[time-nuts] Oooold GPS receiver discussions Austron 2201 as an xample

paul swed paulswedb at gmail.com
Wed Nov 11 19:31:54 EST 2015


Magnus
Indeed the almanac seems to be the problem. I can see what it thinks should
be in view. Its not been easy to backout if the satelliets are behind or in
the future. My time mis-alignment even though the closks correct within 1
second and the same for my location accuracy. I can manually tell it to
track a satellite and it does. Though even at that it seems to have issues
I speculate as poor signal to noise.
So may have to do some digging.
Thanks
Paul.

On Tue, Nov 10, 2015 at 9:06 PM, Magnus Danielson <
magnus at rubidium.dyndns.org> wrote:

> Paul,
>
> On 11/10/2015 08:58 PM, paul swed wrote:
>
>> There is a good discussion about old GPS receivers that have been running.
>> Its the NAVSTAR proteus thread.
>> Very good details in that thread about some issues. Such as the 1024 week
>> rollover and that the receiver should still keep working. Though the date
>> and time would be wrong.
>>
>
> I was surprised that it resurfaced, as we have been beating that issue to
> quite some depth multiple times here.
>
> Given that theory I fired up the 2201. This is the unit that had no down
>> converter and I home brewed one using another GPS receiver. Put simply it
>> all worked.
>>
>> But its always been a bit magical. Wait for a full moon, jump 3 times
>> counter clockwise...
>> Never made sense as to why it worked or really controlling it
>> consistently.
>> (Why even care for this pain in the ....? Because it has a really good
>> frequency offset measuring system.)
>> Over time its become more difficult to get it to work even though the down
>> converter is having no problem acquiring and locking to satellites.
>>
>> I think and its a theory its because its looking for SVNs but with the
>> wrong PRN. (these would be retired SVN/PRN sequences)The trigger was
>> looking at the satellite it wanted and realizing the PRN was not accurate
>> and the satellite wasn't even up.
>> I need to do some more digging but perhaps over time there are fewer and
>> fewer correct PRNs that align to the SVN.
>> Does this make any sense at all?
>>
>> There seems to be no way to tell the system that SVN X = PRN Y
>>
>
> Why should there be? In the GPS signal structure, the SVN number has _no_
> bearing on achieving lock. It's all PRN codes, and as SVN numbers increase
> with launches, the PRN codes is reassigned.
>
> What is relevant is the almanac indicating the rough orbit of all 32 PRNs.
> As you wake up from a long and deep slumber, any remains of almanac will be
> mostly outdated. Once you have locked onto a single bird, it takes you 12.5
> min to recover the full almanac. If you have or don't have a hint of where
> you are can lead you off track for some time if you have old data.
> Depending on how good the recovery algorithms are, the time to lock can
> vary a lot.
>
> It might be that it just needs to sit there until it locks a bird, and
> then acquire channel after channel.
>
> Some receivers needs a proper reset to get going with good speed.
>
> The only other fake out approach as a test will be to find a PRN thats up
>> and see if that aligns to a SVN.
>> Then manually enter that SVN as a search.
>>
>
> No need.
>
> 73 de Magnus SA0MAD
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