[time-nuts] Lassen IQ receiver APRS beacon pictures

[Robert Darlington]

On Sun, Oct 12, 2008 at 9:23 PM, Mike Monett <XDE-L2G3 at myamail.com> wrote:

<snip?

   Too bad  about losing the 1PPS. Maybe you can persuade  them  to get
some spares.

I added the PPS line today.  2 wires off the connector (PPS and ground) to
an SMA connector and some epoxy.  It took 10 minutes but wish I thought to
add it from the get go.


>  >  Here's the board: http://www.losalamostech.com/gps_board.jpg
>
>  Thanks - that gives a good idea how bug the GPS is.
>
>  But don't the connectors add a lot of weight?
>

It adds some weight.  I have another board with a Lassen SQ receiver that
I'll probably end up using for the balloon project.


>
>  > Here's their gadget:
>
>  >http://www.lanl.gov/orgs/tt/pdf/techs/hands_off_gun.pdf
>
>  Shades of Star Trek. We can see it happening right before our eyes.


Yep, and it was designed nearly 5 years ago.  My company got the contract to
clone the device and design/build anything that was impossible to find.  Our
clones were sent out as demo units by the technology transfer group at the
lab.  I guess it's so companies can license the technology and develop
commercial units.  We made one clear, and one transparent blue.  The most
fun was polishing up the parts that were made on an ObJet printer.  They
were yellow but polished up clear.  Very cool.


>
>  >> 2. How  well  do  they work indoors? Do you  have  to  be  near a
>  >> window? What about inside a multi-story building?
>

I've been running it for the past 24 hours with the antenna in a west facing
window.  I'm picking up 4-5 satellites and only getting a 2D fix.  6 allows
for a 3D fix, but it might be more of a reception issue than number of
satellites.  I would think 4 would be sufficient.

<snip>


>  >> 4. Have  you ever taken the shield off and looked  inside?  If so,
>  >> can you  see the crystal oscillator and tell whether it is  a bare
>  >> crystal or a complete oscillator module?
>

http://www.nerdhouse.org/gallery/v/gps

<snip>

 I'm very  interested  in antennas these days. I  know  nothing about
>  them, but  find they can have a lot to do with time errors.  Such as
>  multipath from passing cars.
>
>  What happens if a pigeon sits on your GPS antenna?
>

I imagine that it would attenuate the signal sufficiently to lose a fix, but
the realtime clock would take over.  I setup the Lassen iQ to only give a
PPS signal when it has a satellite fix so in this case, it should stop.

  > The really  interesting thing is that the manual  states  that the
>  > PPS output  is  within  50ns of reality. I plan  on  using  one to
>  > discipline that  Efratom  rubidium  standard  I  was  asking about
>  > earlier in  the  week.  By  the  way,  the  connector  from Mouser
>  > electronics came in and I'm ready to go, just waiting on  the part
>  > to come in from China.
>
>  Me too! I just got news that my rubidium has already  shipped, which
>  is surprising. PayPal hasn't cleared yet, so I guess he has a lot of
>  confidence in the system.
>
>  Which Efratom do you have? And who are you dealing with in China?
>

It's an Efratom FRS-C purchased on eBay from "fluke.l",
http://myworld.ebay.com/fluke.l/   He's got nearly 500 positive feedback
ratings so I figure it's a good risk.   $100 delivered.

A couple weeks ago I picked up an HP time mark generator for calibrating my
oscilloscopes.  It has a 10MHz TCXO inside but can accept an external 10MHz
reference source.  Buttons on the front divide the signal, there's also an
amplifier built in, etc.  So before adjusting anything on the scope, but
after hooking it up and playing with it, I started wondering how accurate it
was -nevermind the parallax error associated with using my Mk. I eyeball
viewing the CRT!   Then I started thinking about calibrating it against
another time mark generator I had in the cabinet and wondered how I could
know if that was accurate.  Then I started thinking about how I could trust
any clocks and to what precision I need for my measurements.  I guess I
caught the bug late in the game but when I got it, it hit rather fast.
Today I was questioning why the PPS period of this GPS was reading
1.000002527 seconds (it was fluctuating by about 300ns too!) on an HP
frequency counter from the 80s.  Using the PPS signal to gate it I was
getting 10,000,0025 to 10,000,027 counts pretty consistantly using the
internal 10MHz source.  Sounds like it needs adjustment, but I don't dare
fiddle with it till I feel a lot more comfortable in this area.

The good news?   Both the time mark generator and the frequency counter will
accept a 10MHz source.   The Efratom unit spits out a 10MHz TTL signal that
should be just fine, especially after using a GPS PPS signal for feedback.
I have a lot to learn in this area but thankfully my business partner knows
a lot about this stuff.  He was one of the Navy/DoD electronics weenies that
serviced *the* clocks and was responsible for portable units in case ground
stations were destroyed.  This would allow for surface fleets to contact
subs, and military coordination in general in the case of nuclear war.  A
true keeper of the time during the late 80s early 90s.

-Bob