ETL 1110-2-540
30 Sep 96
should include information about the continuing meteorolo-
system operation or to collect data between normally sched-
gical conditions.
uled interrogation times. Two-way communication also
means that more equipment is required at remote sites (as
(b) In rapidly responding watersheds, meteorological
opposed to one-way systems), since both receive and transmit
forecasts may be the only effective tools to provide adequate
functions must be supported. Power requirements are higher
flood warnings. Waiting for observed rainfall and river
with interrogation systems, since radio receivers must be on
measurements in these situations introduces too much of a
continuously to listen for interrogation requests.
delay. The flood may come and go before the message gets
out.
(d) Reports from an interrogation system inherently
"lag" the monitored event. In situations where field
Once
c. Data
transmission
subsystems.
conditions change rapidly, reports may not get to the user in a
hydrometeorological parameters are measured in the field,
timely manner. For example, if 3-hr interrogation intervals
they must be transmitted to a central location for analysis.
are used and a major rainstorm occurs in the first hour after a
The type of forecast situation will dictate the most appropriate
scheduled interrogation, two additional hours will pass before
method for data transmission. Obviously, speed and
the user gets the new data. Even if the interrogation
reliability are critical to flash flood warning - preparedness
schedule is immediately increased, a significant amount of
programs. Several ways of automatically collecting data from
potential response time is lost. In addition, intensity infor-
remote locations are in use. They include interrogation, event
mation can be lost since the data are interpreted as occurring
reporting, timed reporting, and various combinations of these
over a 3-hr period instead of concentrated in the first hour.
techniques. Each technique has its place. Which technique to
Such a lag in reporting and loss of intensity information can
use depends on the application and individual preferences.
be important in a critical response situation like a flash flood.
(1) Interrogation.
Interrogation techniques are,
(2) Timed reporting. Timed reporting systems are one-
perhaps, the oldest of the automated data collection
way communication systems. Data reports are transmitted
techniques. Here, a central station "calls" a remote data
from remote sites at regularly scheduled intervals without
location and "asks" for the data. The remote site responds by
prompting from a central computer. Each remote location is
sending back the data. Common examples include telephone
allotted a small "window" of transmission time. At the
and radio-based interrogation systems. A computer may use
appointed time, the remote site turns on its transmitter and
a telephone circuit or radio link to contact a remote site to
sends the data. If, for some reason, the remote site misses its
request data. The equipment at the remote site responds to
scheduled transmission, it must wait until the next trans-
the request and returns the data to the computer. A group or
mission time to send the data.
network of remote sites are contacted sequentially. When the
computer finishes talking to one site, it contacts the next
(a) The GOES satellite system is a good example of a
station.
timed reporting system. Each data platform in the network
has a designated time to transmit. The data platform sends
(a) Interrogations or requests for data from a network of
data to the satellite that relays it to the ground receiving
remote gaging locations are normally set up on a routine fixed
station. This approach requires accurate clocks at the remote
schedule. Fixed intervals can be set for every 30 min, every
sites to choreograph the intricate "data dance" through the
hour, every 6 hr, every 24 hr, etc. At the scheduled
network. Without accurate clocks, the data reporting sched-
interrogation time, the central computer automatically calls
ule would quickly turn chaotic.
each site and collects the data. When a data collection cycle
has completed, the computer will wait for the next appointed
(b) Timed reporting systems are excellent for routinely
time to begin the collection cycle again.
collecting data on a set schedule. One-way communication
for timed reporting also lowers power requirements. How-
(b) If the data show that field conditions are changing
ever, since the systems communicate in one direction only,
rapidly, the interrogation schedule can be changed to retrieve
they lack the flexibility to respond to changing field
data more frequently. Sometimes the schedule is changed
conditions.
manually by an operator. Sometimes the computer is pro-
grammed to detect opportunities to increase or decrease
(3) Event reporting. Event reporting systems also com-
interrogation frequency automatically, as conditions warrant.
municate in one direction only. However, the hallmark of
these systems is their ability to generate data reporting rates
(c) Interrogation systems are two-way communication
that are directly proportional to the rate of change of the
systems. Central data collection sites and remote data sites
monitored conditions. Event reporting systems generate data
have both receive and transmit capabilities. One has the
reports from remote locations as soon as "events" are
opportunity to contact a remote site at anytime to verify
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