ETL 1110-2-540
30 Sep 96
telephone access is not always available for remote gages,
connections limit the GOES system's effectiveness for time
particularly in mountainous or rural areas. Even when it is
critical situations like flash flood detection.
generally available in these areas, the costs to get telephone
(3) Meteorburst.
Although not often used,
service directly to the data collection site may be prohibitive.
meteorburst communication techniques have been used
successfully to collect data for hydrologic purposes. This
(2) Satellite. Large area data collection is often accom-
technique uses naturally occurring meteor trails to augment
plished using satellites. Data collection platforms can be
long distance communication.
deployed hundreds or thousands of miles from the central
computer. Data are transmitted to a satellite in an earth orbit
(a) Every day, the earth's atmosphere undergoes
and relayed to a ground receiving station. A single satellite
billions of high-velocity encounters with pieces of cosmic
can relay data from anywhere in its "viewing" area to the
debris otherwise known as meteors. As they enter the
ground station.
atmosphere, friction converts their kinetic energy to heat. The
temperature of the incoming particles rises and their surfaces
(a) Two types of satellite systems are in general use:
begin to erode, leaving behind a trail of ions. These trails
polar orbiting and geostationary. Polar orbiting systems use
have the property that incident radio energy is either reflected
satellites in a relatively low orbit roughly 805 mm (500 miles)
or reradiated. Meteorburst communication systems use this
above the earth's surface. These systems are constantly
property to relay radio signals.
moving with respect to a fixed location on the earth's surface.
The ARGOS system, jointly administered by France and the
(b) A typical meteorburst system is an interrogated
United States, is an example of this type operation. Low orbit
system. A central computer station sends its signals toward
satellites like ARGOS pass overhead approximately twice per
the top of the atmosphere where meteor trails are likely.
day. When the satellite comes within range, a data collection
Though the life of a meteor trail is short (a few milliseconds
platform transmits its information to the satellite for temporary
to a few seconds or so), an individual trail can retain its pecu-
storage. When the satellite passes within range of a ground
liar radio relay capacity long enough for a signal to bounce
station, the data are retransmitted to the ground station. Low
off the trail and back down to a remote data platform. The
orbit satellites are generally used in applications where time is
remote platform must immediately respond by sending its data
not a critical factor.
along the same path before the trail disintegrates. Although,
this communication technique appears to be a "hit or miss"
(b) The most commonly used satellite system for hydro-
proposition, a sufficient number of trails occur to keep the
logic data collection is the GOES system. This system
average waiting time for a usable trail on the order of
employs satellites deployed in a high earth orbit approxi-
minutes.
mately 37,000 km (23,000 miles) above the equator. At this
altitude, an orbiting satellite moves at the same angular
(c) Meteorburst systems in use today include the
velocity as a point on the earth's surface. This allows the
SNOTEL system in the western United States, the
satellite to remain in a fixed position relative to points on the
hydrologic data collection system for Alaska, and a tsunami
earth's surface.
detection system on the coast of British Columbia. The
SNOTEL system maintains remote data platforms throughout
(c) The GOES system is primarily a timed reporting
the mountainous western states to monitor snowpack
system. At precisely defined times, the remote data collec-
conditions to forecast future water supplies. In Alaska,
tion platforms "wake-up" and transmit their data to the
statewide hydrometeorological conditions are monitored
satellite for immediate relay to the ground station. Generally,
hourly. Tide stations linked to a central computer by meteor
these transmission times are scheduled once every 3 hr. To
communications stand watch for sudden changes in ocean
handle emergencies, an event reporting capability is also
levels along the Canadian west coast that may precede a
available. If the data collection platform detects an unusual
tsunami.
event, the platform spontaneously transmits its information,
(d) Meteorburst systems are appropriate for long
which is immediately relayed to the ground station.
distance communications without an installed relay such as a
radio repeater or a satellite. Up to 2,000 km can be covered
(d) The GOES system is excellent for retrieving data
in a single "hop." Meteorburst systems also work extremely
from large areas. However, since ground stations are
well in far northern latitudes where geostationary satellite
relatively expensive, small users must rely on state and
coverage is limited. However, the responsiveness of a meteor
Federal agencies to operate them. Data are often distributed
communications system still relies on the occurrence of
from the ground station computer by telephone. The
appropriate meteor trails. The occurrence of usable trails
expense, timed reporting limitations, and telephone
varies daily and annually. If an emergency such as a
4-8
Previous Page
