THE
FEDERAL RADIONAVIGATION PLAN
and
THE
FAA AIR TRAFFIC CONTROL
SYSTEM
OF THE FUTURE
Submitted To
THE DOT/DOD
USERS REVIEW
OF THE FEDERAL
RADIONAVIGATION PLAN
by:
LANGHORNE BOND
(919) 542-6614
I.GPS
VULNERABILITY:THE ASSUMPTIONS CHANGE
During
the past year it has become clear that the GPS signal is subject to a variety
of interferences which can make the signal useless for navigation purposes.
For
most civil users of the GPS signal, the signal’s vulnerability is an annoyance,
albeit an expensive one.But for
aircraft and marine navigation, loss of the GPS signal for even short periods
of time can affect schedules and revenues, and can even be catastrophic.
II.SOLE
MEANS:THE NAVIGATION OF THE FUTURE
BY GPS
The
federal government, under the leadership of the White House science and
technology advisors has urged the USDOT to adopt the satellite GPS signal
as the primary guidance for navigation.Initially
it was thought that the GPS signal was perfectly reliable and invulnerable,
and therefore that no other radionavigation
signals would be needed.GPS would
be acceptable as the “sole means” of navigation.
The
consequences of this (erroneous) assumption were that all of the expensive
ground based radio navaids for planes and
vessels, such as LORAN, VOR/DME, ILS, NDB, etc., could be decommissioned
and scrapped.The savings to the federal
budget would be impressive.
The
use of GPS as a “sole means” of navigation is embedded in the Presidential
GPS directive, the 1996 Federal Radionavigation
Plan, the FAA’s Air Traffic Control System of the future, and in the Coast
Guard’s maritime navigation plan.The
International Civil Aviation Organization (ICAO) has accepted the GPS sole
means logic.
The
recognition of GPS’s vulnerability, and therefore that GPS cannot be relied
on as the sole means of navigation, has destroyed the foundation of every
one of the plans.They will be revised.
III.FOR
EVERY PHASE OF FLIGHT
The
potential for loss of the GPS signal means that alternate/back-up systems
must be in place to provide radionavigation
service to permit aircraft to fly to their destination and to land safely
under all conditions of weather and traffic.An
alternate/back-up system for every phase of flight is needed.
To
date the focus has been on retaining the Loran signal, which according
to the 1996 Federal Radionavigation Plan
(and also the 1994 Plan, after a sudden change from the 1992 FRP) is scheduled
for termination in the year 2000.
But
LORAN, as well as VOR/DME, is an en route system capable only of non-precision
approaches with higher minimum descent altitudes.Neither
LORAN nor VOR/DME used alone can be used for low ceiling precision
approaches.
The
ubiquitous ILS systems provide this service.They
must be retained.
IV.RETAINING
THE GROUND BASED NAVAIDS
The
FAA and DOT planners are now beginning the healthy process of discussing
which of the ground based navaids should
be retained as a “back-up” or “complement” to GPS.There
is an unspoken assumption that the original decommissioning plan was about
right, and that only a skeleton system of navaids
need be retained.
Not.The
pilot unions and general aviation groups will be heard on this issue. The
airport lobbies and members of Congress will speak up when ILS’s
are to be removed.There is a genuine
aviation safety issue here:can aviation
safety be assured with a severely reduced navaid
system when GPS is gone?Probably
not.
Here
is a good starting point for the discussion:Since
the GPS signal can be lost, the navaids
that could be removed pre-GPS are the same as those that could be removed
post-GPS.
Each
of the present navaid systems presents a
different service, user, and cost picture.Here
is a brief summary for each of the three principal navaid
systems.
A.VOR/DME
There
are more than 1,000 VOR/DME transmitters in the
VOR/DME
technology is inherently inefficient, and relatively costly.The
main drawback is that the signal is very high frequency and therefore line
of sight.It disappears over the
horizon, and there are large chunks of mountainous, low altitude or rural
On
the other hand, the VOR/DME system serves nearly all the significant airports
in the
Furthermore,
the
It
has long been recognized among ATC professionals that the
Turning
off, or pruning, the VOR/DME system will prove to be a slow grudging process.But
a good case can be made for some reduction.
B.LORAN
The
LORAN navaid signal is at a low frequency
and therefore follows the curve of the earth.It
is available to the surface of the earth over all the
By
every standard—coverage, accuracy and operational cost—LORAN is significantly
superior to VOR/DME.There is only
one obstacle—aircraft equipage.Even
though more than 100,000 general aviation aircraft are LORAN equipped,
two thirds of the aircraft are not.And
none of the air carrier fleet is LORAN equipped.
There
is a special, more acute, problem with LORAN in marine navigation.The
LORAN signal, which follows the surface of the earth, is the only
alternate back-up signal to GPS for marine navigation.Since
the cut off date of Y2K for LORAN was published in the 1994 Radionavigation
Plan, sale of marine LORAN receivers was chilled. Unless the Y2K LORAN
termination date is abandoned, there will be tankers full of crude oil
and toxic chemicals, and bulk carriers full of explosive fertilizers, navigating
in
LORAN
is the alternate navigation system of the future.It
should be retained and modernized by DOT.The
decision to do this will immediately invigorate world-wide LORAN development,
modernization and expanded presence.
C.ILS
The
Instrument Landing System (ILS) navaid
provides precision guidance through bad weather right down to the runway.Next
to the jet engine, ILS was probably the most important safety advance in
aviation history.There are nearly
1,200 ILS’s in the
The
ILS is the last critical navaid in aircraft
flight, permitting the plane to return safely to the ground, even in otherwise
lethal weather.Because the alternate/back-up
system must cover every phase of flight, full ILS service must be
maintained.
Will
it be possible to scrap some of the ILS transmitters?That
question will be answered by pilots and by airport managers.There
will be great resistance.
V.SHARPENING
THE GPS SIGNAL
In
the early, bright days of GPS planning there was the mind-set that the
dependability and coverage of the GPS signal could provide a radionavigation
signal for every phase of flight, and that all the ground based navaids
could be scrapped.Expensive, technically
challenging programs would be launched by FAA to increase the accuracy
of the GPS signal to permit low DH precision approaches to replace even
the ILS.
Two
types of systems were proposed:wide
area and local area systems.
A.WIDE
AREA AUGMENTATION SYSTEM (WAAS)
The
WAAS was originally designed as a means for transmitting integrity messages
as an augmentation to basic GPS.That
purpose has been expanded to include differential refinement of the basic
GPS signal in space to the Category I (CAT I) precision approach standard,
close to a garden-variety ILS with a 200’ DH.WAAS/GPS
will be available everywhere in the
A
CAT I approach requires a signal accuracy of 4 meters or less.The
initial tests of WAAS did not meet a CAT I standard, and the Hughes contract
requires delivery of only a 7.6 meter vertical accuracy, which would not
support a CAT I approach to a 200-foot DH.However,
more recent tests are said to indicate WAAS/GPS can be brought within 4
meters.This indicates good technical
work by Hughes and FAA.
However,
there are a number of caveats to the WAAS/GPS project, to wit:
·Cost
Saving.The vulnerability of the GPS
signal has eliminated the possibility of scrapping many, and probably any,
of the existing ILS’s.Neither
WAAS/GPS (nor LAAS/GPS) can now claim the original estimates of cost avoidance.
·Service.There
are more than 1,000 ILS’s in the
·Qualifying
Airports.A signal in space is only
one of numerous requirements for a certified CAT I approach.Other
requirements include runway length and width, runway marking and lighting,
and obstacle clearance. Obstacle
clearance is especially expensive, and perhaps impossible to establish.As
an example,
·Since
the ILS service will be retained at the top 630 airports, the WAAS approach
signal will benefit only the smaller airports.WAAS
precision approaches are therefore a benefit primarily to general aviation.
·The
cost of WAAS has escalated from an estimated $500 million to a life cycle
cost of $3 billion.The benefits
have contracted.The economic rationale
must be recalculated.
B.LOCAL
AREA APPROACHES.A second GPS
sharpening project is underway at FAA to improve GPS signal accuracy to
a higher standard than WAAS.The Local
Area Augmentation System (LAAS) would have a limited range of 25-30 miles
and would provide a signal accuracy to permit CAT II and CAT III approaches.Cat
II would provide a 100’ DH and CAT III would even permit autoland.
CAT
II and CAT III are required and usable only by commercial air carriers
since almost no other civil user has this requirement or capability.The
latest plan is to provide 143 LAAS systems in the largest metropolitan
hubs, where they would replace ILS systems providing the same service.
However,
because GPS vulnerability requires an alternative/back-up service for every
phase of flight, ILS systems will remain in service.The
planned LAAS systems are largely duplicative of ILS.
There
are other local area approach systems to improve GPS.The
SCAT/MIG systems are based on differentially corrected GPS signals and
are designed to provide a CAT I signal at an airport for a very reasonable
price—about $300,000 to $400,000 compared to a CAT I ILS for about $500-700
thousand.If the purchaser is willing
to live with GPS vulnerability, which may be acceptable at a small airport,
there could be a useful role for some local GPS precision approach systems
in the rural US or in the developing world.
VI.DIFFERENTIAL
GPS USING LORAN COMMUNICATIONS
The
Europeans are well aware of the vulnerabilities of GPS and of the necessity
of a secure back-up/alternate ground-based system.They
have formed and are implementing an elegant, low cost solution—EUROFIX.
Under
the leadership of Prof. Durk van Willigen,
GPS differential messages have been combined with the standard LORAN signal.The
differential data is simply added to the signal at the existing
LORAN stations.The combined signal
provides the DGPS communication to improve GPS accuracy when it’s available,
and an independent navigation signal for vessels and aircraft when it’s
not.
The
results are encouraging.The Eurofix/LORAN
differential signal has been demonstrated to provide accuracy within 5
meters, and it could be improved to as sharp as 1 meter.This
is better than WAAS (<7 meters) and perhaps as accurate as LAAS (about
1 meter).
The
cost of the EUROFIX solution applied to the
The
US LORAN station chain is aging and it does need to be upgraded.Upgrading
the LORAN stations would require a one-time expenditure of $75-100 million,
and would not be repeated for at least 20 years.The
annual operating cost would fall from $18 million to $6 million.
VII.SURVEILLANCE
GOES DARK:SINGLE THREAD INTERDEPENDENCE
WITH GPS
Dependence
on the GPS signal is not limited to the navigation system.Satellite
dependence has crept into the controller’s world as well.
In
the ATC system of the future, the aircraft broadcasts directly its position
(ADS-B, based on GPS).
Therefore,
if the GPS signal is lost, the controller’s picture is lost!This
is the ultimate in ATC vulnerability:a
single hand held jammer can destroy not
only all GPS navigation for a radius of 200 kilometers, but also the controller’s
ability to see the aircraft as well.
The
current ATC surveillance system relies on a network of radar installations
which are independent of each other and which incorporate two different
types of sensors—primary radar (a “skin paint”)
and secondary radar, which is transponder beacon based.This
system may look inelegant to the satellite technician, and expensive to
the budgeter.But it is very reliable
and therefore, very safe.
At
least two steps should be taken to minimize the spread of the GPS
virus into the controller’s suite:
·any
downlink of aircraft position should be based on a multi-sensor navigation
receiver, i.e., GPS plus LORAN, or GPS plus VOR/DME, or GPS plus INS/FMS.When
GPS is lost there will be a back-up fix for the controller as well as the
pilot.
·retain
the secondary (beacon) radar systems.
VIII.THE
NEXT STEPS FOR DOT
A.NO
SOLE MEANS FOR AIRCRAFT
The
DOT should state publicly that the GPS signal is vulnerable and cannot
be a sole means of navigation for aircraft.
B.CHANGE
FRP NOW
The
DOT should immediately amend the 1996 Federal Radionavigation
Plan by deleting all termination dates for navaids,
including LORAN, VOR/DME, and ILS.
C.UPGRADE
LORAN
The
DOT should declare the LORAN system a permanent navaid
and should upgrade all 29 stations with modern, low maintenance, solid
state components.Blink and GPS differential
correction should be added.
D.FIX
MARINE RADIONAVIGATION
The
DOT should reject GPS sole means marine navigation and should require GPS
and LORAN receivers on all large vessels, on all HAZMAT cargo vessels,
and on all passenger-carrying vessels of
E.LORAN
AS A DISTRIBUTION CHANNEL FOR GPS CORRECTIONS AND INTEGRITY
The
DOT should add the differential GPS signal to all US LORAN stations.
F.MAKE
ILS PERMANENT
FAA
should declare ILS a permanent precision approach and should initiate an
open dialogue with users about eliminating some ILS’s.
G.CONTINUE
VOR/DME
FAA
should affirm the continuation of VOR/DME and should initiate an open dialogue
with users about reducing the number of VOR/DME sites. The air carriers
will shortly need only the DME signal to drive their FMS systems.Therefore,
the carriers should state how many DME’s
they will need in the future.The
general aviation community is now using VOR/DME, as well as LORAN, but
LORAN is a complete substitute for VOR/DME.FAA
should open a dialogue with the general aviation community about transitioning
all GA aircraft to LORAN and, at some future point, decommissioning VOR.
This
would lead to an ultimate terrestrial system with DME, LORAN, and ILS.
H.REASSESS
WAAS
All
of the permutations of GPS are radio signals which make it possible to
determine an accurate point in space.The
unique requirements of aviation are now known to limit the benefits and
cost savings of WAAS.But radio location,
of which radionavigation is only a part,
has other useful roles for WAAS.
The
benefits to all users of WAAS should be evaluated before a final
disposition of WAAS is made.
I.LORAN
APPROACHES
FAA
should direct its ATC procedures office to establish LORAN non-precision
approaches.
J.LORAN/GPS
RECEIVERS
FAA
should develop the technology for low cost LORAN/GPS integrated receivers
for general aviation aircraft.
K.MULTI-SENSOR
FREE FLIGHT
The
Flight 2000/Free Flight Program should be modified slightly to accept GPS,
INS, VOR/DME & LORAN.This
includes the Conflict Probe Project.
L.NO
CONTROLLER GPS VIRUS
FAA
should modify any use of satellites for surveillance to eliminate sole
reliance on GPS and to maintain independence between navigation and surveillance.
M.EVALUATE
ADS-B
FAA
should initiate an independent assessment ADS-B to be certain the independence
between navigation and surveillance can be assured and that GPS vulnerabilities
are not a safety problem in the surveillance area.
N.ICAO.
FAA
should notify ICAO that GPS as a sole means of navigation involves very
significant risk and that alternate/back-up systems
should be provided. FAA should initiate action in ICAO to recognize LORAN
and variants such as the Russian/CIS “Chayka”
system as internationally-standardized navaids.