A Differeitial Global Positioning System for Flight
Inspection of Radio Navigation Aids
Dr. D. Alexander Stratton Parker
Gull Electronics
BIOGBAF’EY
‘“cnRin”~rqllircr~-aircraft
Dr. Stratton Q an eq#neef wit& Parb Iiammn
GxpruadouJr Gull EIledmh DMdon, a k=mg
man- of wionics md fad systems. His
responsibilities hxhde rcsea& and development of
avionic3 WigafiOrl~ syjtems for night inspectioe. His
Interests include satelhte navigmth, llrcran gUrd=a
and control, and flight safety. He received a R Sci.
degree in Aemautkal En&wing from Rensselaer
Pal- Institute in 1985, and be rcaivcd an M.A
in 1989 a n d a PLD in l992 In MezhanicaI a n d
Aerospaa Engineering horn Princha Univusity.
kwiueetimcsmorc-thaathtnavigatiooaid
itself. Ovuthcyearqmanygovunmentsbaveadopkd
manuanyqeratcd optical theodoiitcs for aircraft
positio+ wbi& arc limited by visibility, turbu.Ienc;
and operator performance. Portable laser and infra-red
tracking systczl& wilic?l offer improved fcahlrcs at
higher w arc used &onaUy. W&h rquirements to
inspect thousands of radio navigation facilities worldwide, the FAA has abandoned ground-based tracking
systems in favor of Automatic Flight Inspstion Systems
(AFIS). lkxe systems, East developed by Par&r Gull
engineers ‘m 1973, use on-board Inertial Navigation
Systems (ES) and other airborne ensors for
positioning thereby &hating FYtathcr dependency,
hibilkylimitations, and ground qnipment [2,3], Parker
Gun~inscrvicew~~FAAperformthebuDrof
ail ilight inspdon in the US. today, and Park Gull
AFIS arc used by several international governments,
including the Japan Civil Aviation Bureau.
-cr
‘l%h paper describes a unique rppiication of Global
Positioning System (GPS) t&no&g to the flight
iqxctioa dradio wigati~ adds. Ground and flight
test results wlidate t h a t DlfXcrenthl GPS @X’S)
teC!lnology can m e e t the siringat aaxlraq
rlqdmlcllte d flight illspccti~ lnduding those for
cattgoly Ill landiTl.g systems. A llniqac method d
ughiy-lwcnxate 8knlft po5stiolling is described that
usucommexiaiGPSrrcJvwmddounotrquirePa&theL2 frequ~,or8mbiguity&atiorr’orr*
ffy.‘ A M gulcratioll of poctabk mg?lt inspedfoa
sJStUll~‘buedlUlthistCChlldoglW~plaa
anaknome ground trackhg quipm-t commonty
lascd for !Ii$lt insp&hn amhide tbc united stat&L
FlighthtmPltsdtbfrp8per~~DGPs
provides pasitiouai 8axuacy qliv8httothbIghcstQ=wf4&t~~rpstems~--dw.
INTRODUCX’ION
Tbc Fcdcrai Ation Admiktdoa (FAA) a n d
insunahonal agulcks pafonn ili#lt insp4on of their
radio naYi@ion aids to comply with htaBational civil
Aviation Organitatioa (ICAO) rqukcmcau [l]. Plight
tdhgQplbility-tbCsystun’SlCUUaCyIIlUStbCat
Diifcxnfiai GPS technology prov@ a new alternative
forflightins@onthatokrsmanyoftheadvantages
ofAFISatlowcrcos~ ADGPS-bsedFli&Ins@oa
Systan (DGPS-FE) cmpioys a grorxkd unit at a hcd
hxation that tckmctcrs G P S measurcmaltstoan
aihorneullit~i1). Tkaiholxunitusesthc
tc!anctcxzddatatoaxxfitso&oardGi’S
Iuznrem~~~arc~correIatcdto
thostcxpfknctdonthc~ PrcYiousrcse3r&
a@oying spc&hd dual-m GPS rur.ks and
*udoiitc$hassh~tbatDGPscantrackairuaftia
reai time with cuatimaa-Icvci &ccuraq [WJ. systems
baxd an simpler singie-t?equcny l.ccckswithlJarrow
comlator spa&g have achkved sub-mcta auaxaq in
reai time [&9]. Diffcrcntiai GPS has been tested for a
valkty of appliut.iong notably for use in combinalion
wilh INS for ilight inspc&n [log].
-
1
DGPS-Based Plight Inspwtioa System
Figure L
Co-a
This paper desaax a difkreat approach to highlyacaKatcaircraftpositioningforflightiaspebion. The
approach uses commercial aingkfrqueocy GPS
receivers, and it proviti robust accuracy without INS,
laser trach& or theodolitcs. This paper presents the
rc.suhs of an independent research and development
program that is proving the viabiity of the DGPS-FIS
concept. Flight tests have been performed comparing a
prototype DGPSFR3 with dual-frquency survey-grade
DGPS quipmeat and a thtxxlolite tracker. The results
establish that the prototype’s accuracy clearly is
sufficient for flight inspxtion of Category III landing
aids. The high accuracy and portability of this
technology make it suitable for a variety of emerging
flight inspection requirements [l2].
INERTIAL NAVIGATION SYSTEM PO!3lTIONING
The effective use of INS is critical to suozxfully flight
inspecting Instrument Landing Systems (ES) without
ground equipment. The error characteristics of ring
laser gyro INS are quite stabk, making it feasible to
measure its errors in flight [U]. The highestquality INS
error estimates are ma& from position 6xes ma&
during low-altitude passes over surveyed runway
thresholdmar~ TwosuchErcscanbcobtaincdas
the aircraft is frown over each end of the runway
foIlowing an ILS approach. Vertical position bii is
d&rmine.dtomPcaua~ofoocftnsingaradio
altimeter, whik similar acwacy is obtained in the
horizontat plane using a camera pasitioning system (or
a manual pfoc&uc that provides somewhat less
accuraq) 1141. A second fix enables dctwmination of
drift rate to approximately two 6t per minute. Tbcst
corrections cnabk accurate analyak of IIS immcdiateiy
following the approach
DIF’PRRE3TIALGPSPOSPPIONINGTECHNOU)GY
A new approach to DGPS derives moic born the
system’s stabiity (ix., its rcpcatabk accuracy) than it
does horn abwlute accuracy. Parker Gull’s approach
aststbcarctIlcntstabiioftheGPSLIcom’apluuc
tomcctallfl.ightinsprxtioaacwacya&ria~ut
INS or ground trackers. DGPS position solutions
combii airborne GPS measurements with ground GPS
data tclcmetcfcd to the airuaft as it flies a pr&&on
approach. Iike the INS solutions of API& the DGps
solutions arc improved with the aid of a single mnway
fix, providing an immediate past-approach emluation of
the landing system. Plight test results have established
that this new approach easily meets the fquircd
auxuacy criteria - in fact, it exceeds them by wide
margins. Furthermore, the carrier-phase solutions are
immune to multipath and to adverse satellite geometry
that affect convtntionaf approach= to DGPS.
Canwn&nal GPS Pcdiimbg
The GPS is dcsigd to enable accurate geodetic
positioning with a low-cost re.&ver. GPS receivers lock
on to a set of satellites and demodulate their ranging
C/A oxkzg producing range estimates called pseudo
rwzga. Quality receivers also can provide occwrtuloted
wrier pharc of integrated Doppler measurements that
art based on the received phase of the satellite carrier
signal. The conventional method for positioning with
GPS is to use the pscudc+rangcs from four or more
satellites to triangulate position and prexise time.
&cause of deliberate errors introduced into GPS as
well as secondary environmental effects, the horizontal
positional acwacy of the C/A code ranging is limited
to about 100 m. GPS Carl be used alone for inspection
of en-route navigation aids, but higher acwacy is
needed to provide the accuracy for precision landing aid
verification (cg for ILS).
DiierentiaJ positioning provides increased accuracy
because ex~ors cxpcricnczd by nearby fcccivw5 a r e
almost identical A receiver fixed at a known position
can t&meter pseudorange corrections to nearby GPS
users, or the ground component can &meter its
pseudorang? directly to an airbomc component that
computes the corrcctio~ By using tckmetacd psc&
rPnggaIhfbOfIlCsystU?l~positio~With~~CUlfa~
ofonetothr~metersinthoho&mtalplaneandtwo
to six meters in altituk &pendi.ng on WeRite
gcom&y. These code DGPS solutions do not provide
fur&iuU vwi.icaI accuracy for flight inspec&joL
MW fcfldons from obstnldorls ncaf t h e
antennas dc-comelate the pscu&rangcs. Mullipar.h can
bcmihgatcdbyllsingfeccivuswithnaKOwcOlTtla!af
spacing but the multiplicative effects ofadvme satdlitc
geometry can not be. A more substa&f acauacy
improvement is obtained by using the accumulated
car&r phase measurements.
..
GvGr Phare T&g
receiver acumulatts the difference between the
phase of the carrier from each satellite and the phase of
its local oscillator. Auwnulated carrier phase can be
viewed as a bii estimate of the satellite to receiver
distance, with an unknown integer ombiguify (ix7 bias)
that quah the integer number of l9-an. wavelengths
from the satellite to the receiver at the time of &k-on.
If these integer ambiguities can be nsolvcd (ix7
detennined exactly), the position solutions are accurate
toasman~onofthccvrierwavelengthfor~bng
asthereceivers&ntainsatellitebck Onewayto
resolve ambiies is to make several guwses as to the
axr&ambiguityvaluuusingpse&rangesandsingle
outthecorrectguessba5edon lmcizs& measurement
epochs. The simplest ‘static’ resolution methods rquirc
that both antennas are stationary during this process.
Improved ‘On-The-Fly” (OTP) ambii resolution
techniques can work in real time with one antenna in
motion The most reliable OTP methods rquirc dualfrquency receivers [4,5,7j or GPS-like transn&ions
from ground ‘pseudolites’ [6j.
A GPS
Fortunately, flight inspection dots not require these
Instead, t h e
computationally-intensive techniques,
ambiies a be 6xed (i.e., estimated) at the runway
threshold, and the stabii of the carrier phase can be
used to position backwards from the 6x point. While
incorrect ambiguity estimates cause a slight divergence
over time, the divergence is at the subdecimeter level
over several minutes. On the other hand, conventional
position ti can be exploited in an (optional) ambiguity
resolution process that, once suc4xs&r& eliminates the
need for runway fixes while lock is maintained. Because
they do not use the ranging code+ tier-phase
solutions are virtually immune to multipath This also
enhances their capability to detect GPS multipath errors
during flight inspection of GPS approaches.
Furthermore, carrier-phase degradation is so small that
these solutions are accurate even when the satellite
geomcWW=.
A hybrid +t.ioning approach, carrier-smoothed code,
exploits the low noise of the carrierphaseandthelongterm~acyoftherangingcodcs. Acarrier-smoothed
code solution combines the OuTiu phase and pseudor a n g e s u s i n g a c o m p l e m e n t a r y tilter. T h e
complementary 6lter reduas muhipath s&CmWly
andithasnoambiiesto rcsok Canicr-smd
adetbolutionshave~ciult acCura~tobeusedwith
a radio altimeter to fix the carrier-phase ambii
Mxxatorytestresultsverifythehighlevelofrepeatable
accuraq available from the carrier phabc, and they also
indicate the difficulty in relying on code DGPS for flight
inspection. Static and bwdynamic tests were performed
at Parker Gull in April, 1994. The laboratory test
con6guration consisted of two NovAtel Model 95lR
GPS installed in two desk-top computers (Fii 2). GPS
antennas were installed on the roof of Gull Plant g,
located on Marcus Blvd., Smithtown, NY.
Fwe 2 Laboratory Test Configuration.
Static test results for repeatable accuracy of carrier
phase DGPS and absolute acwacy of code and carrier.
s m o o t h e d sohltious are summarized in Table L
Centimeter-level repeatable accuracy was seen for ail
t&s of carrier-phase positioning. Cycle slips (a
potential source of carrier phase error) were not
observed during the entire test period. Code and
carrier-smoothed code solutions were a&ted by
multipath refl&ons from nearby air conditioning ducts
As later flight test results confirm, an accuracy
improvement is gain4 when the antennas can be placed
away born obstrudi~ Fwe 3 compares the relative
accuracy of code and carrier-phase solutions in North
and East positioe The code sohuions for North and
East position (solid and dashed lines, respectively)
exhibit noise levels at the meter level. The centimeterlevelerrorsofthe carrier-phase solutions are not
. . .
wleonFs3.
.
‘*’
Ho&~ntal2D:2cm
Tablcl StaticTestResults.
C*’ indicatts qantity not used by Parker Gull system)
Fwe 3. Gxie and Carrier-Phase DGPS Solutions.
To test the dynamii: tracking capabilities of the carrierphase solutions, a turntable was positioned on the roof
with a GPS antenna mounted 6” from the ccntcr of
rotation. The second antenna was mounted in a choke
Carrier-phase
ring ground plane 24’ away.
measurements were made as the potter’s wheel was
spun manually. Fwe 4 compares the calculated North
vs. East position of the moving antenna (each denoted
by an X) with the actual antenna position (the circle).
The solution tracks within 2 an throughout the test
Fwe 5 amtains the time responses. The antenna
cxpcrienad an average of approximately l/6 g and a
n&mum of l/2 g acceleration during the fastest spin
sequence (the last five spins).
Scn&itytoRw~wayFkBias
Asen&ivQanaly&sh~thatthcaccuraqofcarricrphase solutioas is not degraded even when the threshold
fix&poor. Amatrixofsol~onswascomputedwith
horizontal fix errors of 0,2, and 4 meters, and with
ve&alExurorsofO,l,and2metus (Thescare
much pea&x a~ors than would normally occur). While
rbiasinthefucausesacorrespcmdingbiiinthc
ambi@ity-fixed S o l u t i o n , n o furtheS significant
degradatioosare~ F~6preSentshv*
sigma residual drift errors ova a fourteen-minute test
puiod;foftlleworstca5ethc2dRMsho~~
+tionexrofis17cm,whikthc maximum vertical
positioa~is&xr~ Thus,anoccasionalbadEixwill
manifestitsclfasahyperbolicexr~thatiseasily
id&&d by an eqerienccd flight inspcdion t&k
.
t
I
I
-mm
-10 op
IIEastFcdtIal
0
10
igurt 4. North Vs. East Position for Circle Test.
QIP
yQnrc 5. The response for Circle Ttst
Fwe 6. Eff& of Large F= Errors on Ambii-
FDed Sohtk~
DGPS FUGHTTESTING
T&h S’
A prototype DGPS flight refercnct system was
assembled and flight tested at the Ohio University
Airport on September l-5 1994. The prototype’s
airborne module consisted of a 486 notebook computer
and docking station containing a NovAtel 95l.R 10channel GPSCard and a Syn&sized Netlink Radio Data
System (SNRDS) provided by GLE Electronics. The
ground station consisted of a NovAtel215l.R receiver,
386 notebook computer, and a second SNRDS. The
Ohio University Avionics Center (OUAC) was
contracted to provide au aircra@ pilot., truth system, and
technical assktanw for the tests. The Center has a
twenty-year history of reswch, d e v e l o p m e n t ,
instahion, and prelimhary flight inspection of
navigation aids and avionics systems.
The OUAC operated two tracking systems to provide
truth data: Ashtech 212 GPS receivers in the aircraft
and on the ground logged data throughout the test
period for post-flight DGPS positioning. A tracking
theodolite ah was operated during some of the
approaches. The A&tech DGPS solutions have been
us4xl as the primary source of truth data for the
accuracy tvdluations of this paper. The Ashtech system
has been selected because of its proven high accuracy as
well as its continuous tracking capabii. The Ashtech
system has been evahated at the FM Technical
Centeis laser range, conhmiag its accuracy to the limit
of the laser-tracker’s performance (about 1 meter) [Sj.
A&tech’s PNAV post-proc&ng software provides
centimeter-level acavacy by rcsohhg the exact integer
ambii in the Ll carrier data. Currently, OUAC is
evaluating the tracker data to confirm that good
agreement was obtained bctsvczn the Ashtech DGPS
and the tkodolite tracking system. The OUAC uses
this theodo& system regularly for Bight inspection of
ILS and Miqowave Landing Systems (MIS).
DGPS FZighf Gidax
For these tests, the Center’s Piper Saratoga was
out&ted with Parker Gull’s prototype DGPS and Ohio
Unive+s Interferomctric Flight Reference/Autoland
(ERA) ~+em [4], which provided vertical and lateral
flightguidance. Forthel%stportionofthetcsts,thc
IFRA used data from an Ashtech 212 dual-frquency
P+ode tracking GPS reahr for positioning [q. For
the second portion of the t% the NovAtel receiver in
Parker GuIl’s DGPS provided GPS data to the IFRA for
positioning, using a separate ground station set up by
OUAC Dr. Bob Iilley, Director of OUAC, piloted thi
Saratw through six sets of low approaches to Runway
25. Dr. L&y noted that the m provided excelleat
guidane with both configuchns. Real-time DGPS
solutionswuetcsted,anddatawasloggcdTorpost-test
analyze Fwes 7,8, and 9 present the vertical, lateral,
and longitudinal flight pro6h.
ACCURACY E’VALIMl’IONS
Post-proc#sed DGPS solutions of the Parker Gull
system hawz been compared to the Ashtech PNAV
soluth~ Summary &a&tics are presented in Table 2
0vc.raJ.l accuraq (2uRM.S for all stahtics) of CarTiersmoothed axle solutions is 0.75 meters cros+track and
03 meters along-track. Along-track error is somewhat
smah because GPS gcomchy is more favorable in the
East-West dire&x~ Acarracyof ambii-fixed carrier
phase xhtions ovex a two-minute propagation the is
q&l to the fix bias plus a few centimeters. Fwe 10
prwxts the composite residuals of carrier-smoothed
vertical ark-phax &htioll owr sixtcul runs is u
centimcttrs,wilichoccursatl.5nmfrolllthethreahold
Even with a one-ft radio altimeter bias added, this
represents an error of 0.01’ - quivaht to the best
tracking ems in use for flight inspihon today.
u
Ii
...................................................
ar~aA
, .............-.......
:“._. ...
I........
“.“.a.............
“...” ..“.‘:. .....”.
.............
Figurt 7. Composite VerticaI Flight Profiles.
. . . . .
-8
I
..; . . . . .
. : . .
I
8
;I
nbQ& ;I
q
II
Fwe 10. Composite Cross-Track Error of Carriersmoothed code solutiolls.
....
.
.
.
a
1
_...._.._.._._............................. .
.
.
~.._......:.........:.........~.........:.....
Figure 11. Composite Change in Crars-Track Error of
Canicr-Phase Solutions.
.._.....; ......
__; ......... . .....
:Qu.re 9. Composite Lcmgi~dinal Flight Profiles.
code solutions in cross-track direcfioe F-w 11 and
I2 present composites af the change in residuals of the
carrier-phase sohtions in cross-track and vertical
directiong respectively. The worst-case drift in the
AsrrobustntsstcstofthcambiiWsoluti~a
solution is initialized with a one-m&r vcztical bias (far
greater than is nom&y czpuienccd). The carrier-
phase solution is propagated over a six-minute period
ccm&ingofadownwindtmn,outboundflight,turnon
final, and return to the threshold (Fii 14, altitude is
shown as a dotted line). Upon return to the threshold,
the sdution has drifted by only ten centimeters (Fii
14). Thisimiicattsthatthercceivcr maintained
00lltinuousbckonthecarlicr phase throughout the
maneuver. Italsosuggcitsthatrepcatedrmwayfirw
maybennnc#ssaryforaDGPS-based-inspection
syhm cmathcambi&icshavcbcenrcsohd
I
Fwe 12 Composite Change in Vertical Error c
Carrier-Phase Solutions.
I
m+
igure l3. Robustness of Vertical Carrier-Phase
Sohtion to One-Meter Initial F= Error.
Solution
Evaluated
CroSS-track
20 accuracy
20 accuracy
vertical 2u WorsMasc cross- worstcasc
accuracy
vertical en-or
track vMr
Carrier-Phase
Frx+2an
Fu+3cm
Fm+6cm
Fa+Scm
Fix+l3an
Carriersmoothed code
0.75 m
030m
la m’
1.07 m
1.87 m’
Code’
1.47 mg
0.63 m’
ti2 m’
211 m*
3.95 ’
AlOng-hpCk
Table 2 Static Test Results.
C*’ indicates quantity not us4 by Parker Gull system)
CONCLUSIONS
ACKNOWLXDGEMENTS
Taken together, these results uxfirm that differential
GPS can me& ail flight inspcxtion requirements, without
inertial systems or survey-gade equipment. A new
approach to prehe po&iou@ offers amsiderable
operational advan&es compared to theodolites and
lasertrackers. Onceabasestationiscstablishednear
theairportunderhspection,aDGPSbasedFIScan
provide the same capabiWes as a fuIly-automatic flight
@e&m system. combii DGPS with established
xunway fix procedures greatly inaeases the system’s
r o b u s t n e s s over conveational D G P S techniquts,
particularly when the sateI& gtomchy is sub-optimal.
Incombinationwithc5tablishcd~procedurcs,aDGPSFT!S can compute position acauately withod any use of
rang&code& ThisindependenceGmbtexploitedby
a new geueration of portabk tracking systeins designed
for flight hpection of global navigation satellite
syst-
Thisworkwasassistedgreatlybythefixultyandstaffof
the Ohio Uniwhty Avionics Center, particuMy ProL
Frank van Grass, Dr. Bob Lilley, and Dr. David Diggle.
NovAtel Co~unications, Ltd., of Calgary, Canada
provided GPS quipmeut fix the tests, and GLB
Electronics, Inc., of Buffalo, NY provided data radio
quipmeaL
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