GPS vs GPS+Galileo - Page 1
Forum section: electronics / general

author: Electro Fan
author_rank: Super Contributor
timestamp: July 13, 2024, 01:45:34 am
content: I have experimented with GPSDOs for about 5 years. I have two BG7TBL units with 2018-06-05 date codes, both GPS only. I just received a similar model (with no date code) that supports GPS and Galileo. I'm curious to see if the double GNSS offers any advantages.I'm using one of the 2018 units as a Ref input to the counter and the new GPS+Galileo unit is providing the signal input on Channel 1 of the counter. Not sure how much aging or drift might have occurred with the 2018 unit or how accurate either is, but I'm hoping when the new unit settles in that they will display similar frequencies. I also just received a tinyPFA so hopefully after studying the manual and various videos I can better determine what is doing what. I'm sure I'll have some questions and would be happy to hear any advice.

author: mendip_discovery
author_rank: Super Contributor
timestamp: July 13, 2024, 01:19:39 pm
content: A few years ago I tried to get my head around the uncertainties. I think you might fall into the timing differences between the two different systems, though the average between them might be an improvement.

author: Electro Fan
author_rank: Super Contributor
timestamp: July 14, 2024, 12:18:00 am
content: Just getting started but in Lady Heather the only new infoI see so far is a column after PRN and before AZ that shows either G or S. It looks like 16 current satellites have a G and 1 has a S. Any chance G stands for Galileo and S stands for GPS?I take it back. With the GPS-only GPSDOs the G and S designations have been there all along. So, maybe I don't have Lady Heather properly set up to show both the GPS and and Galileo satellites?Looks like I can see both GPS and Galileo satellites in u-center. u-center seems to see1926 satellites,Lady Heather saw 16 (even though I tried changing /si= to both 17 and 18; maybe 18 is the max for si?)Lady Heather now shows 22-23 satellites in the PRN table (but not in the Satellite Positions view). So it's just a matter of how to determine which are GPS and which are Galileo.Also, fwiw, I happen to see a current HDOP of 0.75. I'm pretty sure that's lower by ~0.15 than any HDOP I've seen over the years with the GPS only GPSDOs.As I'm writing Lady Heather now shows a HDOP of 0.68; not sure if it's due to the extra satellites or maybe I just didn't notice it go that low previously.Net, net: after the edits above, mostly just curious to know if there is a way to see see/distinguish the Galileo satellites in Lady Heather?ThxEdit: for /uo= is 18 still the correct value?

author: Electro Fan
author_rank: Super Contributor
timestamp: July 14, 2024, 04:41:06 am
content: I found some of the answers in the Heather manual (duh).Using the SI command with a + enables two columns, so now plenty of real estate to display more satellites.However, even after reading the fine manual I can't figure out how to get SG command to do much of anything. It shows A= All, D = receiver default, G = GPS, S = SBAS, E = Galileo, etc but none of those values when entered seem to display any new/different info; so no good way found yet to see which satellites are GPS vs Galileo.Maybe in the column next to the PRN G means GPS and S means SBAS? In which case no Es are showing for Galileo? Just guessing...If anyone knows how to get a BG7TBL GPSDO that is configured for GPS & Galileo - to display the Galileo satellites in Lady Heather, please feel free to say. ThanksPS, I saw this but it might be just over the horizon for me....https://portal.u-blox.com/s/question/0D52p0000CXrykpCQB/enable-gpsgalileosbas-only

author: Electro Fan
author_rank: Super Contributor
timestamp: July 20, 2024, 10:41:06 pm
content: Quote from: mendip_discovery on July 13, 2024, 01:19:39 pmI'm still observing. This is with the Galileo + GPS GPSDO as the Ref in for the counter and the GPS-only as the input to Channel 1. I've reset and watched this several times. I get some variation that I mostly attribute to HVAC room temperature cycling, but it's been pretty consistent with these numbers. Along the line of your comment it seems that the average is doing well but it's hard to know the contribution of the two GPSDOs and the counter.Question: how much if at all does GPSDO signal strength impact the performance of the 10 MHz ref signal? Sometimes with tree obstruction the GPSDO signal strength indicators can fluctuate from 1 bar at the lowest to 3 bars at the most. If I had a perfect view of the sky would the GPSDO 10 MHz signal be any more accurate or stable, or it's either above the threshold or below the threshold?

author: Leo Bodnar
author_rank: Frequent Contributor
timestamp: August 01, 2024, 08:03:24 am
content: I have found that adding Galileo into the mix of GNSS adds phase "wobble" in 100-1000 sec interval. You would need to plot ADEV against a stable source like a good Rubidium to catch that.If your goal is to investigate GNSS performance then you probably want to reduce the time constant in your GPSDO control loop so that OCXO is not masking the ADEV region you are interested in.Leo

author: Melt-O-Tronic
author_rank: Frequent Contributor
timestamp: August 17, 2024, 04:42:07 pm
content: I've been trying to answer the same question. I can say anecdotally using my Racal 1992 as a crude TIC that including GLONASS and/or Beidou really degrades PPS jitter. Watching on an oscilloscope, Galileo appears to improve it, but my recollection is consistent with Leo Bodnar's observation.I just got a nanoVNA-H4 and turned it into a tinyPFA, so I'm experimenting with that. Here are some early findings (as a rank amateur) on this question. I need to redo my Galileo-only trace to the same time scale as the others, so take this with a small grain of salt. After retesting, I expect that line to follow the others as closely.For what it's worth, a word about my setup:GPS Antenna-- Taoglas A.41.A.301111 antenna mounted on the roof with a clear 360° LOS except for two metal vents several feet away. These have a small effect on the horizontal pattern, but I don't think it's significant for timing. This comes into my lab to a distribution amplifier consisting of a bias-T, Minicircuits LNA and Minicircuits 6-way splitter.GPS Receiver-- I have two LEA-M8T's, a NEO-6M and a Jupiter T connected to the distribution box with equal length RG316 jumpers. These tests were all done with LEA-M8T #1. I spent >2 years collecting RINEX and post-processing through NRCAN to determine my antenna's position and I keep the M8T in position lock (timing) mode.Reference-- I'm using one of my Racal 1992's (I call this one 1992A) which has the high stability OCXO option as my reference. I put that signal through a 10 dB attenuator and a 10 MHz low pass filter that has very little phase shift at 10 MHz. It is not disciplined to GPS, but has been recently adjusted to be pretty close. In the one case on the chart where I tested at 10.24 MHz (to get to a multiple of the GPS's internal clock), I used my Agilent E4433A as the reference. It's locked to the Racal 1992A.