![](media/image1.jpeg){width="1.625in"
height="1.2680555555555555in"}Karl Winner

Sr. Systems Engineer

Advanced Navigation and

Positioning Corporation

Hood River OR USA

e-mail: kwinner@anpc.com

**Application of the Transponder Landing System**

**To Achieve Airport Accessibility**

[]{.mark}

# Abstract

The Transponder Landing System (TLS) is a precision approach landing
system designed for use at airports where rough terrain or real estate
constraints make Instrument Landing System (ILS) installation
cost-prohibitive. All aircraft that are equipped to fly an ILS and are
equipped with a transponder can also use TLS. TLS determines the
aircraft's position in space from signals emitted by the aircraft's
transponder, using time and angle measurements at ground-based sensors.
ILS-like localizer and glide slope corrections are computed to guide the
aircraft to the desired course. The pilot can fly a precision approach
to Category I minimum decision heights, just like flying an ILS. TLS is
inherently easier to site than ILS and therefore can be less costly to
install and to successfully flight inspect. TLS can provide Category I
signals-in-space without extensive ground conditioning.

# Background

ILS glide slope equipment can be challenging to site at airports that
are located near rough terrain. Lateral ground plane requirements are
sometimes costly to satisfy where antenna tower offset criteria are
applied to comply with Obstacle Free Zone surfaces. At some airports,
upslope of terrain below the proposed approach path precludes the use of
ILS glide slope equipment without extensive earth removal to reduce
multipath.

ILS localizer performance can be diminished by multipath from large
buildings located on the airport property itself. Restrictions to
aircraft and vehicular movement can cause significantly reduced airport
throughput when critical area protective measures are in effect.

What is needed is a new precision landing system that can measure the
aircraft location and compensate for the effects of ground-based
multipath. Critical areas associated with ground sensors and
transmitters should be designed to have minimal operational impact.

# Introduction

From a pilot's perspective, a TLS approach is similar to an ILS
approach, except that transponder operation with an IFR transponder code
is required. The aircraft must be equipped with an ILS localizer and
glide slope receiver, a localizer and glide slope Horizontal Situation
Indicator (HSI) or Course Deviation Indicator (CDI), and a Mode 3/A
compatible transponder. The ground-based TLS determines the location of
the aircraft by interrogating the aircraft transponder and then
measuring the transponder Time-Of-Arrival (TOA), azimuth angle and
elevation angle with two Angle-of-Arrival (AOA) sensors. Once the
location of the aircraft is determined, the amount of localizer or glide
slope correction is determined to guide the aircraft back to the desired
course. This guidance information is transmitted throughout the TLS
service volume to the tracked aircraft using the VHF localizer and UHF
glide slope signals modulated with 90 and 150 Hz tones. The pilot can
fly a TLS precision approach to Category I minimum decision heights, as
supported by TERPS and approach lighting.

All TLS glide slope and localizer signal parameters comply with ICAO
Annex 10^\[1\]^ standards for Category I ILS, including service volume,
displacement sensitivity, clearance, alignment, modulation, frequency
and crossing height.

ILS frequency engineering principles also apply to TLS.

Flight inspection of TLS is very similar to flight inspection of ILS
with some differences described in *FAA Order 8200.40 Flight Inspection
of the Transponder Landing System*.

# TLS components & subsystems

## Interrogator Antenna

TLS mode 3/A interrogation coverage has been designed to trigger
transponder replies within the standard service volume. Side Lobe
Suppression (SLS) is transmitted to suppress transponder replies outside
the coverage volume. TLS interrogator emissions exhibit low pulse
repetition frequency and low transmit amplitude to yield virtually no
impact on the operation of nearby Secondary Surveillance Radars (SSR) or
Aircraft Collision Avoidance Systems (ACAS)^2^. The interrogator
antennas are shown in Figure 1.

![](media/image2.jpeg){width="2.74375in" height="2.09375in"}

Figure 1: Interrogation and SLS antennas are shown; each antenna is
vertically polarized with 6 driven, radiating elements. Main lobe peak
is +3 degrees above the horizon. The assembly is 2 m tall.

## Elevation Measurement Subsystem

The transponder reply time-of-arrival (TOA) and elevation
angle-of-arrival (AOA) is measured using an interferometer array of four
receiving antenna. By designating the lowest antenna as a Reference
signal and applying differential carrier phase measurement techniques to
the received signal at each of the other three antennas (each
differenced with the reference signal), three measurements of the angle
of arrival can be computed.

Two or more transponder signal wavefronts arriving at an antenna will
sum vectorially based on their phase and amplitude with a resultant
phase $f_{R}$ as shown in equation 1.

> $f_{R} = \text{tan}^{- 1}\left( \frac{a_{1}\text{sin}(f_{1}) + a_{2}\text{sin}(f_{2})}{a_{1}\text{cos}(f_{1}) + a_{2}\text{cos}(f_{2})} \right)$
> eq. 1
>
> where:
>
> $a_{1}$,$f_{1}$ are the amplitude and phase of the direct path signal.
>
> $a_{2}$,$f_{2}$ are the amplitude and phase respectively of the
> multipath signal.

The most straightforward method to achieve an accurate measurement of
quantity$f_{1}$ is to reduce the amplitude $a_{2}$ of the multipath
signal. To reduce the amplitude of the ground-based multipath signal,
the antennas used for the TLS elevation array have been selected to have
a steep gain from --3 degrees to +4 degrees about the horizon, as shown
in Figure 3. The steep slope significantly reduces the amplitude of
reflections below the horizon relative to those above the horizon.
Aircraft on final approach will generally be around 3° above the horizon
and the most significant multipath returns from an ideal (flat) ground
plane surface will have a reflection angle of approximately -3°.
Referring to Figure 2, this 6° separation centered about the horizon
provides about 12.5 dB suppression of the multipath from the direct path
signal.

![](media/image3.wmf){width="2.88125in" height="2.3402777777777777in"}

Figure 2: Elevation antenna vertical pattern. Each antenna element is a
vertical array of 6 wavelengths in height to reduce the amplitude of
ground-based multipath.

![](media/image4.png){width="2.879861111111111in"
height="3.7333333333333334in"}

Figure 3: The elevation measurement assembly is shown with 4 antennas,
each one vertically polarized with 10 receive elements. Main lobe peak
is 7 degrees above the horizon. The assembly is 9 m tall.

The elevation AOA subsystem installed at Pullman, Washington USA is
shown in Figure 3. This view is from the front of the array with the
runway to the right side of the Picture. Since each of the three
measurement antennas is spaced progressively further from the Reference
antenna, associated measurement data becomes increasingly more accurate
(better in resolution) and has diminished cycle size. As a result of the
inherent resolution associated with each phase center aperture, the
antennas have been designated Low, Medium and High, in reference to the
resolution of the AOA measurement associated with each antenna.

Figure 4 illustrates a possible ground profile from which the following
reflection producing surfaces can be identified:

-   A flat ground plane extending out from the array. A common variation
    of this profile includes a lateral tilt immediately in front of the
    AOA array.

-   A valley and hill, significant enough in size that there is a large
    surface area inclined toward and visible to the top antenna.

Referring to Figure 4 and the two types of surface profiles indicated,
Pullman Washington airport has lateral slope directly in front of the
elevation measurement antenna with hills rising to 1.6 degrees under the
approach path. Data collected at Pullman is shown in Figure 5 for the
High-resolution AOA measurement (blue -- with 2 cycle wraps) and a
single TOA (black -- steadily decreasing from 8.7nm to 2.6nm). This data
is compiled from a single level approach with the aircraft on course
centerline at constant altitude (ILS-2 profile). The AOA measurement has
low and high frequency components.

![](media/image5.wmf){width="5.774305555555555in"
height="2.7583333333333333in"}

Figure 4: Multiple ground planes with respect to the elevation
measurement antenna.

![](media/image6.png){width="5.727777777777778in"
height="3.1326388888888888in"}

Figure 5: TOA and AOA data from the elevation measurement sensor at
Pullman ,WA USA. A high frequency ripple is present on the AOA
measurement (blue). AOA measurement cycle wrap occurs near interrogation
time 13455.

The frequency of the error is a direct consequence of the antenna
geometry and in particular the height of the antennas above the
reflection point. Slowly varying error can be easily accounted for with
a simple multipath compensation data file and algorithm. A description
of this processing method is discussed.

**Multipath Compensation Data File.** The antennas cannot completely
eliminate all multipath signals so there will be a remaining low
frequency phase error at each antenna characterized by reflections from
ground surfaces. Since the AOA Sensor measures the differential phase
between the two antennas, the resulting error in the measurements is the
differential phase error between the two antennas. Any errors common to
both antennas will be canceled due to the difference measurement
process.

Multipath Compensation processing of the AOA measurements further
reduces the effects of multipath to provide a smooth glide slope signal.
After the system is installed, a Multipath Compensation data file is
generated by gathering AOA data correlated to theodolite data. A
theodolite is placed near the runway such that the theodolite eyepiece
coincides with the geometric glide path. Data is gathered from the
theodolite with the system operating and tracking an aircraft. The
aircraft conducts several level crossings established near the approach
course centerline and at the outer marker intercept altitude, typically
near 1600 feet above ground for a 3.0 degree glide path. The level
crossing provides data as shown in Figure 5, and the comparison with the
theodolite reference is displayed in Figure 6. The TLS Multipath
Compensation process provides a site-specific look-up table containing
corrections to apply to the AOA measurements. During system operation,
the measurement processing algorithms apply the look-up table to the AOA
measurements to ensure that the actual Path Angle (PA) and Threshold
Crossing Height (TCH) correspond to the desired PA and TCH.

Additional theodolite data collection with an aircraft established near
the glide path can be used to establish multipath dependencies in range
and azimuth. A data file can then be generated that contains
compensation data as a function of range and azimuth in addition to
elevation.

The TLS multipath compensation processing directly results in
symmetrical glide slope displacement sensitivity (50%+/-2% at all sites)
and reduced glide slope structure.

![](media/image7.png){width="5.647916666666666in" height="3.11875in"}

Figure 6 : Measured elevation in degrees (abscissa) vs error in counts
(ordinate). Divide counts by 100 to get degrees (approximately). Error
established by computing difference between theodolite estimate of
aircraft elevation and High-resolution measurement. The least squares
best fit (blue line) through data represents the compensation data file.

**Multipath Compensation by Antenna Placement.** At sites where
up-sloping terrain exists under the approach path, there will be a small
angular separation between the direct transponder reply and multipath.
At The Dalles, WA USA, the angular separation is small enough that high
frequency differential phase error will be superimposed on the low
frequency error as shown in Figure 7. The low frequency error can be
modeled and compensated by a simple data file and algorithm, however it
is not practical to do so with the high frequency error.

![](media/image8.wmf){width="2.939583333333333in" height="2.55in"}

Figure 7: Differential Phase Error for High Resolution measurement; TLS
facility at The Dalles, WA USA.

An effective mitigation for the high frequency error is to add a second
antenna; picking a location such that its phase error will be opposite
in sign and similar in magnitude to the error of the High-resolution
measurement. An antenna has been added to the TLS for this purpose.
Because it is mounted below the High-resolution antenna, resulting in
less angle-of-arrival resolution, it is referred to as the
Medium-resolution antenna. Figure 8 is a plot of the Medium-resolution
measurement differential phase error at The Dalles, WA USA.

![](media/image9.wmf){width="2.9631944444444445in"
height="2.4541666666666666in"}

Figure 8: Differential Phase Error for Medium Resolution measurement;
TLS facility at The Dalles, WA USA.

When compared to the High-resolution phase error in Figure 7, The Medium
measurement is seen to be significantly out of phase and therefore
largely able to cancel these errors. The combined signal is shown in
Figure 9 where it is apparent that much of the high frequency error has
been removed from the two signals. The remaining high frequency error is
time varying due to its dependence on the aircraft position and it is
treated essentially as noise in the TLS Kalman filter algorithm.

![](media/image10.wmf){width="3.004166666666667in"
height="2.6118055555555557in"}

Figure 9: Combined Differential Error for High and Medium Resolution
measurements at The Dalles, WA USA. The high frequency differential
error present in both the individual measurements has been eliminated in
the combined data.

**Aircraft Elevation Track Estimate.** The initial elevation estimate is
computed from the Low resolution AOA measurement. This elevation
estimate will be enhanced by the greater accuracy of the other two
measurements from the Medium and High-resolution antennas. Again
referring to Figure 3, a typical installation is shown which also
includes a Medium resolution antenna just below the uppermost
High-resolution antenna. The cycle ambiguity of the Medium and
High-resolution measurements must be resolved.

The algorithm that resolves the cycle ambiguity is based on finding the
minimum residual (minimum difference) between the Low-resolution
angle-of-arrival estimate and the Medium-resolution estimate, factoring
in the integer cycle k. There are multiple potential elevation solutions
for the Medium-resolution measurement but the minimum residual between
the Low and any Medium resolution aoa estimate within ±½ of a
Medium-resolution cycle will result in the correct cycle for the Medium
resolution. The actual implementation of this algorithm works in the
space $Dl$as defined by equation 2.

> $\text{find }k|k\lambda + Dl < \frac{1}{2}h_{c}$eq. (2)
>
> where: $h_{c}$ is one Medium-resolution cycle size in meters.

In order to filter noise and provide a robust glide slope estimate, the
Low-resolution and Medium-resolution systems are independently tracked
using a linear kalman filter with the state defined as
$\overline{x} = \begin{bmatrix}
Dl & \overset{˙}{D}l
\end{bmatrix}^{T}$ where$\Delta l$is defined in equation 2. Once the
Medium cycle is initialized, the same cycle ambiguity resolution is
applied to select a High-resolution measurement. By using linear
processing of the measurement tracks, there is significantly more stable
track behavior.

## Azimuth Measurement Subsystem

The transponder reply time-of-arrival (TOA) and azimuth angle-of-arrival
(AOA) is measured using an interferometer array of three receiving
antenna. Using the same AOA measurement techniques described above for
the elevation subsystem, an azimuth angle-of arrival is measured by the
subsystem shown in Figure 10. The right most pictured antenna as
designated the Reference, the middle antenna designated the Low, and the
remaining antenna designated the High. The AOA sensor is visible behind
and between the Low and High antennas.

![](media/image11.png){width="2.9118055555555555in"
height="2.183333333333333in"}

Figure 10: The azimuth measurement assembly is shown with three
antennas. Each antenna is vertically polarized with 6 receive elements.
Main lobe peak is +3 degrees above the horizon. The assembly is 2 m
tall, 105 Kgs (230 lbs) and mounted on frangible couplings.

Ground-based multipath has little effect on the azimuth tracking
accuracy because the antenna phase centers are all the same height above
the reflection plane. Although reflections from vertical surfaces of
buildings are attenuated by the 3dB beam width of 30 degrees, the
primary mitigation of this source of multipath is provided by siting
this antenna array near the GPI such that vertical reflection sources
are kept at the horizontal edge of the antenna pattern.

**Aircraft Azimuth Track Estimate.** The azimuth position of the
aircraft is first established using Differential Time Of Arrival (DTOA)
to determine the aircraft position on a hyperbolic arc. This position
can then be used to resolve the cycle ambiguity on the Low-resolution
AOA measurement and establish a Kalman filter estimate (track) of
aircraft azimuth. The Low-resolution track is then used to resolve the
High-resolution measurement cycle ambiguity and establish a Kalman
filter track based on the High-resolution measurement.

## Aircraft Guidance Uplink 

After computing an estimate of the aircraft position in azimuth,
elevation and range, RF guidance is generated to guide the aircraft to
the desired approach. A three-dimensional track defined as
$\overline{x} = \begin{bmatrix}
x & y & z & \overset{˙}{x} & \overset{˙}{y} & \overset{˙}{z}
\end{bmatrix}^{T}$, where x and y are developed from the TOA measurement
and azimuth AOA, and altitude z is developed from the elevation AOA
measurement. Given this altitude, the three-dimensional track is updated
with a kalman filter resulting in an accurate estimate of the total
track state. Using the method of computing a three-dimensional track
state, the array can be placed at various locations with respect to the
aircraft touchdown point.

A composite field pattern that is amplitude modulated by a 90 Hz and a
150 Hz tone is generated that emulates ILS guidance at the tracked
aircraft position. This composite modulation signal is input to the
localizer and glide slope RF transmitters where it is used to modulate
the carrier at the appropriate frequency. Since both the VHF and UHF
guidance are composite signals, the Difference Depth Modulation (DDM) is
immune to the degrading effects of multipath, although signal fade can
occur in response to site-specific terrain masking. The uplink antenna
locations are shown in Figure 11. The base station shelter, also shown
within Figure 11, contains the interrogation transmitter, guidance
transmitter and CPU's.

![](media/image12.png){width="2.7777777777777777in" height="3.28125in"}

Figure 11: Uplink antenna tower with VHF localizer and UHF glide slope
antennas. The tower is 9 m tall. Climate controlled base station shelter
houses transmitters and CPU's.

## Monitoring Critical Parameters

The Integrity Monitoring (IM) has been designed in accordance with
system safety principles and airworthiness requirements within SAE ARP
4754^3,4^ that are also applied to airborne navigation systems.
Potential hazards resulting from Hardware/Software or System failure
modes have been eliminated or mitigated by implementing self-test and
redundant systems. The TLS software has achieved Level B design
assurance in accordance with RTCA DO-178B^5^.

The integrity level for TLS has been established as meeting or exceeding
the level 2 integrity value specified in table C-2 of ICAO Annex 10. The
IM examines variations in the ground equipment that could adversely
affect the accuracy and availability of guidance to a landing aircraft.
The IM has been designed such that guidance to the aircraft will cease
in the event of degradation to the ground equipment or if the monitor
itself should fail. The Cal/BIT (Calibration / Built In Test) component
of the IM is shown in Figure 12. This assembly is installed in front of
the azimuth and elevation measurement antenna arrays and transmits a
test signal that can be used to evaluate the accuracy of the TOA and AOA
subsystems.

![](media/image13.jpeg){width="2.879861111111111in"
height="2.1527777777777777in"}

Figure 12: The Calibration / Built In Test transmitter assembly is
shown. It provides a 1090 MHz signal used to monitor the accuracy of the
measurement subsystems. The assembly is 2 m tall, 40 Kgs (90 lbs) and
mounted on frangible couplings.

## Error Budget and Siting Criteria

A basic advantage of a ground-based measuring system is that there is a
much greater freedom associated with equipment siting as compared to
present day ILS (notwithstanding the possible emergence of frangible ILS
glide slope equipment). Computations can easily correct for the AOA
elevation equipment offset from the GPI. TLS equipment siting is derived
by the application of an error budget to all sources of error that
contribute to measurement degradation. Both range and angle errors must
be accounted for as the final localizer and glide slope guidance is
derived from both range and angle measurements. The primary range error
is caused by the transponder encoding delay and is accounted for in the
Error Budget and Geometric Dilution of Precision (GDOP) modeling.
Frangible, lightweight and low profile equipment design allows safe
placement within obstacle clear areas adjacent to runways and taxiways.
This siting flexibility also helps position critical areas to minimize
their impact on airport operations.

Table 1: Primary Glide Slope Alignment Error Sources

  -------------------------------------------- --------------------------
  [alignment error source]{.smallcaps}         [2σ error
                                               (deg)]{.smallcaps}

  [aoa sensor]{.smallcaps}                     0.12

  [critical area multipath]{.smallcaps}        0.08

  [siting, snow, standing water]{.smallcaps}   [0.08]{.smallcaps}

  [rss error (incl gdop)]{.smallcaps}          0.19

  [uplink transmitter]{.smallcaps}             0.03

  [total]{.smallcaps}                          7.5% Path angle
  -------------------------------------------- --------------------------

The TLS TOA measurement accuracy is 10ns (2σ). After accounting for
critical area considerations and service volume requirements for
off-bore site performance, the elevation and azimuth sensors must be
installed at least 100 meters apart. Standardization of a service volume
width substantially smaller than the +/-35 degrees recommended by Annex
10 would allow the sensor spacing to be much closer.

# System Validation

## Flight Inspection Results

During Flight Inspection, the ground technician can set TLS
signal-in-space parameters using a software tool. The ground technician
can easily adjust course width, path width, glide path angle and
localizer alignment.

Path angle alignment measured during the course of 3 months at Madras OR
is reported in Figure 13. The Figure shows a mean path angle of 3.03°
with weekly results staying within +/- 0.04 degrees of the mean.

![](media/image14.wmf)Figure 13: Glide slope angle measured twice per
week during 3 months at Madras OR.

A TLS Localizer type Directional Aid (LDA) approach with glide slope has
been commissioned at Subic Bay, Philippines. The Subic Bay LDA has an
offset of 8.87 degrees.

Results from US FAA Flight Inspections are shown in Table 2.

Glide slope flight check using a ground-based theodolite are shown in
Figure 14.

Table 2: TLS flight inspection results at six TLS facilities.

+-----------------+-------------------------+-------------+-----------+
| **Site**        | ##                      | **Glide     | **        |
|                 | ## Site Characteristics | slope /     | Localizer |
|                 |                         | structure   | structure |
|                 |                         | as % of     | % of      |
|                 |                         | tolerance** | to        |
|                 |                         |             | lerance** |
+-----------------+-------------------------+-------------+-----------+
| Madras, OR USA  | Ideal / flat            | 3.0° / 30%  | 8%        |
+-----------------+-------------------------+-------------+-----------+
| The Dalles, WA  | Hill elevation 2.8° @ 3 | 4.0° / 50%  | 15%       |
| USA             | nm from threshold       |             |           |
|                 | directly under approach |             |           |
+-----------------+-------------------------+-------------+-----------+
| Subic Bay,      | Terrain rising under    | 3.1° / 66%  | 40%       |
| Philippines     | approach such that 3.1° |             |           |
|                 | glide path offset 8.9   |             |           |
|                 | degrees is required.    |             |           |
|                 | Airport security fence  |             |           |
|                 | near azimuth sensor     |             |           |
|                 | causes multipath.       |             |           |
+-----------------+-------------------------+-------------+-----------+
| Watertown, WI   | 3% Upslope in front of  | 3.6° / 50%  | 15%       |
| USA             | elevation sensor with   |             |           |
|                 | hill rising to 1.8°     |             |           |
|                 | directly under the      |             |           |
|                 | approach. No property   |             |           |
|                 | to install ILS          |             |           |
|                 | localizer.              |             |           |
+-----------------+-------------------------+-------------+-----------+
| FAA Tech        | Gully with 6% grade     | 3.0° / 30%  | 8%        |
| Center, NJ USA  | between equipment and   |             |           |
|                 | runway; elevation       |             |           |
|                 | sensor is 600 feet from |             |           |
|                 | runway.                 |             |           |
+-----------------+-------------------------+-------------+-----------+
| Pullman, WA USA | 10% lateral grade in    | 3.0° / 56%  | 15%       |
|                 | front of elevation      |             |           |
|                 | sensor, hills under the | 80% near Pt |           |
|                 | approach rising to 1.6° | C; FAA      |           |
|                 |                         | result was  |           |
|                 |                         | 38 uamps    |           |
|                 |                         | near Pt C   |           |
|                 |                         | and will    |           |
|                 |                         | recheck     |           |
+-----------------+-------------------------+-------------+-----------+

![](media/image15.png){width="6.159722222222222in"
height="2.3027777777777776in"}

Figure 14: Glide Slope structure and alignment at Pullman WA. Upper plot
shows the theodolite plotted in red with the TLS elevation AOA track in
blue. Lower plot shows the difference and the ICAO tolerance (box).
Glide path angle 3.02 degrees with 17 and 20 microamps recorded in zone
2 and zone 3 respectively.

#   {#section .unnumbered}

# Conclusion

TLS provides accurate glide slope signals over terrain that can be cost
prohibitive to ILS installation. Multipath mitigation is accomplished
using directional antennas. The effects of multipath are further reduced
though a process of calibration that yields a database of
angle-of-arrival offsets used during system operation to correct
ground-based multipath induced error.

Standardization of an azimuth volume substantially smaller than the 35
degrees recommended by annex 10 would further increase TLS siting
flexibility.

The operational impact of critical areas associated with the TLS azimuth
and elevation sensors can be minimized when designing the placement of
TLS equipment near a runway and taxiway.

Computation of the uplink guidance can easily incorporate a correction
for the TLS equipment offset from the glide path intercept point. The
path angle and TCH are determined by a site-specific database. The
apparent emanation point for the localizer can also be projected a great
distance from the TLS azimuth AOA equipment. The TLS localizer's
proportional guidance volume width is independent of runway length, and
can be nominally set to 6 degrees at all airports where TLS is
installed.

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# REFERENCES

1.  [International Standards and Recommended Practices and Procedures
    for Air Navigation Services: Aeronautical
    Telecommunications]{.underline}, Annex 10 to the Convention on
    International Civil Aviation; Fifth Edition, Volume I, Part I \--
    Equipment and Systems; International Civil Aviation Organization;
    1996.

2.  [The Effect Of Transponder Landing System Emissions On ATC
    Performance]{.underline}, DOD Joint Spectrum Center, November 1997.

3.  [Certification Considerations for Highly Integrated or Complex
    Aircraft Systems,]{.underline} ARP 4754, Systems Integration
    Requirements Task Group, Society of Automotive Engineers 1996.

4.  [Guidelines and Methods for Conducting the Safety Assessment Process
    on Civil Airborne Systems and Equipment]{.underline}, ARP 4761, S-18
    Committee, Society of Automotive Engineers, 1996.

5.  [Software Considerations in Airborne Systems and Equipment
    Certification,]{.underline} Document No. RTCA/DO-178B -- December 1,
    1992.