Concept for Multiple Operations at Non-Tower Non-Radar airports during
instrument meteorological conditions

Victor A. Carreño, NASA Langley Research Center, Hampton, Virginia

# Abstract

A concept for multiple operations during Instrument Meteorological
Conditions (IMC) at non-tower, non-radar airports is described. The
objective is to provide an automated service which will support
separation assurance for aircraft operating in the airport airspace.
This type of service will enable the use of a large number of airfields
which currently have limited use in IMC. The service must be provided
with minimal infrastructure and at low cost.

The concept is based on a centralized automated airport management
module and distributed, on-board navigation tools. The airport
management module serves as an arbiter and sequencer. It receives
requests from aircraft via a data link and grants or denies access. The
airport management module also provides estimated times of arrival when
access is granted and an "expect further clearance" time when access is
denied. On-board avionics tools provide situational awareness and
generate advisories to be able to meet estimated times of arrival.

The concept is being developed such that operations into and out of the
non-tower, non-radar airport are compatible with the existing National
Airspace System (NAS). A system simulation has been developed based on
this concept. This paper describes the system functionality, system
requirements, and operations. Preliminary results of the system
simulation with various aircraft mixes, wind speed and directions, and
arrival rates are presented.

The work presented in this paper does not describe the SATS HVO concept
of operations \[1\]. It is a feasibility study and used to develop
methods for verification.

# 1. Introduction

Air transportation in the United States and the rest of the world is
mostly performed by commercial carriers. Since deregulation, commercial
transportation has evolved into a hub-and-spoke system. The
hub-and-spoke system has clear operational and economical advantages for
the airlines over a point-to-point system. However, the hub-and-spoke
system also has some significant drawbacks, which have become more
accentuated as air travel has increased in the last decade. Saturation
of hub airports with resulting delays, the need for connections when
traveling between medium or small cities, and the increase in overall
door-to-door travel time are some of the major concerns.

Although there are more than 5400 public use airports in the United
States \[2\] the majority of these airports are underutilized.
Underutilization is due to lack of commercial service, lack of
facilities, and a public perception that small airplanes are not a
viable mode of transportation. General aviation aircraft are mostly used
for recreation or for special purposes.

Airports without facilities such as Instrument Landing Systems, control
towers, radar coverage, etc. are limited in the number and types of
operations they can support. A person using such airports may not have a
high level of confidence that they will be able to arrive at their
destination and that they will arrive on time to meet their commitment.
The loss of productivity and inconvenience may preclude business and
other travelers from using these airports.

Providing air traffic services at airport which currently lack such
services will significantly expand the options for travelers in the
medium to small markets. Emerging and existing technologies such as
Automatic Dependent Surveillance-Broadcast (ADS-B) \[3\], Global
Positioning System (GPS) \[4\], Wide Area Augmentation System (WAAS)
\[5\] and others could provide the necessary technology and
infrastructure to develop air traffic services at reduced cost, as
compared to radar installations and other current methods of traffic
surveillance and navigation aids.

## Operations to non-tower non-radar airports

Airports without radar coverage rely on procedural separation during
Instrument Meteorological Conditions (IMC). IMC exists when conditions
are below weather minima specified for the class of airspace. Under this
condition, only instrument rated pilots can fly under instrument flight
rules. Procedural separation is used where surveillance data is not
available or in uncontrolled airspace. In non-radar airports, Air
Traffic Control (ATC) provides separation services to Instrument Flight
Rule (IFR) aircraft by a method of "one-in/one-out." That is, only one
IFR aircraft is given access to the airspace at a given time. Once
access is granted to an aircraft to enter the airspace, no other IFR
operation is permitted until the aircraft granted access departs the
airspace or closes its flight plan. As expected, this method results in
an underutilization of the airport facility.

In \[6\], Conway and Consiglio describe a system to automate the
one-in/one-out method of procedural separation. This system proposes a
special designation airspace around the airport, a ground based system
which automates the method for procedural separation, a data
communication link between the ground based automation and aircraft
intending to use the airport facility, protocols governing access to the
airspace, and procedures for transition between the in-route airspace
and the airport airspace.

Tobias and Scoggins \[7\] describe an automation system, based on
artificial intelligence, which attempts to replicate traditional ATC IFR
services. The system generates routes and vectoring to provide
sequencing and separation into and out of the airport airspace. VHF
radio communication is used to transmit synthesized voice commands to
aircraft.

In this paper, we describe a concept for multiple operations to
non-tower, non-radar airports during IMC. This concept does not automate
typical ATC services like in Tobias and Scoggins' work and does not
provide ground base aircraft vectoring. Rather, it is an automated
arbiter and sequencer that, together with state broadcasting and
enhanced traffic displays, allows the pilot to accept responsibility for
separation.

The concept is based on a special designation airspace called the Self
Controlled Area (SCA), a ground based automation system called the
Airport Management Module (AMM), data communication between the AMM and
the aircraft operating to and from the airport facility, aircraft data
broadcast of state information, and on-board navigation tools and
navigation displays with traffic information.

Section 2 describes the operations concept and discusses possible ways
to incorporate the specially designated airport facility into the
present National Air Space (NSA). Section 3 describes the simulation.
Section 4 gives simulation results of the concept implementation.
Section 5 is a summary and future work.

# 2. Concept of Operations

The system operates with four main components: 1. A Self Controlled Area
surrounding the airport facility where pilots accept responsibility for
separation; 2. An Airport Management Module which receives request from
aircraft to access the SCA and grants or denies access; 3. A data
communication link between aircraft and the AMM; 4. State data broadcast
from aircraft capable of operating in the SCA and associated on-board
navigation and traffic displays.

## The Self Controlled Area

The Self Controlled Area (SCA) is a three dimensional airspace
surrounding the airport facility. Depending on geography and location,
the shape of this volume could vary. For illustration purposes and for
the experimental results presented in this paper, we have selected a
cylinder of 12 nautical mile radius and 4000 feet height above ground
level, as shown in Figure 1.

![](media/image1.png){width="3.0215277777777776in"
height="1.6354166666666667in"}

Figure 1. Self Controlled Area

The design of the SCA includes a generic GPS standard instrument
approach procedure and a "T" structure as shown in figure 2.

![](media/image2.png){width="3.2in" height="3.423611111111111in"}

Figure 2. Basic "T" Design

For the "T" configuration, aircraft approaching the terminal area from
the base right shall proceed directly to the IAF_R when granted access;
approaches from base left shall proceed directly to the IAF_L;
approaches from the straight in area shall proceed directly to the IF.

It is important to note that aircraft entering the SCA accept
responsibility for separation. The system automation does not provide
vectoring and commands for separation.

## Airport Management Module (AMM)

The AMM is a centralized automated system which will typically reside in
the airport grounds. The AMM is an arbiter to the Self Controlled Area.
It receives requests from aircraft wanting to enter the SCA and grants
or denies access. It will only grant access to the SCA when the
requesting aircraft will be time separated with all other aircraft
already given access.

The AMM grants or denies entry into the SCA based on nominal approach
paths, aircraft performance calculations, and possible conflicts with
other aircraft already given access to the SCA. The AMM maintains a
database of aircraft types and performance characteristics for the
aircraft types.

To determine grant or denial of access, the SCA is divided into 6
regions, shown in Figure 3. The SCA is divided into regions to obtain
simplified access criteria, without overly constraining the airspace.

![](media/image3.png){width="3.196527777777778in"
height="3.526388888888889in"}

Figure 3. SCA Access Regions

The entry criteria are based on the operating region of the aircraft.
For example, an aircraft entering region 1 will have to be time
separated at the SCA boundary, IAF_R, IF, FAF, and RT, with any other
aircraft entering region 1. However, an aircraft entering region 1 will
only have to be time separated at the IF, FAF, and RT with an aircraft
entering region 5.

When the AMM grants access to the SCA, it generates Estimated Time of
Arrivals (ETAs) to the SCA boundary, all fixes, and the runway
threshold, which the pilot might use as guidelines for navigation. When
a request to enter the SCA is denied, the AMM produces an Expect Further
Clearance (EFC) time, which is an approximate time when the aircraft
will be given access.

## Data Communication Link

The data communication link is a two-way link, which allows aircraft to
send requests to the AMM and receive from the AMM request decisions,
ETAs and EFC information.

## State Data Broadcast

Aircraft operating or intending to operate in the SCA, broadcast their
state information. Aircraft also receive broadcasts from other aircraft
and this information is used for situational awareness, navigation and
conflict detection. Data broadcast could be based on ADS-B or other
aircraft data broadcast standard.

## Operations within the Air Traffic Control (ATC) system

The automated airport access system could be integrated within the ATC
system by means of a procedural transition between the ATC controlled
airspace and the automated airport. In addition to the SCA, the system
requires designated holding fixes outside of the SCA. Figure 4
illustrates a possible geometry for such a system.

![](media/image4.png){width="3.1944444444444446in"
height="2.5277777777777777in"}

Figure 4. SCA Integrated for Operations with ATC

The following example is the typical steps a pilot will follow when
operating from a non-automated airport (KNWX) to an automated airport
(KAYZ).

1\. Pilot files a flight plan to fly from KNWX to KAYZ using holding fix
BAMBY.

2\. ATC controls aircraft departure.

3\. ATC vectors aircraft to KAYZ and clears aircraft to BAMBY when fix
is not occupied.

4\. As aircraft approaches BAMBY (within r nautical miles), it requests
to the AMM entry into the SCA.

5a. If access is immediately granted to the SCA, the pilot notifies ATC
that he is leaving the BAMBY holding fix and proceeding to the SCA. ATC
terminates services and the pilot accepts responsibility for separation.
BAMBY holding fix becomes unoccupied and ATC can clear other aircraft to
this fix.

5b. If access is denied to the SCA, the aircraft enters the holding
pattern at BAMBY holding fix. The fix will remain occupied until the
aircraft notifies ATC that it is proceeding to the SCA. It is also
possible for ATC to clear more than one aircraft to a holding fix at
different altitudes. For example, clear to BAMBY at 3000 and clear to
BAMBY at 4000.

## Operations in a free-flight environment.

In a free-flight environment, flight crews are responsible for flight
separation in the National Air Space \[8\]. Transition between the
free-flight airspace and the SCA is as follows:

1\. Aircraft approaches automated airport.

2\. Aircraft requests to the AMM entry into the SCA.

3a. If access is granted to the SCA, the aircraft proceeds directly to
designated initial fix.

3b. If access is denied, aircraft must remain outside the SCA and
maintain separation from other aircraft in the vicinity.

# 3. Simulation

A simulation has been developed to evaluate the feasibility, efficiency,
and performance of the concept. It simulates operations where the SCA is
within a free-flight environment. It is a batch simulation (no pilots
are involved) and implements the AMM decision logic, an onboard
navigation advisor which gives heading, altitude, and speed advisories,
together with models of aircraft dynamics (including winds), and models
of pilots. Figure 5 depicts the main blocks of the simulation.

![](media/image5.png){width="3.196527777777778in"
height="3.013888888888889in"}

Figure 5. Simulation Block Diagram

The simulation is written in Java® and each aircraft is an instantiation
of an object for a given aircraft type. In this experiment, only two
types of aircraft have been implemented: a Cessna 172 and a LearJet 45.
These aircraft are meant to be representative of slow piston aircraft
and fast jet or turbo prop aircraft. Hence forth in this paper, piston
aircraft refers to slow aircraft and jet to fast aircraft. The speed
profiles used for these aircraft are shown in Figure 6. The speed
profiles show 3 speeds: The initial speed, nominal arrival speed (nomA),
and nominal final speed (nomF). The initial speed is the calibrated air
speed (CAS) of the aircraft when it requests entry into the SCA. It is
assumed that the initial speed is always equal to or greater than nomA.

The speed profiles are used by the AMM and the onboard navigation
advisor. The AMM uses the speed profile, together with path and winds,
to calculate ETAs to boundaries and fixes. The onboard navigation
advisor uses the speed profile to generate speed guidance to meet ETAs.

![](media/image6.png){width="3.1993055555555556in"
height="2.8118055555555554in"}

Figure 6. Speed Profiles

## AMM operation

The AMM receives requests to enter the SCA and grants or denies access.
The system is based on a first-come-first-serve method of arrival. To
implement access control, the AMM maintains two aircraft lists: An
access request list, and an access granted list. When the AMM receives a
new request, it puts the request in the request list. Based on the type
of the aircraft, the nominal estimated time of arrivals (ETAs) are
calculated. These ETAs are to the SCA boundary, initial arrival fix
(IAF), intermediate fix (IF), final arrival fix (FAF), and runway
threshold (RT). The AMM then compares the ETAs of the requesting
aircraft with the ETAs of all aircraft in the access granted list. The
requesting aircraft ETAs must maintain a T~s~ time separation with the
ETAs of all other aircraft in the access list and the time separation
must be such that overtaking cannot occur in the SCA. Access will be
granted when, for each aircraft already granted access to the SCA, the
requesting aircraft will be ahead by T~s~ on all relevant points
(fixes), or behind by T~s~ on all relevant points. T~s~ is the required
time separation. In mathematical terms, access is granted when the
following predicate holds true:

$$\begin{array}{r}
\forall j:((\forall\text{fix}:\text{ETA}_{\text{fix}}^{\text{req}} - \text{ETA}_{\text{fix}}^{j} \geq T_{s})\text{OR} \\
(\forall\text{fix}:\text{ETA}_{\text{fix}}^{\text{req}} - \text{ETA}_{\text{fix}}^{j} \leq - T_{s}))
\end{array}$$

where *j* is the index of aircraft in the access list, *req* is the
requesting aircraft and *fix* are all the points where time separation
is to be maintained.

When access is granted, the requesting aircraft is removed from the
request list and put into the access granted list. The AMM broadcasts a
request granted message, which includes initial arrival fix assignment
and ETAs to SCA boundary and fixes. The aircraft is expected to proceed
directly to the assigned initial arrival fix as described in GPS
procedures for T approaches, Federal Aviation Regulations/Aeronautical
Information Manual (FAR/AIM) \[9\].

When access is denied, the requesting aircraft is kept in the request
list and an expect further clearance (EFC) time is calculated and
broadcast to the requesting aircraft. The aircraft is expected to remain
outside of the SCA.

The AMM calculates the ETAs in the following manner:

-   It receives, from aircraft transmissions, the aircraft heading and
    location.

-   It receives the aircraft ground speed or calculates ground speed
    from consecutive locations.

-   From ground speed, heading, and prevailing winds, it calculates the
    aircraft true air speed (TAS).

-   Using the calculated TAS, the AMM calculates the aircraft ground
    speed should the aircraft were to turn to the IAF. This speed is the
    initial speed.

-   The nominal path is calculated.

-   Using the speed profile for the type of aircraft and the nominal
    path, the AMM generates ETAs to the SCA boundary, all fixes, and
    runway threshold.

## On-Board Navigation Advisor

The on-board navigation advisor module generates heading, altitude, and
speed guidance to navigate the nominal arrival path. When the aircraft
has been cleared to the SCA, the navigation module uses the published
approach procedure, geographical data and assigned IAF to generate the
guidance. The heading, altitude, and speed guidance will follow a
nominal speed profile.

The navigation module will give guidance to enter into a holding pattern
when access to the SCA has been denied.

## Pilot Model

The pilot module is a low fidelity model of a pilot. It receives current
state information from the aircraft dynamics module. It also receives
heading, altitude, and speed guidance from the navigation module. With
this information, the pilot model makes changes to the aircraft bank
angle, horizontal speed and vertical speed to conform to the nominal
path. The pilot model introduces a level of uncertainty to the
simulation. The pilot model has a reaction delay time to the guidance
and it does not perform changes when the difference between the current
state and the guidance is bellow a set threshold. Pilot uncertainty
produces aircraft paths that are off the nominal path within error
bounds.

## Generation of Aircraft and Arrival Rate

Arrival rates can be selected at the beginning of the simulation. To
make the simulation more realistic, aircraft are not generated in the
vicinity of the target airport. In the simulation, aircraft \"take off\"
from airports 30 to 400 miles away from the target (destination) airport
and fly towards the target airport. The distance and origination of the
aircraft are random which produces a quasi-random arrival rate.

For example, at a 10 aircraft per hour arrival rate, 15 aircraft could
arrive in a 15 minute interval and 5 aircraft in the next 1 hour and 45
minutes interval. The arrival rate selected is the *average* arrival
rate over the simulation duration.

# 4. Results

The results presented in this section are for an automated airport
within a free flight environment. Aircraft request entry into the SCA
approaching the airport from any direction outside the SCA periphery.
Time separation is set at 3 minutes and pilots request entry when at 25
nautical miles from the center of the SCA and at or below 8 thousand
feet AGL (Above Ground Level). Runway orientation is north to south with
aircraft landing to the south (runway 18).

Results show average and maximum delay as a function of various aircraft
mixes, arrival rates and wind conditions. Delay is the difference
between the time when an aircraft request entry into the SCA and the
time when access is granted. Results presented in this section are for a
48 hour period of operation. The first three graphs, figures 7, 8 and 9,
are average and maximum delay as a function of average arrival rate for
100/0, 50/50, and 0/100 percent piston/jet ratios, respectively. The
vertical axis for these three graphs is on a logarithmic scale
(hours:min:sec) which shows in all cases an exponential and possibly
double exponential growth in delay as a function of arrival rate.

![](media/image7.png){width="3.1972222222222224in"
height="2.245833333333333in"}

Figure 7. Delay vs. Landing Rate, 100% Piston, 20 Knot Wind out of the
South

![](media/image8.png){width="3.1972222222222224in" height="2.26875in"}

Figure 8. Delay vs. Landing Rate, 50% Piston 50% Jet, 20 Knot Wind out
of the South

![](media/image9.png){width="3.1972222222222224in" height="2.26875in"}

Figure 9. Delay vs. Arrival Rate, 100% Jet, 20 Knot Wind out of the
South

For 100% piston and 100% jet, an average arrival rate of 15 per hour
produces average delays of approximately 4 minutes. This figure is
comparable to the average delay rate for commercial operations in Europe
(4 minutes) \[10\] and below the average delay rate for commercial
operations in the U.S. (9 minutes) \[11\].

Further increase in arrival rate towards the theoretical maximum of 20
per hour (for 3 minutes time separation) produces delays which are
judged to be unreasonable. In the case of a 50 percent piston/jet mix,
the jet class suffers a disproportioned number of delays and delays are
judged to be too high even at rates of 12 arrivals per hour. A possible
solution to the excessive delays for fast aircraft is to base the
request distance on the type of aircraft. Faster aircraft can request
entry into the SCA farther away than slower aircraft. In essence, a
request based on time-to-go rather than on distance-to-go.

Figure 10 shows delays as a function of aircraft mix for an average
arrival rate of 10 per hour. For jet aircraft, average and maximum
delays increase for any piston/jet mix over an all jet scenario. For
piston aircraft, average and maximum delays are reduced for a 10/90
piston/jet mix over an all piston scenario. In general, piston aircraft
delays are not significantly affected by the inclusion of jet aircraft.
Jet aircraft delays, however, are detrimentally affected to a high
degree by the inclusion of slow aircraft operations. Again, taking into
account the aircraft performance in the request procedure could
alleviate this problem.

![](media/image10.png){width="3.1958333333333333in"
height="2.2944444444444443in"}

Figure 10. Delay vs. Aircraft Mix, 10 Landings/Hour Average, 20 Knot
Wind out of the South

Figures 11 and 12 show the impact of winds on system performance. Winds
from 0 up to 20 knots do not affect the system delays. 30 and 35 knot
winds produce a small increase on average delays for both piston and jet
aircraft and higher maximum delays.

![](media/image11.png){width="3.1972222222222224in" height="2.26875in"}

Figure 11. Delay vs. Wind Speed, Wind out of the South, 10 Landings/Hour
Average

Wind direction (Figure 12) has a small impact on average system
performance. However, the data shows that a 20 knot cross wind caused a
jet aircraft to have a delay of 38 minutes and 4 seconds and a piston
aircraft to have a delay of 33 minutes and 6 seconds (wind out of the
East, 20 knots). Wind out of the west, also a cross wind, resulted in a
piston aircraft having a delay of 27 minutes and 23 seconds.

![](media/image12.png){width="3.1958333333333333in"
height="2.2805555555555554in"}

Figure 12. Delay vs. Wind Direction, 20 knot wind, 10 Landings/Hour
Average

It can be argued that wind has a bigger impact on piston (slower)
aircraft and the observed maximum delay on the jet aircraft is a result
of the jet aircraft queuing behind the slower aircraft.

# 5. Summary

A concept for automated access services to non-tower, non-radar airports
during IMC was presented. To investigate the feasibility of the concept,
a batch simulation has been developed.

Simulation shows that time separation with implicit sequencing is an
effective way to provide automated traffic management in the terminal
area. Mixed operations, where fast aircraft and slow aircraft are
sharing approach paths, produced higher average and maximum delays than
operations with a single type of aircraft.

Access requests based on distance to airport penalizes fast aircrafts. A
method based on time to airport will probably result in a more equitable
system.

In mixed operations, 10 aircraft per hour average landing rate appears
to be the upper acceptable limit. For fast aircraft using the facility,
arrival rates of up to 18 aircraft per hour average landing rate
produces acceptable results. This is close to the theoretical maximum of
20 aircraft per hour average landing rate.

Preliminary simulations with landing and take off operations show
improvement in delays when total operation rates are compared to landing
rates. However, this depends on methods used to separate arriving from
departing flights. For example, if departing flights can be vertically
separated from arriving flights on regions 1 and 6, then reduced delays
can be achieved for constant operation rates. Dedicated departure
corridors also produce reduced overall delays and higher throughputs.
These considerations are dependent on geography and airspace design.

In conjunction to the simulation work, a verification using formal
mathematical techniques \[12\] is being conducted to show that the
concept is safe. This is accomplished by mathematically showing that
time separation at fixes and the method to grant or deny entry into the
SCA guarantees geometrical separation in the airspace.

The concept presented in this paper was developed to show feasibility
and to explore methods for verification. It is not the concept of
operation being developed for the SATS HVO \[1\] program. The concept
presented in this paper is expected to evolve and change as constraints
are better defined and more results from simulation and experiments are
obtained.

# References

\[1\] Small Aircraft Transportation System (SATS) High Volume Operation
(HVO) Operational Concept: Nominal Operations, NASA Langley Research
Center, June 2003.

\[2\] U.S Bureau of Transportation Statistics.

\[3\] Minimum Operational Performance Standards for 1090 MHz Automatic
Dependent Surveillance Broadcast (ADS-B), RTCA Paper No.
145-00/SC186-152, RTCA, May 12, 2000.

\[4\] Global Positioning System Standard, Positioning Service
Performance Standard, Assistant Secretary of Defense for Command,
Control, Communication and Intelligence, October, 2001.

\[5\] Wide Area Augmentation System, Executive Summary, Independent
Review Board, Federal Aviation Administration, January 2001.

\[6\] Conway, S.R. and Consiglio, M.; A Method of Separation Assurance
for Instrument Flight Procedures at Non-Radar Airports. AIAA Guidance,
Navigation, and Control Conference & Exhibit, AIAA2002-4448, Aug 2002.

\[7\] Tobias, L. and Scoggins, J.L.; Time-Based Air Traffic Management
Using Expert Systems, NASA TM-88234, Ames Research Center, April 1986.

\[8\] RTCA, 1995, Final Report of the RTCA board of directors' select
committee on free flight, Washington D.C.

\[9\] Federal Aviation Regulations /Aeronautical Information Manual,
Jeppesen Sanderson Inc. 1999.

\[10\] Delays to Air Transport in Europe, EUROCONTROL/ECAC, Central
Office for Delay Analysis, October, 1999.

\[11\] Barnhart, Cynthia and Bratu Stephan, National Trends in Airline
Flight Delays and Cancellations, and the Impact on Passengers,
presentation, March 2001.

\[12\] Butler, R.W., et. al.; NASA Langley's Research and Technology
Transfer Program in Formal Methods. Available from
http://shemesh.larc.nasa.gov/fm/fm-pubs-larc.html