LAAS Ground Facility Design Improvements to Meet Proposed Requirements for Category II/III Operations

Stanford University has developed a Local Area Augmentation System (LAAS) ground facility prototype known as the Integrity Monitor Testbed (IMT) to demonstrate the feasibility of LAAS precision approaches under Category I conditions. While the Category I IMT is essentially complete, research on IMT algorithms continues to improve its performance so that it can eventually meet Category II/III approach requirements. To the extent possible, it is desirable to satisfy Category II/III requirements with modifications to the existing single-frequency (L1) LAAS architecture in order to provide Category II/III initial operational capability (IOC) before the second civil frequency (L5) is present on a sufficient number of GPS satellites. This will also provide a backup operational mode in a future dualfrequency LAAS if either L1 or L5 is interfered with. This paper addresses IMT improvements to detection of satellite signal deformation, code-carrier divergence monitoring of both potential satellite failures and ionosphere spatial anomalies, and position-error monitoring at a “remote” monitor receiver that is some distance away from the existing reference receiver antennas. With these relatively-limited modifications to the existing Category I LGF architecture, significant performance improvements are demonstrated. While the degree to which these improvements are sufficient depends on changes now being considered to the Category II/III requirements, we believe that, with further refinement, they will be sufficient to provide acceptable IOC and dual-frequency backup availability. 1.0 Introduction The Stanford University Integrity Monitor Testbed (IMT) is a Local Area Augmentation System (LAAS) Ground Facility (LGF) prototype that meets the requirements to support civil aviation operations up to and including Category I precision approach. The IMT includes monitor algorithms designed to detect all failure modes of concern to the LAAS Signal-in-Space along with fault-handling logic known as Executive Monitoring (EXM). The IMT has been tested under both nominal conditions and with injected failures to verify its ability to meet the Category I requirements, as specified in the FAA LGF Specification [1]. Details of the existing Category I IMT design and performance can be found in previous papers [2,3,4,8]. While the IMT is designed to maximize integrity and continuity performance for Category I, significant enhancements will be needed to improve performance sufficiently to meet the tighter Category II/III precision approach and landing requirements, which are now being revised by RTCA SC-159 WG-4. The existing LAAS MASPS [5] requirements for Category II/III specify a level of integrity risk that is 20 – 40 times tighter than that for Category I, while the continuity risk requirement is tighter by a factor of 4 – 8. Meeting both of these tightened requirements simultaneously is a challenge for integrity monitoring. In addition, the LGF time-to-alert when hazardous failures occur is reduced from three seconds to approximately one second, which mandates faster monitor response without harming continuity. This paper describes three enhancements to the IMT that are being designed and tested to see if they can meet the projected Category II/III LGF requirements. Section 2.0 discusses monitoring of satellite signal deformation, which has been enhanced by adding a fourth “ultranarrow” (0.05 chip spacing) correlator pair to the three included in the Category I baseline “SQM 2b” monitor [7]. Another element of SQM, the code-carrier divergence monitor, is the focus of Section 3.0. This monitor has been significantly enhanced by implementing a Cumulative Sum or CUSUM algorithm, which significantly speeds up detection of anomalies that take time to build up in smoothed pseudoranges before they threaten users. Section 4.0 discusses the use of a remote monitor receiver to more tightly bound the user protection Figure 1: “2-Order Step” Signal Deformation Example and Corresponding Correlation Peak levels assumed in the LGF to enhance overall continuity risk and enhance EXM. Taken together, these improvements demonstrate that a significant fraction of the gap between Category I and Category II/III requirements can be satisfied by relatively minor upgrades to the existing LAAS ground system. 2.0 Enhanced Signal Deformation Monitoring One potential failure mode of concern to LAAS is a subtle failure of the signal generating hardware onboard the satellite may distort the incoming signal. These anomalous or “evil” waveforms (EWFs, otherwise known as “signal deformations”) distort the correlation function generated within a GPS receiver. This affects codetracking loops and leads to erroneous pseudorange measurements. Furthermore, for receivers of different configurations (i.e., discriminator types, correlator spacings, and front end bandwidths) these correlation peak distortions result in different pseudorange errors. Since user receivers vary and differ from the reference receivers, these errors cannot, in general, be differentially corrected. The ICAO EWF threat models this class of integrity faults as a combination of both digital and analog failure modes on the satellite signal-generating hardware [6]. The digital parameter ∆ models a lead (or lag) of the falling edge of the C/A code chip transition. The parameters fd and σ model the frequency and damping of a (2nd-order) failed, analog filter response. The 2ndorder response is given by: Figure 1 illustrates an example of these waveforms for fd = 3 MHz, σ = 0.8 MNepers/sec, and ∆ = 0.3. Figure 2 depicts, to scale, the correlator spacings used for SQM. For SQM2b, which has been validated as sufficient to protect all airborne receiver designs permitted in the LAAS MOPS [8], the early-to-late spacings are 0.1, 0.15, and 0.2 chips wide; the code tracking loop employs the pair at 0.1 chips. To increase detection capability, Cat II/III SQM analysis will employ an additional correlator spacing pair (at 0.05 chips). This is made possible by the availability of receivers such as the NovAtel OEM-4 receiver, which can (with speciallydesigned firmware) provide 8 correlator outputs for each of 12 different satellites. Nominal thermal noise and multipath at LAAS installations cause distortions of the correlation peak which can conceal the presence of an EWF. Figure 3 illustrates this nominal distortion of the correlation peak for a single satellite pass. For this figure, actual correlation peak measurements were taken using a singlechannel, 48-correlator receiver with the full set of spacings shown in Figure 2. Figure 2: “SQM 2b” Multicorrelator Receiver and Addition of 4th Correlator Pair -1.5 -1 -0.5 0 0.5 1 1.5 0 0.2 0.4 0.6 0.8 1 Correlation Peaks Code Offset (chips) N or m al iz ed A m pl itu de 0 1 2 3 4 5 6 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 C/A PRN Codes