A Differeitial Global Positioning System for Flight Inspection of Radio Navigation Aids

Dr. D. Alexander Stratton Parker Gull Electronics

BIOGBAF’EY ‘“cnRin”~rqllircr~-aircraft tdhgQplbility-tbCsystun’SlCUUaCyIIlUStbCat Dr. Stratton Q an eq#neef wit& Parb Iiammn kwiueetimcsmorc-thaathtnavigatiooaid GxpruadouJr Gull EIledmh DMdon, a k=mg itself. Ovuthcyearqmanygovunmentsbaveadopkd man- of wionics md fad systems. His manuanyqeratcd optical theodoiitcs for aircraft responsibilities hxhde rcsea& and development of positio+ wbi& arc limited by visibility, turbu.Ienc; avionic3 WigafiOrl~ syjtems for night inspectioe. His and operator performance. Portable laser and infra-red Interests include satelhte navigmth, llrcran gUrd=a tracking systczl& wilic?l offer improved fcahlrcs at and control, and flight safety. He received a R Sci. higher w arc used &onaUy. W&h rquirements to degree in Aemautkal En&wing from Rensselaer inspect thousands of radio navigation facilities world- Pal- Institute in 1985, and be rcaivcd an M.A wide, the FAA has abandoned ground-based tracking in 1989 and a PLD in l992 In MezhanicaI and systems in favor of Automatic Flight Inspstion Systems Aerospaa Engineering horn Princha Univusity. (AFIS). lkxe systems, East developed by Par&r Gull engineers ‘m 1973, use on-board Inertial Navigation -cr Systems (ES) and other airborne ensors for positioning thereby &hating FYtathcr dependency, ‘l%h paper describes a unique rppiication of Global hibilkylimitations, and ground qnipment [2,3], Parker Positioning System (GPS) t&no&g to the flight Gun~inscrvicew~~FAAperformthebuDrof - iqxctioa dradio wigati~ adds. Ground and flight ail ilight inspdon in the US. today, and Park Gull test results wlidate that DlfXcrenthl GPS @X’S) AFIS arc used by several international governments, teC!lnology can meet the siringat aaxlraq including the Japan Civil Aviation Bureau. rlqdmlcllte d flight illspccti~ lnduding those for cattgoly Ill landiTl.g systems. A llniqac method d Diifcxnfiai GPS technology prov@ a new alternative ughiy-lwcnxate 8knlft po5stiolling is described that forflightins@onthatokrsmanyoftheadvantages usucommexiaiGPSrrcJvwmddounotrquireP- ofAFISatlowcrcos~ ADGPS-bsedFli&Ins@oa a&theL2 frequ~,or8mbiguity&atiorr’orr* Systan (DGPS-FE) cmpioys a grorxkd unit at a hcd ffy.‘ A M gulcratioll of poctabk mg?lt inspedfoa hxation that tckmctcrs GPS measurcmaltstoan sJStUll~‘buedlUlthistCChlldoglW~plaa aihorneullit~i1). Tkaiholxunitusesthc anaknome ground trackhg quipm-t commonty tc!anctcxzddatatoaxxfitso&oardGi’S lascd for !Ii$lt insp&hn amhide tbc united stat&L Iuznrem~~~arc~correIatcdto FlighthtmPltsdtbfrp8per~~DGPs thostcxpfknctdonthc~ PrcYiousrcse3r& provides pasitiouai 8axuacy qliv8httothbIghcst- a@oying spc&hd dual-m GPS rur.ks and Q=wf4&t~~rpstems~--dw. *udoiitc$hassh~tbatDGPscantrackairuaftia reai time with cuatimaa-Icvci &ccuraq [WJ. systems INTRODUCX’ION baxd an simpler singie-t?equcny l.ccckswithlJarrow comlator spa&g have achkved sub-mcta auaxaq in Tbc Fcdcrai Ation Admiktdoa (FAA) and reai time [&9]. Diffcrcntiai GPS has been tested for a insunahonal agulcks pafonn ili#lt insp4on of their valkty of appliut.iong notably for use in combinalion radio naYi@ion aids to comply with htaBational civil wilh INS for ilight inspc&n [log]. Aviation Organitatioa (ICAO) rqukcmcau [l]. Plight 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 1 auxuacy criteria - in fact, it exceeds them by wide Figure L DGPS-Based Plight Inspwtioa System margins. Furthermore, the carrier-phase solutions are Co-a immune to multipath and to adverse satellite geometry This paper desaax a difkreat approach to highly- that affect convtntionaf approach= to DGPS. acaKatcaircraftpositioningforflightiaspebion. The approach uses commercial aingkfrqueocy GPS Canwn&nal GPS Pcdiimbg receivers, and it proviti robust accuracy without INS, laser trach& or theodolitcs. This paper presents the The GPS is dcsigd to enable accurate geodetic rc.suhs of an independent research and development positioning with a low-cost re.&ver. GPS receivers lock program that is proving the viabiity of the DGPS-FIS on to a set of satellites and demodulate their ranging concept. Flight tests have been performed comparing a C/A oxkzg producing range estimates called pseudo prototype DGPSFR3 with dual-frquency survey-grade rwzga. Quality receivers also can provide occwrtuloted DGPS quipmeat and a thtxxlolite tracker. The results wrier pharc of integrated Doppler measurements that establish that the prototype’s accuracy clearly is art based on the received phase of the satellite carrier sufficient for flight inspxtion of Category III landing signal. The conventional method for positioning with aids. The high accuracy and portability of this GPS is to use the pscudc+rangcs from four or more technology make it suitable for a variety of emerging satellites to triangulate position and prexise time. flight inspection requirements [l2]. &cause of deliberate errors introduced into GPS as well as secondary environmental effects, the horizontal INERTIAL NAVIGATION SYSTEM PO!3lTIONING positional acwacy of the C/A code ranging is limited to about 100 m. GPS Carl be used alone for inspection The effective use of INS is critical to suozxfully flight of en-route navigation aids, but higher acwacy is inspecting Instrument Landing Systems (ES) without needed to provide the accuracy for precision landing aid ground equipment. The error characteristics of ring verification (cg for ILS). laser gyro INS are quite stabk, making it feasible to measure its errors in flight [U]. The highestquality INS DiierentiaJ positioning provides increased accuracy error estimates are ma& from position 6xes ma& because ex~ors cxpcricnczd by nearby fcccivw5 are during low-altitude passes over surveyed runway almost identical A receiver fixed at a known position thresholdmar~ TwosuchErcscanbcobtaincdas can t&meter pseudorange corrections to nearby GPS the aircraft is frown over each end of the runway users, or the ground component can &meter its foIlowing an ILS approach. Vertical position bii is pseudorang? directly to an airbomc component that d&rmine.dtomPcaua~ofoocftnsingaradio computes the corrcctio~ By using tckmetacd psc& altimeter, whik similar acwacy is obtained in the rPnggaIhfbOfIlCsystU?l~positio~With~~CUlfa~ horizontat plane using a camera pasitioning system (or ofonetothr~metersinthoho&mtalplaneandtwo a manual pfoc&uc that provides somewhat less to six meters in altituk &pendi.ng on WeRite accuraq) 1141. A second fix enables dctwmination of gcom&y. These code DGPS solutions do not provide drift rate to approximately two 6t per minute. Tbcst fur&iuU vwi.icaI accuracy for flight inspec&joL corrections cnabk accurate analyak of IIS immcdiateiy MW fcfldons from obstnldorls ncaf the following the approach antennas dc-comelate the pscu&rangcs. Mullipar.h can bcmihgatcdbyllsingfeccivuswithnaKOwcOlTtla!af DIF’PRRE3TIALGPSPOSPPIONINGTECHNOU)GY spacing but the multiplicative effects ofadvme satdlitc geometry can not be. A more substa&f acauacy A new approach to DGPS derives moic born the improvement is obtained by using the accumulated system’s stabiity (ix., its rcpcatabk accuracy) than it car&r phase measurements. does horn abwlute accuracy. Parker Gull’s approach . . GvGr Phare T&g

A GPS receiver acumulatts the difference between the Mxxatorytestresultsverifythehighlevelofrepeatable phase of the carrier from each satellite and the phase of accuraq available from the carrier phabc, and they also its local oscillator. Auwnulated carrier phase can be indicate the difficulty in relying on code DGPS for flight viewed as a bii estimate of the satellite to receiver inspection. Static and bwdynamic tests were performed distance, with an unknown integer ombiguify (ix7 bias) at Parker Gull in April, 1994. The laboratory test that quah the integer number of l9-an. wavelengths con6guration consisted of two NovAtel Model 95lR from the satellite to the receiver at the time of &k-on. GPS installed in two desk-top computers (Fii 2). GPS . If these integer ambiguities can be nsolvcd (ix7 antennas were installed on the roof of Gull Plant g, ‘*’ detennined exactly), the position solutions are accurate located on Marcus Blvd., Smithtown, NY. 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 dual- frquency receivers [4,5,7j or GPS-like transn&ions from ground ‘pseudolites’ [6j.

Fortunately, flight inspection dots not require these computationally-intensive techniques, Instead, the ambiies a be 6xed (i.e., estimated) at the runway Fwe 2 Laboratory Test Configuration. 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 Static test results for repeatable accuracy of carrier over several minutes. On the other hand, conventional phase DGPS and absolute acwacy of code and carrier- . position ti can be exploited in an (optional) ambiguity smoothed sohltious are summarized in Table L resolution process that, once suc4xs&r& eliminates the Centimeter-level repeatable accuracy was seen for ail need for runway fixes while lock is maintained. Because t&s of carrier-phase positioning. Cycle slips (a they do not use the ranging code+ tier-phase potential source of carrier phase error) were not solutions are virtually immune to multipath This also observed during the entire test period. Code and enhances their capability to detect GPS multipath errors carrier-smoothed code solutions were a&ted by during flight inspection of GPS approaches. multipath refl&ons from nearby air conditioning ducts Furthermore, carrier-phase degradation is so small that As later flight test results confirm, an accuracy these solutions are accurate even when the satellite improvement is gain4 when the antennas can be placed geomcWW=. 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) A hybrid +t.ioning approach, carrier-smoothed code, exhibit noise levels at the meter level. The centimeter- exploits the low noise of the carrierphaseandthelong- levelerrorsofthe carrier-phase solutions are not . . . term~acyoftherangingcodcs. Acarrier-smoothed wleonFs3. code solution combines the OuTiu phase and pseudo- ranges using a complementary tilter. The complementary 6lter reduas muhipath s&CmWly andithasnoambiiesto rcsok Canicr-smd adetbolutionshave~ciult acCura~tobeusedwith a radio altimeter to fix the carrier-phase ambii 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 carrier- phase 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 ring ground plane 24’ away. Carrier-phase 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~thatthcaccuraqofcarricr- phase solutioas is not degraded even when the threshold fix&poor. Amatrixofsol~onswascomputedwith horizontal fix errors of 0,2, and 4 meters, and with t I I IIEastFcdtIal ve&alExurorsofO,l,and2metus (Thescare -mm -10 op 0 10 QIP much pea&x a~ors than would normally occur). While rbiasinthefucausesacorrespcmdingbiiinthc igurt 4. North Vs. East Position for Circle Test. ambi@ity-fixed Solution, no 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

. 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 The OUAC operated two tracking systems to provide assembled and flight tested at the Ohio University truth data: Ashtech 212 GPS receivers in the aircraft Airport on September l-5 1994. The prototype’s and on the ground logged data throughout the test airborne module consisted of a 486 notebook computer period for post-flight DGPS positioning. A tracking and docking station containing a NovAtel 95l.R 10- theodolite ah was operated during some of the channel GPSCard and a Syn&sized Netlink Radio Data approaches. The A&tech DGPS solutions have been System (SNRDS) provided by GLE Electronics. The us4xl as the primary source of truth data for the ground station consisted of a NovAtel215l.R receiver, accuracy tvdluations of this paper. The Ashtech system 386 notebook computer, and a second SNRDS. The has been selected because of its proven high accuracy as Ohio University Avionics Center (OUAC) was well as its continuous tracking capabii. The Ashtech contracted to provide au aircra@ pilot., truth system, and system has been evahated at the FM Technical technical assktanw for the tests. The Center has a Centeis laser range, conhmiag its accuracy to the limit twenty-year history of reswch, development, of the laser-tracker’s performance (about 1 meter) [Sj. instahion, and prelimhary flight inspection of A&tech’s PNAV post-proc&ng software provides navigation aids and avionics systems. centimeter-level acavacy by rcsohhg the exact integer ambii in the Ll carrier data. Currently, OUAC is DGPS FZighf Gidax evaluating the tracker data to confirm that good agreement was obtained bctsvczn the Ashtech DGPS For these tests, the Center’s Piper Saratoga was and the tkodolite tracking system. The OUAC uses out&ted with Parker Gull’s prototype DGPS and Ohio this theodo& system regularly for Bight inspection of Unive+s Interferomctric Flight Reference/Autoland ILS and Miqowave Landing Systems (MIS). (ERA) ~+em [4], which provided vertical and lateral flightguidance. Forthel%stportionofthetcsts,thc ACCURACY E’VALIMl’IONS IFRA used data from an Ashtech 212 dual-frquency P+ode tracking GPS reahr for positioning [q. For Post-proc#sed DGPS solutions of the Parker Gull the second portion of the t% the NovAtel receiver in system hawz been compared to the Ashtech PNAV Parker GuIl’s DGPS provided GPS data to the IFRA for soluth~ Summary &a&tics are presented in Table 2 positioning, using a separate ground station set up by 0vc.raJ.l accuraq (2uRM.S for all stahtics) of CarTier- OUAC Dr. Bob Iilley, Director of OUAC, piloted thi smoothed axle solutions is 0.75 meters cros+track and Saratw through six sets of low approaches to Runway 03 meters along-track. Along-track error is somewhat 25. Dr. L&y noted that the m provided excelleat smah because GPS gcomchy is more favorable in the guidane with both configuchns. Real-time DGPS East-West dire&x~ Acarracyof ambii-fixed carrier solutionswuetcsted,anddatawasloggcdTorpost-test phase xhtions ovex a two-minute propagation the is analyze Fwes 7,8, and 9 present the vertical, lateral, q&l to the fix bias plus a few centimeters. Fwe 10 and longitudinal flight pro6h. 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 ......

ar~aA I , i...... -...... :“._....I...... “.“.a...... “...” ..“.‘:...... ”......

Figurt 7. Composite VerticaI Flight Profiles.

. . . . . -8 I I 8 ;I ;I q II ..; . . . . . nbQ& Fwe 10. Composite Cross-Track Error of Carrier- . : . . smoothed code solutiolls......

a _...._.._.._._...... 1 ~.._...... :...... :...... ~...... :..... Figure 11. Composite Change in Crars-Track Error of Canicr-Phase Solutions.

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, :Qu.re 9. Composite Lcmgi~dinal Flight Profiles. the sdution has drifted by only ten centimeters (Fii 14). Thisimiicattsthatthercceivcr maintained 00lltinuousbckonthecarlicr phase throughout the code solutions in cross-track direcfioe F-w 11 and maneuver. Italsosuggcitsthatrepcatedrmwayfirw I2 present composites af the change in residuals of the maybennnc#ssaryforaDGPS-based-inspection carrier-phase sohtions in cross-track and vertical syhm cmathcambi&icshavcbcenrcsohd directiong respectively. The worst-case drift in the I I

m- + Fwe 12 Composite Change in Vertical Error c igure l3. Robustness of Vertical Carrier-Phase Carrier-Phase Solutions. Sohtion to One-Meter Initial F= Error.

Solution CroSS-track AlOng-hpCk vertical 2u WorsMasc cross- worstcasc Evaluated 20 accuracy 20 accuracy accuracy track vMr vertical en-or

Carrier-Phase Frx+2an Fu+3cm Fm+6cm Fa+Scm Fix+l3an Carrier- 0.75 m 030m la m’ 1.07 m 1.87 m’ smoothed code Code’ 1.47 mg 0.63 m’ ti2 m’ 211 m* 3.95 ’

Table 2 Static Test Results. C*’ indicates quantity not us4 by Parker Gull system)

CONCLUSIONS ACKNOWLXDGEMENTS

Taken together, these results uxfirm that differential Thisworkwasassistedgreatlybythefixultyandstaffof GPS can me& ail flight inspcxtion requirements, without the Ohio Uniwhty Avionics Center, particuMy ProL inertial systems or survey-gade equipment. A new Frank van Grass, Dr. Bob Lilley, and Dr. David Diggle. approach to prehe po&iou@ offers amsiderable NovAtel Co~unications, Ltd., of Calgary, Canada operational advan&es compared to theodolites and provided GPS quipmeut fix the tests, and GLB lasertrackers. Onceabasestationiscstablishednear Electronics, Inc., of Buffalo, NY provided data radio theairportunderhspection,aDGPSbasedFIScan quipmeaL provide the same capabiWes as a fuIly-automatic flight @e&m system. combii DGPS with established REFERENCES xunway fix procedures greatly inaeases the system’s robustness over conveational DGPS techniquts, 1 In~oaalstandprds, Recommadedlklcti~ particularly when the sateI& gtomchy is sub-optimal. and Mures for Air Navigatioa Schcea, Annex 10 Incombinationwithc5tablishcd~procedurcs,aDGPS- to the Convention on International Civil Aviation, FT!S can compute position acauately withod any use of Iutemathal Civil Aviation Organhtion, Montreal, rang&code& ThisindependenceGmbtexploitedby canada,Aprisl=. a new geueration of portabk tracking systeins designed I for flight hpection of global navigation satellite syst- = 2 Litman, B. lmpkmentatioa of Automatic l.0. Feit, C., ‘Accurate Positioning in a Flight Iwqxction

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7. Blomenhofer, H. Walsh, D, and Hein, G, ‘On-The- Fly Canier Phase Ambiguity Resolution for Re.& Airaaft Lanw w of the Sii International Technical Meeting of the Satellite Divisioa of the Insthtc of Navigation, p. 821X30, Salt Lake City, UT, Sept 2x4,1993.

8. Hun&y, W, Rawson, S, and Cmrtncy, G, Vlight Evaluation of a Basic C/A &de Differential GPS for Category I Recision Approae m of the Fifth Intcrnatronal Tech&al Meeting of the Satellite Division of the Institute of Navigation, p. 6374, Atbeqatrqut, NM, w- 1&18,l993.

9. LachapcIlc, G, Cannon, M, and 4 G, l Ambii ResoIution On-the-Fly - A Comparisoa of P Code and High Performana C/A Code dcccivcr TechnoIog& l’mmbgsdthcFiPChI.ntunationalT~ Meting d the Satellite Division of the hstltutc of Navigation, p. 6374, AIbequerque, NM, Sept. l&l& 1993. .s. RELEASE FORM %lo-“i Session:

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