Global Navigation Satellite Systems, Inertial Navigation, and Integration. Mohinder S. Grewal

PRN codes allows users independent access to multiple GPS satellite signals on the same carrier frequency. The signal transmitted by a particular GPS signal can be selected by generating and matching, or correlating, the PRN code for that particular satellite. All PRN codes are known and are generated or stored in GPS satellite signal receivers. For legacy GPS there are two PRN codes transmitted from each satellite. The first PRN code from each GPS satellite, sometimes referred to as a precision code or P‐code, is a relatively long, fine‐grained code having an associated clock or chip rate of f₀ = 10.23 MHz. A second PRN code from each GPS satellite, sometimes referred to as a clear or coarse acquisition code or C/A‐code, is intended to facilitate rapid satellite signal acquisition and handover to the P‐code. It is a relatively short, coarser‐grained code having an associated clock or chip rate of f₀ = 1.023 MHz. The C/A‐code for any GPS satellite has a length of 1023 chips or time increments before it repeats. The full P‐code has a length of 259 days, during which each satellite transmits a unique portion of the full P‐code. The portion of P‐code used for a given GPS satellite has a length of precisely one week (seven days) before this code portion repeats. Accepted methods for generating the C/A‐code and P‐code were established by the satellite developer (Satellite Systems Division of Rockwell International Corporation) in 1991 [17].

Navigation signal. The GPS satellite bit stream includes navigational information on the ephemeris of the transmitting GPS satellite and an almanac for all GPS satellites, with parameters providing approximate corrections for ionospheric signal propagation delays suitable for single‐frequency receivers and for an offset time between satellite clock time and true GPS time. The legacy navigational information is transmitted at a rate of 50 baud. Further discussion of the GPS and techniques for obtaining position information from satellite signals can be found in chapter 4 of Ref. [18].

      Precise positioning service (PPS). Formal, proprietary service PPS is the full‐accuracy, single‐receiver GPS positioning service provided to the United States and its allied military organizations and other selected agencies. This service includes access to the encrypted P(Y)‐code.

      Standard positioning service (SPS). SPS provides GPS single‐receiver (stand‐alone) positioning service to any user on a continuous, worldwide basis. SPS is intended to provide access only to the C/A‐code and the L1 carrier.

      1.2.1.3 Modernization of GPS

      GPS IIF, GPS IIR–M, and GPS III provide the legacy and new modernized signals. These may include L2 civil (L2C) signal and the L5 signal (at 1176.45 MHz) modulated by a new code structure, as well as, the M and L1C codes. These modernized GPS signals improve the ionospheric delay calculation, ranging performance, ambiguity resolution, and overall PVT accuracy.

      The GPS Ground Control Segment monitors the GPS signals in space, interfaces with the US Naval Observatory for timing information, and has remote monitor/uplink transmitter sites throughout the globe. Over the years, the GPS GCS has been upgraded and the Next‐Generation Operational Control System (OCX) will monitor all legacy and modernized GPS signals to provide for enhanced PVT solutions for the user segment. See Sections 4.2.8 and 10.5.5.5 and Ref. 18, Chapter 4.

      1.2.2 Global Orbiting Navigation Satellite System (GLONASS)

      A second system for global positioning is the Global Orbiting Navigation Satellite System (GLONASS), placed in orbit by the former Soviet Union and now operated and maintained by the Russian Republic [19,20].

      1.2.2.1 GLONASS Orbits

      GLONASS has 24 satellites, distributed approximately uniformly in three orbital planes (as opposed to six for GPS) of 8 satellites each. Each orbital plane has a nominal inclination of 64.8° relative to the equator, and the three orbital planes are separated from each other by multiples of 120° right ascension. GLONASS orbits have smaller radii than GPS orbits, about 25 510 km, and a satellite period of revolution of approximately 8/17 of a sidereal day.

      1.2.2.2 GLONASS Signals

The legacy GLONASS system uses frequency‐division multiplexing of independent satellite signals. Each GLONASS satellite transmits two navigation signals in the L1 and L2 frequency bands, corresponding to f₁ = (1.602 + 9k/16) GHz and f₂ = (1.246 + 7k/16) GHz, where k = −7, −6, … 5, 6 is the satellite number. These frequencies lie in two bands at 1.598–1.605 GHz (L1) and 1.242–1.248 GHz (L2). The L1 code is modulated by a C/A‐code (chip rate = 0.511 MHz) and by a P‐code (chip rate = 5.11 MHz). The L2 code is presently modulated only by the P‐code. The GLONASS satellites also transmit navigational data at a rate of 50 baud. Because the satellite frequencies are distinguishable from each other, the P‐code and the C/A‐code are the same for each satellite. The methods for receiving and analyzing GLONASS signals are similar to the methods used for GPS signals. Further details can be found in the patent by Janky [21].

      1.2.2.3 Modernized GLONASS

      The first of next‐generation GLONASS‐K satellites was first launched on 26 February 2011 and continues to undergo flight tests. This satellite transmits the legacy FDMA (frequency division multiple access) GLONASS signals and a L3OC code‐division multiple access (CDMA) signal at a frequency of 1202 MHz. Other GLONASS CDMA signals are under development within the legacy L1 (L1OC signal) and L2 (L2OC signal) bands.

      1.2.3 Galileo

      The Galileo system is satellite‐based navigation system currently under development by the European Union (EU). This development has completed definition and development phases and is nearly complete with launching operational satellites to achieve a 30 satellite constellation. Galileo operates in the L‐band with MEO satellites at height slightly above the GPS MEO satellites (23 222 km for Galileo versus 20 180 km for GPS). Galileo satellites operate in three orbital planes at an inclination angle similar to GPS. Galileo operates in three spectral bands known as E1 (1559–1592 MHz), E5 (1164–1215 MHz), and E6 (1260–1300 MHz).

      1.2.3.1 Galileo Navigation Services

      The EU intends the Galileo system to provide various levels of services.

      Open service (OS). The OS provides signals for positioning and timing, is free of direct user charge, and is accessible to any user equipped with a suitable receiver, with no authorization required. The OS provides dual‐frequency operation in the L1/E1 and L5/E5 frequency bands. The Galileo E1 L1C signal centered at 1575.42 MHz is compatible with the modernized GPS L1C signal transmitted by GPS III satellites. The Galileo E5a signal at 1176.45 MHz is part of a combined AltBOC signal. Modernized GNSS receiver equipment may use a combination of Galileo and GPS signals, thereby improving performance in severe environments such as urban canyons and heavy vegetation.

Commercial service (CS). The CS service is intended for applications requiring performance higher than that offered by the OS. Users of this service pay a fee for the added value. CS is implemented by adding two additional signals to the OS signal suite. The additional signals are protected by commercial encryption, and access protection keys are used in the receiver to decrypt the signals. Typical value‐added services include service guarantees, precise timing, multifrequency ionospheric delay measurements, local differential correction signals for very high‐accuracy positioning applications, and other specialized requirements. These services will be developed by service providers, which will buy the right to use the multifrequency commercial signals from the Galileo operator.

      Public regulated service (PRS). The PRS is an access‐controlled service for government‐authorized applications. It is expected to be used by groups such as police, coast guards, and customs. The signals will be encrypted, and access by region or user group will follow the security policy rules applicable in Europe. The PRS will be operational