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From: Tim Watts <tw_usenet@dionic.net>
Newsgroups: comp.os.linux.misc,comp.sys.raspberry-pi
Subject: Re: Keeping 'order' without RTC.
Date: Wed, 04 Mar 2015 07:31:59 +0000
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Xref: csiph.com comp.os.linux.misc:13858

On 04/03/15 00:50, Martin Gregorie wrote:
> On Tue, 03 Mar 2015 22:27:23 +0000, J G Miller wrote:
>
>> So alternative sources of time could be DVB-t (using a DVB-t card in a
>> PCI, grab the time from that and use it to seed the LAN ntpd) or in a
>> similar manner from DVB-s if a satellite dish is available, or from a
>> GPS handheld receiver.
>>
> Yes, down in ZA I'd probably go for a GPS receiver, but one of the
> 'blind' receivers, e.g a Garmin GPS35 or GPS18, rather than a handheld.
> These output an NMEA 183 sentence every second, typically at 4800 baud by
> default but are configurable to higher speeds if required. The GPS35-HV
> model is set up to communicate over a single D-9 connector and wants a
> 12v power supply. From memory the other models use TTL levels.
>
> You can find blind GPS receivers on eBay and can probably find used ones
> in ZA - ships chandlers or yacht clubs would be good places to start
> asking around.
>
> But you're right: Mr Glurr may not agree about the benefits of GPS as a
> time source.
>
>

For a Raspberry a USB GPS would probably be cheaper and simpler - 
bearing in mind the GPS will probably have to be mounted in the roof 
(depending on whether the roof is insulated in foil faced insulation) or 
near a window on the end of a long cable (USB lengths permitting).