**Potential Interference to GPS from**

**UWB Transmitters:**

**Test Plan \-- Version 4.5**

**Phase 1:**

Accuracy Test for Aviation Receivers and

Reacquisition Time Test for Land Receivers

Ming Luo, Dennis Akos, Sam Pullen, Per Enge

Stanford University

Table of Contents

1.0 Background 2

2.0 Introduction to Test Plan
\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...6

3.0 Test Scope 8

4.0 Overview of Test Procedure [10](#overview-of-test-procedure)

4.1 Calibration [10](#calibration)

4.2 Receiver Normalization [10](#receiver-normalization)

4.3 Receiver Operating Points [10](#receiver-operating-points)

4.4 UWB Interference Measurements [11](#uwb-interference-measurements)

4.5 Reporting [11](#reporting)

5.0 Accuracy Test Procedure for Aviation Receivers
[13](#__RefHeading___Toc480186566)

5.1 Broadband Random Noise Normalization
[13](#broadband-random-noise-normalization)

5.2 Procedure for Testing Potential UWB Impact on GPS Accuracy
[13](#__RefHeading___Toc480186568)

6.0 Reacquisition Time Test Procedure for Land Receivers
[15](#__RefHeading___Toc480186569)

6.1 Broadband Random Noise Normalization
[15](#broadband-random-noise-normalization-1)

6.2 Reacquisition Time Test with UWB Noise
[16](#reacquisition-time-test-with-uwb-noise)

Appendices:

A.
Figures\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\.....17

B.
Calibration\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\.....24

# 1.0 Background

The Global Positioning System (GPS) is fundamental to the critical
infrastructure of the United States (US) and internationally. GPS is a
fully operational service that provides a global source for accurate
timing and positioning, 24 hours a day. GPS is presently used by
aviation for the en-route and non-precision landing phases of flight.
GPS is currently used within the US for precision approach and landings
and is in the final stages of approval as a national and international
standard. Companion GPS-based applications for runway incursion and
ground traffic management are also underway. Additionally, GPS-based
public safety systems and services are fielded. Planned or newer
systems, such as Enhanced 911 (E911) and personal location and medical
tracking devices are soon to be commercially available. Additional
future systems are planned for land, marine and space applications. The
US telecommunications and power distribution systems are dependent upon
GPS for network synchronization timing. Further, GPS is a powerful
enabling technology that has created new industries and new industrial
practices fully dependent upon GPS signal availability and continuity.
Several critical industries, both aviation and non-aviation, would incur
adverse impact if there were degradation to GPS signal continuity and
availability.

UWB technology is based on very short pulses of radio energy. Its wide
signal bandwidth yields excellent multipath immunity. UWB technology has
potential in a variety of applications including communication and
ranging, and is expected to see increased civil use in the future. The
UWB technology was the focus of the Notice of Inquiry (NOI) of the
Federal Communications Commission (FCC) under the Office of Engineering
and Technology (OET) entitled \"Notice of Inquiry in the Matter of
Revision of Part 15 of the Commission\'s Rules Regarding Ultra-Wideband
Transmission Systems\", FCC Docket Number (No.) 98-208/ET No. 98-153.

Because GPS has a pivotal role in so many critical systems that the
public depends upon for its safety and well being, it is necessary to
determine what the potential for interference is from ultra-wideband
(UWB) systems to GPS. Preliminary analysis and testing has indicated a
potential for interference from some types of UWB sources to GPS
reception. These preliminary findings call for the performance of
controlled testing to determine the nature and extent of the potential
for interference to GPS from selected UWB parameters in order to assure
public safety and safety-of-life. Without test results, such an
assurance cannot be made with full confidence since preliminary analysis
has shown a potential for interference from UWB to GPS and other
systems, including fielded aviation systems.

The aviation community has a large body of developed and published
technical standards for GPS and defined interference criteria making it
logical to initiate the first phase of testing for aviation based on
this large body of work. Additionally, due to the critical role of many
non-aviation GPS-based applications, this test phase also addresses some
issues of land receivers.

This test phase selects the metric of accuracy performance and GPS
signal reacquisition time. Aviation receivers meeting published
specifications are used in the accuracy measurement phase; a land
receiver will be used for the reacquisition testing. A GPS simulator
provides the GPS input and the UWB parameters are provided by a
prototype UWB waveform generator where the various UWB waveform
parameters can be varied independently in a controlled manner. These
metrics were considered appropriate for the first phase of testing.
Accuracy measurements also include the deleterious effects of cycle
slips, and are an appropriate metric not only for precision approach but
other demanding applications as well, for example, machine guidance.

Reacquisition, while important to many aviation applications, is a
critical performance metric for dynamic, real-time land applications,
such as emergency medical response vehicles, other public safety
vehicles and in-vehicle navigation. Reacquisition is also a critical
performance metric for marine applications in harbor and harbor-approach
areas. Particularly under extreme weather conditions, these systems can
be the lifeline of a successful search-and-rescue situation or can be
the measure preventing the initial event of the accident.

A full testing program would include not only aeronautical systems, but
systems critical to land and sea operations. We note that systems such
as radio astronomy and private sector systems should be looked at to
determine whether there is potential for interference from UWB systems
operating under any proposed rules. Test results can be inculcated into
the technical rules, support appropriate regulatory actions and other
associated decisions. It is also important to consider the current role
that GPS plays in the consumer market. Since many UWB proposals are for
consumer-grade products, it is important to assure that already existing
GPS-based consumer products are included in an appropriate manner in the
analysis and decision-making process.

The first phase of the test program concentrates on the aeronautical
applications of GPS L1 signal, centered at 1575.42 MHz. These tests are
necessary to evaluate the impact that UWB device emissions could have on
safety-of-life aeronautical systems that are based on the GPS Standard
Positioning Service (SPS), the Wide Area Augmentation System (WAAS), and
the Local Area Augmentation System (LAAS). Allowable levels of
interference are already specified in the LAAS Minimum Performance
Standards (MASPS) and the WAAS and LAAS Minimum Operating Performance
Standards (MOPS) interference "masks". Appropriate reference documents
include:

1.  *Assessment of Radio Frequency Interference Relevant to the GNSS,*
    January 27, 1997 (RTCA DO-235).

2.  *Minimum Aviation Performance Standards for the Local Area
    Augmentation System,* September 28,1998 (RTCA/DO-245).

3.  *Minimum Operational Performance Standards for Global Positioning
    System/Wide Area Augmentation System Airborne Equipment,* October 6,
    1999 (RTCA DO-229B or the GPS/WAAS MOPS).

4.  *Minimum Operational Performance Standards for GPS Local Area
    Augmentation System Airborne Equipment,* January 11, 2000*, (*RTCA
    DO-253 or the GPS/LAAS MOPS).

5.  *Technical and Performance Characteristics of Current and Planned
    RNSS (space-to-earth) and ARNS Receivers to be Considered in
    Interference Studies in the 1559 to 1610MHz,* International
    Telecommunications Union (ITU) Document 8/83-E, April 29,1999

6.  International Civil Aviation Organization (ICAO) Global Navigation
    Satellite System Panel (GNSSP) SARPs, *Resistance to Interference
    Section B.3.7*

7.  *Technical Standard Order C129, Airborne Supplemental Navigation
    Equipment Using the Global Positioning System (GPS*), TSO C129,
    USDOT Federal Aviation Administration, December 1992.

8.  *Global Positioning System - Standard Position System Signal
    Specification*; 2nd Edition; June 2, 1995.

Table 1 highlights the parameters used to derive the limits on
out-of-band (OOB) emissions from Mobile Satellite Service (MSS) Mobile
Earth Terminal (MET) in order to protect aeronautical GPS receivers used
for Cat I precision approaches.

## Table 1. GPS L1 Receiver RFI Susceptibility Link Budget 

# for Single MSS MET Interference for Category I Landings

^1^ This value was determined for Mobile Satellite Services (MSS) only,
for the 1559-1610 MHz band.

^2,\ 3^ This parameter was determined for one MSS emitter and one GPS
receiver onboard an aircraft for Category I; it may not be appropriate
for all pertinent aviation or non-aviation operational scenarios.

^4^ This margin will be absorbed by other aeronautical services.

As noted in Table 1, the total RFI susceptibility limit is --140.5
dBW/MHz. RTCA SC-159 is currently finalizing the link budget for
Category II/III approaches and landings that will be similar in nature.
It is expected that aeronautical interference sources external to GPS
and the additional receiver hardening required for Category II/III
approach and landings will consume the entire 5.6 dB aeronautical
services margin. This 5.6 dB margin results in a C/N~0~ margin of only
3.2 dB for the LAAS application (as detailed in Reference 5, Annex 5).

Due to the adoption of a -70 dBW/MHz limit by the FCC for the MSS MET,
the total level of -146.1dBW/MHz is taken up by the MSS earth-to-space
services leaving no margin for the UWB emissions or other new
technologies that may be proposed in the future. In order to appreciate
why the GPS Category I link budget has a lack of margin it is necessary
to provide additional background on the allowed RFI allocation process
and the integrity monitoring design of the GPS receiver.

For a MOPS-compliant GPS receiver (i.e., the receiver operates at the
minimum standard), the significance of the susceptibility limit is that
any combined *non-aeronautical* RFI exceeding -146.1dBW/MHz is likely to
cause an alert leading to loss of continuity. In other words, the
performance of minimally MOPS-compliant receivers will fall short of
requirements and may generate Harmful and Misleading Information (HMI)
in the absence of navigation alert. The MOPS specifies that all combined
non-aeronautical RFI below -146.1 dBW/MHz shall not cause a loss of
continuity. GPS receivers that surpass the MOPS requirements must issue
a loss-of-continuity alert when RFI exceeds -146.1 dBW/MHz and a
navigation hazard is present; the hazard must be detected and alerted so
that users are not threatened by it.

The aeronautical community is concerned because there is no margin
available in the -140.5dBW/MHz susceptibility limit for non-aeronautical
RFI from other sources such as UWB devices since all available margins
were allotted to a single MSS MET. For instance, there also is no margin
for the World Radiocommunication Conference of 1997 (WRC- 97) Inmarsat
proposal to operate space-to-Earth MSS satellites in the 1559-1567 MHz
band. The issue is still on the WRC-2000 agenda.

These statements are true even for a device that conforms to Part 15
limits. For example, the FCC spurious emissions of a Part 15 device must
be below -71 dBW/MHz in the GPS band. This results in an RFI level of
-147.5 dBW/MHz at 100 feet. See Table 1. Since MSS METs and UWB device
emissions may combine at the aircraft, the resulting RFI level would be
-143.56 dBW/MHz. After including 5.6 dB of aeronautical signals, the RFI
level would be -138 dBW/MHz, or 2.5 dB above the allowed level of ‑140.5
dBW/MHz. This reduces the safety margin reserved for aviation use to an
unacceptable level.

Furthermore, the above RFI scenario does not include any effects from
multiple MSS METs, multiple UWB devices, VHF harmonics, or other
systems. It identifies a receiver-emitter proximity for a single,
critical aeronautical application i.e. Category I precision approach and
landing. The range of aeronautical use of GPS has evolved and requires
examination of the range of the receiver-emitter proximity to assure
that this range and the other parameters listed (see Table 1) protect
all aeronautical use of GPS. Further, these parameters must be examined
for appropriate non-aviation operational scenarios to assure that
appropriate public safety services will be protected. To achieve this
work, the appropriate operational scenarios must be developed to provide
the framework into which the technical results of testing can be
applied. This is true for any service, aviation or non-aviation.

It is planned to include study of the aggregate effect of multiple UWB
emitters in a later study phase, pending funding. Certainly to determine
the appropriate protection limits for systems that may be potentially
affected, the aggregate effect must be somehow determined.

The above discussions described the link-budget margin for receivers
used in a given aeronautical safety-of-life scenario. For
non-aeronautical applications the scenarios are under discussion.
Critical scenarios also include non-aviation safety-of-life and public
safety services, such as ambulance and E911 services. In the ambulance
scenario the possibility arises where terrestrial GPS receivers, MSS
hand-held cell phones and UWB devices may operate simultaneously at very
close ranges. If interference between these systems occurs, all services
can be adversely impacted not only technically but economically as well.

Importantly, appropriate operational scenarios be developed for aviation
and non-aviation applications. The test plan will collect interference
effects data using both aeronautical and non-aeronautical receivers that
when combined with the appropriate protection limits will allow the
analysis of any appropriate scenario.

> **2.0 Introduction to Test Plan**

The goal of this test plan is to characterize the interference effects
of UWB emissions on various types of aviation and non-aviation GPS
receivers in a controlled test environment. Some UWB emissions could be
quite noise-like while others may have more discrete spectral lines in
the vicinity of GPS. An RFI equivalence concept was developed to relate
the interference impact of UWB signals on GPS over this range of UWB
emissions to that of a known and well understood RFI source, i.e.,
broadband noise. The method chosen for this test plan is to determine
the UWB interference effect for a given set of emission parameters that
is equivalent to a known portion of the broadband noise input which
causes the GPS receiver to just meet its performance criterion. A
significant level of broadband noise is input to give a representation
of the actual GPS environment.

The test criteria consist of pseudorange measurement accuracy for
aviation receivers and reacquisition time for non-aviation receivers.
The pseudorange accuracy criterion for aeronautical GPS receivers is a
standard deviation of less than 15cm[^1]. The equivalence concept test
methodology consists of inserting broadband noise into the GPS receiver
and increasing its level until 15 cm of pseudorange standard deviation
is indicated. The broadband noise source is then reduced 2 dB and the
UWB emission level is increased by varying one of the UWB parameters
(e.g. power) until there is a 15 cm pseudorange standard deviation
indication. The above procedure is repeated with the broadband noise
source reduced by 4 dB instead of 2 dB. Another UWB parameter (e.g. PRF)
is chosen and the entire sequence repeated until all UWB parameters have
been investigated. From this interference effect data, a profile of
those UWB parameters that have the most significant effect on GPS
accuracy performance will emerge.

This process provides accuracy data at three different levels of
broadband noise (100%, 63%, and 40% of the critical noise input) in
combination with three different levels of UWB RFI (0%, 37%, 60%). These
data capture the RFI effects on the GPS receiver that can be used in
external derivations of the UWB protection level appropriate for GPS. An
equivalent process is used for a non-aeronautical receiver with a one
second acquisition time as the test criterion.

Three potential benefits from determining the equivalence of UWB
transmissions with broadband noise are:

1)  a simple test procedure;

2)  interference effects data that can use information from specific
    interference encounters (e.g., range, antenna orientation and gain,
    source motion) and UWB source information to determine compatible
    UWB scenarios that satisfy the protection limit; and,

3)  if during the broadband noise equivalence test, a 4 dB increase in
    broadband noise also corresponds to a 4 dB increase in the UWB
    transmitter power, for the same accuracy degradation value (15 cm)
    then UWB source may be classified as noise-like. In such cases a
    simple calculation of broadband noise sources can determine UWB
    protection limit.

It should be noted that this test plan does not:

1)  define the UWB protection limits; or,

2)  define the interference scenarios.

Also a separate effort, not a part of this test, is necessary to
determine effective UWB emission measurement techniques, since existing
methods (e.g., FCC Part 15) tailored for older technologies are likely
inadequate. As testing proceeds, detailed notes will be taken and
developed into appendices if warranted to clarify the details of the
various aspects of this testing approach.

Further testing for GPS must include at a minimum other receiver types
such as fielded aviation equipment based on TSO C129 standard, include
the aggregate effect of multiple UWB emitters, and address the additive
affect of other systems and their out-of-band emissions. Note that it is
important to test with actual UWB equipment to validate these results
and add additional parameters reflective of current UWB technology.
Future testing should be accomplished to look at discrete and continuous
spectra, noting that some UWB equipment is a combination of the two.

> **3.0 Test Scope**

The test plan for this phase of testing includes an accuracy test for
aviation receivers and a reacquisition time test for land receivers, and
these tests will be sequenced as follows:

1)  Accuracy test for aviation receiver #1

2)  Accuracy test for aviation receiver #2

3)  Reacquisition time test for land receiver #3

4)  Accuracy test for aviation receiver #1 with a pseudolite sharing the
    channel

5)  Reacquisition time test for land receiver #4

6)  Reacquisition time test for land receiver #3 (or #4) with a
    pseudolite sharing the channel

In all cases, the tests will quantify the RFI impact of UWB signals
relative to that of a known amount broadband random noise. In this plan,
broadband random noise will refer to continuous noise from a noise diode
that has power spectral density much broader than the RF/IF bandwidth of
the GPS receiver. Such noise is used to model thermal noise in the
receiver, sky noise and any other wideband interference process *other
than* UWB. UWB signals also have bandwidths that are greater than the
front end of the GPS receiver, but they have an additional structure
that may cause their RFI effect to be very different than broadband
random noise.

The receiver's C/N estimator will not be used to estimate total noise
power for the following reasons. First, any given GPS receiver\'s C/N
estimator may respond differently to broadband random noise than another
receiver's estimator. Second, the estimators may respond differently
depending on the UWB signal parameters.

Pseudorange measurement accuracy, acquisition/reacquisition times, and
loss-of-tracking threshold are the four important performance metrics to
GPS users. For this test phase, the metric selected is accuracy
performance in an aviation receiver. The most demanding precision
approach operations require a pseudorange measurement standard deviation
of less than 15cm. Pseudorange measurement accuracy is influenced by
degradations from both code and carrier tracking. As such it is the most
sensitive metric for the aviation applications.

Acquisition/reacquisition time is an important metric for most land
users. For example, in-vehicle navigation and emergency vehicles need to
quickly reacquire GPS after signal loss and develop a new position
estimate. For this reason, emergency land applications require
reacquisition times of approximately 1 second. The reacquisition tests
described here assume that only one satellite is lost and must be
reacquired.

These tests are crafted to provide input to a separate process that
considers the operational scenarios that might place UWB and GPS
equipment in proximity. Such scenarios may include the use of GPS to
provide position reports with all E911 calls. They may also include the
use of GPS to avoid runway incursions, or the use of GPS during the
precision approach of aircraft. Each scenario has a link budget that
*assumes* that the presence of certain types of interference. The test
described will not develop the scenarios or the associated link budgets.
Rather, they will provide data on the interference effects of various
combinations of UWB signal parameters.

The RFI effect of the UWB signal will be sensitive to the details of the
UWB signal design. Some of these trends are depicted in Figure 1. We
anticipate that our interference measurements will reflect the following
quantitative trends:

-   Pulse Repetition Frequency (PRF): If the pulses are sent at a very
    low rate compared to the front end bandwidth of the GPS receiver,
    then the interference will be smaller than that due to UWB operation
    at high PRFs. Most GPS receivers have bandwidths between 2 MHz and
    24 MHz. If the UWB PRF is less than 2 million pulses per second
    (MPPS), then the pulses will still be distinct at the output of the
    receiver front end and interference will probably be relatively
    small. If the UWB PRF is higher than the bandwidth, then the GPS
    front end will smear the pulses together and the interference effect
    will probably be larger. GPS receivers are well known to have lower
    sensitivity to pulsed interference and higher sensitivity to
    continuous interference.

-   No Modulation: If the PRF is high, then the interference effect will
    depend on the UWB modulation. Some UWB signals may not be modulated.
    In this case, the signal is a pulse train with a constant time
    between pulses. This case is shown in Figure 2 and results in the
    line spectrum also shown in Figure 2. The GPS spectrum for the C/A
    code also has a line spectrum. UWB interference will be greatest
    when the UWB lines fall on top of the GPS spectral lines. UWB
    interference will be small when the UWB lines fall between the GPS
    lines. This spectral coincidence is difficult to predict and the UWB
    effect on GPS will be very variable.

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-   Pulse Modulation: If the UWB pulses are modulated randomly or with a
    long code, then the line spectrum will disappear. This effect is
    shown in Figure 3, which shows the amplitude spectrum for a UWB
    pulse train without modulation, with on-off-keying (OOK) and with
    pulse position modulation (PPM). If modulation is used with
    sequences that are continuous and have high PRFs, then the
    interference effect will be similar to white noise of equal power.

-   Pulse Bursting: As shown in Figure 2, UWB pulses may be transmitted
    in bursts with a prescribed on-time and off-time. If the duty cycle
    (fractional on-time) is less than 40 percent or so, then we expect
    that the effect of one UWB transmitter on a GPS receiver will be
    reduced. The interference effect will also depend on the on-time of
    the pulse bursts.

-   Pulse Shaping: As shown in Figure 4, the overall UWB spectrum
    depends on the pulse shape. The pulse can be crafted so that the UWB
    spectrum avoids certain critical bands.

All of these trends must be validated and quantified. To that end, these
tests will vary the UWB signal parameters and determine how the UWB to
broadband random noise equivalence depends on the UWB signal parameters.
This test philosophy is depicted in Table 2 and Figure 5, which show
four loops on the UWB signal parameters. The first loop simply varies
the modulation from: no modulation to random OOK to random PPM. The
second loop transmits pulse bursts with varying duty cycle. The third
loop varies the UWB pulse repetition frequency (PRF). The final loop
captures the effect of pulse shaping by varying the UWB power. These
tests simply treat the UWB power level in the GPS band as an independent
parameter.

Table 2: UWB Signal Parameters to be tested.^1^

  ----------------------------------- -----------------------------------
  UWB Signal Parameter                Range

  Power (dBW/MHz)                     As need to introduce the
                                      interference effects described
                                      below^2^

  Pulse Repetition Frequency (MHz)    0.1, 1.0, 20.0

  Modulation                          None, random OOK, random PPM^3^

  Burst Duty Cycle (%)                10, 50, 100

  Burst On-Time                       0.1 millisecond (msec), 1 msec, 10
                                      msec
  ----------------------------------- -----------------------------------

^1^ The permutations listed in the table represent the current plan.
Different values may be selected based on the early test results.

^2^ The UWB test pulse spectra are depicted in Figure 4, where the pulse
amplitude is controlled to introduce a known amount of UWB noise power
in the GPS band.

^3^ The random PPM will be such that no spectral lines remain and the
spectrum is continuous.

# 4.0 Overview of Test Procedure

###### 4.1 Calibration

We now describe the overall test procedure that is depicted in Figures 6
through 8. As shown in Figure 6, the test begins with calibration of:
the GPS signal generator and signal path, the broadband random noise
source and the UWB signal source. This procedure is described in the
Appendix and will not be further detailed in the body of the test plan.

###### 4.2 Receiver Normalization

Next, the receiver is normalized using the Test Setup shown in Figure 9.
Accuracy and reacquisition time are measured as a function of input
noise where the noise is entirely due to broadband random noise with no
UWB component. This step establishes receiver performance in the absence
of UWB noise and provides a baseline for later comparison.

All noise power measurements will be made using a bandpass filter that
is based on the interference masks in the WAAS and LAAS Minimum
Operational Performance Standards (MOPS). This measurement filter has a
noise bandwidth of approximately 20 MHz. All accuracy and reacquisition
time measurements will be made as a function of the noise power (N~0~)
as measured at the output of this standard filter. A current NTIA test
program will relate increase in receiver noise using various receiver
bandwidths as a function of UWB parameters.

The results from the receiver normalization will sample the curves shown
in Figures 10 and 11. As shown, both accuracy and reacquisition time
will degrade with increasing noise power. Each data point will require
many measurements to establish statistical confidence. For the accuracy
normalization, the number of measurements per sample will be large
enough to provide a 95% confidence at the 1-centimeter level. For the
reacquisition time normalization, the number of measurements will be
large enough to provide a 95% confidence at the 0.5-second level.

The time required to establish these levels of confidence is receiver
dependent. The samples must be statistically uncorrelated, and the time
between such uncorrelated samples depends on the bandwidth of the
receiver's tracking loop. Hence, this tracking bandwidth will be
determined for each receiver under test and used to determine the time
required to test each receiver.

To minimize test time, the accuracy tests will use code minus carrier
measurements, where the code will not be smoothed by the carrier. These
*unsmoothed* errors are greater than the errors using carrier smoothing.
Moreover, the 15-centimeter (cm) requirement is based on 100 seconds of
carrier smoothing. Hence, the 15-cm requirement must be inflated by the
factor, *k*, shown in Figure 9. This factor is given by the noise
equivalent bandwidth of the loop providing the unsmoothed measurements
divided by the noise equivalent bandwidth with 100 seconds of carrier
smoothing. This factor must be determined with care, because the ratio
of these noise bandwidths is not necessarily given by the inverse of the
ratio of their stated time constants.

###### 4.3 Receiver Operating Points

The normalization curves depicted in Figures 10 and 11 will be used to
determine the operating point for the UWB interference measurements. The
accuracy test will be operated near the noise power required for an
accuracy of *k*15 centimeters. This power is denoted
![](media/image1.wmf). The reacquisition test will be operated near the
noise power required for a reacquisition time of 1 second. This power is
denoted *N^\*^~REACQ~*. These operating points shall be determined to an
accuracy of +/- 0.5 dB.

For accuracy, the UWB interference tests are initiated at broadband
random noise powers given by *N^\*^~ACC\ ~* - 2dB and *N^\*^~ACC\ ~* -
4dB. For reacquisition time, the UWB interference tests are initiated at
broadband random noise powers given by *N^\*^~REACQ\ ~* - 2dB and
*N^\*^~REACQ\ ~* - 4dB.

###### 4.4 UWB Interference Measurements

The UWB interference measurements are shown in Figure 7 and the Test
Setup is shown in Figure 12. For future testing, the setup also has the
capability to include signals from a pseudolite. As shown, UWB noise
power is added to the broadband random noise. These tests are designed
to provide data points on curves such as those shown in Figure 13 for
accuracy and Figure 14 for reacquisition time. In both cases, the
broadband random noise power (N~0~) is decreased so that the noise power
is at the operating points discussed above. From that operating point,
UWB power is introduced to increase the total noise power (N~0~+N~UWB~).
As shown in Figures 13 and 14, this degradation may or may not cause the
performance curves to follow the curves for broadband random noise
alone, and the exact trajectory will depend on the UWB signal
parameters. If the specific UWB waveform has a more deleterious effect
than broadband random noise, then the UWB trajectory will be higher than
the broadband random noise curve. If the parameters are such that the
UWB signal is less damaging than broadband random noise, then the UWB
trajectory will fall under the broadband random noise curve. Both
situations are depicted in Figures 13 and 14.

The UWB portion of the total noise power (N~0~+N~UWB~) will be changed
in 1 dB steps. UWB noise power will be measured in the same standard
filter described above. This practice requires that the UWB PRF be less
than 20 Mpps. If the pulse rate is greater, then the UWB spectral lines
may fall outside of the filter passband and the results will be
unreliable.

As before, each sample will require many measurements to establish
statistical confidence. For the accuracy tests with UWB, the number of
measurements per sample will be large enough to provide a 95% confidence
at the 1-centimeter level. For the reacquisition time tests with UWB,
the number of measurements will be large enough to provide a 95%
confidence at the 0. 5-second level. The time required for the UWB
interference measurements will be receiver dependent and the bandwidth
of the receiver under test will be used to determine the test time. Once
again, code-carrier measurements will be used to minimize the time
required for the accuracy tests.

###### 4.5 Reporting

For each set of UWB signal parameters, we will report the following
parameters of significance:

1)  UWB power (N~UWB~) portion of the total noise power (N~0~+N~UWB~)
    required to degrade the accuracy to *k*15 cm.

2)  Accuracy as measured by code minus carrier when *N~0~ + N~UWB~ =
    N^\*^~ACC~*. In other words, record the accuracy when the noise
    power including UWB noise is equal to the previously determined
    threshold for broadband random noise only.

3)  UWB portion (N~UWB~) of the total power (N~0~+N~UWB~) required to
    degrade the reacquisition time to 1 second.

4)  Reacquisition time when *N~0~ + N~UWB~ = N^\*^~ACC~*. In other
    words, record the reacquisition time when the signal to noise ratio
    including the UWB noise is equal to the previously determined
    threshold for reacquisition for broadband random noise only. The
    above listed parameters will be determined for both starting points
    N^\*^ - 2dB and N^\*^ - 4dB. We will provide timely inputs to the
    processes that are developing the operational scenarios.

> []{#__RefHeading___Toc480186566 .anchor}**5.0 Accuracy Test Procedure
> for Aviation Receivers**

The accuracy test procedure is described in the following two
subsections. This test procedure is adapted from Section 2.5.8 of RTCA
DO-229B, the *Minimum Operational Performance Standard for Avionics
Using the Wide Area Augmentation System (WAAS).* As described above, it
includes the following steps: calibration, normalization with white
noise only, UWB interference measurements, and reporting. The
calibration is described in the Appendix. Sections 4.1 and 4.2 detail
the broadband random noise normalization and the UWB interference
measurements respectively.

###### 5.1 Broadband Random Noise Normalization 

1)  Set up the test equipment as shown in Figure 9.

2)  The GPS receiver is operated with the minimum rated received
    satellite signal level. Compensation is applied to adjust for room
    temperature, satellite simulator noise output, or the effects of a
    remote antenna preamplifier as needed. In other words, set the GPS
    power (C) to --134.5 dBm+G~LNA~ where G~LNA~ is the gain of any
    equipment that might nominally appear between the antenna and the
    receiver under test.

3)  Broadband random noise is added to the simulated GPS satellite
    signal at the receiver input. Set the center frequency of the
    broadband noise to 1575.42 MHz. The starting value is the
    RTCA/DO-229B MOPS level for initial acquisition. Adjust the
    broadband noise power such that the noise power is --103.5
    dBm+G~LNA~ as measured in the standard filter described earlier. The
    gain G~LNA~ accounts for the gain that appears between the antenna
    and the receiver under test. As a rough check on power levels,
    measure the carrier to noise density (C/N~0~) as reported by the
    receiver. This (C/N~0~) should be approximately 33 dB-Hz.

4)  Let the GPS receiver track the satellite and reach steady state (for
    at least 10 seconds).

5)  Measure the unsmoothed pseudorange and estimate the one-sigma
    pseudorange error by computing the standard deviation ~r~ of the
    code-minus-carrier test statistic after removing a 2^nd^-order
    polynomial fit of the mean. Use the sample size required to achieve
    the confidence levels described above. Also recall that the
    unsmoothed pseudorange error is larger than the smoothed pseudorange
    error by a factor of *k*. This factor is the ratio of the noise
    bandwidth for the code loop to the noise bandwidth when 100 seconds
    of carrier smoothing is used.

6\) Increase the broadband random noise power in 1 dB steps until the
variance just exceeds the *k*15 cm accuracy limit. Record the noise
power setting (N^\*^~ACC~).[]{#__RefHeading___Toc480186568 .anchor}
Record also the C/N indicator from the GPS receiver.

###### 

###### 5.2 Procedure for Testing Potential UWB Impact on GPS Accuracy 

1)  Setup the test equipment as shown in Figure 12 without the
    pseudolite.

2)  Set the noise attenuator to 2 dB below the value obtained in Section
    4.1, Step 6 (N^\*^~ACC~).

3)  Select one set of UWB signal parameters from the test matrix
    described earlier and set the UWB noise power (N~UWB~) 10 dB below
    the broadband random noise power (N~0~).

4)  Let the GPS receiver track the satellite and reach steady state (for
    at least 10 seconds).

5)  Measure the unsmoothed pseudorange and estimate the one-sigma
    pseudorange error by computing the standard deviation ~r~ of the
    code-minus-carrier test statistic after removing a 2^nd^-order
    polynomial fit of the mean. Use the sample size required to achieve
    the confidence levels described above and recall that the unsmoothed
    pseudorange error is larger than the smoothed pseudorange error by a
    factor of *k*.

6)  Increase the UWB power until the *k*15 cm pseudorange variance is
    just exceeded. Record that power setting. Record also the C/N
    indicator from the GPS receiver. Also find and record the accuracy
    when the total power (UWB plus broadband) equals the threshold power
    for broadband noise alone.

7)  Change the UWB signal parameters to the next values in the test
    matrix and repeat steps 3) through 6) until all *n* combinations of
    UWB signal parameters are exhausted. For this initial test phase,
    *n=81.*

8)  Set the noise attenuator to 4 dB below the value obtained in Section
    4.1, Step 6 (N^\*^~ACC~) and repeat steps 3) through 6) to obtain a
    second set of data points for the *n* cases.

> []{#__RefHeading___Toc480186569 .anchor}**6.0 Reacquisition Time Test
> Procedure for Land Receivers**

The reacquisition time test procedure is described in the following two
subsections. This test procedure is adapted from Section 2.5.6 of RTCA
DO-229B, the *Minimum Operational Performance Standard (MOPS) for
Avionics Using the Wide Area Augmentation System (WAAS)*. These tests
assume that only one satellite is lost and needs to be reacquired. As
such, the receiver is assumed to have a good estimate of its time offset
relative to GPS time and the expected Doppler offset of the lost
satellite. However, the receiver must search over all possible values of
code phase.

Similar to the accuracy test, the reacquisition time test includes the
following steps: calibration, normalization with broadband random noise
only, UWB interference measurements, and reporting. The calibration is
described in the Appendix. Sections 5.1 and 5.2 detail the broadband
random noise normalization and the UWB interference measurements.

###### 6.1 Broadband Random Noise Normalization 

1)  Set up the test equipment as shown in Figure 9. Connect the
    simulator clock to the receiver clock. This connection provides the
    time information to the receiver that is assumed in the
    reacquisition time tests described in Section 2.5.6 of the MOPS.

2)  The GPS receiver is operated with the minimum rated received
    satellite signal level. Compensation is applied to adjust for room
    temperature, satellite simulator noise output, or the effects of a
    remote antenna preamplifier as needed. In other words, set the GPS
    power (C) to --134.5 dBm+G~LNA~ where G~LNA~ is the aggregate gain
    of any equipment that might nominally appear between the antenna and
    the receiver under test.

3)  Add broadband random noise to the simulated GPS satellite signal at
    the receiver input. Set the center frequency of the broadband noise
    to 1575.42 MHz. The starting value is the RTCA/DO-229B MOPS level
    for initial acquisition. Adjust the broadband random noise power
    such that the noise power is --103.5 dBm+G~LNA~ as measured in the
    standard filter described earlier. The gain G~LNA~ accounts for the
    gain that nominally appears between the antenna and the receiver
    under test. As a rough check on power levels, measure the carrier to
    noise *density* (C/N~0~) as reported by the receiver. This (C/N~0~)
    should be approximately 33 dB-Hz.

4)  Let the GPS receiver track the satellite and reach steady state (for
    at least 10 seconds).

5)  Attenuate the GPS signal so that the receiver loses lock.

6)  Introduce a 50 meter step in simulated pseudorange over 10 seconds
    while the signal is not being tracked by the receiver under test.

7)  Remove the attenuation of the GPS signal and measure the time until
    the receiver reports code phase lock continuously for 10 seconds.

8)  Repeat steps 4) through 7) until the sample size provides the
    confidence levels described above.

9)  Increase the broadband random noise power by 1 dB and repeat
    steps 4) through 9) until the noise power (N~0~) is slightly greater
    than the threshold power ![](media/image2.wmf) for the reacquisition
    time specification of 1 second.

###### 6.2 Reacquisition Time Test with UWB Noise

1)  Setup the test equipment as shown in Figure 12 without the
    pseudolite.

2)  Set the noise power to 2 dB less than the threshold noise power
    (N^\*^~REACQ~) determined in the broadband random noise tests
    described in Section 5.1.

3)  Select one set of UWB signal parameters from the test matrix
    described earlier and set the UWB noise power (N~UWB~) 10 dB below
    the broadband random noise power (N~0~).

4)  Let the GPS receiver track the satellite and reach steady state (for
    at least 10 seconds).

5)  Attenuate the GPS signal so that the receiver loses lock.

6)  Introduce a 50 meter step in simulated pseudorange over 10 seconds
    while the signal is not being tracked by the receiver under test.

7)  Remove the attenuation of the GPS signal and measure the time until
    the receiver reports code phase lock continuously for 10 seconds.

8)  Repeat steps 4) through 7) until the sample size provides the
    confidence levels described earlier for reacquisition time.

9)  Increase the UWB noise power by 1 dB and repeat steps 4) through 9)
    until the total noise power (N~0~+N~UWB~) is slightly greater than
    the power required to obtain a 1 second reacquisition time. Record
    the UWB power (N~UWB~). Also find and record the reqcauisition time
    when the total power (UWB plus broadband) equals the threshold power
    for broadband noise alone.

10) Change the UWB signal parameters to the next values in the test
    matrix and repeat steps 4) through 9) until all UWB signal
    parameters are exhausted.

11) Set the broadband random noise power to *N^\*^~REACQ~* - 4 dB and
    repeat steps 4) through 10) to obtain a second set of *n* values of
    UWB power settings.

# Appendix A: Figures

# 

![](media/image3.wmf){width="4.948611111111111in"
height="2.7909722222222224in"}

# 

**Figure 1: UWB Signaling**

![](media/image4.wmf){width="4.497916666666667in"
height="2.859027777777778in"}

# 

**Figure 2: Different UWB Signals**

# 

# 

# 

# 

# 

# ![](media/image5.wmf){width="4.561111111111111in" height="2.8958333333333335in"}

# 

**Figure 3: UWB Amplitude Spectrum**

# 

# 

![](media/image6.wmf){width="4.270833333333333in"
height="2.1465277777777776in"}

**Figure 4: A UWB Signal**

# 

# 

# 

# ![](media/image7.wmf){width="4.2875in" height="3.2256944444444446in"}

# 

**Figure 5: Loops for UWB Signal Parameters**

# 

# 

# 

# ![](media/image8.wmf){width="4.707638888888889in" height="3.002083333333333in"}

# 

**Figure 6: Overall Test Flow (1 of 3)**

# 

# 

# ![](media/image9.wmf){width="4.767361111111111in" height="2.9631944444444445in"}

# 

**Figure 7: Overall Test Flow (2 of 3)**

# 

# 

# 

# 

# 

# ![](media/image10.wmf){width="3.5805555555555557in" height="1.1895833333333334in"}

# 

**Figure 8: Overall Test Flow (3 of 3)**

# 

# 

# 

# 

# 

# ![](media/image11.wmf){width="4.124305555555556in" height="2.8118055555555554in"}

**Figure 9: Test Setup for Receiver Normalization**

# 

# 

![](media/image12.wmf){width="4.040277777777778in"
height="2.936111111111111in"}

**Figure 10: Receiver Normalization for Pseudorange Accuracy Test**

![](media/image13.wmf){width="3.9402777777777778in"
height="2.8756944444444446in"}

**Figure 11: Receiver Normalization for Reacquisition Time**

![](media/image14.wmf){width="3.8986111111111112in"
height="3.604861111111111in"}

**Figure 12: Full Test Setup**

# ![](media/image15.wmf){width="4.604166666666667in" height="2.9375in"}

# 

# 

**Figure 13: Pseudorange Accuracy as UWB Power is Added**

**to Increase the Total Noise**

![](media/image16.wmf){width="4.311805555555556in" height="2.9375in"}

**Figure 14: Reacquisition Time Increase as UWB Power is Added**

**to Increase the Total Noise Power**

# Appendix B: Calibration

###### **B.1 GPS signal calibration**

######## B.1.1 Measure signal power at the output of the GPS simulator

This step calibrates the GPS simulator and the cable. It provides the
relationship between the simulator's specified power lever and the
readings at the power meter or spectrum analyzer. If a spectrum analyzer
is used, calibrate the spectrum analyzer with a power meter as
necessary.

Procedure:

-   Turn off the PRN code of the GPS simulator.

-   Sweep the power level setup of the GPS simulator.

-   Measure the signal strength at the spectrum analyzer.

-   Plot the calibration chart (see Figure B2 for an example).

![](media/image17.wmf){width="4.143055555555556in"
height="3.0993055555555555in"}

######## B.1.2 Calibrate GPS Power with splitters and combiners

As shown in Figure B3, the test setup will use splitters and combiners.
Hence, we need to calibrate their effects. It is assumed that the
impedance of a GPS receiver is 50 ; thus there is no power reflection.

Procedure:

-   Select PRN code at the GPS simulator.

-   Sweep the power level of the simulator.

-   Plot received power vs. simulator setup power (to generate a plot
    like Figure B2).

-   Rotate the location of the spectrum analyzer to calibrate each port
    (for each receiver).

-   It may be preferable to just check a few points instead of sweeping
    the entire power range.

Notes:

1)  We can also calibrate the setup as an equivalent 6-port net using
    the network analyzer.

2)  To maintain the characteristics of the net close to their calibrated
    status, we plan to build an enclosure to keep the above components
    and their connections fixed.

> 3\) The circuit can be balanced by adding calibrated pads.

###### **B.2 UWB calibration**

######## B.2.1 Snapshots of UWB transmitted signal

Record pulse shape in the time domain and spectrum in the frequency
domain for each transmitter (for the selected parameters only).

######## B.2.2 UWB transmitted power vs. setup power

This procedure calibrates the measured UWB output power vs. the
transmitter setup power.

Procedure:

-   Set the UWB to the no-modulation mode and PRF = 20MHz (TBC).

-   Sweep the UWB power level by adjusting the attenuator.

-   Measure the signal strength at the spectrum analyzer.

-   Plot the calibration chart (for an example, see Figure B5).

######## ![](media/image18.wmf){width="4.794444444444444in" height="3.5854166666666667in"}  {#section-45 .unnumbered}

######## 

######## ![](media/image19.wmf)  {#section-47 .unnumbered}

######## B.2.3 UWB transmitted power through splitter and combiner

This setup takes into account component losses (and other effects) in
the automatic measurement setup (see Figure 12). This step is similar to
Figure B3. As noted before, calibration using the network analyzer may
be equivalent.

######## Figure B6: UWB Power through Splitter and Combiner

Procedure:

-   Sweep the power level of the UWB transmitter.

-   Plot received power vs. simulator setup power (to generate a plot
    similar to Figure B5).

-   Rotate the location of the spectrum analyzer to calibrate each port
    (for each receiver)

-   We may be able to just check a few points instead of sweeping the
    entire power range.

[^1]: Reference Document 4, paragraph 2.3.6.8.1, page 34