GPS modernization

The United States' Global Positioning System (GPS), having reached Fully Operational Capability on July 171995 [ [http://www.navcen.uscg.gov/gps/geninfo/global.htm Global Positioning System Fully Operational ] ] 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. The target date is 2013 with incentives offered to the contractors if they can complete it by 2011.

New Navigation Signals

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.

Technical Details

* 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 1500 milliseconds.
** 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 is a non-data sequence (it does not contain 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-200D.

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 a user can calculate their position 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, or approximately 100 times stronger. 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), planned to be implemented with first GPS IIF launch (2008).

* 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 carrier to improve tracking
* Enables greater civil interoperability with Galileo 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.

*NASA has requested that Block III satellites carry laser retro-reflectors. [cite web |url=http://ilrs.gsfc.nasa.gov/docs/retromtg_060406.pdf |title=ILRS Meeting on Retroreflector Arrays] 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 is 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). [cite web |url=http://ilrs.gsfc.nasa.gov/docs/retromtg_060406_slides.pdf |title=Slides from ILRS Meeting on Retroreflector Arrays | month=April | year=2006]

ee also

* GPS signals

Notes

References

* [http://www.gpsworld.com/gpsworld/article/articleDetail.jsp?id=12195&pageID=1&sk=&date= The Modernized L2 Civil Signal] . GPS world article from 2001.
* Lazar, Steven. " [http://www.aero.org/publications/crosslink/summer2002/index.html Modernization and the Move to GPS III] ." "Crosslink", Summer 2002. Pages 42-46. [http://www.aero.org/publications/crosslink/summer2002/07.html HTML Version]
* " [http://www.navcen.uscg.gov/gps/modernization/TheNewL2CivilSignal.pdf The New L2 Civil Signal] " presentation.
* [http://www.mitre.org/work/tech_papers/tech_papers_00/betz_overview/betz_overview.pdf Technical Paper]
* [http://www.gpsworld.com/gpsworld/article/articleDetail.jsp?id=154869&pageID=1&sk=&date= Getting to M] GPS world article on Mcode
* [http://www.globalsecurity.org/space/systems/gps_3.htm GPS III]
* [http://www.globalsecurity.org/space/systems/gps_3-ocx.htm GPS III Operational Control Segment (OCX)]
* [http://gps.faa.gov/gpsbasics/GPSmodernization-text.htm FAA GPS Modernization Page]
* [http://www.engadget.com/2006/01/25/government-turns-up-volume-on-gps/ Engadget News Post, regarding L2C activated]
* [http://www.navcen.uscg.gov/cgsic/meetings/summaryrpts/46thMeeting/15%20CGSIC%20GPS%20Prog%2046.ppt Presentation Containing image] "(to be integrated)"
* [http://www.navcen.uscg.gov/cgsic/meetings/summaryrpts/38thmeeting/Clarke.ppt Clarke Presentation] "(to be integrated)"