtotal moves from £121.49 to £148.07 inc VAT for H1 + H2 + H3. All suppliers UK: The Pi
Hut, DigiKey UK, RS Components UK.

Receiver alternatives, recorded

The M9N is retained. Its spec — 25 Hz with four concurrent constellations — turns out to be
hard to beat without tripling the price and accepting a rate-for-constellations trade.

Candidate	Bands	Max rate	Protocol	UK inc VAT	Outcome
NEO-M9N	L1	25 Hz, four constellations	UBX	£69.59	Retained
NEO-F10N	L1+L5	10 Hz	UBX	~£60	Rejected: rate
LG290P	quad	20 Hz	NMEA/RTCM	£182.40	Rejected: protocol, RTK
ZED-F9P	L1+L2	9–25 Hz, config-dependent	UBX	~£205	Unproven, not rejected

LG290P fails twice. No UBX: standard NMEA carries time to 10 ms where UBX carries it to
the nanosecond, which would undo much of what crossing_time.cpp exists to achieve. And its
headline feature is RTK — a stated non-goal, and unusable anyway while the device must work
fully offline.

ZED-F9P is recorded as unproven rather than rejected, deliberately. Its datasheet gives
the max navigation rate as a table of six values by constellation configuration — PVT
9/10/20/20/16/25 Hz, RTK 7/10/15/14/13/20 Hz — and the pattern says the higher rates come from
running fewer constellations. If 25 Hz means GPS alone, then at full concurrency it may fall
below the 20 Hz minimum, and the concurrency traded away is itself a reception benefit. I could
not resolve which configuration gives which rate; the PDF table flattened on extraction.
Anyone revisiting must settle that before ordering.

Mounting guidance, which is worth more than the part number

• The magnet mount needs the steel roof as a ground plane
• Not inside the cabin — windscreen attenuation plus A-pillar multipath is the exact error
  this selection exists to reduce
• Retention at track speed: an antenna leaving a car at 120 mph endangers everyone else on
  circuit. Tether it, or use the AA.171 adhesive/screw-mount body for a permanent fit.

Docs only; repo-check passes.