ALPS - Alternative Positioning System

Global Navigation Satellite System (GNSS) combined with Space Based Augmentation Systems (SBAS) and Ground Based Augmentation Systems (GBAS) cover not only the en-route airspace, but extend its application towards a continuous gate-to-gate navigation. One problem with GNSS compared to ground-based systems is the low received signal strength at the user antenna. Therefore, intentional or unintentional radio frequency interference can lead to a local or even regional level system failure. Likewise, increased ionospheric activity can disrupt GNSS-based services. Consequently, alternate means for providing position, navigation and time services during GNSS outages are being explored.

ALPS project focus is to look for a backup solution modularly composed of existing ground based navigation infrastructure, new ground based systems and airborne sensors.

Objective

Figure 1. ALPS block diagram

The goal of ALPS is to provide resources for users in the event of GNSS outages, so that navigation operations can continue to be carried out in order to allow the traffic volume to be processed at the specified performance level. The major objectives of the project are mentioned below:

Design of alternate navigation system based on the specified navigation performance for aviation (RNP 0.3)

Development of algorithms, modeling of various error sources and definition of integrity concepts Validation of the overall concept through software simulation and flight tests Presentation of findings to international aviation authorities to be incorporated in future decision making process for APNT.

The design and development of new systems and algorithms are based on near term as well as predicted future air traffic density. Different systems might use different sensors with dissimilar errors, and hence the analysis and statistical modelling of the errors is necessary to define the integrity concept. The performance of the overall system under-consideration is evaluated using simulation tools. As reliable navigation of an aircraft is extremely important for safety, it is imperative to test the systems and the algorithms in real life scenarios through flight trails. The abstract level concept of the ALPS project can be explained via the block diagram shown in figure 1.
Traffic simulations will be used to analyze the impact of GNSS outages and a reversion to the proposed APNT system on a local and regional level.

Challenges

Figure 2: Position Error in East-North-Up

Ground based navigation systems have different challenges compared to the space based GNSS system. Some of the major challenges are identified below:

For ground based navigation infrastructure, multipath is a dominating source of error. As multipath depends on the environment, modeling it is challenging.
Providing multiple ranges to an aircraft requires a dense ground infrastructure. Therefore we are considering the combination of multiple systems and sensors.
Combining measurements from various systems and sensors with different error characteristics requires new algorithms for determining integrity bounds.
Time synchronization of ground stations for pseudoranging system is critical, and in absence of GNSS, it requires a new method/infrastructure.

ALPS emphases on addressing the first three issues through innovative ideas to ensure safety and efficiency of air traffic in the long term.

To evaluate the performance of algorithms, proposed improvements and new systems, a simulation tool is used, developed specifically for ALPS project. Figure 2 shows the result of position error in local coordinate system using this simulation tool. Simulation helps us in assessing the performance theoretically with immediate results, before testing the algorithms in flight trails which is expensive and time consuming.

Figure 3: Aircraft used for flight trails

To demonstrate the functionality, robustness and accuracy of the proposed system in dynamic operation, flight tests will be performed. To assess the system performance we will install 4 ground stations and additional sensors in the aircraft and then fly the aircraft with variation in altitude, attitude and distance from the ground station. The aircraft used is shown in figure 3 and one of the ground stations is shown in figure 4.