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the responsibility of
the Federal Government. Research by HSOAC shows that users can mitigate short-term GPS
disruptions (e.g., inability to read a GPS signal) with various strategies, ranging from using local
backup capabilities to delaying operations until GPS is restored. The HSOAC report, Analyzing a
More Resilient National Positioning, Navigation, and Timing (PNT)
Capability,
1
provides an
analysis of some mitigations used by specific industries to respond to a temporary disruption or
1 The 2017 National Defense Authorization Act mandated a study “to assess
and identify the technology-neutral
requirements to backup and complement the positioning, navigation, and
timing [PNT] capabilities of the Global
Positioning System [GPS] for national security and critical infrastructure.” DHS had HSOAC conduct this study.
v
loss.
Recognizing the ability to mitigate the negative impacts of temporary disruptions, the remainder
of this report will primarily address mitigation against long-term or
permanent disruption or loss
of GPS PNT capabilities. While the probability of long-term GPS disruption
is low, it is feasible,
and prudent risk management demands taking steps to minimize the negative impacts of such an
event.
In reviewing and analyzing HSOAC’s report, DHS went beyond simply
identifying system
specifications and systems that could provide PNT if GPS were unavailable.
The department
sought to understand how the introduction of non-GNSS PNT systems would
affect the security
and resilience of critical infrastructures that are dependent on GPS for
PNT. A special area of
emphasis was to evaluate the reasons a user would choose to adopt non-GNSS services when
GPS was available. Without end user adoption, the provisioning of services
does not change the
risk associated with loss of GPS. We frame this report through that risk analysis perspective.
Through the course of its analysis, DHS identified key findings and considerations. These
findings are critical in contextualizing the department’s assessment and recommendations in the
critical infrastructure community and risk landscape. Below are key
findings:
1. GPS is not the only source of PNT data. Other sources are currently available for
purchase, and include alternate space-based systems and constellations, terrestrial
beaconing systems, time-over-fiber, cellular and wireless signals, and local terrestrial
systems.
2. Whatever the source of the PNT, it is incumbent on users to apply the principles found in
Executive Order 13905, Strengthening National Resilience Through Responsible Use of
Positioning, Navigation, and Timing Services. By applying these principles
uses can
reduce the risk associated with the disruption or manipulation of PNT
services.
3. Unless non-GPS PNT sources are free/low-cost or provide a unique benefit deemed
valuable by the user and not found in GPS and other currently available sources, there is
no reason to assume users will adopt new non-GPS PNT sources more widely
than they
have today. However, user behavior could be modified through subsidies or regulatory
requirements.
4. The critical infrastructure sectors heavily reliant on PNT (meaning disruption would
cause significant costs, delays, or degradation of functions and service) include
communications, information technology, transportation, emergency services, energy,
surveying and mapping, and financial services.
5. The critical infrastructure sectors highlighted in this report are
heavily reliant on PNT
services, but their requirements differ significantly. Some sectors require very precise
timing, while in others position and navigation precision is more important.
6. Critical infrastructure systems that would cease to operate due to GPS disruptions will do
so because of design choices associated with a lack of information, cost, efficiency, and
other considerations—not because of a lack of available options. In other words, business
decisions, the lack of a Federal mandate, and potentially an
underappreciation of the risk
associated with GPS dependence are factors in the lack of resilience to GPS disruption.
vi
7. New non-GPS PNT systems that are designed without considering existing
PNT
systems—including their capabilities, limitations, and why they were adopted
in some
industries and not others—may simply compete with existing systems rather
than fill
perceived backup gaps.
8. The position and navigation functions in critical infrastructure are so diverse that no
single PNT system, including GPS, can fulfill all user requirements and applications.
Because of this, DHS could not identify generic specifications for a
national backup.
Position and navigation backups must be application-specific and must be developed in
coordination with industry owners and operators.
9. While position and navigation requirements are complex, timing
requirements are simple,
with a minimal acceptable precision of anywhere between 65-240 nanoseconds. This
level of precision supports all critical infrastructure requirements and is expected to meet
future requirements, including 5G.
Recommendations
Based on these key findings and considerations, our analysis of the current
PNT ecosystem, and
the goal of mitigating risk wherever possible, DHS offers the following recommendations to
address the nation’s PNT requirements and backup or complementary capability gaps:
1. Temporary GPS disruptions: End users should be responsible for mitigating temporary
GPS disruptions. For example, the Federal Aviation Administration maintains sufficient
PNT capabilities to assure the continued safe operation of the national airspace, albeit at a
reduced capacity, during GPS disruptions. The Federal Government can
facilitate this
mitigation for various critical infrastructure sectors, but should not be solely responsible
for it.
2. PNT Diversity and Segmentation: The Federal Government should encourage adoption
of multiple PNT sources, thus expanding the availability of PNT services
based on
market drivers. Encouraging critical infrastructure owners and operators to adopt multiple
PNT systems will diffuse the risk currently concentrated in wide-area PNT services such
as GPS. Federal actions should focus on facilitating the availability and adoption of PNT
sources in the open market.
3. System Design: PNT provisioning systems, assets, and services must be designed with
inherent security and resilience features. Critical Infrastructure systems
that use PNT
services must be designed to operate through interference and to identify
and respond to
anomalous PNT inputs. These attributes are applicable to the PNT receivers
and the
systems that use them.
4. Pursue Innovation that Emphasizes Transition and Adoption: Incorporating
PNT
signal diversity into the PNT ecosystem should be pursued with an emphasis
on research
and development that prioritizes successful transition and adoption into existing GPS
receivers, taking into account factors such as business case considerations, financial
costs, technical integration, and logistical deployment.
Further explanations of these findings and recommendations are included in
the body of this
report.
vii
Positioning, Navigation, and Timing (PNT) Backup and
Complementary Capabilities to the Global Positioning
System (GPS)
Table of Contents
I. Legislative Language .............................................................................................................
1
II. Background and Key Assessments......................................................................................
2 Background...............................................................................................................................................
2
Previous Assessments...............................................................................................................................
2
Conclusions from Previous Assessments..................................................................................................
3
III. Positioning Navigation and Timing Landscape................................................................ 3 Overview...................................................................................................................................................
4
Existing and Emerging PNT Capabilities.................................................................................................
4
IV. Analysis of Alternatives ......................................................................................................
5
Timing Backup..........................................................................................................................................
6
Positioning and Navigation Backup..........................................................................................................
8
Understanding the Augmentation Market Space ....................................................................................
10
V. Backup Considerations .......................................................................................................
10
Temporary Disruptions...........................................................................................................................
10
Long-Term Disruptions ..........................................................................................................................
11
Conclusion ..............................................................................................................................................
12
VI. Departmental Plan for Meeting the Requirements ........................................................ 13
Department of Homeland Security .........................................................................................................
13
A. List of Abbreviations/Acronyms ........................................................................................
14
B. Bibliography ........................................................................................................................
15
C. User Needs Framework ....................................................................................................
16
1
I. Legislative Language
NATIONAL DEFENSE AUTHORIZATION ACT (NDAA) 17,
PUBLIC LAW 114–328
Sec. 1618. backup and complementary positioning, navigation, and
timing capabilities of global positioning system.
(a) STUDY.—
(1) IN GENERAL.—The covered Secretaries shall jointly conduct a study to
assess and
identify the technology-neutral requirements to backup and complement the positioning,
navigation, and timing capabilities of the Global Positioning System for national security and
critical infrastructure.
(2) REPORT.—Not later than one year after the date of the enactment of this Act, the
covered Secretaries shall submit to the appropriate congressional committees
a report on the study
under paragraph (1). Such report shall include—
(A) with respect to the Department of each covered Secretary, the identification of
the respective requirements to backup and complement the positioning, navigation, and
timing capabilities of the Global Positioning System for national security
and critical
infrastructure;
(B) an analysis of alternatives to meet such requirements, including, at a minimum—
(i) an analysis of appropriate technology options;
(ii) an analysis of the viability of a public-private partnership to
establish a
complementary positioning, navigation, and timing system; and
(iii) an analysis of the viability of service level agreements to operate a complementary positioning, navigation, and timing system; and
(C) a plan to meet such requirements that includes—
(i) for each such Department, the estimated costs, schedule, and system
level technical considerations, including end user equipment and integration considerations; and
(ii) identification of the appropriate resourcing for each such Department
in
accordance with the respective requirements of the Department, including domestic or international requirements.
(b) SINGLE DESIGNATED OFFICIAL.—Each covered Secretary shall designate a
single senior
official of the Department of the Secretary to act as the primary representative ofsuch Department for
purposes of conducting the study under subsection (a)(1).
(c) DEFINITIONS.—In this section:
(1) The term ‘‘appropriate congressional committees’’ means—
(A) the congressional defense committees;
(B) the Committee on Science, Space, and Technology, the Committee on Transportation and Infrastructure, and the Committee on Homeland Security of the House
of Representatives; and
(C) the Committee on Commerce, Science, and Transportation and the Committee
on
Homeland Security and Governmental Affairs of the Senate.
(2) The term ‘‘covered Secretaries’’ means the Secretary of Defense, the Secretary of
Transportation, and the Secretary of Homeland Security.
2
II. Background and Key Assessments
Background
Since GPS became available for civilian use, numerous studies have assessed
the economic
impact of GPS, vulnerabilities of GPS end user equipment, consequences of
GPS disruptions,
and what systems could backup and complement GPS. Detailed in the “Previous Assessments
Section” is a non-exhaustive selection of previous works relevant to this effort selected to
highlight the long-term concerns regarding the growing dependence on GPS.
This report is a
summary and analysis of the HSOAC’s 2018 report, Analyzing a More Resilient National
Positioning, Navigation, and Timing (PNT) Capability. HSOAC’s report
contains data and
literary analyses of a large library of reference material to support their
own conclusions
regarding GPS dependency and PNT backups/alternate systems. DHS used HSOAC’s report to
inform and guide the findings and recommendations presented in this report. Previous Assessments
1997
The Presidential Commission on Critical Infrastructure issued a report
titled Critical
Foundations: Protecting America’s Infrastructures. The report stated that “[p]ossible exclusive
reliance on GPS and its augmentations, combined with other complex interdependencies, raises
the potential for ‘single point failure’ and ‘cascading effects.’”2 While
this was primarily
referring to the use of GPS in the national airspace, it has become increasingly more applicable
to other critical functions required to operate U.S. critical
infrastructure.
The findings of this report influenced the development of National Security Presidential
Directive (NSPD)-39 that directed the Secretary of Transportation to:
develop, acquire, operate, and maintain backup position, navigation, and
timing
capabilities that can support critical transportation, homeland security,
and other
critical civil and commercial infrastructure applications within the United States,
in the event of a disruption of the Global Positioning System or other space-based
positioning, navigation, and timing services, consistent with Homeland
Security
Presidential Directive-7, Critical Infrastructure Identification, Prioritization, and
Protection, dated December 17, 2003.3

2011
DHS published a National Risk Estimate related to GPS stating:
After a nine-month review, U.S. Government and private sector experts
concluded that portions of the Nation‘s critical infrastructure are increasingly
reliant on GPS and GPS-based services. In the short term, the risk to the nation is
2 President’s Commission on Critical Infrastructure Protection, Critical Foundations: Protecting America’s
Infrastructures (Washington, D.C.: United States Government, 1997), A-19. 3 United States Government, Fact Sheet: National Security Presidential
Directive 39: U.S. Space-Based Position,
Navigation, and Timing Policy (Washington, D.C.: United States Government, 2004).
3
assessed to be manageable. However, if not addressed, this threat poses increasing
risk to U.S. national, homeland, and economic security over the long term.4

2017
The semi-governmental advisory body Innovate UK issued a report on the
economic impact to
the United Kingdom associated with a five-day disruption to global
navigation satellite systems
(GNSS). The report estimates a daily economic impact $1.25B to the UK
economy.5

2019
The U.S. Department of Commerce and DHS, through RTI International,
estimated that the
economic impacts to the U.S. economy caused by a 30-day loss of GPS would be
$1 billion per
day and could be 50 percent higher if the disruptions occurred at the least opportune time.6

Conclusions from Previous Assessments
All research as of the release of this report shows that dependence on GPS
and other GNSS
continues to grow. There have been no meaningful efforts to address the unabated adoption and
use of GPS, and increasingly foreign GNSS, in U.S. critical infrastructure. Though the 2017 and 2019 reports used different methodologies and assessed
two economies
with significantly different GDPs, they concluded that GPS disruptions would have negative
impacts that would likely exceed $1 billion a day to their respective economies. As currently
designed, the infrastructure in the UK and U.S. would suffer significant degradation and
economic impacts should GPS or GNSS services be disrupted. Those negative economic impacts
would be spread across a wide variety of critical infrastructures and a wide variety of
applications of GPS or GNSS.
Both reports also confirmed the wide variety of uses and the unique ways GPS and other PNT
sources are integrated in the operation of critical infrastructure that are broader than initially
expected. For example, a greater economic loss is caused by disruption of
cargo throughput at
maritime ports that use automated container handling equipment, which
require GPS to function,
than from navigation issues created by the loss of GPS. Improving navigation
to the port
provides fewer benefits than improving the container handling equipment’s ability to operate in
the absence of GPS. These types of insights help determine where sources of risk lie and where
to focus mitigation efforts and stimulate the availability of non-GNSS PNT services.
III. Positioning Navigation and Timing Landscape
4 United States Department of Homeland Security, National Risk Estimate:
Risks to U.S. Critical Infrastructure from
Global Positioning Disruptions (Washington, D.C.: United States Department
of Homeland Security, 2010), 3. 5 Greg Sadlier et al., The economic impact
on the UK of a disruption to GNSS (London, UK: London Economics,
2017), iii.
6 Alan O’Connor et al., Economic Benefits of the Global Positioning System (GPS) (Research Triangle, NC: RTI
International, 2019), ES-4.
4
Overview
Based on the research cited in the previous section and HSOAC’s report, it
is apparent that the
long-term, global disruption of GPS capabilities would have wide-ranging negative impacts on
the global economy and the daily lives of people around the world.
GPS and foreign GNSS are the primary PNT services that industries use to enhance operations.
However, despite the billions of dollars invested by the United States in
GPS, it does not meet all
the U.S. PNT needs. Where GPS cannot fulfill end user requirements and where there are
sufficient drivers (i.e., economic, safety of life, security), industries
and the public sector have
developed and employed additional capabilities to fill PNT gaps. This report summarizes these
use cases and additional PNT technologies that are already available to
address gaps in PNT
services. The report highlights where additional technologies may be
available.
Existing and Emerging PNT Capabilities
The following is a list of emerging and existing PNT capabilities, excluding GPS. This is not an
all-inclusive list, rather it is intended to show the diversity in the PNT ecosystem.
• Foreign GNSS (Galileo [EU], GLONASS [Russia], and Beidou [China]): Increasingly, receiver manufacturers are including multiple GNSS
constellations in their
equipment. Manufacturers claim that using multiple constellations together provides
better accuracy and improves operations in environments with limited sky visibility as the
receiver is likely to have more satellites in view. There is also a belief
that multiconstellation receivers provide resilience against failure of a single GNSS constellation.
While this may be true in some respects, all GNSS are subject to the similar phenomenology that can disrupt their reception, including intentional
jamming, spoofing,
and natural disturbances such as ionospheric disturbance caused by space weather. The
signal from a foreign GNSS could be intentionally or unintentionally compromised
causing system degradation or shutdown. This feature introduces exposure to threats,
particularly in the area of critical infrastructure.
• Satellite-Based Augmentations and Ground Reference Stations: These systems are
designed to work with GPS and other GNSS to increase position and navigation accuracy. Some systems can provide real-time accuracy 100 times better than
GPS alone,
with some capable of delivering different levels of accuracy depending on
the
subscription. Industry recognizes the value of these increased capabilities, justifying the
increased costs associated with more expensive receivers and subscriptions
to gain access
to better accuracy. Examples of public and private augmentations include:
o Ground-Based Augmentation System and Wide-Area Augmentation System
o StarFire (Manufacturer - John Deere)
o OmniSTAR (Manufacturer - Trimble/Fugro)
o CenterPoint RTX (Manufacturer - Trimble)
o U.S. Continuously Operating Reference Stations
5
o TerraStar
• Existing PNT Services: Private companies are constantly seeking ways to deliver PNT
services to meet customer needs. These services may provide complementary
PNT
functions to GPS by expanding PNT capabilities— including cross checks to validate
data—or extending PNT services to GPS denied or degraded environments. For example,
GPS/GNSS does not work indoors effectively, while other systems do.
Therefore, some
non-GNSS services are more suitable for location-based services in built-up areas. Of
note, many of the below services would be available for a fee in contrast to GPS service,
which is free to end users. The additional cost is a barrier to adopting one
of the below
services as a backup to GPS, and disincentives system diversity. Listed
below are some
of the private sector efforts to provide PNT services:
o Low Earth Orbit Satellites offering positioning and timing services: This service is currently available to U.S. critical infrastructure operators for
a fee.
These service providers claim that this system can deliver precision time
that
meets all identified timing requirements in critical infrastructure. This system also
offers a positioning and navigation service, however current accuracy levels are
less than GPS.
o Metropolitan Beaconing Systems (MBS): FirstNet is exploring the
possibility of
deploying a MBS to meet the Enhanced 911 requirement for 3D location of emergency calls in the largest metropolitan service areas. If fielded, and
the
system performs according to specifications, all critical infrastructure
timing
requirements in the serviced area could be met. The system may also be
capable
of meeting indoor and outdoor position and navigation requirements. This service
would be fee based.
o Time Over Fiber: Precision Time Protocol (IEEE 1588) has advanced in accuracy. New protocols have demonstrated sub-nanosecond time transfer over short distances (10s of kilometers), with sub-microsecond over greater distances.
As this technology progresses, it will enable companies to offer time-as-a-service
for a fee over fiber networks.
o High Precision Positioning: Numerous sites around the country have
deployed
highly precise positioning systems that do not rely on positioning data from GPS.
These systems cover limited areas but can provide positioning data significantly
better than GPS. DOD, DOT, and private entities purchase these systems to
meet
specific operational requirements.
IV. Analysis of Alternatives
HSOAC Research indicates that critical infrastructure systems that would
cease to operate
without GPS do so because of design choices, cost factors, increasing efficiency, or other
considerations—not because of a lack of available additional means to
navigate, determine
location, or synchronize. If future critical infrastructure systems are engineered and operated
based on the same design choices, providing additional PNT sources will not change the risk
associated with the unavailability of GPS. Unless the additional PNT
services provide unique
benefits deemed valuable by the user community, or regulators mandate that users secure
6
resilience to long-term GPS failure, there is no reason to assume businesses will act differently
than they do today.
In acknowledging the availability of existing and emerging PNT services,
based on research
conducted by HSOAC, DHS sought to determine how deployment of additional, federally
supported PNT system(s) or requirements would change the risk associated
with the loss of GPS
for critical infrastructure. The precision of those alternative
capabilities, and the analysis
HSOAC used to arrive at them, are listed in subsequent sections. Following
the technical
specifications, this report discusses adoption-related requirements.
Timing Backup
Requirements
GPS timing is exceptionally accurate, delivering time within billionths of a second to users
around the world. According to GPS.gov’s website, GPS Accuracy Q&A page,
“[t]he U.S.
Government distributes Coordinated Universal Time (UTC) as maintained by the U.S. Naval
Observatory (USNO) via the GPS signal in space with a time transfer accuracy relative to UTC
(USNO) of =40 nanoseconds (billionths of a second), 95% of the time.” Applications receive the
time directly from GPS—no other inputs are required.
Based on multiple studies, the most stringent timing requirements for
critical infrastructure are
listed in Table 1.
Table 1
Function Requirement
Electricity 1.0µs Phasor Measurement Unit*
Wired and Wireless Communications 1.5µs for 4G LTE Network Backbone
50–200 Parts Per Billion Frequency Stability
Emergency Services (FirstNet) 1.5µs for 4G LTE Network Backbone
50–200 Parts Per Billion Frequency Stability
Financial Services 50µs**
*Currently not used for operations
**Regulatory requirement
µs = microsecond(s) (millionth of a second)
The precision time delivered by GPS exceeds the need of all critical infrastructure for the
foreseeable future. Cellular networks require timing accuracy of 1.5µs for
4G. As the nation
transitions to 5G, those requirements may become more precise (~240 nanoseconds) but are not
expected to exceed GPS’s capabilities. Since the telecommunication timing requirement is
nationwide and is also the most precise requirement used by critical infrastructure it can serve as
a baseline requirement for timing services accuracy.
This means that any system serving as a timing backup or alternate must
provide a minimum of
1.5µs accuracy and 50–200 parts per billion frequency stability to maintain support to the
communications sector.
7
While the Electricity Sector appears to have a more stringent requirement, utilities can continue
to operate without that level of precision due to their strategic implementation of timing
requirements. While the electric industry has sought to benefit from the availability and precision
of GPS, they have not created a dependence on GPS. Should GPS signals become unavailable,
the electric grid will continue to operate. This is an effective model for
the use of GPS in critical
applications. The system takes advantage of increased precision to increase efficiencies while
still being able to operate in the absence of GPS, albeit at reduced efficiency.
Alternative Systems
Table 2 depicts proposed timing solutions submitted by industry to DHS
during a Request for
Information (RFI) in December 2018. According to industry, there are several systems that can
meet or exceed all timing requirements for U.S. critical infrastructure
(those indicated in green).
Each proposal has unique characteristics that could impact industry use. For example, some
systems are available nationwide while others are local or regional (see
Table 3).
Table 2*
Proposed Solutions Precision Timing Requirements
5G 4G-LTE
Network
Phasor
Measurement
Unit
Financial
Services
µs = microseconds ~240
Nanosec. 1.5 µs 1 µs 50 µs
eLORAN Meets Precision Meets Precision Meets Precision Meets Precision
Locata Meets Precision Meets Precision Meets Precision Meets Precision
Network Time Protocol Not close to req. precision Not close to req.
precision Not close to req. precision Not close to req. precision
NextNav Meets Precision Meets Precision Meets Precision Meets Precision Precision Time Protocol Meets Precision Meets Precision Meets Precision
Meets Precision
Satellite Time and Location (STL) Meets Precision Meets Precision Meets Precision Meets Precision
NIST WWVB Radio Not close to req. precision Not close to req. precision Not close to req. precision Precision within factor of 5
Meets Precision Precision within factor of 5 Not close to req. precision *System performance parameters are as reported by the submitter and have not been validated by the
government. Actual system performance must be assessed as part of any acquisition effort.
Table 3
Proposed Solutions Availability Coverage
eLORAN Not operational in U.S. TBD
Locata Commercial use Local
Network Time Protocol Commercial use Local – National
NextNav Commercial use Regional
Precision Time Protocol Commercial use Local - National
Satellite Time and Location Commercial use National
NIST WWVB Radio Commercial use National
8
One area of concern is dependence on GPS to synchronize to UTC. Any system designed to
backup or complement GPS must not have a GPS connection in its timing supply chain.
Positioning and Navigation Backup
Requirements
While determining timing requirements was relatively straight forward,
position and navigation
is complex with no single application, like telecommunications for timing,
that can be used as a
baseline for position and navigation backup capabilities. To identify the end-user requirements,
HSOAC conducted market analysis and developed functional use cases to understand the level of
precision required for specific applications and how those applications
obtain position and
navigation data.7

Table 4 contains the leading position and navigation applications and key capabilities enabled by
GPS-based services identified in the Food and Agriculture Sector.
Table 4
Applications GPS Mapping GPS Piloting Variable Rate Tech
Key Functions
• Can treat each part
of the field
differently
• Can pinpoint
problems in specific
patches of land
through soil and
other analysis
• Enables targeting of
pests identified by
aerial images
• Uses GPS to more
accurately steer
equipment
• Enables farm work to
extend after daylight
• Can ensure
equipment does not
disturb crops
• Used to ensure all land
has planted seeds or
are not replanted
• Can better target
fertilizer use and apply
appropriate amount
• Can better use
pesticides to target
pests when attacking
specific areas of field
Initial Metrics Sub 1 meter (1m) Sub 1m Sub 1m
Fee-Based
Services
Yes, software and
hardware for GPS
mapping is an external
service
Yes, software for auto
steering requires monthly
or yearly subscription
service; fees are $900–
3,000 a month
Yes, requires yield
measurements and other
software to target
pesticide, fertilizer, and
other material application
This example shows that farmers can pay for sub-meter accuracy to enhance mapping, vehicle
piloting, and variable-rate application of chemicals. To obtain these levels
of precision and the
enhanced efficiencies, the farmer must purchase user equipment and pay a subscription. If either
GPS or the augmentation is unavailable, the ability to conduct the task is impaired. However,
there are several considerations that determine whether a farm adopts

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