Newbie: GPS Time - Page 1
Forum section: electronics / general

author: Bicurico
author_rank: Super Contributor
timestamp: May 30, 2019, 08:01:00 pm
content: Hi,I purchased from AliExpress a USB GPS module. It cost 7.98 US$ with free shipping and supports GLOSNASS.First thing I had to do in my "VMA Simple Spectrum Analyser" software (https://vma-satellite.blogspot.com/2017/03/vma-simple-spectrum-analyser-download.html) was to add support for the $GNGGA command. The receiver is really neat and my goal was to use it, apart from my software, with Lady Heather for accurate time measurement.Of course Lady Heather does not recognize the COM11 port and I have not figured out how to change that. No sweat, I will try later and read the documentation.But what I noticed, and hence this post, is that the $GNRMC command returns a time (HHMMSS), which is around 4 seconds behind any online web based time server like:https://www.worldtimeserver.com/orhttps://time.is/pt/First I thought this is the lag from the server to my browser rendering the information, but then it occured to me that 4 seconds is far too long a lag and "time.is" even tells the user about the maximum lag expected (in my case +/-0.005 seconds).So the question is: who is wrong?1) The USB GPS receiver? Takes too long to process the data from GPS satellites? And the lag comes from the device itself?2) My browser (Chrome)?3) The internet time server?4)Thanks,Vitor

author: Bicurico
author_rank: Super Contributor
timestamp: May 30, 2019, 08:03:35 pm
content: COM11 is recognized by starting Lady Heather like this: heather /11So this question is solved.Regards,Vitor

author: Bicurico
author_rank: Super Contributor
timestamp: May 30, 2019, 08:04:43 pm
content: Now I see that the time shown by Lady Heather is on spot!I think Lady Heather does some compensation - I remember vaguely reading about it...But still I would like to know how that is done.Thanks,Vitor

author: dkozel
author_rank: Regular Contributor
timestamp: May 30, 2019, 08:10:55 pm
content: GPS time is missing some, but not all, leap seconds. So it's not UTC or TAI!http://leapsecond.com/java/gpsclock.htm

author: Bicurico
author_rank: Super Contributor
timestamp: May 30, 2019, 09:06:18 pm
content: I see! That makes sense!Thank you for the explanation.Now I found it on Lady Heather, too: 4th line in upper left corner - UTC ofs: 19Regards,Vitor

author: texaspyro
author_rank: Super Contributor
timestamp: May 31, 2019, 02:10:18 am
content: Quote from: dkozel on May 30, 2019, 08:10:55 pmThe GPS time scale has no leapseconds. GPS time started at midnight between 5 and 6 Jan 1980. The number of leapseconds since then is the "UTC offset". This is sent by the satellites, but some receivers don't output it. If it is not sent by the receiver Heather can get it from a user specified value or an educated guess.Some GPS receivers can be configured to send either GPS or UTC time (a few can send local time if you tell it your time zone). If a receiver only sends UTC time then Heather can convert it to GPS time via the UTC offset setting.Heather has commands for displaying either GPS or UTC based times. It also has commands for setting your time zone (for local time display) and daylight saving time corrections. Heather also has special time zone names for displaying time in various other formats like TAI and various astronomical time scales... including Mars time.