```

[Federal Register Volume 89, Number 248 (Friday, December 27, 2024)]
[Notices]
[Pages 105542-105553]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 2024-30760]

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DEPARTMENT OF COMMERCE

National Telecommunications and Information Administration

[NTIA-2024-0005]
RIN 0660-XC065

Impact of L-Band MSS `Direct-to-Device' Operations on GPS

AGENCY: National Telecommunications and Information Administration, 
Department of Commerce.

ACTION: Notice; request for comment.

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SUMMARY: The National Telecommunications and Information Administration 
(NTIA) is seeking information and public comment on the potential 
impact on the GPS L1 signal at 1575.42 MHz of the increasing deployment 
of mobile devices capable of operating on satellite systems in the L-
band at 1610-1660.5 MHz. Under current FCC rules, these devices are 
permitted to operate subject to a substantially less restrictive out-
of-band emission limit than similar devices transmitting on the same 
frequencies and connecting to terrestrial base station systems. NTIA is 
seeking public comment on the risk of interference posed by this 
increased deployment of mobile devices capable of operating on L-band 
satellite systems at 1610-1660.5 MHz, as well as potential mitigation 
options to safeguard GPS systems while facilitating the potential 
benefits of direct-to-device services (D2D services).

DATES: Written comments must be received on or before February 10, 
2025.

ADDRESSES: All electronic public comments on this action, identified by 
Regulations.gov docket number NTIA-2024-0005, may be submitted through 
the Federal e-Rulemaking Portal at https://www.regulations.gov. The 
docket established for this request for comment can be found at 
www.Regulations.gov, NTIA-2024-0005. To make a submission, click the 
``Comment Now!'' icon, complete the required fields, and enter or 
attach your comments. Additional instructions can be found in the 
``Instructions'' section below, after SUPPLEMENTARY INFORMATION.

FOR FURTHER INFORMATION CONTACT: Please direct questions regarding this 
Request for Comment to Ashley Davenport at [email protected] with 
``Impact of L-band MSS D2D Operations on GPS'' in the subject line. If 
submitting comments by U.S. mail, please address questions to Ashley 
Davenport, National Telecommunications and Information Administration, 
U.S. Department of Commerce, 1401 Constitution Avenue NW, Washington, 
DC 20230. Questions submitted via telephone should be directed to (202) 
482-0297. Please direct media inquiries to NTIA's Office of Public 
Affairs, telephone: (202) 482-7002; email: [email protected].

SUPPLEMENTARY INFORMATION:

Background

    NTIA, located within the Department of Commerce, is the Executive 
Branch agency that is principally responsible by law for advising the 
President on telecommunications and information policy issues and 
managing the federal use of spectrum. The Global Positioning System 
(GPS) is one such federal system. NTIA is beginning to see increasing 
deployment of consumer equipment capable of operating on Mobile-
Satellite Service (MSS) networks with operations in the L-band near the 
L1 signal of the Global Positioning System (GPS) at 1575.42 MHz. Some 
of these devices are traditional mobile phones or Internet of Things 
(IOT) devices that typically operate on terrestrial networks in mobile 
bands outside the L-band, while others are being designed as lower-cost 
IOT or other devices operating primarily, if not exclusively, with 
satellites (collectively, D2D devices). The services enabled by these 
devices (D2D services) could provide substantial benefits to the 
public, including during times of emergency, while also supporting 
important federal government missions. The purpose of this Request for 
Comment is to secure public input on the risk of interference into GPS 
posed by wider deployment of D2D services in the L-band and potential 
mitigation opportunities.
    When the FCC developed MSS rules for the L-band, there was an 
expectation that comparatively few user devices would be deployed 
relative to ubiquitous terrestrial mobile handsets. Therefore, the out-
of-band emission (OOBE) limit for L-band devices operating in the MSS 
is substantially more relaxed (by more than 30 dB) than the requirement 
later developed for user devices that would communicate with 
terrestrial base stations in the L-band. The stricter OOBE limit for 
devices operating with terrestrial base stations was based in large 
part on the expectation that there would be a much larger number of 
such devices. The wide deployment of devices subject to the MSS limit 
thus presents an unexpected increase in the risk of interference to 
nearby GPS receivers. Attached hereto is a technical analysis conducted 
by the U.S. Department of Transportation, in conjunction with other 
U.S. Government departments and agencies, that more fully describes the 
basis for concern.
    NTIA and other U.S. Government departments and agencies are 
cognizant that the initial business case for satellite operation of the 
new devices appears to focus on situations in which the user is 
outdoors and terrestrial wireless service is unavailable and that these 
situations will often arise in areas with less intensive GPS use for 
critical service. The extent to which this is the case and will 
continue to be the case in the future remains unclear, as is the extent 
to which the delivery of such a service

[[Page 105543]]

might involve the devices transmitting in the L-band at other times. We 
seek to understand these issues and their connection to interference 
risks.
    One of the situations of greatest concern involves the use of such 
a device while on an airplane given the proximity of the device to GPS 
receivers and the importance of those receivers to aircraft navigation 
systems. We seek information on this scenario, in particular, and on 
the need for mitigation options that go beyond the existing regulatory 
prohibition against operating the device on an airplane and 
corresponding labeling requirements.

Request for Comments

    Through this Request for Comment, we hope to gather information on 
the following issues and questions. These questions are not exhaustive, 
and commenters are invited to provide input on relevant questions not 
asked below. Commenters are not required to respond to all questions. 
When responding to one or more of the questions below, please note in 
the text of your response the number of the question to which you are 
responding. Commenters should include a page number on each page of 
their submissions. Commenters are welcome to provide specific 
actionable proposals, rationales, and relevant facts.
    All comments received are a part of the public record and will 
generally be posted to Regulations.gov without change. All personal 
identifying information (e.g., name, address) voluntarily submitted by 
the commenter may be publicly accessible. Anyone submitting business 
confidential information should clearly identify any business 
confidential portion of a comment at the time of submission, file a 
statement justifying nondisclosure and referring to the specific legal 
authority claimed, and provide a non-confidential version of the 
submission.
    For comments submitted electronically containing business 
confidential information, the file name of the business confidential 
version should begin with the characters ``BC.'' Any page containing 
business confidential information must be clearly marked ``BUSINESS 
CONFIDENTIAL'' on the top of that page. The corresponding non-
confidential version of those comments must be clearly marked 
``PUBLIC.'' The file name of the non-confidential version should begin 
with the character ``P.'' Any submissions with file names that do not 
begin with either a ``BC'' or a ``P'' will be assumed to be public and 
will be made publicly available through https://www.regulations.gov.

Questions

    1. We seek further information from parties deploying or planning 
to deploy D2D service in the L-band regarding:
    a. the actual out-of-band emissions of the devices;
    b. the potential for limiting those emissions further;
    c. the typical duration and frequency of occurrence of L-band 
transmissions supporting D2D services (including any automatic 
transmissions);
    d. expected geographic deployment patterns based on initial 
offerings of D2D services, as well as planned and/or potential service 
expansions (for example, to what extent might they operate indoors as 
well as outdoors or in areas where terrestrial wireless service is also 
available);
    e. any other information relevant to an evaluation of the 
likelihood that an L-band device will cause harmful interference to a 
nearby GPS receiver;
    f. the ability to limit or preclude satellite operations to and 
from a device when terrestrial service is available, including 
incidental transmissions;
    g. the ability to limit L-band operations and attempted operations 
when on an airplane or near other critical GPS receivers (including 
information about the effectiveness of ``airplane mode'' or other 
similar settings);
    h. the options for educating users regarding the risks of operating 
near critical GPS receivers, including on-screen warnings and the 
likely effectiveness of these options; and
    i. any other mitigations that might reduce the risk of harmful 
interference to a nearby GPS receiver.
    2. We seek information from GPS manufacturers and users regarding:
    a. any documented instances of harmful interference from MSS L-band 
devices;
    b. their concerns regarding the increased probability of harmful 
interference; and
    c. any mitigations that might be effective in reducing the risk of 
harmful interference while minimizing impacts on MSS service delivery.
    3. We seek information on industry standards and performance 
improvements achieved by the GPS industry over the past ten years to 
self-protect the various categories of GPS receivers to minimize 
receiver blocking and overload.
    a. Information on separation distances due to receiver selectivity, 
compared with the separation distances shown in the DoT technical 
analysis due to MSS L-band mobile earth station out-of-band-emissions 
(OOBE).
    b. GPS receiver overload limits (i.e., GPS receiver input power 
tolerance thresholds and separation distance(s)) from MSS mobile earth 
station operations from Globalstar, Inmarsat, etc., in the vicinity of 
different categories of GPS receivers.
    4. We seek comment on the Department of Transportation technical 
analysis. Parties should feel free to submit any alternative technical 
analysis.

Sean Conway,
Deputy Chief Counsel, National Telecommunications and Information 
Administration.

Analysis of Potential Interference to GPS/GNSS From Emerging L-Band MSS 
D2D and IoT Services November 25, 2024

1 Overview

    This paper assesses the potential impact of emerging L-band mobile 
satellite service (MSS) direct-to-device (D2D) and Internet of Things 
(IoT) services on Global Positioning System (GPS)/Global Navigation 
Satellite System (GNSS) receivers that are to receive interference 
protection in the 1559-1610 MHz Radionavigation Satellite Service 
(RNSS) allocation. The analysis considers only the emissions from 
terrestrial devices (e.g., D2D handsets and IoT equipment) up to the L-
band MSS satellites. The main finding of this initial analysis as 
detailed in this paper is that emissions from L-band MSS D2D and IoT 
user equipment operating at 1610-1660.5 MHz, operating in compliance 
with the Federal Communications Commission (FCC) rules can result in 
interference to GPS/GNSS receivers operating in the 1559-1610 MHz RNSS 
band. The risks of interference to GPS/GNSS from L-band MSS Mobile 
Earth Stations (MES) are growing rapidly as D2D and IoT services are 
increasingly included in high-volume, daily-use consumer and industrial 
products (e.g., Apple phones and tablets).
    Within this paper, MSS emissions in the 1559-1610 MHz RNSS band are 
hereafter referred to as out-of-band emissions (OOBE). Both OOBE and 
the fundamental transmissions in the 1610-1660.5 MHz MSS band are 
considered. The OOBE results apply to all L-band MSS operators. The 
fundamental transmission analysis in this paper was performed using 
available data for Ligado's specific uplink frequency bands of 1627.5-
1637.5 MHz and 1646.5-1656.5 MHz. Results for other operators are not 
provided in the paper but are expected to be worse for

[[Page 105544]]

operators using frequencies closer to the RNSS band.
    FCC rules for L-band MSS mobile Earth stations that are applicable 
to emerging D2D and IoT services are summarized in Appendix A. A 
scenario in which MSS D2D equipment is operated onboard a commercial 
aircraft is presented in Appendix B.

2 Impact Assessments

2.1 Emissions in the GNSS Band

2.1.1 D2D Handset and IoT Device Emissions

    D2D handset or IoT device emissions that fall within the 1559--1610 
MHz RNSS allocation can cause degradation to GNSS receiver signal-to-
noise-density (C/N0). The FCC rules for satellite services, contained 
within Part 25 of Title 47 of the Code of Federal Regulations (47 CFR 
part 25),\1\ include requirements applicable to D2D handset or IoT 
device emissions. 47 CFR 25.216, in particular, requires that the 
effective isotropic radiated power (EIRP) levels of MES with assigned 
frequencies within 1610 and 1660.5 MHz ``. . . shall not exceed-70 dBW/
MHz, averaged over any 2 millisecond active transmission interval, in 
the band 1559-1605 MHz.''
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    \1\ https://www.ecfr.gov/current/title-47/chapter-I/subchapter-B/part-25.
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    Assuming a GNSS receiver noise floor of -201.5 dBW/Hz,\2\ Table 1 
provides the separation distances needed to limit GNSS receiver C/N0 
degradation to 1, 3, 6, and 10 dB assuming an isotropic receive 
antenna, free space path loss, and assuming a 3 dB polarization loss 
for linearly polarized transmissions being received by a right-hand 
circularly polarized antenna.
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    \2\ See, for example, ITU-R M.1903, https://www.itu.int/dms_pubrec/itu-r/rec/m/R-REC-M.1903-1-201909-I!!PDF-E.pdf. Five of 
11 GNSS receiver types characterized in this recommendation have a 
system noise temperature of 513K, which equates to a -201.5 dBW/Hz 
noise floor. This level, -201.5 dBW/Hz, is also a standard noise 
floor assumption for certified GNSS equipment on aircraft; see, 
e.g., RTCA DO-235C.

         Table 1--C/N0 Degradation vs Separation Distance--GNSS Receiver With Isotropic Receive Antenna
----------------------------------------------------------------------------------------------------------------
                                                                 Received interference     Required separation
                   Degradation to C/N0 (dB)                         level  (dBW/Hz)                (m)
----------------------------------------------------------------------------------------------------------------
1.............................................................                   -207.4                     79.1
3.............................................................                   -201.5                     40.3
6.............................................................                   -196.8                     23.3
10............................................................                   -192.0                     13.4
----------------------------------------------------------------------------------------------------------------

    Using a model for the relative antenna gain pattern of a GNSS high 
precision receiver (HPR) towards a vertically polarized source at 
1575.42 MHz \3\ and assuming a maximum boresight gain of +3 dBic, 
Figure 1 shows the area around a single D2D device where the receiver 
could experience a 1, 3, or 10 dB C/N0 degradation. The plot assumes 
the D2D or IoT device is 2 m above the ground and that free space path 
loss conditions exist.
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    \3\ See Section 4.1.3.1 of the Department of Transportation 
(DOT)'s Global Positioning System (GPS) Adjacent Band Compatibility 
Assessment Final Report, https://rosap.ntl.bts.gov/view/dot/35535. 
Vertically polarized emissions for the D2D or IoT device is assumed.
[GRAPHIC] [TIFF OMITTED] TN27DE24.241

Figure 1. Impact on HPR Due to -70 dBW/MHz Emissions in 1559-1605 MHz

    The required lateral separations to keep a high precision GNSS 
receiver C/N0 degradation with the modeled antenna below 1, 3, or 10 dB 
shown in Figure 1 are slightly less than the results for a receiver 
with isotropic receive antenna in Table 1. This observation can be 
explained by the fact that a typical HPR antenna may have a gain 
towards a vertically polarized source at the horizon that is below 0 
dBi. The results for other GPS/GNSS receiver classes are slightly worse 
(i.e., requiring additional separation for the same C/N0 degradation) 
since their noise floors are similar to HPR devices but they

[[Page 105545]]

typically have higher antenna gain at low elevation angles (see Table 
2). Sample calculations for the separation distances in Tables 1 and 2 
are provided in Appendix C.
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    \4\ https://docs.fcc.gov/public/attachments/FCC-20-48A1.pdf.

 Table 2--Separation Distances from D2D Handset or IoT Device Necessary
     To Limit C/N0 Degradation for Various GPS/GNSS Receiver Classes
------------------------------------------------------------------------
                                                           Required
                                                         separation to
                                    Noise floor, N0       limit C/N0
     GPS/GNSS receiver class          (dBW/Hz) *       degradation to 1,
                                                       3, and 10 dB  (m)
                                                              **
------------------------------------------------------------------------
High precision (HPR)............  -201.5............  52.2 (1 dB), 27.5
                                                       (3 dB), 10.1 (10
                                                       dB)
General Location/Navigation       -200.5............  59.6 (1 dB), 31.0
 (GLN).                                                (3 dB), 10.9 (10
                                                       dB)
Cellular (CEL)..................  -200.5 to -201.5..  99.8 to 112.0 (1
                                                       dB), 50.9 to 57.1
                                                       (3 dB), 16.9 to
                                                       19.0 (10 dB)
Certified Aviation *** or         -201.5............  66.7 (1 dB), 34.6
 General Aviation (GAV).                               (3 dB), 12.2 (10
                                                       dB)
------------------------------------------------------------------------
* Based upon receiver system noise temperature values in ITU-R M.1903.
** Using representative antenna gain patterns for each receiver class
  from https://rosap.ntl.bts.gov/view/dot/35535, and assuming the
  transmit antenna is at 2 m height above ground.
*** See Appendix B for an assessment of a scenario in which the D2D
  equipment is operated by a passenger on a commercial aircraft.

    A lower emission limit would reduce the impact area. For instance, 
Figure 2 shows the impacted area for HPR GNSS devices if the EIRP 
emissions in 1559-1605 MHz were reduced to -105 dBW/MHz (as required 
for Ligado ancillary terrestrial component [ATC] handset transmissions 
per the April 2020 FCC Order and Authorization \4\).
[GRAPHIC] [TIFF OMITTED] TN27DE24.242

Figure 2. Impact on HPR due to -105 dBW/MHz Emissions in 1559-1605 MHz

    For a GNSS device with an isotropic receive antenna and -201.5 dBW/
Hz noise floor, Figure 3 presents required separation distances for 1, 
3, 6, and 10 dB C/N0 degradations as a function of D2D and IOT device 
emission levels in 1559-1605 MHz.

[[Page 105546]]

[GRAPHIC] [TIFF OMITTED] TN27DE24.243

Figure 3. Required Separation Distance vs D2D Emissions in 1559-1610 
MHz To Limit GNSS Receiver C/N0 Degradation To 1, 3, 6, or 10 dB

    The above results are similar to those provided to DOT GPS ABC 
Workshops by Greenwood Telecommunications Consultants LLC \5\. 
Greenwood additionally examined the impacts of multiple transmitters in 
the referenced presentations and recommended -105 dBW/MHz as a Ligado 
mobile device EIRP limit in 1559-1605 MHz.
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    \5\ See https://rosap.ntl.bts.gov/view/dot/34708 and https://rosap.ntl.bts.gov/view/dot/34710.
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2.2 Emissions in the Adjacent Band--Fundamental Emissions

2.2.1 D2D Handset and IoT Emissions

    D2D handsets and IoT devices may operate with many center 
frequencies and at various power levels within the 1610-1660.5 MHz MSS 
band. In this section, one example is provided applicable to Ligado D2D 
operations. As noted earlier in Section 1, other L-band MSS operators 
are providing D2D/IoT services using frequencies closer to the 1559-
1610 MHz RNSS band and may also utilize higher uplink transmit power 
levels. These services could therefore have a potentially greater 
impact on GPS/GNSS.
    Again, as just one example of D2D/IoT operations, Ligado D2D 
handsets are expected to have a maximum EIRP of 23 dBm, use vertical 
polarization, and only operate in the 1627.5-1637.5 MHz and 1646.5-
1656.5 MHz bands. These are the same emission characteristics as Ligado 
ATC handsets.
    Extensive results for the impact of vertically-polarized, 23 dBm 
handset emissions at 1630 MHz are presented in Appendix I of the DOT 
GPS ABC Assessment Final Report (see footnote 3 for a hyperlink to this 
report). Some of these results, for the most sensitive device within 
each receiver class, are reproduced below. In these figures ``ITM'' 
corresponds to a 1 dB CNR degradation, ``LOLL'' corresponds 
to loss of lock on low-elevation angle satellites, and 
``LOLH'' corresponds to loss of lock on high elevation angle 
(i.e., all) satellites. GLN = ``General Location/Navigation'' and TIM = 
``Timing'' GNSS devices.

[[Page 105547]]

[GRAPHIC] [TIFF OMITTED] TN27DE24.244

[GRAPHIC] [TIFF OMITTED] TN27DE24.245

[GRAPHIC] [TIFF OMITTED] TN27DE24.246

    IoT devices may transmit significantly higher power levels. For 
instance, on April 5, 2023, the FCC granted a blanket license to Skylo 
Technologies for up to 1,000,000 IoT devices to communicate to the 
Ligado Skyterra-1 and Inmarsat satellites in the 1525-1559 MHz and 
1626.5-1660.5 MHz MSS bands.\6\ The license permits EIRPs of up to 10W 
for IoT transmissions to Skyterra-1 and the Inmarsat MSS satellites. 
This EIRP limit is 50 times greater than the Ligado ATC handset EIRP 
limit of 200 mW and could impact sensitive GNSS devices at distances of 
up to almost 1 km. Each of the impact contour plots shown above would 
have the impacted region expand by a factor of 7 (since free space path 
loss falls off with distance to the transmitter squared and the IoT 
devices are approximately a factor of 7 squared more powerful than 23 
dBm).
---------------------------------------------------------------------------

    \6\ https://docs.fcc.gov/public/attachments/DOC-392372A1.pdf.
---------------------------------------------------------------------------

    Importantly, the impact of the fundamental D2D handset and IoT 
device transmissions on GPS/GNSS receivers varies significantly with 
receiver model. The results presented above apply to the most sensitive 
device of all of those tested within each receiver class by DOT. As 
detailed in

[[Page 105548]]

Appendix B of the DOT GPS ABC Assessment Final Report (link in footnote 
3), for the HPR tand GLN receiver classes approximately a 50 dB 
difference in interference levels resulting in a 1 dB C/N0 degradation 
at 1630 MHz was observed between the most and least sensitive receiver.

3 Differences Between L-Band D2D and IoT Services and Historical Use of 
the Mobile Earth Stations

    There have been MSS operations in the L band for over two decades 
now. Given that there have been no reports of interference from these 
operations to GNSS during this time, a reasonable question is: why 
should the GNSS community be concerned with interference at this point 
in time? There are several reasons for concern:
     Legacy L-band mobile earth stations (MESs) are expensive 
and sparse in number compared to anticipated D2D and IoT devices
    [cir] We reviewed non-D2D/IoT products available from several 
prominent satellite equipment vendors,\7\ and L-band MES equipment are 
on the order of several hundred dollars or more. The equipment cost, 
and the additional costs for the required satellite service plan and 
usage fees limits the number of devices in use and also limits their 
operation in highly populated areas (where cellular communications is 
an available and cheaper option).
---------------------------------------------------------------------------

    \7\ See, e.g., https://www.remotesatellite.com/collections/msat-tracking/ and https://satellitephonestore.com/catalog/search?q=msat.
---------------------------------------------------------------------------

    [cir] For comparison, D2D devices are anticipated to include 
millions of cellular phones that are owned and used on a daily basis by 
most U.S. citizens. Although D2D operations are expected to be mainly 
used when the users are out of cell coverage, with millions of devices 
in use it is likely that some transmissions will occur in populated 
areas (e.g., in cellular dead zones or when there is a radio 
propagation blockage in the direction of the cell tower). It is also 
possible that small D2D devices might be operated onboard aircraft (see 
Appendix B).
    [cir] In addition to legacy MES and cellular phones with D2D 
capabilities, there are also emerging standalone L-band MSS D2D/IoT 
devices. Just one company, Skylo, has alone already received a blanket 
license for up to 1M IoT devices capable of transmitting to Skyterra-1. 
The Motorola Defy satellite link is a new D2D product that became 
available for purchase in the United States in June 2023. This device 
is small (3.4 x 2.5 x 0.5 inches), battery-powered (with an advertised 
battery life of up to 4 days), and lightweight (2.5 ounces). It 
connects to iOS and Android smartphones through Bluetooth and enables 
smartphone users to send and receive text messages through MSS 
geostationary satellites, including those operated by Ligado and 
Viasat/Inmarsat. This device retails for $150 including a one-year 
service plan.\8\
---------------------------------------------------------------------------

    \8\ See, e.g., https://www.rei.com/product/227381/motorola-defy-satellite-link.
---------------------------------------------------------------------------

     Most existing MES include a GPS tracking capability. The 
vendor would thus have a need to ensure that MES emissions in the 1559-
1605 MHz band are far below the -70 dBW/MHz level permitted by 47 CFR 
25.216 in order to protect their own equipment's GPS operation. It is 
not clear that IoT device manufacturers would have the same incentive 
to achieve far better OOBE performance than required by the FCC rules, 
especially considering the market motivation to make IoT devices 
extremely low-cost.
     The FCC rules (47 CFR 25.285) prohibit MES capable of 
transmitting in the 1610-1660.5 MHz band from operating on civil 
aircraft unless its use is approved by the FAA or pilot. Further 25.285 
requires that such MES cannot be ``. . . sold or distributed to users 
unless it conspicuously bears the following warning: `This device must 
be turned off at all times while on board aircraft.' '' Unfortunately, 
the E-LABEL Act permits such warnings to be made available to the users 
of electronic devices digitally through a screen on a device 
display.\9\ This digital display can oftentimes be difficult for users 
to find. For instance, on the iPhone 14, users would have to navigate 
to a ``Legal & Regulatory'' screen two levels deep under ``Settings'' 
to find the warning required per 47 CFR 25.285 against using on-board 
aircraft (see Figure 9).
---------------------------------------------------------------------------

    \9\ https://www.govinfo.gov/content/pkg/COMPS-11324/pdf/COMPS-11324.pdf.

---------------------------------------------------------------------------

[[Page 105549]]

[GRAPHIC] [TIFF OMITTED] TN27DE24.247

Figure 9. The Warning Against Use of the iPhone 14 Satellite Feature on 
Aircraft is Compliant With the FCC Rules After the E-LABEL Act but 
Obscured Two Levels Deep in the Settings Menu (Settings/General/
Legal&Regulatory)

4 Summary

    This paper assessed the potential impact of emerging L-band MSS D2D 
and IoT services on GPS/GNSS.
    If D2D or IoT devices are minimally compliant with 47 CFR 25.216, 
out-of-band emissions from single or multiple D2D devices could degrade 
GNSS receiver performance at distances up to 50 m or more. Additional 
filtering cannot mitigate the problem since the interference is 
overlapping with the GNSS signals in frequency.
    The fundamental emissions of L-band MSS terminals, including MES, 
D2D and IOT devices operating in the 1610-1660.5 MHz band can result in 
impact to GNSS devices at considerable distances. In this paper, as one 
example, 200 mW devices transmitting in 1627.5-1637.5 MHz were assessed 
to impact GPS/GNSS receivers at distances of up to 140 m. The FCC has 
authorized over 10 million mobile devices operate in L-band with EIRP 
of up to 10 W and at frequencies closer to the 1559-1610 MHz band. 
These devices could cause GNSS receiver degradation at distances 
exceeding 1 km. The impacted distances are highly variable across GNSS 
receivers, with the values cited above applying to the most sensitive 
receivers tested by DOT in 2016.
    Finally, the paper identifies reasons why emerging L-band D2D and 
IoT operations may be problematic for GNSS even though reliant on 
existing MSS authorizations. Importantly, the primary concerns raised 
in this paper are applicable to MES, D2D and IoT devices with 
connectivity to any L-band MSS satellites with uplinks in the 1610-
1660.5 MHz band. Thus, these issues are relevant to, e.g., Inmarsat/
Viasat, Iridium, Ligado, and Globalstar MSS operations.
Appendix A. FCC L-Band MSS Mobile Earth Station Rules
The FCC Rules for satellite services (47 CFR part 25) include the 
following provisions related to L-band MSS operations:
     25.216--provides limits on mobile earth station emissions 
in the 1559-1610 MHz ARNS band. Per 25.216(c), ``The e.i.r.p. density 
of emissions from mobile earth stations placed in service after July 
21, 2002 with assigned uplink frequencies between 1610 MHz and 1660.5 
MHz shall not exceed -70 dBW/MHz, averaged over any 2 millisecond 
active transmission interval, in the band 1559-1605 MHz. The e.i.r.p. 
of discrete emissions of less than 700 Hz bandwidth from such stations 
shall not exceed -80 dBW, averaged over any 2 millisecond active 
transmission interval, in the 1559-1605 MHz band.''
     25.285--prohibits operation of Earth stations capable of 
transmitting in the 1.5/1.6 GHz MSS frequency bands on board civil 
aircraft ``unless the device is installed in a manner approved by the 
Federal Aviation Administration or is used by the pilot or with the 
pilot's consent''. Further, portable Earth stations operating in these 
bands cannot be ``. . . sold or distributed to users unless it 
conspicuously bears the following warning: `This device must be turned 
off at all times while on board aircraft.' ''
     25.287--provides requirements for mobile Earth stations 
operating in the 1530-1544 MHz and 1626.5-1645.5 MHz bands to ensure 
compliance with Footnote 5.353A in 47 CFR 2.106 and US315.
Appendix B. MSS D2D Use on Commercial Aircraft
Overview
    One location that many people encounter that is often without 
cellular or WiFi service is when flying aboard a commercial aircraft. 
Low-cost D2D equipment may easily be taken aboard an aircraft and a 
passenger may operate this equipment (without knowing it is illegal to 
do so) in the United States

[[Page 105550]]

from a window seat. They might do so for a variety of reasons, e.g., to 
stay in touch with family and friends through text conversations while 
on the flight.
Potential for Interference
    As noted in Section 2.1.1, MSS D2D devices are permitted to emit 
broadband noise in the GPS L1 band with an effective isotropic radiated 
power (EIRP) of up to -70 dBW/MHz from 1559-1605 MHz.\10\ The path loss 
at the GPS L1 frequency from a vertically polarized antenna within a 
commercial aircraft to the right-hand circularly polarized GPS antenna 
on top of the aircraft fuselage can be as low as 54 dB.\11\ This value 
was measured by Delta Airlines for a Boeing 737-800 aircraft. NASA, in 
cooperation with United Airlines, has measured values as low as 65 dB 
for Boeing 737 and 747 aircraft.\12\ Figure 1 presents an example of 
these NASA path loss measurement results for a 737-200 aircraft 
measured from each window seat and also just inside each aircraft door.
---------------------------------------------------------------------------

    \10\ See 47 CFR 25.216.
    \11\ See, e.g., Section E.5.2 of RTCA Document DO-235C, 
``Assessment of Radio Frequency Interference Relevant to the GNSS L1 
Frequency Band,''
    \12\ See for instance https://ntrs.nasa.gov/api/citations/20030067884/downloads/20030067884.pdf and other NASA reports 
referenced within RTCA DO-235C.
[GRAPHIC] [TIFF OMITTED] TN27DE24.248

Figure 1. Measured Path Loss at 1575 MHz From Vertically Polarized 
Antenna Inside a Boeing 737-200 Aircraft to the Top-Mounted GPS Antenna

    Commercial aircraft GPS avionics are typically certified to able to 
tolerate in-band interference with a maximum power spectral density of 
-140.5 dBW/MHz at the output of the passive antenna element while 
meeting all performance requirements after initial acquisition.\13\ The 
interference from MSS D2D equipment could be much higher than this 
maximum tolerable level. For instance, for the worst-case location in a 
737-800 aircraft measured by Delta Airlines, the -70 dBW/MHz 
permissible D2D equipment EIRP level within 1559-1605 MHz would result 
in -124 dBW/MHz received interference (16.5 dB higher than the required 
avionics tolerance). A more typical path loss from a window seat to the 
GPS antenna, based upon NASA's measurements (see Figure 2), is on the 
order of 70 dB. D2D equipment with a -70 dBW/MHz EIRP would still 
result in an excessive level of interference, -140 dBW/MHz, just above 
the maximum tested level for certified avionics.\14\
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    \13\ See, e.g., Federal Aviation Administration Technical 
Standard Orders (TSO)-C145, -C146, -C161, -C196, -C204, -C205 and 
also International Civil Aviation Organization (ICAO) GNSS Standards 
and Recommended Practices (SARPs). Certified GPS equipment are less 
tolerant of interference during initial acquisition and are only 
required to perform this function with in-band interference at a 
level of -146.5 dBW/MHz.
    \14\ Note that the International Telecommunication Union (ITU) 
recommends a 6 dB safety margin between permitted interference 
levels and maximum tolerable levels for safety applications of GNSS 
such as aircraft navigation. See, e.g., ITU-R M.1903. Application of 
this safety margin would result in a much larger, 6.5 dB, excess of 
interference in this scenario.
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Legality of Operating MSS D2D Equipment on Civil Aircraft
    The FCC rules (47 CFR 25.285) prohibit operation of MSS equipment 
capable of transmitting in the 1.5/1.6 GHz MSS frequency bands on board 
civil aircraft ``unless the device is installed in a manner approved by 
the Federal Aviation Administration or is used by the pilot or with the 
pilot's consent''. Further, portable Earth stations operating in these 
bands cannot be ``. . . sold or distributed to users unless it 
conspicuously bears the following warning: `This device must be turned 
off at all times while on board aircraft.' ''
    Unfortunately, many consumers are not familiar with the FCC rules. 
Furthermore, the E-LABEL Act permits warnings such as that required in 
47 CFR 25.285 to be made available to the users digitally through a 
screen on a device display. This E-LABEL provision makes it unlikely 
that the warning will be seen by most users. For instance, despite the 
Motorola Defy Satellite Link dongle being compliant with 47 CFR 25.285, 
nowhere in the user manual is it mentioned that the equipment should 
not be operated on an aircraft. The manual includes a cryptic section 
entitled ``Viewing the E-label'' that informs the user that an ``E-
label'' can be found under a ``Settings'' menu within the Bullitt 
Satellite Messenger app used to send text messages on the users' 
smartphone. Only a user that bothers to look at this screen would find 
the warning shown towards the bottom of Figure 2.
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[[Page 105551]]

Figure 2. The Warning Against Use of the Motorola Defy Satellite Link 
Aboard an Aircraft is Compliant With the FCC Rules, but Obscured in a 
Smartphone Application Settings Screen Unlikely To Be Noticed by Most 
Consumers

Potential Mitigations
Equipment Performing Better than FCC Rules
    The -70 dBW/MHz EIRP emission limit for broadband noise in the 
1559-1605 MHz band is required by the FCC rules (47 CFR 25.216) for all 
L-band MSS transmitters including D2D devices. It is possible that some 
devices may perform substantially better than this limit. One Motorola 
Defy device that was tested for FCC equipment authorization appears to 
have broadband emissions that are ~10 dB better than the -70 dBW/MHz 
limit (see Figure 3). Unfortunately, this is not sufficiently lower to 
preclude the concern of interference if operated aboard a civil 
aircraft described above.
[GRAPHIC] [TIFF OMITTED] TN27DE24.250

Figure 3. Measured Motorola Defy Satellite Link Broadband Emissions 
From 1559-1610 MHz From FCC Equipment Authorization Filing. Device 
Appears To Have Lower Emissions Than the FCC-70 dBW/MHz Limit in 1559-
1605 MHz by ~10 dB.

Time Duration of Emissions
    Currently available MSS D2D devices only permit short messages to 
be sent (e.g., ~140 characters for the Motorola Defy Satellite Link). 
MITRE has obtained two Motorola devices to measure the time duration of 
emissions (both the fundamental within the MSS band and also the out-
of-band emissions within the 1559-1610 MHz GPS/GNSS band).
    Noting that the use of GPS for aircraft navigation has some very 
strict continuity requirements, it is unlikely that even a short 
duration of emissions will be tolerable. For instance, for precision 
approach operations the International Civil Aviation Organization 
(ICAO) has adopted a continuity requirement that the probability that 
GPS is lost over an exposure period of 15 seconds must not exceed 8E-6. 
Furthermore, certified GPS avionics are not required to be able to 
continue to track GPS signals in the event that broadband interference 
exceeds -140.5 dBW/MHz, except if pulsed with pulse widths less than 
125 microseconds and duty cycle less than 1%. Once tracking is 
disrupted, reacquisition within 20 seconds is only required if the 
receiver continues to track a set of satellites with good geometry and 
up to 5 minutes if not.
Summary
    This appendix has identified a concern of interference from MSS D2D 
devices to GPS avionics on commercial aircraft. Such devices have only 
very recently become available at very low cost in the United States. 
This concern is currently unmitigated from such devices that do not 
operate with significantly lower RNSS band emission limits than the FCC 
RNSS emission limit contained within 47 CFR 25.216 for L-band MSS 
transmitters, including D2D and IOT equipment.
Appendix C. Separation Distances: Sample Calculations
    Table 1 assumes a GNSS receiver with a right-hand circularly 
polarized isotropic receive antenna and a noise floor, N0, of -201.5 
dBW/Hz referenced to the output port of the antenna. MSS MES emissions 
are assumed to be at the 47 CFR 25 limit of -70 dBW/MHz (= -130 dBW/Hz) 
and the MES is assumed to use a vertically polarized antenna. As an 
example of the calculations resulting in the separation distances 
within Table 1, consider the first row. A degradation of 1 dB in C/N0 
results when the received interference power density, I0, at the output 
of the GNSS receiver antenna reaches a level of -207.4 dBW/Hz.

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    This received interference power density occurs at a distance, d, 
of 79.1 m using free space propagation loss for the GNSS center 
frequency of 1575.42 MHz with corresponding wavelength, [lambda] = 
0.1903 meters:

I0 = -130 dBW/Hz--20 log10(4[pi]d/[lambda])--3 = -207.4 dBW/
Hz

    The subtraction of 3 dB in the above equation is to account for 
polarization mismatch loss between an assumed vertically polarized MSS 
MES signal and the right-hand circularly polarized GNSS receive 
antenna.
    Table 2 presents refined separation distance results using an 
average of representative measured GNSS receiver antenna gain patterns 
from Section 4.1.3.1 of the Department of Transportation (DOT)'s Global 
Positioning System (GPS) Adjacent Band Compatibility (ABC) Assessment 
Final Report, https://rosap.ntl.bts.gov/view/dot/35535. Vertically 
polarized emissions and a height of 2 meters for the MSS MES are 
assumed. As an example of the calculations, the first row in Table 2 
assumes an HPR receiver with a -201.5 dBW/Hz noise floor. From the 
DOT's ABC report, and based upon measurements of multiple HPR antennas, 
Figure 1 below shows a modeled GNSS HPR receiver antenna gain pattern 
for a vertically polarized source. At a lateral distance of 52.2 meters 
the gain of this antenna on the ground towards a MSS MES at 2 m height 
is approximately -6.6 dBi. With this geometry, the slant range distance 
is ~52.2 meters and the received interference strength is:

I0 = -130 dBW/Hz--20 log10(4[pi]d/[lambda])--6.6 = -207.4 
dBW/Hz, resulting in a 1 dB increase over N0 (as explained in the 
footnote)

    Note that since the GNSS antenna gain pattern that was used in this 
instance is based upon measurements with a vertically polarized source, 
no further polarization mismatch adjustment is required.
[GRAPHIC] [TIFF OMITTED] TN27DE24.232

Figure 1. Representative HPR Antenna Gain Pattern Towards Vertically 
Polarized Source

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[FR Doc. 2024-30760 Filed 12-26-24; 8:45 am]
BILLING CODE 3510-60-P
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