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21st century technology
– now: Global Satellite system for signal leakage
By Ken Eckenroth, Vice
President of Engineering, Cable Leakage Technologies
The purpose of this article is threefold: To provide information about
the current relationship between CATV and GPS; point out similar
expressions and terms that are interchangeable; and provide information
about policies and technical data which will influence the cable
industry in the future. Because previous trade articles have been
written on GPS history and theory, this paper will try to avoid
repeating or recycling old material.
Introduction
GPS is a satellite-based radionavigation aid deployed by the U.S.
Department of Defense that is primarily aimed to support the military.
However, GPS is also made available for use by the general public and
commercial entities to obtain accurate positioning.
In February 1993, the 18th "block-two" satellite was set into orbital
plane B-1. There are now 22 block-one and block-two satellites in orbit
out of a scheduled 24. Six launches are scheduled for 1993, which will
complete the constellation and begin phasing out the older block-one
satellites. For the first time in history, 24-hour three-dimensional
coverage is possible. There are absolutely no lapses in two-dimensional
coverage now.
Also, newly completed GPS algorithms are making constellation jumps,
where a segment of the plotted course jumps to one side or the other
(see Figure 1), a thing of the past. Jumps occur when a GPS receiver
tracks different satellite constellations as they become visible. These
jumps, when combined with selective availability (S/A) errors, can
produce a messy course on a digital map. Multipath errors are still a
problem, but they occur significantly less frequently and are usually
single points instead of a segment.
Maintenance
concepts
Let's back up and look at two concepts that are fundamental to CATV
service: Demand maintenance (D/M) and preventive maintenance (P/M). D/M
is reactive - which results in random pattern, like outages and
priority service calls. It doesn't matter what part of town they occur,
the course of action is to 'Drop what you're doing and proceed
immediately to that new location." The other side of the coin is P/M,
which is methodical and well planned. A tech could have 25 service
calls, 20 amps to sweep, 30 leaks to fix, or 100 miles of plant to ride
out, but his day is planned ahead of time, which is more efficient. A
tech may have to respond to an outage, but he will return to his P/M
duties.
GPS has two categories that correspond with the D/M and P/M formats:
Real time and post processing. The idea of real time GPS is to
supplement beepers and two-way radios with fleet management. The idea
of post processed GPS is to enhance the productivity of P/M by
streamlining the planning.
Surveyor
accuracies
First, let's talk about how surveyors achieve accuracies of just a few
centimeters. A sophisticated GPS receiver will measure not only the
pseudoranges (code), but also the carrier phase. Utilizing differential
techniques, the carrier phase yields centimeter precision compared to
meter level precision offered by the pseudoranges. The carrier phase
measurements are made ambiguous by an unknown integer number of carrier
cycles - the so-called "carrier phase ambiguity."
This carrier phase ambiguity remains constant over time as long as the
receiver is phase-locked to the incoming signal. No similar ambiguities
exist for the pseudoranges. If we can correctly identify the carrier
phase ambiguity number, we can convert the carrier phase measurements
into very precise range measurements. This integer ambiguity resolution
is the goal behind several different post processed procedures,
including: static differential positioning; pseudo-kinematic surveying;
stop-and-go surveying; and rapid static surveying. While a detailed
discussion of these procedures is beyond the scope of this paper, the
concept to remember is the difference between pseudorange (code) and
carrier phase receiver abilities. The terms pseudorange and code are
interchangeable in this application. All GPS receivers interpret code,
but only high-end GPS receivers interpret code and carrier.
Time
concepts
The next concept is the difference between GPS time and UTC (universal
coordinated time). UTC time is measured in seconds and is referenced to
the London time zone. This time zone is not to be confused with the
absolute longitude coordinates 0 degrees 0 minutes 0 seconds, which
runs north and south through London. This time zone encompasses the
United Kingdom. GPS time is a product of a satellite's atomic clock,
with nanosecond accuracy. A sophisticated GPS receiver would utilize
this in time transfer technology, which is an entirely different field.
How does the FAA (Federal Aviation Administration) plan to use GPS? The
answer falls into two categories: Accuracy and integrity. Civilian GPS,
or SPS (Standard Positioning Service), has a stated accuracy of 100
meters (95 percent of the time). This is acceptable to the aviation
community for en route and non-precision flight operations, but not for
airport precision approaches. Three technical approaches for accuracy
are under study - differential GPS; augmenting GPS signals with other
navigation aids (top candidates are GLONASS [Russian GPS], Loran-C, and
inertial navigation systems); and real-time kinematic carrier phase
tracking.
The challenge here is to take this centimeter accuracy beyond the
stationary mode and develop methods for dynamic civil aviation
operations (ambiguity resolution on the fly). There is a new technique
that shows great promise. It's called "widelaning the dual frequencies"
and is solving the ambiguity resolution in 1 to 3 seconds.
GPS at this time cannot meet the FAAs' requirement for integrity. This
is the ability of the system to provide timely warnings to the user
when the system should not be used for navigation. These integrity
requirements are 30 seconds for en route flight, 10 seconds for
terminal areas and non-precision approaches, 6 seconds for certain
precision approaches, and 1 to 2 seconds for precise approaches leading
to actual runway touchdowns.
Minimum Operational Performance Standards (MOPS) set by a
Washington-based commission, which affect aviation on a global scale,
are calling for the 10-second integrity warning. Inmarsat is proposing
an integrity channel via its 3rd-generation satellites beginning in
1995. It would uplink in C-band and downlink in L-band frequencies
adjacent to GPS frequencies. This is called the Bent Pipe effect.
When one is dealing with something as critical as integrity, the
question, "What if there is a total failure of the system?" must be
asked. RAIM (receiver autonomous integrity monitoring), which would
choose between other augmented systems seems to be the conventional
answer. The GPS
process for aviation is evolving toward a GNSS (global navigation
satellite system).
Horizontal
accuracies
Horizontal accuracy is a subject that does apply to cable operators.
Government-stated specifications for SPS (civilian) is 100 meters (95
percent of the time) with S/A on; PPS (precise positioning system) or
military spec is 18 meters (95 percent of the time). This 95 percent
spec is a huge cushion. Realistically, it's more like 99.5 percent.
Every once in a while you'll see a huge jump that probably only the
Dept. of Defense could explain. S/A does not affect the PPS. So, does
the PPS represent what the civilian service would look like if S/A was
turned off?
Remember, PPS is a product of the P code placed on both the L-1 and L-2
frequencies (see Figure 2). The dual frequencies allow the receiver to
correct for ionospheric errors. The higher frequency will have a
greater loss. The difference can be measured to determine the
ionosphere loss. The C/A code is only on the L-1 frequency. Single
frequency signal processing incorporates a mathematical model called
the Klobucar model. This eliminates half the ionosphere errors but not
all of them.
There's another error that is the product of S/A, called the
"observables." These are the satellites that are in view for the
individual GPS receivers when making differential corrections from a
base station to a rover receiver. There may be, for example, eight
satellites in the sky over an immediate area. The base station GPS
receiver may see all eight while the rover may see only seven. This is
because buildings and other obstructions may block the view of the
rover. The percentage of observable error is the combination of how
long an obscured satellite (for the field unit) is on the horizon and
whether or not S/A is cranking on that satellite at that time, and
whether a leak (event) occurs at that time.
Consequently, each receiver has its own unique navigation solution
(prime course). This means that if (up to 100 meter error causing) S/A
were removed, accuracy is slightly worse than 18 meters because half of
the ionospheric error still exists for a single-frequency GPS unit.
Those who have ever seen a plotted course on a digital map with S/A
working know that the goal is to be on the right street. Most streets
are 300 to 400 feet apart. That meas accuracy of 150 feet or less would
be sufficient to put the path on the right street. When you're dealing
with the integrated navigation solutions (latitude and longitude)
instead of the raw data (pseudoranges, range rates, etc.), S/A's effect
is quasi-directional. The plotted course could be up to 300 feet above,
below, left, or right of the actual prime course. Integrated
differential corrections would produce a spec in the neighborhood of 25
meters (99 percent of the time because of the observables and multipath
errors). Remember, these are worst case numbers and the norm would be
15 meters (street width accuracies).
There is a way to get this type of accuracy by using differential GPS
receivers. The most accurate and expensive models produce 2- to 5-
meter accuracy, but because they work with GPS raw data, they work in
real-time only. The differential systems which utilize integrated
solutions are more reasonably priced and work in the post processed
realm. Consumers will buy the accuracy that applies to them. That's why
it's important to understand all the facts.
Government policy re: S/A
There is another way to remove S/A, and that is to simply turn it off
during peace time. S/A at one time was degrading GPS accuracies to 500
meters (95 percent of the ti-e). The Cold War threat of missiles being
locked onto the signal was a real concern. In 1983 the Department of
Defense reduced accuracy to 100 meters. Additional emphasis was placed
on the civil use of GPS after the downing of flight KAL 007 by the
Soviet Union. The Senate, after condemning the act, called for a
speeded up timetable because GPS benefits public safety.
In this context, maybe the S/A error should be tied directly to defense
conditions. Zero or minimum threat should equal no S/A. Medium threat
would equal medium S/A, and so on.
Anti-spoofing
Anti-spoofing (A/S) is another product of the DOD that provides
military protection against fake signals. A/S works by encrypting P
code to Y code. P code is commercially used by civilians, but only
military personnel have the Y code encryption keys (see Figure 2). It's
important to understand the difference between S/A and A/S in the fact
that A/S does not affect the average civilian GPS user. Most people
familiar with GPS would say S/A is here to stay. However, there are
many who believe the DOD is doing the American taxpayer a great
disservice by operating S/A during peace time. CATV is now part of the
long list of industries affected by the operation of S/A.
Conclusion
GPS technology promises some amazing things to different consumer
groups. Unfortunately, some of these groups will have to wait for some
answers. Fortunately, one of these groups isn't CATV.
Copyright © 2004 Cable Leakage Technologies. All Rights
Reserved.
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