**Methodology for Determining Compatibility of GPS L5 with Existing
Systems and Preliminary Results**[^1]

Christopher Hegarty, Taehwan Kim, Swen Ericson

*The MITRE Corporation, Center for Advanced Aviation System Development
(CAASD), McLean, VA*

Patrick Reddan, Thomas Morrissey

#### Zeta Associates, Fairfax, VA

A.J. Van Dierendonck

#### AJ Systems, Los Altos, CA

## 

## BIOGRAPHIES

Dr. Christopher Hegarty is a Project Team Manager at The MITRE
Corporation. He received his B.S. and M.S. from Worcester Polytechnic
Institute, and his D. Sc. from The George Washington University. He was
a recipient of the 1998 ION Early Achievement Award, and currently
serves as Editor of *Navigation*: *Journal of the Institute of
Navigation* and as co-chair of RTCA SC159 Working Group 1 (2^nd^ Civil
GPS Frequency)*.*

Taehwan Kim is a Lead Engineer at The MITRE Corporation. He received his
B.S. in mathematics from the Seoul National University, M.S. in computer
science from the University of South Carolina, Columbia, and is
currently working toward his Ph.D. EE at The University of Maryland,
Baltimore County. Since 1987, he has held several industry positions in
the areas of radar and satellite communication engineering.

Swen Ericson is a System Development Engineer at The MITRE Corporation.
He received his B.S. degree in Civil Engineering from The University of
Miami, FL in 1992, and M.S. in Surveying Engineering from Purdue
University in 1994. He has recently been involved with GPS modernization
studies at MITRE/CAASD.

Patrick Reddan received his B.E.E.E. from Manhattan College and the
Degree of Engineer (EE) as well as Master in Engineering Administration
from George Washington University. He joined Zeta Associates in 1986 and
is currently providing support to the FAA WAAS program development.
Particular areas of interest include system architecturing as well as
signal processing and interference mitigation techniques.

Dr. Thomas Morrissey received his BSEE from the University of Notre
Dame, his SMEE from the Massachusetts Institute of Technology, and his
Ph.D. in EE from the University of Notre Dame. He served in the US Army
from 1968-1970, worked in private industry from 1971-1975, and held
research and analytical positions with the Central Intelligence Agency
from 1975-1998. He joined Zeta Associates in November 1998 and is
currently providing support to FAA programs.

Dr. A.J. Van Dierendonck received a BSEE from South Dakota State
University and MSEE and Ph.D. from Iowa State University. Currently, he
is self-employed under the name of AJ Systems and is a general partner
of GPS Silicon Valley. In 1993, Dr. Van Dierendonck was awarded the
Johannes Kepler Award by the Institute of Navigation Satellite
Navigation Division for outstanding contributions to satellite
navigation. For 1997, he was awarded the ION Thurlow award for
outstanding contributions to the science of navigation. A.J. has 25
years of GPS experience and is a Fellow Member of the IEEE. He currently
serves as working group co-chairman of the RTCA SC159 Working Group
(WG1) for the 2^nd^ Civil GPS Frequency.

**ABSTRACT**

This paper describes the methodology being used by an ad hoc working
group of the Interagency GPS Executive Board (IGEB) Third Civil Signal
Implementation Steering Group to address the feasibility of coexistence
between the third civil GPS signal and existing systems at or near
1176.45 MHz. The paper also describes preliminary results of applying
this methodology to known emitters in the U.S. and other regions of the
world.

**INTRODUCTION**

On January 25, 1999, U.S. Vice President Al Gore announced that two new
civil GPS signals will be added to future GPS satellites \[1\]. The
second civil signal will use the C/A codes currently used at GPS L1
(1575.42 MHz) and will be located at GPS L2 (1227.6 MHz). The third
civil signal, which is intended to meet the needs of critical
safety-of-life applications, will be located at 1176.45 MHz (L5). The
second and third civil signals are anticipated to be included on GPS
satellites that will be launched beginning in 2003 and 2005,
respectively.

To address issues related to the implementation of the third civil
signal, the Interagency GPS Executive Board (IGEB) has established a
steering group and four ad hoc working groups (see Figure 1). This paper
describes the initial findings of ad hoc Working Group 1 (WG1) that was
chartered to "...address the feasibility of providing a protected
safety-of-life GPS signal suitable for civil aviation use while allowing
existing, authorized systems to coexist..." \[2\]. More specifically,
this paper:

1.  Describes existing systems operating at or near 1176.45 MHz.

2.  Introduces the methodology being used by ad hoc WG1 to determine
    compatibility.

3.  Presents preliminary results using this methodology.

![](media/image1.wmf)**Figure 1. IGEB Third Civil Signal Implementation
Steering Group Organization**

**EXISTING SYSTEMS OPERATING AT OR NEAR 1176.45 MHz**

Major existing systems operating at or near 1176.45 MHz include:

1.  Aeronautical systems operating between 960 and 1215 MHz -- including
    Distance Measuring Equipment (DME) and Tactical Air Navigation
    (TACAN) systems, that operate throughout the band, and the systems
    that operate at 1030 and 1090 MHz, including Secondary Surveillance
    Radars (SSR), Traffic Collision and Avoidance System (TCAS),
    Identify Friend or Foe (IFF) and planned Automatic Dependent
    Surveillance -- Broadcast (ADS-B). The Federal Aviation
    Administration (FAA) operates approximately 1000 DME and TACAN
    beacons. The U.S. Department of Defense (DoD) operates an additional
    173 TACAN beacons \[3\].

2.  Joint Tactical Information Distribution System/Multi-functional
    Information Distribution System (JTIDS/MIDS) operating between 969
    and 1206 MHz -- a spread-spectrum digital communications system used
    by the DoD and other nations to exchange data among military
    platforms. JTIDS is currently deployed on about 600 platforms in its
    Class 2 form. They are installed on F-14 fighters, a squadron of
    F-15 fighters, Navy E-2Cs, ships, E-3 airborne early warning
    platforms, Modular Control Equipment, ABCCC, Rivet Joint, and Joint
    STARS platforms. MIDS is under development as the next-generation
    version of JTIDS and will eventually be deployed on about 2400 U.S.
    platforms by the year 2005.

3.  Military and civil radars operating between 1215 and 1385 MHz --
    including about 250 systems in the U.S. used for long-range primary
    air traffic control (ATC), the North Warning System (NWS), drug
    interdiction, and other applications.

One common characteristic of all of these systems is that they are
pulsed. This commonality facilitates the coexistence of these systems
with L5, since GPS receivers can be made very robust against pulsed
interference. Details on these systems are provided in the following
subsections.

### DME/TACAN

DME and TACAN are navigation systems that provide airborne interrogators
with distance measurements from ground transponders via pulse ranging
(TACAN provides azimuth information as well). DME and TACAN operate over
much of the 960 -- 1215 MHz band with a channel spacing of 1 MHz. Of the
four modes (X, Y, W, and Z) used by DME/TACAN, only the X-mode replies
that span 1151 -- 1213 MHz fall in-band with respect to the L5 signal.
The X-mode ground transponders (beacons) transmit pulses in pairs with a
pulse separation of 12 μs and a repetition rate of around 2700 pulse
pairs per second (ppps) for DME, and 3600 ppps for TACAN. These
repetition rates are maintained even when aircraft interrogations are
not being received. The pulse shape is nominally gaussian (see Figure
2). The one-half voltage pulse width is 3.5 μs.

The ground beacons transmit with a peak Effective Isotropic Radiated
Power (EIRP) ranging from 100 to 10000 W. Typical antenna gain patterns
are shown in Figure 3. Most (around 87.5 percent) of the radiated power
is contained within a 0.5 MHz bandwidth centered on the 1 MHz channels.

DME and TACAN beacons are situated throughout the U.S. to provide
aircraft navigation for en route through non precision approach phases
of flight. In certain regions, many DME/TACANs are visible at high
altitudes, notably the East Coast, and airspace surrounding Chicago and
Los Angeles. For example, Figure 4 shows the DMEs and TACANs with
frequency allocations from 1157 -- 1209 MHz surrounding Harrisburg,
Pennsylvania. A simulated sample of the signal that would be received by
an airborne GPS/WAAS receiver with a 20 MHz passband at 40,000 ft above
Harrisburg is shown in Figure 5. For reference, the assumed noise floor
in the passband is at --97 dBm. As will be shown, this level of
interference is not acceptable. As a solution to this problem, WG1 has
considered reassigning the frequencies of some or all of the DME/TACANs
that currently fall within 10 MHz of L5.

###### Other Aeronautical Systems

WG1 has found other aeronautical systems (e.g., DME/TACAN interrogators,
SSR, IFF, TCAS) operating in the 960 -- 1215 MHz band to be very minor
contributors to the electromagnetic environment around 1176.45 MHz.
These systems do not operate inband to L5 and are pulsed with low duty
cycles.

![](media/image2.wmf){width="3.25in" height="2.2506944444444446in"}

Figure 2. DME/TACAN X-Mode Pulse Pair

![](media/image3.png) ![](media/image4.jpeg){width="1.575in"
height="2.379166666666667in"}

Figure 3. Typical DME (Left) and TACAN (Right) Antenna Gains vs.
Elevation Angle (Peak Gain Minus Cable Losses is 6-9 dBi)

![](media/image5.wmf){width="3.3979166666666667in"
height="2.0479166666666666in"}

**Figure 4. DME/TACANs Surrounding Harrisburg, Pennsylvania**

![](media/image6.wmf){width="3.4993055555555554in"
height="2.529166666666667in"}

**Figure 5. Signal Received by an L5 GPS/WAAS Receiver 40,000 ft Above
Harrisburg**

###### JTIDS/MIDS

JTIDS and MIDS operate over 51 frequencies between 969 and 1206 MHz (see
Figure 6). Airborne platforms equipped with JTIDS/MIDS may have top and
bottom mounted antennas (see Figure 7 for a typical bottom-mounted gain
pattern).

![](media/image7.wmf){width="3.2465277777777777in"
height="0.41041666666666665in"}

**Figure 6. JTIDS/MIDS Carrier Frequencies**

![](media/image8.wmf){width="2.486111111111111in"
height="2.486111111111111in"}

**Figure 7. F-15 JTIDS Bottom Mounted Antenna Gain vs. Elevation Angle**

To ensure compatibility between JTIDS and other systems (e.g., DME,
TACAN, secondary radars) in the 960 -- 1215 MHz Aeronautical
Radionavigation Service (ARNS) band, the National Telecommunications and
Information Administration (NTIA) has levied a set of electromagnetic
compatibility (EMC) requirements upon JTIDS transmissions \[4\],
including the pulse spectrum rolloff specification shown in Figure 8.
Importantly, although the rolloff specification only calls for the noise
floor to be 60 dB down from the carrier beyond 15 MHz from the carrier
frequency, in order to meet EMC monitoring requirements to protect
systems operating at 1030/1090 MHz, JTIDS equipment as currently
implemented must maintain a noise floor at least 67.4 dB lower than the
carrier \[5\].

![](media/image9.wmf){width="3.248611111111111in"
height="2.9541666666666666in"}

**Figure 8. JTIDS Pulse Power Rolloff Specification \[4\]**

NTIA EMC requirements call for JTIDS and MIDS terminals to monitor their
emissions. Each terminal is required to inhibit transmissions when
emissions exceed specified limits. JTIDS and MIDS terminals monitor
emissions in accordance with one of four operator selected EMC
protection modes. These modes are:

-   100/20 Full EMC Protection mode

-   100/50 Full EMC Protection mode

-   Exercise protection mode

-   Combat mode

In the two full EMC protection modes, 100/20 and 100/50 are percentages
of 396,288 pulses allowable in a 12 second interval by all terminals and
a single terminal operating in a geographic area, respectively. In the
U.S., the geographic area (GA) is defined to be a circle with a 200 nmi
radius. Many nations in Europe, including Belgium, Denmark, France, The
Netherlands, Germany, Greece, Italy, Norway, and Turkey have smaller GAs
of 100 nmi radii or less, while others (e.g., U.K.) have larger GAs.

There are several other emission characteristics that are monitored in
the full EMC protection modes, including notably a 200 W transmit power
limit, uniform frequency distribution across the 51 frequencies, and
maximum emission power in bands surrounding the frequency pair 1030/1090
MHz.

For exercises in which the full EMC Protection modes are too
restrictive, the DOD must coordinate with the FAA to use the Exercise
protection mode. This mode is typically used for large scale training
exercises, and during peacekeeping missions. The DOD would like to have
the 200 nmi GA reduced in the U.S. to avoid the need for coordinating
smaller exercises.

Finally, the combat mode is used in operations in which a transmission
inhibit would deny warfighting forces needed threat information or
jeopardize an engagement with hostile forces. This mode is used in
situations where all the features and transmission power of the
terminals are needed.

###### RADARS

Out-of-band (OOB) emissions from radars operating between 1215 -- 1385
MHz in the U.S. was initially a significant concern for WG1. Although L5
is almost 40 MHz below 1215 MHz, the extremely high radiated power
levels of many of these systems (tens of kilowatts to Megawatts) results
in non-negligible OOB power levels at 1176.45 MHz, even through the
antenna sidelobes. Furthermore, some of the radars have transmit modes
that include very long (on the order of 1 ms) pulses that could be
especially disruptive to GPS L5. OOB spectrum measurements obtained for
several of the radars indicated that peak OOB emissions are only 55 dB
below inband power levels.

Closer scrutiny of the OOB emission characteristics of the radars
revealed that OOB emission are characterized by short (less than a few
hundred nanoseconds) spikes of power, corresponding to the rise and fall
times of the radars' inband pulses. These spikes dominate the spectrum
measurements. In between these spikes, the OOB emissions are much lower.

With proper accounting for the time characteristics of radar OOB
emissions, radars were not found by WG1 to be significant contributors
to the electromagnetic environment around 1176.45 MHz.

**SIGNAL-IN-SPACE ASSUMPTIONS**

As shown in Figure 1, IGEB Third Civil Signal Implementation Steering
Group ad hoc Working Group 3 (WG3) has been chartered to define the
requirements for the new signal. WG3 has, in turn, been relying on RTCA
Special Committee 159 Working Group 1 (SC159 WG1) to achieve consensus
on the signal design. This section summarizes the most recent signal
design \[6\] that has been recommended by SC159 WG1 and endorsed by WG3.
Further background on the development of the signal design may be found
in \[7\].

The proposed GPS L5 signal is comprised of a pair of carriers (inphase
and quadrature) at 1176.45 MHz that are Binary Phase Shift Keying (BPSK)
modulated by:

-   10.23 Mchip/s spreading sequences -- The inphase and quadrature
    carrier channels are BPSK modulated by different sequences (see
    \[6\] for details of the spreading sequences). Length 10230
    sequences will be employed, resulting in a 1 ms repetition period.

-   The inphase signal is BPSK modulated by data. The data rate is 50
    bps. A rate ½ forward error correction (FEC) code is employed,
    yielding 100 symbols per second (sps).

-   The inphase signal is BPSK modulated by a 1 kHz Neumann-Hoffmann
    sequence to improve symbol synchronization and cross-correlation
    properties.

The proposed WAAS L5 signal structure is similar, except that only a
single carrier is used, and the data rate is 250 bps. A rate ½ FEC code
is employed yielding 500 sps.

The GPS L5 minimum received signal power into a 0 dBic antenna at or
near the surface of the earth will be --154 dBW. This level is 6 dB
higher than the specified minimum power for the C/A code on L1 and
facilitates coexistence with the emitters analyzed in this study. For
GPS, one-half the power (-157 dBW) is provided to each carrier component
(inphase and quadrature). For WAAS signals at L5, WG1 has been assuming
the signal power will be at either --154 dBW or --151 dBW (since these
satellites have not yet been designed, there is greater flexibility).
All WAAS power is devoted to the inphase carrier.

## RECEIVER ASSUMPTIONS

To facilitate coexistence, it is assumed that GPS/WAAS L5 receivers will
employ some form of pulse detection and pulse blanking circuitry as well
as better out-of-band filtering relative to current civil equipment
operating at L1. The pulse detection circuitry detects when pulsed
energy is being received above a certain threshold that is set relative
to the nominal level of noise in a receiver's passband. When a pulse is
detected, the input signal level is zeroed ("blanked"). Blanking is
equivalent to excluding a portion of the received signal from the
complex correlation sums generated by the receiver. Although the
blanking threshold would be set adaptively relative to the nominal noise
floor in a practical implementation, WG1 has conservatively assumed a
fixed input level threshold of --86.5 dBm.

The assumed out-of-band filtering requirement is shown in Figure 9. A
combination of radio frequency (RF) and Intermediate Frequency (IF)
filters provides a total of 5.5 dB/MHz attenuation for interfering
signals that fall beyond 10 MHz of L5. The maximum attenuation is 70 dB.
Participants in RTCA SC159 WG1 have recommended use of a higher (5 dB)
noise figure for L5 GPS/WAAS receivers than is specified in \[8\] for
L1. This recommended increase is due to the anticipated greater filter
insertion loss resulting from the more stringent filtering requirement.
Combined with a 100 K sky noise, the 5 dB noise figure results in a
receiver noise floor at --200 dBW/Hz.

![](media/image10.wmf){width="3.347916666666667in"
height="1.8270833333333334in"}

Figure 9. Assumed GPS/WAAS L5 Receiver RF/IF Filter Response

## METHODOLOGY FOR DETERMINING COMPATIBILITY

The methodology for determining compatibility consists of determining
the degradation to a GPS/WAAS L5 receiver's post-correlation
signal-to-noise power density ratio (S/N~0~) due to the presence of
interfering signals. An acceptable level of degradation is defined to be
one that yields 33.7 dB-Hz. This S/N~0~ is attained at L1 for WAAS under
the following conditions:

-   Minimum received signal power of --161 dBW into a 0 dBic antenna

-   --4.5 dBic antenna gain towards a desired low-elevation angle
    satellite

-   2 dB receiver implementation loss

-   Maximum interference level of --122.5 dBm/MHz (-116.5 dBm/MHz to
    protect all receiver functions, including acquisition, from \[8\]
    adjusted by a 6 dB safety margin \[9\])

In the absence of interference, a minimum S/N~0~ of 39.5 dB-Hz is
expected (-154 dBW received power - 4.5 dBic antenna gain - 2 dB
implementation loss - (-200) dBW/Hz noise floor = 39.5 dB-Hz).
Compatibility is achieved at a geographic location, from a WG1
standpoint, if signals from existing systems do not result in more than
a 5.8 dB degradation to the starting L5 S/N~0~ (39.5 -- 5.8 = 33.7).

An equation for computing the S/N~0~ degradation to an L5 receiver is:

$$\begin{array}{r}
\frac{S}{N_{0,\text{eff}}} = \text{39}\text{.}5 + \text{20}\text{log}(1 - \text{PDC}_{B}) \\
 - \text{10}\text{log}(1 - \text{PDC}_{B} + \sum_{i = 1}^{N}\text{10}^{\left( \frac{R_{i}}{\text{10}} \right)})
\end{array}$$

Where

$(R_{i})_{\text{dB}} = P_{i} + \text{97}\text{dBm} + \text{10}\text{log}\text{dc}_{i}$,

PDC~B~ (pulse duty cycle -- blanker) is the total duty cycle of all
pulses strong enough to activate the blanker, N in the total number of
low-level undesired received signals (i.e., those not strong enough to
trip the blanker), P~i~ is the peak received power of the i-th undesired
signal, and dc~i~ is the duty cycle of the i-th low-level signal. The
right-hand terms of the above equation are:

-   1^st^ term -- S/N~0~ is the absence of interference

-   2^nd^ term -- Lost signal power due to the blanker

-   3^rd^ term -- Adjustment of the noise floor due to: (1) suppression
    of thermal noise by the blanker, and (2) noise floor contribution
    due to low-level pulses and continuous interference.

Use of the equation to predict an L5 receiver's performance in the
presence of many undesired pulsed signals is only valid when the pulses
are very short relative to the minimum predetection integration time
used by the receiver (1 -- 10 ms, depending on the mode of operation).
This constraint is satisfied for DME/TACAN, JTIDS/MIDS, and most radar
OOB emissions.

Note that the equation conservatively assumes that low-level pulses do
not coincide with strong pulses that trip the blanker. Figure 10 plots
the acceptable combinations of strong pulses and low-level interference
(low-level pulses and continuous interference). Note that if there are
no low-level pulses present, the blanker permits the receiver to operate
in the presence of undesired signals with very high duty cycle (over 70
percent). Of course, any environment that includes high-level pulses in
some regions, will also have larger regions with many low-level pulses.
Thus, the likely operating point of the curve in Figure 10 is towards
the right side -- lots of low-level pulses and a low total duty cycle of
high-level pulses.

### 

It is very important to carefully define the meaning of "duty cycle" for
both PDC~B~ and dc~i~. Although the usual first step of defining duty
cycle as the product of pulse width (PW) and pulse repetition frequency
(PRF) is unambiguous, PW definitions vary greatly among systems. For
instance, DME PWs are defined by half-voltage, whereas JTIDS pulses are
defined by 90 percent power. The definitions that must be used to yield
correct results using the above formula are:

$$\begin{array}{r}
\text{dc}_{i} = \text{PW}_{i} \times \text{PRF}_{i} \\
\text{PW}_{i} = \frac{1}{P_{i}}\int_{- \infty}^{\infty}{p(t)\text{dt}}
\end{array}$$

for low-level pulses where p(t) is the instantaneous received power of
the pulse.

As an example, the instantaneous received power of a perfectly
gaussian-shaped (not the case in reality, but a good first
approximation) DME pulse can be written as:

$$p(t) = P_{i}e^{- {\alpha t}^{2}}$$

where α = 4.5 × 10^11^ s^-2^. The half-voltage pulse width is 3.5 μs,
but PW~i~ as defined above is 2.6 μs.

For strong pulses, pulse widths should be defined by the time period
that the instantaneous received power exceeds the blanker threshold.

### 

### ![](media/image11.wmf){width="3.2979166666666666in" height="2.6486111111111112in"}

Figure 10. Acceptable Combination of High-Level Pulses and Low-level
Interference

### 

### PRELIMINARY RESULTS

Using the assumptions and methodology described in the previous
sections, interference analyses were performed using the GPS RFI
Environment Evaluation Tool (GREET), a software tool developed by The
MITRE Corporation's Center for Advanced Aviation System Development.
Interference scenarios in both the conterminous U.S. (CONUS) and Europe
were evaluated on a half-degree grid. Maps showing signal-to-noise ratio
degradation at grid points of different flight altitudes were generated
for different emitter configurations (see Figures 11-20).

Figures 11-13 show signal-to-noise degradations in the U.S. for various
flight levels. Each of these plots includes effects from:

-   DME/TACAN ground beacons

-   JTIDS/MIDS -- Calculations for each grid point reflect the effects
    of a 200 nmi JTIDS GA centered at that grid point assuming maximum
    allowable pulse density in the 100/50 Full EMC Protection mode.

-   OOB emissions from radars

-   Aeronautical emitters -- A conservative allowance for emissions from
    DME/TACAN interrogators, SSR interrogators and transponders, TCAS,
    and ADS-B was included.

Note that signal-to-noise degradation is very dependent on altitude. At
40,000 ft above mean sea level (MSL), many ground-based emitters can be
seen. Figure 11 indicates an unacceptable level of degradation at this
altitude over large regions in CONUS. At 25,000 ft MSL (Figure 12),
fewer emitters can be seen and the situation improves. At 5,000 ft above
ground level (AGL) (Figure 13), the environment is very clean.

The high altitude electromagnetic environment surrounding 1176.45 MHz is
dominated by DME/TACAN, as can be seen by comparing Figure 11 that
includes all emitters with Figure 14 that only includes the effects of
DME/TACAN. A very promising solution identified by WG1 is to reassign a
subset of the inband DME/TACANs. Figure 15 plots signal-to-noise
degradation due to all emitters with the inband DME/TACANs reassigned to
other frequencies in the 960 -- 1215 MHz band. The plot indicates that,
with this DME/TACAN reassignment, an acceptable environment for L5 may
be achieved throughout CONUS at this flight level.

Figures 16 -- 20 show a similar sequence of plots for Europe. Figures
16-18 illustrate the effects of DME/TACAN, JTIDS/MIDS, and other
aeronautical emitters on L5 at various altitudes over Europe. These
figures should not be considered comprehensive as they do not include
radars (authors did not have data available), and they assume a
JTIDS/MIDS environment roughly consistent with a 100 nmi GA throughout
Europe (not the case in reality, see earlier discussion on GAs). Similar
to the results for the U.S., it appears that an unacceptable
electromagnetic environment for L5 exists at high altitudes. DME/TACAN
is the dominant contributor to this environment at high altitudes. As
with the U.S., DME/TACAN frequency reassignments appear to be a
promising solution for Europe.

![](media/image12.png){width="3.245833333333333in"
height="1.9104166666666667in"}

Figure 11. Total GPS Receiver Postcorrelation S/N~0~ Degradation at
40,000 ft MSL over CONUS

![](media/image13.png){width="3.245833333333333in"
height="1.9104166666666667in"}

Figure 12. Total GPS Receiver Postcorrelation S/N~0~ Degradation at
25,000 ft MSL over CONUS

![](media/image14.png){width="3.245833333333333in"
height="1.9104166666666667in"}

Figure 13. Total GPS Receiver Postcorrelation S/N~0~ Degradation at
5,000 ft AGL over CONUS

![](media/image15.png){width="3.245833333333333in"
height="1.9104166666666667in"}

Figure 14. GPS Receiver Postcorrelation S/N~0~ Degradation from
DME/TACAN at 40,000 ft MSL over CONUS

![](media/image16.png){width="3.245833333333333in"
height="1.9104166666666667in"}

Figure 15. Total GPS Receiver Postcorrelation S/N~0~ Degradation at
40,000 ft MSL over CONUS, DME/TACAN Emitters at 1176.45 ±10 MHz
Reassigned

![](media/image17.png){width="3.245833333333333in"
height="1.9104166666666667in"}

Figure 16. Total GPS Receiver Postcorrelation S/N~0~ Degradation at
40,000 ft MSL over Europe

![](media/image18.png){width="3.245833333333333in"
height="1.9104166666666667in"}

Figure 17. Total GPS Receiver Postcorrelation S/N~0~ Degradation at
25,000 ft MSL over Europe

![](media/image19.png){width="3.245833333333333in"
height="1.9104166666666667in"}

Figure 18. Total GPS Receiver Postcorrelation S/N~0~ Degradation at
5,000 ft AGL over Europe

![](media/image20.png){width="3.245833333333333in"
height="1.9104166666666667in"}

Figure 19. GPS Receiver Postcorrelation S/N~0~ Degradation from
DME/TACAN at 40,000 ft MSL over Europe

![](media/image21.png){width="3.245833333333333in"
height="1.9104166666666667in"}

Figure 20. Total GPS Receiver Postcorrelation S/N~0~ Degradation at
40,000 ft MSL over Europe, DME/TACAN Emitters at 1176.45 ±10 MHz
Reassigned

### 

### CONCLUSIONS

This paper has described the methodology being used by an ad hoc IGEB
working group to evaluate compatibility of the new civil GPS signal at
1176.45 MHz with existing systems operating at or near this frequency.
Preliminary results indicate that on the surface of the earth and at low
altitudes, no modifications to existing systems in the U.S. will be
necessary to protect the GPS L5 service. At high altitudes, many more
emitters are visible and present an unacceptable level of interference
to the new signal. A promising solution to this problem is to reassign
DME/TACAN frequencies that fall in-band to the new GPS signal.

**ACKNOWLEDGMENTS**

The authors would like to acknowledge the many contributions to this
paper from the other participants in the IGEB Third Civil Signal
Implementation Steering Group Ad Hoc Working Group 1. Special thanks to
Dr. John Barrows (consultant to the FAA) and Mike Biggs (FAA) for
information on DME/TACAN, and John Ashley (Sentel Corporation) for
analyzing interference from on-board aeronautical systems.

**REFERENCES**

1.  Anon., *Vice President Gore Announces New Global Positioning System
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2.  Anon., *Plan for Implementation of the Third Civil GPS Signal*,
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3.  Anon., *1996 Federal Radionavigation Plan*, U.S. Department of
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4.  NTIA, October 1994, JTIDS EMC Features Guidelines, SPS WG-1
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5.  Leiter, M., 31 March 1998, "The Ratio of Peak-to-RMS Noise Power in
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6.  Van Dierendonck, A.J., and J.J.Spilker, *Proposed Third Civil GPS
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9.  Anon, *Modification of preliminary draft new Recommendation ITU-R
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[^1]: ^\*^ This paper is based on system analysis studies performed for
    the FAA Satellite Program Office (AND-730). This paper reflects the
    views of the authors, who are responsible for the accuracy of the
    facts, analyses and suggestions presented herein, and does not
    reflect the official views or policy of the FAA.