1. Overview

The United States' Global Positioning System (GPS) reached Fully Operational Capability on July 17, 1995,^[1] completing its original design goals. However, additional advances in technology and new demands on the existing system led to the effort to modernize the GPS system. Announcements from the Vice President and the White House in 1998 initiated these changes. In 2000, the U.S. Congress authorized the effort, referred to as GPS III.

The project involves new ground stations and new satellites, with additional navigation signals for both civilian and military users, and aims to improve the accuracy and availability for all users.

Raytheon was awarded the Next Generation GPS Operational Control System (OCX) contract on Feb 25, 2010.^[2]

2. Development

Block IIIA satellites use Lockheed Martin's A2100 bus structure. The propellant and pressurant tanks are manufactured by Orbital ATK from lightweight, high-strength composite materials.^[3] Each satellite will carry 8 deployable JIB antennas designed and manufactured by Northrop Grumman Astro Aerospace^[4]

The first GPS III satellite was originally scheduled for launch in 2014,^[5] but in December 2016 the Director of the U.S. Air Force's Global Positioning Systems Directorate announced the first satellite will launch in the spring of 2018.^[6] In March 2017, the US General Accounting Office stated "Technical issues with both the GPS III satellite and the OCX Block 0 launch control and checkout system have combined to place the planned March 2018 launch date for the first GPS III satellite at risk."^[7] The delays have been caused by a number of factors, primarily due to issues found in the navigation payload.^[8][9] Further launch date slippages were caused by the need to complete qualification testing and validation of the new SpaceX Falcon 9 rocket modification that is planned to deliver the first GPS III satellite to its orbit. As of August 2018, this launch is scheduled for December 2018.^[10]

Future Block III variants are planned to incorporate additional capabilities. They include Distress Alerting Satellite System (DASS) capabilities for search and rescue, as well as satellite crosslinks for rapid command and reduced age of data.^[11]

On April 27, 2016, Space Exploration Technologies Corp. (SpaceX), in Hawthorne, California was awarded an $82,700,000 firm-fixed-price contract for launch services to deliver a GPS III satellite to its intended orbit. This launch service contract will include launch vehicle production, mission integration, and launch operations for a GPS III mission. The locations of performance are Hawthorne, California; Cape Canaveral Air Force Station, Florida and McGregor, Texas. The work is expected to be completed by July 31, 2018.^[12]

On September 21, 2016, the U.S. Air Force exercised a $395 million contract option with Lockheed Martin for the ninth and tenth Block IIIA space vehicles, expected to be available for launch by 2022.^[13]

3. New Navigation Signals

3.1. Civilian L2 (L2C)

One of the first announcements was the addition of a new civilian-use signal to be transmitted on a frequency other than the L1 frequency used for the existing GPS Coarse Acquisition (C/A) signal. Ultimately, this became known as the L2C signal because it is broadcast on the L2 frequency (1227.6 MHz). It can be transmitted by all block IIR-M and later design satellites. The original plan stated that until the new OCX (Block 1) system is in place, the signal would consist of a default message ("Type 0") that contains no navigational data.^[14] OCX Block 1 with the L2C navigation data was scheduled to enter service in February 2016,^[15][16] but has been delayed until 2022 or later.^[17]

As a result of OCX delays, the L2C signal was decoupled from the OCX deployment schedule. All satellites capable of transmitting the L2C signal (all GPS satellites launched since 2005^[14]) began broadcasting pre-operational civil navigation (CNAV) messages in April 2014, and in December 2014 the Air Force started transmitting CNAV uploads on a daily basis.^[14][18] The L2C signal will be considered fully operational after it is being broadcast by at least 24 space vehicles, currently projected to happen in 2021.^[14] As of October 2017, L2C was being broadcast from 19 satellites.^[14]

The L2C signal is tasked with providing improved accuracy of navigation, providing an easy-to-track signal, and acting as a redundant signal in case of localized interference.

The immediate effect of having two civilian frequencies being transmitted from one satellite is the ability to directly measure, and therefore remove, the ionospheric delay error for that satellite. Without such a measurement, a GPS receiver must use a generic model or receive ionospheric corrections from another source (such as a Satellite Based Augmentation System). Advances in technology for both the GPS satellites and the GPS receivers have made ionospheric delay the largest source of error in the C/A signal. A receiver capable of performing this measurement is referred to as a dual frequency receiver. The technical characteristics of it are:

• L2C contains two distinct PRN sequences:
  • CM (for Civilian Moderate length code) is 10,230 bits in length, repeating every 20 milliseconds.
  • CL (for Civilian Long length code) is 767,250 bits, repeating every 1,500 milliseconds (i.e., every 1.5 s).
  • Each signal is transmitted at 511,500 bits per second (bit/s); however, they are multiplexed to form a 1,023,000 bit/s signal.
• CM is modulated with a 25 bit/s navigation message with forward error correction, whereas CL contains no additional modulated data.
• The long, non-data CL sequence provides for approximately 24 dB greater correlation protection (~250 times stronger) than L1 C/A.
• L2C signal characteristics provide 2.7 dB greater data recovery and 0.7 dB greater carrier tracking than L1 C/A.
• The L2C signals' transmission power is 2.3 dB weaker than the L1 C/A signal.
• In a single frequency application, L2C has 65% more ionospheric error than L1.

It is defined in IS-GPS-200.^[19]

3.2. Military (M-Code)

A major component of the modernization process, a new military signal called M-code was designed to further improve the anti-jamming and secure access of the military GPS signals. The M-code is transmitted in the same L1 and L2 frequencies already in use by the previous military code, the P(Y) code. The new signal is shaped to place most of its energy at the edges (away from the existing P(Y) and C/A carriers).

Unlike the P(Y) code, the M-code is designed to be autonomous, meaning that users can calculate their positions using only the M-code signal. P(Y) code receivers must typically first lock onto the C/A code and then transfer to lock onto the P(Y)-code.

In a major departure from previous GPS designs, the M-code is intended to be broadcast from a high-gain directional antenna, in addition to a wide angle (full Earth) antenna. The directional antenna's signal, termed a spot beam, is intended to be aimed at a specific region (i.e., several hundred kilometers in diameter) and increase the local signal strength by 20 dB (10× voltage field strength, 100× power). A side effect of having two antennas is that, for receivers inside the spot beam, the GPS satellite will appear to be two GPS satellites occupying the same position.

While the full-Earth M-code signal is available on the Block IIR-M satellites, the spot beam antennas will not be available until the Block III satellites are deployed. Like the other new GPS signals, M-code is dependent on OCX—specifically Block 2—which is scheduled to enter service in October 2016,^[16][20] but that date does not reflect the two year first satellite launch delay expected by the GAO.^[21][22] Other M-code characteristics are:

• Satellites will transmit two distinct signals from two antennas: one for whole Earth coverage, one in a spot beam.
• Binary offset carrier modulation
• Occupies 24 MHz of bandwidth
• It uses a new MNAV navigational message, which is packetized instead of framed, allowing for flexible data payloads
• There are four effective data channels; different data can be sent on each frequency and on each antenna.
• It can include FEC and error detection
• The spot beam is ~20 dB more powerful than the whole Earth coverage beam
• M-code signal at Earth's surface: –158 dBW for whole Earth antenna, –138 dBW for spot beam antennas.

3.3. Safety of Life (L5)

Safety of Life is a civilian-use signal, broadcast on the L5 frequency (1176.45 MHz). In 2009, a WAAS satellite sent the initial L5 signal test transmissions. SVN-62, the first GPS block IIF satellite, continuously broadcast the L5 signal starting on June 28, 2010.

As a result of schedule delays to the GPS III control segment, the L5 signal was decoupled from the OCX deployment schedule. All satellites capable of transmitting the L5 signal (all GPS satellites launched since May 2010^[23]) began broadcasting pre-operational civil navigation (CNAV) messages in April 2014, and in December 2014 the Air Force started transmitting CNAV uploads on a daily basis.^[24] The L5 signal will be considered fully operational once at least 24 space vehicles are broadcasting the signal, currently projected to happen in 2024.^[23] As of April 18, 2017, L5 was being broadcast from 12 satellites.^[23]

• Improves signal structure for enhanced performance
• Higher transmission power than L1 or L2C signal (~3 dB, or twice as powerful)
• Wider bandwidth, yielding a 10-times processing gain
• Longer spreading codes (10 times longer than used on the C/A code)
• Located in the Aeronautical Radionavigation Services band, a frequency band that is available worldwide.

WRC-2000 added space signal component to this aeronautical band so the aviation community can manage interference to L5 more effectively than L2. It is defined in IS-GPS-705.^[25]

3.4. New Civilian L1 (L1C)

L1C is a civilian-use signal, to be broadcast on the same L1 frequency (1575.42 MHz) that contains the C/A signal used by all current GPS users.

L1C broadcasting will start when GPS III Control Segment (OCX) Block 1 becomes operational, currently scheduled for 2022.^[6][17] The L1C signal will reach full operational status when being broadcast from at least 24 GPS Block III satellites, currently projected for the late 2020s.^[26]

• Implementation will provide C/A code to ensure backward compatibility
• Assured of 1.5 dB increase in minimum C/A code power to mitigate any noise floor increase
• Non-data signal component contains a pilot carrier to improve tracking
• Enables greater civil interoperability with Galileo L1

It is defined in IS-GPS-800.^[27]

4. Improvements

Increased signal power at the Earth's surface

• M-code: −158 dBW / −138 dBW.
• L1 and L2: −157 dBW for the C/A code signal and −160 dBW for the P(Y) code signal.
• L5 will be −154 dBW.

Researchers from The Aerospace Corporation confirmed that the most efficient means to generate the high-power M-code signal would entail a departure from full-Earth coverage, characteristic of all the user downlink signals up until that point. Instead, a high-gain antenna would be used to produce a directional spot beam several hundred kilometers in diameter. Originally, this proposal was considered as a retrofit to the planned Block IIF satellites. Upon closer inspection, program managers realized that the addition of a large deployable antenna, combined with the changes that would be needed in the operational control segment, presented too great a challenge for the existing system design.^[28]

• NASA has requested that Block III satellites carry laser retro-reflectors.^[29] This allows tracking the orbits of the satellites independent of the radio signals, which allows satellite clock errors to be disentangled from ephemeris errors. This, a standard feature of GLONASS, will be included in the Galileo positioning system, and was included as an experiment on two older GPS satellites (satellites 35 and 36).^[30]
• The USAF is working with NASA to add a Distress Alerting Satellite System (DASS) payload to the second increment of GPS III satellites as part of the MEOSAR search and rescue system.^[31]

5. Control Segment

The GPS Operational Control Segment (OCS),^[32] consisting of a worldwide network of satellite operations centers, ground antennas and monitoring stations, provides Command and Control (C2) capabilities for GPS Block II satellites. The latest update to the GPS OCS, Commercial Off-The-Shelf Upgrade #2 (CUP2), was completed on December 1, 2016.^[33]

5.1. Next-Generation Operational Control Segment (OCX)

In 2010, the United States Air Force announced plans to develop a modern control segment, a critical part of the GPS modernization initiative. OCS will continue to serve as the ground control system of record until the new system, Next Generation GPS Operational Control System^[34] (OCX), is fully developed and functional.

OCX features are being delivered to the United States Air Force in three separate phases, known as "blocks."^[35] The OCX blocks are numbered zero through two. With each block delivered, OCX gains additional functionality.

In June 2016, the U.S. Air Force formally notified Congress the OCX program's projected program costs had risen above $4.25 billion, thus exceeding baseline cost estimates of $3.4 billion by 25%, also known as a critical Nunn-McCurdy breach. Factors leading to the breach include "inadequate systems engineering at program inception", and "the complexity of cybersecurity requirements on OCX."^[36] In October 2016, the Department of Defense formally certified the program, a necessary step to allow development to continue after a critical breach.^[37]

OCX Block 0 (Launch and Checkout System)

OCX Block 0 provides the minimum subset of full OCX capabilities necessary to support launch and early on-orbit spacecraft bus checkout on GPS III space vehicles.^[6]

In August 2017, OCX Block 0 was used in the first live-satellite GPS III launch rehearsal.^[38]

The U.S. Air Force accepted the delivery of OCX Block 0 in November 2017, and is using it to prepare for the first GPS launch in 2018.^[39]

OCX Block 1 (Civilian GPS III features)

OCX Block 1 is an upgrade to OCX Block 0, at which time the OCX system achieves Initial Operating Capability (IOC). Once Block 1 is deployed, OCX will for the first time be able to command and control both Block II and Block III GPS satellites, as well support the ability to begin broadcasting the civilian L1C signal.^[6]

In November 2016, the GAO reported that OCX Block 1 had become the primary cause for delay in activating the GPS III PNT mission.^[40]

As of April 2017, OCX Block 1 development was underway.^[41] Block 1 is scheduled for delivery to the United States Air Force in late 2020, and is projected to achieve Ready To Operate (RTO) status in January 2022.^[17][42][43]

OCX Block 2 (Military GPS III features, Civilian Signal Monitoring)

OCX Block 2 upgrades OCX with the advanced M-code features for military users and the ability to monitor performance of the civilian signals.^[35] In March 2017, the contractor rephased its OCX delivery schedule so that Block 2 will now be delivered to the Air Force concurrently with Block 1.^[42] In July 2017, an additional nine months delay to the schedule was announced. According to the July 2017 program schedule, OCX will be delivered to the Air Force in April 2022.^[44]

5.2. Contingency Operations

GPS III Contingency Operations ("COps") is an update to the GPS Operational Control Segment, allowing OCS to provide Block IIF Position, Navigation, and Timing (PNT) features from GPS III satellites.^[6] The Contingency Operations effort enables GPS III satellites to participate in the GPS constellation, albeit in a limited fashion, without having to wait until OCX Block 1 becomes operational (currently scheduled for 2022).

The United States Air Force awarded the 96 million dollar Contingency Operations contract in February 2016.^[45] The GPS Contingency Operations program successfully completed its Critical Design Review (CDR) in November 2016.^[46] Final delivery to the United States Air Force is projected to occur in 2019.^[38]

5.3. Deployment Schedule

6. Satellites

The ten GPS Block IIIA Space Vehicles are scheduled for launch beginning in December 2018, continuing through at least 2023.^[38][43][47]