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 <mlewis...@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: <589bcfe7.4010...@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 _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.