[ntp:questions] Roof antenna, which one, would you bother?

Brian Inglis Brian.Inglis at SystematicSw.ab.ca
Fri Jan 3 19:58:04 UTC 2014


On 2014-01-03 03:54, Ralph Aichinger wrote:
>
> I am currently in the process of remodeling my house
> and a dedicated outdoor/roof mounted GPS antenna
> would be possible to mount without excessive cost.
>
> I probably would not see a huge difference for
> timing purposes, but what would your choice of
> an outdoor GPS antenna/receiver be? I am planning
> to put the NTP server (small Raspberry Pi, nothing
> fancy) into a rack maybe 10 metres of cable length away
> from the roof. Or would you move the NTP host itself
> to the attic and run Ethernet up?
>
> I suppose something like the Garmin GPS 17x (mast mount
> GPS with serial output and PPS, intended for the marine
> market) would work fine, also be robust enough and
> cheap on the used market.
>
> Would mounting just an antenna (active or passive) also
> make sense, or would cable loss be too high?
>
> What would *your* roof top setup be? Cost is of course
> an important consideration, on the other hand, 100
> or 200 Euros for the GPS, antenna and mounting hardware
> is probably OK. I would prefer to do cabling jobs now,
> while I am in the process of doing a lot of "dirty" things.
>
> Or would you not bother at all, and just put some puck into
> the window (which probably works too)?

You may not be safe running your Pi in an uninsulated attic,
as most chips are rated for commercial operation from 0-70C,
which could be exceeded in cold winters or hot summers, due
to heat buildup in the attic plus device self-heating.

All Garmin GPS models are rated for -30C-80C so should be okay
outside.

Most 17x models, other than 17x NMEA 2000, are not high sensitivity
so require clear sky all around, as would be expected in marine use,
and those other models appear to be discontinued and replaced by
high sensitivity 19x. Marine model power varies by model - LVS takes
up to 6V and HVS 9-16V (battery).

You could get a new 18x LVC for US$60 + shipping + customs from
gpscity.ca (cheapest I found) and crimp or solder the GPS leads
and USB (or other 5V) power connector leads into a DE9F.

Instructions are on David Taylor's site at
http://www.satsignal.eu/ntp/FreeBSD-GPS-PPS.htm#usb
I simplified the wiring shown in the above link by connecting
all +5V power leads to pin 6 DSR.

The page above also has a link to an alternate source of the device
already wired to a DE9, with an extra cost USB power adapter, total
cost US$100 + shipping + customs. The page notes that Dave Hart
bought and used these devices.

-- 
Take care. Thanks, Brian Inglis


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