From: Starlink
[mailto:starlink-bounces@lists.bufferbloat.net] On Behalf Of dan via Starlink
Sent: Monday, November 13, 2023
3:50 PM
To: Dave Taht
Cc: Dave Taht via Starlink;
Network Neutrality is back! Let´s make the technical aspects heard this time!
Subject: Re: [Starlink] bluetooth
occupancy sensing
I also have bluetooth occupancy sensors lol. We have a lab test
going of triangulated bluetooth tag locating. So you put a BLE tag on a
device and *3* or more receivers in a space at different locations and heights
which are documented and then BLE tags are triangulated based on RSSI.
[RR] This only works if ALL the sensors are synchronized. Adding
more sensors does not help either, because for every sensor you add that’s
not synchronized, you need to estimate a time bias/offset. And if the oscillators
are not synchronized (in frequency), then you have another parameter to
estimate … the frequency offset. It can get out of hand really fast! The
way this is “normally” done is to “calibrate” the area
around the sensors. What that really means is that “array manifold
vectors” are collected during a calibration phase then used during the
measurement phase to “locate the transmitters” and even that is a “tricky
problem”. Check out MUSIC!!! It’s a half-century old!
Having sensors at various heights allows for tracking even the 'z'
axis. These tags are very cheap, you can buy complete tags for a couple
of bucks, don't even have to build your own, and you can get them built into
cutable (or non-cuttable) wristbands. You can also do short-term
tracking of cell phone beacons, though privacy mode means that you only get a
short 'session' with a phone (because of privacy mode on newer phones) that
isn't paired with something but if you have a phone with a bluetooth headset,
the 'locks' the bluetooth mac address and now you can track the phone anywhere
that the bluetooth headsets follow. You can also track cars which don't
scramble the mac, but you get cars with wifi mac as well.
We can get bluetooth to within inches accurate when it's line of site.
[RR] This requires sub-nanosecond synchronization …
remember it’s a nanosecond/foot (the inverse of the speed of light that
is!) JJ
In a pocket or something it's about a meter because bodies/clothes etc
reduce RSSI unevenly. The purpose of this is a couple of things, 'patient
tracking' in any sort of a facility like nursing home or hospital, and device
tracking, again in a facility with shared hardware like portable EKGs and
handheld XRays etc that get 'misplaced' and staff has to go on a hunt
for. It's also much cheaper than lorawan as BLE transmits many times a second
and runs for years while lora is built for more range and only transmits
intermittently, usually 10-60 minutes to preserve battery.
We're testing mainly on dragino and milesight devices. I'm also having
decent enough luck with mikrotik's knot which can track BLE beacons with high
enough precision. Mikrotik has their own somewhat expensive BLE beacons
also but these are basically universal.
[RR] If you are getting inch accuracy without addressing the
synchronization problem, I’d love to hear how, especially when there are
let’s say 10 BT transmitters to “locate simultaneously”. JJJ