Research on GPS Construction and Technology Development

WU Jun,LIU Chunbao

Beijing Institute of Space Science and Technology Information,Beijing 100094

Abstract:The United States was the first country in the world to develop a satellite navigation system,with rich experience in system management,R&D,operation,and satellite applications industry.It started the construction of the Global Positioning System (GPS) in 1973 and deployed the first satellite in 1978.It has successfully developed and deployed three series of GPS satellites with a total of seven models.The United States is now focusing on the research and development of cutting edge navigation technologies and constellation modernization,replacing old ones with new GPS III series of satellites and actively exploring and verifying the frontier navigation technologies represented through the Navigation Technology Satellite 3 (NTS-3).It is now upgrading the original ground-based operation and control system,actively developing and deploying the GPS Next Generation Operational Control System (GPS OCX),and upgrading the military user equipment supporting Military Code (M-Code).The U.S.attaches importance to multiple measures to improve the service performance of the GPS system and enhance the resilience of the system to provide positioning,navigation,and timing capabilities.In this context,the progress of the construction of GPS and the related technological innovations are separated out and analyzed,which will help the Global Navigation Satellite System (GNSS)solutions summarizing their experience and learning from each other's development to better serve social progress and economic development.

Key words:GPS,key technologies,development trends,cyber security

1 INTRODUCTION

In 2020,the landmark events that would influence the future development of the Global Positioning System (GPS) in terms of the development of key technologies for GPS and satellite navigation include:

1) The first four GPS III satellites are now all in service and modernization of the constellation is well underway[1-2].

2) L3Harris' Navigation Technology Satellite 3 (NTS-3)project has passed the critical design review (CDR) by the U.S.Air Force[3].

3) Deployment of GPS Operational Control System(OCX) Block is entering a critical phase,with the new Operational Control Segment (OCS) capabilities being implemented step by step[4].

4) Multiple measures are being taken to further improve the safety and reliability of GPS user equipment[5-6].

5) The Federal Communications Commission (FCC) approved with conditions Ligado's application to deploy a low-power terrestrial nationwide network in the L-band that will primarily support 5G and Internet of Things(IoT) services.The incident caused great controversy[7].

2 GPS III SERIES SATELLITE

GPS III satellites (as shown in Figure I) will be the mainstay of the U.S.navigation satellite system for the next two decades.Compared with the previous model,the major changes of the GPS III satellites' features and capabilities are:

1) The addition of the globally compatible signal (L1C),which is compatible with global navigation satellite systems enables to improve civilian user connectivity.

2) An increase of satellite design life to 15 years.

3) A three-fold increase in positioning accuracy and up to eight times improved anti-jamming capabilities.

4) Unlike the GPS IIF satellite,the GPS III satellite features an apogee propulsion system.

Figure 1 GPS III satellite

According to the plan,the GPS III series satellite launch mission will be completed around 2034,when the entire navigation constellation will be composed entirely of the GPS III series of satellites,including 10 GPS III satellites and 22 GPS IIIF satellites.

2.1 Deployment of the GPS III

In December 2018,the first GPS III satellite Vespucci was launched aboard a SpaceX Falcon 9 rocket.After more than one year of in-orbit tests,it has joined the GPS constellation in service on January 13,2020.

In August 2019,the second GPS III satellite Magellan was launched aboard the United Launch Alliance (ULA) Delta IV rocket and joined the GPS constellation in service on April 1,2020.After a series of in-orbit tests,the spectral densities of Magellan's navigation signals in the L1,L2,and L5 bands obtained by ground stations indicated that all signals are in good condition and that the clock stability of GPS III satellites is significantly better than that of GPS IIF satellites.

Table 1 Timeline of the first four GPS III satellites

The third GPS III satellite Columbus was successfully launched by a SpaceX Falcon 9 on June 30,2020,and bas been officially providing navigation services since July 27,2020.

The fourth GPS III satellite Columbus was successfully launched by a SpaceX Falcon 9 on November 5,2020,and has been officially providing navigation services since December 1,2020.

2.2 Production Schedule of GPS III

Production such as the Mission Data Unit (MDU) (Figure 2) of GPS III satellites at Lockheed Martin's facility near Denver continues to advance,with commercial production lines completing a range of processes from satellite assembly to testing and storage.

As of June 2020,GPS III SV05 has been completed;GPS III SV06,SV07,and SV08 have been assembled in the production plant for various post assembly tests;while GPS III SV09 and SV10 are in the component assembly process.

Figure 2 Mission Data Unit (MDU) of the eighth GPS III satellite

2.3 Development Progress of GPS IIIF

Lockheed Martin is actively advancing the GPS III Follow on(GPS IIIF) program with an expected production of 22 GPS IIIF satellites.The GPS IIIF satellite is shown in Figure 3.In March 2020,GPS IIIF completed its CDR,demonstrating a level of maturity to move to manufacture,testing,and subsequent delivery.GPS IIIF will continue to be built based on the existing modular GPS III satellites,evolving with new technologies and evolving mission requirements.GPS IIIF is expected to achieve:

1) Regional military protection capability (RMP).

2) Signal in-orbit upgrade and signal reconfiguration capability.

3) Laser retroreflector array (LRA) provides more precise ranging data.

4) Redesigned nuclear blast detection system (NDS).

Figure 3 GPS IIIF satellite

5) New search and rescue payload (SAR).

3 NAVIGATION TECHNOLOGY SATELLITE 3

NTS-3 is a precursor to the future evolution of the GPS system and is designed to demonstrate cutting edge satellite navigation technology and validation to achieve a smooth transition of the GPS system to subsequent models.The concept view of NTS-3 satellite is shown in Figure 4.

In August 2019,the NTS-3 program was selected for the U.S.Air Force Vanguard Program,which received additional resources.In February 2020,the L3Harris Technologies' NTS-3 program passed the Preliminary Design Review (PDR) that defined the spacecraft's delivery path and allowed the program to move into the next phase of development.On June 25,2020,NTS-3 passed the CDR,marking the program's entry into the production,demonstration and test phases.

The NTS-3 payload will be integrated into Northrop Grumman's ESPAStar platform,which is scheduled to be launched into geosynchronous orbit in 2022 to validate the ability to provide complementary services to the current GPS system operating in the medium circular orbit at higher orbits.During this period,the satellite will broadcast experimental signals of the new architecture Positioning,Navigation and Timing (PNT)structure to verify the ability to improve signal anti-jamming performance and flexibility to adapt to complex requirements.The NTS-3 program will also demonstrate advanced operational command and control and agile software defined radio capabilities for the U.S.Army,including in-orbit upgrades and signal reconfiguration,where the related validation technologies will be applied to subsequent GPS satellite models.

Figure 4 Concept view of NTS-3 satellite

In addition,the NTS-3 program has attracted the interest of countries such as the United Kingdom and South Korea.They are closely following the work of the NTS-3 program and are considering the relevance of these efforts to their own navigation constellations.

4 THE NEXT GENERATION OPERATIONAL CONTROL SYSTEM

The goal of GPS OCX is to fully meet the operation,control,and management requirements of the space segment of the modernized GPS system and to ensure the operational safety of the GPS system.In addition to supporting all the functions and capabilities of the satellites in space segment,its most important capability increment is the cyber security capability.Operation and maintenance of the OCX system are shown in Figure 5.

4.1 Application of the Ground Operational Control System under OCX Block 0 Framework

The followings are the descriptions of the process and application of the OCX Block 0 system in terms of the GPS III Launch and Checkout System (LCS),the GPS III Contingency Operations System (COps),and the M-Code Early Use(MCEU).

1) GPS COps received operational acceptance

GPS III COps will fly on the GPS III satellites to be brought into the operational constellation,sustaining current levels of performance and avoiding degradation.It is a modification to the current control segment to operate GPS III satellites' PNT and NDS payloads and maintain limited test M-Code capability until OCX Block 1 delivery.

In June 2019,delivery,support and final testing of the GPS III COps system for the OCX Block 0 system was completed,and the COps was installed at the GPS system's Master Control Station in Colorado and at an Alternate Master Control Station in California.

Figure 5 Operation and maintenance of the OCX system

In March 2020,the Lockheed Martin developed GPS COps upgrade projects was accepted.

2) GPS M-Code passed operational acceptance for early use

The US Space and Missile Systems Center (SMC) passed the operational acceptance of the GPS MCEU on November 18,2020.The acceptance test activities were completed at the Master Control Station at Schriever Air Force Base,Colorado,and the Alternate Master Control Station at Vandenberg Air Force Base,California.Back in July 2020,SMC completed hardware and software modifications such as the corresponding software defined receivers,which now have been installed in six monitoring stations of the GPS system,enabling the current GPS ground control system to distribute,upload,and monitor M-Code messaging or signals within the space segment constellation,which will be used to support the testing of new military GPS user equipment (MGUE).The acceptance of the MCEU enables the ground OCS for the entire GPS constellation to fill the gap for M-Code operations until the full transition to the introduction of the OCX Block 1.The M-Code is the next generation of the US Army's GPS military code,which is more resistant to interference.

3) GPS III LCS supported the launch of the first four GPS III satellites

LCS module of the OCX Block 0 system has supported the launch and early orbital operations of the first four GPS III satellites,as well as in-orbit checkout and test work.

4.2 Application and Development Progress of Ground OCS in OCX Block 1 Framework

Operational control of the entire constellation based on OCX Block 1 included the GPS III (Fly Constellation &GPS III)related validation test.The Space Operations Squadron performed the first station keeping maneuver on the second GPS III satellite Magellan on May 14 at the Schriever Air Force Base in Colorado to validate the Architecture Evolution Plan(AEP) system's ability to operate and control a new model of GPS satellite for orbital maneuvers.

GPS OCX further improves the accuracy of the entire constellation,reduces manpower costs through a higher degree of automation,provides a better response to abnormal conditions in the constellation,further reduces the response time,and improves cyber safety.

The maneuver action required GPS III's signal to be turned off.Turning off the navigation signal while performing the maneuver prevented users from receiving inaccuracies generated by a change in satellite velocity.The maneuver was completed in a special mission area and there were no changes to the normal operations area.This maneuver did no effect the rest of the GPS satellite constellation.

5 MULTI PRONGED APPROACH TO ENHANCE GPS SYSTEM CYBER SECURITY CAPABILITIES

The Space Threat Assessment 2020 published in March 2020 by the Center for Strategic and International Studies(CSIS) well organized the catalog of ways that the essential space-based services Americans rely upon can be degraded or eliminated.The US space assets are in danger from an array of kinetic,non-kinetic,electronic,and cyber threats.

The report identifies jamming and spoofing as the most realistic threat,with 188 times;ASATs (anti satellite weapons) and direct ascent weapons (direct ascent) follow with 179 references.Of all types of satellites that could be threatened,GPS/GNSS (98 mentions) was the most frequent,followed by communications (42 mentions) and reconnaissance satellites(3 mentions) respectively.

5.1 United States' SMC Validates and Improves GPS Cyber Security Capabilities with Digital Twin Technology

In April 2020,SMC conducted a GPS system test for cyber security vulnerabilities using digital twin technology,a move to test and evaluate the GPS system's cyber security capabilities.The US launched 12 GPS IIR satellites between 1997 and 2004,with an initial design life of 7.5 years,of which 8 are still in orbit beyond their service life.While Booz Allen Consulting(BAH) completed the GPS IIR type satellite digital modeling work in 2018,the research team is actively working on testing and evaluating the various subsystems of the intrusion model to find current GPS system cyber vulnerabilities through a series of vulnerability scans and penetration tests.SMC is committed to researching approaches and solutions to detect existing vulnerabilities and to make fixes and improvements in the current satellite project.

5.2 Microchip Upgrades the Firewall Product Software Version of GNSS Signal Receiver

In June 2020,Microchip Technology Inc.released an updated version of its BlueSky GNSS Firewall product software.Version 2.0 of Microchip's BlueSky GNSS Firewall software provides real time analysis of incoming GPS signals,detection of interference and spoofing,enhanced response and fault recovery capabilities,and an increased ability to avoid signal outages.Such upgrades are important for critical infrastructure operators to protect against GPS vulnerabilities and ensure continuous service operation.Such critical infrastructure systems include power utilities,financial services,mobile networks,and transportation,which all rely on GPS delivered timing to ensure ongoing operations.

The 2.0 version BlueSky GNSS Firewall software (Figure 6) can be installed on BlueSky GNSS Firewall devices.When connected to Microchip's TimePictra management software,the critical infrastructure operators also can monitor and analyze GNSS signals at home and abroad.Specific features include a graphical display and advanced threshold settings for GNSS observations such as satellites in view,carrier to noise,position dispersion,and phase time deviation radio frequency (RF) power levels and so on.The upgrade further simplifies the system start up and deployment process.

6 CHALLENGES FOR GPS SERVICE

On April 20 2020,the US FCC announced that it has approved Ligado's application with conditions to deploy a low power terrestrial nationwide network in the L-band that will primarily support 5G and IoT services.

Ligado Networks (Ligado) sought to use a portion of L-band spectrum that was formerly allocated for satellite use to build a terrestrial 5G network focused on IoT services for smart devices.In May 2018,Ligado made significant changes to its spectrum use proposal,as shown in Figure 7.

1) Ligado significantly reduced the power of its base station from the originally requested 32 dBW to 9.8 dBW(a 99.3% reduction).

2) Ligado withdrew its previous application for use of the 1545-1555 MHz band in order to carve out a wider protected band between its downlink operation and the frequencies used by GPS,i.e.to provide a significant (23 MHz) protected band using its own licensed spectrum to achieve spectrum separation.

Figure 6 Data interface of Microchip's 2.0 version BlueSky GNSS Firewall software

Figure 7 Proposal of Ligado (L-band spectrum) changed in May 2018

3) Ligado currently seeks terrestrial use only in the 1526-1536 MHz,1627.5-1637.5 MHz and 1646.5-1656.5 MHz frequency bands.

4) Uninterrupted 24/7 monitoring of its base station transmit power levels and the development of detailed protection mechanisms.

The move immediately sparked significant controversy within the US.In response to the FCC's decision,at least 12 federal agencies,including the U.S.Department of Defense (DoD),Department of Transportation (DoT),Department of Homeland Security (DHS),and the GPS business community,have pushed back hard,with nearly 100 organizations and dozens of members of Congress.They cited the use of adjacent GPS signal (L1) band networks on the ground as seriously interfering with the normal use of GPS signals and receivers,and endangering national security.They has issued statements or written to the FCC requesting that the decision be reversed,but to no avail,leading to a series of heated exchanges within the US Congress.

The Ligardo affair continued to fester,sparking a controversy that spilled over from all sides to Congress,resulting with the House and Senate passing the National Defense Authorization Act (NDAA) for Fiscal Year 2021 in late July,a version of the bill included several provisions directly related to the FCC's decision.Among its provisions in the Act:

1) Requires the DoD to estimate and report to Congress the cost of damage to department systems as a result of the FCC order.

2) Prohibits using department funds to upgrade or modify military equipment to make it resilient to interference caused by broadcasts in the spectrum allocated (the FCC order requires this to be funded by Ligado).

3) Prohibits contracting with any entity using the frequency bands allocated to Ligado unless the Secretary of Defense certifies the use will not interfere with GPS services.

4) Requires the Secretary of Defense to contract with the National Academies of Sciences,Engineering,and Medicine for an independent technical review of the FCC order.

7 CONCLUSION

In summary,it can be seen that the GPS system continues to enhance its leading position in the field of global satellite navigation by technological innovation,while at the same time placing great emphasis on building cyber security capabilities.

7.1 The Role of Satellite Navigation Systems is Significant,but There Are Certain Vulnerabilities in the Services They Provide

The satellite radio navigation signals are already very weak by the time they reach the ground,and receivers need to process complex signals and decode them.In the case of Ligado,there is a greater diversity of receiver types for civilian users and fewer risk reduction options than for military users.The debate between the US Congress,the DoD,the FCC,Ligardo,and others surrounding the terrestrial deployment of L-band 5G network facilities is a primary example of the US' heavy reliance on GPS.

7.2 Technological Innovation and Breakthroughs are the Key to Development of Future Satellite Navigation Capabilities

In recent years,the U.S.has placed great emphasis on technologies such as in-orbit digital waveform generators,advanced high-gain area enhancement antennas,high-performance onboard atomic clocks,and management and control of the GPS constellation through the Inter-Satellite Link (ISL).These technologies will be further demonstrated in satellite models such as GPS IIIF and NTS-3.Research on providing navigation augmentation services based on LEO constellations is also being explored.The above technological innovations and breakthroughs will be the main way to enhance the resilience of the navigation satellite system itself and the services it provides,which will influence and even determine the future global competitiveness of the system as future application scenarios change.

7.3 Cyber Security and Information Assurance are Important Safeguards for the Control of Satellite Navigation System

Cyber security and information assurance are important guarantees for the autonomy and controllability of satellite navigation systems.The ground operational control system and its communications links with the space constellation,and the inter-satellite links between the satellites of the space constellation all belong in the cyberspace category,and their cyber security capability determines whether the system can operate or work in a way that meets demand.