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A CLI dialogue on digital RF tracking/mapping
By Ken Eckenroth
Vice President, Engineering
Cable Leakage Technologies
 
I can’t help but remember listening intently to discussions by Bob Dickinson, Ted Hartson and Bob Saunders at a cumulative leakage index (CLI) seminar in Memphis, Tenn., three years ago.  This industry is like a magnet.  It captures your attention, as does the possibility of a CLI tool employing digital RF tracking/mapping  (DRTM).
        
DRTM is a system developed for quarterly monitoring for signal leakage.  According to Federal Communications Commission regulations, cable operators are required to “provide for a program of regular monitoring for signal leakage by substantially covering the plant every three months.”  Currently, a technician must stop the ride-out to manually record each leak by finding the closest physical address.  DRTM significantly speeds up this process by automatically recording the latitude/longitude location so no stopping is necessary.  Different methods have been developed for optimum speed and quick pole-line distance changes for meaningful monitoring.  Other articles have been written on these procedures, therefore we’ll focus on some new topics.
 
 
What is DRTM?        
There are four basic components to a DRTM system:
1. RF receiver,
2. Navigation system,
3. The interface module, and
4. Digital mapping program.
 
We’ll concentrate on the navigation portion of the system by explaining global positioning system (GPS) fundamentals, history and specifications as well as the near-future potential for CATV applications.
 
GPS is a $12 billion satellite-based, worldwide navigation system developed by the U.S. Department of Defense.  The system utilizes a time-based, spread spectrum signal that produces a Cartesian earth-centered, earth-fixed set of coordinates.
 
This is the most accurate method of positioning to date.  The satellites ephemeris (earth orbits) are monitored by a government control segment twice a day.  The satellites orbit the earth every 11 hours and 58 minutes.  The control segment consists of four monitor stations strategically positioned around the world, an uplink station and a master control station.  Any deviation from a satellite’s ephemeris is immediately detected and included in a data system health message from the satellite.
 
The GPS signal is transmitted in the L-band (RF spectrum from 390 MHz to about 1.5 GHz), which was chosen because the bandwidth allocation was more readily available than other bands.  Also, the ionosphere or space losses to an isotropic antenna are less for L-band as opposed to C-band.
 
The GPS system (Navstar) was preceded by a Navy project known as Timation.  This is the application of atomic clocks for satellite navigation.  Several earlier versions were considered according to size, weight and accuracy.  One of these is the rubidium clock, which is accurate to one part in 1012 (1 nanosecond).  This progressed to what is used today.  The cesium atomic clock is accurate to one part in 1013 (0.1 nanosecond).  The time is so precise it’s almost magical.  It is based on the universal coordinated time (UTC), which is known as Greenwich mean or Zulu time (See Figure 1.).
 
fig 1

Two frequencies are transmitted: L-1 at 1,575.42 MHz and L-2 at 1,227.6 MHz.  These frequencies are then modulated by pseudo-random codes.  The course acquisition (C/A) or gold code is at 1.023 Mbps and the precise (P) code is at 1023 Mbps.  The C/A code repeats every millisecond and the P code repeats every seven days. L-1 and L-2 frequencies were not randomly chosen (10.23 x 120 = 1,227.6 and 10.23 x 154 = 1,575.42).  Most GPS receivers utilize the L-1 frequency and the C/A code.
 
 
The next utility 
Communications and Navigation.  They go together like salt and pepper.  The GPS community is calling this new navigation phenomenon the next utility because of its ability to let you know where you are anywhere in the world.  It seems inevitable that this GPS utility will integrate with other utilities.  Cable is a logical choice since it has many resources that qualify it for this alliance.
 
First of all, let’s discuss GPS accuracy as it’s applied to digital mapping.  Eighteen satellites are in place now.  There will be 24 satellites in six orbital planes, inclined 55o to the equator, spaced 60o apart.  Six planes x 60o = 360o circle.  This constellation is scheduled for completion in 1993.
 
Right now we have 2D or horizontal coverage for 22 hours a day.  Even though there is coverage for 22 hours, at times the geometric dilution of precision (GDOP) isn’t the best.  There is a nominal scale for this.  Anything below six is good.  The 24 satellites in orbit will provide a constant GDOP of two, which is great.  Right now, if the GDOP gets to around eight, which it occasionally does, the horizontal picture will wander somewhat, but you can still see what streets you drove on.
 
fig 2

The earth-centered ranging (along with the GPS user’s location) produces a special polyhedron that influences the GDOP based on its volume.  As the volume increases, the GDOP decreases.  (See Figure 2.)  As more satellites go up, the GDOP and the resolution of digital mapping will improve.  This is comparable to TV or computer monitors’ resolution improving – e.g., high definition TV or Super VGA, respectively.
 
Another thing to remember is that the GIS community is in the process of remapping the world using GPS.  Most of the inaccuracies of our current maps are vertical because of the varying effects of gravity.  Isaac Newton was on to this when he predicted a pendulum clock set in Paris would lose time at the equator.  As the remapping is done, the horizontal as well as the vertical accuracy will improve.  It would be wise to make sure the digital mapping program has upgrades available so the remapping can be realized.
  
Differential GPS
All this leads our discussion to a very hot topic in the GPS world right now – differential GPS (DGPS).  DGPS will provide and average GPS user with 2-5 meter accuracy.  Think about this for a moment – it is accuracy under 15 feet.  Centimeter accuracy also is possible.  This would be a topic for a whole other article.
 
Differential GPS reference stations are the ticket.  What this station does is a simple concept.  It compensates for all inaccuracies (including selective availability).  The DGPS receiver is placed at a known location.  A surveyor provides this information working off a known benchmark.  The receiver now knows the following two things: where it is physically located and what it is receiving from the satellite. Every subsequent measurement taken is mathematically corrected.  This data is the same correction needed for all GPS users in an immediate area of about a 100-mile radius. Since GPS time stamps all of its measurements, it is a simple procedure to correlate the data.
 
Now this brings us to the delivery of this corrected data.  The two methods are post-processed vs. real time.  Post processing applies corrected data to the gathered data (from a vehicle in the field) at the end of the day or gathering session.  The corrections are made before the data is entered into the digital mapping program.  The map then displays 2-5 meter accuracy.  
        
fig 3

The other side of the coin is a real-time application.  The corrected data is transmitted to the immediate 100-mile radius area via UHF or VHF telemetry.  The users in the field receive this data with their radio links and the corrections are applied in real time.  This is valuable for fleet management situations like police, fire, ambulance, etc.
 
There are currently only a handful of these differential reference stations in the country.  The Coast Guard has set up most of them, but there also is a private company in Tennessee offering this service on the subcarrier of an FM station.  The correction signal is a small amount of data that takes very little digital room.  What we’re talking about here is a service that is provided to authorized users for a fee.  It will be desirable for this service to be available nationwide.
 
CATV has a natural infrastructure already in place to provide this service.  Nearly every CATV franchise has a headend tower that would be perfect for transmitting real-time corrections to an area and CATV’s close affiliate ties with TV and radio stations won’t hurt either.  As well, a headend computer environment is perfect.  Backup power supplies and lightning-fast switches equal redundancy and integrity.  All of this is possible along with the benefit that it would be under the watchful eye of a headend tech instead of at an unattended remote location.  The post-processed corrected data would be delivered by telephone modems.  One cable company could provide this service to all the other cable companies in the area as well as other GPS users like police, fire fighters, surveyors and ambulances.  The list of users is growing every day.  Also the CATV world has the necessary expertise to secure this service to authorized users only.
 
CATV is always looking for new sources of revenue as the recent example of digital audio services shows.  Here is a tremendous opportunity right now during differential GPS’s formative years for CATV to get involved.
 
Government support looks good.  An official at the Department of Transportation agrees that the cable industry is the perfect choice for GPS. (See Figure 3.) A DOD official approves of this expedited civilian application of a military creation.  Maybe CATV’s lessons of peaceful coexistence with aviation also could apply to the GPS’s military and civilian communities.  An official at the FCC says there are no restrictions to CATV from providing this service.  This seems to echo the challenge: “Go for it!”
 
The time is now.  Opportunities are usually timely by nature.  This reminds me of the “Larry King Live” session at the 1992 National Show. King talked about Yogi Berra’s solution to a fork in the road.  Yogi said “ take it!” Let’s continue to define this new industry and realize its full potential enhance and serve the CATV community.
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