Page 1 of 52

Consolidated Comments and Reponses for NASCTN’s Draft Test Plan – Impact of LTE Signals on GPS Project

NASCTN convened a panel of technical experts, from NIST and NTIA, to develop a test plan that would meet the objectives of this project:

 Develop a repeatable, calibrated, and well-documented test methodology to investigate the impact of adjacent-band LTE transmissions on GPS receivers 

operating in the L1 frequency band

 Collect data on a representative subset of available GPS devices in order to validate the proposed test methodology.

In May of 2016, NASCTN distributed a draft test plan and feedback form to a cross-section of GPS manufacturers, Federal agencies and spectrum regulators to 

obtain technical feedback on the proposed methodology.

Over a two-month period, NASCTN received 159 comments from the following spectrum regulators, Federal agencies, GPS manufacturers and members of the 

general public:

 Federal Communication Committee (FCC)

 National Telecommunications & Information Administration (NTIA)

 U.S Department of Transportation (DOT)

 National Aeronautics and Space Administration (NASA)

 U.S. Air Force Space Command (AFSPC)

 U.S. Air Force Spectrum Management Office (AFSMO)

 Deere & Company*

 Garmin Ltd.

 Honorable John Stenbit, Dr. Brad Parkinson, Dr. John Betz

 Mr. Larry Chesto

The NASCTN test team reviewed the comments and developed a revised test plan in July of 2016 that addressed the technical issues raised in the comments. The 

adjudicated comments are not attributed to any specific organization or individual.

*The comments submitted by Deere & Company were non-technical in nature and were not included in the comment adjudication. However, Deere & Company also filed these 

comments with the FCC.
Page 2 of 52

Comment 

Number

Level Reference 

within Draft 

Test Plan

Comment Commenter's Recommendation NASCTN Comment Resolution

1 General 

– C

Cover Page The test team responsible for preparing this plan and presumably 

running the test does not include key sufficient PNT expertise. 

Being involved with GPS interference testing for 25 years, only one 

of the names is recognized as being associated with the GPS

community. An online search of the team’s biographies did not 

readily indicate experience with testing of GPS interference 

effects. PNT systems differ from communication systems in 

numerous ways. The original concept for standing up a “National” 

testing organization recognized the concept of bringing in relevant 

experts from across the federal government, not just NIST—

particularly for PNT. 

Formally add technical experts from across the federal 

PNT community to the test team for purposes 

developing and executing this test. Will commit to 

providing technical experts as it has for previous LTE 

impact tests. Recommend seeking experts from other 

agencies as well—particularly Air Force and FAA.

The NASCTN process for obtaining 

feedback on the proposed test plan

resulted in comments from Federal 

agencies, GPS manufacturers, and both 

spectrum regulators (FCC and NTIA).

2 General 

– I

General 

Comment

C/No changes are the mechanism through which the other KPIs 

are affected, and the C/No degradation due to LTE can be

measured precisely with most receivers.

Emphasis must be given to the C/No data as the most 

relevant and accurate measurements.

Although the NASCTN test plan 

includes a number of measurands in 

the study, including C/No, it does not 

make any recommendations on the 

appropriateness of these measurands, 

which is the purview of the spectrum 

regulators.

3 General 

– I

General 

Comment

The plan does not indicate how the internal post--‐ processing 

software of the GPS devices will be handled. Many commercial 

GPS devices have software that will enable internal data 

corrections to be performed to the signal prior to the final location 

and display on a map. Functions such as, “snap to road” must be 

accounted for and either turned off or bypassed in the device. If 

not, the resulting output and positioning information may alter the 

actual results of the DUT.

Provide a section on how NASCTN plans to handle the 

post--‐processing software within the DUT.

This information will be included in the 

final report in compliance with the 

CRADA between NIST and Ligado 

Networks.

4 General 

– I

General 

Comment

The Test Plan demonstrates how to measure the effects of a single 

LTE base--‐station on GPS receivers, but does not consider the 

potential of multiple LTE base--‐station effects on a GPS receiver.

Consider performing tests to demonstrate the 

“aggregate” effects from multiple base--‐stations to a 

single GPS receiver.

This would be a follow-on investigation. 

The amount of power at the DUT 

during these test could represent 

power from an aggregate test of 

devices.
Page 3 of 52

Comment 

Number

Level Reference 

within Draft 

Test Plan

Comment Commenter's Recommendation NASCTN Comment Resolution

5 General 

– I

General 

Comment

One of the most significant requirements of a GPS receiver is the 

ability to acquire and re--‐acquire satellite signals. This capability is 

correlated with C/No. Did not see any tests on how this will be 

performed in the Test Plan.

Consider adding tests to determine loss--‐of--‐lock, initial 

acquisition, and re--‐acquisition of satellite signals.

Initial acquisition, loss of lock, and reacquisition tests added to test plan.

6 General 

– I

Page 4, 2nd 

para, lines 

3--‐5

“...but does not incorporate insight from any preliminary anechoic 

chamber tests that would naturally solidify the test plan.”

There are numerous previous “LTE impacts on GPS” tests 

done in anechoic chambers that produced more than a 

thousand pages of data on a nearly identical system and 

in one of the same anechoic chambers you are 

proposing to use. While I can certainly understand why 

you might not want to replicate those previous tests 

precisely, you certainly can and should “incorporate 

insight” from these previous government anechoic 

chamber tests. Lessons learned from those tests, as well 

as the reports from independent audit reports by Idaho 

National Labs and MIT/LL of the test methodology, can 

help ensure NASCTN’s test “...bridges US resources to 

get to the right national answer.”

The NASCTN test methodology will be 

well documented, so others will be able 

to make comparison with previous 

measurement campaigns.

7 General 

– I

Section 4 

(and 9)

In Section 9, the test plan includes an action "Data analysis and 

report," implying that a report will be issued. Is that a separate 

deliverable from the "test methodology and measurement data" 

referenced here? If so, it should be included here. If a "report" is 

planned, it may be useful to provide a draft outline for such a 

report (in an appendix) since such a draft would provide more 

insight into the information to be documented.

Clearly indicate whether a test report will be issued 

(deliverable) and, if so, include draft outline for test 

report.

Final test report will be developed and 

release in accordance with the CRADA 

between NIST and Ligado networks. An 

outline of the test report will not be 

included in this test plan.

8 General 

– I

Section 4 A clear statement on the availability of the data would be useful 

and will note whether the release period is due to internal review 

of data or due to dictates by the study sponsor. As an observation, 

this draft test plan appears to function primarily as a test plan, but, 

also, in some ways to a project plan in its identification of 

deliverables, etc..

Insert a clear statement on the plan (e.g., timeline, 

restrictions, etc.) for releasing test report/data. (Or, 

consider releasing a clearly defined and concise Project 

Plan to accompany the test plan.)

The handling of proprietary 

information and distribution of the 

NASCTN report, measurement data, 

and data analysis is governed by the 

CRADA signed by NIST and Ligado 

Networks. 

The NASCTN report will include all of 

the information that is publically 

releasable.
Page 4 of 52

Comment 

Number

Level Reference 

within Draft 

Test Plan

Comment Commenter's Recommendation NASCTN Comment Resolution

9 General 

– I

Section 8 A back--‐up plan may be useful to ensure the availability of the 

data that could be lost due to device, process, or other failure in 

the collection and transport of the data. A basic overview of such a 

process would be useful in the test plan (or project plan).

Consider defining and executing a data backup plan 

(e.g., on--‐site replication, transport, etc.).

The raw data collected from DUTs is 

backed-up on a daily basis.

10 I Section 1, 

2nd para. 

Page 5

The statement that the FCC granted a waiver for operation of a 

terrestrial network in 2011 is incorrect. The partial waiver was to 

remove a restriction for an Ancillary Terrestrial component to the 

licensee’s Mobile Satellite Service and was only granted on the 

condition of proving non--‐interference to GPS

Change 1st sentence to read “In January 0f 2011, the 

United States Federal Communications Commission 

granted a partial waiver of the integrated service rule for 

an Ancillary Terrestrial Network, subject to conditions 

which have not yet been met.

Text modified to add clarity.

11 I Section 1, 

2nd para, 

line 4 Page 5

The qualifier about interfering with GPS receiver was not in the 

FCC Order. Presidential Decision Directive requires federal 

agencies to also provide interference spectrum for planned and 

future GPS applications.

Delete the word “existing” from “...would not interfere 

with existing GPS receivers.”

Will make editorial change in final 

report. Existing receivers are used in 

this test.
Page 5 of 52

Comment 

Number

Level Reference 

within Draft 

Test Plan

Comment Commenter's Recommendation NASCTN Comment Resolution

12 C Section 1, 

last para 

Page 5

Concur with this statement, but the list of receivers provided 

elsewhere in the Test Plan is in no way a representative subset of 

available devices. The 2nd NPEF test (2011), focused on just the 

General Navigation category and tested more than 100 different 

types of receivers with a wide and significant range of results.

The sentence is OK as is, provided the list of test 

receivers expands significantly, with input from the 

federal agencies and industry. If not, this sentence 

should be deleted as the current plan does not test 

representative receivers.

The NASCTN test plan includes GPS 

receivers from the following categories: 

 General location and 

navigation

 Precision position

 Precision timing

Other GPS receiver categories, 

including aviation, space-based and 

DoD receivers are beyond the scope of 

the NASCTN project. If additional 

testing is required beyond this project, 

organizations are encouraged to submit 

project proposals to NASCTN. 

NASCTN included a number of GPS 

receivers in each of these categories in 

order to adequately validate the 

proposed test methodology. A major 

goal of the NASCTN project is to 

provide a reproducible test project that 

other organizations can use to perform 

similar tests on other GPS devices 

included in this study. 

Based on the feedback from Federal 

agencies and GPS manufacturers, 

NASCTN has revised its list of GPS 

receivers.
Page 6 of 52

Comment 

Number

Level Reference 

within Draft 

Test Plan

Comment Commenter's Recommendation NASCTN Comment Resolution

13 I DUT list, 

Section 

5.2.1 Page 7

Only 1--‐2 precision receivers to be tested. May want to include 

receivers using codeless/semi--‐codeless techniques. May need to 

transmit both GPS L1P and L2P for this.

Consider expanding DUT list to increase test value. 

Consider simultaneous testing.

The NASCTN test plan includes GPS 

receivers from the following categories: 

 General location and 

navigation

 Precision position

 Precision timing

Other GPS receiver categories, 

including aviation, space-based and 

DoD receivers are beyond the scope of 

the NASCTN project. If additional 

testing is required beyond this project, 

organizations are encouraged to submit 

project proposals to NASCTN. 

NASCTN included a number of GPS 

receivers in each of these categories in 

order to adequately validate the 

proposed test methodology. A major 

goal of the NASCTN project is to 

provide a reproducible test project that 

other organizations can use to perform 

similar tests on other GPS devices 

included in this study. 

Based on the feedback from Federal 

agencies and GPS manufacturers, 

NASCTN has revised its list of GPS 

receivers.
Page 7 of 52

Comment 

Number

Level Reference 

within Draft 

Test Plan

Comment Commenter's Recommendation NASCTN Comment Resolution

14 C Table 1

Page 8

The list of receivers is intended to be representative of the General 

Navigation category, but has 4 of 7 receivers in the “Hiking” 

function which is a very narrow subset of general navigation and 

location. Previously, LightSquared tried to argue that hiking 

receivers were such a niche use they shouldn’t be tested at all. 

While we don’t agree with that, they certainly aren’t 

representative of the entire category. Furthermore, all 4 of these 

are also from the same manufacturer. 1st responder devices in 

particular had a wide range of interference response in previous 

government LTE interference tests. Testing only 1 model can’t 

possibly be “representative”.

Significantly expand the number and types of receivers 

to be tested. Consider adding additional 1st responder 

receivers, Coast Guard devices for marine navigation, 

receivers supporting Positive Train Control and 

intelligent transportation applications, as well as those 

in surveying, agriculture, construction and other areas. 

There are an incredibly diverse set of GPS applications 

and previous tests have shown they do not respond the 

same in an interference environment.

The NASCTN test plan includes GPS 

receivers from the following categories: 

 General location and 

navigation

 Precision position

 Precision timing

Other GPS receiver categories, 

including aviation, space-based and 

DoD receivers are beyond the scope of 

the NASCTN project. If additional 

testing is required beyond this project, 

organizations are encouraged to submit 

project proposals to NASCTN. 

NASCTN included a number of GPS 

receivers in each of these categories in 

order to adequately validate the 

proposed test methodology. A major 

goal of the NASCTN project is to 

provide a reproducible test project that 

other organizations can use to perform 

similar tests on other GPS devices 

included in this study. 

Based on the feedback from Federal 

agencies and GPS manufacturers, 

NASCTN has revised its list of GPS 

receivers.

15 I Describe 

GPS 

simulator 

configuratio

n, Section 

5.2.2 Page 

10

What signals are transmitted, what global gain setting, errors that 

are enabled, etc.

Include some description in plan and details in test 

report.

Details on GPS setup added to the test 

plan.
Page 8 of 52

Comment 

Number

Level Reference 

within Draft 

Test Plan

Comment Commenter's Recommendation NASCTN Comment Resolution

16 I Table 5. GPS 

test levels 

Page 10

Minimum spec power (3GPP standard) is a good test configuration 

baseline, as is the limited exposure. May also consider nominal 

power levels (time varying) scenario to avoid argument that --‐

130dBm or weaker is a stressing case, thus limiting value of test. I 

would prioritize the --‐130dBm case though if time limitations are 

factor. Consider also constant weaker signal scenarios, non--‐static 

scenarios. Make sure receivers are configured to track weak 

signals for limited exposure test (some have minimum signal 

masks).

Please consider. The test includes testing at both a 

nominal level of -128.5 dBm per SV and 

a range of power levels for a limited 

condition.

17 I Section 

5.2.4, 1st 

para. Page 

11

“..the architecture of the proposed LTE system is not known to 

authors of the test plan.” This seems odd as the entity requesting 

the test should have some idea of what their architecture is and 

the choices made in architecture selection affect the interference 

environment.

Recommend delaying finalizing this Test Plan, until the 

proposed LTE architecture is better understood, so we 

are not testing a “generic” LTE that may significantly 

vary from the final system.

The scope of this NASCTN project has 

been negotiated in the CRADA between 

NIST and Ligado Networks. If additional 

tests are needed beyond this project, 

organizations are encouraged to submit 

project proposals to NASCTN.

18 I Section 

5.2.4, last 

para. Page 

11

Must also consider the density of the transmission sources. “LTE 

impacts on GPS” does not mean interference from only one 

transmission source. There are 40,000+ transmission sources and 

our concern is on the interference from the network, not a single 

tower. The proposed density of the network in 2011 had towers 

every .4--‐.7 km. With interference effects predicted at greater 

than 20 km, that means the victim receiver expects to receive 

interference from multiple stations simultaneously. Density of the 

LTE stations can’t really be ignored if the criteria is supposed to be 

“based on end--‐user experience”

Add density of the LTE base stations as part of the LTE 

Signaling Scenario factors.

The scope of this NASCTN project has 

been negotiated in the CRADA between 

NIST and Ligado Networks. If additional 

tests are needed beyond this project, 

organizations are encouraged to submit 

project proposals to NASCTN.

19 I 5.2.4, Figure 

1 Page 12

Although figures are useful, the specific values (functions) should 

be included on the graphic to remove any questions as to the 

specific values.

Provide the specific values for the mask in the figure (or 

add a table).

Specific values will be added in the final 

report.

20 I Section 

5.3.2, 1st 

para Page 

14

Refers to baseline change. Cannot have a baseline with only one 

(1) antenna

This usage of “baseline” might mean the location 

solution with no LTE interference? If so, use different 

words as “baseline” means the vector between a pair of 

antennas to this audience.

Text modified to add clarity.
Page 9 of 52

Comment 

Number

Level Reference 

within Draft 

Test Plan

Comment Commenter's Recommendation NASCTN Comment Resolution

21 E 5.3.3 first 

para, last 

sentence 

Page 15

“..., and a length should be chosen that maximizes these 

variations.”

Maximizes should be minimizes Text is correct. See reference.

22 I Section 

5.3.3 Page 

15

Shows Time interval counter Be sure to use a post--‐processed carrier solution for the 

receiver clock offset to achieve state of the art, which is 

<100 ps accuracy in biased time transfer. Reference 

GRACE and DSAC missions.

The post-processing of the time 

interval counter results will measure 

the impacts to the accuracy as specified 

by the manufacturer of the device. 

23 I Fig 3. Page 

16

Will probably need GPS amplifier, don’t think GSS8000 will provide 

high enough signal level to radiate 3m

Add an appropriate amplifier that will provide adequate 

signal levels for the test to be performed.

Amplifier added to figure.

24 I Section 5.4 

Page 20

Data acquisition dwell times Recommend look at results from ABC testing to see the 

variety of C/No “overshoots” and settling times 

observed.

The NASCTN test methodology will be 

well documented, so others will be able 

to make comparison with previous 

measurement campaigns.

Although the NASCTN test plan 

includes a number of measurands in 

the study, including C/No, it does not 

make any recommendations on the 

appropriateness of these measurands, 

which is the purview of the spectrum 

regulators.

25 E 5.4, Figure ? 

Page 21

Insert figure number and caption. Added figure number and caption.

26 E 5.4, Figure ? 

Page 22

Insert figure number and caption. Added figure number and caption.

27 I 5.4, Figure ? 

Page 22 

(other 

sections as 

well)

It is not clear what the “step” power levels will be in this and other 

processes identified in the test plan. Given the protection limits 

used in traditional sharing recommendations, this value will need 

to be < 1 dB (per ref unit).

Identify the expected "step" power levels for the 

different test processes.

The stepped power levels are chosen 

based on the regions of significant 

device response. This range is initially 

estimated by a preliminary sweep of 

power levels; additional levels are 

tested as needed. This results in a 

lower uncertainty in the data for 

measured power levels.
Page 10 of 52

Comment 

Number

Level Reference 

within Draft 

Test Plan

Comment Commenter's Recommendation NASCTN Comment Resolution

28 I 5.5 Although both suggested methods in Section 5.5 are useful, 

varying the level of the out of band emissions (e.g., Section 5.5.1) 

is necessary for understanding the tolerance of the devices and, 

from a different perspective, the system--‐wide impact that factors 

the distance from the base station. Ideally, however, both the 

absolute OOBE power level and the LTE transmitter power should 

be varied independently to capture both the distance effects and 

the relative transmit power.

At a minimum, use the swept range method (e.g., 

Section 5.5.1) to vary the level of the out of band 

emissions.

Added text for clarity.

29 I 5.5.1 

Emulate 

swept range 

vs 5.5.2 

Study LTE 

In--‐Band 

Coexistence 

Margins 

Page 23

Marginal preference for swept range. However also consider fully 

separate testing of OOBE and in--‐band LTE source (toggle them on 

and off) at swept power to be able to distinguish the effects. 

Mechanism for interference is very different between the in--‐

band and out of band signals.

For consideration. Added text for clarity.

30 C Section 5 

(and 6)

Because the recorded values of key parameters are self--‐ reported 

by each DUT, a procedure should be included to calibrate each 

device’s reporting of key values as a function of both the change in 

RNSS system input power and a change in the external noise (in--‐

band and/or OOBE, AWGN) power in the absence of the LTE signal. 

The calibration will provide insights to the relation to the 

“absolute” (or perceived) DUT noise temperature, any 

quantization of the reported values, and any variation due to input 

levels in the self--‐reported values.

Include procedures to calibrate each DUT’s reporting of 

the key values such as C/N0, as needed, as a function of 

both the change in RNSS system input power and a 

change in the external noise power.

As pointed out in the introduction, GPS 

devices are inherently not metrology 

devices and thus the concept of 

“calibrating the device” is misleading.

The calibration process in the testing 

process applies to setting the RF 

conditions created at the plane of the 

DUT location, including the GPS signal 

power, the LTE power, and the LTE outof-band emission mask. The baseline 

condition, with no LTE power or OOBE 

present is measured over a sufficient 

time period to establish a statistically 

valid baseline.
Page 11 of 52

Comment 

Number

Level Reference 

within Draft 

Test Plan

Comment Commenter's Recommendation NASCTN Comment Resolution

31 C Section 6.2 

Page 24

A critical measure of a GPS receiver is the ability to acquire 

satellites—often in challenging environments such as urban 

canyons—and the time it takes to acquire. The NIST proposed 

methodology assumes the victim receiver is already locked on and 

measures when it loses lock. The more challenging real--‐ world 

situation is to acquire in an LTE environment where the interfering 

signals are already present.

Add to list of response variables: GPS acquisition 

(binary), time to acquire (seconds)

Initial acquisition, loss of lock, and reacquisition tests added to test plan.

31 C Section 6.2 

Page 24

The most important variable to control, or at least to know, is the 

noise temperature of the DUT (with no RFI) in the test setup 

compared to what is expected in operation. Otherwise, a high 

noise level during testing would obscure effects from the LTE 

interference.

Add calibrations of the DUT noise floor under test 

conditions minus the LTE signal. Will need to inject noise 

at a known level. For example, well--‐ calibrated OOBE 

interference could be used for this.

The expected noise temperature in an 

operational setting is not an 

established standard value for all 

devices or all use cases. The 

measurement process includes a 

baseline case that does not include an 

LTE or the proposed OOBE mask – i.e., 

no interference. By testing in a wellcontrolled environment, the results 

data show the DUT response due to the 

added radio activity.

32 I Section 6.2 

Page 24

No tests of acquisition are included If you rely on KPI instead of C/No degradation, you 

should test the receiver function most sensitive to RFI, 

which is a cold--‐start acquisition scenario.

Although the NASCTN test plan 

includes a number of measurands in 

the study, including C/No, it does not 

make any recommendations on the 

appropriateness of these measurands, 

which is the purview of the spectrum 

regulators. Tests of acquisition in the 

with LTE activity have been added to 

the test plan.

33 I Section 6.2

Page 24

Greater specificity and information about the planned variables 

would be useful. For example, as noted in the draft, values such as 

C/No are based on the devices' internal reporting, so such 

"sources" for the data should be documented. As written, the test 

plan is a bit vague in defining the response measures and the 

associated (meta) data defining each. In addition, identifying the 

initial data format concept for the reported data will assist readers 

of the test plan in understanding the actual output of the testing.

Include a detailed table for all planned response 

measures that includes key information such as source 

of value (e.g., external reporting, internal test point, 

external test device measurement, observation, status 

signal, etc.), operating mode of the DUT, and other 

factors. Consider defining (table in appendix?) the 

planned output format (data schema) for the core (raw) 

data.

This information will be included in the 

final report in compliance with the 

CRADA between NIST and Ligado 

Networks.
Page 12 of 52

Comment 

Number

Level Reference 

within Draft 

Test Plan

Comment Commenter's Recommendation NASCTN Comment Resolution

34 I 6.3 (and 

other 

sections) 

Page 25

While recognize that the proposed baseline testing provides 

insights into core effects, other situations and test scenarios 

should be considered and defined as part of this round or future 

testing. Such scenarios include: --‐ DUT in motion --‐ DUT initial 

signal acquisition --‐ Different GPS constellation configurations --‐ 

Test configurations to emulate a network of LTE systems (Note: 

Although the closest, single base station may be the most likely 

situation (and impact) for terrestrial GPS systems, the LTE network 

deployment may have an effect on airborne and space GPS users.) 

--‐ TBD: other scenarios will be based on the types of receivers not 

included in this round

Although probably outside the scope of this baseline 

testing, consider other test scenarios such as DUTs in 

motion, impact on DUT’s initial signal acquisition, other 

types of receivers and their operational scenarios, etc..

The scope of this NASCTN project has 

been negotiated in the CRADA between 

NIST and Ligado Networks. If additional 

tests are needed beyond this project, 

organizations are encouraged to submit 

project proposals to NASCTN.

35 I Appendix A 

Pages 28 --‐ 

29

How is Appendix A to be used. This seems only relevant for OOBE 

signal effects. LTE “in--‐ band” UL/DL effects depend on rx out of 

band filtering and linearity. Will SIR be used separately for OOBE 

and in--‐ band?

Please provide clarification. Additional clarifying test has been 

added.

36 I Appendix A 

Page 29

Specifies receiver noise floor will be characterized. How will this be 

done. System noise temp is important for 1dB C/No loss criteria.

This is critical to translating test results to real--‐ world 

conditions, particularly if unreported high test noise 

floors are used to obscure the effects of RFI on the C/No.

The baseline case provides information 

on device response to the ambient 

noise in the test setup.

37 I Appendix A 

Definition of 

Prs Page 29

Receiver is designed to acquire and track at a given C/No, NOT a 

given received signal power.

The satellite power is independent

from the GPS device performance and 

is the signal the device desires to 

capture and demodulate. 

38 I Appendix A 

Last 

equation 

Page 29

SIR is not important by itself. We need to know the Interference to 

noise ratio to predict the effects on C/No, and we must know the 

Signal to (noise + interference) ratio which shows up along with all 

the additional errors in KPIs.

Rethink this issue. You may be considering the receiver 

system noise during tests vs during actual operations, 

but this needs to be made explicit.

C/No along with other measurands are 

being collected. SIR is information that 

can help in the DUT analysis.

39 E Appendix B 

Page 31

Consists of only a table. Add detail. Title and caption provide information 

on table contents.

40 E Table 9 Page 

32

Spectrum Analyzer not at GPS output. Noise floor depends on 

analyzer bandwidth setting, etc.

Calibrations sections will be updated in 

final report.

41 E Appendix 

C.2 Page 33

Paragraph 2 does not describe the test configuration figure 12. 

Break is at LTE generator output in figure, at amp output in 

paragraph. If active device is in network analyzer test, must set NA 

test power to not compress amp.

Need to fix. Calibrations sections will be updated in 

final report.
Page 13 of 52

Comment 

Number

Level Reference 

within Draft 

Test Plan

Comment Commenter's Recommendation NASCTN Comment Resolution

42 E Table 11 

row 2 Page 

35

“GPS” should be “LTE” Need to fix. Calibrations sections will be updated in 

final report.

43 E Table 12 

(Page 36), 

Table 13 

(Page 37), 

Table 14 

(Page 39)

OOBE is mixed up with “in--‐ band” in several places. Cross 

references are inaccurate. Clarify that measurements are made at 

minimum attenuation setting of step attenuators, which I believe 

is intent.

Need to fix. Calibrations sections will be updated in 

final report.

m I Appendix 

C.4 Page 40

GSS8000 may not produce high enough signal level to make these 

measurements or to radiate. May need GPS amp. May be able to 

use “MON” port for measurements, or rely on simulator spec. Do 

need to characterize simulator source noise accurately.

Please consider. GPS amplifier is included and 

characterized in the testing.

45 E D.1 Page 42 There appears to be a misplaced word (linear vs. circular) in the 

initial sentence.

Revise to remove ambiguity. The text has been corrected to reflect 

the circular polarization.

46 E Table 18 , 

last row, 

last para. 

Page 43

“LTE” should be “GPS” Need to fix. Calibrations sections will be updated in 

final report.

47 I Appendix 

D.3 Page 44

Network analyzer will likely provide more accurate path loss 

measurement.

Please consider. A network analyzer is utilized for the 

path loss measurement.

48 I Appendix 

E.1 Page 46

We calculate the worst case for 3rd order IM products will come 

from the band edges, namely --‐1627.5+1536 + 1637.5 = 1546.0 

MHz

Recommend you select the worst--‐case frequencies 

from the LTE signal to calculate the locations of the IM 

products.

Will consider in the up and downlink 

combination.

1 C N/A Based on experience with the GPS Adjacent Band testing, there are 

a number of elements that should be more fully described in the 

test plan or in a test procedures document to include: (1) defined 

set of test events, (2) minimum data elements to be recorded and 

associated processing, (3) completed equipment list, and (4) venue 

for testing. 

Recommends updated document(s) be circulated for 

review when these additional elements for testing are 

known. 

The updated NASCTN test plan is 

posted on its website 

(https://www.nist.gov/ctl/nationaladvanced-spectrum-andcommunications-test-network-nasctn).
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2 C N/A The NASCTN process stresses the need for openness and 

transparency. Benefited tremendously in its GPS Adjacent Band 

effort by holding public workshops and using a Federal Register

Notice to solicit comments on its draft test plan.

Recommends use of a Federal Register Notice process so 

that all interested parties who would like the 

opportunity to review and comment have the 

opportunity to do so. Also, it creates a public docket for 

comments, increasing openness and transparency. Also

recommend holding public workshops as part of the 

Outreach and Community Feedback process.

The NASCTN process for obtaining 

feedback on the proposed test plan 

resulted in comments from Federal 

agencies, GPS manufacturers, and both 

spectrum regulators (FCC and NTIA).

NASCTN is reviewing notification 

process to improve awareness among 

the stakeholder community.

3 C N/A The test plan states “The output of this project will be a test 

methodology and measurement data from a set of GPS devices 

subject to nearby LTE activity”. 

Recommends further consideration be given to 

describing how the data from this effort will be handled, 

including who will have access to the test results and the 

measurement data.

The handling of proprietary 

information and distribution of the 

NASCTN report, measurement data, 

and data analysis is governed by the 

CRADA signed by NIST and Ligado 

Networks. 

The NASCTN report will include all of 

the information that is publically 

releasable.

4 C N/A Signal acquisition is usually the most fragile GPS receiver function 

in the presence of interference. If any alternative metric (aside 

from the accepted 1 dB) for harmful interference is recommended, 

it must be demonstrated that GPS devices will continue to be able 

to acquire in the presence of interference. Also, operation in the 

presence of other real-world stressed conditions besides lower 

power levels (e.g., dynamics, vibration) should be considered. 

These points were discussed extensively at the workshops in 

conjunction with its GPS Adjacent Band effort and additional 

testing to include signal acquisition.

There is broad interagency support for evaluation of 

signal acquisition testing and testing under stressed 

conditions in general. We would welcome having 

discussions with NIST regarding inclusion of these test 

conditions. 

The scope of this NASCTN project has 

been negotiated in the CRADA between 

NIST and Ligado Networks. If additional 

tests are needed beyond this project, 

organizations are encouraged to submit 

project proposals to NASCTN.

5 I Title The Title of the test plan states it is “LTE Impacts on GPS Test and 

Metrology Plan”. 

In terms of scope, this study is only looking at the impact 

of one particular proposed set of LTE frequencies –

those proposed by Ligado – and only a subset of civilian 

GPS devices (high precision, timing, GLN). Recommend 

limitations of the scope of the study be reflected in the 

title of the test plan, as well as elsewhere in the 

document.

The scope section of the test plan 

provides this material.
Page 15 of 52

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6 E Section 1

Pg. 5 3rd

Paragraph

Insert “conditional” before “waiver” Same as comment Modified text.

7 E Section 1

Pg. 5 3rd

Paragraph

Change “measurement effort” to “study” Same as comment This is a measurement effort.

8 I Section 1

Pg. 5 3rd

Paragraph

This paragraph would benefit from discussion of the Technical 

Working Group (TWG) test results.

Recommends including a description of the TWG results, 

including the widespread interference impact witnessed 

during this testing.

The NASCTN test methodology and 

results will be well documented, so 

others will be able to make comparison 

with previous measurement 

campaigns.

9 I Section 1

Pg. 5 3rd

Paragraph

This paragraph would benefit from discussion of the National 

Space-Based PNT Engineering Forum (NPEF) testing that took place 

in 2011.

Recommend including background material to 

summarize the tremendous amount of additional work 

that was conducted on LightSquared compatibility with 

GPS by the Federal government subsequent to the FCCmandated TWG. See, e.g., 

http://www.gps.gov/spectrum/lightsquared/. 

Background should also include mention of the DOT ABC 

study/testing and what differences there are between 

the proposed test and the ABC test. Willing to have 

follow-up discussions as necessary to provide additional 

information about these tests.

The NASCTN test methodology and 

results will be well documented, so 

others will be able to make comparison 

with previous measurement 

campaigns.
Page 16 of 52

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10 C Section 1

Pg. 5 4th

Paragraph

This section states that the “GPS industry” prefers the 1-dB CNR 

degradation metric. While we understand there has been much 

discussion over the metric to apply for an interference protection 

criterion, feedback and discussion at the DOT GPS Adjacent Band

workshops supported use of the 1-dB criteria by the majority of 

participants, both from government and industry. We note that 

the Space-Based PNT Advisory Board also has recommended use 

of this criteria at their recent meeting and in a letter to the EXCOM

co-chairs. 

Furthermore, this metric is embedded in ITU-R Recommendations 

on protecting RNSS, including M.1903. Further, NTIA directed the 

NPEF to use this metric in Federal Government testing of 

LightSquared conducted in 2011, and the EXCOM endorsed the 1-

dB metric in their January 2012 conclusions regarding the 

compatibility of LightSquared’s proposed network with GPS. See, 

e.g.,https://www.ntia.doc.gov/files/ntia/publications/lightsquared

_letter_to_chairman_genachowski_-_feb_14_2012.pdf

From our perspective, protection of GPS/GNSS from 

interference is critical for safety-of-life applications. The 

1 dB CNR degradation metric provides the interference 

protection criteria. Trying to define harmful 

interference based on “end-user experience” is 

challenging given the diversity of receivers and 

applications and is problematic in ensuring those 

applications are protected from interference.

Although the NASCTN test plan 

includes a number of measurands in 

the study, including C/No, it does not 

make any recommendations on the 

appropriateness of these measurands, 

which is the purview of the spectrum 

regulators.

11 C Section 2

Pg. 5 1st

Paragraph 

The stated objective is “to establish a test methodology to 

investigate the impact of LTE transmission on GPS receivers”. This 

objective statement is far too broad to communicate accurately 

what distinguishes this test plan from the many test plans for 

earlier LightSquared-GPS tests.

Recommend the objective section should make it clear 

that this test plan focuses only on impact of one 

proposed LTE network on GPS – specifically the one 

proposed by Ligado. The specific frequencies and power 

levels to be tested should be mentioned here.

The scope section provides details on 

the overall NASCTN test.

The NASCTN final report will include a 

discussion of the limitations of the tests 

performed as well as a summary of the 

underlying assumptions.
Page 17 of 52

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12 C Section 2

Pg. 5 3

rd

Paragraph

The test plan states that “a representative subset of available 

devices will be tested...”. However, later on in the test plan it is 

clear that only a small number of receivers representing a small 

number of civilian application categories will be tested. 

Received feedback through several public workshops on 

the importance of testing receivers representing a wide 

number of GPS/GNSS applications. Categories of 

receivers included in the DOT testing were: aviation 

(non-certified), general location and navigation, cellular, 

timing, high-precision and networks, and space-based 

receivers. Recommend that NIST include receivers from 

each of these categories in the NASCTN testing. 

Otherwise this limitation in scope should also be 

addressed in the objectives section.

The NASCTN test plan includes GPS 

receivers from the following categories: 

 General location and 

navigation

 Precision position

 Precision timing

Other GPS receiver categories, 

including aviation, space-based and 

DoD receivers are beyond the scope of 

the NASCTN project. If additional 

testing is required beyond this project, 

organizations are encouraged to submit 

project proposals to NASCTN. 

NASCTN included a number of GPS 

receivers in each of these categories in 

order to adequately validate the 

proposed test methodology. A major 

goal of the NASCTN project is to 

provide a reproducible test project that 

other organizations can use to perform 

similar tests on other GPS devices 

included in this study. 

Based on the feedback from Federal 

agencies and GPS manufacturers, 

NASCTN has revised its list of GPS 

receivers.

13 C Section 3

Pg. 6 1st

Paragraph

This section states in the 1st sentence that the objective of the test 

is to establish the impact of LTE signals on GPS devices”. 

In terms of scope, this study is only looking at the impact 

of one particular proposed set of LTE frequencies –

those proposed by Ligado – and only a subset of civilian 

GPS devices (high precision, timing, GLN). Recommend 

limitations of the scope of the study be reflected in the 

title of the test plan, as well as elsewhere in the 

document.

The scope section modified to add 

additional content and clarity.

The NASCTN final report will include a 

discussion of the limitations of the tests 

performed as well as a summary of the 

underlying assumptions.
Page 18 of 52

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14 I Section 3

Pg. 6 3rd 

Paragraph

It is not the OOBE that makes the handset emissions of concern –

it is the potential for very close proximities of the handsets to the 

GPS receivers, for which both coupling of the fundamental and 

OOBE can cause deleterious effects.

Recommend modifying text to highlight this point. Depending on the input filtering of the 

DUT, the OOBE may emit energy in the 

operating bandwidth of the DUT. 

15 C Section 3

Pg. 6 4th

Paragraph

A significant limitation in the scope of the test plan is only 

considering GPS and not multi-GNSS. Feedback received from GPS 

receiver manufacturers during the DOT GPS Adjacent Band 

workshops is that most civil/commercial receivers now 

incorporate multi-GNSS signals. 

In order to be representative of current user equipment 

and for the testing to be rigorous, the scope should 

include GNSS signals from other constellations. 

Understand that FCC rules require licensing of nonFederal receive-only equipment operating with foreign 

satellite systems, including receive-only earth stations 

operating with non-U.S. licensed radionavigationsatellite service (RNSS) satellites. However, in reality, 

widespread GNSS equipment has been deployed over 

the past decade or more. Also, this licensing rule does 

not apply to federal user equipment such as those GNSS 

receivers being used in significant volumes for highprecision applications by NASA, USGS, NOAA, etc.

The scope of this NASCTN project has 

been negotiated in the CRADA between 

NIST and Ligado Networks. If additional 

tests are needed beyond this project, 

organizations are encouraged to submit 

project proposals to NASCTN.

16 C Section 3

Pg. 6 4th

Paragraph

It is stated that LTE “will” be present in the bands listed. “Will” should be replaced with “has been proposed” 

since currently the FCC has not decided that the licensee 

can proceed with operating the proposed network.

Text has been updated accordingly.

17 C Section 3

Pg. 6 6th

Paragraph

The draft test plan indicates that “Aviation, space-based, cellular, 

and DoD devices of all categories are not included”.

Received feedback through several public workshops on 

the importance of testing receivers representing a wide 

number of GPS/GNSS applications. Categories of 

receivers included in the DOT testing were: aviation 

(non-certified), general location and navigation, cellular, 

timing, high-precision and networks, and space-based 

receivers. Recommend NIST include receivers from each 

of these categories in the NASCTN testing. Otherwise 

this limitation in scope should also be addressed in the 

objectives section.

The scope of this NASCTN project has 

been negotiated in the CRADA between 

NIST and Ligado Networks. If additional 

tests are needed beyond this project, 

organizations are encouraged to submit 

project proposals to NASCTN.
Page 19 of 52

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18 I Section 3

Pg. 6 6th

Paragraph

Why would modification to test procedures be required for these 

other categories of receivers?

Received feedback through several public workshops on 

the importance of testing receivers representing a wide 

number of GPS/GNSS applications. Categories of 

receivers included in the DOT testing were: aviation 

(non-certified), general location and navigation, cellular, 

timing, high-precision and networks, and space-based 

receivers. Recommend NIST include receivers from each 

of these categories in the NASCTN testing. Otherwise 

this limitation in scope should also be addressed in the 

objectives section.

The scope of this NASCTN project has 

been negotiated in the CRADA between 

NIST and Ligado Networks. If additional 

tests are needed beyond this project, 

organizations are encouraged to submit 

project proposals to NASCTN.

19 C Section 4

Pg. 7

The test plan states “The output of this project will be a test 

methodology and measurement data from a set of GPS devices 

subject to nearby LTE activity”. 

Recommends further consideration be given to 

describing how the data will be handled, including who

will have access to the test results and the measurement 

data.

The project is governed by a CRADA 

that was negotiated between NIST and 

Ligado Networks. The NASCTN final 

report will include all information that 

is publically releasable.

20 I Section 5.1 

and 5.2.2

Pg. 7 and 

Pg. 10

Not including atmospheric impairments will lead to larger than 

necessary position errors by the GPS receivers under test.

All GPS devices apply corrections for ionospheric and 

tropospheric delays. Setting these errors to zero in the 

simulated signals will lead to larger errors than 

necessary. For ionospheric errors, GPS devices apply 

corrections in several different ways: (1) singlefrequency standalone receivers use the broadcast 

Klobuchar coefficients in the navigation data, (2) dualfrequency receivers apply corrections based upon the 

difference in delay seen on L1/L2 pseudoranges, (3) 

differential receivers apply corrections from WAAS or 

another differential system. For the troposphere, every 

device applies corrections using models, etc. If you set 

the simulator to not apply ionospheric delays, there is a 

problem that the ionospheric parameters in the 

emulated GPS navigation data cannot be set to produce 

a 0 m correction. If the simulator does not emulate 

tropospheric errors, you cannot make most GPS devices 

NOT apply their corrections so you’ll end up with 

pseudorange errors of up to 25 m (for SVs emulated at 

low elevation angles for an emulated user location near 

0 m MSL)

The scope of this NASCTN project has 

been negotiated in the CRADA between 

NIST and Ligado Networks. If additional 

tests are needed beyond this project, 

organizations are encouraged to submit 

project proposals to NASCTN.
Page 20 of 52

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21 I Section 

5.2.1

Pg. 7

It is stated that the “first round of testing will focus on the top two 

or three devices” in each category but no definition is provided for 

“top” or any evidence that the devices selected meet that 

definition.

Please provide a definition for “top” devices and 

evidence that the devices selected for test are “top” by 

that definition. Address inconsistencies with later 

statements in the test plan that devices would be 

selected based around practicalities (e.g., what was 

available and could produce the desired data). Clarify 

different rounds of testing to be conducted.

The NASCTN test plan includes GPS 

receivers from the following categories: 

 General location and 

navigation

 Precision position

 Precision timing

Other GPS receiver categories, 

including aviation, space-based and 

DoD receivers are beyond the scope of 

the NASCTN project. If additional 

testing is required beyond this project, 

organizations are encouraged to submit 

project proposals to NASCTN. 

NASCTN included a number of GPS 

receivers in each of these categories in 

order to adequately validate the 

proposed test methodology. A major 

goal of the NASCTN project is to 

provide a reproducible test project that 

other organizations can use to perform 

similar tests on other GPS devices 

included in this study. 

Based on the feedback from Federal 

agencies and GPS manufacturers, 

NASCTN has revised its list of GPS 

receivers.
Page 21 of 52

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22 C Table 1

Pg. 8

It appears that many of the devices to be tested are older devices 

which are not representative of modern designs. If a large number 

of devices were being tested to include newer receivers, especially 

those processing multi-GNSS signals, this would not be a concern. 

However, to only focus on these devices is a limitation of the test 

effort.

The MW810 was introduced in 2007. The eTrex H was 

introduced in 2007 and is discontinued by the 

manufacturer. The Montana 650t is discontinued. It 

doesn’t appear that any of the GLN devices are 

GLONASS-capable, which is the norm for new designs. 

Even if the testing will not address LTE impacts on 

GLONASS signal reception, it is important to test modern 

receivers because the font-end is typically shared 

amongst all GNSS signals tracked. GPS/GLONASS 

receivers using a wider front-end make GPS signal 

reception more sensitive to LTE interference in the 

1626.5 – 1660.5 MHz band. Recommends expanding the 

diversity of devices to be tested, to reflect actual and 

future devices market conditions.

Based on the feedback from Federal 

agencies and GPS manufacturers, 

NASCTN has revised its list of GPS 

receivers.
Page 22 of 52

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23 C Table 5

Pg. 10

It is not stated where these power levels are referenced to. In TS 

37.571-1, these power levels are referenced to the output port of 

a passive antenna within a cellular device. Also, the selection of 

appropriately reduced power levels for “stress” testing is critical to 

the completion of the stated test objectives.

If -130 dBm is intended to be used as the received power level 

for the “nominal” referenced to the output port of a 0 dBic 

antenna, then the power levels should be adjusted for each 

GPS device tested. For instance, high precision GPS receivers 

typically use antennas with gain patterns that vary from +5 

dBic at boresight to -5 dBic at 5 degree elevation angle. This is 

quite different from GPS devices in cellular phones, whose 

antenna gain patterns are closer to 0 dBic (or less) for most 

directions (because the cell phone can be held with any 

orientation). Consideration of the user antenna gain pattern is 

particularly important for the “limited exposure testing”. If a 

high precision antenna is tested in the chamber with the GNSS 

signal generator antenna at boresight, it will provide ~+5 dBic 

gain for all satellites, whereas in the real-world, this receiver 

would see gains as low as -5 dBic towards desired satellites. 

The establishment of baseline C/N0’s in the “limited exposure” 

testing will critically influence the outcome of this proposed 

testing. The authors should give considerable thought towards 

ensuring a suitably stressing environment. A 1-dB degradation 

to the receiver noise floor has EXACTLY the same impact as if 

all GPS satellites decreased transmitted power to 80% of their 

current values. We firmly believe that it would have a 

significant impact on user equipment performance. 

This would not be the case for receivers outside in the clear. 

However, It would be the case driving down a street that is 

tree-line, or for a banking aircraft etc. --- almost all GPS users 

frequently experience periods when one or more tracked 

satellites are seen at C/N0’s that are within 1-dB of their 

break-lock point. This is where the difference in performance 

would be seen – those satellites would not be tracked with 1 

dB lower C/N0. If the test does not create such scenarios, this 

very important finding will be missed.

Plan has been updated with details on 

satellite constellations and power 

levels, both nominal and limited. In the 

absence of well-defined industry 

standards for limited exposure testing 

conditions, the limited conditions in 

this test are set to provide data over a 

range of satellite power levels. Motion 

is an additional factor not included in 

these measurements. 
Page 23 of 52

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24 C Section 

5.2.3

Pg. 11

Feedback received during the workshops strongly recommended 

inclusion of augmentation signals in the MSS band. 

Recommend augmentation signals in the MSS band be 

included in the test effort.

The intent of the testing is to focus on 

the LTE impacts on GPS. When 

possible, the DUT will be tested initially 

without augmentation signals to ensure 

that the impacts can be mapped to the 

GPS component of the receiver. 

Augmentation signals such as Wide 

Area Augmentation Signals (WAAS), 

Real Time Kinematic (RTK) solutions 

(e.g., signals from a secondary 

receiver), have been enabled.

The signal constellation will include two 

WAAS satellites. RTK units will be 

tested.

25 I Section 

5.2.4

Pg. 11

Unclear why a 96 dB attenuation selected for the downlink? This 

would lead to -34 dBm being received from a 62 dBm base station. 

Much higher power levels were seen during earlier LightSquared

testing when 3 towers were lit up in Las Vegas. In those tests, 

received powers of up to -14 dBm were seen 2 m off the ground. 

More than predicted using free-space, but rather consistent with 

the two-ray propagation model. Users > 2 m off the ground (e.g., 

UAS) can see far higher power levels.

Please provide a rationale for choosing a propagation 

loss consistent with free space at 1 km for this particular 

test.

This loss factor is utilized in the 

combined up and downlink test, not 

the downlink only case. 

26 I Section 

5.3.2

Pg. 14

The Draft Test Plan states “It is important to recognize that these 

tests are focusing on the change in the baseline due to the LTE 

activity, not on precise locations”. Are the receivers going to be 

reset to run under the same conditions with LTE activity as the 

baseline?

Recommend providing clarification in this section. Test plan modified to add clarity.

27 I Section 

5.3.4

Pg. 19

Encouraged to see recognition in the draft test plan that the 

separation distance between a GPS receiver and LTE handset can 

be less than 1 m and even 0.5m.

Recommend conducting testing using a short (0.5m) 

separation distance.

The scope of this NASCTN project has 

been negotiated in the CRADA between 

NIST and Ligado Networks. If additional 

tests are needed beyond this project, 

organizations are encouraged to submit 

project proposals to NASCTN.
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28 I Section 5.4

Pg. 20

It isn’t clear what is intended by “Power level increments are 

determined by an initial investigation of the DUT behavior and the 

eventual maximum power level sought”. Aren’t all of the devices 

tested under the same conditions? 

Recommend providing clarification in the test plan on 

the power levels to be tested, including increments of 

increasing the power level.

The stepped power levels are chosen 

based on the regions of significant 

device response. This range is initially 

estimated by a preliminary sweep of 

power levels; additional levels are 

tested as needed. This results in a 

lower uncertainty in the data for 

measured power levels.

29 C Section 

5.5.2

Pg. 23

3 meters is stated as an example separation distance for between 

an LTE transmitter and GPS device under test. As stated elsewhere 

in this plan, LTE handsets can be much closer than this. What is the 

rationale for this distance?

Recommend applying a shorter separation distance (less 

than a meter) as discussed in Section 5.3.4.

The scope of this NASCTN project has 

been negotiated in the CRADA between 

NIST and Ligado Networks. If additional 

tests are needed beyond this project, 

organizations are encouraged to submit 

project proposals to NASCTN.

30 I Section 

6.3.1

Pg. 25

Measured data is listed elsewhere in the report to include C/N0, 

pseudoranges, time, etc. However, in this section it is noted that 

the “data” will be processed until there is less than a 1% change. 

This process makes sense for data elements such as C/N0 for a 

fixed interference level, but such a steady-state would never be 

seen for other data elements (e.g., pseudorange, or receiver time 

output).

Recommend this section be written so it is clear in 

describing that the data processing would only be 

applied to certain specific receiver output data. These 

receiver output data should be identified.

Test plan modified to add clarity.

31 C Section 7

Pg. 26

The NASCTN process stresses the need for openness and 

transparency. GPS Adjacent Band benefited tremendously by 

holding public workshops and using a Federal Register Notice to 

solicit comments on its draft test plan. Also coordinated many 

interagency discussions through the NCO.

Recommend use of a Federal Register Notice process so 

that all interested parties who would like the 

opportunity to review and comment have the 

opportunity to do so. This approach creates a public 

docket for comments, increasing openness and 

transparency. Also recommend NIST hold public 

workshops as part of the Outreach and Community 

Feedback process and provide information to the NCO 

to ensure widespread distribution within the 

interagency for review.

The NASCTN process for obtaining 

feedback on the proposed test plan 

resulted in comments from Federal 

agencies, GPS manufacturers, and both 

spectrum regulators (FCC and NTIA).

NASCTN is reviewing notification 

process to improve awareness among 

the stakeholder community.
Page 25 of 52

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32 I App A

Pg. 28

Equation 1 appears to be incorrect. The -10 log10(Br/Bi) terms do not appear to make sense. 

If a 10 MHz LTE signal is received by a 2 MHz GPS C/Acode receiver, one would expect the received power (if 

it were defined to be that power seen within the GPS 

device under test) to be attenuated by 7 dB rather than 

enhanced. But -10*log10(2/10) = +7 dB. Also, please 

note that this equation is not particularly useful for 

adjacent band interference. If the 10 MHz LTE is not 

cofrequency with a 2 MHz C/A-code device, then a 

correction for Br/Bi does not seem appropriate at all. For 

adjacent-band interference we are concerned primarily 

with the fundamental component of the interference 

signal compressing the front-end of the GPS device 

under test. For this reason, this equation and the entire 

appendix come across as spurious.

Additional clarifying text have been 

added to the test plan.

33 I App D

Pg. 42 

It is noted that the LTE antenna is linearly polarized whereas the 

transmit antenna is linear. It is speculated that “circular” was 

meant in the latter instance. Note also that Ligado had at one time 

planned to use +/-45 deg cross-pol antennas (see the Jan 2012 FAA 

LightSsquared report at www.gps.gov)

Recommend fixing the 1st sentence. Also, the EIRP in 

3GPP GPS Power Level appears to be missing a unit (e.g., 

an ‘m’ or ‘W’ after dB)

Test plan modified to address 

comment.

1 - Para 5.3.5 

on OOBE

The first thing to is to address Unwanted Emissions as OOBE does 

not include harmonic emissions, parasitic emissions, 

intermodulation products and frequency conversion products 

especially when narrowband signals are used by multiple users. A 

narrow band spike at the GPS frequency is not desired.

- A prototype of the proposed system is 

not currently operational, so we cannot 

collect information on unwanted 

emissions. The unwanted emissions are 

assumed to be below the OOBE levels 

considered here, but that would need 

to be verified on a prototype system.
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2 - - The 2nd thing is to perform a live spectrum analysis of Ligado 

signals and use this information for setting up your simulation. See 

definitions below and always refer to unwanted emissions in the 

1559-1610 MHz. Unwanted Emissions in this band are very 

important.

- We generally agree that a live spectrum 

analysis of Ligado signals would be 

beneficial for informing the test plan. 

However, no system is currently 

deployed or functionally operational, 

and thus spectrum captures are not 

possible. Therefore, we cannot capture 

the unintentional emissions. The next 

best surrogate is to utilize an industry 

LTE waveform generator in 

combination with noise generators to 

create the OOBE. 

3 - - The 3rd thing to consider in your simulation is propagation, near 

and far field.

- The LTE signal will be amplified to 

compensate for the separation distance 

between the transmitting antenna and 

the DUT. The amount of gain is 

calculated relative to a free space farfield propagation path, thus assumes 

an r2 path loss. Near field path loss 

factors are a great deal more complex 

than far-field path loss, including a loss 

of assumption on antenna gain 

patterns, which are based on far-field 

behavior, and the interaction between 

the DUT and the transmitting antenna. 

The near-field configuration is outside 

of the technical merits of this test 

setup.

4 - - The 4th thing to consider is the difference between antenna gain 

towards interference and the gain towards the GPS satellites (you 

may have this covered by GPS satellites of various power levels.) 

Remember that GPS is officially a 24 satellite system and 

one does not see all satellites (12) most of the time, so 

testing with 4 to 6 satellites only should be considered.

In section 5.2.2, the test plan describes 

both a nominal and Limited Exposure 

condition. The Limited Exposure 

considers only 8 satellites in view, with 

6 power levels at least 5 dB below the 

nominal 128.5 dBm.
Page 27 of 52

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1 Informa

tive

Section 1, 

paragraph 4

Some additional background information w.r.t. the use of 1 dB 

change in carrier-to-noise density ratio (C/N0):

This metric was introduced as the critical test criterion initially in 

the TWG test efforts performed by the GPS industry and then used 

in the NPEF follow-on testing performed over the 2011-2012 

timeframe. Prior to the introduction of this metric as the sole test 

measurand, the standard criteria for GPS interference testing was 

an impact to position accuracy (typically pseudorange, but many 

receivers don’t report this measurand), loss-of-lock w.r.t. a single 

satellite (aka “break-lock”), and reacquisition time (time required 

to reacquire a lost satellite once the interference signal is 

removed). Each of these three metrics can be directly correlated 

to an impact on one or more of the four basic GPS performance 

parameters of accuracy, availability, integrity, and reliability. 

There is considerable debate over whether a 1 dB C/N0

degradation criterion should be used in lieu of these traditional 

metrics, and if so, whether the metric should apply to receivers 

that extend their passbands beyond the RNSS allocation, 

particularly since no direct correlation has been made between 

this test metric and a degradation to any of the four 

aforementioned core GPS receiver performance expectations.

None – Background Information Only -

2 Importa

nt

Section 1 

and briefing 

slides of 

6/10/16.

Break-lock is a traditional GPS susceptibility measurand that is 

important to include because it’s typically available across all GPS 

receivers.

Include satellite break lock in the list of measurands to 

be collected.

Loss of lock is included in test but data 

is device dependent.
Page 28 of 52

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3 Suggesti

on

Section 3, 

paragraph 

5. 

The GPS receiver categorization proposed is consistent with what 

has been done in past efforts, but the results from the TWG and 

NPEF testing seem to indicate that a categorization based on 

receiver RF architecture may be more appropriate than 

categorizing based on application. For example, categorizing 

according to narrowband and wideband (SPS) GPS receivers may 

be more appropriate given that the trend in all of the previous test 

efforts seems to indicate that the primary impediment to 

achieving EMC are those GPS receivers that utilize wide pass bands 

so as to either capture the entire transmitted GPS signal (including 

the residual sidelobes), or to more readily accommodate the 

reception of GPS augmentation signals, multiple GPS signals (e.g., 

L2C and/or L5) and/or satellite signals associated with other GNSS 

constellations.

Consider re-categorization or subcategorization 

according to GPS receiver RF front-end architecture.

The scope section now points out that 

a categorization by the basic receiver 

architecture, i.e., narrowband versus 

wideband, may be a better predictor of 

device performance in the presences of 

out-of-band emissions. 

4 Importa

nt

Section 3, 

paragraph 

9.

Agree that the DUT (i.e., GPS receiver) is generally not a 

metrology-grade piece of equipment and that it can contribute 

significantly to the overall measurement uncertainty budget, 

particularly when C/N0 is considered as a primary metric. For 

example, there is no standard model/algorithm used in GPS 

receivers for estimating the C/N0. Rather, there are at least five 

different algorithms that are utilized by GPS receivers for this 

estimation. Information obtained from the open literature 

suggests that there can be considerable variation between these 

algorithms which will impact on the uncertainty budget when 

using a GPS receiver to estimate the C/N0 level.

Further, relevant to the point of comment #1, a 1-dB variation in 

the carrier level (and thus also in the C/N0) is a common and 

recurring occurrence in normal GPS operation, absent any increase 

in noise. The GPS Interface Specification stipulates that the 

received signal level will vary by at least 2 dB in normal operation 

and thus will also impact on the uncertainty budget when using a 

GPS receiver to estimate the C/N0 level. 

Include a component in the measurement uncertainty 

budget to account for the variation among carrier and 

noise estimation algorithms utilized by the different GPS 

DUTs and to account for the natural variation of the GPS 

downlink signals.

A study of the uncertainty due to 

different carrier and noise estimation 

algorithms would be useful. Due to 

time limitations, it is not clear that a 

thorough analysis can be completed 

during this effort.
Page 29 of 52

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5 Importa

nt

Section 

5.2.1, Table 

4.

While it is recognized that including GPS development boards 

within the suite of test receivers provides additional test flexibility, 

extreme care should be exercised when assessing the interference 

susceptibility of such developmental/prototype applications, 

particularly if such boards are not shielded, since it may be difficult 

to ascertain the interference ingress.

Steps should be taken to ensure that any interference 

identified to development boards be attributable to 

ingress via the receive antenna port and not through 

other means attributable to a lack of a shielded 

enclosure.

Development boards are no longer 

included in the list of DUTs. They will 

only be utilized for test setup 

diagnostics.

6 Importa

nt

Section 

5.2.2, Table 

5.

The power level settings for the emulated GPS satellites appears to 

be too low. For the nominal case, the proposal to set all satellites 

to a level of -130 dBm is not representative of a normal “real 

world” configuration. The current GPS Interface Specification 

states that the L1 navigation signal strength at end-of-life (EOL), 

worst-case, will be -158.5 dBW (-128.5 dBm) for all SV Blocks. In 

normal operation the satellite signal strengths are 3-5 dB hotter 

than the EOL levels. This situation is compounded in the “Limited 

Exposure” case in that the received GPS power levels are proposed 

to be set to levels that are at, or below (by as much as 5.5 dB), the 

worst-case EOL levels. Also question the limitation to 6 satellites 

in the GPS constellation emulation. With the expansion of the GPS 

constellation from 24 to 30 satellites, in normal operation there 

are typically at least 10-12 GPS satellites in view of a user’s 

location and most modern receivers are capable of processing at 

least 12 satellites simultaneously.

Include a mix of GPS satellite power levels to include at 

least one at EOL spec level of -128.5 dBm with the 

remaining SV levels set to represent a distribution over 

‘real world’ levels. This will enable a comparison of 

some measurands/metrics over best-case, nominal, and 

worst-case constellation configurations. Emulate more 

than six satellites in the simulation (10-12 being more 

realistic). The lack of satellite availability or received 

signal degradation due to signal blockage should be 

considered on a case-by-case basis in the follow-on 

operational scenario-based link budget analyses.

The “limited” case provides a range of 

reduces power levels to simulate 

additional signal loss in the propagation 

path.

7 Importa

nt

Sections 

5.3.2 and 

5.3.3, 

Figures 2 

and 3.

The block diagrams representing the LTE emulation component of 

the applicable test set-up depicts a “Custom OOBE Filter” in the 

transmission chain. The characteristics of this filter will be critical 

in understanding the final test results.

Provide technical specifications and operational 

characteristics for the custom filter used in the test 

program.

OOBE filter design will be documented 

in the NASCTN Test Report.

8 Importa

nt

Section 

5.3.3, 

paragraph 

6.

It is stated that the LTE signal power will be incremented until the 

receiver loses lock. Is the loss of lock relative to a single GPS SV 

being tracked, to one of the SV’s being used in the solution, or to 

all SV’s in view? Typically, the first satellite that the receiver loses 

lock with is the one with the lowest received power level, which is 

the point of comment #6 (i.e., if the satellite received power is set 

to levels lower than EOL levels, then the associated results can 

reflect a bias.

Clarify. Loss of lock is dependent on device 

generated reports.
Page 30 of 52

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9 Editorial Pages 21 

and 22. 

There are no captions associated with the two figures on these 

pages, to include the figure numbers that are called out in the 

relevant text.

Include captions with figure numbers. The captions have been corrected.

10 Importa

nt

Appendix A, 

A.3 –

Equation 1.

The ‘Lfilt’ term in the equation represents the attenuation of the 

transmitted fundamental signal due to the RF filtering employed 

by both the source transmitter and the victim receiver” (i.e., 

receiver selectivity). This term is also known as the “OffFrequency Rejection” (OFR) component of the FrequencyDependent Rejection (FDR) term.

Change to clarify that receiver selectivity is also a 

relevant component of this interference link budget 

parameter. 

Additional clarifying text have been 

added to the test plan.

11 Suggesti

on

Appendix A, 

A.3 –

Equation 1.

The term “10log10 (Br/Bi)”, which is also known as the On-Tune 

Rejection (OTR) component of the FDR, is superfluous when 

considering adjacent-channel interactions with a noise-like 

emission source. In these cases, it’s typically assumed that Bi = Br

and thus this term reduces to zero in the worst-case.

For the adjacent-channel (band) interactions under 

consideration, this term can be left out of the equation 

(perhaps with some explanation).

Additional clarifying text have been 

added to the test plan.

1 - NASCTN 

Draft, page 

4

(We) agree that any tests, if conducted, should involve a 

“transparent, well-calibrated test method.” Transparency also 

requires making publicly available input from any other federal 

agencies prior to, throughout, and after the testing process. 

In addition, data derived from any testing – including “raw,” preanalysis data as well as processed, post-test data and information 

– must similarly be publicly available to ensure that the rights of all 

parties are protected.

In the interest of such transparency, any process should 

make publicly available all comments sent to NIST 

concerning the test plan and the testing, including all 

correspondence with the test’s sponsor that have led to 

this stage. Any sponsorship arrangements should also be 

a matter of public record.

In addition, data derived from any testing – including 

“raw,” pre-analysis data as well as processed, post-test 

data and information – must similarly be publicly 

available to ensure that the rights of all parties are 

protected

NASCTN will post the comments, 

NASCTN’s response, and a copy of the 

revised test plan on its website.

The handling of proprietary 

information and distribution of the 

NASCTN report and measurement data 

is governed by the CRADA signed by 

NIST and Ligado Networks. 
Page 31 of 52

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Comment Commenter's Recommendation NASCTN Comment Resolution

2 - NASCTN 

Draft, page 

4. 

NASCTN 

Draft, 

Section 6.1, 

page 24.

The test plan also notes as a goal “sound and statistically valid data 

retrieval and processing.” The test plan also acknowledges “a risk 

of selection bias in the test results” as well as the likelihood that 

conclusions based on such data “will not be rigorously 

generalizable to the population of all devices.” (We) have concerns 

that, given that only seven devices will be tested from the 

“outdoor” segment of the General Location-Navigation (“GLN”) 

device market, such “bias” will be likely to occur; seven devices are 

in no way statistically representative of the tens of millions of GLN 

devices in the outdoor, marine, aviation, automotive, and fitness 

segments. The devices proposed for testing also do not represent 

the latest makes and models that incorporate multi-GNSS 

technology.

The NASCTN test plan includes GPS 

receivers from the following categories: 

 General location and 

navigation

 Precision position

 Precision timing

Other GPS receiver categories, 

including aviation, space-based and 

DoD receivers are beyond the scope of 

the NASCTN project. If additional 

testing is required beyond this project, 

organizations are encouraged to submit 

project proposals to NASCTN. 

NASCTN included a number of GPS 

receivers in each of these categories in 

order to adequately validate the 

proposed test methodology. A major 

goal of the NASCTN project is to 

provide a reproducible test project that 

other organizations can use to perform 

similar tests on other GPS devices 

included in this study. 

Based on the feedback from Federal 

agencies and GPS manufacturers, 

NASCTN has revised its list of GPS 

receivers.

3 - NASCTN 

Draft, page 

4. 

NASCTN 

Draft, 

Section 4, 

page 7

The test plan also advocates a “clear path from measurement setup to data collection to processed results.” At the same time, it 

notes that “pass/fail criteria will not be discussed nor will 

conclusions be drawn by the testing team on the data collected.”

Despite this second statement, (we) urge public release 

of all NASCTN material related to these tests to ensure a 

“clear path” is followed.

The NASCTN final report will not only 

include the results of the tests, but will 

also provide the detailed test process 

used in the NASCTN measurements so 

that other organizations will have an 

opportunity to reproduce the tests or 

extend them to other GPS receivers. 
Page 32 of 52

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Comment Commenter's Recommendation NASCTN Comment Resolution

4 - NASCTN 

Draft, 

Section 

5.2.3, page 

11.

In addition, the test plan notes that augmentation signals will be 

tested on an “as needed” basis or “time permitting.” (We) are 

concerned that test results will be questionable if expected signals 

– such as WAAS or RTK for high precision devices – are not 

present. If devices and systems are not tested in the configurations 

and with the signals needed to function in their intended modes, 

the “clear path” may be obstructed.

The intent of the testing is to focus on 

the LTE impacts on GPS. When 

possible, the DUT will be tested initially 

without augmentation signals to ensure 

that the impacts can be mapped to the 

GPS component of the receiver. 

Augmentation signals such as Wide 

Area Augmentation Signals (WAAS), 

Real Time Kinematic (RTK) solutions 

(e.g., signals from a secondary 

receiver), have been enabled.

The signal constellation will include two 

WAAS satellites. RTK units will be 

tested.
Page 33 of 52

Comment 

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Comment Commenter's Recommendation NASCTN Comment Resolution

5 - NASCTN 

Draft, page 

4.

NASCTN 

Draft, page 

5.

Public release of all data is needed to ensure NASCTN’s goal to 

“inform discussions between different interests on proper 

measurement requirements” is upheld. The test plan also notes 

that the GPS industry prefers use of an interference metric based 

on a 1 dB decrease in the carrier-to-noise density ratio (C/N0), 

whereas potential users of spectrum adjacent to GPS prefer a 

definition of interference premised on end-user experience. 

Absent use of the 1 dB metric, We have concerns about the 

conclusiveness (and certainly the universal applicability) of 

assessment of interference based on just one or two aspects such 

as position accuracy and/or timing. The possibility and implications 

of interference must be considered not only for accuracy, but for 

integrity, continuity, and availability. 

Further, in measuring timing accuracy, in addition to position 

accuracy, NASCTN’s proposed test plan fails to consider velocity, 

another key output of most GPS devices. If those conducting tests 

aim to measure key performance indicators other than the 

fundamental metric of a 1 dB degradation in C/N0, they must 

measure all outputs that are significant to users of GNSS systems.

(We) participated in the early planning of the DOT “Adjacent Band 

Compatibility” Study and support that proceeding’s use of the 

appropriate metric (1 dB metric), its vetting through public and 

government input, its transparency and government sponsorship, 

and its broader and more meaningful sample of devices. Most 

importantly to (us), in terms of advancing the prompt expansion of 

broadband service, the DOT testing has already been completed 

and analysis is underway.

The handling of proprietary 

information and distribution of the 

NASCTN report and measurement data 

is governed by the CRADA signed by 

NIST and Ligado Networks. 

The objective of the NASCTN project is 

to establish a test methodology to 

investigate the impact of LTE 

transmission on GPS receivers. NASCTN 

will not be making any 

recommendations about interference 

levels or defining interference 

protection criteria.

Although the NASCTN test plan does 

include a number of measurands in the 

study, including the C/N0 measurand, it 

does not make any recommendations 

on the appropriateness of these 

measurands, which is the purview of 

the spectrum regulators. 
Page 34 of 52

Comment 

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Level Reference 

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Comment Commenter's Recommendation NASCTN Comment Resolution

1 Critical No 

Reference 

Given

Changing the measure of interference from the internationally 

recognized standard, even if done based on sound testing, will 

require DOD to rewrite the Joint Capability Integration and

Development System (JCIDS) Requirements documents for its 

ongoing acquisition of Military GPS User Equipment. This will add 

significantly to acquisition timelines and will certainly add 

significant cost for components of critical weapon systems in each 

of the Services (Army vehicles, Navy ships and Air Force jets). 

Depending on the standard chosen, it is also likely to require 

retrofitting of ~2 million already fielded military receivers designed 

to work within the current interference standard - presuming 

retrofitting can even be done as a technical matter. Obviously 

none of the UAVs (“drones”) will function without reliable GPS.

The House and Senate Armed Services Committees and 

the Defense Appropriations subcommittees of each 

Chamber must be involved in proposals to change 

interference standards, as they are significant 

stakeholders in DOD resourcing decisions. The scope of 

this program is not insignificant; the MGUE program 

alone was funded at $142+M in 2016, and is 

programmed for $1,061,867,000 over the FYDP (2018-

2022). (AFSPC/JA Addition)

The objective of the NASCTN project is 

to establish a test methodology to 

investigate the impact of LTE 

transmission on GPS receivers. NASCTN 

will not be making any 

recommendations about interference 

levels or defining interference 

protection criteria.

2 Importa

nt

Preface, 

Page 4, 

Paragraph 3

The third para states the comparison of results between this test 

and previous testing (presumed to be government) are out of 

scope for this effort. To be academically complete this test plan, 

as a deliverable, should address differences in anechoic chamber 

testing methodology with previous testing

As a deliverable, include an addendum that addresses 

the differences between this test plan’s anechoic 

chamber testing and other testing (e.g. 2011 NPEF test 

plan and DoT ABC Assessment test plan)

The NASCTN test methodology will be 

well documented, so that others will be 

able to make comparison with previous 

measurement campaigns.

3 Critical Preface, 

Page 4, Last 

Paragraph

In the interest of transparency, the test plan needs to identify who 

NASCTN solicited comments from and how comments received are 

adjudicated

As a deliverable, include an addendum of groups or 

entities NASCTN solicited comments from, who 

responded and how their comments were adjudicated

The NASCTN process for obtaining 

feedback on the proposed test plan has 

resulted in comments from Federal 

agencies, GPS manufacturers, and both 

spectrum regulators (FCC and NTIA).

NASCTN will post the comments, 

NASCTN’s response, and a copy of the 

revised test plan on its website. 

4 Importa

nt

Preface, 

Page 4, Last 

Paragraph

The NASCTN process for soliciting comments needs to include 

standard government and public mechanisms for ensuring all 

affected stakeholders are afforded the opportunity to participate. 

This ensures NASCTN’s process is open, transparent, inclusive and 

documented in the proper, associated forums

NASCTN should use the IRAC to solicit comment from 

federal agencies and the Federal Registry for public 

comment.

The NASCTN process for obtaining 

feedback on the proposed test plan has 

resulted in comments from Federal 

agencies, GPS manufacturers, and both 

spectrum regulators (FCC and NTIA).
Page 35 of 52

Comment 

Number

Level Reference 

within Draft 

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Comment Commenter's Recommendation NASCTN Comment Resolution

5 Background, 

Page 5

The background does not address how NASCTN was engaged by 

the commercial sector to develop a test plan, the source/amount 

of funding, and the substance of the agreement as it relates to 

scope and content of the test plan. This detail is important for the 

reader to fully understand and appreciate the context of how the 

test plan is being influenced by this agreement.

Include a paragraph under Background that explains the 

engagement from the commercial sector that is driving 

this effort and the substance of any agreement with 

NACSTN that is influencing assumptions, methodology, 

constraints and participation in this effort. For 

transparency, the agreement with the commercial entity 

should be appended to the test plan and resulting 

report.

The project is governed by a CRADA 

that was negotiated between NIST and 

Ligado Networks. The NASCTN final 

report will include all information that 

is publically available.

6 Importa

nt

1 

Background, 

page 5, 3rd 

para

Para states that a consensus was not reached on what constitutes 

GPS receiver interference but does not qualify what constitutes a 

consensus. Para only identifies the GPS industry as having a 

preference for the 1 dB IPC

Qualify what is believed to be constitute a consensus. 

Government testing, facilitated by the PNT EXCOM, 

along with GNSS related ITU recommendations 

formulated the government’s assessment of what 

constitutes permissible GPS receiver interference (ref 

NTIA Report 05-432, para 5.3)

As evident by comments on the test 

plan, not all stakeholders fully accept 

that a 1 dB change in C/No is the best 

measure for determining interference.

7 Critical 2, Objective, 

Page 5

The test objective is only clear on testing a range of LTE power 

levels on GPS receivers. Later in the document it is clarified that 

testing will only be performed in a steady state condition. There is 

no discussion on how GPS receiver acquisition and reacquisition is 

or is not being considered.

Clarify in the test objectives what modes of operation, 

both LTE and GPS, are being tested. Explain what modes 

are not being tested and why.

Time to first fix and GPS receiver 

reacquisition has been added to the 

test plan, where appropriate.

8 Importa

nt

3 Scope, 

page 6, 6th 

para

The para identifies what categories of GPS receivers will not be 

tested but does not address any future requirement or plan to do 

so

Specify if, then when, untested GPS receiver categories 

will be addressed

The scope of this NASCTN project has 

been negotiated in the CRADA between 

NIST and Ligado Networks. If additional 

testing is needed beyond this project, 

organizations are encouraged to submit 

project proposals to NASCTN.
Page 36 of 52

Comment 

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Level Reference 

within Draft 

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Comment Commenter's Recommendation NASCTN Comment Resolution

9 Importa

nt

3 Scope, 

page 6, 8th 

para

Later in the test plan it is also stated GPS receivers will be tested in 

a GPS acquired, steady state. If this para is to capture the 

constraints associated with the tested GPS receivers then this 

needs to be treated with the same level of detail afforded LTE

Para needs to fully explain the limitations and 

constraints being assumed within this particular test 

plan. Para should explain rational for only addressing a 

GPS acquired steady state GPS receiver performance. 

Para should explain rational for not exploring a means to 

measure 1 dB IPC metrics along with performance 

metrics at higher interference levels. 

Time to first fix testing has been added 

to the test plan, i.e., data will be 

collected on the time it takes the GPS 

receiver to establish satellite lock in the 

presence of LTE signals. C/No will be 

collected for all tested LTE power 

levels, assuming the device reports 

C/No. An important goal of this effort 

is to provide an open set of data that 

can assist all stakeholders in their 

discussions on the proposed LTE 

system impacts on GPS receiver 

performance.

10 Importa

nt

Table 1, 

page 8

An Apple iPad Air 2 (WiFi only) is included in the pool of general 

navigation and location devices. This device does not include a 

GPS chip; it is dependent upon an external device like a Bluetooth 

capable GPS receiver (e.g. Bad Elf)

Remove and replace with a GPS capable tablet The list of GPS devices considered in 

this study has been modified, including 

the removal of the Apple iPad Air 2. 

11 Critical Section 7, 

page 26

The test plan states comments will be solicited from “the 

engineering community within federal and non-federal groups and 

entities.” 

The test plan should list specifically which organizations 

and at what level were coordinated with. And, if those 

organizations submitted comments to the test plan, a 

comment resolution matrix dispositioning each and 

every comment should be sent out to all who 

commented on the test plan. This is essential to 

maintain “a transparent, well-calibrated test method” as 

stated on page 4 of the test plan.

The NASCTN process for obtaining 

feedback on the proposed test plan has 

resulted in comments from Federal 

agencies, GPS manufacturers, and both 

spectrum regulators (FCC and NTIA).

NASCTN will post the comments, 

NASCTN’s response, and a copy of the 

revised test plan on its website 

(https://www.nist.gov/ctl/nationaladvanced-spectrum-andcommunications-test-network-nasctn).

12 Critical Section 8, 

page 26

The test plan does not address disposition of raw test data. The 

test plan states the measurements “will be physically removed by 

NASCTN personnel at the end of the measurement period.” 

To maintain the transparency sought by NASCTN, the 

raw test data should be made available to stakeholders 

as identified by the PNT EXCOM. The CRADA between 

NASCTN and Ligado should also be updated to reflect 

this. If so desired, organizations receiving raw test data 

could be required to sign Non-Disclosure Agreements 

(NDAs) to protect proprietary Ligado data. 

The handling of proprietary 

information and distribution of the 

NASCTN report and measurement data 

is governed by the CRADA signed by 

NIST and Ligado Networks. Release of 

data will be controlled by relevant 

statutes and the terms of the CRADA.
Page 37 of 52

Comment 

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within Draft 

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Comment Commenter's Recommendation NASCTN Comment Resolution

13 Critical Section 9, 

page 26

The project tasks as listed do not reflect disposition of individual 

comments to commentating entities.

Update the Project Task list to include formal disposition 

of comments and rationale for adjudication.

The NASCTN process includes the 

collection, adjudication and posting of 

the comments along with the 

distribution of a revised test plan.

1 Importa

nt

Section 1 

Background 

in the 

second 

paragraph 

end of first 

sentence.

It is not clear that only GPS receivers operating in the tracking 

mode are being considered (e.g., no acquisition or TTFF). 

Change first sentence as follows:

In support of these functions, this NASCTN effort focuses 

on potential impacts of proposed Long Term Evolution 

(LTE) activities adjacent to the L1 GPS band receivers 

operating in tracking mode.

Time to first fix is now included and the 

sentence has been edited to reflect this 

change.

2 Editorial Section 1 

Background 

last 

paragraph

“This implies that a 1 dB increase to the noise floor, as measured 

by any receiver in a shielded or direct-wired environment, is 

considered interference.”

Suggest the following to replace the sentence:

“That most GPS receivers operating over a wide range of 

GPS signal levels and receiver types and designs will not 

suffer performance degradation (due to interference), if 

the interference signal level at the input to the GPS 

receiver is at least 6 dB below receiver noise. “

It is not obvious that the two 

statements are equivalent. No change 

can be made to the test plan without 

further discussion.

3 Importa

nt

Section 2 

Objective in 

the first 

sentence

It is not clear that only GPS receivers operating in the tracking 

mode are being considered (e.g., no acquisition or TTFF). 

Change the first sentence as follows:

The objective of this project to establish a test 

methodology to investigate the impact of LTE 

transmission on GPS receivers operating in tracking 

mode.

Text has been updated to reflect 

inclusion of TTFF.

4 Critical Table 5 The GPS L1 C/A signal power level of -130 dBm is not correct. In Table 5 change -130 dBm to -128.5 dBm.

Should all of the GPS signal power levels be set to the 

same level?

Table updated.

5 Importa

nt

Table 1 and 

Table 2

Should increase the number of high precision receivers tested Reduce the number of general navigation receivers by 

two and add two high precision receivers

Devices list has been changed to reflect 

the inclusion of additional high 

precision receivers.

6 Importa

nt

Section 

5.3.2

Filter response of custom OOBE filter in Figure 2 is missing Include the measured response of the custom OOBE 

filter 

The NASCTN test report will include the 

measurement of filter response.

7 Importa

nt

Section 

5.5.1 and 

5.5.2

Sections 5.5.1 and 5.5.2 describe two approached for varying the 

power levels for the in-band and OOBE signal levels for the LTE 

downlink and uplink test signals. Both approaches address 

different issues related to GPS interference.

Retain both approaches to varying the power levels for 

the LTE test signals.

Due to time constraints, the approach 

with fixed OOBE and varying LTE power 

is given priority.
Page 38 of 52

Comment 

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Comment Commenter's Recommendation NASCTN Comment Resolution

8 Critical Section 6.2 A key performance parameter is the loss of lock between the GPS 

receiver and a GPS satellite. This condition occurs when an 

interfering signal reduces the carrier-to-noise density ratio to such 

an extent that the GPS receiver can no longer adequately 

determine pseudorange.

Add GPS receiver break-lock power level to the list of 

Key Performance Indicators. The break-lock condition is 

as reported by the GPS receiver.

Added but is DUT specific depending on 

data provided.

9 Importa

nt

Section A.3 

first 

paragraph, 

second 

sentence

The deployment model is important to assessing the aggregate 

power levels.

Add a footnote to the second sentence:

The aggregate interference will be determined by the 

deployment model which establishes antenna couplings 

and minimum separation distances.

A footnote has been added to the text 

to the effect of the comment.

10 Importa

nt

Section A.3 

Equation 2

There is a parameter in the link budget for the desired signal for 

polarization mismatch loss. For the desired link would there a loss 

for polarization mismatch? 

Delete the parameter for loss due to polarization 

mismatches from Equation 2.

The desired signal may have a 

polarization mismatch if the 

polarization on the receiver is not 

identical to the transmitted 

polarization. For example, an elliptically 

polarized receive antenna will have 

some polarization loss when the 

desired signal is linearly polarized. (The 

propagation loss does not incorporate 

this loss in this case.)

11 Editorial Table 10 Table 10---What is the meaning of an AWGN propagation 

condition?

Please explain or add reference. This condition has been removed. The 

additional signals, L1C pilot, Pseudo Y, 

and M-code, that will be present have 

been added. 

12 Editorial No 

description 

of the data 

format for 

the 

measureme

nt results

Will the measurement data be presented for each satellite in the 

test?

Add a new section describing the format of the output 

data

The data available is dependent on the 

DUT and will be provided in the final 

report.
Page 39 of 52

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13 Editorial Appendix D. 

D.1 p.42

The text below is misworded:-

...spectral masks are specified for linear polarization, but the 

transmit antenna is designed for linear polarization...If the 

transmit antenna radiates true RHCP...

Suggest revising text as shown below:

...spectral masks are specified for linear polarization, but 

the transmit antenna is designed for circular

polarization...If the transmit antenna radiates true 

RHCP...

{Ref. Table 7 antenna (RCHP for anechoic chamber) ETSLindgren 3102}

The text has been corrected to reflect 

the circular polarization.

1 Critical No 

Reference 

Given

The comment feedback ROE mentions to consider the test plan 

scope before commenting however the scope of the testing effort 

should be coordinated with the federal stakeholders and PNT 

EXCOM. Since the PNT EXCOM is already performing licensee 

agnostic testing through the DOT ABC effort to produce 

appropriate regulatory level decision making results NASCTN 

should support the conclusion of this testing before attempting to 

start a separate effort. 

Re-evaluate scope or consider delaying or canceling 

testing until after completion of the DOT ABC effort. At 

a minimum, explain how the NASCTN testing relates to 

the ABC testing, why it is necessary in light of the ABC 

effort, and how it adds to and is not duplicative of the 

ABC testing. 

The scope of this NASCTN project has 

been negotiated in the CRADA between 

NIST and Ligado Networks. If additional 

testing is needed beyond this project, 

organizations are encouraged to submit 

project proposals to NASCTN.

The final test process and 

measurement results of the DOT ABC 

test campaign have not been made 

public, so it is inappropriate for 

NASCTN to comment on this on-going 

effort.

2 Critical No 

Reference 

Given

The NASCTN test plan misleads the reader/audience into thinking 

that NASCTN is conducting its own independent test effort to 

evaluate GPS compatibility when in reality it is hosting 

R&A/Ligado’s re-accomplishment of their testing with a 

government observer. At best, since Ligado intends to file their 

test results as a public notice with the commission, this entire 

effort is redundant and unnecessary. At worst, this activity will be 

misconstrued to imply the US Government endorses the testing 

and conclusions made by a biased private company over the 

Executive Committee established to coordinate the use of PNT 

across the US government. 

Re-evaluate purpose of testing or at a minimum be more 

open about the process and minimal value of the results

The NASCTN test plan was developed 

by an independent team of researchers 

from NIST and ITS, not by Roberson & 

Associates or Ligado Networks. Also, all 

measurements and data analysis 

associated with this project will be 

performed by NIST and ITS personnel. 
Page 40 of 52

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3 Critical No 

Reference 

Given

This testing would produce a laundry list of criteria that can be 

used instead of the 1 dB criteria for evaluating service level GPS 

compatibility. These other protection criteria are not supported by 

the domestic or international regulatory framework or the various 

government agencies and PNT community that support the PNT 

EXCOM. In addition, none of the proposed criteria represent 

adequate and objective protection criteria to evaluate service level 

compatibility. A bigger picture study would need to take place on 

what constitutes an appropriate GPS protection criteria and the 

assumptions necessary to evaluate them before collection is 

accomplished.

Flawed testing yields flawed data. Re-evaluate purpose 

of testing. If the question is “what is an appropriate 

metric to measure GPS compatibility?” consider 

undertaking a study that evaluates the merits of any 

other proposed metrics and their applicability to all 

aspects and users of GPS as well as what assumptions 

should be applied when evaluating them. This 

background framework is necessary to ensure data 

collection efforts produce valid and usable data to the 

GPS and spectrum management communities. Experts 

from the PNT community should be involved in this 

effort from the onset.

Although the NASCTN test plan does 

include a number of measurands in the 

study, including the C/N0 measurand, it 

does not make any recommendations 

on the appropriateness of these 

measurands, which is the purview of 

the spectrum regulators.

4 Importa

nt

No 

Reference 

Given

The focus of the test plan is restricted only to GPS and ignores 

other GNSS system effects from the proposed Ligado operations. 

Failure to address other GNSS systems seems to run counter to 

both existing PNT Policy (NPSD-39) and National Space Policy.

Re-evaluate scope, add other GNSS signals. The US 

National Space Policy indicates “the US shall engage with 

foreign GNSS providers to encourage compatibility and 

interoperability.” Creating and institutionalizing a new 

interference standard by effectively repurposing 

spectrum for uses incompatible with current 

internationally accepted interference standards violates 

this policy. Before accepting a new interference 

standard, it is necessary to engage with foreign GNSS 

providers – even presuming the proposed test plan were 

not flawed. 

The scope of this NASCTN project has 

been negotiated in the CRADA between 

NIST and Ligado Networks. If additional 

testing is needed beyond this project, 

organizations are encouraged to submit 

project proposals to NASCTN.
Page 41 of 52

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5 Critical No 

Reference 

Given

Since Ligado representatives designed and are funding the test, 

and will conduct it, with NASCTN observers simply ratifying the 

test plan was executed, the NASCTN testing will produce results 

similar in nature to what the private Ligado testing produced, 

which sets up a situation of FCC and NTIA having two similar 

results based on receiver performance effects (NASCTN and 

Ligado’s independent testing) with the third “outlier” then being 

the AF and Federal agency supported DoT ABC test and analysis 

effort that, like the previous NPEF test, uses the 1 dB IPC. The 

proposed NASCTN testing can be used to undermine the DoT’s 

ABC effort and the criteria that underpins the previous National 

Space-Based PNT Systems Engineering Forum (NPEF) testing from 

2011 and the January 2012 EXCOM letter that was based on the 

NPEF testing.

NASCTN should consider delaying this testing and review 

existing published NPEF test results to become familiar 

with current valid testing on GPS compatibility. Further, 

given the suspended lawsuit LightSquared filed against 

the US in the Court of Federal Claims on 11 July 2014, 

which has only been dismissed without prejudice, it is 

likely that Ligado will sue the US for additional costs and 

lost profit, claiming reliance on the NASCTN testing, 

should it not get the approval it seeks from the FCC 

based on the results – or, if it does get approval and 

then have it revoked due to actual interference 

experienced in national critical infrastructure by Ligado 

operations. 

The NASCTN test plan was developed 

by an independent team of researchers 

from NIST and ITS, not by Roberson & 

Associates or Ligado Networks. Also, all 

measurements and data analysis 

associated with this project will be 

performed by NIST and ITS personnel. 

6 Critical No 

Reference 

Given

The NASCTN testing itself appears contrary to the PNT EXCOM 

letter from January 2012 that indicated no further testing was 

warranted at that time to evaluate the Lightsquared proposal. The 

key parameters for the Lightsquared base stations that drove the 

EXCOM conclusion in January 2012 are the same as the NPEF 

tested in 2011. The 2011 NPEF testing and January 2012 EXCOM 

conclusion remain valid since the proposed adjacent band use by 

Ligado has not really changed from what was tested in 2011. The 

difference is that the NASCTN test ignores the international 

standard for interference used by the ITU, and formerly accepted 

by the FCC, and proposes instead a measure which is untested, has 

no basis in science, and is designed to accommodate a single 

emitter (Ligado). 

Postpone this effort and support the DOT ABC test 

effort. If the goal is to explore potential interference 

standards which might serve as a legitimate alternative 

to the current internationally accepted standard, the 

objective should so state - and the effort must engage 

the larger GNSS community, both domestically and 

internationally, regarding that goal. Per OMB Circular A119, implementing 15 USC 272(b), as well as Executive 

Order 12866 Section 1(b)(8) Federal Agencies shall use 

technical standards developed or adopted by voluntary 

consensus bodies unless they are inapplicable or 

impractical. Further, 19 USC 2532(3) require Agencies to 

use existing international standards in compliance with 

US obligations assumed with the World Trade 

Organization Agreement on Technical Barriers to Trade 

Agreement unless they create “fundamental technical 

problems”, in which case the US Trade Representative 

must be consulted, OMB A-119, 5(h). If no suitable 

standard exists, and the agency decides to develop its 

own, it must report this to OMB, for further reporting to 

Congress, OMB A-119 Section 10. (AFSPC/JA Addition)

The objective of the NASCTN project is 

to establish a test methodology to 

investigate the impact of LTE 

transmission on GPS receivers. NASCTN 

will not be making any 

recommendations about inference 

levels or defining interference 

protection criteria.
Page 42 of 52

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7 Critical Page 5, 

Paragraph 3

On numbered Page 5, third paragraph, it is clear the Plan authors 

lack a basic understanding of the use of interference protection 

criteria (IPC), which is used to prevent interference levels from 

reaching “harmful” levels and, consistent with U.S. Space Policy, 

sustain the RF environment for GPS. The Plan mischaracterizes the 

conclusions of the FCC-mandated Technical Working Group (TWG), 

which with the exception of LightSquared and its partners and 

supporters (e.g., Sprint, which had an agreement with 

LightSquared at the time), all supported use of the 1 dB 

IPC. Further, the Plan, in the same paragraph, notes that: “This 

implies that a 1 dB increase to the noise floor, as measured by any 

receiver in a shielded or direct-wired environment, is considered 

interference.” This is exactly the case...the 1 dB IPC is used to 

examine interference effects from one system into another using 

the domestic and internationally recognized criteria for making 

such assessments. It is unclear what this statement is trying to say 

other than that the authors seem to lack an understanding of 

interference and spectrum management. Instead, the Plan will

collect “measurands” of receiver performance that are consistent 

with Ligado’s independent test purposes or provide options to use 

criteria other than the well-established 1 dB IPC to protect GPS. 

Engage PNT spectrum experts and spectrum 

professionals to gain a better understanding of 

interference protection criteria and reevaluate and 

revise the test plan criteria to ensure that the testing 

does not use inappropriate criteria or undermine other 

USG test efforts and National Space Policy.

The objective of the NASCTN project is 

to establish a test methodology to 

investigate the impact of LTE 

transmission on GPS receivers. NASCTN 

will not be making any 

recommendations about inference 

levels or defining interference 

protection criteria.

8 Importa

nt

Page 5, 

Paragraph 3

The Plan, on Page 5, third paragraph, also asserts that many 

modern receivers cannot provide measured C/No information. 

This is not the case, or is misleading, because we know the NPEF 

testing in 2011 did measure C/No degradations from GPS 

receivers.

Remove Sentence “To complicate matters, many 

modern GPS receivers do not readily provide Key 

Performance Indicators such as C/N0”

The language has been clarified to 

point out that how the C/No value is 

estimated on the devices is not welldefined. In addition, there is no agreed 

upon standard calibration process for 

C/No.
Page 43 of 52

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9 Critical Page 5 There is no mention of the two rounds of NPEF testing from 

2011...only the TWG effort on numbered Page 5, second 

paragraph. The NPEF test and analysis effort, which had the full 

support of federal agency GPS stakeholders, tested a significant 

number of receivers used for different applications and used the 

standard 1 dB IPC to assess compatibility with GPS receivers. By 

comparison, the proposed NASCTN testing uses a small set of 

receivers using criteria (“measurands”) that are vague and without 

explanation of purpose and seem aimed at measuring levels of 

interference that cause some form of degradation to the receiver, 

which would be at or above harmful levels.

Update the background material to include these efforts 

and how their conclusions influenced the development 

of this test plan. If they were not considered, take the 

time to review them before attempting a new GPS 

measurement effort and learn from the methods/results 

already presented to better frame this and future 

efforts. If the true goal is to explore new interference 

criteria, then begin a dialogue with knowledgeable 

representatives of the PNT user community, including 

users of PNT timing in critical national infrastructure, 

and the international GNSS community. (AFSPC/JA 

Addition)

The objective of the NASCTN project is 

to establish a test methodology to 

investigate the impact of LTE 

transmission on GPS receivers. NASCTN 

will not be making any 

recommendations about inference 

levels or defining interference 

protection criteria.
Page 44 of 52

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10 Critical Page 5, Final 

Paragraph

This sentence mentions that a “representative subset of available 

devices will be tested...”. There is no further explanation 

throughout the document as to how it was determined this subset 

is representative of anything or which entities had any input in 

making a determination that the number of devices and the 

applications they support are in fact adequate for the testing. Of 

particular concern to DOD, military receivers are not adequately 

represented. 

Reach out to the PNT EXCOM and community experts to 

determine the appropriate set of devices for GPS 

compatibility testing. Review the work the DOT 

accomplished before developing the ABC test plan and 

how they created a more representative receiver set. 

Expand the receiver set to be tested, including in 

particular precision and timing receivers and those 

functions that depend on GPS timing. This is no small 

matter. Well known location uses aside, GPS timing is 

ubiquitous, and critical to the US financial system (for 

processing ATM, check, bank-to-bank and Federal 

Reserve transactions as well as the New York Stock 

Exchange), the utility grid, and phone and data 

communications networks. No testing is included in the 

Ligado/NASCTN plan to assess the interference these 

critical infrastructure components would suffer, 

although it is clear from the testing done in 2011 (using 

internationally accept interference standards), that 

interference can be expected. Note: DOD is prohibited 

by 10 USC 2281 from concurring in a test plan which will 

yield inadequate data regarding the impact on military 

receivers, since assured PNT is now critical to Joint 

operations. Flawed test plans which yield flawed data 

that becomes a basis for spectrum use decisions would 

directly and adversely impact the military potential of 

GPS. (AFSPC/JA addition)

The NASCTN test plan includes GPS 

receivers from the following categories:

 General location and 

navigation

 Precision position

 Precision timing

Other GPS receiver categories, 

including aviation, space-based and 

DoD receivers are beyond the scope of 

the NASCTN project. If additional 

testing is required beyond this project, 

organizations are encouraged to submit 

project proposals to NASCTN.

NASCTN included a number of GPS 

receivers in each of these categories in 

order to adequately validate the 

proposed test methodology. A major 

goal of the NASCTN project is to 

provide a reproducible test project that 

other organizations can use to perform 

similar tests on other GPS devices on 

included in this study. 

Based on the feedback from Federal 

agencies and GPS manufacturers, 

NASCTN has revised its list of GPS 

receivers.
Page 45 of 52

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11 Critical Page 6, 

Paragraph 6

The limitations of the NASCTN testing are explained...e.g., not 

covering space-based receivers, military receivers, aviation, or 

even cellular receivers. The much more comprehensive NPEF 

testing, as well as the ABC test and analysis effort, do cover these 

receivers. It therefore seems the NASCTN testing will be of very 

limited utility to the overall discussion of impacts to GPS from the 

proposed Ligado use of the MSS L-band spectrum since most 

receiver types and applications will have been ignored. Reach out 

to the PNT EXCOM and community experts to determine the 

appropriate set of devices for GPS compatibility testing. 

Reach out to the PNT EXCOM and community experts to 

determine the appropriate set of devices for GPS 

compatibility testing. 

The NASCTN test plan includes GPS 

receivers from the following categories:

 General location and 

navigation

 Precision position

 Precision timing

Other GPS receiver categories, 

including aviation, space-based and 

DoD receivers are beyond the scope of 

the NASCTN project. If additional 

testing is required beyond this project, 

organizations are encouraged to submit 

project proposals to NASCTN.

Based on the feedback from Federal 

agencies and GPS manufacturers, 

NASCTN has revised its list of GPS 

receivers.

12 Critical Page 7, 

Paragraph 1

It is not clear how the data will be distributed and who owns the 

end product. If the output data is not subject to public peer 

review and scrutiny, it can be misused or misrepresented. More 

information on how the data will be handled and reviewed by 

agencies and the public is necessary to understand the risks 

associated with undertaking the NASCTN test effort and to ensure 

transparency in the test process.

Re-evaluate purpose of testing and be more open about 

the process, including using a Public Notice process to 

ensure all stakeholders and the public in general have an 

opportunity to comment on issues affecting a system, 

such as GPS, that has such enormous value to the 

broader National interest. Handling of proprietary data 

is also not articulated. It will be impossible to plan a 

representative test if receiver manufacturers do not 

cooperate – and without assured protection of 

proprietary data they cannot. 

The handling of proprietary 

information and distribution of the 

NASCTN report and measurement data 

is governed by the CRADA signed by 

NIST and Ligado Networks. 

DUT manufacturers provided technical 

information and support, as needed, to 

the execution of the test plan.

13 Critical Section 

5.2.3, 

Paragraph 1

Not testing the effects of receivers that are designed to receive the 

MSS signal in the 1525-1559 MHz is a serious omission as some of 

the fundamental problems involved in the Ligado-GPS issue stem 

from the requirement to receive the augmentation signal 

anywhere in the MSS band from 1525-1559 MHz.

Re-evaluate purpose and scope of the testing, to include 

test cases that stress the effects augmentation signals 

received in the MSS band.

At least six precision timing and 

precision location devices will be 

tested. In general, these devices 

represent wider-band architectures. 
Page 46 of 52

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14 Critical Section 

5.3.2, 

Paragraph 1

It is stated that “It is important to recognize that these tests are 

focusing on the change in the baseline due to the LTE activity, not 

on the precise locations.” It is precisely this measure of 

degradation, the “user experience” criteria that federal agencies 

do not support and that is advocated only by Ligado and its 

supporters. “User experience” is not an interference protection 

criterion. This is the fatal flaw in the NASCTN testing and why the 

effort could be used to undermine the DoT ABC test and analysis 

effort and the previous NPEF testing in 2011 as well as the 

rationale for the January 2012 PNT EXCOM letter.

Consider undertaking a study that evaluates the merits 

of any other proposed metrics and their applicability to 

all aspects and users of GPS as well as what assumptions 

should be applied when evaluating them. This 

background framework is necessary to ensure data 

collection efforts produce valid and usable data to the 

GPS and spectrum management communities. Experts 

from the PNT community should be involved in this 

effort from the onset. It is inappropriate to agree to a 

test plan designed around a single commercial product. 

(AFSPC/JA Addition)

The objective of the NASCTN project is 

to establish a test methodology to 

investigate the impact of LTE 

transmission on GPS receivers. NASCTN 

will not be making any 

recommendations about inference 

levels or defining interference 

protection criteria.

Although the NASCTN test plan does 

include a number of measurands in the 

study, including the C/N0 measurand, it 

does not make any recommendations 

on the appropriateness of these 

measurands, which is the purview of 

the spectrum regulators.

15 Importa

nt

Section 5.5 There is discussion regarding the swept LTE levels with two types, 

one with the fixed OOBE and the other with changing OOBE. It is 

unclear why the OOBE should be changing for different LTE 

transmit power, which seems contradict what was filed by Ligado? 

Further explanation should be provided for this section.

Provide further explanation on why different OOBE 

levels would be used.

Additional text has been added to 

explain the two different scenarios. In 

both cases, the agreed upon OOBE 

mask is the same. The first case 

assumes a fixed physical distance 

between the GPS device and the LTE 

emitter, e.g. 3 m. In the second case, 

the distance between the LTE emitter 

and the GPS device is assumed to 

change with the LTE device 

transmitting at maximum power. 

16 Critical Section 

6.3.1, 

Paragraph 4

It is stated that additional metrics will be investigated, apparently 

on the fly and with no pre-planning, to determine steady state 

performance of the receiver. This kind of arbitrary statement and 

process is both unscientific and unsound and should be thought 

out and planned for in a much more thorough fashion.

Re-evaluate purpose of testing, take a more scientifically 

sound and rigorous approach to any test effort. 

Expedited data collections without a clear framework of 

assumptions and methodology are not likely to produce 

useful results.

The available measurands are device 

dependent, and as devices are added 

based on feedback, the list of 

measurands has become better 

defined. Key measurands are listed in 

the test plan, and will be collected if 

extracting from the device is possible.
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17 Critical Section 7, 

Page 26

The outreach plan should include publication of the Test Plan in 

the Federal Register and solicitation of public comments as was 

the case with the DoT ABC effort. This is the only way to ensure 

transparency in the process...by using a well-known vehicle to 

gather input from the public.

Re-evaluate purpose of testing and be more open about 

the process, including using a Public Notice process to 

ensure all stakeholders and the public in general have an 

opportunity to comment on issues affecting a system, 

such as GPS, that has such enormous value to the 

broader National interest.

The NASCTN process for obtaining 

feedback on the proposed test plan has 

resulted in comments from Federal 

agencies, GPS manufacturers, and both 

spectrum regulators (FCC and NTIA).

NASCTN is reviewing notification 

process to improve awareness among 

the stakeholder community.

18 Importa

nt

No 

Reference 

Given

The measurement of interference effects should be explained in 

context...e.g., that interference impacts/effects on the receiver 

constitute harmful interference to the receiver and sound 

spectrum management practice and procedure is aimed at 

preventing such levels from occurring in the first place.

Engage PNT spectrum experts and spectrum 

professionals to gain a better understanding of 

interference protection criteria and reevaluate and 

revise the test plan criteria to ensure that the testing 

does not use inappropriate criteria or undermine other 

USG test efforts and National Space Policy.

The objective of the NASCTN project is 

to establish a test methodology to 

investigate the impact of LTE 

transmission on GPS receivers. NASCTN 

will not be making any 

recommendations about inference 

levels or defining interference 

protection criteria.

1 - - To justify replacing the 1 dB criterion for tolerable interference will 

require a very extensive justification looking at many worst case 

GPS/GNSS operational situations. By not using the widely accepted 

and previously employed 1 dB degradation criterion, NASCTN is 

taking on a daunting task. Working with the many receivers, 

receiver operating modes and operating conditions needed to 

identify and adequately measure other key performance indicators 

will require an extensive amount of resources and time to be done 

correctly. Finding cases and conditions where the impact is 

acceptable is not sufficient; the proposed testing must explore all 

relevant cases and conditions to demonstrate that the impact is 

not unacceptable. It appears that they do not have the needed 

resources and time, and instead are planning to use an overly 

simplistic test that will not provide the needed information. 

Although the NASCTN test plan does 

include a number of measurands it the 

study, including the C/N0 measurand, it 

does not make any recommendations 

on the appropriateness of these 

measurands, which is the purview of 

the spectrum regulators.
Page 48 of 52

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2 - - If NASCTN does not have the resources and time to do this for a 

large number of receivers and conditions, then it would be better 

to do this thoroughly for a small number of receivers that previous 

testing has shown to be particularly sensitive to LTE interference, 

rather than use an overly simplistic approach on a larger number 

of receivers. Note especially the change from “GPS receivers” to 

satnav receivers. The test plan has taken a first step in this respect 

by including WAAS receivers, but signals from other satnav 

systems are already being used extensively in the U.S., and other 

signals will be used as well, consistent with U.S. policy. 

Plan should add goal of answering the key question: 

determining the level of LTE interference that can be 

accepted by satnav receivers operating satisfactorily 

under all relevant conditions. This fundamental goal 

should be added to the Preface.

The objective of the NASCTN project is 

to establish a test methodology to 

investigate the impact of LTE 

transmission on GPS receivers. NASCTN 

will not be making any 

recommendations about inference 

levels or defining interference 

protection criteria.

3 - - When we asked during the telecon, we were told that it is not the 

objective of this effort to find what margin is left for LTE when 

satnav receivers are operating in stressed conditions. If the tests 

do not provide this result, then they do not provide the needed 

information for assessing LTE compatibility with satnav receivers. 

No one source of interference can take up all the margin, which 

receivers rely upon to handle a large set of different stresses, 

including other sources of interference. Many of the following 

comments involve more detailed technical aspects of this 

observation and recommendation, pointing out several of the 

ways that this test plan does not meet the objective it states in the 

first sentence of its Section 3 “to develop a rigorous testing 

methodology and collect supporting data to establish the impact 

of LTE signals on GPS devices.” Instead, the current test plan only 

establishes the impact on a limited number of GPS devices 

operating in their most interference-resistant mode under 

relatively unstressed conditions. 

As a follow-up, during the Interagency Discussion on 10 June, 

NIST/NASCTN indicated they would welcome suggestions to help 

define the envelope of more stressed conditions. This is a 

welcome change, but there is no way to respond to this new 

opportunity before the 13 June comment deadline.

Plan should address highly stressed conditions – the 

“envelope” conditions.

The fact that the test population is 

limited does not imply that the test is 

not rigorous. Key attributes of a 

rigorous test are repeatability, 

calibration of measurements, and the 

control of confounding sources of 

variability. 

The number of devices required for a 

statistically meaningful result that 

completely covers all categories and 

use cases of devices is beyond the 

scope of this effort. 

The definition of a stressed condition is 

highly subjective. The limited 

conditions proposed here are derived 

from reviewing previous tests and 

solicited feedback. To the best of our 

knowledge, there is no consensus on 

stressed condition parameters.
Page 49 of 52

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4 - - The test plan currently involves getting a device under test (DUT) 

into steady state tracking mode, then evaluating the effect of 

different levels of interference on the tracking performance. Since 

steady state tracking is usually the most robust, interferenceresistant mode of receiver operation, the results of this test do not 

adequate assess LTE impacts on receiver operation. Instead, for 

each interference configuration, including the baseline with no 

interference, tests should evaluate receiver performance in every 

mode and state of operation, defining and observing the Key 

Performance Indicators appropriate to that mode and state of 

operation. This would be an expansion to the flowcharts shown on 

pages 21 and 22 of the test plan. As examples, cold start

acquisition should evaluate the time to first fix, carrier phase 

differential receivers should evaluate time for ambiguity 

resolution, and many receivers should be tested for error-free 

reading of the data message. Handling rising and setting satellites, 

as well as satellites disappearing and appearing due to blockage, 

are additional conditions under which the effects of interference 

must be evaluated.

Plan should include all receiver classes, receiving modes 

and states, particularly those that are well known to be 

most sensitive to interference, e.g. acquisition (cold 

start) and reacquisition.

Loss of lock has been included in the 

test plan, and time to first fix (TTFF) has 

been added as well. Other conditions, 

modes, and setting will be considered 

by the Test Master as a test 

modification as appropriate.

5 - - The test plan states that receivers will be tested when stationary. 

Many receiver functions are stressed by the acceleration, jerk, and 

vibrations associated with receiver motion. Receiver susceptibility 

to interference should be evaluated under significant dynamics, 

not only the easy case of no dynamics. As a minimum, vehicle 

dynamics should be included for devices that might be used in 

vehicles.

Plan should include moving receivers. The scope of this NASCTN project has 

been negotiated in the CRADA between 

NIST and Ligado Networks. If additional 

testing is needed beyond this project, 

organizations are encouraged to submit 

project proposals to NASCTN.

6 - - The test plan does not include any other sources of interference, 

despite the fact that satnav receivers often operate under 

conditions where there is some level of out of band and in band 

interference. Stressing levels of other interference should be used

during the testing in order to assess how much additional 

interference from LTE can be tolerated. As examples, there is and 

will be intrasystem interference from other GPS signals and 

satellites, intersystem interference from current and future satnav 

systems, interference from FCC Part 15 devices, interference from 

UWB devices, as well as other spectrum uses in adjacent bands.

Plan should include other sources of interference. The scope of this NASCTN project has 

been negotiated in the CRADA between 

NIST and Ligado Networks. If additional 

testing is needed beyond this project, 

organizations are encouraged to submit 

project proposals to NASCTN.
Page 50 of 52

Comment 

Number

Level Reference 

within Draft 

Test Plan

Comment Commenter's Recommendation NASCTN Comment Resolution

7 - - While the specified minimum received power for C/A signals is -

158.5 dBW (or -128.5 dBm), received power levels can be much 

less than that value due to receive antenna gain, blockage due to 

foliage or construction materials, or tracking of reflected signals 

when the direct path is blocked. Also, while many GPS satellites 

are usually visible to a receiver with open view of the sky, visibility 

can be reduced to four satellites or fewer (in which case altitude 

hold and/or clock hold may be used) due to blockage from 

buildings or terrain. 

Plan should include various received power levels and 

numbers of satellites.

The proposed limited exposure does 

include a reduced number of satellites 

and reduced power levels.

8 - - The test plan indicates it will test one receiver at a time. Yet 

differential systems require multiple receivers—at least one 

reference receiver, or base, and one user receiver, or rover. Some 

modern differential systems (e.g., networked differential systems) 

may require several reference receivers. The effect of interference 

must be assessed when all of these receivers are exposed to the 

interference, and thorough testing should examine different 

interference conditions for each of the reference receivers and 

user receivers.

Plan must include multiple receivers simultaneously, at 

least in some cases.

RTK systems that utilize two receivers 

will be included.

9 - - Proposed test plan seems to address only receivers for GPS C/A 

signals and WAAS signals. GPS L1C signal receivers should also be 

evaluated. Further, there are other satnav systems with signals in 

this band, and they may be used in the U.S. At a minimum, Galileo 

PRS receivers, or their surrogates, should be tested as well.

Plan should include receivers for more satnav signals, 

including L1C and from other GNSS. These advanced 

signals are the basis for many high productivity

applications.

The scope of this NASCTN project has 

been negotiated in the CRADA between 

NIST and Ligado Networks. If additional 

testing is needed beyond this project, 

organizations are encouraged to submit 

project proposals to NASCTN.
Page 51 of 52

Comment 

Number

Level Reference 

within Draft 

Test Plan

Comment Commenter's Recommendation NASCTN Comment Resolution

10 - - While Appendix A in the test plan suggests use of signal to LTE 

interference ratio (SIR) as a metric, this metric will be of little use. 

At low LTE interference levels, receiver performance is only an 

affine function of SIR, since thermal noise and other interference 

have observable effects. At high LTE interference levels, receivers 

may respond nonlinearly to interference. Thus, the same SIR, with 

different levels of desired signal levels, can produce very different 

results in the receiver. Furthermore, there are four power levels of 

interference (power in the designated uplink and downlink bands, 

and power in the satnav band from uplink and downlink 

transmissions), and received satnav signals have different power 

levels as shown in Table 5 of the test plan. Thus, there is no such 

single quantity known as SIR.

Plan should focus on absolute received power levels, not 

signal to interference ratio

The absolute power levels (assuming a 

0 dBi antenna) present at the DUT will 

be known. In addition, these test 

include a scenario with energy in both 

an uplink and downlink band 

simultaneously. To the best of our 

knowledge, this condition has received 

minimal consideration in previous 

testing.

11 - - While the test plan does not address it, there are also significant 

technical issues concerning extrapolation of test chamber results 

to operational conditions. Variations in path loss, transmit and 

receive antenna gains, and overlapping transmitter coverage are a 

few examples of the many issues that need to be addressed in 

performing an adequate extrapolation.

Plan should address how test data will be extrapolated 

to operational conditions.

The comment applies to the systems 

analysis that should accompany any 

well-engineered proposed deployment. 

Data from the NASCTN test, amongst 

others, could be included in that 

analysis.
Page 52 of 52

Comment 

Number

Level Reference 

within Draft 

Test Plan

Comment Commenter's Recommendation NASCTN Comment Resolution

12 - - Given that the current plan for testing will not involve all 

representative and realistic stresses, and that only a limited 

number of receivers will be tested, and then only in their most 

robust modes, the results will not provide useful results 

concerning how much LTE interference can be tolerated by satnav 

receivers. Buried on page 24 of the test plan is the admission that, 

“the conclusions that can be drawn from this test will be limited to 

the specific set of devices under test, and will not be rigorously 

generalizable to the population of all devices.” Currently the 

limitations of this testing are not clearly stated in the test plan’s 

Preface, Background, or Scope. These limitations should be 

highlighted and detailed prominently in the Test Plan and Test 

Report. Otherwise, it will be easy for these limitations to go 

unrecognized, and consequently for any test results to be 

misinterpreted as actually informing about the level of LTE 

compatibility with satnav receivers, when they really only provide 

limited insights: for a small number of receivers, under relatively 

benign conditions, when the receivers are operating with ample 

margin.

At a minimum, the test plan and test report should 

clearly and prominently highlight limitations of the 

testing, and the resulting restrictions on drawing 

conclusions from the tests.

The NASCTN final report will include a 

discussion of the limitations of the tests 

performed as well as a summary of the 

underlying assumptions.

13 - - If the objectives of this test plan include the recommendation 

from comment 2: “To develop a technically and operationally valid 

way to determine the level of LTE interference that can be 

tolerated by satnav receivers operating satisfactorily under all 

relevant conditions,” then there are many constructive 

suggestions that satnav experts can provide. Some of them are 

outlined here, but NASCTN’s willingness to consider and 

incorporate them, only indicated today, does not allow time to 

provide them before the comment deadline on 13 June.

To have credibility with the PNT community, it is clear 

that real PNT expertise must be added to the test team. 

If the plan is to answer the real question, the satnav 

community can provide assistance.

The NASCTN process for obtaining 

feedback on the proposed test plan has 

resulted in comments from Federal 

agencies, GPS manufacturers, and both 

spectrum regulators (FCC and NTIA).

14 - - The test plan should be subject to a regular, formal comment 

process, since its results are likely to influence a decision that will 

potentially affect hundreds of millions of PNT users. Note that the 

manufacturers generally do not represent the major classes of 

users.

The test plan review process should be open and formal. The NASCTN process for obtaining 

feedback on the proposed test plan has 

resulted in comments from Federal 

agencies, GPS manufacturers, and both 

spectrum regulators (FCC and NTIA).