May be Rubidium Atomic Frequency Standard will help, not so expensive.
Cheers

Rodolfo

-----Mensaje original-----
De: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu]En nombre de
Bob Axtell
Enviado el: Martes, 01 de Enero de 2013 02:44 a.m.
Para: Microcontroller discussion list - Public.
Asunto: Re: [EE]Re: GPS Modules


On 12/31/2012 9:12 PM, RussellMc wrote:
>> The field units are mains powered and draw hundreds of Watts each, so
>> the added power consumption is not an issue.
>>
>> As long as the GPS locks onto the time within 5 minutes, I think the
>> client will be happy.  They are also fixed position.  Would this help?
>>
>> Maybe using the GPS to correct an on-board RTC would be a better option?
> Off the cuff low practical experience comments:
>
> GPS sounds like a good fit.
> A typical GPS (all modern?) maintain  ephemeris (short term) and
> almanac (long term) satellite constellation descriptions to allow
> location of the satellites. If the system is always on the time signal
> is essentially continous. If you turn the system on regularly and
> frequently (say hourly to maybe a few hours) the ephemeris information
> is usually relevant enough  to allow reacquisition in under about a
> minute and sometimes much less. If you turn the system on daily or
> weekly it can take minutes to acquire enough satellites to start
> producing information. (1 =3D somewhere apparently :-), 2 =3D a torus,  3
> =3D where you are or out closer to lunar orbit, 4 =3D where you are, 5+ =
=3D
> better.).
>
> Away from cities you are unlikely to seldom experience catastrophic
> loss of enough satellite signals to lose the time signal.
>
> So, if you have a RTC that has enough stability for at least 10
> minutes (not exactly hard even on the far side of the black stump on a
> hot Australian summer day) then a GPS fix "when required' will keep it
> stable enough.
> An RTC less stable than that or no RTC would benefit from permanent
> GPS operation.
>
>
>           http://en.wikipedia.org/wiki/GPS_signals
>
>
>        R

h
I am involved with solar tracker design. In order to determine sun=20
position, I must determine the exact UTC time. We keep a supercap-backed=20
RTC which is updated daily at 4AM.

After a lot of research, I determined that WWVB ("Atomic" time service=20
at 60khz) was too sensitive to cellphone and lightning interference. A=20
pity, since WWVB receivers use only a milliamp or less, while GPS use as=20
much as 100ma. We are forced to use GPS receivers.

We keep the GPS unpowered until 4AM local time, which is usually a quiet=20
period. We keep the unit powered until we receive at least 3 good nmea=20
time passes. I then average those, update the RTC, then turn off the GPS.

The time signal is accurate enough for solar purposes from just one=20
satellite. As a consequence, we use older less-sensitive $16 GPS modules=20
crammed into a tiny plastic, sealed waterproof box. We get time signals=20
in 3 minutes or so, but we check the source to make sure the pass is=20
from a satellite.

-- Bob A

--=20
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist

--=20
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
.