[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
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Received interference Required separation
Degradation to C/N0 (dB) level (dBW/Hz) (m)
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1............................................................. -207.4 79.1
3............................................................. -201.5 40.3
6............................................................. -196.8 23.3
10............................................................ -192.0 13.4
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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
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Required
separation to
Noise floor, N0 limit C/N0
GPS/GNSS receiver class (dBW/Hz) * degradation to 1,
3, and 10 dB (m)
**
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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)
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* 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).
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\6\ https://docs.fcc.gov/public/attachments/DOC-392372A1.pdf.
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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).
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\7\ See, e.g., https://www.remotesatellite.com/collections/msat-tracking/ and https://satellitephonestore.com/catalog/search?q=msat.
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[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\
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\8\ See, e.g., https://www.rei.com/product/227381/motorola-defy-satellite-link.
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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).
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\9\ https://www.govinfo.gov/content/pkg/COMPS-11324/pdf/COMPS-11324.pdf.
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[[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.
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\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.
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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