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  • Abstract

Evolution of the Global Navigation SatelliteSystem (GNSS)

This growing civil aviation system is expected to replace a significant number of ground based navigation systems and allow for more efficient use of the world wide airspace.

The Global Navigation Satellite System (GNSS) is the worldwide set of satellite navigation constellations, civil aviation augmentations, and user equipment. This paper reviews the current status and future plans of the elements of GNSS as it pertains to civil aviation. The paper addresses the following satellite navigation systems: the U.S. Global Positioning System (GPS), Russian GLONASS, European Galileo, Chinese Compass, Japanese Quasi Zenith Satellite System, and Indian Regional Navigation Satellite System. The paper also describes aviation augmentations including aircraft-based, satellite-based, ground-based, and ground-based regional augmentation systems defined by the International Civil Aviation Organization. Lastly, this paper details typical user equipment configurations and civil aviation applications of GNSS including navigation, automatic dependent surveillance, terrain awareness warning systems, and timing.

SECTION I

INTRODUCTION

The International Civil Aviation Organization (ICAO) defines the Global Navigation Satellite System (GNSS) as “a worldwide position and time determination system that includes one or more satellite constellations, aircraft receivers and system integrity monitoring, augmented as necessary to support the required navigation performance for the intended operation” [1]. This paper reviews the current status and future plans for the components of GNSS and civil aviation applications.

SECTION II

CONSTELLATIONS

Current international GNSS standards for civil aviation—ICAO's Standards and Recommended Practices (SARPs) [1]—address only two core constellations: the U.S. Global Positioning System (GPS) and the Russian Federation's GLONASS. The ICAO Navigation Systems Panel (NSP), chartered with updating the GNSS SARPs, has on its current work program the addition of material on Galileo, an emerging European satellite navigation system.

This section describes the GPS, GLONASS, and Galileo programs. Also addressed are planned future satellite navigation systems, which may or may not be adopted internationally for civil aviation use.

A. Global Positioning System (GPS)

The GPS [2], [3], [4], [5] is a satellite navigation system operated by the United States. The GPS program began in the early 1970s [2], [3]. Eleven developmental prototype GPS Block I satellites were built by Rockwell International and launched from Vandenberg Air Force Base in California between February 1978 and November 1985 (one Block I was destroyed in a launch failure). These were followed by operational satellites: nine Block II satellites launched in 1989 and 1990, 19 Block IIA satellites (see Fig. 1) launched between 1990 and 1997, and 13 Block IIR satellites launched between 1996 and 2004 (one, the first, Block IIR satellite was destroyed in a launch failure). At the time of the writing of this paper, six of eight modernized Block IIR (IIR-M) satellites have been launched. The Block II and IIA satellites were built by Rockwell International, and the Block IIR and IIR-M satellites were built by Lockheed Martin Corporation and its navigation payload subcontractor, ITT Aerospace/Communications.

Figure 1
Fig. 1. Block IIA satellite.

In total, 57 GPS satellites have been successfully placed in orbit, with 31 currently operational. The operational satellites include 13 Block IIA, 12 Block IIR, and six Block IIR-M satellites. The capabilities of these satellites and future blocks will be described later in this section.

The operational GPS satellites are nominally maintained within 24 orbital slots [6]. These slots reside within circular orbits inclined 55 with respect to the equatorial plane. Four slots are contained in each of six orbital planes, A–F (see Table 1), with an orbital radius of 26 559.7 km. The constellation design includes asymmetrical spacing in argument of latitude between satellites within each plane, which was determined to provide robustness in performance against satellite failures [7]. Excess satellites are typically launched into locations adjacent to slots that contain satellites expected to require replacement the soonest.

Table 1
Table 1 Nominal GPS Constellation Slot Locations [6]

GPS presently provides two services—one for civilian users referred to as the Standard Positioning Service (SPS) and one available only to authorized users (primarily the U.S. military, and the militaries of U.S. allies) referred to as the Precise Positioning Service (PPS). The United States has pledged to make the GPS SPS available for civil aviation use on a continuous worldwide basis, free of direct user fees, with a minimum of six years' advance notice to be provided in the event that this service will be terminated. This commitment was initially made by the administrator of the Federal Aviation Administration (FAA) in 1994 [8]. The commitment to provide GPS SPS service was reiterated in 2007 [9], with an additional commitment made at that time to provide GPS satellite-based augmentation system (SBAS) services in North America, free of direct user charges, through the FAA's Wide Area Augmentation System (WAAS) (see Section III-A for a description of SBASs, including WAAS).

At one time, the accuracy of the SPS was intentionally degraded using a technique referred to as selective availability (SA), which was observed to be implemented as a pseudorandom dithering of the satellite clock that could be removed only by PPS receivers with knowledge of the generation algorithm and cryptographic keys [3]. On May 1, 2000, the intentional degradation of SPS performance due to SA was ceased [10]. More recently, in September 2007, the United States announced that the capability to implement SA will be removed from future GPS satellite procurements [11].

The specified accuracy of the GPS SPS is 13 m, 95% for horizontal positioning and 22 m, 95% for vertical positioning [6]. This specification is for the signal-in-space (SIS) only (i.e., it does not include errors due to the atmosphere, multipath, or user equipment) and is based upon a global average. Actual performance is typically significantly better than the specification. For instance, the observed 95% horizontal and vertical positioning accuracies for 28 GPS SPS receivers distributed throughout North America from October 1–December 31, 2007, were 2.5 and 4.9 m, respectively [12]. Further, the data reported in [12] include all real-world errors, whereas the accuracy specification in the SPS Performance Standard [6] only includes SIS errors.

Code-division multiple access (CDMA) is utilized for all the GPS signals, i.e., all of the satellites broadcast their signals upon the same carrier frequencies. The Block I through Block IIR satellites broadcast navigation signals upon two carrier frequencies, referred to as Link 1 (L1) at 1575.42 MHz and Link 2 (L2) at 1227.6 MHz. For these satellites, two direct-sequence spread-spectrum (DSSS) signals with rectangular symbols are broadcast in phase quadrature on L1 [13]. The coarse/acquisition (C/A) code signal has a 1.023 MHz chipping rate and the Precision (P) code signal has a 10.23 MHz chipping rate. The C/A code is generated using length-1023 Gold codes [14], which repeat every millisecond. The P code is a week long when unencrypted but is normally encrypted to deter spoofing, and when it is, it is referred to as the Y code. An identical P(Y) code signal is also broadcast on the L2 carrier. Both the C/A and P(Y) code signals are further modulated by the same 50 bps data. This 50 bps data stream includes information required for navigation including the ephemeris, clock corrections, and health information for the broadcasting satellite, as well as almanac data for the entire constellation.

The Block IIR-M satellites (see Fig. 2) introduced two new navigation signals—a new military signal on L1 and L2 referred to as the M code [15] and a new civil signal on L2 referred to as L2C [13], [16]. Both of these new signal types have advanced designs that include a dataless signal component and forward error correction of the navigation data to enable robust tracking and data demodulation by user equipment. L2C uses DSSS modulation with rectangular symbols and a 1.023 MHz chipping rate. ICAO decided against including L2C within the GNSS SARPs in 2002 because the L2 band is shared with radiolocation (radars), fixed services, and mobile services in some regions, and reception of L2C without interference from other services could thus not be ensured worldwide.

Figure 2
Fig. 2. Block IIR-M satellite (courtesy of Lockheed Martin Corporation).

The M code uses DSSS modulation with a 5.115 MHz chipping rate and a spread-spectrum symbol that is two cycles of a 10.23 MHz square wave. This DSSS modulation variant is referred to as binary offset carrier (BOC) [17] and may alternatively be viewed, and in practice may be generated, as the product of 1) an ordinary DSSS signal using rectangular symbols and 2) a square wave subcarrier.

After the eight Block IIR-M satellites are launched, follow-on satellites, referred to as Block IIF satellites, will be launched. The Block IIF satellites [18] are being built by Boeing, and 12 of these vehicles are anticipated to be placed in orbit beginning in 2009. The Block IIF satellites will add an additional civil navigation signal on a new carrier frequency. The new carrier frequency, at 1176.45 MHz, and signal are both referred to as Link 5 (L5) [19], [20]. L5 is generated with DSSS modulation using rectangular symbols and a 10.23 MHz chipping rate. As with the other modernized GPS signals (e.g., L2C and M code), L5 includes a dataless signal component and forward error correction of the navigation data for robust tracking and data demodulation.

The L5 signal is located within the 960–1215 MHz band that is used worldwide by civil aviation for distance measuring equipment (DME). DME provides aircraft with range measurements to ground sites using high-power pulsed transmissions from airborne interrogators and ground-based beacons. The 960–1215 MHz band is also used by other high-power pulsed systems, including the Joint Tactical/Multifunctional Information Distribution System (a military tactical communication system), Tactical Air Navigation, and secondary surveillance radars. For this reason, pulse blanking [21] is envisioned for GPS L5 avionics, and the L5 signal is designed with a much higher minimum received power level of −154.9 dBW [20] than the other civil GPS signals. For comparison, the GPS C/A code and L2C signals have minimum specified received power levels of −158.5 and −160 dBW, respectively [13]. All of the GPS civil signal minimum power levels are specified at the output terminals of a 3 dBi linearly polarized user antenna at worst normal orientation. Although airborne GPS antennas are nominally right hand circularly polarized like the broadcast GPS signals, low-profile antennas tend to be dominantly linearly (vertically) polarized towards low-elevation angle satellites.

The procurement of the GPS satellites to follow the Block IIFs is currently under way. The next-generation GPS vehicles are referred to as Block III. These spacecraft will broadcast an additional L1 civil signal (L1C), which will employ a signal that is created using a BOC modulation with a 1.023 MHz chipping rate and time-multiplexed mixture of symbols that are derived from 1.023 and 6.138 MHz square wave subcarriers [22], [23]. The Block III satellites are anticipated to be launched beginning in 2014.

Fig. 3 illustrates the evolution of the GPS signals from the Block I through the Block III satellites. Shown on the figure are the normalized power spectral densities of the various GPS signals in decibels. For reference in viewing the figure, the bandwidths spanned between the first spectral nulls of the P(Y) code and L5 signals are each 20.46 MHz.

Figure 3
Fig. 3. Evolution of GPS signals.
Figure 4
Fig. 4. GLONASS-M satellite.

The GPS satellites are monitored, commanded, and controlled by a ground network referred to as the GPS control segment (CS). The CS includes a master control station (MCS) at Schriever Air Force Base in Colorado, a global set of monitor stations, and ground antennas. The CS has recently undergone two significant transitions. The first major transition was the addition of a number of new monitor stations, which began in the Legacy Accuracy Improvement Initiative (L-AII) program, taking the total number of monitor stations from six to 14, with a future anticipated total of 17 [4]. The second major transition, completed in October 2007, consisted of upgrading of the MCS from a legacy system based upon an IBM mainframe computer to a more modern system based upon a distributed Sun workstation configuration. This MCS modernization program is referred to as the Architecture Evolution Plan (AEP). A further evolution of the CS is planned in a program referred to as OCX. The Air Force is in the process of selecting a contractor for the OCX program to develop and build the next-generation CS.

B. GLONASS

GLONASS is a satellite navigation system operated by the Russian Federation. The first GLONASS satellite was launched in October 1982. A total of 81 GLONASS and 14 modified GLONASS (GLONASS-M) satellites have been launched successfully thus far. However, the GLONASS satellites have a short design life (1 to 3 y [4]). After peaking briefly at 24 satellites in 1995, the constellation has decayed and at the time of the writing of this paper consists of only 13 operational satellites.

Fortunately, the GLONASS program has been reinvigorated in recent years. The Russian Federation passed Decree Number 587 in August 2001, which called for the GLONASS constellation to be rebuilt within a decade. Twenty-four satellites have been launched since 2001, and current plans call for 24 operational satellites by 2010 [24].

The GLONASS constellation nominally consists of 24 satellites in three orbital planes, with an inclination angle of 64.8 and an altitude of 19 100 km [25]. A Walker [26] 24/3/1 constellation design is employed, where the notation “Walker T/P/F” denotes a constellation of T satellites in circular orbits equally divided and equally spaced within P planes with an offset in mean anomaly between the first satellite in each plane of 360× F/T.

The current GLONASS satellites broadcast navigation signals in two subbands of L-band referred to as L1 and L2. A frequency-division multiple access (FDMA) design for the signals are utilized, with the L1 carrier frequencies given byFormula TeX Source $$f_{K1}=f_{01}+K\cdot\Delta f_{1}$$ and the L2 carrier frequencies byFormula TeX Source $$f_{K2}=f_{02}+K\cdot\Delta f_{2}$$where f01 = 1602 MHz, f02 = 1246 MHz, Δ f1 = 0.5625 MHz, Δ f2 = 0.4375 MHz, and K is the channel number. The channel numbers originally spanned from zero to +13, but to protect the radio astronomy service in the 1610.6–1613.8 and 1660–1670 MHz bands, Russia has committed to migrate away from using the upper channels. A range of channel numbers from −7 to +6 is indicated in [25], but current plans call for a broader end-state channel range of −7 to +9 [24]. The currently operational satellites use channels from −2 to +6. Since there are more satellites in the nominal GLONASS constellation than there are channels, two GLONASS satellites may transmit upon the same channel number if they are in antipodal slots within the same orbital plane [25]. It should be noted that all of the GLONASS signals broadcast by the same satellite are coherently derived from the same clock and the L2 carrier frequencies are 7/9 the L1 frequencies.

The GLONASS satellites broadcast two direct DSSS navigation signals with rectangular symbols on the L1 carriers [25]. The standard accuracy signal, intended for civil use worldwide, is generated using a length-511 maximal length sequence and a 0.511 MHz chipping rate. The high accuracy signal has a 5.11 MHz chipping rate and is only intended for use by the Russian Ministry of Defence and entities it authorizes. Navigation data are modulated upon the signals at 50 bps, without forward error correction. The 20-ms data symbols are Manchester encoded. The former Union of Soviet Socialist Republics offered GLONASS for civil aviation use, free of direct user fees, to ICAO in 1988 [4]. On May 18, 2007, Russian President Vladimir Putin signed a decree reiterating the offer to provide GLONASS civil signals, free of direct user fees, to the world.

On the GLONASS satellites, only the high accuracy signal is broadcast on the L2 carriers. The GLONASS-M satellites (see Fig. 4) broadcast both the high and standard accuracy signals on the L2 carriers, and further have an improved design life of 7 y [24].

The next generation GLONASS satellites are currently in development. The GLONASS-K satellites are anticipated to be launched beginning in the 2009–2010 timeframe [24], and will broadcast navigation signals in an additional sub-band. The new sub-band is referred to as L3. The GLONASS L3 carrier frequencies will be 94/125 the L1 carrier frequencies, placing them in the range of 1202–1208 MHz. The civil L3 signals will use DSSS modulation with rectangular chips, like the L1 and L2 civil GLONASS signals, but will employ a much higher chipping rate on the order of 4 MHz. The evolution of the GLONASS FDMA signals is shown in Fig. 5.

Figure 5
Fig. 5. Evolution of GLONASS FDMA signals.
Figure 6
Fig. 6. Artist's view of a Galileo satellite. European Space Agency, J. Huart (reprinted with permission).

A recent development in GLONASS is that CDMA signals, in addition to the FDMA signals described above, are being considered for the GLONASS-K satellites and beyond. The FDMA approach for GLONASS, with each signal on a separate carrier frequency, leads to slightly more complex user equipment. Further, group delay variations across the passband of receivers can result in biases in the measurements made from one signal to the next. These biases can impact the achievable accuracy unless sophisticated calibration techniques are employed. CDMA GLONASS signals at 1575 and 1176 MHz are tentatively planned for GLONASS-K and beyond, utilizing signal designs similar to the GPS L1C and L5 signals [24].

The GLONASS system architecture includes a ground control segment with ten monitor stations distributed throughout Russia, and additional facilities to command and control the satellites. GLONASS provides user equipment with their positions in the Earth Parameter System 1990 (PZ-90), whereas ICAO standards require that navigation systems provide user location in the World Geodetic System 1984 (WGS-84) coordinate system used by GPS. In the past, a transformation was required to convert from PZ-90 to WGS-84 coordinates. In September 2007, an adjustment was made to the PZ-90 system to tie it more tightly with the International Terrestrial Reference Frame 2000 (ITRF 2000) [24]. WGS-84 is routinely adjusted to maintain close alignment with the ITRF, and differences in terrestrial coordinates between the two systems are now consistent to within 2 cm. Efforts are under way to determine the appropriate transformation, if a transformation is indeed necessary, to convert from PZ-90 to WGS-84 after the September 2007 adjustment.

C. Galileo

Galileo is a planned European satellite navigation system. Galileo is specifically designed for civil and commercial purposes and will be interoperable with the other radio-navigation systems. This will be beneficial to all users as they will be able to use more satellites for redundancy and higher accuracy. A Galileo satellite is illustrated in Fig. 6.

Services

Four distinct navigation services and one service to support search and rescue operations have been identified to cover the widest range of users needs, including professional users, scientists, mass-market users, safety of life, and public regulated domains. The Galileo navigation services can be enhanced on a local basis through combination with local elements for applications with more demanding requirements and depending on specific environmental characteristics.

The Open Service (OS) results from a combination of open signals. This provides position and timing performances commensurate with the ones offered by other GNSS constellations. The Safety of Life Service (SoL) improves the open service performance by providing timely warnings to the users when it cannot guarantee to meet certain margins of accuracy (integrity). It is envisaged that a service guarantee will be provided for this service. The Commercial Service (CS) provides access to two additional signals, to allow for a higher data rate throughput and to enable users to improve accuracy through advanced processing techniques. The Public Regulated Service (PRS) provides position and timing to specific users requiring a high continuity of service, with controlled access. The performance requirements for the OS and SoL services, which are anticipated to be used for civil aviation applications, are summarized in Table 2. The definitions of the performance parameters related to accuracy, integrity (e.g., integrity risk, alert limit, and time to alert), and continuity are provided in Section V of this paper.

Table 2
Table 2 Performance Requirements for the GALILEO Open and Safety-of-Life Services

Infrastructure

The core of the Galileo system will be a global constellation of 27 satellites in three medium Earth orbit (MEO) orbital planes inclined at 56 to the equator at about 23 000 km altitude in a Walker 27/3/1 configuration [27]. Each plane will have one active spare, able to cover for any failed satellite in that plane. Thus far, one test satellite named GIOVE-A was launched in December 2005, and a second test satellite, GIOVE-B, was launched in April 2008. Four in-orbit validation satellites are anticipated to be launched in 2009, and the entire constellation will be populated by approximately 2013.

A number of interconnected ground facilities will allow the accomplishment of the various services, including two Galileo control centers, five monitoring and control stations, and five mission uplink stations (ULSs) to enable global coverage without interruptions. The Galileo control centers comprise two separate types of facilities: a ground control segment (GCS) and a ground mission segment (GMS). The GCS will support spacecraft and constellation maintenance, whereas the mission segment is directly handling the navigation system control. The GCS will use a global network of nominally five tracking, telemetry, and control stations to communicate with each satellite. The GMS collects data from a global network of around 30 Galileo sensor stations to monitor the navigation signals of all satellites on a continuous basis and communicates with the Galileo satellites through a global network of mission ULSs, installed at five sites. The GMS is in charge of orbit and time synchronization functions as well as the provision of integrity data for the SoL service.

Signals

The Galileo navigation signals are transmitted in the four frequency bands indicated in Fig. 7. These four frequency bands are referred to as E5a, E5b, E6, and E1. They provide a wide bandwidth for the transmission of the Galileo signals. The Galileo frequency bands have been selected in spectrum allocated globally for radionavigation satellite services (RNSS) and, in addition to that, the E5a, E5b, and E1 bands are included in the allocated spectrum for aeronautical radionavigation services (ARNS), employed by civil-aviation users, and allowing dedicated safety-critical applications. The frequency bands are also either overlapping or contiguous to frequency bands used by other GNSS constellations. This will favor the combined use of several constellations to increase performance and robustness of the navigation services offered to the user communities.

Figure 7
Fig. 7. Galileo frequency bands.

CDMA is used within each frequency band. The Galileo signals are all coherently generated from the same clock and employ either DSSS modulation with rectangular symbols or BOC modulations. The signals that support the OS and SoL services include a signal in the E1 band with the same spectral characteristics as the GPS L1C signal [28], and signals in the E5a and E5b bands that resemble a pair of DSSS modulations with rectangular symbols and 10.23 MHz chipping rates. More detailed descriptions of the Galileo signals may be found in [4] and [29].

D. Compass

The BeiDou/Compass Navigation Test System (BNTS) is the first Chinese satellite navigation system. This system, which closely mirrors the design of the failed U.S. Geostar system, is capable of providing two-dimensional position accuracies on the order of 20–100 m using two-way range measurements between the user equipment and geostationary satellites [4]. The system also provides low-rate bidirectional communications and differential GPS/GLONASS services. Three BNTS satellites were launched in October 2000, December 2000, and May 2003 into geostationary orbits at longitudes of 80 E, 140 E, and 110.5 E, respectively [4], [30].

China is planning to enhance their satellite navigation system's capabilities into a system capable of providing accurate three-dimensional positioning worldwide. The end-state system is referred to as the Compass/BeiDou Navigation Satellite System (CNSS). The space segment of CNSS will consist of 30 MEO satellites at an altitude of around 21 490 km and five geostationary (i.e., circular orbits in the equatorial plane at an approximate altitude of 35 786 km such that the satellite circles the Earth at precisely the same rate the Earth rotates and thus appears stationary to an observer on the Earth) satellites. Two services are planned. The Open Service will provide accuracies of 10 m in positioning, 0.2 m/s in velocity, and 50 ns in time dissemination [30]. The second service is the Authorized Service, which is only intended for entities authorized by the Chinese government (e.g., the Chinese military). An experimental test CNSS satellite was launched into geostationary orbit in February 2007, and the first Chinese MEO satellite navigation satellite was launched in April 2007. CNSS services are anticipated to be available in China and neighboring countries by 2008, and will be expanded globally in the succeeding years with the population of the MEO constellation.

The BNTS transponders utilize uplink frequencies in the 1610–1626.5 MHz band and downlink frequencies in the 2483.5–2500 MHz band. CNSS is a CDMA-based system with DSSS signals planned on four carrier frequencies: 1207.14 MHz (shared with GALILEO E5b), 1268.52 MHz (shared with GALILEO E6), 1561.098 MHz (E2), and 1589.742 MHz (E1). Note that, as for GALILEO, this frequency plan is influenced by GPS—the set of carrier frequencies are all derivable from the GPS fundamental clock frequency of 10.23 MHz.

The signal design for CNSS has not yet been published by the Chinese. Various organizations, however, have been observing the signals broadcast by the first CNSS MEO [31], [32], [33]. Based upon these measurement campaigns, the first MEO has been broadcasting DSSS signals with rectangular chips on E2, E5b, and E6. Two signals in phase quadrature, each with a 2.046 MHz chipping rate, have been observed on E2. Two 10.23 MHz chipping rate signals in phase quadrature have been observed on E6. Signals with 2.046 and 10.23 MHz chipping rates, in phase quadrature, have been seen on E5b. The received CNSS power levels have been reported to be significantly stronger than typical received GPS signal power levels [33]. No observations have yet been reported for E1. It is not clear whether the signals broadcast by the first CNSS MEO are representative of the final design.

E. Quasi-Zenith Satellite System (QZSS)

The Quasi-Zenith Satellite System (QZSS) [4], [34] is a satellite navigation system that is being developed by the government of Japan. QZSS is not intended to provide a standalone navigation capability, but rather to improve the performance of GPS in Japan, particularly in urban environments where buildings obscure visibility of much of the sky.

The planned QZSS constellation consists of three satellites in elliptical orbits at geosynchronous altitude (around 35 786 km) inclined 45 to the equatorial plane in three orbital planes with the same ground track. The ground track forms a figure-eight pattern with the northern portion of the ground track covering a much smaller geographical area than the southern portion due to an eccentricity of orbit of around 0.099. Inclined geostationary orbits, such as are used by QZSS, are sometimes referred to as tundra orbits and allow a small number of satellites to provide good coverage over a limited geographic region. The central line of the ground track is at 135 E in longitude. A program objective is to launch the first satellite in 2009, and the second and third satellite within several years to follow [34].

The QZSS satellites will broadcast six CDMA navigation signals on four carrier frequencies [35]. The carrier frequencies are 1575.42 MHz (common with GPS L1 and Galileo E1), 1278.75 MHz (common with Galileo E6), 1227.6 MHz (common with GPS L2), and 1176.45 MHz (common with GPS L5).

F. Indian Regional Navigation Satellite System (IRNSS)

The Indian Regional Navigation Satellite System (IRNSS) [36] is a satellite navigation system planned by India. The system is being implemented by the Indian Space Research Organisation. The overall system will consist of seven satellites. Three of the satellites will be placed in geostationary orbits with longitudes of 34 E, 83 E, and 132 E. The four remaining satellites will be placed at geostationary altitudes, but in tundra orbits inclined 29 with respect to the equatorial plane, such that the subsatellite points on Earth trace figure-eight patterns with centers on the equator at 55 E and 111 E in longitude. The intended service volume is bounded in longitude between 40 E and 140 E and in latitude between 40 S and 40 N, and the anticipated system accuracy is 20 m. Current plans call for the first satellite to be launched in 2009, three additional satellites to be launched by the end of 2010, and the entire constellation to be operational by 2011.

Three services are planned for IRNSS. The IRNSS Standard Positioning Service will be based upon DSSS signals with rectangular chips and a 1.023 MHz chipping rate broadcast at 1191.795 and 2491.005 MHz. A Precise Positioning Service will operate using the same carrier frequencies and DSSS modulation type, but with a higher 10.23 MHz chipping rate. Restricted Services will also be provided via a 10.23 MHz chipping rate DSSS signal on the 1191.795 MHz carrier frequency.

SECTION III

AUGMENTATIONS

A. Satellite-Based Augmentation System (SBAS)

A satellite-based augmentation system provides differential corrections, integrity parameters, and ionospheric data over a given region. An SBAS consists of a ground network of monitoring stations that collect GPS measurements. The receivers in the ground network are dual frequency—capable of tracking the GPS L1/L2 C/A code and L2 P(Y) code signals to determine the electron content of the ionosphere integrated along the signal paths from the visible satellites. Semicodeless processing techniques [37] are used to track the encrypted P(Y) code signals. Some SBAS ground networks are capable of additionally monitoring GLONASS L1 signals, but due to the current state of the GLONASS constellation this capability has not been utilized. Error corrections and integrity data are then computed by a centralized facility. This information is then broadcast to the end users through a geostationary (GEO) satellite link.

Current-generation SBAS GEOs broadcast directly on the GPS L1 carrier frequency of 1575.42 MHz. The SBAS signal resembles the GPS C/A code signal, but a higher data rate of 250 bps is employed with forward error correction encoding to enable all the requisite system data to be provided to the user. Current-generation user equipment process GPS C/A code and SBAS signals on L1 only. SBAS is standardized internationally by ICAO [1].

Several SBAS systems are either already operational or in development [38]. These include the WAAS [39], [40], [41] in the United States, the European Geostationary Navigation Overlay Service (EGNOS) [42] in Europe, the Multifunctional Transport Satellite (MTSAT)-Based Augmentation System (MSAS) [43] in Japan, and the GPS and GEO Augmented Navigation (GAGAN) system in India. All SBAS systems can be received by a unique airborne receiver compliant to the internationally recognized RTCA DO-229 standard [44].

The U.S. WAAS, using a ground network with monitors throughout the United States and transponders on two Inmarsat-3 GEOs at 178 E and 54 W, was declared in August 2000 to be offering continuous service for nonsafety applications. In July 2003, WAAS was commissioned for safety-of-life services. WAAS services have recently migrated to two second-generation GEOs located at 133 W and 107 W (see Fig. 8), and the WAAS ground network has been expanded into Canada and Mexico. The second-generation WAAS GEOs broadcast signals at both the GPS L1 and L5 carrier frequencies, although presently the L5 signal is only for use by the WAAS ground network. It is envisioned that global SBAS services will eventually migrate towards supporting dual-frequency user equipment [41], [45].

Figure 8
Fig. 8. Footprints of second-generation WAAS GEOs.

EGNOS service is based upon three GEOs: a European Space Agency Artemis satellite at 21.5 E and two Inmarsat-3 GEOs at 15.5 W and 64.5 E. The EGNOS ground network has been fully deployed, and the system is expected to be commissioned for safety-of-life operations by 2009.

The Japanese SBAS-MSAS utilizes two GEOs and a ground network distributed throughout Japan plus monitor and ranging stations in Canberra, Australia, and Honolulu, HI. MTSAT-1R was launched in February 2005 and is located at 140 E. MTSAT-2 was launched in February 2006 and is located at 145 E. The MSAS system was commissioned for safety-of-life services in September 2007.

GAGAN is currently under development [46]. The design includes eight reference stations distributed throughout India. At present an Inmarsat-4 GEO is being used for system testing. The system is anticipated to be fully operational by 2010.

B. Ground-Based Augmentation System (GBAS)

A ground-based augmentation system (GBAS) provides differential corrections and integrity data for the GPS or GLONASS open signals using redundant reference stations situated at an airport and a very high-frequency data (VHF) data broadcast. GBAS is intended to provide area navigation in the terminal area and support Category I through III precision approach operations. A detailed description of the GBAS concept and various implementations, including the U.S. Local Area Augmentation System, program may be found in [47].

C. Ground-Based Regional Augmentation System (GRAS)

The ground-based regional augmentation system (GRAS) [48] is a blending of the SBAS and GBAS concepts. GRAS utilizes a distributed set of reference stations and centralized processing sites to compute differential GNSS corrections and integrity data, like SBAS. However, rather than using geostationary satellites to broadcast the data to users like SBAS, GRAS relies on a VHF broadcast using the GBAS physical link and message format. GRAS is currently being developed in Australia and is anticipated to be fully available in 2009.

D. Aircraft-Based Augmentation System (ABAS)

ICAO defines an aircraft-based augmentation system (ABAS) as “an augmentation system that augments and/or integrates the information obtained from the other GNSS elements with information available on board the aircraft.” ABAS includes methods to provide integrity monitoring through either the exploitation of redundant GNSS measurements referred to as receiver autonomous integrity monitoring (RAIM) (see Fig. 9 and [49]) or through the use of onboard sensors (e.g., barometric altimeters, inertial navigation systems, other navigation systems). ABAS also includes the use of other onboard sensors to enhance continuity, availability, or accuracy over that provided by the other elements of GNSS [50].

Figure 9
Fig. 9. Illustration of RAIM concept through an analogy of (a) a two-dimensional problem involving noisy measurements of a linear relationship and (b) the four-dimensional problem of solving user position and clock error in GNSS.
Figure 10
Fig. 10. Multi-mode receiver, approximately 7.85 × 5 × 14 in3, 15 lbs (courtesy of Rockwell-Collins).
SECTION IV

AIRCRAFT RECEIVERS

A. Air Transport

Air transport aircraft typically carry redundant multimode receivers (MMRs) as the onboard GNSS sensor (see Fig. 10). As of April 2002, it was estimated that more than 16 000 MMRs had been purchased for use in the worldwide air transport fleet [51]. These receivers are referred to as multimode because they also provide other navigation sensor functionality. Two major form factors in use include the digital MMR [52] and the analog MMR [53]. The digital MMR provides GNSS, instrument landing system (ILS), and optional microwave landing system (MLS) receiver capabilities within a single unit. A typical analog MMR additionally provides very high-frequency omnidirectional range (VOR) and marker beacon receiver functionality.

Although some MMRs include hardware to process GLONASS signals, this capability is largely a growth path, and current generation MMRs rely primarily if not exclusively on the GPS L1 C/A code signals for their GNSS functionality. Many fielded MMRs do not include SBAS and GBAS functionality, but newer products are including these capabilities. The majority of air transport aircraft operators have thus far displayed greater interest in GBAS than in SBAS due to the greater perceived operational benefits for the former system versus the latter. All fielded MMRs, at a minimum, use RAIM for integrity monitoring.

A typical integration of MMRs within an air transport aircraft's navigation system is shown in Fig. 11. Redundant GNSS and VOR/ILS antennas supply the requisite inputs to the redundant MMRs. The GNSS antennas are top-mounted on the aircraft for good visibility of the satellites, typically near the centerline of the fuselage, fore of the wings to avoid blockage and multipath from the wings and tail structure. A common form factor for airborne GPS antennas is specified in [54]. This form factor calls for a conformal antenna that is 4.7 ×2.9× 0.75 in3, with the height dimension (0.75 in) only accounting for the portion of the unit protruding above the fuselage. Additional inputs to the MMRs may be supplied from the flight management system (FMS) or other navigation sensors for initialization purposes, as well as from control units (not shown) for, e.g., mode selection and channel tuning. The outputs of the MMRs are provided to the FMS and also to flight displays, autopilot, and terrain awareness warning system (TAWS). The FMS may implement integrity monitoring or performance enhancement of the GNSS input through cross-checking or blending with other available navigation sensor inputs.

Figure 11
Fig. 11. Typical integration of MMR within air transport aircraft navigation system.

B. Regional/Business

There are a wide range of GNSS avionics configurations within regional/business aircraft. Larger aircraft within this category often include sophisticated navigation systems, similar to those described for air transport aircraft above but with the GNSS sensor typically installed as a separate unit (as opposed to being integrated within a MMR). Smaller regional/business aircraft may utilize panel mount GNSS sensors, as is common with general aviation aircraft installations to be described in the following section.

C. General Aviation

Although high-end general aviation aircraft may include distributed navigation systems similar to those employed by regional/business aircraft, a more common configuration is the use of a panel mount unit (see Fig. 12). A typical panel-mount unit integrates GPS/SBAS with ILS/VOR, and VHF communications functionality. It has been estimated that well over 100 000 panel mount receivers have thus far been sold by one prominent manufacturer. More than 30 000 of these units include SBAS functionality.

Figure 12
Fig. 12. Panel mount general aviation GPS receiver, approximately 6.25 × 4.60 × 11.0 in3, 9.5 lbs (courtesy of Garmin).
SECTION V

AVIATION APPLICATIONS

A. Navigation

The primary application of GNSS for civil aviation is as a navigation sensor in instrument meteorological conditions for all phases of flight: departure, en route, nonprecision approach, and precision approach. GNSS offers many benefits over traditional navigation aids, including facilitating area navigation (RNAV)—the ability to fly arbitrary routes rather than being constrained by the location of ground navigation facilities. Other benefits include the provision of improved navigation services in areas that are not presently covered by ground navigation aids and the possibility to alleviate some of the expense of maintaining expansive networks of ground navigation facilities.

Performance requirements for navigation sensors generally fall within four categories: accuracy, integrity, continuity, and availability. Accuracy is the degree of conformance between true aircraft position and that position estimate provided by the navigation sensor. Since navigation sensor errors are probabilistic, accuracy requirements are typically specified as horizontal and vertical position error levels that are achieved with high probability (e.g., 95%).

Integrity is the ability of navigation system to provide timely warnings when the system cannot be safely used for navigation. Integrity requirements for safety-critical navigation applications are commonly specified using three parameters: 1) an alert limit, 2) time-to-alert, and 3) integrity level or probability of hazardously misleading information. An alert limit is the maximum allowable navigation system position error before safety would be unacceptably compromised if the user is not promptly notified. The time-to-alert is the maximum allowable period from the onset of an out-of-tolerance condition until an alert is provided. The integrity level or probability of hazardously misleading information is the maximum acceptable probability of occurrence of an out-of-tolerance condition without a timely alert.

Continuity is the capability of a navigation system to perform its function without unscheduled interruptions during the intended operation. Availability is the fraction of time during which a system is usable to perform an intended operation.

Table 3 summarizes ICAO's GNSS signal-in-space performance requirements. Note that although the GPS SPS can meet the accuracy requirements for many phases of flight, it cannot achieve the integrity requirements for any phase of flight without augmentation (ABAS, SBAS, or GBAS). For instance, the integrity requirements within the GPS SPS Performance Standard are based upon the possible occurrence of up to three major service failures per year, each of 6 h in duration, where a major service failure is defined as the presence of a large (over 30 m) range error for measurements to a satellite without the user being able to detect this from the satellite's broadcast navigation data [6].

Table 3
Table 3 ICAO GNSS Signal-in-Space Performance Requirements

Most of the operations listed in Table 3 have been defined for decades and may be performed using traditional ground navigation aids where available. En route through nonprecision approach applications only require horizontal position estimates from the navigation sensor because these operations rely on vertical position estimates from an onboard barometric altimeter. The approach with vertical guidance (APV) operations are newly defined to tailor operations to the capabilities provided by GNSS. Currently available user equipment can achieve the integrity requirements for en route through non-precision approach using RAIM, SBAS, or GBAS, whereas SBAS or GBAS is required for vertically guided operations except for aircraft with sophisticated ABAS capabilities (e.g., barometric vertical navigation). Researchers are currently exploring whether RAIM techniques can be applied to meet precision approach (e.g., APV or Category I) requirements (see, for example, [55]).

Many nations have approved GNSS operations in instrument meteorological conditions. Fig. 13 depicts nations that have approved aviation operations using GPS, as compiled by the FAA from data available circa 2005. An emerging concept in navigation at the present time is required navigation performance (RNP), which is defined to be RNAV operations with navigation containment and monitoring [56]. Many RNP procedures have been developed or are planned worldwide with GNSS as one enabling technology.

Figure 13
Fig. 13. Nations that have approved GPS for aircraft navigation in instrument meteorological conditions (courtesy of the Federal Aviation Administration).

A number of countries, including the United States, are planning to decommission significant numbers of ground-based navigation aids in the future as the GNSS infrastructure advances. A key concern with the increasing reliance on satellite navigation is the vulnerability of GNSS signals to unintentional or intentional radio frequency interference. Prudent means to address this concern include the retention of a subset of existing ground-based navigation aids and the development of operational procedures to mitigate the impact of an event in which GNSS service is lost over a large geographic area.

B. Automatic Dependent Surveillance

Automatic dependent surveillance (ADS) is a concept whereby aircraft continually transmit position, intent, and other data to air traffic service facilities or to other aircraft. ADS provides a number of benefits over radar-based surveillance. These benefits include the provision to air traffic controllers of the location of aircraft in areas where radar coverage is infeasible or impractical, e.g., oceanic and remote airspace. ADS systems can also supply aircraft intent information, i.e., the planned trajectory of the aircraft, which is not available from radars. Modern ADS implementations also allow pilots to view the locations of nearby aircraft on cockpit displays, enhancing situational awareness.

There are two main types of ADS systems. The first type, referred to as ADS-addressed (ADS-A) or ADS-contract (ADS-C), involves transmitting an aircraft's location to a single air traffic services recipient over a point-to-point data link. The second type is ADS-broadcast (ADS-B), in which an aircraft continually broadcasts its position over a data link to air traffic services and other nearby aircraft. GNSS is the most commonly used onboard position sensor for the various ADS services that are currently implemented worldwide.

The use of ADS for civil aviation was first studied in-depth upon the establishment in 1983 of the ICAO Special Committee on Future Air Navigation Systems (FANS) [57]. At that time, GPS and GLONASS were just two of several navigation inputs for ADS that were considered. Other candidates included OMEGA, inertial navigation systems, VOR, DME, and Loran-C. ADS-C implementations using GPS as the primary navigation input were tested in the early 1990s and implemented in some regions of the world shortly after the certification in 1995 of Boeing's FANS-1 navigation system. Airbus later developed an ADS-C and GPS-capable avionics package, FANS-A, which was first certified in 2000 on the Airbus A340/A330 family of aircraft. Implementations involving FANS-1 and FANS-A equipped aircraft combined with compatible ground systems are collectively referred to as FANS-1/A. FANS-1/A ADS-C implementations follow standards developed by RTCA [58] and ARINC [59] that are based upon a dedicated data link connection between each equipped aircraft and air traffic service provider and are still in operation.

The second form of ADS, ADS-B, is currently being implemented in many areas of the world. ADS-B equipped aircraft continually broadcast their position, intent, and other information over a data link to nearby air traffic facilities and other suitably equipped aircraft. The broadcasting function is known as ADS-B Out, whereas the function enabling an aircraft to listen to ADS-B broadcasts from other aircraft and air traffic facilities is referred to as ADS-B In.

The FAA is implementing ADS-B services throughout the United States. A contract was awarded to ITT Corporation in August 2007 to deploy an ADS-B ground infrastructure throughout the United States, and ADS-B services are anticipated to be available in all areas with current secondary radar coverage by 2013. The FAA has recently proposed mandating ADS-B Out equipage for all aircraft in Class A, B, and C airspace in the National Airspace System and Class E airspace at or above 10 000 ft mean sea level over the 48 contiguous states and the District of Columbia, as well as airspace surrounding the busiest U.S. airports and portions of Class E airspace over the Gulf of Mexico [60]. Within the U.S. proposal, aircraft flying at or above flight level 240 (FL240) would be required to meet ADS-B performance requirements using the 1090 Extended Squitter (1090ES) broadcast link [61], whereas aircraft flying below FL240 would be required to meet ADS-B Out requirements with either the 1090ES link or the Universal Access Transceiver (UAT) broadcast link [62].

Many other countries, including Australia, Canada, and the member states of the European Union, are using ADS-B operationally or have demonstration programs underway. Other countries, including China, Fiji, Hong Kong, India, Indonesia, Mongolia, New Zealand, Singapore, and Thailand, are planning trials in the near future [63].

C. Terrain Awareness Warning Systems

Controlled flight into terrain (CFIT), in which a perfectly functioning aircraft is inadvertently flown into the ground, water, or an obstacle, has historically been a leading cause of aviation fatalities. Various technologies referred to as TAWS, ground proximity warning systems, or ground collision avoidance systems have been developed with great success to reduce the occurrence rate of CFIT [64]. Early technological solutions to CFIT employed onboard sensors including radio altimeters, air data systems, and inertial sensors to detect hazardous conditions (e.g., excessive sink rate with respect to terrain clearance) and provide the crew with aural and visual warnings.

Modern TAWS implementations add the use of GNSS with an onboard terrain database to provide a forward-looking capability, and in some cases also depict terrain in the vicinity of the aircraft on a cockpit display. A forward-looking terrain awareness capability has been mandated by ICAO, since January 1, 2007, for all turbine- or piston-engined aircraft with a maximum certified takeoff mass in excess of 5700 kg or authorized to carry more than nine passengers [65].

D. Timing

The GNSS is capable of disseminating precise time and frequency. This capability is utilized by many applications including a number of aviation systems. As one example, the UAT is an aeronautical data link that supports ADS-B. Currently available airborne UAT equipment, which require knowledge of Coordinated Universal Time (UTC) within 5 ms [62], use GPS for timing.

SECTION VI

SUMMARY AND CONCLUSIONS

The GNSS presently consists of a fully populated GPS constellation, a partially populated GLONASS constellation, two operational SBASs (WAAS in the United States, and MSAS in Japan), a small number of operational GBASs, and a large amount of installed user equipment. The majority of currently installed user equipment only utilize the GPS L1 C/A code signals and, to a lesser extent, SBAS L1 signals, in conjunction with RAIM and other onboard sensors to meet integrity requirements for navigation.

Within the next decade, it is anticipated that modernized GPS signals will be broadcast, the GLONASS constellation will become fully populated, and additional satellite navigation systems including the European Galileo, Chinese Compass, Japanese QZSS, and Indian IRNSS will become operational. These additional ranging sources will greatly increase the accuracy, continuity, availability, and robustness of GNSS. Additionally, in the forthcoming years, additional SBASs (e.g., EGNOS and GAGAN) are expected to be commissioned and a greatly increased number of GBAS ground facilities deployed.

These emerging GNSS capabilities will offer tremendous benefits to the civil aviation community worldwide, including the opportunity to decommission portions of the costly ground-based navigation aid infrastructure and the enabling of many new procedures to allow for more efficient use of the airspace.

Challenges for the future include decisions on capabilities to include within avionics to maximize user benefits (e.g., if a multitude of satellite navigation systems are deployed, which GNSS signals should be processed?) and the selection of evolution paths for the GNSS components. Some aspects of the latter topic are addressed in [55].

Acknowledgment

The authors would like to thank the following individuals for helpful inputs to this paper: G. McGraw and J. Wichgers of Rockwell-Collins; T. Murphy of Boeing; D. Benson, R. Braff, J. P. Fernow, V. Massimini, K. Markin, J. Nickum, and R. Strain of The MITRE Corporation; and G. Thompson, D. Burkholder, and K. Alexander of the Federal Aviation Administration.

Footnotes

Manuscript received nulldate; revised June 07, 2008. Current version published nulldate.

C. J. Hegarty is with The MITRE Corporation, Bedford, MA 01730 USA (e-mail: chegarty@mitre.org).

E. Chatre is with the GNSS Supervisory Authority, Brussels, Belgium (e-mail: eric.chatre@gsa.europa.eu).

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Authors

Christopher J. Hegarty

Senior Member, IEEE

Christopher J. Hegarty (Senior Member, IEEE) is a Director with The MITRE Corporation, Bedford, MA where he works primarily on aviation applications of GPS. He is currently President of The Institute of Navigation and Chair of RTCA, Inc.'s Program Management Committee. He coedited/coauthored Understanding GPS: Principles and Applications, 2nd ed. (Norwood, MA: Artech House, 2006).

Dr. Hegarty received the 2005 ION Kepler Award and the 2006 Worcester Polytechnic Institute Hobart Newell Award.

Eric Chatre

Eric Chatre graduated as an Electronics Engineer from Ecole Nationale de l'Aviation Civile, Toulouse, France, in 1992.

From 1994 to 2001, he was with the Air Navigation Service Provider, Toulouse, working on implementation of satellite navigation in civil aviation. He has since then been working for the EGNOS and Galileo programmes and is now part of the GNSS Supervisory Authority in charge of Mission Definition and Certification aspects.

Cited By

Worldwide Vertical Guidance of Aircraft Based on Modernized GPS and New Integrity Augmentations

Proceedings of the IEEE, vol. 96, issues (12), p. 1918–1935, 2008

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