NTIA Seeks Comment on L-Band MSS Operations Impact on GPS

Content

ACTION:

Notice; request for comment.

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 adavenport@ntia.gov 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: press@ntia.gov.

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

  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.

1. 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.

1. 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.

1. 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

  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.”

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.

| Degradation to C/N0 (dB) | Received interference level
(dBW/Hz) | Required separation
(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.

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

  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.

| GPS/GNSS receiver class | Noise floor, N0
(dBW/Hz) * | Required separation to limit C/N0
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 (GLN) | −200.5 | 59.6 (1 dB), 31.0 (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 General Aviation (GAV) | −201.5 | 66.7 (1 dB), 34.6 (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).

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.

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.

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,
“LOL L ” corresponds to loss of lock on low-elevation angle satellites, and “LOL H ” corresponds to loss of lock on high elevation angle (i.e., all) satellites. GLN = “General Location/Navigation” and TIM = “Timing” GNSS devices.

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).

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

  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 ○ 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).

○ 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).

○ 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)

• 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).

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

  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.

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)

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.

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.

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.

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, λ = 0.1903 meters:

I0 = −130 dBW/Hz—20 log 10 (4πd/λ)—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 log 10 (4πd/λ)—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.

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

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

Footnotes

(1) https://www.ecfr.gov/current/title-47/chapter-I/subchapter-B/part-25.

(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.

(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.

(4) https://docs.fcc.gov/public/attachments/FCC-20-48A1.pdf.

(5) See https://rosap.ntl.bts.gov/view/dot/34708 and https://rosap.ntl.bts.gov/view/dot/34710.

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

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

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

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

(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.

(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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