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Signal leakage: The next logical
step:
By Ken Eckenroth, Vice President of Engineering and Mike Ostteen, Vice
President, Cable Leakage Technologies
In the CATV world, as in most business environments, economics is the
driving force. Operators are constantly searching for ways to cut costs
and utilize resources without sacrificing quality. By now, the
differences between the yearly certification and quarterly monitoring
methods are more pronounced. Much work and many resources are spent
during the day-to-day monitoring and repairs on any plant.
Mobile
mapping
Unfortunately, the presentation of the accumulated data leaves much to
be desired. One method to correct this is a mobile mapping system. With
prices in such systems varying from around $10,000 to $20,000, one must
find a system best suited to leakage monitoring needs.
A mobile leak detection and mapping system allows operators to choose
the best way to utilize available resources. Operators can, for
example, shuffle the equipment between service technicians for data
collection during their normal routines virtually eliminating the
ride-out tech. Or, a continuous ride-out of the entire plant using
minimally trained personnel can be performed.
With either method, the cost savings are tremendous because of the ease
and quickness of data accumulation. The line tech and his bucket truck
are not required to accumulate the data; line techs can be sent
straight to problem areas with a digital map of the RF leaks found. At
this time, the tech peaks out the leak at 10 feet and records anything
over 50 micro(GREEK LETTER u)V/m for use in the "I over infinity"
formula.
Any mobile detection and mapping system has three major components.
They are the RF detection device, vehicle tracking device and the
interface device, along with its software which correlates the
collected data into useful information.
RF
leak detection
Let's consider the RF leak detection device first. In an imperfect
environment where the pole line varies in distance from the road, it's
impractical to expect that a technician or installer with a meter set
to go off at a certain threshold can properly monitor a plant for
leaks. While a spectrum analyzer might cover the entire spectrum of
signals available, it's also expensive overkill for testing the
magnitude of the leak.
We believe that using a frequency-specific meter such as Wavetek's
CLM-1000 is more beneficial to the collecting of accurate data. These
devices have the ability to tune in to any frequency (50 MHz to 550
MHz) chosen for monitoring.
It also has another feature which makes it invaluable for mobile leak
detection - the ability to calibrate distance from the pole line into
its RF measurements. By using this function, monitoring is meaningful
as well as accurate. By connecting a remote, hand-held terminal to the
unit and using function keys, the driver can maintain a fairly accurate
vehicle-to-pole distance for proper RF signal strength. With a properly
calibrated meter and a good antenna, we have found that measurements
made during the ride-out are very similar to the readings taken by
peaking the signal on both its axes at 10 feet.
The interface/data collection aspect of this package demands certain
things. First, the field equipment needs to be commercially rated for
long trouble-free use in a bumpy, hot/cold environment. Second, the
equipment needs to be user-friendly. A complicated and cumbersome
system would collect more dust than data. Three, the system needs to be
upgradeable. The fast-paced CATV, computer and navigation communities
necessitate an open architecture in hardware and software.
Presenting
the data
Maybe the most exciting aspect of mobile RF/axis detection is the
presentation of the collected data. Instead of collecting addresses of
leaks located in a daily log to be condensed and manipulated later, a
map of the cable plant, complete with the approximate location of the
leaks and their magnitude, is generated minutes after completion of the
rideout.
Perhaps the most interesting part of this type of system is the means
of navigational tracking. CLI has brought two of these to CATV, which
are Loran and GPS.
GPS, or global positioning system, is a $1.2 billion satellite network
created by the United States Department of Defense (DOD). When in full
operation, it will have six to nine satellites in view from anywhere in
the world, at all times. By far it is the most sophisticated form of
navigation on the planet. Seven dimensions can be derived from its
information - three dimensions of position and three dimensions of
velocity, as well as one dimension of precise time. (The time is so
accurate it's almost magical.)
Loran has been around for decades. Loran C operates at around 100 kHz.
Unlike VBF signals which have "line of sight" properties, Loran signals
hug the earth and travel hundreds of miles. Location is then determined
by triangulating on three or more stations in a chain.
Much has been written on both. Both are excellent sources for
latitude/longitude or 2D location detection. But, for vehicle tracking
purposes, the GPS wins hands down. Even with significant improvements
and advanced calibration techniques giving it accuracy of 150 feet to
300 feet, the Loran can't be compared to the GPS.
Because of the relatively tight densities of some neighborhoods, the
Loran system is not equipped to handle the job as well as the GPS.
Also, Loran has problems with high voltage lines which make it a poor
candidate for vehicle tracking. Loran does have the advantage of
working well under dense foliage, where GPS (which is in the "L" band,
at 1575.42 MHz) has trouble. However, if any light can get through,
generally a moving vehicle can successfully track its path.
GPS
offerings
The GPS provides a 3D (altitude), and a 4D (real time) that may have a
CATV application in the future. For vehicle tracking purposes, 2D is
sufficient. Street width accuracy (about 50 feet) is very common and to
be expected with GPS. Unbelievable accuracies of one centimeter using
GPS technologies such as P-Code, Carrier Aided Tracking and
differential technology are also available. However, with price tags up
to $66,000, these are expensive and unnecessary for this application.
The important thing is to have equipment that is upgradeable to this
accuracy if it is needed in the future.
GPS
concerns
There are concerns about GPS, including the fact it is an incomplete
system, having 17 of the 24 satellites in operation. However,
sufficient coverage exists in North America at this time for most
tracking purposes. This includes 22 hours a day of 2D coverage and 16
hours of 3D. Additional satellites are scheduled for deployment this
year with a total completion date in 1993. The most recent satellite
was launched on February 23 and put into service March 21. A computer
program is available for monitoring satellite position and
availability.
Selective
availability
Another concern about GPS is a low level government test called
"selective availability," or SA. The Department of Defense will
intentionally degrade the signal to 300 foot accuracy using SA to
ensure that in times of war, a foreign country cannot use GPS for
hostile purposes. The DOD peaks SA infrequently, however, and
information on whether or not it is activated is available from a Coast
Guard telephone number.
A frequently asked question about GPS is: How many channels are
necessary? GPS receivers ranging from one to 24 channels are available
(there is even a 36-channel model). Anything over six channels is
generally designed for surveyors for precise measurements, whereas a
portable handheld receiver would be sufficient for a lost hiker. A GPS
satellite broadcasts a sophisticated pseudo-random code along with a
satellite system data health message which takes up to 30 seconds to
complete.
In vehicle tracking, an update every one to five seconds is needed. A
six-channel GPS, then, is the best choice for a high dynamic situation
like vehicle tracking. In residential areas, 90-degree and 180-degree
turns are common and come quickly while driving a vehicle. Also,
continuous six-channel receivers minimize an effect called GDOP
(geometric dilution of precision).
The satellite's location in the sky will provide better geometric
angles at certain times. A six-channel system tracks all the available
satellites and picks the ones with the best angles.
This is much like a pool player selecting the best angle for a shot. A
six-channel receiver also provides a better signal-to-noise ratio than
lesser receivers. They compare their channels to each other and
calibrate out interchannel biases. Another possible source of problems
are multipath errors, which are similar to cable television's
"ghosting" effects. This problem, which can cause substantial position
errors, is corrected by advanced signal processing techniques using
optimizing filter algorithms.
New
directions
The GPS industry has some exciting new products soon to be released on
the market that use GYRO technology for dead-reckoning techniques.
Dead-reckoning refers to the ability of the GPS to track itself using
vehicle movement sensed by the GYRO to detect the direction and
velocity of the vehicle heading during momentary lapses of coverage.
This will significantly help tracking in downtown areas dominated by
highrise buildings.
The future of cable vehicle tracking could go in several directions. RF
snapshots, fleet management, contouring, design system integration,
differential and dead-reckoning seem to be the most likely candidates.
The CATV community will decide which technologies are applicable and
may come up with a few of its own.
Copyright © 2004 Cable Leakage Technologies. All Rights
Reserved.
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