I am doing similar tests, seeing similar results. I have 5 x NEO-M8T's, 3 M6T', 4 Skytrack Venus 838Lp-T's. Mostly have Toaglas triple band antennas on PCB groundplanes with LNA-SAW-LNA conditioning in line. I have built a three arm test rig (arm < =1.5 M) for testing RTK measurments, etc. Hope to deploy it at local USGS office RTK calibration/test range. So far the performance of the M8T's seems impressive. Usually 29 sats in view from basement windowsill (inside) with woods 15 M away. Facing south. Dave, KD0EAG Message: 5 > Date: Wed, 8 Feb 2017 21:11:51 -0500 > From: MLewis <mlewis000@rogers.com> > To: Discussion of precise time and frequency measurement > <time-nuts@febo.com> > Subject: [time-nuts] u-blox NEO-M8T GPS initial tracking test > Message-ID: <589BCFE7.4010704@rogers.com> > Content-Type: text/plain; charset="utf-8"; Format="flowed" > 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

wow Five M8Ts. And here I was feeling extravagant having splurged for one... I'm only seeing between 20 and 25 signals, but there's a bank of concrete&steel brick-clad buildings opposite. And typically with the majority of signals coming from north of my building, they're coming in off those buildings. Given the building orientation, the reported location during survey is interesting (attached). With freezing rain yesterday, then a drop in temperature, one, the other, or both, resulted in last night's sats at 20 degrees and 35 degrees on heading 320 showing a strong signal, likely off the freezing-rain coated office tower. I may throw a piece of RF foam in LOS from that building to the antenna to see if that can sufficiently block that one source. Isn't multipath fun. I believe the M8T is supposed to be able to directly replace a M7T or M6T. By chance did you drop the M8T onto a M6T or M7T board? The only thing I haven't been able to do is the GNSS configure and enable of GAL. Even with the assigned channels well under the 32 max, it won't accept GAL configurations. Seeing there was no GAL sequence in the text GNSS config file (others show their id an a sequence of zeros), I even configured Beidou as GAL, generated the GNSS config file and edited it to change the Beidou id from 3 to 2 for GAL. Loading the file populates with the text config file settings, except that it treats the 2/GAL labelled sequence as its original Beidou as if the id is still 3. Any tips on configuring/enabling for GAL? Michael On 09/02/2017 6:12 AM, David Witten wrote: > I am doing similar tests, seeing similar results. I have 5 x > NEO-M8T's, 3 M6T', 4 Skytrack Venus 838Lp-T's. Mostly have Toaglas > triple band antennas on PCB groundplanes with LNA-SAW-LNA conditioning > in line. I have built a three arm test rig (arm < =1.5 M) for testing > RTK measurments, etc. Hope to deploy it at local USGS office RTK > calibration/test range. > > So far the performance of the M8T's seems impressive. Usually 29 sats > in view from basement windowsill (inside) with woods 15 M away. > Facing south.