[time-nuts] u-blox NEO-M8T GPS initial tracking test

MLewis mlewis000 at rogers.com
Wed Feb 8 21:11:51 EST 2017

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 

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:

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 
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!

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.

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 


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