GPSDO - Mystery of the extra satellite

[jpb]

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.

[testpoint1]

GPS splitter problem ? does it include amp ?

[jpb]

GPS splitter problem ? does it include amp ?

Thank 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.pdf
I'm wondering if it is a setting in Lady Heather (perhaps the number of displayed satellites is different - I'll investigate).

[jpb]

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!

[jpb]

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)

[texaspyro]

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.   

[testpoint1]

if you use active GPS antenna, you can add the 3.3V power within the splitter, this will increase the SNR.

[jpb]

if you use active GPS antenna, you can add the 3.3V power within the splitter, this will increase the SNR.

Thank 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?

[testpoint1]

if you use active GPS antenna, you can add the 3.3V power within the splitter, this will increase the SNR.

Thank 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?

is this a spiltter or combiner ?  if the GPS antenna port already have 3.3V, that is OK for active antenna.

[jpb]

It is one of these hp58516a s
http://www.to-way.com/tf/hp58516a.pdf
They 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.

[mfro]

... to hide the location (of my secret volcanic lair) so had to redact manually ...

If that's your goal, you'd better blacken a little more - with the remaining information, it's still entirely possible to locate you

[jpb]

... to hide the location (of my secret volcanic lair) so had to redact manually ...

If that's your goal, you'd better blacken a little more - with the remaining information, it's still entirely possible to locate you

I had wondered about that Given sun azimuth and so on - but it would be quite hard work.

[testpoint1]

so, your receiver must have 5V feed to the splitter, then the 5V will pass to active GPS antenna.

[texaspyro]

If that's your goal, you'd better blacken a little more - with the remaining information, it's still entirely possible to locate you

A 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.

[jpb]

so, your receiver must have 5V feed to the splitter, then the 5V will pass to active GPS antenna.

I'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.

[jpb]

If that's your goal, you'd better blacken a little more - with the remaining information, it's still entirely possible to locate you

A 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.

That is good, I was just about to relocate my volcano which would have been expensive! (Got to keep ahead of that Bond chappie.)

[testpoint1]

so, your receiver must have 5V feed to the splitter, then the 5V will pass to active GPS antenna.

I'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.

from the manual. at least 4.5V

[jpb]

so, your receiver must have 5V feed to the splitter, then the 5V will pass to active GPS antenna.

I'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.

from the manual. at least 4.5V

Thanks 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).