Re: [time-nuts] nuts about position (Michael Wouters)

[Jerome Blaha]

Real-time GPS <0.3m accuracy is quite possible with RTKLIB and < $35 of new 
equipment, some patience, a laptop/android phone, and a real-time RTCM3 
correction station stream (free) or with post-processing using ground stations 
and actual satellite path (Ephemeris).
 
Disclaimer: I am neither a surveyor nor claimed GPS expert, but I play one on 
TV with my day job, so consult a surveyor if you are having to survey in WGS84 
datum and convert to NAD or some other reference system, especially if testing 
against old surveyed reference points.  For instance here in the SF Bay Area, 
the land is moving North 28.31mm/yr and West 21.87mm/yr with respect to the 
WGS-84 (GPS Position) datum, which uses satellites orbits and the earth center 
as their reference.   If you were to survey your Telescope position in WGS-84 
in the Bay Area, wait 15 years and survey again, the telescope will appear to 
have moved Northwest by 53.7cm and will have moved down by 1.1cm (Assumptions 
taken from SLAC de-trended data here: 
https://www.unavco.org/data/gps-gnss/derived-products/position-timeseries/SLAC_timeseries_cleaned_detrended.png)

This is why most surveying is performed not in WGS-84 but in a datum that 
remains fixed to a tectonic plate as it moves, such as NAD83.  The 83 refers to 
the date or year (epoch) when the land position or survey marker was at that 
location, because even surveyed positions can move over time within that same 
tectonic reference frame such as along fault-lines, etc. 

Getting back, RTKLIB is a brilliant piece of open-source software to allow 
sub-meter (<5cm) phase-based positioning and my hat goes off to Tomoji Takasu 
from Japan who wrote this and keeps updating it.  Very few GPS receivers can 
measure phase shift, so this is where a specialized L1 + L2 survey receiver is 
needed or you find a cheap L1 GPS chip capable of raw phase and code, such as 
the u-blox M6T or M8T.


RECEIVER:
With regards to a GPS + Glonass raw receiver, the cheapest ublox M8T legitimate 
supplier seems to be over at csgshop.com in Northern Europe.  He has the real 
M8T GPS for $75 and I secretly believe he might also be one of the test 
engineers for u-blox. I say real, because there are knock-offs on ebay.

Why did I mention $35?  Well, you can find an M6T gps-only with raw ublox 
output on Amazon and elsewhere called LEA-6T that should get the job done if 
you add a metal ground plane. This little thing works surprisingly well when 
tested for repeatable RTK fixes within 5cm and you can even do a real-time RTK 
fixes with only an Android phone and the older RTK lib App on GooglePlay. (M8T 
not supported)  Don't feel you have to use a newer M8T with GPS + Glonass, as 
usually a GPS-only solution will be superior to a GPS + Gloanss, because 
Glonass seems to have a little more noise to it. (Discussion for another time) 


USGS SURVEY MARKERS
While we're at it, let's mention that there are survey points all over the 
United States where you can test the accuracy of your newly working RTK GPS 
system.  These are markers placed by the USGS and are typically reported in NAD 
or other American datum format.  These must be converted to WGS-84 datum for 
you to compare with GPS positions and there are tools out there if you do a 
Google search for converting datums to WGS-84, just pay attention to what 
year(epoch) you are converting to and it gets tricky to wrap your head around 
conversion between different Ellipsoids and Geoids (ground height).
https://www.ngs.noaa.gov/NGSDataExplorer/


RTKLIB:
RTKLIB has multiple tools to allow streaming, real-time, logging, 
post-processing, and graphing RTK GPS fixes.  Typically raw GPS (code & phase 
measurements) are converted to RINEX data format before post-processing.  RINEX 
is a standard and allows for raw GPS code and phase measurements between 
different GPS manufactures into one format for processing in third-party 
software.  

RTKLIB works in Windows and is a little tough to get started with; thankfully 
there is another very nice guy, Tim Everett, who has written many articles at 
rtkexplorer.com on accuracy of cheap RTK gps chips interfaced with RTKLIB as 
well as tutorials on getting started with pretty pictures.  As a side note, you 
may be tempted to try a newer ublox M8N; however many M8N chips are firmware 
disabled to not give raw phase anymore without firmware hacking.  (The M8T 
still works with RTKLIB 2.4.3)


SATELLITE PATH (EPHEMERIS)
When typical GPS receivers are running in real-time, they download an 
approximate ephemeris of where the satellite path flies within a meter or two. 
The final ephemeris is the best true path that the GPS satellite actually flew, 
down to 10cm, however these files are not available until more than a week 
after you collect your raw survey by going to a government website such as 
(https://www.ngs.noaa.gov/orbits/orbit_data.shtml)   These post-processed 
orbital paths are computed from hundred of surveyed ground 

Re: [time-nuts] Very Accurate Delta Time RF Pulse

[Jerome Blaha]

Thank you all who responded including Bob, Attila, Vlad, Brooke, and Chris for 
some great suggestions.

This is a fun side project of mine to passively detect RF emitters based upon 
strongest nearby signal using ToA pulses from cheap log power sensors or 
perhaps the Watson-Watt method.  The hope is to use it in a vehicle with 
sufficient antenna spacing and time pulse accuracy to create a neighborhood 
plot with strongest TX locations.  

Yes, there are major issues to be overcome.  The super wide band input has no 
tuner and will pickup massive noise from many near-field sources, such as 
wi-fi, Bluetooth, or phones, however some can be filtered as noise.  
Additionally, very few omni antennas cover such a large input range and I don't 
think CW signals will be detected properly, as they don't use a distinct 
rising-edge pulse.
 
I'm leaning toward what Bob suggested with a single shot Ghz counter possibly 
with some type of pulse start/stop timer or a double input A/D with GS/s 
buffers that can be stopped and momentarily read off whenever a new strong 
signal is detected or after a set time each second.  Vlad mentioned a phase 
comparator AD8302, which would also be interesting and allow for analog or 
possibly digital wideband multi-frequency comparison using phase.  The AD8302 
apparently comes with its own internal double log power RF input, which could 
save on purchasing additional power sensor ICs as well.

Best Regards,

-Jerome 
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[time-nuts] Very Accurate Delta Time RF Pulse Measurements

[Jerome Blaha]

Hi Guys,

This is a little outside of time-nuts scope, but not by much.  I'm interested 
in finding the time between two rising edges above a set threshold with 
preferably nS or high ps timing accuracy.  Can this be simply done with a few 
programmed Microchip PICs or with a good short term OCXO clock?  The issue I 
see is that a 10Mhz timing reference with 1 cycle difference in time yields 
100ns resolution, which is far too large, so maybe a PIC can solve this.

This weekend project would be a multi-element antenna array, each with a 
super-fast response log peak power detector fed into several PICs for time of 
arrival.  Whenever a nearby high energy RF pulse is detected, the time of 
arrival between two antenna elements and hence the direction toward the TX 
could be roughly computed.  Some typical log peak detectors have an 8ns input 
pulse response time, so I'm hoping that rise times are similar between multiple 
detectors, negating the delayed response.

There are time of arrival/AoA systems out there with synthetic doppler, phased 
arrays, correlative interferometers, and phase comparators, but it would be 
interesting to accomplish super wideband AoA timing on two rising pulses with 
relatively cheap parts. 

Thanks,

-Jerome
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[time-nuts] FTS 4050 Cesium Time Reference Manual

[Jerome Blaha]

Does anyone by chance have a copy of the FTS 4050 Cesium Time Reference 
Operation or Service manual in PDF?  Or is an equivalent Cesium unit out there 
who's manual can be used as a surrogate for setup and operation?

I have a request in with support from Symmetricom (Microsemi), however there is 
no joy on that route.  I'm also open to scanning someone's hard copy manual 
into PDF and returning promptly with beer money, if that helps.

Thanks,

-Jerome

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Today's Topics:

   1. A Symmtricom GPSDO Board (Arthur Dent)


--

Message: 1
Date: Sun, 5 Jun 2016 10:46:38 -0400
From: Arthur Dent 
To: time-nuts@febo.com
Subject: [time-nuts] A Symmtricom GPSDO Board
Message-ID:

Content-Type: text/plain; charset=UTF-8

I have two similar Symmetricom GPSDO boards but with a slightly different parts 
layout that were manufactured in about 2012. The OCXO in the first one is 
4.1950 Mhz and there is a 10 Mhz VCO next to it.

The second one has a Symmetricom SA22-c
Rb oscillator. The large blank spot in question is for the version of these 
that would have the Rubidium oscillator and the copper foil is part of the heat 
sink for that version.

I haven’t checked the output frequency of the SA22-c but it is probably the 
same as the OCXO version.

-Arthur


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[time-nuts] Generating a solid PPS from 10Mhz source

[Jerome Blaha]

Hey Guys,

Is there an easy circuit to build that can consistently deliver a 1 PPS from a 
10MHz source with excellent resolution and repeatability?  My first application 
is to test different 10MHz oscillators without a TIC always attached and then 
compare the PPS output change over time against a master GPSDO PPS with an 
HP53132A.

The circuit used for PPS generation would have to deliver consistent PPS output 
with preferably not more than 100ps noise or jitter, assuming a perfect source. 
 I'm totally guessing that for this resolution, the PPS would have to be 
generated and accurate to within 0.001Hz every second.  If this is too 
difficult, maybe the integration time can be increased to generate one pulse 
every 10second or every 100,000,000.00 cycles?

Finally, is a square 10Mhz reference any better in this case than a sinusoidal 
input for generating the PPS?

Thanks,
Jerome

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