What Are LEO Satellites and Why Are They Good for PNT?

GPSGlobal Positioning System is a navigation satellite system. See also and other GNSSGlobal navigation satellite system (GNSS): A general term describing any satellite constellation that provides positioning, navigation, and timing (PNT) services on a global or regional basis. See also operating in Medium Earth Orbit (MEO) are the primary domain for Positioning, Navigation and Timing (PNTPosition, Navigation, and Timing: PNT and map data combine to create the GPS service.) services. But these signals are weak and subject to interference, both intentional and unintentional.

Out-of-domain solutions such as LEOLEO - Low Earth Orbit. LEO refers to the orbit that most artificial satellites travel within. LEO is closer to earth compared to other satellite constellations such as GPS and GEO satellite systems. LEO is approximately 1,200 miles above Earth or less, meaning the satellites in this orbit travel at high speeds (i.e. 15,000 mph) and can orbit the Earth in less than 2 hours. Most human spaceflight missions have ocurred within LEO. satellites, terrestrial wireless infrastructure, network time transfer, and signals of opportunity are necessary to provide an essential backup for PNT-dependent systems and safeguard our national critical infrastructure. The necessity of these essential backup systems was emphasized in a February 2020 PNT Executive Order.

What Does “Out of Domain” Mean?

Most PNT resources are delivered through a primary domain of government-owned and operated GNSS satellite constellations which are generally in Medium Earth Orbit (MEO), such as GPS (United States), Galileo (European Union), GLONASS (Russia), and BeiDou (China).

Alternative systems that are capable of backing up and augmenting GNSS are considered out-of-domain. The four categories of out-of-domain resources that can deliver PNT data are:

1. Low Earth Orbit (LEO) — Satellite timing and location data from an orbit in space about 25x closer than GNSS
2. Network Time Transfer — precise timing data from synchronized clocks across a high-speed computer network
3. Terrestrial Wireless Infrastructure — timing and location data from ground-based equipment and support operations across a specific geographic region
4. Signals of Opportunity — location data innovatively derived from radio signals not originally engineered for navigation purposes

The ability to leverage diverse sources of reliable PNT data, including out-of-domain resources, is essential to ensure ongoing resilience and provide an essential backup for GNSS.  These and other resilient PNT strategies will help ensure the continuous operations of PNT-dependent systems and safeguard our national critical infrastructure.

What Do These Diverse Systems Do?

LEO Systems. LEO constellations have different operational features and performance characteristics than MEO systems, such as increased signal strength and enhanced security. LEO-based solutions are also by nature global, offering worldwide coverage, 2D/3D positioning, and precise timing in one system.

Although some resilient PNT solutions in LEO constellations are still in development and others have been proposed, Satellite Time and Location (STL) from Satelles is the only alternative PNT solution in this category that is currently operating at technology readiness level (TRL) 9, with a multi-year track record of providing reliable service to customers.

Terrestrial Wireless Infrastructure. This category includes technologies that require ground-based equipment and support operations across multiple regions to achieve full coverage. Metropolitan Beacon System (MBS) is a leading technology that delivers 2D/3D location and timing for indoor and urban environments where GPS is either unavailable or significantly degraded. Available in a growing number of U.S. markets, MBS provides ultra-precise 3D positioning that can pinpoint first responders within a building.

Technologies from other providers may offer alternative PNT in the future based on signals from different types of beacons, base stations, and other equipment in various topologies. For example, Enhanced Loran (eLoran) — descended from legacy LORAN technology — requires radio transmitters and antennas nationwide for timing and significant supplemental infrastructure to support 2D location.

Network Time Transfer. These technologies deliver precise timing by synchronizing clocks across a high-speed computer network, typically relying on Gigabit Ethernet or SONET. For example, Precision Time Protocol (PTPPrecision Time Protocol is a protocol used to synchronize clocks throughout a computer network. On a LAN network, PTP can enable the clocks on each server to be synchronized within a sub-microsecond range, thus making it suitable for demanding applications that require precise timing and control. PTP is standardized within IEEE-1588v2.) solutions achieve clock accuracy in the sub-microsecond range. The White Rabbit Protocol (born out of research led by CERN) is capable of delivering precise timing in the sub-nanosecond range.

Network time transfer can be deployed worldwide but requires a high-speed connection to operate at any given location. This technology is also limited to timing applications because it does not provide location information.

Signals of Opportunity. These technologies leverage radio signals not intended for navigation. For example, they derive location information from signals emanating from devices all around us, such as LTE, Wi-Fi, and Bluetooth signals.

Signals of opportunity (SOOP) sources of PNT require specialized hardware that can acquire and process signals from across the radio spectrum. Some providers not only integrate multiple radio signals but also merge SOOP with other technologies to fine-tune 2D location — and in some cases, provide a degree of 3D positioning.

Opportunistic navigation requires a user to be in a signal-rich environment. For example, there are fewer signals in the middle of an ocean than there are in a large city, thereby making SOOP an attractive solution in urban areas. This technology is also limited to location applications because it does not provide timing information.

Benefits of LEO Satellites

LEO constellations have different operational features and performance characteristics than MEO systems, such as increased signal strength and enhanced security. Satelles’ LEO satellite-based STL service offers several distinguishing features, such as:

High Power Signals

STL signals use a unique high-power channel that, when combined with the proximity of LEO satellites (compared to GPS satellites in medium Earth orbit) and signal coding gain, produces a signal that is 1,000 times (30 dB) stronger than any GNSS signal.

Continuous Signals from Everywhere in the Sky

LEO satellites travel at speeds of about 17,000 mph (an eight-minute horizon-to-horizon transit), resulting in carrier frequency variations due to Doppler effects. With satellites rapidly traversing the sky, the continuously changing signal allows the STL transmission to reach receivers in challenging locations. STL also emanates from the Iridium constellation, whose polar orbits ensure global coverage including at both poles (where the GPS signal is weak).

3D Positioning

With multiple satellites in a LEO constellation, Satelles can provide 3D location information as well as precise timing. Compare that to wide-area ground-based PNT systems — most notably those that rely on terrestrial wireless infrastructure — which only offer the potential to deliver 2D positioning and typically require deploying supplemental infrastructure.

Less Susceptible to Attacks

Unlike land-based systems, satellite systems are effectively impervious to terrorist attacks from non-state actors, and  the impact of a state actor attack on a satellite in a constellation would have far less impact than an attack on a land-based system.

Protection from Space Phenomena

At their lower orbit altitude, LEO satellites are better protected from space disturbances (e.g., solar storms), and are far less susceptible to the effects of internal charging and surface charging, which can cause permanent damage to the electronic components of space vehicles operating in MEO or GEOGeostationary Earth Orbit is a circular orbit trajectory that is geosynchronous with Earth's rotational movement. Because it matches Earth's rotation, a satellite in a geostationary orbit appears to be stationary to an observer on the ground. orbits.[JH1] [KV2]

LEO satellites are better shielded from these phenomena because they orbit below the Earth’s magnetosphere and inner Van Allen Belt (planetary radiation belt).

Leveraging Out-of-Domain Sources of PNT to Ensure Resilience

With STL from Satelles, users gain uninterrupted access to PNT sources that backup GPS and strengthen the resilience of our national critical infrastructure. STL also offers:

Levels of stability, reliability and trust required by commercial enterprises and government entities across a range of critical infrastructure applications.

It is commercially available today, without additional investment in infrastructure. The STL antenna installs directly in the server rack — no rooftop access is needed.

Available by subscription, STL is both affordable and effective, offering urban and rural coverage everywhere.

To see if STL or LEO PNT is right for your critical infrastructure, contact us today.