Anticipating issues with multipath, signal levels and a limited skyview,
along with discovering that the tallest building in my skyview with a
military computer centre has various stealth antennas around its roof, I
went with:
- u-blox NEO-M8T, mainly for its sensitivity, multi-GNSS (GPS, GLO & GAL
(and Beidou)), and multipath handling, and
- Tallysman TW4722 "Accutenna" wideband (GPS/GLO/Bei/GAL) active antenna
with LNA-SAW-LNA filter, using a dual-feed patch element, for 'enhanced
multipath rejection' & 'excellent out of band signal rejection'.
Powered up for the first time, my u-blox NEO-M8T connected through USB
to the u-blox u-center V8.24. It reported two satellites, with the
antenna sitting inside a concrete/metal/brick building, behind two panes
of glass, closed metal blinds, two feed below grade. So before
installing it outside, I figured lets see what it does on the window
sill behind glass.
Setup: facing the SE, only ~45% of a full skyview with a bank of
buildings opposite and parked cars between, antenna on 2" of wood with
the 100 mm circular stainless ground plane almost touching the inside
pane of glass, approximately 18" above grade, backed by the closed metal
blinds.
With GPS, GLO, SBAS & WAAS enabled, it picked up twenty satellites, most
with a very usable signal. But around 2/3 were north of my building, not
line-of-sight (LOS) in my skyview. So as expected, there's a lot of
multipath (MP) going on. Reported location was initially within an 2 m x
8 m oval, but as the various satellites travelled and their multipath
changed, the footprint expanded to 20 m x 120 m, then a 200 m trip to
the south and back, before roaming a 4 m by 20 m oval. Altitude started
+/- 5 m, but then ranged over 80 m. Apparently doable for upper micro
second accuracy, but not for nano second accuracy.
With the M8T set to test for the antenna connection of open circuit,
this removed power from the active antenna. Of course all of the signal
levels dropped dramatically. To my surprise, I observed that the signal
levels from both the LOS satellites and non-LOS satellites were not only
low, but very similar in amplitude, with only a few even lower. With the
amp back on, I observed that the LOS satellite signals and the MP
satellite signals were distinctly separate in amplitude. Now I
understood what a multipath-filtering active antenna would do for me! An
arbitrary 30db signal level threshold set in the M8T promptly had it
ignoring most of the satellites not LOS in my skyview.
The u-center SkyView showed those remaining were some satellites at the
horizon to my NW, with u-center WorldView placing them over the Bearing
Strait down through the Aleutian chain. Their signals had to be coming
in over the building and reflecting back at the window, most likely
straight back off of the tall building to the SE. I set the M8T's
elevation exclusion at 15 degrees and that filtered those sources out.
Fifteen degrees also excludes LOS satellites while at a low elevation,
thereby ignoring signals prone to low-angle multipath off the buildings,
parked cars & ground. As seen on the u-center Skyview: closer/higher
satellites on that NW heading didn't get that same reflection; signals
on the equivalent heading on the other side of north couldn't reflect
off the park to the SW. The elevation exclusion may also be of benefit
as it excludes those satellites low to the horizon, hence with the
longest signal paths through the atmosphere?
The above settings combined with the TW4722 antenna leave the M8T with
only LOS signals from within the actual skyview to seek its timing
solutions from.
I did a quick test with the glass removed. LOS satellites gained 3 to 8
db. MP satellites dropped 5 to 8 db. In any event, it wasn't enough for
a meaningfully change. It just meant a satellite coming into range did
so a few seconds sooner, or dropped out a few seconds sooner, with no
material change in coverage. And not relevant, as I've got a fixed
survey and a minimum of three satellites for timing solutions.
I was going to be doing a stealth antenna install outside, but given how
well it's done, I leaving it inside behind glass:
Good:
I've got a survey dialed in within 1/3 metre and fixed.
Between GPS & GLO, I get between three to eight LOS satellites providing
their timing solutions to the M8T. And that is before adding in GAL
coverage.
This setup with the M8T & TW4722 also receives a SBAS sat and/or a WAAS
sat, usually both, along with an EGNOS available.
The u-center tools are largely intuitive, the visuals meaningful & useful.
The antenna is secure indoors, and I won't have to brush the snow off
the antenna!
Bad:
I don't get to play with RF absorbing foam to custom shield the low- and
high-angle multipath off of the surrounding buildings, parked cars & ground.
Next:
A com cable to the interface on the NEO-M8T breakout board.
A dedicated NTP server (well, a PC).
Learning a setup for Lady Heather, using the USB to COM driver, just
because.
Michael
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