**Civil GPS/WAAS Signal Design and Interference Environment at 1176.45
MHz: Results of RTCA SC159 WG1 Activities**

Christopher Hegarty

*Federal Aviation Administration, Washington, D.C.*

A.J. Van Dierendonck

#### AJ Systems, Los Altos, CA

## 

## BIOGRAPHIES

Dr. Christopher Hegarty received his B.S. and M.S. from Worcester
Polytechnic Institute, and his D. Sc. from The George Washington
University. He has been with The MITRE Corporation since 1992, most
recently as a Project Team Manager. In August 1999, he began a one-year
assignment as Civil GPS Modernization Project Lead with the FAA through
the Intergovernmental Personnel Act. 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 (3^nd^ Civil GPS Frequency)*.*

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. Recently, he
was inducted into the GPS Joint Program Office's GPS Hall of Fame. He
currently serves as working group co-chairman of the RTCA SC159 Working
Group (WG1) for the 3^nd^ Civil GPS Frequency.

**ABSTRACT**

This paper describes the contributions of RTCA Special Committee 159
Working Group 1 (3^rd^ Civil GPS Frequency) towards the implementation
of a third civil GPS signal at 1176.45 MHz (L5). These contributions
came primarily in two areas. First, Working Group 1 (WG1) was relied
upon by the Interagency GPS Executive Board (IGEB) to develop consensus
on the design of the third civil signal. Second, WG1 developed a set of
receiver assumptions, and participated in the development of a
methodology, for analyzing the compatibility of the third civil signal
with existing systems operating at or near L5.

**INTRODUCTION**

In July 1998, to promote the development of a second civil GPS signal,
an ad hoc working group in RTCA Special Committee 159 (SC159) was
established. In October 1998, this ad hoc working group became a
full-fledged SC159 working group, known as Working Group 1 (WG1).
Initial activities of WG1 included establishing requirements for the new
signal and developing an RTCA position on the location of the carrier
frequency of this signal (RTCA was one of several organizations that
recommended that the second civil signal be located within the 960 --
1215 MHz aeronautical band).

On January 25, 1999, U.S. Vice President Al Gore announced that not just
one, but 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 2005
and 2007, respectively.

Shortly after Vice President Gore's announcement, SC159 WG1 was provided
an expanded role in the implementation of the L5 signal. This expanded
role came about in February 1999 through the establishment of a Third
Civil Signal Implementation Steering Group within the Interagency GPS
Executive Board (IGEB). The Steering Group developed a plan \[2\] for
the implementation of the L5 signal that included a strong tie (see
Figure 1) between SC159 WG1 and an ad hoc working group of the IGEB
(IGEB WG3) that was chartered to define the technical characteristics of
the third civil GPS signal. As will be discussed later in this paper,
SC159 WG1 also had significant interaction with another ad hoc IGEB
working group (IGEB 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..." \[2\].

This paper summarizes the accomplishments of SC159 WG1 towards:

-   Developing consensus on the signal design for GPS and Wide Area
    Augmentation System (WAAS) signals to be broadcast at L5.

-   Defining receiver characteristics to be used in analyzing the
    compatibility of the L5 signals with existing systems operating at
    or near L5.

![](media/image1.wmf){width="3.3020833333333335in"
height="1.9666666666666666in"}**Figure 1. IGEB Third Civil Signal
Implementation Steering Group Organization**

**SIGNAL DESIGN**

In August 1999, the IGEB endorsed a proposed design \[3\] for the third
civil signal. The GPS L5 signal will be comprised of a pair of carriers
(in-phase and quadrature) at 1176.45 MHz that are Binary Phase Shift
Keying (BPSK) modulated by:

-   10.23 Mchip/s spreading sequences -- The in-phase and quadrature
    carrier channels are BPSK modulated by different sequences (see
    \[3\] for details of the spreading sequences). Length 10230
    sequences will be employed, resulting in a 1 ms repetition period.

-   The in-phase 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 subframe structure is
    described in \[4\].

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

An overview of the generation of the GPS L5 signal is provided in Figure
2.

![](media/image2.wmf){width="3.3055555555555554in"
height="0.8159722222222222in"}

**Figure 2. GPS L5 Signal Generation**

The Federal Aviation Administration (FAA) also plans to eventually
implement a WAAS signal at L5. A design for the WAAS L5 signal structure
has been proposed \[4\] that is similar to the GPS L5 signal, 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 of the L5 signal with existing systems. For GPS,
one-half the power (-157 dBW) is provided to each carrier component
(in-phase and quadrature). The WAAS L5 signal minimum received signal
power will also likely be specified at --154 dBW. All WAAS power is
devoted to the in-phase carrier.

The signal design summarized above and described in detail in \[3,4\]
has benefited from many inputs, notably the proposal for the dataless
channel or "coherent carrier" component of the signal originally made in
\[5\]. Numerous earlier iterations of the signal design were presented
at WG1 meetings to solicit feedback. In return, WG1 participants
analyzed virtually every aspect of the signal design and provided
feedback for improvement of the design. The following subsections
summarize some of the main design features that were analyzed.

###### COHERENT CARRIER

The dataless channel was originally proposed to allow carrier phase
tracking of the signal in low signal-to-noise conditions. Figure 3 \[6\]
illustrates the cycle slip performance of the GPS L5 signal as compared
to the standard BPSK signal (e.g., the C/A code). The figure plots the
probability of a cycle slip in a 1-second interval versus
post-correlation signal-to-noise density, S/N~0~. Note that to achieve a
cycle slip probability of 10^-5^ per second, roughly 3.5 dB less
signal-to-noise is required for the GPS L5 signal as compared to the
standard BPSK signal. A 10 Hz Phase Locked Loop (PLL) tracking the
dataless channel is assumed for the GPS L5 signal and a 10 Hz Costas
loop is assumed for the standard BPSK signal. This relative comparison
does not include consideration of dynamics and oscillator jitter. Adding
consideration of these important effects would widen the cycle slip
performance difference between the GPS L5 signal and the standard BPSK
signal, since the optimal bandwidth for a PLL, considering noise,
dynamics, and oscillator jitter, will be lower than that for the Costas
loop.

![](media/image3.wmf){width="3.301388888888889in"
height="2.4770833333333333in"}

Figure 3. Cycle Slip Performance of L5 with PLL Compared to Cycle Slip
Performance of L1 C/A Code

**DATA DEMODULATION**

Although the cycle slip performance is better for the new signal
structure than the L1 C/A code, rms carrier phase tracking errors are
actually larger over the range of S/N~0~'s of interest \[6\]. This
observation initially led WG1 to be concerned over the performance of
coherent data demodulation when the coherent reference is the noisy
carrier phase measurement. For instance, with a 10 Hz PLL tracking the
dataless channel, phase errors vary with a time constant of 0.1 s, or
ten channel symbols. WG1 considered the provision of additional power to
the data channel (beyond one half of the available signal power).
However, simulation of the data demodulation performance revealed that
the rate ½ FEC scheme selected is very robust to losses in recovered
symbol energy over strings of symbols.

Figure 4 \[7\] is a very interesting graphical derivation of the optimal
division of power between the data and dataless channels for the L5
signal. Two curves are plotted on the figure, based on simulation
results. The first is the required S/N~0~ to provide a word error rate
(WER) of 0.001 versus the fraction of total signal power devoted to the
dataless channel. The second curve is the required S/N~0~ to provide a
cycle slip rate of 10^-5^/second. Note that, given these assumed
performance requirements, the optimal division of power between the data
and dataless channels is close to one-half to each.

![](media/image4.wmf){width="3.303472222222222in"
height="2.4027777777777777in"}

Figure 4. Determination of Optimal Division of Power Between the Data
and Dataless Channels

![](media/image5.wmf){width="3.3472222222222223in"
height="2.2888888888888888in"}

**Figure 5. Acquisition Performance**

**ACQUISITION**

Importantly, although the coherent carrier significantly improves
tracking thresholds for the GPS L5 signal, it does not facilitate
acquisition, which is inherently a non-coherent function. As compared to
the L1 C/A code, acquisition of the GPS L5 signal requires a search
through ten times as many code phase possibilities. However, the lower
cross-correlation values between the L5 signal codes reduces the dwell
time required to search each code phase cell with a fixed probability of
missed detection by around a factor of four \[8\]. Figure 5 \[8\] plots
the probability of correct detection versus post correlation
signal-to-noise density, C/N~0~, with the dwell times set so as to
achieve a missed detection probability of 0.001. Note that the required
C/N~0~ for a 0.9 probability of detection is approximately the same for
the GPS L5 signal and the L1 C/A code.

**RANGING PRECISION**

The GPS L5 signal structure will provide improved ranging precision in
the presence of multipath (see Figure 6 \[4\]), as compared to the L1
C/A code. The higher chipping rate also improves ranging precision in
the presence of noise and interference. In the presence of additive
white Gaussian noise, the GPS L5 signal structure provides an
approximate factor of three improvement in root mean square (rms)
tracking errors over the L1 C/A code (assuming both signals are limited
to 24 MHz). This improvement is due to the roughly three-fold larger rms
bandwidth of the GPS L5 signal, as compared to the L1 C/A code.

![](media/image6.wmf){width="3.2430555555555554in"
height="2.216666666666667in"}

**Figure 6. Multipath Performance Comparisons**

**INTERFERENCE ENVIRONMENT FOR L5**

Major existing systems operating at or near 1176.45 MHz include \[9\]:

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 \[10\].

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. Additional details on these systems may be found in \[9\].

## RECEIVER CONSIDERATIONS

After being briefed on the technical characteristics of the emissions
from the existing systems operating nearby in frequency to L5, WG1
participants agreed that modest receiver design changes would be highly
desirable to facilitate coexistence. The two changes addressed by the
group were: (1) the incorporation of some form of pulse detection and
blanking circuitry (or any other technique that provides equivalent
performance), and (2) 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.

The WG1 consensus on out-of-band filtering requirements for L5 equipment
is shown in Figure 7. 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. WG1 participants have recommended use of a
higher (5 dB) noise figure for L5 GPS/WAAS receivers than is specified
in \[11\] for L1 (see Figure 8 \[12\] for an illustrative comparison of
L1 and L5 front-ends). 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 \[12\].

![](media/image7.wmf){width="3.347916666666667in"
height="1.8270833333333334in"}

Figure 7. Assumed GPS/WAAS L5 Receiver RF/IF Filter Response

![](media/image8.wmf){width="3.3048611111111112in"
height="1.2583333333333333in"}

Figure 8. Illustrative Implementation of L1/L5 Receiver Front-End

## COMPATIBILITY ANALYSES

The IGEB Third Civil Signal Implementation Steering Group ad hoc Working
Group 1 (IGEB WG1) developed a methodology for determining compatibility
of L5 with exsting systems \[9\]. RTCA SC159 WG1 provided assistance to
IGEB WG1 in two important ways. First, SC159 WG1 provided assumptions
regarding receiver characteristics (i.e., the values described
previously in this paper for noise floor, selectivity, and blanking
performance). Second, there was considerable interaction between the
IGEB and RTCA during the development of the compatibility methodology.
This methodology 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 (for
acquisition). 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 \[11\]
    adjusted by a 6 dB safety margin \[14\])

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

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

is the ratio of peak non-blanked pulse power to precorrelation noise
power, 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.

### 

Using the assumptions and methodology described in the previous
sections, interference analyses were performed for the IGEB using a
software tool developed by The MITRE Corporation's Center for Advanced
Aviation System Development known as the GPS RFI Environment Evaluation
Tool (GREET). Interference scenarios in the conterminous U.S. (CONUS)
and other regions of the world 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 9-13).

Figures 9-10 \[13\] show signal-to-noise degradations for the world for
two flight levels. Each of these two plots includes effects only from
DME/TACAN ground beacons. Note that signal-to-noise degradation is very
dependent on altitude. At 5,000 ft above ground level (AGL) (Figure 9),
very few emitters can be seen and the environment is very clean. At
40,000 ft above mean sea level (MSL), many ground-based emitters can be
seen. Figure 10 indicates an unacceptable level of degradation at this
altitude certain regions in CONUS, Europe, and Japan.

Figures 11-13 show signal-to-noise degradations in the United States for
these same two flight levels. Figures 11 and 13 include the effects from
all known emitters including:

-   DME/TACAN ground beacons

-   JTIDS/MIDS

-   Out-of-band emissions from radars

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

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 12 that only includes the effects of
DME/TACAN. A very promising solution identified by IGEB WG1 is to
reassign a subset of the inband DME/TACANs. Figure 13 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.

![](media/image9.wmf){width="6.954166666666667in"
height="4.1097222222222225in"}

**Figure 9. GPS Receiver Postcorrelation S/N~0~ Degradation at 5,000 ft
AGL from DME/TACAN**

![](media/image10.wmf){width="6.954166666666667in"
height="4.1097222222222225in"}

**Figure 10. GPS Receiver Postcorrelation S/N~0~ Degradation at 40,000
ft MSL from DME/TACAN**

![](media/image11.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/image12.png){width="3.245833333333333in"
height="1.9104166666666667in"}

Figure 12. GPS Receiver Postcorrelation S/N~0~ Degradation from
DME/TACAN at 40,000 ft MSL over CONUS

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

Figure 13. Total GPS Receiver Postcorrelation S/N~0~ Degradation at
40,000 ft MSL over CONUS, DME/TACAN Emitters at 1176.45 ±10 MHz
Reassigned

### SUMMARY

This paper has described the contributions of RTCA SC159 WG1 (3^rd^
Civil GPS Frequency) towards the implementation of a third civil GPS
signal at 1176.45 MHz. These contributions came primarily in two areas.
First, Working Group 1 (WG1) was relied upon by the Interagency GPS
Executive Board (IGEB) to develop consensus on the design of the third
civil signal. Analyses performed by WG1, described in this paper, have
facilitated IGEB consensus on the signal structure described in \[3\].

As its second major contribution to the implementation of a new civil
signal at L5, WG1 developed a set of receiver assumptions, and
participated in the development of a methodology for analyzing the
compatibility of the third civil signal with existing systems operating
at or near L5.

Preliminary results from applying this methodology indicate that on the
surface of the earth and at low altitudes, no modifications to known
existing systems 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 participants in RTCA SC159 WG1. Special thanks to Dr. Tom
Morrissey (Zeta Associates), Bob Erlandson (Rockwell-Collins), Tae Kim
and Swen Ericson (MITRE) for providing some of the figures presented in
this paper. Also acknowledged are Tom Stansell, Richard Keegan and Dr.
Charles Cahn, all representing Leica, for their suggestions and support
with regard to the coherent (dataless) carrier component of the L5
signal.

**REFERENCES**

Many of the references listed below are presentations that were made at
RTCA SC159 WG1 meetings. These presentations may be obtained from the
WG1 co-chairs (ajvd@aol.com or Chris.Hegarty@faa.gov).

1.  Anon., *Vice President Gore Announces New Global Positioning System
    Modernization Initiative*, Press Release, The White House, Office of
    the Vice President, January 25, 1999.

2.  Anon., *Plan for Implementation of the Third Civil GPS Signal*,
    Interagency GPS Executive Board, February 9, 1999.

3.  Spilker, J.J., and A.J. Van Dierendonck, "Proposed New Civil GPS
    Signal at 1176.45 MHz," *Proceedings of The Institute of Navigation
    ION GPS '99*, Nashville, Tennessee, September 1999.

4.  Van Dierendonck, A.J., and J.J.Spilker, "Proposed Third Civil GPS
    Signal at 1176.45 MHz: In-Phase/Quadrature Codes at 10.23 MHz Chip
    Rate," *Proceedings of The Institute of Navigation Annual Meeting*,
    Cambridge, Massachusetts, June 1999.

5.  Stansell, T., C. Cahn, and R. Keegan, *Lc Signal Structure*,
    Memorandum to A.J. Van Dierendonck, February 12, 1999.

6.  Hegarty, C., *Evaluation of the Proposed Signal Structure for the
    New Civil GPS Signal at 1176.45 MHz*, The MITRE Corporation, Working
    Note WN99W34, McLean, Virginia, June 1999.

7.  Morrissey, T., *Forward Error Correction for GPS L5 Data*, RTCA
    SC159 WG1, London, July 20-21, 1999.

8.  Van Dierendonck, A.J., *L5 Acquisition Considerations*, RTCA SC159
    WG1, Washington, D.C., June 7, 1999.

9.  Hegarty, C., T. Kim, S. Ericson, P. Reddan, T. Morrissey, and A.J.
    Van Dierendonck, "Methodology for Determining Compatibility of GPS
    L5 with Existing Systems and Preliminary Results," *Proceedings of
    The Institute of Navigation Annual Meeting*, Cambridge,
    Massachusetts, June 1999.

10. Anon., *1996 Federal Radionavigation Plan*, U.S. Department of
    Defense and Department of Transportation,
    DOT-VNTSC-RSPA-97-2/DOD-4650.5, July 1997.

11. Change 3 to RTCA/DO-229B Minimum Operational Performance Standards
    for Global Positioning System/Wide Area Augmentation System Airborne
    Equipment, RTCA Paper No. 061-98/SCI59-781, March 31, 1998.

12. Erlandson, R.J., *L5 Receiver Implementation Aspects*, RTCA SC159
    WG1, London, July 20-21, 1999.

13. Kim, T., and S. Ericson, *Analyses of GPS L5 Receiver Radio
    Frequency Interference from DME/TACAN, JTIDS/MIDS, ARNS, and Radar*,
    RTCA SC159 WG1, London, July 20, 1999.

14. Anon, *Modification of preliminary draft new Recommendation ITU-R
    M.\[RNSS.CHAR\], Technical Characteristics of Current and
    Prospective RNSS (Space-to-Earth) and ARNS Receivers to be
    Considered in Interference Studies in the Band 1559-1610 MHz*, U.S.
    ITU-R Working Party 8D, Document USWP8D/13 (Rev. 5), April 12, 1999.