Terrain Referenced Navigation System Research Articles

Error modelling method of extended Kalman filter‐based terrain referenced navigation system for integrity assurance under nominal conditions

Navigation safety is further emphasised for terrain referenced navigation (TRN) systems used in middle-range unmanned aerial vehicles (UAVs) than those in cruise missiles flying through mountainous areas, because middle-range UAVs may fly to regions and cities containing both combatants and civilians. For safety assurance of TRN systems, this paper proposes a method to compute protection levels (PLs) to assure the required level of integrity under nominal conditions. For obtaining PLs of TRN systems, all the components of the TRN measurement errors should be identified and overbounded. This study models every error component: the radar altimeter error, the vertical error in the terrain elevation database, the lateral offset induced error, and the interpolation error. Based on the characteristics of TRN systems using terrain information for obtaining navigation solutions, this study developed new models for the lateral offset induced error and the interpolation error as a function of terrain roughness by examining the effect of terrain roughness on the error sources. The simulation results demonstrate that the latitude and longitude PLs obtained by applying the proposed models can sufficiently overbound the navigation solution error for TRN systems under nominal conditions; especially the proposed method shows remarkable performance in regions with low terrain roughness.

A Pragmatic Approach to the Design of Advanced Precision Terrain-Aided Navigation for UAVs and Its Verification

Autonomous unmanned aerial vehicles (UAVs) require highly reliable navigation information. Generally, navigation systems with the inertial navigation system (INS) and global navigation satellite system (GNSS) have been widely used. However, the GNSS is vulnerable to jamming and spoofing. The terrain referenced navigation (TRN) technique can be used to solve this problem. In this study, to obtain reliable navigation information even if a GNSS is not available or the degree of terrain roughness is not determined, we propose a federated filter based INS/GNSS/TRN integrated navigation system. We also introduce a TRN system that combines batch processing and an auxiliary particle filter to ensure stable flight of UAVs even in a long-term GNSS-denied environment. As an altimeter sensor for the TRN system, an interferometric radar altimeter (IRA) is used to obtain reliable navigation accuracy in high altitude flight. In addition, a parallel computing technique with general purpose computing on graphics processing units (GPGPU) is applied to process a high resolution terrain database and a nonlinear filter in real-time on board. Finally, the performance of the proposed system is verified through software-in-the-loop (SIL) tests and captive flight tests in a GNSS unavailable environment.

Improvement of terrain referenced navigation using a Point Mass Filter with grid adaptation

We propose a Point Mass Filter (PMF) with a grid adaptation algorithm suitable for Terrain Referenced Navigation (TRN) systems having linear system models and highly nonlinear measurement models. The TRN systems are once again attracting attention, with the development of related hardware and sensors that can replace the Global Positioning System (GPS) in GPS denied environments. However, conventional TRN systems implemented by the Extended Kalman Filter (EKF) have some limitations, when operated in a featureless terrain area. A global nonlinear Bayesian estimation method referred to as a PMF has been applied, to overcome these drawbacks. This method, however, also has some limitations and unclear steps. Thus, we propose an adaptive PMF method for TRN systems. This algorithm can overcome the divergence problem, when a vehicle having large initial position error flies over flat or repetitive terrain for a long time. The effectiveness and the performance improvement of the proposed PMF are verified through computer simulation for TRN, compared to the EKF and two conventional PMF methods.

A Terrain Referenced UAV Localization Algorithm Using Binary Search Method

This study focuses on localization of Unmanned Aerial Vehicles (UAV) since permanent navigation has vital significance to support position information and to avoid getting lost. Actually, there exist effective aeronautical navigation systems in use. Inertial Navigation System (INS) and Global Positioning System (GPS) are two representatives of the most common systems utilized in traditional aerial vehicles. However, an alternative supporter system for UAVs should be mentioned since INS and GPS have serious deficiencies for UAVs such as accumulated errors and satellite signal loss, respectively. Such handicaps are coped with integrating these systems or exploiting other localization systems. Terrain Referenced Navigation (TRN) could be a good alternative as a supporter mechanism for these main systems. This study aims to localize a UAV accurately by using only the elevation data of the territory in order to simulate a TRN system. Application of the methodology on a real UAV is also considered for the future. Thus assumptions and limitations are designed regarding the constraints of real systems. In order to represent terrain data, Digital Elevation Model (DEM) with original 30 meter-resolution (Eroglu and Yilmaz 2013) and also synthetically generated 10 meter-resolution maps are utilized. The proposed method is based on searching the measured elevation values of the flight within the DEM and makes use of simulation techniques to test the accuracy and the performance. The whole system uses sequences of elevation values with a predefined length (i.e. profile). Mainly, all possible profiles are generated and stored before the flight. We identify, classify and sort profiles to perform search operations in a small subset of the terrain. During the flight, a measured flight profile is searched by the Binary search method (Eroglu 2013) within a small neighborhood of corresponding profile set.

지형 험준도를 고려한 프로파일 기반 지형참조항법과 관성항법의 결합 알고리즘

In recent years alternative navigation system such as a DBRN (Data-Base Referenced Navigation) system using geophysical information is getting attention in the military navigation systems in advanced countries. Specifically TRN (Terrain Referenced Navigation) algorithm research is important because TRN system is a practical DBRN application in South Korea at present time. This paper presents an integrated navigation algorithm that combines a linear profile-based TRN and INS (Inertial Navigation System). We propose a correlation analysis method between TRN performance and terrain roughness index. Then we propose a conditional position update scheme that utilizes the position output of the conventional linear profile type TRN depending on the terrain roughness index. Performance of the proposed algorithm is verified through Monte Carlo computer simulations using the actual terrain database. The results show that the TRN/INS integrated algorithm, even when the initial INS error is present, overcomes the shortcomings of linear profile-based TRN and improves navigation performance.

Design of a TRN Tracking Loop: a Study on GPS Multipath Mitigation Strategies

Terrain Referenced Navigation (TRN) is based on the comparison of terrain altitude measurements with a reference map. Similar to code acquisition and tracking in Global Positioning System (GPS), the TRN system needs to find and track a particular sequence of measurements in a larger dataset. In our earlier work, these correspondences have been exploited to design a GPS inspired algorithm for TRN. The tracking loop is implemented as an early-late correlator, based on the DLL functional principle. Differences between GPS and terrain data in terms of signal properties are often based on the analysis of an ‘ideal’ GPS signal. In reality a GPS signal suffers from disturbances such as multipath and interference. This paper focuses on the identification and evaluation of potential GPS multipath mitigation strategies to be used to partly mitigate the issues caused by the non-ideal terrain signal. Results for the TRN implementations are described in a comparative manner. The content of this paper was presented during the European Navigation Conference 2011 in London.

Dual Airborne Laser Scanners Aided Inertial for Improved Autonomous Navigation

A dead-reckoning terrain referenced navigation (TRN) system is presented that uses two airborne laser scanners (ALS) to aid an inertial navigation system (INS). The system uses aircraft autonomous sensors and is capable of performing the dual functions of mapping and navigation simultaneously. The proposed system can potentially serve as a backup to the Global Positioning System (GPS), increase the robustness of GPS or it can be used to coast for extended periods of time. Although the system has elements of a conventional TRN system, it does not require a terrain database since its in-flight mapping capability generates the terrain data for navigation. Hence, the system can be used in both non-GPS as well as unknown terrain environments. It is shown that the navigation system is dead-reckoning in nature since errors accumulate over time, unless the system can be reset periodically by the availability of geo-referenced terrain data or a position estimate from another navaid. Results of the algorithm using a combination of flight trajectory data and synthesized ALS data are presented.

Terrain-Referenced Precision Approach Guidance: Proof-of-Concept Flight Test Results

ABSTRACT: Aircraft precision approach guidance systems require accurate positioning relative to the runway with a high level of integrity. Most precision approach guidance systems in use today require ground-based electronic navigation components with at least one installation at each airport, and in many cases multiple installations to service approaches to all qualifying runway ends. A terrain-referenced approach guidance system is envisioned to provide precision guidance to an aircraft without the use of ground-based electronic navigation components installed at the airport. An Airborne Laser Scanner (ALS), when integrated with an Inertial Measurement Unit (IMU) and a Digital Elevation Model (DEM) with approximately a 1 m horizontal resolution and elevation accuracy on the order of 30 cm RMS, can provide navigation and guidance information in support of aircraft precision approach and landing operations. The proof-of-concept terrain-referenced navigation system provides horizontal and vertical position estimates with accuracies on the order of one meter RMS and capitalizes on the position and integrity benefits provided by the Global Positioning System (GPS) Wide Area Augmentation System (WAAS) to reduce the terrain navigator's initial position search space size and to reduce the required size of the high accuracy/high resolution database. This paper describes the proof-of-concept aircraft approach system and presents results from flight tests where the approach system is used to provide real-time guidance cues to the pilot.