Analysis of DME/TACAN Interference on the Lower L-Band

Introduction Although GPS was originally developed for military usage, during the last decade its usefulness for civil navigational purposes became evident. Especially in aeronautical applications it is a valuable system to improve the effective usage of the airspace and air traffic safety. But although the accuracy has been improved a lot in the last decades through aeronautical research but there is still a remaining problem: The integrity of the system is not good enough. This gap is closed for non precision or CAT I like approaches by the usage of WAAS but the integrity limitations outlaws high precision approaches with low or without visibility. The GALILEO system wants to provide a solution for this problem by offering a “Safety Of Life” (SOL) service. To realise this service the E5 Band (1164-1214 MHz) has been issued. Also the GPS system is heading with its second civil frequency L5 into the lower L-Band. One strong argument for the usage of this band was that it showed already an ITU frequency protection for aeronautical applications. But this band has been used long time for the distance measuring equipment DME/TACAN. Luckily, DME is transmitting very short pulses (3.6?s) but the transmission power reaches values up to 2 kW. Having in mind that a navigation satellite transmits 50 -100 W typically and taking further into account the difference in distance (an aircraft can be as close as 0.1km to the DME station but is about 24000 km away from the satellite) it becomes clear that a satellite signal reception is impossible while a DME station is transmitting its pulse. To prevent the SATNAV receiver from being disturbed usually the “pulse blanking technique” is used. With this technique the receiver input is switched off when a pulse is detected. Due to the short pulse duration the decrease of the navigation accuracy is only small. But when the pulse rate increases because several DME stations are received simultaneously and during high traffic load the receiver is struggling with the satellite signal which is in this case interrupted too often. Motivation In order to define test procedures for new receivers, which fulfil the aviation requirements, the EUROCAE has developed a case of artificial DME interference at the assumed hot spot over Franfurt (Germany) and incorporated this in its Minimum Operational Performance Standards (MOPS). The handicap of this artificial test scenario is the lack of information about its measurement basis . Neither the real power levels of DME interference caught by a skywards looking antenna nor the real number of pulses per second are exactly known. Although the range of DME to be received with a DME receiver is widely known, its range of interfering a satellite navigation system is not yet known properly. The Measurement Campaign To answer these questions the German Aerospace Centre (DLR) has conducted a real world measurement. To carry the measurement equipment an experimental aircraft Dassault Falcon F 20E was chosen. Since DME interference is expected to be worst in high altitudes due to many visible DME stations a 300 km wide pattern over the estimated European hotspot over Frankfurt was flown in FL 390. During the experiment we recorded the full E5 band covering a bandwidth of 100 MHz. To allow correlation of the measurements between different altitudes the flight pattern was repeated at other interesting altitudes: FL 390 for the application of intercontinental traffic FL 300 for European traffic FL 150 for propeller driven aircrafts (turboprop traffic) FL 50 for terminal traffic The resulting measurement data has grown up to 15 TB due to the huge bandwidth while recording. This data is now being analysed. We have separated the single DME stations decoded the twin pulses and identified the stations. We will show the interference range of the DME stations in the lower L-band on satellite navigation. We will show how multiple DME stations share one frequency. We will show the pulse load on the different channels and how a receiver can deal with this interference. We will show the identified DME and TACAN stations We will compare our finding with the preliminary EUROCAE MOPS test signals. With this work we want to improve to DME interference forecast and we are targeting to define a new model on DME interference being based on measurements. Acknowledgements: This work has been promoted by the European Union in the ANASTASIA project and is currently evaluated under an ESA contract.