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1 INTRODUCTION
At the end of the 20th century, the dynamic
development of microelectronics caused the
popularization of unmanned aerial vehicles (UAVs),
alsocalleddrones. The UAV is an aircraft that does
not require crew onboard and is unable to take
passengers. The UAV can be remote control by
a
human operator or autonomously by onboard
computers.Hence,theideaoftheUAVhasitsrootsin
thesecondhalfofthelastcentury,whentheremote
controlled models of aircraft, cars, or ships were
mainlyofahobbynature.TheV1flyingbombandV
2
rocketfromthe2ndWorldWarcanberegardedas
theUAVprototypes.Inthepostwarperiod,research
intothedevelopmentofthistechnologyforthearmy
needswasconductedmainlyintheUSAandUSSR.
Thisdevelopmentwasdirectlyrelatedtotheconquest
of the cosmos in which unmanned
spacecrafts, i.a.,
artificialsatellites,wereused.
Initially,duetolegalrestrictions,theUAVswere
used mainly in the armed forces [1–3]. Originally,
their main area of applications was image, optical,
and radar recognition. These are socalled
surveillanceUAVs,e.g.,theNorthropGrummanRQ
4 Global Hawk. Then, the UAVs
were also used to
transposing weapons. This kind is called as a
unmanned combat aerial vehicle (UCAV), e.g., the
General Atomics MQ9 Reaper also called the
Errors of UAV Autonomous Landing System for
Different Radio Beacon Configurations
J
.M.Kelner&C.Ziółkowski
M
ilitaryUniversityofTechnology,Warsaw,Poland
ABSTRACT: At the turn of the 20th and 21st centuries, development of microelectronics and microwave
techniquesallowedforminimizationofelectronicdevicesandsystems,andtheuseofmicrowavefrequency
bands for modern radio communication systems. On the other hand, the global navigation satellite system
(GNSS)havecontributedtothepopularizationofradionavigationincivilianapplications.Thesefactorshada
directimpactonthedevelopmentanddisseminationofunmannedaerialvehicles(UAVs).Intheinitialperiod,
theUAVswereusedmainlyforthearmyneeds.Thisresultsalsofromthelegalaspectsofthe
UAVuseinthe
airspace. Currently, commercial UAVs for civilian applications, such as image recognition, monitoring,
transport, etc., are presented increasingly. Generally, the GNSS system accuracy for the UAV positioning
duringaflightisenough.However,theGNSSuseforautomatictakeoffandlandingmaybeinsufficient.The
extensive,groundbased
navigationsupportsystemsusedatairportsbymannedaircrafttestifytothese.Inthe
UAVcase,suchsystemsarenotusedduetotheircomplexityandprice.Forthisreason,thenoveldedicated
takeoffandlandingsystemsaredeveloped.Theproposaloftheautonomouslandingsystem,whichisbased
ontheDopplereffect,waspresentedin2017.Inthiscase,thesquarebasedbeaconconfigurationwasanalyzed.
ThispapershowstheinfluenceofvariousbeaconconfigurationsintheDopplerbasedlandingsystemonthe
positioningerrorduringtheUAVlandingapproach.
http://www.transnav.eu
the International Journal
on Marine Navigation
and Safety of Sea Transportation
Volume 13
Number 2
June 2019
DOI:10.12716/1001.13.02.22
430
PredatorB.Currently,theUAVsarewidelyusedon
thecivilianmarket.Manyprivatecompaniesprovide
variousservicesusingthe UAVs,e.g.,inthe fieldof
energetics [4–7], agriculture [8,9], forestry and fire
detection [10,11], water management [12] and flood
detection [13], environmental protection [14], search
andrescue[15],radio
communication [16], transport
[17],etc.
The growth of the UAV market has also
contributed to the development of other unmanned
platforms,suchasunmannedground(UGVs),surface
(USVs), and underwater vehicles (UUVs). The main
recipients of this market sector are still the armed
forces of various countries. In a report
[18], the
European Commission indicates the importance of
this technology in the economic and technological
development of countries, especially in the civilian
sector[19].Accordingto the presented forecasts, the
estimated value of the UAV market in 2019 will be
expectedtoreacharound1112billiondollars.In2015
and
2016,thedomesticmarketwasestimatedat165
and200millionPolishzlotys,respectively[20].
Inliterature,wecanfindsynonymousfortheUAV
such as unmanned aerial system (UAS) or remotely
piloted aircraft systems (RPAS). Sometimes, the
differencesbetweentheUAVandUASareindicated.
Then, the UAV is
referred to itself aircraft platform,
while the UAS includes also other system
components,suchasthe groundbased flight control
system. In this case, the RPAS is a synonym of the
UAS. Currently, the RPAS is more widely used in
militaryterminology,especiallyintheNorthAtlantic
Treaty Organization (NATO) and
European Defense
Agency (EDA). In the literature, various
classifications of the UAVs are presented. They
consider different technical aspects or applications.
From the viewpoint of this paper, the UAV term is
referencedtoavertical takeoff and landing (VTOL)
aircraft, unlike a conventional takeoff and landing
(CTOL)aircraft
requiringarunway.Intheremainder
ofthepaper,theVTOLisconsideredasasynonymof
theVTOLUAV.
Globalnavigationsatellitesystems(GNSSs)[21,22]
arecommonlyusedinUAVnavigation.Inaddition,
remote control and video transmission from the
aircraft to the remote UAV operator allows safe
displacement of
the drone. However, this solution
may not be sufficient to takeoff and landing
approach. This is a significant problem, notably for
theautonomousUAVs.Thetakeoffandlandingare
theaircraftflightstagesthatrequirespecialprecision
in steering and navigation. For manned aircraft, the
pilot onboard
has more control over the plane or
helicopter.Ontheotherhand,dedicatedtakeoffand
approachsystemsareusedatlargecivilandmilitary
airports.Theinstrumentlandingsystem(ILS),tactical
air navigation system (TACAN) [23], or European
Geostationary Navigation Overlay Service (EGNOS)
[21,22]areexamplesofradiolocalarea
augmentation
systems (LAASs). They are very important in bad
weather conditions with limited visibility, e.g., fog,
snowfall, or rain. Generally, the LAASs are not
availabletoUAVmajority.Therefore,itisimportant
todevelopsuchsolutions,especiallyforautonomous
drones.
In2016,weproposedalandingsystemonavessel
for the manned and unmanned VTOL [24]. This
system involves the use of terrestrial radiobeacons
(RBs)andadedicatednavigationreceiver(NR)placed
onboard aircraft. In this case, the signal Doppler
frequency (SDF) location method [25–27] is used to
estimatetheaircraftpositionrelativetoalandingpad
on the
ship. This solution [24] is based on the SDF
applications dedicated to inflight navigation and
CTOLlandingapproach,whichareshownin[28]and
[29], respectively. Here, the analyzed concept of the
landingapproachsystemfortheunmannedVTOLis
a system modification shown in [30]. The proposed
solution
has been developed for the landing pad in
hardtoreach places such as oil platforms, vessels,
islands,orskyscraperroofs.In[30],weassumedthat
RBsarelocatedbasedonasquare.Thepurposeofthis
paper is to assess the impact of selected
configurations of the RBs on the
VTOL positioning
accuracy.
The remainder of the paper is organized as
follows. Section 2 describes the SDFbased
autonomous landing approach system. Assumptions
andsimulationscenariosarepresentedinSection3.In
Section4,theobtainedsimulationresultsareshown.
In this case, the positioning accuracy of the VTOL
UAV
fordifferentRBconfigurationsisanalyzed.The
summaryisinthefinalpartofthepaper.
2 AUTONOMOUSLANDINGAPPROACH
SYSTEMFORVTOLUAV
The spatial structure of the SDFbased autonomous
landing approach system for the VTOL is shown in
Figure1.
Figure1.Spatialstructureofautonomouslandingapproach
systemforVTOL[30]
ThegroundpartofthesystemconsistsoffourRBs
and a measuring receiver (MR). In the solution
discussed in [30], we assumed that four RBs are
locatedbasedonthesquarearoundthelandingpad.
EachRBisequipped with a signal generator, power
amplifier, and transmitting antenna placed at
the
stand. Additionally, the RB can be equipped with a
rubidium or cesium frequency standard that will
increasethefrequencystabilityoftransmittedsignals.
ThisisimportantfromtheviewpointoftheSDFused
[31]. Three RBs, i.e., RB1, RB2, and RB3, transmit
harmonic signals at defined
frequencies f1, f2, and f3,
respectively. At frequency f
4, the RB4 transmits a
431
modulatedsignalusingdifferentialphaseshiftkeying
(DPSK).Ineachtransmittedframe,informationabout
the location coordinates of the individual RBs and
theirfrequencycorrectionsaresent.Thesecorrections
aredeterminedbasedonlocalmeasurementscarried
outbytheMRlocatednearRB4.
The NRs placed on the VTOLs
are the receiving
part of the system. Each VTOL is equipped with
typical elements of the navigation system, namely a
GNSS receiver and inertial navigation system (INS).
Thisallowstocarry outacontrolledorautonomous
UAV flight phase. Whereas, the NR provides
positioning the VTOL near the landing pad
and its
landingapproach.TheNRistunedtothefrequency
bandonwhichoperatetheRBs.Thismeansthatthe
NRoperationbandincludesthecarrierfrequenciesf
1,
f
2,f3,andthemodulatedsignalbandatthefrequency
f
4. The NR is made in softwaredefined radio (SDR)
technology[32,33].ThismeansthattheNR provides
signal processing and determining the estimated
positionsoftheUAVrelativetothelandingpad.For
each RB, the Doppler frequency shift (DFS) is
determinedeveryspecifiedtimeperiodΔTbasedon
the
receivedsignalwiththedurationofTS.Then,the
UAV coordinates are determined based on discrete
instantaneousDFSsaggregatedinatimewindowT
A.
The method of the DFS determination for the
harmonicsignalispresentedin[25–27].Inthecaseof
the modulated signal from RB4, after subband
filtering,informationframesaredemodulatedandthe
instantaneous DFSs are estimated based on a
methodologyshownin[34].Adetaileddescriptionof
the
autonomous landing approach system and
estimatingtheVTOLcoordinatesbasedontheSDFis
containedin[30].
The presented system can be classified as precise
shortrangeradionavigationsystems. It can be used
duringtheUAVlandingapproach,aswellasitstake
off from the landing field and
inflight in an area,
where is a radiorange between the NR and RBs. A
keyadvantageoftheproposedsolutionisitsnarrow
bandwithrelativelyhighpositioningprecision.Inthe
case of systems based on time measurement, e.g.,
[35,36],obtainingcomparableaccuracywouldrequire
theuseofa
muchwiderband.Wepointoutthatthe
frequencyallocationforthistypeofdedicatedsystem
is a serious problem. For the developed system,
unlicensedfrequencybands,e.g.,dedicatedtotheWi
Fiincludedinindustrial,scientificandmedical(ISM)
bands,canbeusedforthispurpose.Inthiscase,
the
emission of the harmonic signals with more power
than the emission average in the band does not
constituteasignificantinterferencetoothersystems.
3 SCENARIOANDASSUMPTIONSFOR
SIMULATIONSTUDIES
Inascenarioshownin[30],weassumedthattheRBs
are placed on the basis of the square
with a side
lengthequalr(seeFigure1).Inthispaper,weanalyze
three configurations of the RB positions based on
otherregularpolygons,i.e.atriangle,pentagon,and
hexagon. A common feature of all configurations is
the radius R of a circumscribed circle. Figure 2
presentstheanalyzed
RBconfigurationstogetherwith
a reference configuration based on the square (see
Figure2(b)).
Figure2. Analyzed spatial configurations of RBs based on
regular polygons: a) triangle, b) rectangle, c) pentagon, d)
hexagon
In addition, in simulation studies, we assume
similarassumptionsasin[30],i.e.,
landing point at O is the origin of the local
coordinatesystem;
the system configuration based on the regular
polygon consists of K RBs, where K
=3,4,5,6, for
the regular triangle, rectangle (i.e., square),
pentagon, and hexagon, respectively (see Figure
2);
assuming the distance r
=40m [30] between
neighboring RBs in the square configuration, the
radiusR
28.3misabasefordeterminingtheRB
coordinates with respect to the point O in each
configuration; the location coordinates of the
individualRBsfortheanalyzedconfigurationsare
contained in Table 1; the height of the RB
transmittingantennasish
T=zk=2mfork=1,..,K;
thesystemoperatesintheISMbandusedbythe
WiFi,i.e.,2.4GHz;ineachconfiguration,thekth
RB transmits the harmonic signal at f
k, where
k
=1,..,K–1; while, the Kth RB emits the DPSK
signal at f
K; these frequencies are determined as
follows: f
K(kHz)=2399800+50K+100 and
f
k(kHz)=2399800+50(k–1) for k=1,..,K–1; the
bandwidth of the DPSK signal is equal to
B
T=80kHz;
theNRoperatesatthefrequencyf
R=2.4GHzwith
thereceptionbandB
R=500kHz;
for a Doppler curve (DFSs versus time) analysis,
thetimewindowT
A=5.0sisused;
in an electromagnetic environment, an additive
whiteGaussiannoise(AWGN)isoccurred,anda
leveloftheemittedsignalsatthefarthestpoint(L)
ofananalyzedtrajectoryisensuredbyasignalto
noiseratioequaltoSNR
=8dB;
the VTOL flight between the L and P points is
carriedoutataconstantaltitudehL=50mwitha
velocity v=72km/h=20m/s (see Figure 3); then,
theflightceilingislowered;
thelengthoftheanalyzedVTOLflightroute,i.e.,
thedistancebetweentheLandPpoints,isequalto
d=400m.