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This paper investigates the potential vertical guidance performance of global positioning system (GPS)/wide area augmentation system (WAAS) user avionics sensor when the modernized GPS and Galileo are available. This paper will first investigate the airborne receiver code noise and multipath (CNMP) confidence (_{air}_{air}_{air}

Presently, the only fully operational Global Navigation Satellite System (GNSS) is the Global Positioning System (GPS), which was developed, implemented, and is operated by the US Department of Defense (DoD) to provide position, velocity, and time information to users worldwide [

The GNSS is undergoing substantive changes that will enhance its capabilities in all applications. This future GNSS includes three key elements:

While the Federal Aviation Administration (FAA) WAAS works well under nominal conditions, it is susceptible to the ionospheric disturbance and to the satellite outages. Because of the addition of the new frequencies (L2 and L5) and the new constellation (Galileo), WAAS should be modernized to take advantage of the modernized GPS and Galileo. Therefore, the objectives of this paper are to assess the following two questions. First, can WAAS achieve 20 m Vertical Alert Limit (VAL) (

Accordingly, this paper is organized as follows: Section 2 discusses the system analysis assumptions. The performance of the upgraded WAAS is discussed in Section 3. Section 4 discusses the performance of the L1–L5 dual-frequency WAAS. The possible benefit from the improved signal model is discussed in Section 5. In Section 6, the dual-mode WAAS (GPS and Galileo) is investigated. Section 7 presents a summary and some concluding remarks.

The availability of WAAS is determined by the confidence bounds on position errors and the satellite geometry. The computation of confidence estimates for the corrections to various error sources are defined in the WAAS Minimum Operational Performance Standard (MOPS) [

The weighting matrix, ^{th}

As a result, the inverse of

The variance of the vertical position estimate is the third diagonal element of the position estimate covariance matrix,

_{3,3} is the variance of the vertical position estimate.

The VPL (Vertical Protection Level) is
_{V,PA}^{−7}), the tolerable probability of HMI (Hazardously Misleading Information), provided that the error distribution is a zero mean Gaussian [

The simulation tool in this paper is the MATLAB^{®} Algorithm Availability Simulation Tool (MAAST) [

WAAS contains three segments: control segment, space segment, and user segment [

WAAS has been upgraded to better meet the needs of civil aviation users. Since IOC, 13 new reference stations have been added to the WAAS network, with five of them added in Mexico, four in Canada, and another four added in Alaska. These new reference stations expand the WAAS service coverage. By adding new reference stations to Mexico and Canada, WAAS becomes an international system.

The operating principle of ionospheric correction of the WAAS is to employ a set of reference stations to monitor the GPS signals so as to come up with corrections. Similar to the Nyquist sampling theorem, a key to the success of the approach is that the reference stations and ionospheric grid points (IGPs) must be dense enough to account for the variation of the ionosphere. Thus, the estimation of the ionospheric delay would be benefited from more reference stations. In addition, the IGP mask limits the WAAS precision approach and landing service region, because WAAS users must obtain the real-time ionospheric corrections in order to perform the vertical guidance [

The FAA also plans to add a new geostationary satellite (PRN-133) in 2010, which will be at 98°W. The new additional GEO is to ensure that all users in WAAS service volume will have at least two GEOs in view, and the new additional GEO could improve the satellite geometry for better positioning and continuity.

As described in the first section, GPS will add a new civil frequency, L5 at 1176.45 MHz, in the ARNS band. This new civil GPS signal combined with current L1 will improve the performance for GPS users by enabling them to estimate and mitigate the ionosphere delay. Recall that ionospheric delay currently is the largest obstacle for the GPS to become the primary navigation aid in civil aviation.

An L1–L5 dual-frequency GPS user avionics sensor can estimate the ionospheric delay directly (^{th}

_{i,flt} term is based on the minimum User Differential Range Error (UDRE) [_{i,UIRE} term is based on the minimum GIVE value of 3 m [_{i,air} term uses the Airborne Accuracy Description (AAD-B) model defined in [_{i,tropo} term is defined in [_{i,flt} and _{i,tropo} terms are identical to those of the L1 single frequency GPS/WAAS user, and the _{i,air,L1–L5} term is defined in [_{i,air} but is significantly smaller than _{i,UIRE}. As indicated in _{i,flt} term is the dominant error component for the L1–L5 dual-frequency GPS/WAAS user avionics sensor.

The new civil frequency at L5 will have more signal power than the current civil signal at L1 [

For an L1–L5 dual-frequency WAAS user avionics sensor, the dominant term in the confidence calculation of

Galileo is the Europe’s contribution to a global navigation satellite infrastructure. Galileo is expected to reach the Full Operational Capability (FOC) in 2013. In June 2004, the European Union and the United States signed an agreement to envisage the compatibility and interoperability of GPS and Galileo. In other words, one will be able to calculate a position with the same receiver from any of the satellites in both systems. It will make the user more robust to the loss of the satellites.

WAAS should also take advantage of the new satellite constellation. Therefore, WAAS should provide corrections to Galileo as well as GPS. In the service volume analysis, this paper treats Galileo satellites the same as GPS satellites but in different orbits. With a combined GPS and Galileo constellation, the MAAST simulation shows that the number of satellites in view over a twenty four hour period is more than twenty for each simulation time step (five minutes). This is about twice the number with GPS only. One could expect significant improvement in the geometry for the position estimation. _{i}

This paper extends the same analysis to investigate the aviation navigation performance of Japanese MSAS and European’s EGNOS.

This paper investigated the vertical guidance performances of different phases of GPS/WAAS user avionics sensor for the next 15–20 years. First, this paper showed the performance of the IOC WAAS user avionics sensor. This paper then showed that upgraded WAAS could provide LPV (50 m VAL) service to the GPS/WAAS user avionics sensors in all of CONUS and in most of Alaska with 38 reference station, three GEOs, and an expanded IGP mask. Second, with the second civil signal (L5) WAAS could provide APV II (20 m VAL) service to the GPS/WAAS user avionics sensors in most of CONUS, Alaska, and Canada. Because the ionosphere is currently the largest error source on GPS, the second civil frequency provided a significant improvement on the GPS/WAAS user avionics sensor performance. An L1–L5 dual-frequency user avionics sensor could estimate the ionospheric delay directly and then subtract this estimation from the pseudorange observations. This direct use of the dual-frequency signals will be more accurate and offer higher availability. Because the new civil signals will have stronger power than current signal, this paper therefore lowered the floor of the residual user receiver noise and multipath error to evaluate if the GPS/WAAS user avionics sensor could meet 12 m VAL (CAT I). Unfortunately the L1–L5 dual-frequency GPS/WAAS user avionics sensor with the enhanced signal model could not meet the CAT I requirement (VPL > 12 m VAL).

This paper then analyzed the VDOP improvement from the new satellite constellation composed of the combination of GPS and Galileo. It is shown that the dual-mode (GPS + Galileo) and dual-frequency WAAS user avionics sensor could provide CAT I service to users in most of CONUS, Alaska, and Canada. Importantly, the VPL values are less than 10 m VAL (a more stringent landing requirement) in most of CONUS and Canada, which is a significant improvement. Finally, this paper also presented the analysis results for the similar aviation navigation performance enhancement to Japanese MSAS and European’s EGNOS. The results are equally encouraging.

This paper treated Galileo satellites the same as GPS satellites, but in different orbits, the MAAST simulation results for the dual-mode (GPS + Galileo) and dual-frequency WAAS user avionics sensor might be optimistic. It does not consider and model the time reference difference between GPS and Galileo signals (group delays) and the coordinate difference between these two systems. These differences might have some impact at performance.

MAAST was intended as an efficient and effective tool for algorithm development. It is strictly deterministic, and does not model asset failures in a probabilistic manner. Despite these limitations, the results of this paper show that the performance of WAAS can be dramatically improved with the upgraded system which features new additional civil signal (L5), and a new additional satellites constellation (Galileo).

The work in this paper was supported by the National Science Council in Taiwan under research grant NSC 98-2221-E-006-122, and the author gratefully acknowledges this support. Author would also like to thank Todd Walter and Sherman Lo from Stanford University for their thoughtful comments. This paper is an extended work from the one presented in the Institute of Navigation National Technical Meeting 2005, San Diego, California, USA, January 24–26, 2005 [

The 99.9% LPV availability contours of the current WAAS. The left plot is the MAAST simulation result (a 24 hour average of LPV availability using the actual almanac data of August 15th, 2009), and the right plot is the actual performance of the WAAS (a three month average of LPV availability, July 1st to September 30th, 2009).

The 99.9% VPL contour of IOC WAAS. The VPL values in CONUS are greater than 20 m, and some places are higher than 50 m (LPV VAL).

The 99.9% VPL contour of the upgraded WAAS. The VPL values in all of CONUS and in most of Alaska are less than 50 m (LPV VAL).

The 99.9% VPL contour of the upgraded WAAS with more IGP in Alaska. In comparison with

The 99.9% VPL contour of the upgraded WAAS with extended IGP and three GEOs. In comparison with

The minimum user error components as a function of satellite elevation angle. The left figure is the minimum L1 single frequency GPS/WAAS user error components, and the right figure is the minimum L1–L5 dual-frequency GPS/WAAS user error components.

The 99.9% VPL contour of the L1–L5 dual-frequency GPS/WAAS user avionics sensor. The VPL values in most of CONUS, Alaska, and Canada are less than 20 m (APV II VAL).

The 99.9% VPL contour of the dual-frequency GPS/WAAS user with an improved signal model. The VPL values could not meet the CAT I requirement.

The 99.9% VDOP contour for WAAS with GPS alone. The VDOP values are from 1–4 in CONUS, and the VDOP values are from 1–2 in Alaska.

The 99.9% VDOP contour for WAAS with GPS and Galileo. The VDOP values are from 1–2 in all of CONUS, and in all of Alaska.

The 99.9% VPL contour of the dual-mode (GPS + Galileo) and L1–L5 dual-frequency WAAS user avionics sensor. The VPL values in most of CONUS, Alaska, and Canada are less than 12 m (CAT I VAL). Importantly, The VPL values are less than 10 m in most of CONUS and Canada.

The 99.9% VPL contours of the MSAS user avionics sensor. The left figure is for the current MSAS user avionics sensor, and the right figure is for an L1–L5 dual-frequency MSAS avionics sensor with GPS and Galileo.

The 99.9% VPL contours of the EGNOS user avionics sensor. The left figure is for the current EGNOS user avionics sensor, and the right figure is for an L1–L5 dual-frequency EGNOS avionics sensor with GPS and Galileo.