Artist's impression of a GPS Block III satellite in orbit
|Country of origin||United States|
|Operator||US Space Force|
|Bus||Lockheed Martin A2100|
|Design life||15 years (planned)|
|Launch mass||3,880 kg (8,550 lb) |
|Dry mass||2,269 kg (5,002 lb)|
|Power||4480 watts (end of life)|
|Regime||Semi-synchronous Medium Earth orbit|
|On order||10 |
|Maiden launch||23 December 2018 |
|Last launch||5 November 2020 |
|Derived from||GPS Block IIF|
GPS Block III (previously Block IIIA) consists of the first ten GPS III satellites, which will be used to keep the Navstar Global Positioning System operational. Lockheed Martin designed, developed and manufactured the GPS III Non-Flight Satellite Testbed (GNST) and all ten Block III satellites. The first satellite in the series was launched in December 2018. The tenth and final GPS Block III launch is projected in 2023.
The United States' Global Positioning System (GPS) reached Full Operational Capability on 17 July 1995, 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. 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.
Block III satellites use Lockheed Martin's A2100 satellite bus structure. The propellant and pressurant tanks are manufactured by Orbital ATK from lightweight, high-strength composite materials. Each satellite will carry 8 deployable JIB antennas designed and manufactured by Northrop Grumman Astro Aerospace
Already delayed significantly beyond the first satellite's planned 2014 launch, on 27 April 2016, SpaceX, in Hawthorne, California, was awarded a US$82.7 million firm-fixed-price contract for launch services to deliver a GPS III satellite to its intended orbit. The contract included launch vehicle production, mission integration, and launch operations for a GPS III mission, to be performed in Hawthorne, California; Cape Canaveral Air Force Station, Florida; and McGregor, Texas. In December 2016, the Director of the U.S. Air Force's Global Positioning Systems Directorate announced the first satellite would launch in the spring of 2018. 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". The delays were caused by a number of factors, primarily due to issues found in the navigation payload. Further launch date slippages were caused by the need for additional testing and validation of a SpaceX Falcon 9 which ultimately launched the satellite on 23 December 2018. On 22 August 2019, the second GPS III satellite was launched aboard a Delta IV.
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.
On 21 September 2016, the U.S. Air Force exercised a US$395 million contract option with Lockheed Martin for the ninth and tenth Block III space vehicles, expected to be available for launch by 2022.
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. OCX Block 1 with the L2C navigation data was scheduled to enter service in February 2016, but has been delayed until 2022 or later.
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) 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. 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. As of October 2017, L2C was being broadcast from 19 satellites. 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. Its technical characteristics are:
It is defined in IS-GPS-200.
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 as two GPS signals 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 was scheduled to enter service in October 2016, but which has been delayed until 2022, and that initial date did not reflect the two year first satellite launch delay expected by the GAO.
Other M-code characteristics are:
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 28 June 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) 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. The L5 signal will be considered fully operational once at least 24 space vehicles are broadcasting the signal, currently projected to happen in 2024.
As of 18 April 2017, L5 was being broadcast from 12 satellites.
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.
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. 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.
It is defined in IS-GPS-800.
Increased signal power at the Earth's surface:
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.
The GPS Operational Control Segment (OCS), 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, Architectural Evolution Plan 7.5, was installed on 16 November 2018.
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 (OCX), is fully developed and functional.
OCX features are being delivered to the United States Air Force in three separate phases, known as "blocks". 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 US$4.25 billion, thus exceeding baseline cost estimates of US$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". In October 2016, the Department of Defense formally certified the program, a necessary step to allow development to continue after a critical breach.
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.
Block 0 completed two cybersecurity testing events in April and May 2018 with no new vulnerabilities found.
In June 2018, Block 0 had its third successful integrated launch rehearsal with GPS III.
The U.S. Air Force accepted the delivery of OCX Block 0 in November 2017, and is used it to prepare for the first GPS launch in December 2018.
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 as support the ability to begin broadcasting the civilian L1C signal.
In November 2016, the GAO reported that OCX Block 1 had become the primary cause for delay in activating the GPS III PNT mission.
Block 1 completed the final iteration of Critical Design Review (CDR) in September 2018. Software development on Block 1 is scheduled to complete in 2019, after which the Block 1 software will undergo 2.5 years of system testing.
OCX Block 2 upgrades OCX with the advanced M-code features for military users and the ability to monitor performance of the civilian signals. 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. 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.
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. 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 US$96 million Contingency Operations contract in February 2016. As of September 2018, software development was complete, and component integration testing was scheduled to be completed the next month. Operational acceptance testing is scheduled for January 2020.
|Command & Control||Satellites Delivering Navigation Data|
|December 2018 ||OCX Block 0||Block II||Block III
(Launch and Checkout only) 
|Block II||OCS and OCX operate in parallel|
|January 2020 ||Contingency Operations||Block II|
|January 2023 ||OCX Block 1 and OCX Block 2||Block II & Block III||OCS no longer used, L1C transmissions begin, full GPS III functionality achieved.|
|Library resources about |
GPS Block III