Air Navigation with Global Navigation Satellite Systems and The

Home , Air navigation, GNSS Augmentation

3-1-2 Air Navigation with Global Navigation Satel- lite Systems and the Ionospheric Effects

SAKAI Takeyasu, MATSUNAGA Keisuke, YOSHIHARA Takayuki, and SAITO Susumu

Global navigation satellite system(GNSS) is now used in air navigation. Ionospheric delay in radio wave propagation is one of the most serious problems in using GNSS. Since not only the accuracy but also the safety is very important in air navigation, augmentation systems are necessary to ensure the safety. SBAS(Satellite-based Augmentation System), which covers a wide area with geostationary satellites, or GBAS(Ground-based Augmentation System) which covers a local area around an airport is among those augmentation systems. They are now partially in operation. Local ionospheric delay gradient is a threat to these systems. Measures to protect users from the threat of the ionospheric gradient prevent the advanced use of GNSS in air navigation. If ionospheric gradients were effectively detected without fail and users were timely notified about it, GNSS would be utilized in a more advanced way in air navigation. Keywords Air navigation, Global navigation satellite system, Satellite-based augmentation system, Ground-based augmentation system, Ionospheric anomaly

1 Introduction 2 Ionospheric delays and global navigation satellite systems In recent years, global navigation satellite systems (GNSS) as typified by the Global GPS is a global navigation satellite system Positioning System (GPS) have come into administered by the U.S. The GPS receiver advanced use in the air navigation industry. monitors radio signals broadcast from at least Ionospheric delays can be a major source of four GPS satellites (global navigation satel- error in GPS usage and how to correct those lites) and then measures distances to those delays poses a key challenge. Both safety and satellites, thereby computing the current loca- accuracy are of critical importance to air navi- tion of the receiver［1］. The earth’s atmosphere gation. Several augmentation systems between global navigation satellites (e.g., GPS designed to assure safety have already been satellites) and ground-based receivers affects developed and put to practical use on a limited radio wave propagation. Plasma present in the scale. ionosphere that forms part of the earth’s This report discusses the concept of atmosphere slightly varies the refractive index integrity—an essential element of air naviga- of radio waves, resulting in the velocity of tion applications based on a global navigation radio wave propagation in the ionosphere satellite system—and how to assure integrity. being varied from the velocity of light in the It then introduces the augmentation systems vacuum. Because satellite positioning uses the used with global navigation satellite systems radio wave propagation time to estimate dis- and the measures built into those augmenta- tance, changes in the velocity of radio wave tion systems to address ionospheric effects. propagation are directly related to positioning

SAKAI Takeyasu et al. 231 errors. Refractive index n and group refractive index n' in the plasma can be expressed in equations as:

（1）

（2） where, e denotes the quantum of electric charges, ne the electron density, me the electron mass,ε0 the electric constant, and f the frequency. Here, phase velocity v and group Fig.1 Group delays per TECU relative to velocity v' of radio waves can be expressed in frequencies equations as:

（3） sity between the satellite and receiver, and assume identical absolute values with opposite signs. Because both TEC and frequency are （4） positive values, the phase delay is always neg- ative, so that the phase advances in plasma. Given the relation n < 1, n'> 1, the phase of This means that the phase of the carrier mea- radio waves in the plasma advances while the sured by GPS advances (shortens) while the propagation of information is delayed. code pseudo-range lengthens. The total num- Because the propagation time equals the ber of 1016 electrons per square meter is inverse of velocity of propagation integrated expressed as 1 TECU. Figure 1 plots the group between the satellite and receiver, group delay delays per TECU relative to frequencies, indi- τ caused by ionospheric plasma can be cating 0.16 m, 0.27 m and 0.29 m at GPS L1 expressed as: (1.57542 GHz), L2 (1.22760 GHz) and L5 (1.17645 GHz), respectively. The typical TEC （5） during the solar maximum and minimum in spring and autumn in the vicinity of Japan is This value can be converted to distance as: 70 TECU in the daytime and 15 TECU at nighttime. This corresponds to delays of 11 m, 19 m and 20 m in the daytime and 2.4 m, （6） 4.0 m and 4.4 m at nighttime at frequencies L1, L2 and L5, respectively. The phase delay may be converted to distance as: 3 Impact on GPS and counter- measures

（7） 3.1 Impact on GPS As explained in the foregoing section, the where, λ denotes the radio wavelength. Obvi- ionospheric delay present in the code pseudo- ously, the group delay distance and phase range of a GPS satellite signal is proportional delay are proportional to the total electron to TEC along the path of radio wave propaga- content (TEC) integrated by the electron den- tion and generally gets larger with a lower-ele-

232 Journal of the National Institute of Information and Communications Technology Vol.56 Nos.1-4 2009 vation-angle satellite. While ionospheric half of a cosine function peaking at 14 hours delays are manifested as range errors on indi- (local time), to a daytime ionospheric delay. vidual GPS satellites, these delays adversely The amplitude and period of the cosine func- affect the ultimate positioning solution or the tion are each represented in a cubic polynomi- accuracy of estimating the observation point al of the geomagnetic latitude, and a total of coordinate location and receiver clock, as eight coefficients are conveyed by navigation errors. How the range error in the code pseu- messages. An ionospheric thin-shell model is do-range caused by an ionospheric delay postulated for calculating ionospheric delays affects these estimates depends on the number and its point of intersection with the path of of satellites available and satellite location GPS radio wave propagation is called an (geometry), but its impact is generally known ionospheric pierce point (IPP). Note that the to worsen positioning accuracy in the vertical ionospheric delay model introduced above direction. Although DOP (dilution of preci- uses the geomagnetic latitude of the IPP of sion) is available as an index to assess the each GPS satellite observed. A lower-eleva- impact of GPS satellite location on the posi- tion satellite tends to post a larger ionospheric tioning solution by assuming equal range delay. This effect is expressed by multiplying errors present in the pseudo-ranges of GPS each ionospheric delay by an inclination factor satellites, the practical job of assessing a that allows for the angle of incidence of GPS degraded positioning solution in which ionos- radio waves at the IPP. pheric delays are factored should consider that the effects of range errors caused by ionos- 3.3 Differential correction process pheric delays are varied among GPS satellites. The usefulness of corrections implemented In GPS-based precision surveys, ionos- using an ionospheric delay model like the one pheric delays are typically corrected using the introduced above generally diminishes as carrier phase of two frequencies (L1 and L2). ionospheric delays involved in the observed In air navigation applications concerned with values deviate farther from the model. As a real-time processing and safety, the ionospher- solution to enhanced positioning accuracy, a ic correction with carrier phase of L1 and L2 differential correction method is used for gen- is not directly used, since code pseudo-ranges erating corrections in real time from actual are mainly used and the L2 band is not global- observation data, coupled with transmission to ly protected as a navigation band. In some user stations also in real time to boost posi- augmentation system implementations, how- tioning accuracy. This differential correction ever, the ionospheric correction with the L1 process uses GPS reference stations of known and L2 signals may be used to generate ionos- locations and calculates range errors involved pheric delay correction information. in the code pseudo-range from the observed values, thereby extracting such inherent com- 3.2 Correction based on navigation mon errors as GPS satellite orbit error, GPS messages satellite clock error, ionospheric delay and tro- Navigation messages that are broadcast pospheric delay for use as correction informa- from GPS satellites include GPS satellite orbit tion. information, plus ionospheric correction para- The differential correction process can be meters. As a means of ionospheric delay cor- broken down into local and wide-area differ- rection based on navigation messages in the ential correction according to intended use. point positioning with code pseudo-ranges, Local differential correction, useful in the daily variations are approximated on an ionos- vicinity of reference stations, generates cor- pheric delay model (Klobucher model［2］) rection information as common error informa- derived by adding a constant nighttime ionos- tion without distinguishing ionospheric delays pheric delay of 5 ns (1.5 m), plus the upper from error sources, such as GPS satellite orbit

SAKAI Takeyasu et al. 233 errors and tropospheric delays. Local differen- zation (ICAO) instituted the Global Naviga- tial correction tends to yield high accuracy due tion Satellite System (GNSS) panel in 1993 to to its relative technical simplicity, but its cor- drive its pursuit of global navigation satellite rection information characteristically degrades systems. The panel termed a global navigation as users are located far away from the refer- satellite system that provides performance ence stations. Wide-area differential correction available for civil air navigation purposes as involves a network of reference stations GNSS, and introduced an international stan- deployed within the service coverage to break dard known as GNSS Standards and Recom- down the range errors involved in these code mended Practices (SARPs) in November pseudo-ranges by error source, and thus gener- 2001［3］. According to GNSS SARPs, GNSS ates correction information for transmission to is defined to encompass ground receivers and the users. For this reason, measurements using ground monitoring facilities, as well as artifi- the dual-frequency carrier phase of L1 and L2, cial satellites. It is configured of GPS or and the gridding of correction information are GLONASS (a core system) with an augmenta- typically used with regard to ionospheric tion system. GNSS SARPs also define the per- delays. formance requirements to be fulfilled by GNSS. 4 GPS for air navigation Three kinds of augmentation systems are defined as follows: (i) SBAS (satellite-based 4.1 Air navigation and integrity augmentation system), which transmits aug- The usefulness of GPS for aircraft naviga- mentation signals in wide-area on a continen- tion has been recognized from the beginning, tal scale by means of geostationary satellites, but a key challenge was how to assure integri- (ii) GBAS (ground-based augmentation sys- ty. Integrity here refers to the ability to guar- tem), which provides local augmentation on antee that a navigation system is error-free, VHF waves targeting areas around airports, and promptly issue a warning in case the sys- and (iii) ABAS (aircraft-based augmentation tem becomes unfit for navigation purposes system), which provides augmentation only due to, for example, the failure to provide with the aid of equipment mounted on-board intended positioning performance. Securing aircraft. Among these, SBAS and GBAS integrity calls for a means of immediately secure a high degree of integrity by monitor- detecting any abnormal signal transmitted ing GPS signals with ground monitor stations, from a GPS satellite and reporting it to users. and also transmit correction information to GPS positioning accuracy is adequate for offer high user positioning accuracy. SBAS is aircraft navigating en route. However, GPS equivalent a wide-area differential correction only has five monitor stations located world- system as mentioned earlier; GBAS is equiva- wide, making it insufficient for immediately lent to a local differential correction system. detecting satellite failures in real time and ABAS is intended to assure integrity by reporting those failures to users. Moreover, verifying the consistency of information col- each national government is responsible for lected by on-board equipment, such as with providing a means of air navigation (with the the aid of redundant GPS signals, but only Japan Civil Aviation Bureau being responsible works for navigation in the horizontal direc- in this country), and full-scale dependence on tion because the system cannot secure ade- the U.S. navigation system (including the quate integrity for positioning information in issue of assuring integrity) would prove diffi- the height direction. SBAS and GBAS are dis- cult. cussed below from the perspective of the Keen to work out international standards provider of the means of navigation. for civil aviation including navigation systems, the International Civil Aviation Organi-

234 Journal of the National Institute of Information and Communications Technology Vol.56 Nos.1-4 2009 4.2 Integrity assurance scheme is assumed functional in the current airspace GNSS SARPs define the performance (Fig. 2). If user positioning errors are likely to requirement for integrity as the probability of exceed the alert limit for some reason, the pro- user receiver positioning errors being held tection level rises proportionately to exceed within an alert limit or the prompt posting of the alert limit, thereby allowing for abnormal positioning errors to the user in case user conditions to be detected and posted to users receiver positioning errors exceed the alert If the protection level calculated by a user limit. In other words, this refers to the proba- receiver for a given airspace exceeds the alert bility of events where positioning errors are limit, then GNSS is made unavailable in that held below the alert limit not occurring, with- airspace. Accordingly, a lower protection level out being notified to the user. This probability setting would make a navigation system easier is defined as 1－10－7 per hour for aircraft en (more accessible) to use (more available), but route or 1－2 • 10－7 for aircraft approaching and this entails a tradeoff with the integrity landing on a runway. requirement for meeting the user positioning The alert limit requirement would vary as error-protection level relation. a parameter of in-flight airspace, and its value had not been determined in the stages of aug- 4.3 Integrity threats mentation system development. For this rea- A certain level of hardware reliability is son, the confidence limit for user positioning required to assure integrity, but the availability errors occurring in a service volume has been of error-free information to users assumes assigned a significance level of 10－7 or less on essential importance. If a system is shut down this augmentation system. This confidence due to inadequate hardware reliability, system limit is called a “protection level,” and the continuity would suffer, not integrity. From augmentation system transmits information the standpoint of integrity, it would be better allowing user receivers to calculate the protec- to shut down the system instead of delivering tion level. The augmentation system must invalid information to users as far as efficien- ensure that the probability of user positioning cy is concerned. The kind of invalid informa- errors exceeding the protection level is held tion that threatens integrity or could produce a below 10－7. Under this framework, when the user positioning error exceeding the protection protection level is below the alert limit, GNSS level is called “hazardous misleading information (HMI).” The user positioning error-protection level relation discussed above must always be met at any point whatsoever in a service area. This means that the integrity requirement must be met in even the worst case among multiple cases, rather than being met by the collective average of all those cases. A very high level of integrity is sought (107 hours = 1,141 years), which virtually inhibits the occurrence of user positioning errors beyond the protection level. In the practical process of authenticating SBAS or GBAS as a navigation system, it is necessary to ensure that user positioning errors do not exceed the protection level and are kept sufficiently small. Assuring integrity entails the work of Fig.2 Protection level and alert limit assessing the probability and size of HMI rela-

SAKAI Takeyasu et al. 235 tive to each individual positioning error, and factors; the spike in distributions apart from taking relevant measures as appropriate. Even normal distributions represents lower-frequen- error sources that can cause positioning errors cy events that could invoke major errors. Cor- would not translate into HMI when easily rectable events are marked blue; uncorrectable detectable by a ground station. Moreover, events are marked red. As can be seen from even hardly detectable error sources would be the lower part of the diagram, SBAS and of no concern if only leading to minor posi- GBAS apply correction information to make tioning errors. Easily detectable errors sources normal distributions appear more compact, are typified by GPS satellite clock and orbit while delivering valid integrity information to errors; hardly detectable ones involve tropos- eliminate any threats that could definitely pheric propagation delays. resulting in HMI, thereby protecting the users. Each individual factor identified to translate into HMI is called a “threat.” Measures 4.4 Ionospheric storm effect must be taken to prevent each individual threat The most serious of all prevailing integrity from resulting in HMI. Among these mea- threats is the effect from propagation delays sures, the “monitoring” process is designed to associated with ionospheric storms. Although suit the characteristics of a given threat and be ionospheric storms may not occur frequently, implemented within SBAS or GBAS. When they occur at other than a negligible level. threats are detected by monitoring, actions are Ground monitor stations do not necessarily invoked, such as increasing the protection detect ionospheric storms as they occur, with level, to prevent threats from resulting in such storms being manifested as major user HMI. positioning errors that could result in an HMI The upper panel of Fig. 3 shows a concep- event where the user positioning error exceeds tual image of the probability distributions of the protection level. GNSS positioning errors. Normal distributions SBAS and GBAS provide protection (marked green) in the middle designate posi- against this problem by assuming the continu- tioning errors that may result from various ing presence of ionospheric disturbances in generating integrity information, and also by factoring a sufficient margin into the protection level to discourage users from using a possibly hazardous satellite. The worst case of ionospheric storms is always assumed based on past records of ionospheric observation data. Moreover, monitors are installed to be able to deal with HMI conditions they have been observed in the past. Because these precautions are taken, the protection level tends to rise, leaving room for improvement concerning availability. One cause of degraded availability is monitor stations located too sparsely in preparation for a worst case scenario, given their inability to identify normal conditions other than ionospheric storms. Given the insight into the status Fig.3 Concept of positioning error distrib- ution of ionospheric storms from the standpoint of (Upper) GNSS without augmenta- space weather, a normal protection level free tion (Lower) Effect of the augmentation from concern over anticipated ionospheric system storms could possibly be drastically lowered

236 Journal of the National Institute of Information and Communications Technology Vol.56 Nos.1-4 2009 for improvement. Table 1 Maximum update times and availability times of SBAS ionospheric correction messages 5 Actual augmentation systems

5.1 SBAS (MSAS) 5.1.1 How SBAS works SBAS broadcasts GNSS augmentation information from geostationary satellites to broad sets of users. SBAS consists of ground equipment, geostationary satellites and user receivers. points (IGPs) of geographic longitude and lati- Normally, multiple ground monitor sta- tude. (At latitude higher than 55˚, the IGP lay- tions are distributed at intervals of about sev- out varies.) The following types of SBAS eral hundred kilometers to collect data with messages relevant to ionospheric correction receivers enabled for dual-frequency signals are broadcast: originating from a GNSS core system. Satel- Type 18: IGP service status flag lite-specific orbit/clock errors and ionospheric Type 26: IGP vertical delay estimate (@L1) delays are estimated from the data collected, (0 to 63.875 m, 0.125 m unit) and integrity information, SBAS satellite and post-correction vertical error ranging data and correction information are (residual) variance (index) generated, which are then uplinked to an Type 10: Time degradation parameters SBAS satellite. User receivers calculate ionospheric SBAS signals have the same frequency delays and residual variances by assuming the and C/A code modulation as GPS-L1, and are ionosphere as being a thin shell at an altitude assigned pseudo-random noise (PRN) num- of 350 km. First, the longitude and latitude of bers from 120 to 138. This allows signals orig- the ionospheric pierce point (IPP) of satellite inating from the SBAS satellite to be used as signals received by users from each satellite GPS-like ranging signals. Augmentation infor- are calculated. The vertical delays/residual mation is broadcast at 250 bps, with each mes- variances at the surrounding IGPs in the longi- sage consisting of 250 bits (preamble (8), tude/latitude plane are then subjected to bilin- message type ID (6), data area (212) and CRC ear interpolation for calculating values at IPP, (cyclic redundancy check) code (24)). which are then multiplied by an inclination User receivers use the augmentation infor- factor according to satellite elevation angle for mation received from the SBAS satellite to working out the ranging correction/residual work out positioning and integrity informa- variance values. The residual variance is fur- tion. Tropospheric errors are calculated within ther multiplied by a time degradation factor or the receivers using a model. other factor to perform positioning and calcu- 5.1.2 SBAS ionospheric correction late the protection level based on satellite SBASs now in service or under develop- location as described in Section 4.2. ment target single-frequency aviation user Each SBAS message has a maximum receivers and users who conduct positioning update time and a user availability time calculations based on the pseudo-ranges of the defined as provider service requirements. GNSS core satellites and SBAS satellites. The Table 1 lists the requirements for ionospheric methodology is formulated as an ICAO inter- correction messages. national standard［3］. How to generate SBAS messages is left to SBAS is intended for users in a wide area, the discretion of each SBAS service provider, with ionospheric delay information being but system implementation/verification must broadcast as values at 5˚ x 5˚ ionospheric grid be done so as to meet the integrity require-

SAKAI Takeyasu et al. 237 ments defined in Section 4. bubbles for the deployment of ground stations. 5.1.3 Current status of SBAS Enhanced SBAS ionospheric correction SBASs now in service (2009) include performance should benefit from efforts to MSAS (in Japan)［4］, WAAS (in the U.S.) and explore system capabilities tailored to initial EGNOS (in Europe), while GAGAN (India) is detection and prediction, including the devel- under development. Figure 4 shows an exam- opment of ionospheric models useful under ple of the deployment of MSAS ground sta- the conditions outlined above, deploying tions, along with surrounding IGP and IPP ground stations based on those models, and distributions. detecting and handling disturbance phenome- As explained in the foregoing section, na. SBAS is dedicated to providing a broad range of services, but remains vulnerable to con- 5.2 GBAS strained performance in low-geomagnetic-lati- 5.2.1 How GBAS works tude regions of intense ionospheric activity A landing guidance system based on a sin- due to the delivery of ionospheric correction gle-frequency differential GPS positioning information such as the values of IGP at 5˚ lat- principle, GBAS is intended for use in the itude, as well as the update rate (600-second vicinity of an airport (with a minimum cover- message validity). Consequently, SBAS age requirement of approx. 40 km). Each encounters difficulty in offering a high level GBAS has three to four GPS reference sta- of integrity (1－10－7) while minimizing the tions (GBAS reference stations) deployed on protection level in the presence of small spa- the premises of an airport to generate informa- tial/temporal changes in ionospheric delays or tion on correcting common errors (e.g., GPS spatially small disturbances such as plasma orbit errors and ionospheric delays present in code pseudo-ranges of GPS satellite signals, residual error parameters by error source for determining the reliability of positioning solu- tions calculated by aircraft), and then broad- casting all such information to aircraft on the VHF band (108 to 118 MHz) as augmentation messages like those listed in Table 2 (VHF Data Broadcast (VDB)). Each aircraft per- forms a positioning process based on the correction information, and calculates the protec-

Table 2 Contents of augmentation messages generated by GBAS

Fig.4 MSAS ground station layout, IGP location, and examples of IPP distributions □ denotes an MSAS ground station (at Sapporo, Hitachi-Ota, Tokorozawa, Kobe, Fukuoka and Naha, with the stations at Hitachi-Ota and Kobe serv- ing as geostationary satellite uplink stations). ● denotes a 5˚x 5˚ longitude/ latitude ionospheric grid point (IGP). ▲ denotes the IPP of a GPS satellite monitored by a ground station at a given time (at altitude of 350 km).

238 Journal of the National Institute of Information and Communications Technology Vol.56 Nos.1-4 2009 tion level from residual error parameters to operations up to Category III, thus enabling assess in real time whether it is held within precision guidance down to a height of 0 m. tolerances to determine the reliability of its 5.2.2 GBAS ionospheric delays positioning solution. Because GBAS ionospheric delays exist Pseudo-range errors that may exist at both almost to an equal extent in the psuedo-range GBAS reference stations and aircraft to an data received by both GBAS reference sta- equivalent extent include GPS satellite orbit tions and aircraft, most have been thought errors, satellite clock offsets, ionospheric removable. But GBAS implements a smooth- delays and tropospheric delays. GBAS ing process for the code-pseudo range called removes most of these common errors. Other “carrier smoothing” (using a time constant of possible errors such as receiver clock errors, 100 seconds) to cut multipath errors present in receiver noise and multipath errors are the code-pseudo range based on the carrier reduced by averaging the multiple sources of phase of the GPS signal. As expressed in GPS received data from GBAS reference sta- Equations (6) and (7), ionospheric delays tions located more than 100 m apart to gener- involved in the code pseudo-range and carrier ate more accurate and reliable augmentation phase have the same magnitude but opposite information. polarities, and ionospheric delay contained in The possible range of guidance supported the code pseudo-range and that contained in by landing guidance systems to enable the pre- the carrier phase up to 100 seconds before are cision approach of aircraft is specified by matters of concern［5］. Therefore, if a spatial ICAO regarding visibility, landing system per- gradient of an ionospheric delay (hereinafter formance and other factors, and with landing “ionospheric gradient”) is encountered as an system requirements defined for three levels aircraft lands after making the final approach of the approach and landing phases (Cate- from a point several ten kilometers apart from gories I to III)［3］. Category I, for example, a GBAS reference station as shown in Fig. 5, supports the initial stage of landing system it will affect GPS satellites passing over that requirements. Landing systems falling in Cat- region. egory I should provide precision guidance up The ionospheric gradient (dI/dx) is nor- to a height of about 60 m (200 ft.)—the so- mally expressed as a rate of ionospheric delay called decision height (DH)—at which the change for each horizontal kilometer, and pilot decides whether to land or not. indicated in mm/km units. Assuming that the The positioning principle of GBAS allows the system to deliver significantly high accuracy and its performance has long been veri- fied. In the meantime, integrity (i.e., measure of assuring the validity of generated correction information) has loomed as a key challenge. Category I, for example, defines the probability of not entering a hazardous state due to invalid correction information as being 1－2 • 10 －7 per approach, thereby dictating an extremely high level of reliability. For this and other reasons, mitigation algorithms have been developed for both risk extraction and assessment as needed. In the U.S., preparations are now underway to launch GBAS Category I service. Enhanced ground and on-board equip- Fig.5 GBAS and ionospheric delay spatial ment may eventually allow GBAS to support gradient

SAKAI Takeyasu et al. 239 ionospheric gradient does not change over tively［8］. These values are based on ionos- time and disregarding GPS satellite behavior, pheric gradients associated with SED observa- maximum δI of range errors in the satellite’s tions made in North America on November line of sight (slant) direction as corrected by 20, 2003. GBAS and derived from the ionospheric gra- On the ionospheric threat model, a GBAS dient can be generally expressed in Equation reference station may detect ionospheric gra- (8) as: dients as sharp time-related changes in an ionospheric delay. Called “CCD monitoring,” （8） this method of detection uses time-related changes in CCD (code-carrier divergence), where, x denotes the separation between the which is derived by subtracting the pseudo- GBAS reference station and aircraft. The first range associated with the carrier phase from term on the right side designates a range error the code pseudo-range, by leveraging the fact similar to typical differential GPS resulting that the code pseudo-range and carrier phase from a spatial change in the ionospheric delay; have virtually the same magnitude but oppo- the second term designates the effect of carrier site polarities［9］. smoothing. In the U.S., GBAS safety design is A situation may arise, however, where an implemented based on the settings of an ionospheric front has yet to reach a reference ionospheric gradient maximum of 425 mm/km station or no error has been detected at a and a horizontal distance of 6 km from a GBAS reference station in sync with IPP GBAS reference station to the decision height velocity, resulting in only aircraft being affect- (DH). The range error maximum estimated at ed by ionospheric gradient error. To address DH by solving Equation (8)—with smoothing this possible situation, a method called “geom- time constantτ of 100 s and aircraft velocity v etry screening” has been employed in the U.S. of 0.070 km s－1 assigned—is calculated to be According to this method, conceivable range about 8.5 m［6］. Complications involving errors are postulated by assuming that ionos- adverse conditions such as fewer GPS satel- pheric gradient errors not detectable by a lites are generally known to worsen the ulti- GBAS reference station always exist on the mate positioning solution. As a result, some aircraft, and a subset of GPS satellites that actual vertical errors are found to exceed a could bring unallowable positioning errors is vertical warning limit of 10 m at DH, despite a identified on the ground, so that information vertical protection level not exceeding 10 m, with deliberately exaggerated residual errors and thus suggest the need to employ methods will be broadcast to prevent the protection of countering GBAS integrity threats and miti- level from being used beyond the alert limit gating risks［7］. when an aircraft attempts positioning calcula- 5.2.3 Ionospheric risk assessment and tions in that mix of GPS satellites［8］. mitigation In Japan, observations of any ionospheric In the U.S., the ionospheric gradients gradients in excess of several hundred mm/km (called “storm enhanced density” or SED) like those observed in the U.S. have yet to be resulting from magnetic storms have been reported, but the concept of GBAS ionospher- modeled as an ionospheric front and three ic risk assessment should allow for the signifi- parameters (ionospheric gradient magnitude, cant plasma bubbles frequently observed in slope width, and velocity) have been isolated low-geomagnetic-latitude regions, as well as and delimited to build an ionospheric threat SED. Two ionospheric fronts varying in polar- model. As a result, the U.S. has adopted possi- ity arise from a single plasma bubble; two or ble ionospheric gradient maximums of more plasma bubbles generate far more such 425 mm/km and 375 mm/km for high-eleva- fronts. Therefore, a perspective of review dif- tion and low-elevation GPS satellites, respec- ferent from that of assessing vertical error

240 Journal of the National Institute of Information and Communications Technology Vol.56 Nos.1-4 2009 impact using SED should be worthwhile. In 6 Conclusions addition, the frequency of plasma bubble occurrence is found characteristically higher Recent years have witnessed rapid compared with that of SED. advances in the pace of utilizing GPS-based With these matters taken into considera- navigation systems in the air navigation indus- tion, Electronic Navigation Research Institute try, but how to correct ionospheric delays is building an ionospheric threat model suit- poses a key challenge relative to GPS usage. able for Japan, as well as probing and devel- Both safe and accurate GPS-based navigation oping mitigation methods. It is also exploring systems are of such critical concern to air nav- the possibilities of ionospheric storm field igation that these systems place emphasis on monitoring, whereby GPS receiving stations assuring integrity. Moreover, because air navi- are installed at the end of each runway or else- gation is meant to address practical needs, it where (apart from a GBAS reference station) must be available at any time and accessible to to detect spatial ionospheric gradient errors in anybody. Hence, these two conflicting require- the runway direction, in addition to detecting ments must be fulfilled to the extent that safe- abnormal time-related changes in ionospheric ty can be maintained. delays by using CCD monitoring and elimi- SBAS and GBAS utilize different methods nating those subsets of GPS satellites that of generating differential correction informa- could bring about major positioning errors, by tion and integrity information for delivery to the geometry screening method. Any GBAS aviation users, in order to provide accurate integrity risks designed to be held within toler- and safe aircraft guidance. Under the circum- ances via these monitoring and mitigation stances, the local gradients of ionospheric methods should require verification, but any delays are the toughest of all sources of error necessary ionospheric storm field monitoring to control when threatening safety. SBAS and should entail GBAS installation, increased GBAS have measures in place to protect to operational constraints, extra costs and other aviation users against the threats of ionospher- needs. This is because not all ionospheric ic gradients, including securing safety margins anomalies are detectable by a ground refer- to meet relevant international standards and ence station. This weakness might well be dif- setting system design/operational limits. These fused given the availability of space (ionos- measures, however, in turn impede the pace of phere) weather information that both efficient- more advanced GPS utilization. ly and unfailingly contributes to the detection Developing an ionospheric information of ionospheric gradients. system capable of detecting ionospheric gradients both efficiently and unfailingly, and releasing it on a timely basis could definitely help facilitate more advanced GPS utilization in air navigation applications.

References 01 Sakai, T., “Introduction to GPS technique (in Japanese),” Tokyo Denki University Press, 2003. 02 Klobucher, J. A., “Ionospheric Effects on GPS, Global Positioning System: Theory and Applications,” Vol. 1, B. W. Parkinson, J. J. Spilker, P. Axelrad, and P. Enge (Eds.), American Institute of Aeronautics and Astronautics, 485–515, 1996. 03 “International Standards and Recommended Practices, Aeronautical Telecommunications,” Annex 10 to the Convention on International Civil Aviation, Vol. 1, 6th Ed., July 2006, ICAO. 04 Manabe, H., “MTSAT satellite-based augmentation system (MSAS) in operation (in Japanese),” GPS/GNSS Symposium, pp. 35–39, 2007.

SAKAI Takeyasu et al. 241 05 Christie, J., B. Pervan, P. Enge, J. D. Powell, B. Parkinson, and P.-Y. Ko, “Experimental Observations of Ionospheric and Tropospheric Decorrelation Effect for Differential Satellite Navigation during Precision Approach,” proc. of ION GPS-98, 739–747, 1998. 06 Pullen, S., Y. Park, and P. Enge, “The Impact and Mitigation of Ionosphere Anomalies on Ground-Based Augmentation of GNSS,” the 12th International Ionospheric Effects Symposium, 2008. 07 Luo, M., S. Pullen, T. Walter, and P. Enge, “Ionospheric Spatial Gradient Threat for LAAS: Mitigation and Tolerable Threat Space,” proc. of ION National Technical Meeting 2004, 490–501, 2004. 08 Ramakrishnan, S., J. Lee, S. Pullen, and P. Enge, “Targeted Ephemeris Decorrelation Parameter Infla- tion for Improved LAAS Availability during Severe Ionosphere Anomalies,” proc. of ION National Techni- cal Meeting 2008, 354–366, 2008. 09 Ene, A., D. Qiu, M. Luo, S. Pullen, and P. Enge, “A Comprehensive Ionosheric Storm Data Analysis Method to Support LAAS Threat Model Development,” proc. of ION National Technical Meeting 2005, 110–130, 2005.

SAKAI Takeyasu, Dr. Eng. MATSUNAGA Keisuke Chief Researcher, Communication, Senior Researcher, Communication, Navigation, and Surveillance Navigation, and Surveillance Department, Electronic Navigation Department, Electronic Navigation Research Institute Research Institute Satellite Navigation Satellite Navigation

YOSHIHARA Takayuki, Ph.D. SAITO Susumu, Ph.D. Senior Researcher, Communication, Senior Researcher, Communication, Navigation, and Surveillance Navigation, and Surveillance Department, Electronic Navigation Department, Electronic Navigation Research Institute Research Institute Satellite Navigation Aeronomy, Satellite Navigation

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