Modernization of GPS with new civil signals, and new GNSS constellations that will offer a much larger number of satellite navigation signals, bring new capabilities for navigating airplanes.

The Federal Aviation Administration (FAA) initiated the GPS Evolutionary Architectural Study (GEAS) in late 2006 to plan future navigation architectures, with the goal of creating an architecture capable of providing a service to bring airplanes within 200 feet of the ground anywhere on the globe. The architecture of choice will have implications for near-term planning for the Wide Area Augmentation System (WAAS) and the Local Area Augmentation System (LAAS) and their potential incorporation of the new signals.

In today's augmentation systems, integrity monitoring takes place on the ground. The WAAS and LAAS programs use reference receivers to measure the signal to correct small errors and alert users when faulted conditions may be present. The GPS Operational Control Segment (OCS) also monitors the satellites, identifies faulty satellites, and removes them from service. However, the OCS can take hours to respond to satellite faults, where WAAS and LAAS send alerts within seconds. Further, WAAS and LAAS protect against a larger class of faults and provide firm integrity assurances.

GPS modernization will provide a new civil signal at 1176.45 MHz called the L5 signal. By combining measurements at this frequency with ones from the original L1 frequency at 1575.42 MHz, a user can eliminate the largest current source of uncertainty, in which the ionosphere creates a variable amount of delay between the satellite and the airplane. Measuring the signals at both frequencies allows the removal of this error source. Therefore, future users will avoid this significant error source. As a result, they will enjoy higher availability and be able to operate in regions that are currently unavailable due to extreme ionospheric conditions.

Integrity determination can be made in one of three locations: on the GPS satellite using redundant components and sensors; on the ground using reference monitors; or in the aircraft using redundant signals or sensors. Currently, the satellites perform little integrity monitoring. However, the GPS III program wants to expand that capability, so that future satellites may detect the vast majority of errors and prevent their transmission.

The GEAS seeks to determine the best way to assure integrity of these modernized signals for aviation users. Currently integrity is provided for the L1 signal either by exploiting redundant signals on the aircraft using a technique called Receiver Autonomous Integrity Monitoring (RAIM) or through ground monitoring by WAAS. RAIM is used for Lateral Navigation (LNAV) of aircraft at altitude. WAAS provides both lateral and Vertical Navigation (VNAV) and can be used to bring aircraft to within 200 feet of the ground.

Future architectures may shift more of the integrity monitoring responsibility to the satellite or the aircraft. This article will investigate the relative advantages of certain architectural concepts over others. In particular, we will focus on the issues of time-to-alert (TTA) and required constellation strength.

Architectures

The GEAS has focused on two architecture classes: one where integrity assurance is entirely external to the aircraft, and one where the aircraft exploits redundant signals to meet the TTA requirement. In the first class, integrity messages are broadcast to the airplane within the TTA. For this analysis, all such architectures that achieve this are labeled GPS integrity channels (GICs). The key feature of a GIC is that the signals arrive at the aircraft containing integrity information that meets the TTA. The aircraft does not perform a separate evaluation requiring redundant signals.

The other general architecture class still has integrity information arriving at the aircraft. However, this information arrives outside of the TTA requirement. The aircraft has to make its own integrity determination using this delayed information combined with its current measurements. Here we investigate two forms of RAIM to make this timely integrity determination on the aircraft.

FIGURE 1 GIC architectures. Here integrity is determined external to the aircraft and supplied within the TTA. Integrity may be determined either on the ground or on the satellites or through a combination of both. The integrity information may be broadcast through either geostationary satellites as in WAAS or via the GPS satellites themselves.

There are many possible architectures to assure integrity external to the aircraft.FIGURE 1provides the notional concept of ground-based monitoring and satellite based messaging. WAAS and LAAS are two examples of ground-based architectures. WAAS uses a large geographic network and concentrates the information into a master station to evaluate all of the measurements and determine the necessary corrections and integrityparameters. Conceptually WAAS could be expanded to cover more of the globe. However, it would be difficult to do this and meet the TTA. The existing North American network already has nearly reached the limit for getting information to the user as required. Information from monitoring receivers placed even farther away, with longer communication times, would be challenging to incorporate within the TTA.

As an alternative, many separate SBASs around the world could be expanded to obtain global coverage. In this case, worldwide coverage is achieved by many service providers collectively rather than just one. Another option would be to put enough integrity monitoring into the satellites that they themselves determine integrity and shut themselves off when sufficient integrity cannot be assured.Regardless of the specific implementation, the important feature of any of these GIC architectures, for this study, is that the aircraft is not required to make its own integrity determination. Thus, the GPS satellite constellation need only provide enough satellites and sufficient geometry to afford basic positioning.

In contrast, the RAIM architectures require greater redundancy in the constellation. Not only must there be adequate numbers and geometry to support positioning, but also there must be enough to redundantly support it. That is, positioning must be supported for all satellite subsets formed by removing one satellite. This requires a greater number of satellites to be well distributed about the aircraft.

We next investigate these architectural concepts and their dependence on constellation strength. To do so we need to quantify their Vertical Protection Level (VPL) as a function of measurement confidence and satellite geometry. In order to be used for LPV-200, the VPL must be below 35 meters. Each architectural concept has a different VPL formulation as a function of satellite geometry. The next sections will describe these in more detail.