Adventures in Precision

Adventures with OpenTTP

Getting started with OpenTTP

I am just starting to get going with OpenTTP. This is a set of software (and in theory hardware) to perform time transfer monitoring. This lets you become you very own “time lab” and use the same techniques such as GPS Common View that the pros use (or did before they moved onto better and more precise things…)

Basic preparation

These are the basic steps to getting OpenTTP installed on Debian-like system.

sudo apt-get install git
sudo apt-get install perl libboost-dev libboost-regex-dev libusb-1.0-0-dev libgsl-dev
git clone https://github.com/openttp/openttp.git
cd openttp
git checkout develop

I am doing this on a Raspberry Pi (I am using a Pi 3B+) and things are a little different as OpenTTP doesn’t support the Pi yet. So I have a fork of the repository with support for the Pi (which hopefully will get accepted back into the main project)

sudo apt-get install git
sudo apt-get install perl libboost-dev libboost-regex-dev libusb-1.0-0-dev libgsl-dev
git clone https://github.com/talister/openttp.git
cd openttp
git checkout rpi-support
sudo mkdir -p /mnt/data/pi
sudo chown pi:pi /mnt/data/pi

Build and Installation of OpenTTP

Once the code is checked out, we can go ahead and build and install it (this follows the instructions in the manual but this is only distributed as the LaTeX source):

cd software/system
sudo perl installsys.pl
cd ../gpscv
perl install.pl

That’s the basics of getting the software downloaded and installed; configuration will be the subject of the next article.

Mirror Reflectivity Data

As part of the day job, I needed information on how reflective a freshly coated aluminium surface (a telescope mirror) was. I had no luck finding a well sampled source of data that covered the wavelength range I was interested in (UV/Visible and into a little of the Near IR). I asked a question on Astronomy StackExchange but didn’t really get the answer want I wanted. (This is not a negative on Astronomy StackExchange, which I think is a great site and I am trying to contribute more to).

While reading the NOAO 2018 October Newsletter over lunch (sad, I know), I came across an article that covered the recoating of the 4-meter Blanco telescope (http://www.ctio.noao.edu/noao/content/Victor-Blanco-4-m-Telescope) mirror . This included a theoretical curve for bare Aluminium. So I contacted one of the authors of the article, who sent me a scan of the table from “Reflecting Telescope Optics II” by R. N. Wilson (Amazon link: https://www.amazon.com/Reflecting-Telescope-Optics-Manufacture-Astrophysics/dp/3540603565/)

These data covered 220nm to 30,000nm (0.22 to 30 microns) which was more than I needed. They also had good sampling in the visible range that I was interested. As a bonus it also included Silver and other metals in case I need to model Gemini’s mirrors or some crazy IR spacecraft… So I typed the data for Aluminum and Silver into 2 files and made some quick plots. I am also making the data files available in case they are usable by other people.

Plots

Reflectivity of Aluminium and Silver coatings over the full range of data (220nm to 30000nm; 0.22 to 30 microns)

Zoom in on the reflectivity data in the UV/Visible/NIR region (200-1200nm)

Further zoom in of the optical region (350-1200nm), eliminating the major dip in the silver reflectivity at 315nm

Datafiles

The files of the reflectivity data are available here:

They cover the full range of the original table data from 0.2 to 30 microns.

New Howto for precise time on Windows

New Howto on setting up precise time on Windows

I have created a new Howto on getting precise time (can be as good as microseconds) on Windows. It shows how to configure Network Time Protocol (NTP) and Pulse Per Second (PPS) inputs. Check it out at: https://adventuresinprecision.space/howtos/configuring-ntp-and-pps-on-windows/

Precise GPS positions Howto added

New Howto

I have finally completed work on the Precise GPS Positions Howto which gives details on how to use low cost consumer GPS receivers to give precise positions. I think it also gives some useful background information on how the various GPS signals work. Please check it out and hopefully it will prove useful. Please let me know via a comment !

Recheck of a DMMCheck+

Several years ago I was given a DMMCheck+ as a birthday present. They are very handy little devices which provide AC and DC voltage and current, four resistance values. These are all at about 0.1% precision and frequency and duty cycle are also provided. All of this fits within a battery powered compact 3 inch cube. Unfortunately VoltageStandard.com discontinued the DMMCheck+, but checking the site recently shows that it may be back in production (but currently sold out). I use it to periodically check my HP3478A at home but as part of the check on the precision resistors, I decided to remeasure it with work’s Agilent 34410A to see if it had changed significantly.

DMMCheck+ being tested on the bench for resistance values

Procedure and Measurements

The measurements used a Agilent HP34410A that was available at work. According to the unit, it was last calibrated in January 2007. The number of power line rejection cycles (NPLC) was set to 100. For the AC measurements, the Slow 3 Hz mode was used. 4 wire Kelvin clips were used for the resistance and voltage measurements and Pomona DMM leads were used for the current.

Measuring the 10 kOhm 0.1% precision resistor on the DMMCheck+. Value is 10.00091 kOhm — Measuring the 10 kOhm 0.1% precision resistor on the DMMCheck+

Here are the measured values and the comparison to the supplied spec. sheet. This was measured by VoltageStandard.com with an 8.5 digit 3458A DMM which had been recently calibrated.

DMMCheck+ Measured Values

Measurement	Measured Value (on 2018-04-25)	Measured Value (on 2016-05-01 when supplied)	Difference (ppm)	Notes
DC Voltage	4.99996 V	5.0000 V	8.0
DC Current	0.999936 mA	0.9999 mA	-36.0
AC Voltage	4.99886 V	4.999 V	28.0
AC Current	0.998765 mA	1.0000 mA	1236.5	Incorrect recording?
100 Ω Resistor	100.0838 Ω	100.079 Ω	-48.0	On 100Ω range
	0.100085 kΩ	0.100079 kΩ	-59.9	On 1kΩ range
1 kΩ Resistor	0.999970 kΩ	999.94 Ω	-30.0	On 1kΩ range
	1.00002 kΩ	999.94 Ω	-80.0	On 10kΩ range
10 kΩ Resistor	10.0010 kΩ	9.9996 kΩ	-140.0	On 100kΩ range
	10.00086 kΩ	9.9996 kΩ	-126.0	On 10kΩ range
100 kΩ Resistor	0.100093 MΩ	100.079 kΩ	-139.9	On 1MΩ range
	100.0949 kΩ	100.079 kΩ	-158.8	On 100kΩ range

Summary

With the exception of the AC current, which may have been incorrectly written down, everything shows small changes in two years. The original measurements with the 3458A will have measured the DC voltage to 8 ppm, and resistance to ~10 ppm. However the spec on the 34410A is not as good (about 30ppm on DC volts). It also hasn’t been calibrated in 10 years. Also bear in mind that the 0.1 % specs for the DMMCheck+ corresponds to 1000 ppm (parts per million) for the initial accuracy.

Initial tests with precision resistors

As part of a longer term project to build a (hopefully) stable voltage reference from a LTZ1000 heated buried zener, I ordered some precision Vishay resistors. This post details some fairly basic measurements I made with my home multimeter and a better quality one at work.

Continue reading “Initial tests with precision resistors”

Hello world!

Welcome to Adventures in Precision, a blog to try and document my experiments with getting precision time (making me a time-nut; see mailing list info), electronics (mostly voltage references etc, making me a volt-nut; see volt-nuts mailing list and EEVBlog’s metrology forum) and precise positioning through GPS etc (haven’t found out where the position-nerds hang out yet, but https://rtklibexplorer.wordpress.com/ is a good place).

I hope to make useful references and pages for anyone else who might be interested in these sort of topics.