[time-nuts] FTS4060 Continuing Saga, GPS rain
NE8S
ne8s at earthlink.net
Mon Mar 6 12:17:51 EST 2006
Brooke,
One of my FTS4060's behaved the same way during a power interrupt. It was
the only one that was not on a UPS. After many attempts to reset the logic
while powered up, I finally brought the clock down and 24 hours later,
powered it back up again. As a result, it performed its normal power-up
sequence and the control logic seemed to have reset itself properly. It this
point, it was back to normal. It has been fine ever since.
First time it has happened to me. I am mainly an HP man and very much prefer
the HP clocks and their digital logic structure and methods.
Hope this helps a little.
Kind regards,
Gar Ko
NE8S
CsUTC Observatory
Bio-Nucleonics Laboratory
----- Original Message -----
From: "Brooke Clarke" <brooke at pacific.net>
To: "Tom Van Baak" <tvb at leapsecond.com>; "Discussion of precise time and
frequency measurement" <time-nuts at febo.com>
Sent: Monday, March 06, 2006 10:58 AM
Subject: Re: [time-nuts] FTS4060 Continuing Saga, GPS rain
> Hi Tom:
>
> I think the problem is related to the FTS4060. During the brown out my
> computer and the SR620 counter reset, but the two green LEDs on the 4060
> stayed on, at the time my thought was "cool". But the yellow "Align"
> light behind the door has turned on and so far I have not been able to
> get it to turn off. But after trying a bunch of stuff the jitter value
> is now back in the 9 ns area. I have not been able to find anything in
> the manual about operation with the "Align" light on and with the "Lock"
> and "AC Pwr" LEDs on. Does anyone have any ideas about it?
>
> On the rain thing. I think the key parameter of water that matters is
> the dielectric constant which is about 80. The propagation constant is
> proportional to 1 / SQRT( e ) or about 11% of what it is in air. If
> it's 1 ns per foot in air then it's 8.9 ns per foot in water
>
> I haven't figured out how to calculate the total volume of water in the
> column between the satellites and the receiving antenna. I did find a
> web page that shows the terminal velocity of rain and it varies from
> 0.02 MPH for small drops to 20 MPH for big drops. But this would need
> to be coupled with the rate of rain fall (inches/hour) to determine how
> much water was in the column. Assuming big drops (20 MPH = 352 IPS) and
> 0.1 IPS rain rate says that the rain takes up 1 part in 3520. If the
> height of the rain is 20,000 feet then there's 5.6 feet of rain for a
> delay of 50.5 ns???
>
> Have Fun,
>
> Brooke
>
> Tom Van Baak wrote:
>
>>>Rain should have an effect on the timing of the signal, since the
>>>propagation speed of radio waves through water is different from that
>>>through air. It will also attenuate the signal, causing worse S/N
>>>ratio which would cause the lower-elevation satellites to not be seen.
>>>
>>>
>>
>>David,
>>
>>Maybe we can figure this out. First, the refractive
>>index of water is about 1.3. So I think this means
>>the propagation speed of radio waves in water is
>>down to about 0.75 c, right?
>>
>>Then, how much water are the GPS signals traveling
>>through? Let's assume the typical amount of rain in
>>a heavy storm is a couple of inches. All that water is
>>either puddles already on the ground, drops on their
>>way down, or moisture still in the clouds waiting to
>>come down.
>>
>>The total amount of water in a cross section column
>>of the atmosphere that the GPS signals travel though
>>is thus a couple of inches total, max. Let's assume
>>a worst case -- 6 inches.
>>
>>So, those GPS signals go through 20,000 km of
>>empty space and atmosphere containing a total of
>>6 inches of water; in which it slows down by 30%.
>>At a ns/foot, this comes to 25 ps per inch of water
>>content in the air; a total of 150 ps in my worst-case
>>example above.
>>
>>My conclusion is that rain or snow, light or heavy,
>>has no effect, even at the ns level. Can someone who
>>really knows double check this back of the envelope
>>calculation?
>>
>>Thanks,
>>/tvb
>>
>>
>>
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>>
>>
>>
>
> --
> w/Java http://www.PRC68.com
> w/o Java http://www.pacificsites.com/~brooke/PRC68COM.shtml
> http://www.precisionclock.com
>
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