GPSDO - Mystery of the extra satellite - Page 1
Forum section: electronics / general

author: jpb
author_rank: Super Contributor
timestamp: May 25, 2019, 12:50:40 pm
content: I've just bought a second Star 4+ GPSDO (I got it relatively cheap on an auction) so I can do comparative measurements.I've set it going doing its self survey having set tau to 5000 seconds as on my existing one.It is connected to the same GPS antenna via an HP 4 way splitter so I'd expect it to report exactly the same satellites as the original one.It does do so, there is a slight difference in reported SNR but nothing unexpected.But what is weird is it is able to see an extra satellite, number 21, which the original one is not reporting! See attached screen shot.Given that the GPSDOs are the same model it is a little strange unless one of them has got some parameter set differently - I've not done so deliberately. The satellite has a high SNR so it is not a borderline case.The one exception is the antenna delay - I got around to setting this for 16m of cable on both but for the one in hold mode it ignored it so the new one has -80nsecs and the original is still on 0nsecs but I thought this only affected time alignment not the disciplining input.Also, I couldn't find the command in Lady Heather to hide the location (of my secret volcanic lair) so had to redact manually - I know there is such a command but I couldn't locate it.

author: testpoint1
author_rank: Frequent Contributor
timestamp: May 25, 2019, 01:44:36 pm
content: GPS splitter problem ? does it include amp ?

author: jpb
author_rank: Super Contributor
timestamp: May 25, 2019, 02:56:01 pm
content: Quote from: testpoint1 on May 25, 2019, 01:44:36 pmThank you for responding.Yes, it does have an amplifier and matching to simulate antennas for the ones that don't supply the power to the antenna (that is one difference the original one is supplying the power to the active antenna).It is one of these:http://www.to-way.com/tf/hp58516a.pdfI'm wondering if it is a setting in Lady Heather (perhaps the number of displayed satellites is different - I'll investigate).

author: jpb
author_rank: Super Contributor
timestamp: May 25, 2019, 03:06:57 pm
content: I've just found the command for private location - it is GLP. Then GLM (altitude in m) turns the location back on again.Lady Heather is a great program but it has so many features that it is sometimes difficult to locate the one you want!

author: jpb
author_rank: Super Contributor
timestamp: May 25, 2019, 03:24:17 pm
content: Mystery solved.Both were set to display up to 14 satellites but the new one seemed to display more.I changed both to display only GPS satellites and set the number of satellites as up to 20.They now both display the same set of satellites (though they seem to still display some S ones and not all G ones)

author: texaspyro
author_rank: Super Contributor
timestamp: May 26, 2019, 02:10:25 am
content: The receivers chose the "best" satellites from what is in view to use for their calculations. The decision of what sats to use can be dependent upon when they started up, the phase of the moon, and the price of unicorn horn in China, etc.

author: testpoint1
author_rank: Frequent Contributor
timestamp: May 26, 2019, 02:21:56 am
content: if you use active GPS antenna, you can add the 3.3V power within the splitter, this will increase the SNR.

author: jpb
author_rank: Super Contributor
timestamp: May 26, 2019, 08:46:56 am
content: Quote from: testpoint1 on May 26, 2019, 02:21:56 amThank you for the suggestion but I not quite sure what you mean.The splitter is designed to take the dc for the antenna at port 1 so the GPSDO is supplying the power there. The other ports have the dc blocked but are designed to take current so that the attached GPSDOs think they are supplying an antenna.Are you suggesting that I use a separate power supply and a dc block?

author: testpoint1
author_rank: Frequent Contributor
timestamp: May 26, 2019, 11:02:16 am
content: Quote from: jpb on May 26, 2019, 08:46:56 amis this a spiltter or combiner ? if the GPS antenna port already have 3.3V, that is OK for active antenna.

author: jpb
author_rank: Super Contributor
timestamp: May 26, 2019, 11:49:21 am
content: It is one of these hp58516a shttp://www.to-way.com/tf/hp58516a.pdfThey are designed for active antennas - it is not just a simple splitter. It generally works well, the only drawback is it is designed for GPS L1 and is not quite broadband enough for more general GNSS receivers. At the moment I'm only using GPS anyway so it doesn't matter.

author: mfro
author_rank: Regular Contributor
timestamp: May 26, 2019, 11:51:42 am
content: Quote from: jpb on May 25, 2019, 12:50:40 pmIf that's your goal, you'd better blacken a little more - with the remaining information, it's still entirely possible to locate you

author: jpb
author_rank: Super Contributor
timestamp: May 26, 2019, 11:55:22 am
content: Quote from: mfro on May 26, 2019, 11:51:42 amI had wondered about thatGiven sun azimuth and so on - but it would be quite hard work.

author: testpoint1
author_rank: Frequent Contributor
timestamp: May 27, 2019, 02:56:24 am
content: so, your receiver must have 5V feed to the splitter, then the 5V will pass to active GPS antenna.

author: texaspyro
author_rank: Super Contributor
timestamp: May 27, 2019, 03:50:40 am
content: Quote from: mfro on May 26, 2019, 11:51:42 amA guy that supposedly knows a lot about solar based navigation said that with enough work, one could indeed get an approximation of the location from that data, but it would not be accurate enough to be useful... particulaly with just a single reading. If you are paranoid enough, use the SI command to select a two column sat info table, the sun/moon info does not get drawn.

author: jpb
author_rank: Super Contributor
timestamp: May 27, 2019, 11:41:26 am
content: Quote from: testpoint1 on May 27, 2019, 02:56:24 amI'm not sure what the feed level is from my GPSDO (it is either 5V or 3.3V), but my antenna is very flexible and can accept between 3V and 12V I think.

author: jpb
author_rank: Super Contributor
timestamp: May 27, 2019, 11:42:55 am
content: Quote from: texaspyro on May 27, 2019, 03:50:40 amThat is good, I was just about to relocate my volcano which would have been expensive! (Got to keep ahead of that Bond chappie.)

author: testpoint1
author_rank: Frequent Contributor
timestamp: May 27, 2019, 02:27:42 pm
content: Quote from: jpb on May 27, 2019, 11:41:26 amfrom the manual. at least 4.5V

author: jpb
author_rank: Super Contributor
timestamp: May 27, 2019, 03:37:03 pm
content: Quote from: testpoint1 on May 27, 2019, 02:27:42 pmThanks for pointing this out, I'd better check. Though I've been getting the same sort of SNR with it as I got with a direct connection so if it is a bit low perhaps it is more forgiving than the spec sheet or perhaps the GPSDO is providing 5V (a lot of them do even when the internal logic is 3.3V).