EAAers Put TheAccuracy Back InBy BUDD DAVISSON

It's getting a little mind bogglingtrying to keep up with all that's new inaviation and Oshkosh is guaranteed tosquare the boggle-factor. We're hardlyinside the arches before the new stuffseems to steam roll over us like a tidalwave, challenging us to assimilate itall. Clearly an impossible task.

An unfortunate part of this tidalwave of new developments is that thehigh profile and obvious advances,like new aircraft designs, sometimescast a shadow that makes it difficultto keep track of the smaller, less ob-vious developments. Such is the casewith the Differential GPS experi-ments being demonstrated on thefield at Oshkosh '93.

Not many noticed the GPS notationon the tails of the Cessna 172 and Bo-nanza parked inside the entry arches.Even fewer, if any, noticed the innocu-ous looking box mounted on the gableend of the communications building

with what looked like a cellular tele-phone antenna coming out of it.Nothing hinted at the magnitude of theexperiment they were demonstrating.

The box and the airplanes were im-portant because, if nothing else, theyshow what happens when a trio ofEAAers are charged with solving a tech-nical problem facing aviation. As isusually the case, they came up with a so-lution quickly, simply and inexpensively.

The particular problem this trio wasaddressing is actually two-fold. Thefirst is coming up with a newer, less ex-pensive, yet accurate system to replacethe old UHF ILS technology. The sec-ond challenge is, assuming GPS is acandidate for ILS type approaches, de-veloping a technology that takes GlobalPositioning System (GPS) technologyand refines it so it is accurate enoughfor Category One (CAT 1) approaches.CAT 1 approaches require 200 footceilings and 1/2 mile visibility.

Refining GPS and making it work inthe CAT 1 environment was the chal-

lenge facing Don Taylor (CumulusConsultants, 5645 Rico Dr., Boca Ra-ton, FL 33487), Jim Cook (Palm BeachAvionics, 3950 Airport Rd., Boca Ra-ton, FL 33431) and Rudy Penteado(Genese, Inc., 3300 University Dr.,Coral Springs, FL 33065).

Don has been EAA's representativeto the RTCA (Radio Technical Commit-tee for Aeronautics) committee inWashington, DC for the past year or soand one of the things the RTCA taskforce has been looking at is GPS tech-nology, as an approach medium. Rudyand Don had known each other forsome time and Don had flown the mov-ing map software Rudy had developedfor use in coupling lap top computersto existing GPS units. Jim, an avionicsexpert and electronics manufacturingtype, rounded out the trio when, at theexact moment(!) SPORT AVIATIONEditor Jack Cox was tell ing Don heshould get to know Jim Cook, Jim flewright past Don's window in his Ques-tair Venture. Seems like some things

Rudy Penteado and the Hewlett-Packard 95LX computer used as the moving map screen in the Genese system.56 OCTOBER 1993

are meant to be!Don had already received approval

from the EAA to set up differentialGPS demonstrations on Wittman Air-port parallel to those being conductedby the FAA and Wisconsin DOT whichutilized technology and equipment de-veloped for the purpose by TrimbleNavigation. The Wisconsin DOT wasinvestigating the feasibility of settingup special CAT 1 GPS approaches forsome airports under its jurisdiction.Both set ups were aimed at the samesolution, only the Taylor/Cook/Pen-teado approach was typical EAA: Findthe latest COTS (Commercial Off TheShelf) technology and solve the prob-lem for the least amount of money.

GPS has become the navigationalbuzz word of the decade and has veryeffectively swept every other form ofnavigation, with the possible exceptionof the whiskey compass, under the car-pet. However, the entire GPS concept,being basically a military guidance sys-tem, has been forcefully degraded bythe government to remove much of theaccuracy making it useless for precisionapproaches. Originally, under opti-mum conditions, it would give accuracydown to about half a meter (18 inches)with a few meters possible in the worsecase scenario.

The military has encrypted the signalin such a way that you need a P-codereceiver to get optimum accuracy andthey aren't selling those things at sur-plus stores and aren't likely to. So,about the best a normal GPS unit candeliver is 200-300 feet horizontally and400-500 feet vertically.

There is a way, however, to take thediluted signal and correct it to regainthe original accuracy in a specific loca-tion. This is done by differential lycorrecting the signal using a knowngeographical location as a datum point.

The technology isn't all that com-plex and the hardware, although itwould have seemed to be somethingfrom Buck Rogers a few years back, isnothing more than the normal micro-processors we've come to accept aspart of everything from GPS units tocoffee makers.

Basically, a point on the ground isaccurately surveyed. This gives aknown location in relation to the GPSsatellites. Then, since the signal speedis known, a time-enroute for the signalfrom given satellite is computed. Let'ssay, using a grossly out of scale exam-ple, the calculated t ime is 400milliseconds door-to-door (you don'ttime it with a sweep second hand!).However, when the signal "finally" ar-rives at our GPS receiver at thesurveyed location, we find it took anagonizingly slow 450 milliseconds.This means it is 50 milliseconds late,

GPS receiver and wireless modem in the Bonanza used in the Oshkosh demonstrations.

so we automatically enter that devia-tion in the mini-computer softwarethat's hiding inside the GPS unit atthat location. This software has beencarefully programmed to take the dif-ferences from all of the satellites andassemble a sort of electronic compassdeviation card for GPS units workingat that location. Using the internal de-viation card, the signals from thesatellites can be differential ly cor-rected so the indications on the GPSread-out are accurate.

So, now that we have a way of gen-erating a set of signals that have beencorrected for the military degrada-tion, what good are they to us sittingon the ground?

They don't do us any good at all sit-ting on the ground, but if we could getthose same signal corrections into aGPS unit that 's mounted in an air-plane, that airplane would be capableof achieving the original military accu-racy of the GPS system.

So, utilizing another one of thoseincreasingly common miracles of mod-ern communication, a gizmo that isbasically a wireless modem, we data-link the deviation table up to any GPSunit that happens to be equipped witha wireless-modem operating on thesame frequency as ours.

A modem on board the a i r c ra f tworks with an on-board computer/controller to feed the corrected signalsinto the aircraft's GPS and, voila, in-stant accuracy. Let's see Buck Rogersdo any better than that!

Those are the basic concepts, butputting them into action at a reason-able dollar is what the GPS Trio wasaiming at.

Their final product is a two part ap-proach which includes a base stationthat is really nothing more than a spe-cial GPS receiver equipped with awireless modem. The second piece is amatching modem and GPS unit in theairplane, which is dialed into the groundstation so it can receive the signals.

The airplane also is equipped with a

Rosetta Microsystems encoder. TheRosetta encoder is incorporated in anumber of brand name systems anddiffers from others in that rather thanworking str ict ly in 100 foot incre-ments it is also capable of the one footaccuracy required for precision ap-proaches. Equally as important, it alsohas a computer-type serial port. Byhard-wiring that port to the GPS inthe airplane, a barometrically cor-rected altitude signal is always beingfed directly to the GPS receiver.

The barometrically correct encodersignal is useful because it acts like asatellite perched directly above the air-craft at all times. This gives constantread out of accurate vertical informa-tion which, when combined with thetriangulation of the GPS satellites, givesextremely accurate three-dimensionalinformation to the GPS output device.

The output device utilized by theGPS Trio is a commonly available palmsized computer, the Hewlett-Packard95 LX. The HP 95 has a specially pro-grammed card developed by Rudy thatlets it take the navigational signalscoming from both the encoder and theGPS and display them as a movingmap on the computer's little 2" x 4"display. The entire unit is clipped tothe yoke like an approach plate.

But that's not all!The moving map only takes half the

screen. The other half is a completeHSI that displays all necessary navdata. Then, if that isn't enough, apush of a button replaces the HSI withan RMI or an ADF read out.

And some of us still haven't figuredout northerly turning errors on the oldwhiskey compass.

Okay, enough theory! When wewere sitting in the C-172 on the line atOshkosh, running simulated programsthrough the unit, it dawned on us ...the best way to understand the systemwas to fly it. So, Don slid in the leftseat and we saddled up on the rightfor a hands-on demo.

The scale of the moving map can beSPORT AVIATION 57

changed through a wide range, from3/10ths of a mile (the scale is so huge itonly shows a section of the runway) to42 miles, which, at Oshkosh put every-thing from Green Bay, Fond du Lac andthe surrounding area on the map.

As soon as we started taxiing, therunway depiction started moving. Atthe beginning, the map was orientedwith North up. However, the instant weexceeded 3 kts., the display changed toa "windshield" type of depiction, whereeverything is oriented the way the pilotwould see it out the windshield.

The base station (with the case removed).

It's almost uncanny to be taxiingdown a taxiway with the runwayshowing on the map and moving justthe way we see it out the windows.It's even weirder to watch the displayas we turn off the taxiway to positionand hold on the runway. The mapmoves much smoother and in muchmore real time than I had expected.This is the result of getting positionupdates several times a second.

Once in the air, we headed south-bound, with me watching the movingmap. We increased the scale so wecould see Oshkosh and as far away asFond du Lac. Our position on the mapwas indicated by a continually movingpair of crossed lines and we simplyflew to the intersection where wewanted to go.

The ILS approach path to the run-way was depicted on the screen as alarge "V" shaped funnel, and our jobwas to line up and fly down that fun-nel. Since we could clearly see wherewe were in relationship to the outermarker, we just drove to the outermarker and turned inbound.

Since the HSI needle functions as a58 OCTOBER 1993

classic ILS localizer needle, it was asimple matter to use the map as a wayto get us in the ball park, and use theHSI to fine tune it. The glideslope in-formation is a vertical scale with amoving pointer on the bottom left cor-ner of the HSI and, until it is withinrange, it is "flagged" as being in-op bya box of stripes that lies over it.

From the point of intercepting theILS to decision height, it was just likeflying a normal HSI guided approach,except the HSI was a LCD read-out ona $400 handheld computer rather thana panel mounted gauge costing tentimes that amount. All of the display'smovements were very, very smoothand didn't have any kind of "toy" jerk-iness to them.

The GPS-demo Bonanza has thesame basic equipment, but it is cou-pled to the autopilot and thesmoothness of the signals reallyshows. It centers the glideslope andmaintains it so smoothly it wouldshow up even the smoothest pilot.

It should be mentioned that any kindof approach can be programmed intothe software by the computer key pad.It doesn't necessarily have to overlaythe ILS, as it did at Oshkosh. Anythingabout the approach, from the glideslopeangle to the designated point of touchdown, can be custom tailored. In otherwords, if a guy wants to set up an ap-proach to his mountain runway thatrequires a super-steep angle to a touchdown right next to the trees, all he hasto do is punch in the parameters.

Also, the ground receiver that is cor-recting the signals doesn't have to be atthe airport where the approaches arebeing flown. In fact, one of the con-cept's strong points is that itsapproximate 30 mile range (line ofsight) can be utilized to design ap-proaches to any airport within range.The coordinates of the touch downpoint and parameters of the approachare all that need to be entered. Also,there is no need for front/back courseconsiderations since front course ap-proaches can be programmed in foreach end of the runway and then se-lected at will by punching a couple ofbuttons. The flexibility in approach de-sign is astounding.

The foregoing is why the WisconsinDOT was interested in the concept.Using one unit at Oshkosh, they couldnot only put approaches at all fourrunway ends, but they could also pro-vide Special CAT 1 approaches atAppleton and Fond du Lac.

Those who want to use the SpecialCAT 1 approaches would have to havethe ability to receive the corrected sig-nals from the ground station via datalink and the frequency of the data linkis, right now, one of the big questions

to be settled.One possibility for a data link fre-

quency is the VOR band (112-117.95mHz), as well as others in the regularVHF band and FM sub-carrier ranges.However, one of the leading candi-dates is using transponder frequencies,like 1030 mHz. If a universal fre-quency is assigned for the purpose, thesame airborne receiver could be usedin a number of locations. Otherwise, ameans for tuning the data link receiverto the local frequency would have tobe provided.

The project, as approached by Tay-lor, Cook and Penteado fallssomewhere between being an altruisticmove to better aviation and a commer-cial venture. On the one hand theunits they have built are proof of con-cept systems aimed at giving the FAAmore input to aid them in making theirdecisions. Their systems show whatcan be done using off the shelf goodiesto control costs.

Cook and Penteado also have a com-mercial leaning to their endeavors,since they intend to manufacture com-plete differentiated GPS ground andair systems under the name GeneseNavigation Systems. Because of theircommercial aspirations, they had a rel-atively firm handle on what thesystems would cost.

In round numbers, to set up a com-plete approach package for an airport,the ground equipment would cost wellunder $10,000. This includes a specialGPS receiver, the software to drive it,a data link unit and a battery back-upto keep the system on-line for 12 hoursafter a power interruption. This is thesystem they had running at Oshkosh.

Each airplane would have approxi-mately $3,000 worth of stuff on boardtotal. However, some of that equip-ment may already be installed there,since this cost number includes theright kind of altitude encoder and theGPS unit. If they are already on board,the additional cost comes down. Pos-sibly under $1,000 and most of that isfor the GPS antenna, the data link mo-dem and the palm-sized computer($400 plus $400 program card).

What the guys didn't mention intheir sales pitch is that once this thingis installed in the airplane, it automat-ically has the HSI, RMI and ADFdisplays because this stuff is in thesoftware. If these capabilities are im-portant to the pilot, that fur therreduces the amount of money he hasto invest in navigational gear.

A really neat off-shoot of the wholeGPS data link thing is that the samedata link could be used for a lot of dif-ferent purposes. For one thing, itcould up-link real time weather info.Even more fascinating is its possibili-

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ties for collision avoidance. If eachaircraft had its own data link capabili-ties on the same frequency, then eachairplane's position and altitude couldbe displayed on every other such sys-tem within range. This would giveeach pilot his own ATC radar screen tomonitor and he'd always know whowas around him. Through the soft-ware, it would even be possible totailor the display so only those aircraftwithin, say, 2,000 feet of your altitudeor with threatening headings would bedisplayed. The possibilities are limitedonly by the imagination of the com-puter programmers like Rudy.

So, where do we go from here? Itlooks as if most of the balls are in theFAA's court. In the first place, al-though they appear to be behind theconcept of d i f f e r e n t i a l GPS ap-proaches, they haven't totally throwntheir arms around it. They also have alot of homework to do, like decidingwhat the data link frequency will be,setting up parameters for the base sta-tions, etc., etc.

As everything stands right now,everything we've said here is experi-mental and can ' t be applied to acertified airplane in IFR conditionsand can't be slaved to a certified au-

topilot at all. It will be months beforethere are TSO'd units available fromcommercial manufacturers, which mayinclude Rudy and Jim's company alongwith the known brand names. But theunits won't be worth much until theFAA gets into the act and gives every-one some firm design parameters todesign to. Without them, the base sta-tions can't be put into operation.

As it is now, through the good oldEAA approach of using your head andwhat's sitting on the shelf, the GPSTrio of Taylor, Cook and Penteadohave given us all something importantto get our electronic teeth into. *

SPORT AVIATION 59