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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.).
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.
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.
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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