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docs(bom): verify the 25 Hz claim against the data sheet - #169
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Every alternative examined traded update rate against the number of constellations, and the M9N's own headline figure had been taken on trust from this document rather than checked. Applying that scrutiny only to the alternatives would have been careless. Checked, it holds without qualification. NEO-M9N-00B_DataSheet_UBX-19014285 gives the maximum PVT update rate per constellation configuration as 25 Hz across all five, including GPS + GLONASS + Galileo + BeiDou. The rate does not fall as constellations are added, which is what this project needs and what makes the part unusually well suited to it. Recorded alongside it that the newer u-blox parts are a downgrade for this application, because that is the substitution most likely to be proposed again. The M10 platform is optimised for low power and manages 10 Hz with four constellations, reaching 20 Hz only with three and a raised CPU clock. This device runs from vehicle USB, so the power saving buys nothing and the rate ceiling costs everything. Also recorded why two boards that are easier to buy are not substitutes. The MicroMod NEO-M9N is the same receiver in the wrong packaging: an M.2 edge connector that needs a carrier board it does not include, and a U.FL antenna connector that reintroduces the fragile joint selecting the SMA variant removed, in a vibrating car. The MAX-M10S breakout has the right connectors and half the required update rate. Sourcing is noted as it stands: Pimoroni no longer stock the part and The Pi Hut do not carry it, leaving DigiKey UK or SparkFun direct. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
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Converts the BOM's "25 Hz with four concurrent constellations" from an assertion into a cited
fact, and records why the parts that are easier to buy are not substitutes.
The check, and why it was needed
Every alternative examined — LG290P, NEO-F10N, ZED-F9P, MAX-M10S — turned out to trade update
rate against constellation count. The M9N's own headline figure had been taken on trust from
this document. Applying that scrutiny only to the alternatives would have been careless.
It holds without qualification.
NEO-M9N-00B_DataSheet_UBX-19014285:The rate does not fall as constellations are added. Compare the F9P, whose equivalent row
runs 9/10/20/20/16/25 Hz.
The newer parts are a downgrade here
Recorded with figures, because this is the substitution most likely to be proposed again:
The M10 is newer and worse for this use. It optimises for low power — and this device runs
from vehicle USB, so the saving buys nothing while the rate ceiling costs everything.
Two boards that are easier to buy and still wrong
MicroMod GNSS NEO-M9N (£41.30, The Pi Hut). Same receiver, wrong packaging: an M.2 edge
connector needing a carrier it doesn't include, and a U.FL antenna connector that
reintroduces precisely the fragile joint choosing the SMA variant removed — in a vibrating car,
plus a pigtail to reach the selected SMA antenna. With carrier and pigtail it costs about the
same as
GPS-17285and adds two failure points.MAX-M10S breakout (The Pi Hut). Right connectors, half the required rate.
Sourcing as it stands
Pimoroni no longer stock
GPS-17285and The Pi Hut don't carry it. DigiKey UK remains thesource, SparkFun direct the alternative.
Docs only;
repo-checkpasses.