GPS modernization
Encyclopedia
The United States' Global Positioning System
(GPS), having reached Fully Operational Capability on July 17, 1995, has completed 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, 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.
Lockheed Martin
was awarded the GPS III Space Segment contract on May 15, 2008. The first launch is projected for 2014. Raytheon
was awarded the Next Generation GPS Operational Control System (OCX) contract on Feb 25, 2010.
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:
It is defined in IS-GPS-200.
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 (10x voltage field strength, 100x power). A side effect of having two antennas is that the GPS satellite will appear to be two GPS satellites occupying the same position to those inside the spot beam.
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, tentatively in 2013.
Other M-code characteristics are:
, the first GPS block IIF satellite, continuously broadcast the L5 signal starting on June 28, 2010.
WRC-2000 added space signal component to this aeronautical band so aviation community can manage interference to L5 more effectively than L2
It is defined in IS-GPS-705.
It is defined in IS-GPS-800.
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 Operation Control Segment (OCS) currently serves as the control segment of record. It provides the operational capability that supports global GPS users and keeps the GPS system operational and performing within specification.
OCS successfully replaced the legacy 1970’s-era mainframe computer at Schriever Air Force Base in September 2007. After installation, the system helped enable upgrades and provide a foundation for a new security architecture that supported our troops.
In 2010, the United States Air Force announced plans to develop a modern control segment, which would act as 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 Operation Control System (OCX), is fully developed and functional.
The new capabilities provided by OCX will be the cornerstone for revolutionizing GPS’s mission capabilities, and enabling Air Force Space Command to greatly enhance GPS operational services to our nation's combat forces, civil partners and myriad of domestic and international users.
The GPS OCX program also will reduce cost, schedule and technical risk. It is designed to provide 50% sustainment cost savings through efficient software architecture and Performance-Based Logistics. In addition, GPS OCX expected to cost millions less than the cost to upgrade OCS while providing four times the capability.
The GPS OCX program represents a critical part of GPS modernization and provides significant information assurance improvements over the current GPS OCS program.
On September 14, 2011, the U.S. Air Force announced the completion of GPS OCX Preliminary Design Review and confirmed that the OCX program is ready for the next phase of development.
The GPS OCX program has achieved major milestones and is on track to support the GPS IIIA launch in May 2014.
Global Positioning System
The Global Positioning System is a space-based global navigation satellite system that provides location and time information in all weather, anywhere on or near the Earth, where there is an unobstructed line of sight to four or more GPS satellites...
(GPS), having reached Fully Operational Capability on July 17, 1995, has completed 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, 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.
Lockheed Martin
Lockheed Martin
Lockheed Martin is an American global aerospace, defense, security, and advanced technology company with worldwide interests. It was formed by the merger of Lockheed Corporation with Martin Marietta in March 1995. It is headquartered in Bethesda, Maryland, in the Washington Metropolitan Area....
was awarded the GPS III Space Segment contract on May 15, 2008. The first launch is projected for 2014. Raytheon
Raytheon
Raytheon Company is a major American defense contractor and industrial corporation with core manufacturing concentrations in weapons and military and commercial electronics. It was previously involved in corporate and special-mission aircraft until early 2007...
was awarded the Next Generation GPS Operational Control System (OCX) contract on Feb 25, 2010.
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 is transmitted by all block IIR-M and later design 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. 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 millisecondMillisecondA millisecond is a thousandth of a second.10 milliseconds are called a centisecond....
s. - CL (for Civilian Long length code) is 767,250 bits, repeating every 1500 millisecondMillisecondA millisecond is a thousandth of a second.10 milliseconds are called a centisecond....
s (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 (for Civilian Moderate length code) is 10,230 bits in length, repeating every 20 millisecond
- CM is modulated with a 25 bit/s navigation message with forward error correctionForward error correctionIn telecommunication, information theory, and coding theory, forward error correction or channel coding is a technique used for controlling errors in data transmission over unreliable or noisy communication channels....
, whereas CL is a non-data sequence (it does not contain additional modulated data). - The long, non-data CL sequence provides for approximately 24 dBDecibelThe decibel is a logarithmic unit that indicates the ratio of a physical quantity relative to a specified or implied reference level. A ratio in decibels is ten times the logarithm to base 10 of the ratio of two power quantities...
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.
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
Directional antenna
A directional antenna or beam antenna is an antenna which radiates greater power in one or more directions allowing for increased performance on transmit and receive and reduced interference from unwanted sources....
, 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 (10x voltage field strength, 100x power). A side effect of having two antennas is that the GPS satellite will appear to be two GPS satellites occupying the same position to those inside the spot beam.
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, tentatively in 2013.
Other M-code characteristics are:
- Satellites will transmit two distinct signals from two antennas: one for whole Earth coverage, one in a spot beam.
- Modulation is binary offset carrier
- 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.
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-62USA-213
USA-213, also known as GPS SVN-62 and GPS IIF SV-1 is the first satellite in the Block IIF series of Global Positioning System navigation satellites. It will be used to relay signals for the United States Air Force Navstar Global Positioning System . The satellite was launched at 03:00 GMT on 28...
, the first GPS block IIF satellite, continuously broadcast the L5 signal starting on June 28, 2010.
- Improves signal structure for enhanced performance
- Higher transmission power than L1 or L2C signal (~3dB, 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 world wide.
WRC-2000 added space signal component to this aeronautical band so aviation community can manage interference to L5 more effectively than L2
It is defined in IS-GPS-705.
New Civilian L1 (L1C)
L1C is a civilian-use signal, to be broadcast on the same L1 frequency (1575.42 MHz) that currently contains the C/A signal used by all current GPS users. The L1C will be available with first Block III launch, currently scheduled for 2013.- 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 carrierPilot signalIn telecommunications, a pilot is a signal, usually a single frequency, transmitted over a communications system for supervisory, control, equalization, continuity, synchronization, or reference purposes....
to improve tracking - Enables greater civil interoperability with GalileoGalileo positioning systemGalileo is a global navigation satellite system currently being built by the European Union and European Space Agency . The €20 billion project is named after the famous Italian astronomer Galileo Galilei...
L1
It is defined in IS-GPS-800.
Block III satellite 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
The Aerospace Corporation
The Aerospace Corporation is a private, non-profit corporation headquartered in El Segundo, California that has operated a Federally Funded Research and Development Center for the United States Air Force since 1960...
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
- NASA has requested that Block III satellites carry laser retro-reflectorsRetroreflectorA retroreflector is a device or surface that reflects light back to its source with a minimum scattering of light. An electromagnetic wave front is reflected back along a vector that is parallel to but opposite in direction from the wave's source. The device or surface's angle of incidence is...
. This allows tracking the orbits of the satellites independent of the radio signals, which allows satellite clock errors to be disentangled from ephemerisEphemerisAn ephemeris is a table of values that gives the positions of astronomical objects in the sky at a given time or times. Different kinds of ephemerides are used for astronomy and astrology...
errors. This is a standard feature of GLONASSGLONASSGLONASS , acronym for Globalnaya navigatsionnaya sputnikovaya sistema or Global Navigation Satellite System, is a radio-based satellite navigation system operated for the Russian government by the Russian Space Forces...
, will be included in the Galileo positioning systemGalileo positioning systemGalileo is a global navigation satellite system currently being built by the European Union and European Space Agency . The €20 billion project is named after the famous Italian astronomer Galileo Galilei...
, and was included as an experiment on two older GPS satellites (satellites 35 and 36).
- The USAF is working with NASANASAThe National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...
to add a DASSCospas-SarsatCospas-Sarsat is an international satellite-based search and rescue distress alert detection and information distribution system, established by Canada, France, the United States, and the former Soviet Union in 1979. It is best known as the system that detects and locates emergency beacons...
payload to the second increment of GPS III satellites as part of the MEOSAR system.
Ground Control Segment Improvements
The control segment determines the orbital position of satellites and transmits information to satellites in space to keep the GPS system operational and performing within specification.The Operation Control Segment (OCS) currently serves as the control segment of record. It provides the operational capability that supports global GPS users and keeps the GPS system operational and performing within specification.
OCS successfully replaced the legacy 1970’s-era mainframe computer at Schriever Air Force Base in September 2007. After installation, the system helped enable upgrades and provide a foundation for a new security architecture that supported our troops.
In 2010, the United States Air Force announced plans to develop a modern control segment, which would act as 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 Operation Control System (OCX), is fully developed and functional.
The new capabilities provided by OCX will be the cornerstone for revolutionizing GPS’s mission capabilities, and enabling Air Force Space Command to greatly enhance GPS operational services to our nation's combat forces, civil partners and myriad of domestic and international users.
The GPS OCX program also will reduce cost, schedule and technical risk. It is designed to provide 50% sustainment cost savings through efficient software architecture and Performance-Based Logistics. In addition, GPS OCX expected to cost millions less than the cost to upgrade OCS while providing four times the capability.
The GPS OCX program represents a critical part of GPS modernization and provides significant information assurance improvements over the current GPS OCS program.
- OCX will have the ability to control and manage GPS legacy satellites as well as the next generation of GPS III satellites, while enabling the full array of military signals.
- Built on a flexible architecture that can rapidly adapt to the changing needs of today’s and future GPS users allowing immediate access to GPS data and constellations status through secure, accurate and reliable information.
- Empowers the warfighter with more secure, actionable and predictive information to enhance situational awareness.
- Enables new modernized signals (L1C, L2C, and L5) and has M-code capability, which the legacy system is unable to do.
- Provides significant information assurance improvements over the current program including detecting and preventing cyber attacks, while isolating, containing and operating during such attacks.
- Supports higher volume near real-time command and control capability.
On September 14, 2011, the U.S. Air Force announced the completion of GPS OCX Preliminary Design Review and confirmed that the OCX program is ready for the next phase of development.
The GPS OCX program has achieved major milestones and is on track to support the GPS IIIA launch in May 2014.