Terrestrial radio navigation systems such as VHF OmniDirectional Ranging (VOR) and Distance MeasuringEquipment (DME) have formed the backbone of aviationnavigation for decades. The introduction of GPS hasprovided a dramatic increase in navigation performance,which has enabled Performance Based Navigation (PBN)within NextGen and SESAR. The vulnerability of GPS,however, necessitates an Alternate Positioning Navigationand Timing system or APNT. Terrestrial navigationsolutions such as DME/N are under consideration for thisAPNT role, but now have to perform at a PBN level of,for example, RNP 0.3.The performance requirements of APNT force thetightening of the error budget of terrestrialradionavigation, thereby necessitating a closer look at allerror sources. Analysis of the propagation-induced error,especially multipath, is crucial in determining the feasibleperformance of terrestrial systems. Multipath can becharacterized by lateral multipath (from structures, hills,etc) and longitudinal multipath (from ground reflections).This paper focuses on longitudinal or ground multipathwhich is characterized by relative high signal strength anda very short path length delay. By itself, the groundmultipath does not cause a significant ranging error. Theattenuation of the direct signal that can result fromdestructive ground multipath, however, does significantlyincrease the sensitivity to lateral multipath which canpotentially cause large ranging errors.This paper presents flight test results from December2012 that shows large dips in signal strength andsimultaneous large Time Of Arrival (TOA) errors. Asimplified terrain model is developed that qualitativelymatches the deep nulls in signal strength. Next, awaveform analyzer is presented that decomposes thereceived signal into the direct signal and the dominantmultipath component(s). The large TOA errors areexplained by long delay lateral multipath overtaking theattenuated direct signal due to the ground focusing effect.This paper concludes with several multipath mitigationtechniques.
展开▼
