The addition of automatic satellite communications has now paved the way for operational air traffic control using satellite navigation and communications in accordance with the International Civil Aviation Organization (ICAO) concept for new Communications, Navigation and Surveillance for Air Traffic Management (CNS/ATM). Part of the above concept is Automatic Dependent Surveillance (ADS). This system can be used anywhere on the globe (even in airspace not covered by radar such as over oceanic or sparsely populated areas) by an air traffic control center to determine automatically the current position of aircraft, their intended flight path, and other relevant information held in the aircrafts' onboard navigation systems.
It is the recent successful testing of ADS that will help advance the revolution in further environmental measurements from commercial carriers. The system allows air traffic controllers to track aircraft within the controllers' oceanic airspace on a visual situation display. The system provides an ocean-wide pseudo-radar system that allows greater flexibility in routing--and when fully operational, reduced aircraft separation standards. While weather information has always been essential in flight planning, the greater in-flight flexibility or "free flight' allows immediate in-flight adjustments and choices--thus, the greater necessity for gathering real-time atmospheric information from the aircraft itself. We finally have the positive feedback loop in place for significant operational cost savings due to the availability of up-to-the minute aviation weather parameters.
The FAA has initiated an Aviation Weather Program (the most recent description is provided by Klasinski, et al. (1995). A small subset of that program was the Commercial Aviation Sensing Humidity (CASH) program described by Fleming and Hills (1993). The CASH program evolved into two main activities: the testing of all available water vapor measurement concepts and the creation and implementation of an ascent/descent format for obtaining "profiles" of winds, temperatures, and water vapor. Each of these activities is briefly reviewed below.
a. CASH test program
Accuracy of water vapor measurements on commercial aircraft was important but only one of many factors to be considered in the test program. Since the purpose of the test program was to lead to specifications for the procurement of a Water Vapor Sensing System (WVSS), only a few of the 13 or so identified measurement concepts were actually feasible for real-time commercial aircraft applications. Such a sensor system had to be lightweight, compact, rugged, sensitive, shielded from contaminants, and able to perform reliably at a wide range of temperatures, pressures, Mach numbers and water vapor conditions. The WVSS had to be easily installed on existing aircraft. The system had to perform accurately over at least a three-month unattended period, and any maintenance required at that point in time had to be completed in less than one hour.
The WVSS had to be sufficiently accurate and possess a sufficiently short response time to resolve the vertical structure of the water vapor in the lower troposphere at typical aircraft ascent/descent rates. The vertical structure of water vapor in the lower atmosphere is especially important to aviation for a variety of reasons related to safety, efficiency, and capacity--as well as to the full spectrum of other atmospheric applications: weather prediction, hydrology, atmospheric chemistry, atmospheric pollution, and global change (the global energy and water balance, biogeochemical cycles, etc.)
The test program was fairly complete and included tests in a laboratory chamber, in a wet wind tunnel, and on a well-instrumented research aircraft. The complete test results (Hills and Fleming, 1994) were distributed to all potential bidders for the WVSS procurement phase. These tests revealed potential problem areas and, subsequently, all the responders to the procurement specifications did a good job in addressing these areas. Figure 2 shows results of response time tests conducted in a laboratory chamber. Relative humidity (RH) was started at 5%, rapidly stepped up to 85%, and then back again. A Krypton device was used as a standard in these tests (a very fast response time sensor - but otherwise unacceptable for commercial aircraft use). Figure 3 indicates flight test results from a particular day on the Deutsche Forschungsanstalt for Luft and Raumfahrt (DLR) Falcon 20E jet. Here, the aircraft was flying a level course in and out of clouds--hence the rapid swings in RH values. The standard for this was a Lyman - alpha device (which has a very fast response time but is difficult to keep in calibration, hence not appropriate for commercial aircraft). While the advanced prototype chilled mirror and the Vaisala thin-film capacitor both did remarkably well in capturing the rapidly changing RH signal, the prototype chilled mirror exhibited an "overshoot" condition when going from low to high humidity. A discussion of the winning system and its impact on future water vapor information for atmospheric science is provided in Section 4.
b. New ACARS meteorological formats.
Of the 22,000 wind and temperature ACARS reports received every day, 95% are
at flight level. The few reports available on ascent and descent are usually
obtained at fixed time intervals--too long to provide a "profile" of
information. Since virtually all aviation weather activity is related to
mesoscale phenomena (even flight level winds on the synoptic scale are often
disturbed by mesoscale outflow from convection), obtaining mesoscale profiles
of winds, temperatures, and water vapor was a highly desirable part of the
FAA's Aviation Weather Program and of the CASH program. In designing the new
ascent/descent formats to be used, one had to recognize severe operational
constraints. These are re-framed below in terms of design goals in priority
order: 
(1) maximum vertical measurement resolution
The new "ascent" format is shown in Figure 4
with the default values. The
entire profile from surface to ~18,000 ft. is concatenated and sent as a single
message. This represents a compromise between (1), (2) and (3) above. A
report using the standard or default value of every 6 seconds (the permissible
range is every 3 to 20 seconds) provides data every 300 feet (100 meters) for
typical climb-out rates. The option exists to lower this to 50 meter vertical
resolution (this will be examined during the WVSS Demonstration Program
discussed in the next Section).
(2) minimum communications cost
(3) minimum communications traffic at busy air terminals
(4) greater horizontal measurement resolution at flight levels
(5) further optimization for other operational requirements
Today, the typical carrier provides the ACARS winds and temperature
information by sending a single report every 6 to 7 minutes. The new "enroute"
format has a standard sampling rate of every 3 minutes (with a permissible
range of 1 to 60 minutes). Data are concatenated into six consecutive reports
and sent as a single message as a compromise to (2) and (4) above. At typical
aircraft speeds, the 3-minute sampling interval represents about 40 km
horizontal resolution.
The "descent" format takes into account factors (2), (3) and (5) above. The
aircraft descent into terminal airspace is different from ascent. Moreover, in
adverse weather conditions or heavy traffic, the aircraft final approach and
landing are often subject to delays. Thus, while outgoing traffic is simply
held on the ground during such delays, the incoming aircraft continue to use up
available airspace while in their respective holding patterns. In the case
of no delays, a typical descent will take approximately 20 minutes
(depending upon which of four gateways is assigned for entry to the terminal
airspace). The standard data sampling rate for "descent" is every 60 seconds
(one minute) with 10 reports concatenated and sent as a single message every 10
minutes. This sampling rate is allowed to vary within the format from 20
seconds to 300 seconds. [The rate itself can be changed to match the terminal
situation by an ACARS message].
The "ascent" profile benefits all weather prediction applications. However,
the value of the "descent" data for aviation weather should not be
underestimated. Evans and Ducot (1994) discuss the value (reduced delays) of
comprehensive weather observations within the four transition areas (gateways)
surrounding a terminal. The "descent" profile provides detailed information on
winds, temperatures, and water vapor around and within the four gateways
surrounding the terminal. Moreover, on those occasions when a terminal is
temporarily down or under restricted operations due to adverse weather, the
aircraft measurements have the potential ability to contribute to
determining more accurately the time at which normal terminal operations might
resume. The detailed environmental information available from the aircraft
circling the terminal in various holding patterns, would feed a very localized
dynamic short-range forecast model tailored to that terminal. There is a
growing list of mesoscale models that might be modified for this task.
These new formats are now accepted by the AEEC. They are included as part of
the ARINC 620 specification, and are now being used routinely by United Parcel
Service (UPS).
4. Water Vapor measurements via commercial aircraft.
An extremely focused observational requirement has emerged which involves
determining the four-dimensional structure of the Earth's atmospheric water
vapor field. Water vapor is ubiquitous, energetically important and volatile,
highly variable in space and time and currently poorly measured. Lack of
knowledge of this field has become a major impediment in the mainstream of
socio-economic applications of atmospheric science.
Mesoscale weather systems affect aviation efficiency, capacity and safety.
FAA budget documents, referring to their own statistics, suggest that the
economic impact to the aviation industry is more than one billion dollars per
year. The evolution of mesoscale weather systems is complex. Some scientists,
but not all, feel that water vapor is a key factor in the evolution (many
research articles discuss this, e.g. see Golding, 1990). This industry needs
profiles of winds, temperatures, and water vapor far more frequently than the
current 12-hourly radiosonde network can provide.
Thus, the original intent of the CASH program was to help contribute to this
water vapor measurement challenge. The following paragraphs describe the WVSS
that will be certified by the FAA, the Demonstration Program for the WVSS, and
expansion plans for achieving water vapor profiles worldwide.
a. The water vapor sensing system (WVSS)
Based upon scientific requirements for the ascent/decent region, operational
constraints, and the FAA test results (Hills and Fleming, 1994), specifications
were prepared and a government competitive procurement was conducted. Three
excellent proposals were received which addressed the limitations identified in
the test results. The winning contractor was Lockheed Martin Missiles and
Space Company, Inc. Major characteristics of the WVSS are identified below.
Figure 5 shows the position of the WVSS probe on left side of a B-757 aircraft
(the existing temperature probe is in a similar position on the right hand
side). The WVSS probe is a more advanced probe with better aerodynamic
properties. Both probes extend out from the aircraft skin about 3 inches. A
"can" beneath the aircraft skin houses the conditioning electronics. This
"can" is the same size as used with the angle of attack probe. The future
operational version will combine the temperature and water vapor sensors into a
single probe.
The key features to the air carriers (assuring no impact on their
normal operations) are the WVSS maintenance interval of 12 months (anything
greater than 3 months was acceptable as long as it could be performed at normal
scheduled maintenance check periods), sensor replacement time which is 12
minutes (versus our specification of 60 minutes or less), and system weight
(including cables) which will be less than 5 lbs.
The key features to users of the WVSS data are response time and accuracy.
The response time of the sensor ( a thin-film capacitor where relative humidity
is generally linearly proportional to measured capacitance over most of the
encountered temperature range--see further note below) is 2-3 seconds in the
lowest 20,000 ft. of the atmosphere. Since ascent and descent were the key
areas of interest for the FAA, the accuracy specification was also over this
lower 20,000 ft. The expected accuracy of the WVSS in this range will be 3-5%.
The accuracy at higher levels is discussed below.
Many studies (e.g., the summary of Starr and Melfi, 1991) indicate that
radiosondes have very little value for water vapor measurements at the very
cold temperatures near the tropopause. This is also true of the thin-film
devices, probably the best of the radiosonde sensor technologies. Problems have
been identified at very low relative humidities (RH) and at very high RH values
at these cold temperatures. Lack of good data at the high-end RH values is
especially serious due to the importance of cirrus clouds in any global warming
or cooling scenario.
While the FAA is primarily concerned with lower tropospheric water vapor
(there being little water vapor mass at flight altitudes), the author is
sensitive to the impact of upper troposphere/stratosphere water vapor
measurements for clouds and radiation on extended range weather and climate
predictions. The following discussion, which does not apply to the
lower 20,000 feet layer of primary interest of the FAA (where aircraft speeds
do not exceed Mach 0.4) is relevant to a subset of the atmospheric science
community.
Table 1 is an extract of results from a National Institute of Standards and
Technology (NIST) Report, dated January 31, 1990 and signed by H.G. Semerjian,
which compares Vaisala thin-film humidity measurements with the NIST standard
two-pressure humidity generator. Generally, this comparison is quite good.
However, we do see the Vaisala sensor underestimating the high RH values at
cold temperatures (-20øand -40øC) and note that the tests were
not performed above RH = 82% for -20øC nor above RH = 67% for
-40øC. This was apparently the case because it is difficult to achieve
stable measurements of supersaturation with respect to ice in a laboratory.
Since the saturation vapor pressure with respect to ice is lower than the
saturation vapor pressure with respect to water (the required value for the
World Meteorological Organization's definition of relative humidity),
saturation with respect to ice is achieved in the laboratory first, when the RH
with respect to water is at the lower limits indicated in the Table footnotes
for the temperatures considered. We know that in nature there is virtually
always some degree of supersaturation with respect to ice and in some
situations this supersaturation with respect to ice is significant. This is
due to the lack of sufficient active ice nuclei in nature that leads to the
state of supercooled liquid water. Moreover, we have a few test results with a
frost point hygrometer that indicate that the Vaisala sensor significantly
underestimates the RH in situations where supersaturation with respect to ice
is expected (Heymsfield, personal communication). Other data suggests that
Vaisala humidity measurements are too low relative to other sensors in this low
temperature, high humidity situation, but all current radiosonde sensors are
suspect in the supersaturation with respect to ice situation.
Fortunately, the thin-film measurement in the WVSS (also a Vaisala sensor) has
a significant advantage for the cirrus cloud case (high RH conditions at very
cold temperatures) because of the dynamically heated measurement environment of
the fast moving aircraft at flight level. This makes the actual measured
RH quite a bit lower than the ambient RH of say 90-100%--down in a range of
RH that is accurately measured by thin-film devices--even at very cold
temperatures. Appendix A shows that an ambient or static RH = 100% would be
measured in the duct as approximately 18% RH--this is in the range where
thin-film is accurate at such cold temperatures. For example, Table 2
shows that for T = -20øC, the accuracy in the range of 15% RH to 75% RH
was ñ 1%. Therefore, the WVSS will provide reasonably accurate RH
values near a wet tropopause. Another error will arise from the slower
response time of the sensor at cold temperatures. This error can be partially
corrected if the response time is known, but will still lead to some
uncertainty in rapidly changing conditions.
The above effect of the aircraft's dynamic heating is very positive for
measurements for climate research; on the other hand there is a down side for
very low RH (dry conditions) due to that same dynamic heating effect. We
believe that the lowest RH value that can be measured is 1-2% RH (the
sensitivity limit of thin-film). Thus, using the same example in Appendix A,
the lowest ambient RH that can be computed with accuracy is 11% RH. However,
the WVSS software will actually downlink the mixing ratio (r):
where e is water vapor pressure.
b. The WVSS Demonstration Program
The contract with Lockheed Martin has two deliverables and several options.
The first deliverable is a supplemental type certificate (STC) for the WVSS to
fly on all B-757 aircraft. This aircraft is modern, most carriers using it
have the latest avionics for user-friendly programmable ACARS applications, and
it makes frequent ascents and descents (the industry average is 4
ascent/descents per day which is about how the United Parcel Service (UPS)
B-757 fleet is used). UPS is the carrier who will be a part of the
certification process. The WVSS unit and its software are certified in ground
tests and a flight test.
The contract also calls for 6 WVSS units to be installed on UPS B-757
aircraft. These aircraft operate out of Louisville, Kentucky and fly to
destinations all over the United States. These aircraft will fly the WVSS for
2-3 months each in late 1996. This initial evaluation period is primarily for
a check of the maintainability specification only. Once the government is
satisfied that these units require no further modification, NOAA contract
(co-funded with the FAA) options for an additional 160 WVSS units will be
exercised. The additional units will be added to the UPS fleet and primarily
to other United States air carriers.
When sufficient numbers of WVSS-equipped aircraft are flying, an official
2-year Demonstration Program will begin for the FAA. This program will
determine the impact of ascent/descent profiles of winds, temperatures, and
water vapor on aviation weather products and industry operations. A key use of
the data will be in the Integrated Terminal Weather System (ITWS) soon to be
deployed (cf. Evans and Ducot, 1995). The WVSS information will be a
significant contribution to the radars and automated surface systems in the air
terminal environment--thus contributing to reducing the frequency and severity
of flight delays and to improving the safety of air travel.
Another funder of the WVSS procurement is National Oceanic and Atmospheric
Administration's (NOAA) Office of Global Programs. Data from the WVSS fleet
will support more accurate water vapor flux information and will be used by
scientists involved in the Global Energy and Water Experiment (GEWEX)
Continental-scale International Project (GCIP). Data will be provided
immediately to this project and the full WVSS fleet will fly until at least
until September, 2000 covering the last four years of GCIP. Water and energy
budget studies (with and without the WVSS information) will be performed. The
data will be available through the GCIP data management system.
c. International Expansion Plans
SITA's AIRCOM system (equivalent to ARINC's ACARS), although beginning about a
decade later, is experiencing the same growth (nearly a doubling of message
traffic over each of the last several years). However, the use of AIRCOM for
wind and temperatures is just beginning internationally. Since SITA has VHF
AIRCOM service contracted to 62 airlines in 115 countries and territories, the
potential for world-wide meteorological data via this system is enormous. On
the other hand, supporting all those VHF remote ground stations (RGSs) in all
those locations has led to communication costs per message on the order of 5 to
10 time the comparable costs of ACARS messages. This has inhibited growth for
weather data--but this will change. The transmission costs of the complete
ascent profile shown in Figure 2 is about 20 cents. Even at $2.00 per ascent
profile in Europe, this compares very favorably with the recurring costs of
rawinsondes in Europe which is about $300--though indeed the radiosonde profile
is more complete in detail and height than the aircraft ascent profile.
Average Differences [National Institute of Science and Technology reference - HUMICAP]
The expansion of aircraft profiles of winds, temperature, and water vapor
outside the United States is underway. Verbal agreements exist for the CASH
program to assist the ASDAR Consortium in certifying the WVSS on B-747 aircraft
via British Airways and to assist Meteo France in certifying the WVSS on Airbus
320 aircraft via Air Inter. Negotiations with Australia and Canada are
underway. The ADS development discussed in Section 2 and the observational
requirements of atmospheric science in all countries will slowly lead to
further international involvement.
5. The Commercial Aviation Atmospheric Measurement Program (CAAMP)
In order to meet the observational requirements of the spectrum of geophysical
science applications involving the atmosphere, oceans, and the land surface, a
composite observing system of satellite remote sensing, ground-based remote
sensing, and in-situ observing systems is required. For the atmosphere, the
commercial aircraft can play a leading role in such a composite system--filling
a major gap that exists between radiosondes and satellites. The radiosonde
system with sites separated in space by approximately 400 km and separated in
time by 12 hours gives information quite different than the satellite
information which has much higher resolution horizontally in space and in
time--but which only poorly represents the vertical structure of these
important atmospheric fields.
The commercial aircraft profiles, available at all hours from the airports of
the world, and the relatively high spatial density information provided by
individual enroute airline tracks offer a valuable blend of information to
complement the radiosonde and satellite information. One method of
continuously calibrating the satellite data in space and time (to reduce the
satellite error bars and to extract useful space/time gradient information from
the satellites) was described by Fleming and Hills (1994). A further step of
adding remote sensing from the ground to enhance the composite water vapor
information system was described by Fleming, et. al. (1995).
A new activity, the Commercial Aviation Atmospheric Measurement Program
(CAAMP), of the federal government will encourage government laboratories,
universities and industry to bring the next wave of technological improvements
to observing systems to the commercial aircraft. Efforts are underway to
identify new funds which would be distributed via grants from NOAA, and perhaps
other agencies. Only mature proven measurement concepts would be considered
for adaptation to commercial aircraft. A few examples which have already
surfaced are listed below. In each case, a critical requirement is finding
innovative ways for the sensor system to be self-calibrating or at least
maintain calibration over the length of the maintenance period discussed in
Section 3. The systems described below consist of both in-situ and remote
sensing systems and in all cases have been advanced in terms of
miniaturization, self-calibration, etc., with funding from other sources.
a. Further in-situ measurements
In order to close the loop on sources and sinks of various chemical species,
one needs an effective atmospheric monitoring effort that captures more of the
regional and vertical distribution of the concentration of these chemical
species. Tans, et al. (1996) have developed and automated flask
sampling package consisting of two suitcases. This could easily be adapted for
the package delivery service aircraft which have unused space, in the upper
deck of the B-747 as an example. Ozone profiles may be possible with a
miniature mercury lamp operating in a probe similar to that used for water
vapor. One only needs to package such a sensor along with other sensor devices
so that the carrier operational concerns which were discussed in Section 3 are
met. Two tests have been carried out on private aircraft to 17,000 feet.
Excellent agreement has been found between the sensor and an electrochemical
ozonesonde (Bognar and Birks, 1996, submitted for publication).
New methods for measuring atmospheric water vapor and other atmospheric gases
are possible using Beer's Law and various single mode laser diodes (Silver and
Hovde, 1995). Such measurements can be obtained with the same temperature
probe and "can" arrangement used in the current WVSS (see Figure 3). In fact,
such a sensor system could be retrofitted into the current WVSS configuration
as the next-generation WVSS. in Appendix B shows that such a single mode
laser
diode can easily fit within the space limits of the WVSS "can". Moreover, the
laser diode system would have an extremely fast response time making water
vapor measurements in the upper troposphere and lower stratosphere extremely
accurate.
Similar laser-diodes can be used today for methane (CH4) and, perhaps in the
near future, for nitrous oxide (N2O) and carbon dioxide (CO2) (see Hovde,
et.al., 1995). Future measurements of carbon monoxide (CO) and nitrogen
oxides (NOx) would require further diode laser development.
In-situ turbulence measurements derived from existing vertical acceleration
data on the aircraft can be made without adding any new hardware. Cornman,
et. al. (1995) have an algorithm that provides an atmospheric based
estimate of turbulence (independent of aircraft type) that utilizes information
which is already available on the aircraft. As the processing capability of
the aircraft avionics improves (this will occur in 1996) the information from
the algorithm can be down linked in real-time via ACARS. A demonstration is
expected in 1997.
b. Remote sensing from commercial aircraft
Remote sensing, with forward looking weather radar exists on commercial
aircraft. Other "forward looking" remote sensing technology is being developed
for a variety of applications; e.g. for winds, wind shear, turbulence, etc.
The foundation for the turbulence application goes as far back at 1967 using
unmanned air vehicles (UAVs); e.g., see Spillane and Radok (1971). However, in
this section the discussion is focussed on "downward looking" remote sensing.
Remote sensing, which produces vertical profiles of field quantities, is quite
commonplace on research aircraft. The move to adding this capability of
remote sensing of vertical profiles to commercial aircraft is not far away.
Passenger aircraft are the greater challenge because all those constraints
discussed in Section 3 apply. Thus, remote sensing systems must be made
smaller, have less weight, and have built-in processing power to perform the
necessary calculations in real-time.
The package-carrier aircraft are more flexible in many ways, and some services
may even offer space for equipment--this would be charged based upon actual
carrying weight (fuel costs), the addition of any necessary handling costs, and
the addition of a reasonable profit. Some package-carrier aircraft have
considerable room for such equipment (Arnold Oldach, personnel communication).
The price to the research scientist would be considerably lower than for
research aircraft time. Of course, the commercial aircraft cannot be directed
to a particular location. Some applications may not require this and the
aircraft do provide routine global coverage.
Automated water vapor and temperature profiles over ocean regions can be
obtained from commercial aircraft. The use of Differential Absorption Lidar
(DIAL) techniques for water vapor profiles has been proven on National
Aeronautic and Space Administration (NASA) research aircraft (Browell, et al.,
1991). Another technology is the use of high resolution interferometry for
temperature and water vapor profiles. The automation of the cooling mechanism
for a system providing continuous temperature and water vapor profiles in the
boundary layer (Smith, et al., 1993) has recently been accomplished (Hank
Revercomb, personal communication). There are no severe technical hurdles in
the way of applying both of the above systems to specific commercial aircraft.
Weight reduction, automation of calibration, intelligent cooling systems, and
sophisticated software with accompanying processing power are technically
achievable activities that can lead to such commercial systems. While there
has been a variety of temporary monitoring programs in the past, this
would be organized as an on-going commercial aviation business.
Calibration measurements for satellite-based aerosol and chemical species
monitoring systems are two obvious applications for commercial aircraft.
Optical depths and extinction profiles can be estimated from such simple
spaceborne backscatter measurements with the aid of supplemental information
about the aerosol extinction-to-backscatter ratio. A global climatology of
this critical parameter can be determined through measurements by a combination
of specially configured aircraft-borne and ground-based lidars (Reagan,
1995).
Another exciting possibility is further downsizing of the sensor packages
proposed for lidar-measured winds from satellites. Profiles of winds over the
oceans would be of extreme value to both weather and climate applications. The
United States Air Force has recently tested a Doppler lidar system that
produces real-time three dimensional wind profiles on a C-141 (Overbeck, et.
al. (1996). Housed under the aircraft frame, the use of a shutter would be
required on commercial aircraft to protect the fused-silica window from both
damage and from contamination (e.g., de-icing fluid on the runway during
takeoff and landing). Such a system would provide winds down to the ocean
surface when no clouds are present and down to the cloud level otherwise.
Today, only about 10% of the United States and Canada fleet of 4200 commercial
aircraft are providing real-time information on winds and temperatures. Yet,
this data has had a positive impact on meteorological analysis and prediction.
The United States is the only country in the world with a 3-hour
analysis/forecast cycle--primarily because of the ACARS data. We will soon
begin to see greater numbers of profiles of this information as the new formats
developed for the CASH program become used by more air carriers. By providing
these additional weather communication messages, the carriers help themselves
financially, improve efficiencies and safety for the flying public, and also
help improve many other government weather service functions.
The addition of water vapor information in aircraft atmospheric profiles will
lead to a five-fold increase (compared to radiosondes) in the amount of water
vapor profiles over the United States (counting only ascent from the 166
Demonstration Program aircraft) and an order of magnitude increase in water
vapor information (counting enroute and descent data). The use of the WVSS or
similar devices will spread to other parts of the world.
Expansion of weather observations using the commercial aircraft as the
platform will be encouraged as ADS becomes operational. World population
continues to grow, and more countries are developing to the point where a
greater percentage of their citizens can afford to fly. A McDonald-Douglas
study of a few years ago suggested that world-wide there will be 4700 aircraft
permanently retired and 14,000 new aircraft manufactured over the next 20
years. There will be a significant and needed expansion of weather
information from these additional aircraft. Will there ever be too much data?
Not for meteorologists, but at some terminals the need to reduce the volume of
reports will exist. This can be kept from becoming a problem by one of several
automated methods.
The uses of commercial aircraft for automatic turbulence reports, for the
measurements of chemicals, aerosols, and eventually for winds over the oceans
will all occur in due time. As we see these opportunities for the expanded use
of commercial aircraft, one must remember that the carriers are in business to
move people and/or packages. While the direct benefits of the new
instrumentation to the carriers and the international air traffic control
system may be quantifiable (e.g., of high value for turbulence, winds over the
oceans, etc., and of lesser value for other measured parameters), the carriers
should not be expected to automatically add these measurement systems--even if
all constraints are met and there is no impact on their operations. In some
cases, funding for their involvement will be required. In every application
mentioned in this paper, these costs will be well worth the price.
Acknowledgments. This project was supported by the FAA's Aviation
Weather Program and by NOAA's Office of Global Programs. Special thanks are
due to Tony Bennett (Teledyne) for his support in the 1970's, Mike Russo
(ARINC) and Arnold Oldach (UPS) for guiding us through the AEEC format approval
process, and to Randy Baker (UPS) for his support. Special thanks are also due
to Alan Hills and Bob Gall of NCAR for their early CASH testing work and
encouragement, respectively. Appreciation is extended to Barbara Magill for
typing the manuscript.
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FIGURE CAPTIONS
Figure 1: VHF Data Link Coverage (from SITA as of March, 1995--new remote
ground stations have been added).
Figure 2: Thin-film sensor response intercomparison.
Figure 3: Flight 7 Sensor Comparison, DLR Falcon, July 6, 1993, V=0.4 Mach,
Altitude = 10,000 feet (nominal)
Figure 4: Ascent format for winds, temperatures, and water vapor mixing ratio.
Data at each level is concatenated and sent as a single report (transmission)
via ACARS.
Figure 5: Drawing showing the position of the WVSS probe on a B-757. The probe
extends about 3 inches from the aircraft skin.
r = 0.62197 e/ (P-e)
This is because the mixing ratio is a conserved quantity and (with P and T
measured in the water vapor measurement zone) it is more accurate, as discussed
by Hills and Fleming (1994), to compute mixing ratio than to compute the
ambient RH from the measured RH. One avoids the extra step of calculating es,
static [highly subject to errors in Tstatic as seen in equation (2)] even
though Tstatic is available on the aircraft. The software logic will check for
measured RH ó 2%, calculate the mixing ratio, and set a quality
indicator. The quality indicator is a part of the ACARS water vapor format.
This low-end (dry) error is not so serious for climate research and can be
virtually eliminated with new sensors discussed in Section 5.
TABLE 1. Humidity Comparison (see text for details)
Temperature Humidity Level (% RH )
øC 15 30 45 60 75 98
25 -1.5 -0.8 0.5 1.5 2.5 3.7
5 -1.4 -1.2 -0.7 -0.2 0.2 0.1
-2 -1.4 -1.1 -0.7 -0.2 0.3 -0.4
-20 -1.1 -0.5 -0.7 0.1 0.6 1.9a
-40 -1.4 -0.5 0.3 1.4 2.3b not measured
(a) Humidity level 82% rh
(b) Humidity level 67% rh