The FCC-Approved Ligado Network and Potential Technical Issues for DOD Use of GPS

The FCC-Approved Ligado Network and Potential Technical Issues for DOD Use of GPS

link to page 2 link to page 2

INSIGHTi

The FCC-Approved Ligado Network and
Potential Technical Issues for DOD Use of
GPS
June 1, 2020
On April 20, 2020, the Federal Communications Commission (FCC) unanimously authorized an
application by Ligado Networks LLC (Ligado) to “deploy a low-power [9.8 decibel watts (dBW)]
terrestrial nationwide network in the 1526-1536 MHz, 1627.5-1637.5 MHz, and 1646.5-1656.5 MHz
bands [of the electromagnetic spectrum] that will primarily support Internet of Things (IoT) services.”
These frequency bands are traditionally used for satellite operations. The Department of Defense (DOD)
opposed this decision—along with the Departments of Homeland Security, Transportation, the Interior,
and Justice; the Federal Aviation Administration; and others—due to concerns that Ligado’s proposed
network could interfere with signals from satellites to Global Positioning System (GPS) receivers.
Congress may consider the FCC’s decision, as well as DOD and other federal agency concerns about the
decision, as it conducts oversight of the FCC. Congress may also consider broader issues related to fifth
generation (5G) mobile technologies, such as the allocation of spectrum among competing users and the
impact of spectrum decisions on national security and GPS modernization.
How GPS Works
GPS consists of a constellation of at least 24 satellites in six orbital planes to provide global coverage,
ground-based receivers, and user equipment such as smartphones. The satellites are positioned
approximately 12,500 miles above the Earth’s surface, in what is termed Medium Earth Orbit (MEO).
Because of the distance the radio waves have to travel from MEO to GPS receivers on Earth, the satellite
signals arriving to the earth’s surface are extremely weak—approximately negative 160 dBW (see Figure
1). GPS satellites broadcast two primary signals, which are used for position, navigation, and timing: L1,
an unencrypted course acquisition signal (broadcast around 1575 MHz), and L2, also known as P(Y)
code—the encrypted signal used by the military for improved accuracy (broadcast around 1227 MHz). L2
in turn enables advanced military capabilities like precision-guided munitions. Other military applications
using GPS include navigation systems, communications encryption, and command and control platforms,
such as blue force tracker, which monitors the location of friendly forces. Due to the complexity of L2
encryption, military GPS equipment must first acquire an L1 signal to identify its relative position before
acquiring an L2 signal.
Congressional Research Service
https://crsreports.congress.gov
IN11414
CRS INSIGHT
Prepared for Members and
Committees of Congress

Congressional Research Service
2
Figure 1.Illustrative Sound Levels, by Decibel Watts

Source: https://www.aiha.org/public-resources/noise-hearing-loss.
GPS Modernization
DOD started the GPS modernization program to upgrade GPS features and increase performance. GPS
modernization includes new civilian and military GPS signals to enhance the capabilities of the traditional
L1 and L2 bands. M code, the new military signal, uses a signal across the traditional L1 and L2
frequencies to increase power and provide improved anti-jam capability, more secure and flexible
encryption, and the ability to detect false signals.
To reach initial operational capability, three new segments of M Code for GPS are required: the space
segment, user equipment to receive the new signal, and ground control system to control beam forming
and additional encryption. According to Section 913 of the FY2011 National Defense Authorization Act
(P.L. 111-383), DOD is not authorized to purchase GPS user equipment after FY2017 unless—with
certain exceptions—the equipment is capable of receiving M code from GPS. There have been significant
program delays, however, in all three segments of GPS modernization, and although currently operational
GPS IIF and GPS III satellites are equipped with M code, they do not yet offer the full M code capability.
Potential Technical Issues
According to Under Secretary of Defense for Research and Engineering Michael Griffin, the effect on
GPS of the proposed Ligado network would be akin to attempting to listen to the rustling of leaves (the
GPS signal) while 100 jet aircraft simultaneously took off (Ligado’s potential transmission). Specifically,
the FCC authorized Ligado to operate ground-based transmitters that will generate very strong signals or
noise near the same frequency range where GPS sends its signals. DOD and some civilian GPS users
contend that the level of potential interference would affect not only military applications but also
potentially civilian applications, including aviation, agriculture, telecommunications, and finance, among
others. To mitigate this potential interference, the FCC authorization requires Ligado to operate a 23 MHz
“guard band”—a part of the spectrum designated as a quiet zone to buffer/prevent interference; however,
some analysts argue that the size of the guard band is insufficient to prevent inference.
DOD and the FCC disagree about the standard that should be used to determine “harmful interference” to
GPS. While the FCC used a “performance-based” standard in assessing and approving the Ligado
network, DOD used the International Telecommunication Union (ITU) standard of a 1 decibel (1 dB)

Congressional Research Service
3
increase in noise within the GPS frequency range during its own classified testing in 2016. Using this
more restrictive standard, DOD asserts that the Ligado network would cause harmful interference to GPS.
While GPS modernization could potentially offset interference challenges to military systems that could
be presented by the Ligado network, it would require that all three segments of GPS (space, ground
control, and user equipment) be available to the end users. The modernized GPS receivers equipped with
M code are unlikely to be complete before around 2035. Even when available, some legacy military GPS
receivers will not be upgraded, therefore potentially adversely affecting their performance.

Author Information

John R. Hoehn
Kelley M. Sayler
Analyst in Military Capabilities and Programs
Analyst in Advanced Technology and Global Security

Stephen M. McCall

Analyst in Military Space, Missile Defense, and Defense
Innovation

Disclaimer
This document was prepared by the Congressional Research Service (CRS). CRS serves as nonpartisan shared staff
to congressional committees and Members of Congress. It operates solely at the behest of and under the direction of
Congress. Information in a CRS Report should not be relied upon for purposes other than public understanding of
information that has been provided by CRS to Members of Congress in connection with CRS’s institutional role.
CRS Reports, as a work of the United States Government, are not subject to copyright protection in the United
States. Any CRS Report may be reproduced and distributed in its entirety without permission from CRS. However,
as a CRS Report may include copyrighted images or material from a third party, you may need to obtain the
permission of the copyright holder if you wish to copy or otherwise use copyrighted material.

IN11414 · VERSION 1 · NEW