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[wxsig] Array of X1W-4 sensors to detrime direction and distance ?

Gerry Creager gerry.creager at tamu.edu
Fri Feb 15 13:43:39 UTC 2008


Sorry!  I got busy with CWOP and "real" work!

I got this back from Joe:
Gerry,
The best I can tell (because I don't see one in the sensors), the 
commercial box does not use any sort of precision timing reference 
oscillator.  They simply have a GPS with 1 pps output to create a 
reference a data acquisition board input.  The sensor's clock is 
adjusted to GPS time and the 1 pps triggers a circuit to generate a 
wide-band pulse.  So, as time goes on, data packets are timestamped with 
GPS time and then the 1pps signal shows up to mark the beginning of the 
second.  Vaisala's system basically detects when the RF amplitude 
exceeds a certain threshold (and matches a lightning source signature to 
reduce false alarms).  The packets are sent near-real time to the main 
server which reads the data from all systems and tries to find a "match" 
in the incoming data streams.  This way, you don't really have to worry 
about a few seconds of internet delay....but, of course, you may delay 
finding a solution...say 15 seconds...from when the flash occurred.

The sensor to server data might look something like this...where the 
data points are time, in nanoseconds from computer GPS time.

He also refers to these articles for the research-inclined:
Journal of Geophysical Research
Correlated high-speed video and radio interferometric observations of a 
cloud-to-ground lightning flash by Vladislav Mazur 1997

JGR again
On the retrieval of of lightning radio sources from time-of-arrival data 
by Wiliam J. Koshak 1996

Back to the grind.  Gotta get some WRF data turned into pictures. Today 
and tomorrow look interesting in Texas for severe weather.

gerry

Timestamp 12/12/07 10:01:20 UTC
5600
1030049
199432942
2932740239

If you collect the data from 3 or more sensors, you just try to find a 
fit...it the Chi squared error exceeds your threshold, throw it out as a 
non-event.
Claims on intracloud lightning with the Houston network are in the 
vicinity of 200m.  NLDN errors, with much larger baselines are on the 
order of 700m.  So, it's very possible to produce an amateur grade 
system, with short baselines, having an accuracy of 1/4 mile with the 
right component selection and software.

I'd really go for multiple sites around a given area...say CLL, HOU, 
AUS, SAT, VCT.  For C-G systems, VLF travels a long way.  A short 3 m 
baseline system would require tremendous timing accuracy  hampered by 
receiver bandwidth and still suffer great errors 100 miles out.


Stanley Reynolds wrote:
> Thinking of an array of 4 to 8 shield loop antennas each with it's null pointing in a different direction may provide good directional info vs using vertical antennas.
> 
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-- 
Gerry Creager -- gerry.creager at tamu.edu
Texas Mesonet -- AATLT, Texas A&M University	
Cell: 979.229.5301 Office: 979.458.4020 FAX: 979.862.3983
Office: 1700 Research Parkway Ste 160, TAMU, College Station, TX 77843




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