Vertical Protection Levels for a Local Airport Monitor for WAAS

Certification challenges and higher than anticipated development costs for the Local Area Augmentation System (LAAS) have motivated the investigation of alternative approaches for achieving Category I precision approach and landing service. One proposed alternative is the Local Airport Monitor (LAM). This concept would rebroadcast Wide Area Augmentation System (WAAS) differential corrections in LAAS format while using an on-airport receiver to monitor the WAAS corrections and tighten the broadcast error bounds to the level required for a Category I approach. The LAM proposal would thus merge the capabilities of WAAS and LAAS to achieve Category I with an architecture similar to that proposed for precision approach with the Ground-Based Regional Augmentation System (GRAS). This paper examines a method for implementing the LAM using a range-domain concept for the LAM ground facility. The cornerstone to the implementation is a modified Vertical Protection Level (VPL) equation that takes into account the discrepancy between the WAAS pseudorange corrections and the locally measured pseudorange corrections. This modified protection level equation can be implemented without requiring any changes to existing airborne receiver equipment. Specifically, through a careful manipulation of the VHF Data Broadcast (VDB) message, the aircraft’s existing protection levels are transformed into the desired LAM protection levels. Simulation indicates that a baseline LAM implementation achieves reasonable integrity and availability performance even without a specialized multipath limiting antenna. However, baseline availability and continuity are severely degraded for a Vertical Alert Limit (VAL) below 12 m. Two optional modifications are thus introduced to augment availability in support of a 10 m VAL. The first technique exploits prior knowledge of the WAAS error distribution, and the second computes continuity risk on an ensemble basis, rather than a specific (worst-case) basis. Simulations of these modifications suggest they provide acceptable availability, even for a VAL of 10 m. INTRODUCTION The Local Airport Monitor (LAM) concept is envisioned as an intermediate step between the Local Area Augmentation System (LAAS) and the Wide Area Augmentation System (WAAS) that more economically achieves the integrity requirements for Category I precision approach and landing. The LAM architecture, illustrated in Figure 1, serves two functions. First, the architecture acts as a “WAAS bent pipe” that converts WAAS differential corrections into LAAS format. In this role, the LAM ground station packages final approach segment (FAS) data with locally evaluated WAAS corrections into a conventional VHF Data Broadcast (VDB) message, as defined by the LAAS Interface Control Document (ICD) [1]. In its second function, the LAM ground station directly computes its own local GPS corrections and employs these to monitor the broadcast WAAS differential corrections. By exploiting local monitoring, the LAM can detect threats which WAAS otherwise cannot. Hence the LAM enables a tightening of the WAAS error bounds, which are otherwise inflated to account for local, unobserved events. The advantages of the LAM architecture stem from its combination of local-area and wide-area capabilities. Compared to WAAS, the error bounds for the LAM are tighter in order to enable Category I approach and landing. Compared to LAAS, the monitoring capabilities for the LAM are significantly improved, since the LAM can use WAAS monitors to detect wide-area anomalies such as ionosphere storms. In addition to these technical capabilities, the LAM is also designed to streamline certification. The LAM concept attempts to leverage the differential corrections from an existing, certified system in order to reduce the cost associated with certifying an entirely new suite of LAAS hardware. Although they are derived in the context of WAAS and LAAS, the LAM results presented in this paper are in fact quite general. All of the concepts may be applied to other wide-area differential GPS systems, including both Space Based Augmentation Systems (SBAS) and Ground-Based Regional Augmentation Systems (GRAS). In fact, the local airport monitoring concept has always been perceived as an important component of a GRAS architecture. Regardless of the particular application, a cornerstone for the LAM design is the Vertical Protection Level (VPL), a formal bound on the navigation error derived from the integrity requirement for precision approach. This paper derives a VPL for the LAM that guarantees Category I integrity. Subsequently, this VPL expression is shown to be fully compatible with the broadcast format specified by the LAAS ICD. Hence, when implemented in the LAM, the new VPL equation is completely transparent to any airborne user with an existing LAAS receiver. The new VPL equation serves as the basis for simulations that describe the expected availability and continuity for a Range-Domain Monitoring (RDM) version of the LAM. These simulations indicate that the baseline LAM configuration, using off-the-shelf hardware, cannot achieve acceptable availability and continuity unless VAL is set to 12 m. To enable acceptable performance at the standard LAAS VAL of 10 m, two modifications to the baseline LAM are introduced. The first option uses prior knowledge of the WAAS error distribution to tighten the protection level. The second option uses a modified definition of continuity to achieve an operationally acceptable alarm rate with enhanced availability. Both modifications provide a substantial benefit to LAM performance. FORM OF THE ALTERNATE VPL This section develops a LAM error bound, called the Vertical Protection Level (VPL). The VPL is a confidence limit that describes the largest error that may occur given an allowed integrity risk. To ensure safe navigation, the VPL must remain within an envelope called the Vertical Alert Limit (VAL). If VPL exceeds VAL, then the user treats the LAM corrections as potentially hazardous and therefore unavailable. The VPL expression for the LAM is developed in two steps. First VPL for a WAAS-repeater is considered. A WAAS-repeater facility would simply convert the WAAS correction and error bounds into a LAAS broadcast format [2]. Second, the VPL for the bent-pipe with Local Airport Monitoring (LAM) is considered. LAM uses locally measured GPS pseudoranges to validate the WAAS correction and tighten the WAAS error bound. VPL for “Bent-Pipe” WAAS Repeater The VPL for the simple WAAS repeater resembles that for conventional LAAS [3] and WAAS [4]. This VPL expression for the WAAS repeater treats position error as the weighted sum of Gaussian errors associated with each ranging source. The weighting coefficients, Sv,i, depend on the constellation geometry. The sigma-scaling term, Kffmd, indicates the integrity risk allowance. The standard deviation terms, σW,i, and σair,i, describe the error associated with the WAAS corrections and with the airborne receiver, respectively. ( ) 2 2 2 2 Repeater , , , VPL N ffmd v i W i air i K S ξ σ σ = + ∑ i=1 (1) The WAAS broadcast inflates the true WAAS accuracy by a factor, ξ, in order to protect for unobserved local anomalies. This inflation factor is too large to support the requirements for precision landing. Consequently, the WAAS repeater offers poor Category I availability. VPL for Local Airport Monitoring The LAM improves on the WAAS repeater by exploiting local monitoring. Because the LAM provides local observability of WAAS anomalies, the LAM foregoes WAAS inflation and leverage the true WAAS accuracy, directly. The mechanism for this monitoring process provides a basis for deriving the LAM error distribution and, consequently, the modified VPL expression. Figure 1. Conceptual Diagram for LAM Ground Station Receiver(s) LAM Processor WAAS Message GPS Pseudoranges LAAS ICD-Compliant Message: • Corrections from WAAS • Error Bounds from LAM