MEASUREMENT PLAN TO DETERMINE THE POTENTIAL INTERFERENCE IMPACT TO GLOBAL POSITIONING SYSTEM RECEIVERS FROM ULTRAWIDEBAND TRANSMISSION SYSTEMS

1.0 INTRODUCTION

Recent advances in microcircuit and other technologies have resulted in the development of pulsed radar and communication systems with very narrow pulse widths and very wide bandwidths. One definition of these ultrawideband (UWB) transmission systems is that the emissions have an instantaneous bandwidth of at least 25% of the center frequency of the device. There are several ways of generating very wide signals, including a series of impulses, spread spectrum and frequency hopping techniques. The UWB signals considered in this plan consist of a burst of energy of ideally one positive going cycle (e.g., impulses).

UWB transmission systems can perform a number of useful telecommunication functions that make them very appealing for both commercial and government applications. UWB transmission systems are capable of accurately locating nearby objects, and can use processing technology with UWB pulses to "see through objects" and to communicate using multiple propagation paths. However, the bandwidths of the UWB transmission systems are so wide that, although their average output power, in many cases, is low enough to be authorized under the unlicensed device regulations of the National Telecommunications and Information Administration (NTIA) and the Federal Communications Commission (FCC), some of the systems will intentionally emit signals in bands in which such transmissions are not permitted because of the potential for harmful effects on critical radiocommunication systems.

The Global Positioning System (GPS) is a critical government radiocommunication system that both NTIA and FCC are concerned about. GPS is a dual-use, spaced-based, broadcast-only, radio navigation satellite service (RNSS) that provides universal access to precise position, velocity, and time information on a continuous, worldwide basis, regardless of weather conditions. The GPS constellation consists of twenty-four satellites that transmit both an encrypted code, which is used by U.S. and allied military forces, and an unencrypted code, which is used in a myriad of commercial and consumer applications in the United States and overseas. GPS is presently used by aviation for en-route and non-precision approach landing phases of flight. The Wide Area Augmentation System (WAAS) for Category I precision approach service and the Local Area Augmentation System (LAAS) for Category II/III precision approach service are planned to be available for public use⁽¹⁾. GPS is also in the final stage of approval as an international aviation standard. Companion GPS-based applications for runway incursion and ground traffic management are also underway. Additionally, GPS-based public safety systems and services are being fielded. Planned systems, such as Enhanced 911 (E911) and personal location and medical tracking devices are soon expected to be commercially available. The U.S. telecommunications and power distribution systems are also dependent upon GPS for network synchronization timing. Moreover, GPS is a powerful enabling technology that has created new industries and new industrial practices fully dependent upon GPS signal reception. As it can be seen, both aviation and non-aviation users would incur adverse impact if there was degradation to GPS signal reception.

As a result of the rapid development of the technologies involved in UWB transmission systems, timely policy and regulatory decisions must be made. The FCC has released a Notice of Proposed Rulemaking (NPRM) that proposes regulations to permit the operation of UWB transmission systems on an unlicensed basis under FCC rules⁽²⁾. NTIA recognizes that UWB technology has potential in a variety of applications including communication and ranging, and is projected to proliferate rapidly in future consumer applications. However, GPS has an established pivotal role in many critical systems that the public has grown to depend upon. Therefore it is essential that the potential for interference from UWB transmission systems to GPS receivers be considered.

Preliminary measurements utilizing live GPS satellites in an uncontrolled environment have provided inconclusive results as to the potential for interference from UWB transmission systems to GPS receivers. Therefore, in order to assess the potential interference mechanisms and the extent of any interference to GPS receivers from UWB transmission systems, measurements in a controlled environment are necessary.

2.0 OBJECTIVE AND APPROACH

The primary objective of this measurement plan is to assess the interference potential of UWB signals to GPS receivers. A secondary objective is to promote the establishment of methods for measuring the impact of UWB signals to GPS receivers. The results of these measurements will support efforts to respond to the UWB NPRM.⁽³⁾

The measurements described in this document are designed to provide input to a separate analysis process that will consider the operational scenarios that might place UWB and GPS equipment in proximity. Such operational scenarios will be dependent on both existing and projected GPS and UWB applications and must take into consideration circumstances involving both single and multiple UWB transmission systems. Some examples of the types of GPS applications for which an operational scenario must be considered are: 1) the reporting of caller location when an E911 connection is made, 2) the use of GPS as an integral component of aircraft precision approach guidance systems, and 3) the use of GPS in traffic (e.g., air, maritime, rail, and/or land vehicular) management systems. For each application, a link budget will be developed under assumptions and/or known conditions that are defined by the particular operational scenario.

The measurements described in this document will not develop the operational scenarios or the associated link budgets. Rather, the measurements will define the maximum level of UWB emissions that can be tolerated at the antenna output of each GPS receiver considered. These thresholds will then be used in a link budget analysis for each specific UWB-to-GPS scenario identified, to calculate the maximum permissible output power of a UWB device, under given parameter combinations, that will ensure compatibility with GPS receivers. These operational scenarios, and the associated analyses performed, will be defined in a separate document. The results will be used by NTIA to address frequency management issues such as:

- the identification of any necessary restrictions on UWB transmission systems to prevent interference to GPS receivers;

- an assessment of the need to establish a general limit for UWB emissions in the 1559-1610 MHz RNSS band; and

- an investigation into the cumulative (aggregate) impact of UWB transmission systems on GPS receivers.

The measurement plan described herein will be carried out by personnel from the NTIA Institute of Telecommunication Sciences (ITS) in Boulder, Colorado and the NTIA Office of Spectrum Management (OSM) in Washington, D.C.

3.0 OTHER MEASUREMENT EFFORTS

There are several other measurement efforts underway to assess the potential for electromagnetic compatibility between proposed UWB devices and existing radiocommunication systems. In an attempt to expedite this measurement program, any knowledge or experience gained from these other ongoing measurement efforts will be exploited whenever appropriate. A brief description of each of these efforts is provided below.

3.1 ITS Ultrawideband Measurement Effort

Measurements are now being conducted at the NTIA/ITS laboratories in which the potential for UWB interference to Government radiocommunication systems other than GPS is being assessed. This program includes tasks that are being performed by NTIA ITS and NTIA OSM, and NIST. The objectives of this effort are to: 1) develop measurement procedures to characterize UWB emissions using commercial-off-the-shelf measurement equipment, 2) determine the susceptibility of selected radio receivers by observing the effects of UWB signals in their receiver IF sections, 3) develop one-on-one interference analysis procedures, 4) validate the one-on-one interference analysis procedures with field measurements of selected Federal radio receivers, and 5) demonstrate how multiple UWB emissions add within a receiver. The master plan⁽⁴⁾ and the measurement plan⁽⁵⁾ describing this effort can be found on the NTIA web page located at http://www.ntia.doc.gov.

As a part of this program, high-speed time domain equipment is being used by NIST to provide high resolution impulse measurements. The NIST equipment will be utilized to characterize the UWB generators that will be used in the UWB-to-GPS measurement effort described by this plan.

Spectral content and characterization of pulse amplitude distribution over time provide information which is helpful for understanding the potential for interference effects of UWB signals. Knowledge of energy distribution over time reflects the impulsive versus noise-like traits of the UWB signal, and in turn, helps to assess the likelihood of pulsed interference. Pulse energy distribution can be usefully described by amplitude probability distributions (APDs). Procedures for spectral and APD measurements are currently being developed by ITS as part of this ongoing project. The UWB-to-GPS measurement effort, described in this plan, will gain from this ongoing measurement program by utilizing these APD's in post measurement analyses as appropriate.

3.2 Department of Transportation (DOT)/Stanford University (SU) UWB-to-GPS Measurement Effort

The DOT is sponsoring a measurement effort at SU that is concentrating on assessing the potential for UWB interference to GPS receivers that will conform to the minimum operational performance standards (MOPS) for use in aviation applications such as precision approach and instrumented landings⁽⁶⁾. Also as a part of this effort, SU plans to test one land-based receiver that might be used in a public safety vehicle such as an ambulance, fire engine, or police cruiser.

3.3 Ultrawideband Consortium/Applied Research Laboratories of the University of Texas at Austin Tests for Measuring UWB/GPS Compatibility Effects

The Ultra-Wideband Consortium is sponsoring a measurement effort that is being conducted by the Applied Research Laboratories of the University of Texas at Austin (ARL/UT)⁽⁷⁾. The objective of the ARL/UT measurement program is to measure the behavior of GPS receivers in response to UWB emissions in highly controlled environments. There will be no attempt to identify specific metrics or criteria for assessing GPS performance in the ARL/UT effort. Rather, the program will attempt to collect data relative to all GPS parameters that might be used as performance metrics and then will leave the selection of the applicable metrics and criteria and the associated analyses to be performed by the GPS and UWB communities. For more information concerning the ARL/UT measurement program, see their web page at: http://sgl.arlut.utexas.edu/asd/Cure/.

4.0 MEASUREMENT PLAN

This measurement plan establishes tasks that will be performed to relate UWB signal characteristics to GPS performance criterion. These tasks will include: 1) the identification of GPS receivers to be considered; 2) the identification of UWB signal parameters to be considered; 3) the development of the GPS/UWB measurement methodology; 4) the development of measurement procedures to assess the potential for single source UWB interference; 5) the development of measurement procedures to assess the potential for aggregate UWB interference; and 6) data recording and reporting methods. The measurement procedures developed in this plan will be implemented by ITS personnel to produce the data required by the analysis effort. Figure 1 presents a flow diagram defining this measurement effort. The following paragraphs describe the individual tasks to be performed.

The measurement procedures contained in this plan are intended as guidance. As experience is gained in understanding the characteristics of UWB transmission systems and their transfer properties in GPS receivers, alternative measurement procedures or additional measurements to those described in this plan may be developed. Any modifications to this measurement plan will be coordinated between OSM and ITS.

4.1 Task 1: Identification of GPS Receivers to be Measured

The objective of this task is to identify the GPS receivers to be considered in this measurement effort. Due to time limitations, only a subset of GPS receivers can be tested in this effort. Therefore, it is important that this subset include receivers that are representative of the different technologies being employed (e.g., code-tracking, carrier-tracking, cross-correlator,

narrowly spaced correlators), the different ways in which these technologies are implemented among manufacturers, and the different applications for which they are used (e.g., aviation, maritime, public safety, and surveying).

The receiver bandwidth associated with a GPS receiver is typically dependent on the particular technology employed by the receiver. For example, a C/A-code tracking receiver typically processes a relatively narrow portion (2 MHz) of the GPS signal whereas a receiver that uses a multi-path mitigation technology such as narrowly-spaced multiple correlators often processes much more (~20 MHz) of the GPS signal. Since the susceptibility of a GPS receiver to UWB emissions is likely to be related to the receiver bandwidth, it is important that receivers representative of each of these technologies be examined. As such, the primary criterion that will be used to establish a priority among the candidate receivers proposed for measurement will be the type of technology utilized. A secondary prioritization criterion will then be technology implementation differences from manufacturer to manufacturer. This implies that the intended receiver application will be a tertiary consideration in establishing the measurement priority of receivers. The receiver application differences are best taken into consideration in the operational scenarios to be considered in the analysis effort.

Table 1 provides a list of candidate GPS receivers to be considered in this measurement effort. It is anticipated that each candidate receiver selected for measurement will be provided via loan by the manufacturer along with all relevant supporting technical documentation. In addition, an engineering point of contact will be identified for each receiver provided for measurement. As discussed in Section 3.2, the susceptibility of MOPS-compliant aviation-grade GPS receivers to UWB is being measured in the DOT/SU measurement effort.

4.2 Task 2: Identification of UWB Signal Parameters

The objective of this task is to identify the UWB signal parameters to be considered in the measurement effort. Ideally, the range of UWB signal parameters should be representative of all anticipated UWB devices. However, because of the continuing development of the technologies involved in the design of UWB devices, a complete range of parameters is difficult to define and is nevertheless limited by the capability of the available UWB generators. The generator to be used in these measurements is capable of producing UWB signals as defined in Table 2.

UWB signals are characterized by pulse repetition frequency (PRF), pulsewidth and shape, pulse gating, modulation, and power. Each of these parameters is defined below:

- Pulse Repetition Frequency is the number of pulses per unit time. PRF effects the spectral line magnitude and spacing, and the percentage of time impulses are present.

- Pulsewidth and Shape determine the envelope of the spectral content. As long as the spectral envelope is flat across the GPS band, pulsewidth and shape will have no impact on these measurements.

- Pulse Gating occurs when the pulse trains are transmitted in bursts which are gated on and off for specified periods.

- Modulation schemes employed by UWB devices include none, dithering, pulse-position modulation (PPM), on-off keying (OOK), and combinations thereof. Dithering is the random or psuedo-random spacing of pulses. PPM is the discrete variation in pulse spacing to represent data bits. OOK turns off individual pulses to represent data bits.

- Power can be expressed in peak, median, and/or average. For these measurements, both broadband noise and UWB signal power will be expressed in average power density as measured in a 20-MHz bandwidth (dBm/20 MHz). Peak power can be determined from the APDs produced in the other ITS UWB measurement effort (see Section 3.1).

Table 3 lists the UWB parameters and the range of values chosen for these measurements. Each possible combination will be used to give a total of 32 permutations for each GPS receiver measured. Different values, however, may be selected depending upon early measurement results. It should be noted that no effort will be made in these measurements to intentionally align the UWB spectral lines with GPS spectral lines. The power level of the generated UWB signal will be varied until the specified effect to the GPS receiver is observed.

4.3 Task 3: Development of the GPS/UWB Measurement Methodology

The objective of this task is to develop the measurement methodology, including the performance metric and associated performance criteria, for the GPS receivers to be considered in this plan. In order to assess potential interference effects to GPS receivers from UWB transmission systems, performance metrics and criteria must be established which indicate when the performance of the GPS receiver has been degraded as a result of interference from UWB signals.

After a review of relevant open literature articles, reports, and studies, it was determined that there are no established performance metrics or criteria for the GPS receivers to be considered in this measurement plan. Therefore, to assess the potential for interference from UWB transmission systems, the receiver performance metrics that will be used are based on the loss of signal reception from a GPS satellite and the time required to reacquire a lost satellite in the presence of UWB emissions. A UWB signal has the potential to reduce the carrier-to-noise density ratio (C/N₀) of a given satellite signal to such an extent that the GPS receiver can no longer decorrelate the given satellite signal. This causes the GPS receiver to lose lock on the given satellite. In this measurement plan, this will be referred to as the break-lock point. The break-lock performance metric is applicable to all of the GPS receivers to be considered in this measurement plan.

The criterion to be used to determine the potential interference impact to a GPS receiver from UWB transmission systems must be set less than the break-lock point, but at a level that will still permit reacquisition of the satellite signal. In this measurement plan, this criterion will be set to a level that is 2 dB below the break-lock point of the GPS receiver for the initial assessment. This will also provide a small allowance for any unit-to-unit variability that may

exist, but is not quantified in this measurement effort due to the limited sample of GPS receivers to be measured.

For the measurements to be performed in this plan, the one-sigma pseudorange error will first be determined relative to broadband noise only and then relative to a composite signal consisting of the UWB signal added to broadband noise, as shown in Figure 2. For GPS receivers that employ code tracking only, the pseudorange error is computed from the pseudorange of the simulator minus the measured pseudorange. For GPS receivers that employ code and carrier tracking, the one-sigma pseudorange error is computed from the standard deviation of the code pseudorange minus the carrier pseudorange. The broadband noise and the composite signal power will each be incremented, as the one-sigma pseudorange error is monitored, until the break-lock point of the GPS receiver is reached. Break-lock is represented by the points B (broadband noise only) and D (composite signal) of Figure 2. If there is a significant deviation in the one-sigma pseudorange error prior to break-lock, then that level of broadband noise or composite signal will be recorded in place of the break-lock point. The level of the broadband noise or composite signal will then be decreased by 2 dB from the break-lock point and a reacquisition time measurement will be performed. In this measurement, the GPS signal will be attenuated, causing the receiver to lose lock, at which time a change in pseudorange will be introduced. The attenuation of the GPS signal will then be removed and the receiver must reacquire within the manufacturers specified reacquisition time. If the receiver does not reacquire within specification, then the broadband noise or composite power level will be decreased until the GPS receiver is able to reacquire the satellite within specification (exemplified by points C and E in Figure 2).

To account for other potential sources of interference, such as sky noise, cross-correlation noise from other satellites within the GPS constellation, GPS augmentation systems, and other RNSS systems, the UWB signal will be added to broadband noise. It is difficult if not impossible to estimate the power levels of the individual contributors to broadband noise. Therefore, a single level meant to represent all of the combined contributors will be used. This level of broadband noise will be determined from the minimum C/N₀ required for acquisition of a GPS satellite of 34 dB-Hz.⁽¹⁰⁾ Based on the minimum guaranteed GPS signal power specification for the C/A code of -130 dBm into a 0 dBic gain antenna,⁽¹¹⁾ the maximum broadband noise density level at which satellite acquisition can be ensured is:

N₀ = C - C/N₀ = -130 - 34 = -164 dBm/Hz.

For the measurements defined in this effort, the broadband noise will be measured at the output of a 20-MHz bandpass filter. The broadband noise level, as exemplified by Point A in Figure 2, is then calculated as:

N₀ = -164 + 10 log (20 x 10⁶/1) = -91 dBm/20 MHz.

The use of a broadband noise level based upon the C/N₀ acquisition threshold is supported by computer simulations performed within the International Telecommunications Union- Radiocommunications Sector (ITU-R). The simulations in ITU-R Recommendation M.1477 show how C/N₀ can vary over a 24-hour period, at different user locations, when only sky noise and GPS cross-correlation noise are considered. The results of this simulation indicate that without any additional interference from external sources, and under certain worst case conditions, the C/N₀ level can fluctuate to within 1 dB of the acquisition threshold of 34 dB-Hz.

ITS will develop and validate software for all aspects of these measurements. This will consist of two software packages, one to automate data acquisition and control and the other to perform the data processing. The former will control setup and acquire configuration information about the measurement equipment, as well as acquire data from the GPS receivers. The processing software will convert the raw data obtained during the measurements to a format that can be plotted as shown in Figure 2.

4.4 Task 4: Development of Single Source UWB Interference Measurement Procedures

The objective of this task is to develop measurement procedures for assessing the potential for interference to GPS receivers from a single UWB transmission system. An overview of these measurement procedures is presented below.

Since no definitive pseudorange error or reacquisition time measurement criteria are defined for the GPS receiver types to be measured, the following procedure has been adapted from existing methods and will be used for the measurement of these receiver types. The procedure includes the following steps: 1) calibration; 2) receiver performance measurements with broadband noise only; 3) UWB interference measurements (both range accuracy and reacquisition measurements); and 4) data recording and reporting. The suggested calibration procedure is described in Appendix A. Sections 4.4.1 and 4.4.2 detail the GPS receiver performance and UWB interference measurements, respectively, and Section 4.7 details the data recording and reporting procedures to be used.

The measurement setup for the receiver performance measurement and for the UWB interference measurement is shown in Figures 3 and 4, respectively. As shown in the receiver performance measurement setup, only broadband noise is added to the GPS signal. This broadband noise measurement permits the basic operation of the GPS receiver to be quantified under normal operation. As shown in the UWB interference measurement setup, the UWB power is added to the broadband noise. This broadband noise is introduced to account for existing noise sources other than UWB, such as sky noise, cross-correlation noise generated by GPS satellites other than the one being tracked, GPS augmentation systems, and other contributing radio frequency noise sources such as Mobile Satellite Service (MSS) earth terminals and other RNSS systems. The GPS simulator will be used to simulate the output of one GPS satellite. This approach is appropriate because GPS receivers process each channel independently. Observing the impact of UWB to a single GPS channel simplifies the measurement approach by removing non-pertinent constellation variables (e.g., dilution of precision, rising/setting of satellites, the individual satellite power levels, etc).

The UWB power will be measured in the same 20-MHz bandpass filter used to measure and calibrate the broadband noise source. This will permit convenient comparison between the UWB power and the broadband noise power, since they will both be measured in the same bandwidth. The GPS, UWB, and broadband noise signals will be combined and input directly into the GPS receiver without any further filtering.

4.4.1 GPS Receiver Performance Measurements

1) Set up the measurement equipment as shown in Figure 3.

2) The GPS receiver is operated with the minimum received satellite signal level (C = -130 dBm) assuming a 0 dBic receive antenna gain. If the GPS receiver is normally operated with a separate low noise amplifier between the antenna and the receiver, then the GPS power at the receiver input should be adjusted to C = -130 dBm + G_LNA, where G_LNA is the amplifier gain.

3) Add broadband noise to the simulated GPS satellite signal at the receiver input. The initial broadband noise level should be set to -97 dBm/20 MHz (6 dB below the -91 dBm/20 MHz

described in Section 4.3). The broadband noise to be recorded is that measured at the output of

the 20-MHz bandpass filter with appropriate calibration corrections, and the low noise amplifier gain (if appropriate) to reference this value at the receiver input.

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

5) Measure the code and carrier pseudoranges described in Section 4.3 and estimate the one-sigma pseudorange error. The unsmoothed pseudorange will be measured if available to minimize time between measurements of uncorrelated sample points.

6) Increase the broadband noise in 3-dB increments until the receiver breaks lock with the GPS satellite. Record the one-sigma pseudorange error, the median of the C/N₀ as reported by the receiver, and the broadband noise level at each increment.

7) Repeat Step (6) in 1-dB increments for the 3-dB range before the break-lock point measured in Step (6). The break-lock point determined in this step is denoted as N_BL. Record the code and carrier pseudoranges and the median of the C/N₀ as reported by the receiver at each increment.

8) Let the GPS receiver track the satellite with the broadband noise source turned off (C = -130 dBm + G_LNA). Then input broadband noise into the receiver at N_BL - 2 dB.

9) Attenuate the GPS signal so the receiver loses lock.

10) Introduce a 50-meter step in simulated pseudorange over a 10-second period while the signal is not being tracked by the GPS receiver.

11) Remove the attenuation introduced in Step (9) and measure the time until the GPS receiver

initially reports code phase lock (reacquisition time). Code phase lock should be maintained for at least 10 seconds beyond the initial report.

12) Repeat Steps (8) through (11) for a total of 10 trials. If the receiver reacquires in at least 8 of the 10 trials in a time less than or equal to the manufacturers's specification value for reacquisition time, then the measurement is complete. If the GPS receiver does not reacquire in 8 out of 10 trials, then decrease the broadband noise power level from N_BL - 2dB in 1-dB increments until the GPS receiver successfully reacquires in 8 out of 10 trials. Record the reacquisition time and the broadband noise power level for each trial.

4.4.2 Single Source UWB Interference Measurements

For each permutation of UWB parameters, the following measurement procedures will be utilized:

1) Set up the measurement equipment as shown in Figure 4.

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

4) Add the broadband noise and UWB signal to the simulated GPS satellite signal. Set the broadband noise signal to -91 dBm as measured in the 20-MHz filter with necessary calibration corrections to reference the value at the receiver input and adjusted for G_LNA (as appropriate). The UWB signal is similarly measured in the 20-MHz filter and appropriately referenced to the

receiver input. The initial UWB signal level should be set to -97 dBm/20 MHz (6 dB below the

-91 dBm/20 MHz described in Section 4.3).

5) Measure the code and carrier pseudoranges described in Section 4.3 and estimate the one-sigma pseudorange error. The unsmoothed pseudorange will be measured if available to minimize time between measurements of uncorrelated sample points.

6) Increase the UWB signal level in 3-dB increments until the receiver breaks lock, or the UWB generator power input to the GPS receiver is maximized. Record the one-sigma pseudorange error and the median of the C/N₀ as reported by the receiver and the UWB signal level at each increment. If the UWB generator power input to the GPS receiver is maximized, then skip Step (7) and use the maximum UWB power as N'_UWB for the reacquisition measurements (Steps (8)-(14)).

7) Repeat Step (6) in 1-dB increments for the 3-dB range before the break-lock point measured in Step (6). The break-lock point determined in this step is denoted as N_UWB. Record the code and carrier pseudoranges and the median of the C/N₀ as reported by the receiver at each increment.

8) Let the GPS receiver track the satellite with the broadband noise source and the UWB signal generator turned off (C = -130 dBm + G_LNA).

9) Set the broadband noise level at -91 dBm/20 MHz. Let the GPS receiver track the satellite and reach steady state (at least 10 seconds).

10) Input the UWB signal into the GPS receiver at N_UWB - 2dB. (Or N'_UWB as appropriate).

11) Attenuate the GPS signal so the receiver loses lock.

12) Introduce a 50-meter step in simulated pseudorange over a 10-second period while the signal is not being tracked by the GPS receiver.

13) Remove the attenuation introduced in Step (11) and measure the time until the GPS receiver initially reports code phase lock (reacquisition time). Code phase lock should be maintained for at least 10 seconds beyond the initial report.

14) Repeat Steps (8) through (13) for a total of 10 trails. If the receiver reacquires in at least 8 of the 10 trials in a time less that or equal to the manufacturer's specification value for reacquisition time, then the measurement is complete. If the GPS receiver does not reacquire in 8 out of 10 trials, then decrease the UWB signal power level from N_UWB - 2dB, or N'_UWB, in 1-dB increments until the GPS receiver successfully reacquires in 8 out of 10 trials. Record the reacquisition time and the UWB power level for each trial.

4.5 Task 5: Development of Measurement Procedures for Assessing Aggregate UWB Interference

The objective of this task is to develop the measurement procedures to assess aggregate interference to GPS receivers from UWB transmission systems. Multiple UWB transmission systems, which might individually be tolerated by a GPS receiver, may combine to create an aggregate interference level that could preclude the reliable reception of the GPS signal. It is therefore critical that the potential for aggregate interference from UWB transmission systems be assessed.

In order to develop procedures that will provide consistent, repeatable results, the aggregate interference measurements should be made using a GPS simulator and closed-system measurements. In the NTIA companion measurement effort, ITS is examining the potential interference of UWB transmission systems to aeronautical radionavigation receivers and other potentially susceptible receivers operating in the restricted bands (see Section 3.1).

The aggregate measurements in this plan will consider one of the receivers used in the measurements performed in Section 4.4. Since there are only three UWB signal generators available at the ITS laboratory, there is a limit to the aggregate measurements that can be performed. Table 4 provides a list of the UWB signal parameters to be considered in the aggregate UWB measurements.

With the exception of Measurement Case IV, the following measurement procedures will be utilized:

1) Set up the measurement equipment as shown in Figure 5.

2) The GPS receiver is operated at a signal level of -130 dBm, compensated for calibration and G_LNA relative to the receiver input.

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

4) Add the broadband noise and UWB signal to the simulated GPS satellite signal. The broadband noise signal will be set at -91 dBm as measured in the bandwidth of the 20-MHz filter with calibration corrections to reference the value at the receiver input and adjusted for G_LNA (as appropriate).

5) Set the initial combined power level of each of the three UWB signal generators at -97 dBm as measured in the bandwidth of the 20-MHz filter.

6) Measure the code and carrier pseudoranges and estimate the one-sigma pseudorange error. The unsmoothed pseudorange will be measured if available to minimize time between measurements of uncorrelated sample points.

TABLE 4. UWB Signal Parameters for Aggregate Measurements

7) Increase the combined UWB signal power level in 3-dB increments until break-lock occurs. Record the code and carrier pseudoranges and the median of the C/N₀ as reported by the receiver at each increment.

8) Let the GPS receiver track the satellite with the broadband noise source and the UWB signal generators turned off.

9) Set the broadband noise source to -91 dBm/20 MHz. Let the GPS receiver track the satellite and reach steady state (at least 10 seconds).

10) Input the combined UWB signal into the GPS receiver at a level 2 dB below break-lock point found in Step (7).

11) Attenuate the GPS signal so the receiver loses lock.

12) Introduce a 50-meter step in simulated pseudorange over a 10-second period while the signal is not being tracked by the GPS receiver.

13) Remove the attenuation introduced in Step (11) and measure the time until the GPS receiver initially reports code phase lock (reacquisition time). Code phase lock should be maintained for

at least 10 seconds beyond the initial report.

14) Repeat Steps (8) through (13) for a total of 10 trials. If the receiver reacquires in at least 8 of the 10 trials in a time less than or equal to the manufacturer's specification value for reacquisition time, then the measurement is complete. If the GPS receiver does not reacquire in 8 out of 10 trials, then decrease the combined UWB signal power level in 1-dB increments until

the GPS receiver successfully reacquires in 8 out of 10 trials. Record the reacquisition time and the combined UWB power level for each trial.

For Measurement Case IV, the following procedures will be utilized:

1) Set up the measurement equipment as shown in Figure 5.

2) The GPS receiver is operated at a signal level of -130 dBm, compensated for calibration and G_LNA relative to the receiver input.

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

4) Add the broadband noise and UWB signal to the simulated GPS satellite signal. The broadband noise signal is to be set at -91 dBm as measured in the bandwidth of the 20-MHz filter with calibration corrections to reference the value at the receiver input and adjusted for G_LNA (as appropriate).

5) The initial power level of the UWB signal source #1 is set at -97 dBm as measured in the bandwidth of the 20-MHz filter.

6) Measure the code and carrier pseudoranges and estimate the one-sigma pseudorange error.

7) Increase the combined UWB signal power level in 3-dB increments until break-lock occurs. Record the code and carrier pseudoranges and the median of the C/N₀ as reported by the receiver at each increment.

8) Let the GPS receiver track the satellite with the broadband noise source and the UWB signal generator(s) turned off.

9) Set the broadband noise source to -91 dBm/20 MHz. Let the GPS receiver track the satellite and reach steady state (at least 10 seconds).

10) Input the combined UWB signal into the GPS receiver at a level 2 dB below the break-lock point found in Step (7).

11) Attenuate the GPS signal so the receiver losses lock.

12) Introduce a 50-meter step in simulated pseudorange over a 10-second period while the signal is not being tracked by the GPS receiver.

13) Remove the attenuation introduced in Step (11) and measure the time until the GPS receiver initially reports code phase lock (reacquisition time). Code phase lock should be maintained for 10 seconds beyond the initial report.

14) Repeat Steps (8) through (13) for a total of 10 trials. If the receiver reacquires in at least 8 of the 10 trials in a time less than or equal to the manufacturer's specification value for reacquisition time, then the measurement is complete. If the GPS receiver does not reacquire in 8 out of 10 trials, then decrease the combined UWB signal power level in 1-dB increments until the GPS receiver successfully reacquires in 8 out of 10 trials. Record the reacquisition time and the UWB power level for each trial.

15) The initial power level of UWB signal generator #1 is set to -97 dBm/20 MHz. Set the power level of UWB signal generator # 2 to -97 dBm as measured in the bandwidth of the 20-MHz filter. Repeat Steps (6) through (14).

16) The initial power level of UWB signal generator #1 and #2 is set at -97 dBm/20 MHz. Set the power level of UWB signal generator # 3 at -97 dBm as measured in the bandwidth of the 20-MHz filter. Repeat Steps (6) through (14).

When applicable, the aggregate measurement results will be compared to the single source measurement results.

4.6 Task 6: Data Recording and Reporting

For each iteration of the UWB signal parameters, data will be recorded electronically and on measurement data sheets. The minimum UWB data to be recorded is as follows:

• average power density (dBm/20 MHz) level required to cause the GPS receiver to break-lock with the GPS satellite
• average power density (dBm/20 MHz) level at which reacquisition can occur
• PRF
• % Dither
• % Gating
• Modulation type

Applicable GPS receiver parameters and data will also be recorded. As a minimum these will include:

• Receiver technology type
• Receiver application(s)
• Manufacturer's specified reacquisition time
• Low Noise Amplifier (LNA) gain (if the GPS receiver requires a separate LNA)
• C/N₀ as reported by the receiver at each broadband noise and UWB average power density (dBm/20 MHz) level
• One-sigma pseudorange error at each broadband noise and UWB average power density (dBm/20 MHz) level (not required for the reacquisition measurement)
• Reacquisition time for each reacquisition trial

ITS will prepare a data package for each receiver used in these measurement procedures. This data package will contain, as a minimum, plots of the one-sigma pseudorange error vs. average power density measured in the 20-MHz bandpass filter for both the broadband noise and the composite signal. For each permutation of UWB parameters the composite signal will be plotted on a separate chart. In addition to this graphical data, measurement data will be stored on a writeable compact disc (CD). This data will be stored on the CD in a format which is readily accessible using commercial spreadsheet or database software. The plots and all measurement data sheets will also be stored on the CD (in PDF format).

4.8 SCHEDULE

Table 5 presents the proposed schedule for the UWB-to-GPS measurement effort described in this plan. As can be observed from the table, the schedule proposed therein is extremely aggressive. The measurement schedule shown in Table 5 can be accelerated if additional funding becomes available. However, this schedule severely limits the scope of these tests, particularly with respect to the number of GPS receivers that can be measured and the size of the UWB signal space that can be considered. It must also be noted that the estimated completion dates provided in this schedule does not provide any allowance for unforseen measurement or analysis anomalies that may arise as the program proceeds. Furthermore, this schedule does not account for the time required to conduct the subsequent analyses with the measurement data acquired and to complete a final report formally documenting the measurement and analyses results. The ability to adhere to this schedule is also dependent upon full cooperation from the GPS manufacturers in providing receivers for measurement and technical support as needed.

Table 5. UWB-to-GPS Measurement Schedule

A.0 CALIBRATION

The measurement setup used for the interference measurements can be seen below in Figure A-1. This setup requires calibration of the three main signal sources, along with the cables and attenuators used to connect these signal sources into the measurement setup. The individual splitter, combiner, and bandpass filter to be used in the measurements must also be characterized.

The UWB generator and programmable attenuator will be calibrated using the setup shown in Figure A-2. The UWB generator will be set to maximum power, and the programmable attenuator will be stepped through all possible values. The bandpass filter is to be the same one that will be used in the measurement setup. Power meter readings will be taken at each attenuator setting.

The RF cables used to connect the UWB generator to the attenuator and the attenuator to the RF power meter should be the same cables that are to be used in the measurement setup. This calibration procedure will also be used for the additional UWB generators used in the aggregate measurements described in Section 4.5.

A.2 Calibration of the GPS Simulator and Programmable Attenuator

The GPS simulator and programable attenuator will be calibrated using the setup shown in Figure A-3. The GPS simulator will be set to maximum power, and the programmable attenuator will be stepped through all possible values. The bandpass filter is to be the same one that will be used in the measurement setup. Power meter readings will be taken at each attenuator setting.

The RF cables used to connect the GPS simulator to the attenuator and the attenuator to the RF power meter should be the same cables that are to be used in the measurement setup.

A.3 Calibration of the Broadband Noise Generator and Programmable Attenuator

The broadband noise generator and programable attenuator will be calibrated using the setup shown in Figure A-3. The broadband noise generator will be set to maximum power, and the programmable attenuator will be stepped through all possible values. The bandpass filter is to be the same one that will be used in the measurement setup. Power meter readings will be taken at each attenuator setting.

Figure A-4. Broadband Noise Generator and Attenuator Calibration Set-up.

The RF cables used to connect the broadband noise generator to the attenuator and the attenuator to the RF power meter should be the same cables that are to be used in the measurement setup.

A.4 Characterization of the Splitter, Combiner, and Bandpass Filter

The splitter, combiner, and bandpass filter to be used in the test setup should be characterized on a network analyzer using accepted laboratory procedures. Characterization of any other ancillary equipment required for completion of the measurements (such as amplifiers, RF cables, or RF adapters) will also be conducted in a similar manner.

1. U.S. Department of Transportation and U.S. Department of Defense 1999 Federal Radionavigation Plan (Dec. 1999) at 1-11.

2. In the Matter of Revision of Part 15 of the Commission's Rules Regarding Ultra-Wideband Transmission Systems, Notice of Proposed Rulemaking, ET Dkt. 98-153 (rel. May 11, 2000), Federal Register, June 14, 2000 (Volume 65, Number 115) at 37332-37335. Comments must be submitted on or before September 12, 2000 and reply comments on or before October 12, 2000 (hereinafter "UWB NPRM").

3. The GPS related issues are covered in paragraphs 29, 36, and 47 of the UWB NPRM.

4. NTIA, Ultra-Wideband Signals for Sensing and Communication: A Master Plan for Developing Measurement Methods, Characterizing the Signals and Estimating Their Effects on Existing Systems, June 15, 2000.

5. NTIA, Ultra-Wideband Signals for Sensing and Communication: A Master Plan for Developing Measurement Methods, Characterizing the Signals and Estimating Their Effects on Existing Systems, ITS Ultra-Wideband Measurement Plan, (Master Plan Task 1.2), June 14, 2000.

6. Ming, Luo, Dennis Akos, Sam Pullen, Per Enge Stanford University, Potential Interference to GPS from UWB Transmitters: Test Plan- Version 4.5 Phase1: Accuracy Test for Aviation Receivers and Reacquisition Time Test for Land Receivers, May 1, 2000.

7. Applied Research Laboratories, The University of Texas at Austin, Test Plan for Measuring UWB/GPS Compatibility Effects, July 21, 2000.

8. Department of Transportation, Federal Aviation Administration, Technical Standard Order C-129a, Airborne Supplemental Navigation Equipment Using the Global Positioning System (GPS).⁽⁹⁾

9. Department of Transportation, Federal Aviation Administration, Technical Standard Order C-129a, Airborne Supplemental Navigation Equipment Using the Global Positioning System (GPS).

10. Recommendation ITU-R M.1477, Technical and Performance Characteristics of Current and Planned RNSS (Space-to-Earth) and ARNS Receivers to be Considered in Interference Studies in the Band 1559-1610 MHz.

11. ARINC Research Corporation, Navstar GPS Space Segment/Navigation User Interfaces (Sept. 25, 1997) at 13.