GPS General Introduction

GPS
Title	GPS General Introduction
Edited by	GMV
Level	Basic
Year of Publication	2011

The GPS is the U.S. Global Navigation Satellite System (GNSS) which provides free positioning and timing services worldwide. GPS receivers compute their position in the GPS Reference System using satellite technology and based on triangulation principles (please refer to GNSS positioning). Originally developed for the U.S. military, the incident with the Korean Air Lines Flight 007 led the US Government to decide to make GPS use free for civilian purposes very early in the experimental phase of GPS . The launch of the first Block I Navstar GPS satellite meant the beginning of the deployment of the GPS system on 22 February 1978, followed by the declaration of the Initial Operating Capability in December 1993 with 24 operational satellites in orbit, and the Full Operational Capability in June 1995. GPS is maintained by the United States government and is freely accessible by anyone with a GPS receiver. The Department of Defense is responsible for operating the system, but it also receives national-level attention and guidance through the National Executive Committee for Space-Based Positioning, Navigation, and Timing (PNT).

GPS block IIR-M satellite

GPS Signal Structure

GPS satellite program started to transmit right-hand circularly polarized signals to the earth at two frequencies, designated L1 and L2. The main GPS carrier signal L1, at 1575.42MHz, is modulated by two codes: the coarse/acquisition (C/A) code also known as civilian code and the precision/secure (P/Y) code, reserved by cryptographic techniques to military and authorized civilian users. The GPS L2 signal, centered at 1227.6 MHz, only contains the precise code and it was established to provide a second frequency for ionospheric group delay correction.

The GPS modernization program began in 2005 with the launch of the first IIR-M satellite. Since that moment on, two new signals are transmitted: L2C for civilian users and a new military signal (M code) in L1 and L2 to provide better jamming resistance than the Y code.

Moreover, a new radio frequency link (L5 at 1176.45 MHz) for civilian users has been included in a radio band reserved exclusively for aviation safety services. This signal, available since the launch of the Block IIF satellites (May 28th 2010) has been designed to be compatible with other GNSS systems such as Galileo.

A new signal in L1 frequency band called LC1 has also been included to be interoperable with Galileo E1 signal among others. It is compatible with legacy L1 signal but broadcast at a higher power level and includes advanced design for enhanced performance.

Please refer to GPS Signal Plan article for further details on GPS Signal structure.

GPS Reference Frame

Accurate and well-defined Time References and Coordinate Frames are essential in GNSS, where positions are computed from signal travel time measurements and provided as a set of coordinates. GPS uses the World Geodetic System WGS-84, developed by the US Defence Department, which is a unified terrestrial reference system for position and vector referencing. Indeed, the GPS broadcast ephemeris are linked to the position of the satellite antenna phase centre in the WGS-84 reference frame. Thus, the user receiver coordinates will be expressed in the same ECEF frame. GPS System Time (GPST) is defined by the GPS Ground Segment on the basis of a set of atomic clocks aboard the satellites and in the Monitor Stations. It is not adjusted for leap seconds and it is synchronized with the UTC (USNO) at nanosecond level. The origin epoch of GPS time is 0h UTC (midnight) of January 5th to 6th of 1980.

GPS Services

GPS provides two different positioning services, namely the Precise Positioning Service (PPS) and the Standard Positioning Service (SPS):

1. The Standard Positioning Service (SPS), is a positioning and timing service provided on GPS L1, L2 and L5 frequencies and available to all GPS users. The L1 frequency contains a coarse acquisition (C/A) code and a navigation data message. The L2 frequency contains a CM-code and CL-code signals whereas I5-code and Q5-code signals are transmitted in L5 frequency.
2. The Precise Positioning Service (PPS), is a highly accurate military positioning, velocity and timing service broadcasted at the GPS L1 and L2 frequencies. Both frequencies contain a precision (P) code ranging signal with a navigation data message that is reserved for authorized use by the use of cryptography.

GPS Architecture

GPS architecture is comprised of three segments: a GPS Space Segment, a GPS Ground Segment, and a GPS User Segment. The main functions of the GPS Space Segment are to transmit radio-navigation signals, and to store and retransmit the navigation message sent by the GPS Ground Segment. The GPS Ground Segment is composed of a master control station, a network of monitor stations and ground antennas which upload the clock and orbit errors, as well as the navigation data message to the GPS satellites. Finally, the GPS User Segment consists on the millions of receivers performing the navigation, timing or other related functions.

Differential GPS

Differential GPS is an enhancement to primary GPS constellation(s) information by the use of a network of ground-based reference stations which enable the broadcasting of differential information to the user – also named rover – to improve the accuracy of his position – the integrity is not assured. There are several differential GNSS techniques, such as the classical DGPSs (or DGPS), the Real Time Kinematics (RTK) and the Wide Area RTK (WARTK).

GPS Performances

The levels of performance that the user can expect from GPS are specified in the Standard Positioning Service Performance Standard, and the Precise Positioning Standard. However, the values provided by these documents are very conservative, being the actual performances usually better than these official values. Moreover, the performance obtained with GPS depends strongly on the mode of operation. For instance, a stand-alone receiver that uses only the signals received from the satellites, the levels of performance are:

• C/A-code receivers ~ 5 -10 m.
• P/Y-code receivers ~ 2 -9 m

In case of using GPS in a differential mode, the performances that can be expected are:

• C/A-code DGPS receivers ~0.7 -3 m.
• P/Y-code DGPS receivers ~0.5 -2.0 m.

GPS Future and Evolutions

Aimed at improving the performance for civilian users, the GPS modernization will introduce the following signals:

• L2C (1227.6 MHz: It enables the development of dual-frequency civil GPS receivers to correct the ionospheric group delay. This signal is available since 2005, with the launch of the first IIR-M satellite.
• L5C (1176.45 MHz): It is compatible with other GNSS systems and is transmitted at a higher power than current civil GPS signals, also with a wider bandwidth. This signal is available since the launch of the Block IIF satellites (May 28th 2010).
• L1C (1575.42 MHz): Designed for interoperability with Galileo, it is backward compatible with the current civil signal on L1. This signal is available since the launch of the Block IIIA satellites (first satellite launched December 2018). An enhancement to this L1C signal is being analysed, which is called CHIMERA (Chips Message Robust Authentication). This technique consists on adding encrypted watermarks to the L1C signal that not only let users know when a signal is being spoofed but also makes it possible to authenticate the location of a GPS receiver to another party.

In addition to the civil signals, it is planned to include a new military signal, the M-code, in L1 and L2 frequencies. Regarding the Ground Segment, the new Operational Control Segment (OCX) will replace the current GPS Operational Control System placed at Schriever Air Force Base. The OCX will maintain backwards compatibility with the Block IIR and IIR-M constellation satellites, providing command and control of the new GPS IIF and GPS III families of satellites, and enabling new modernized civil signal capabilities.

GPS Modernization

Credits

This article contains some verbatim paragraphs taken from U.S. governmental web pages. Please see the References section.