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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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Copyright 2009 Cable Leakage Technologies Inc. All Rights Reserved.