The NCDXF/IARU InternationalBeacon Network —Part 2In Part 1, the beacon network was described; this part explainsthe underlying technology including the use of a GPS receiver foraccurate timing.By John G. Troster, W6ISQ and Robert S. Fabry, N6EK

Coordinator Assistant CoordinatorIARU International Beacon Project IARU International Beacon Project82 Belbrook Way 1175 Colusa AveAtherton, CA 94027 Berkeley, CA 94707Photos by John G. Troster, W6ISQ

Part 1, in last month’s QST, pre-sented a short history of the 15-year-old 14.1-MHz worldwide beacon net-

work that we call Phase II. The Phase II net-work introduced two important innovationsin beacons. First, by transmitting at fourpower levels it allowed casual listeners tolearn much more about the robustness of thepropagation path than had previously beenpossible. It’s great fun to know you just hearda 0.1-W signal from halfway around theworld! Second, by sharing a single frequencyamong many beacons, it not only cut downon the spectrum needed by the beacons, butit also simplified listening—the listener doesnot have to tune to a different frequency foreach beacon. In the Phase II network, eachbeacon has a 1-minute time slot once every10 minutes in which it is to transmit. Thedownside of this arrangement is that it takes10 minutes to listen for 10 beacons; manypeople find this too long.

The soon-to-be-operational Phase III net-work will improve on Phase II by allowingmore beacons. It does this by shortening thetransmissions in order to make listening forthe beacons faster. It also will allow the bea-cons to operate on five bands. The presentand possible future locations of beacons inthe Phase III network were shown in the mapin Part 1 of this article.

Time Standard

In order for frequency-sharing to work,each beacon must know precisely when itstime to transmit has come. Each Phase IIbeacon uses a 6-MHz temperature-compen-sated crystal oscillator (TCXO) for its timebase. Each beacon transmits when it is firstput on line, and every 10 minutes thereafter.Each beacon is started manually, and must bestarted within a fraction of a second of thecorrect time for its transmission.

In practice, the Phase II beacon’s internalclock drifts 1 or 2 seconds per month. Thetransmission sequence allows 3 seconds of“guard time” between one beacon’s transmis-sion and the next; after a drift of 3 seconds,

N6EK withan entiremultibandbeaconunit.

a beacon’s transmission may overlap that ofits neighbor. As a practical matter, it is nec-essary to reset each beacon’s internal clockevery four to six weeks, and this resynchron-ization is the most demanding operational as-pect of the Phase II beacon network. We owea great deal of appreciation to the individualamateurs and amateur organizations aroundthe world who have undertaken the responsi-bility of faithfully operating the beacons forso many years.

The other operational problem with thePhase II network is that no distinction is madebetween an intentional synchronizing power-up and a randomly timed power-up after apower failure. If you have heard a beacontransmitting at a wildly incorrect time, a ran-domly timed power-up that has not been yetdiscovered by the beacon operator is thecause.

WWV Timing SolutionA first attempt to solve the synchroniza-

tion problem used the 0.8-second 1500-Hztone broadcast by WWV and WWVH at thestart of each hour to keep the internal clocksynchronized. The transceiver used for the

beacon transmissions was switched to WWV,its audio output was fed into a 1500-Hz de-tector and the detector output was madeavailable to the microprocessor. Spuriousresponses were minimized by measuring theprecise duration of the detected tone and byknowing approximately when the start of thehour should occur. In tests in California thisscheme worked well, but there was a concernthat the scheme could not be depended on towork well in every corner of the world and inevery phase of the sunspot cycle.

GPS Timing Solution

The final synchronization scheme usesthe Global Positioning System (GPS) satel-lites to provide a timing standard. Rather thando this from scratch, it has proven cost-effec-tive to use a commercial unit, the AccutimeGPS receiver made by Trimble Navigation.1

This unit combines the antenna and receiverin a small plastic housing, requires 2 W at12 V dc and provides two outputs: a pulse atthe beginning of every second that is speci-fied to be accurate within 1 microsecond

1Notes appear at end.

Copyright © 1998 ARRL

proportional to the RF output voltage. Themajor functions of the controller includereceiving and interpreting ASCII timingpackets from the GPS receiver, sendingASCII character strings to the transceiver toinitiate frequency switching, controlling thepower level of the transmissions from thebeacon using the ALC of the transceiver andkeying the transceiver to send Morse code.The controller uses the Intel 8748 computer-on-a-chip that was also used for the Phase IIbeacon controller, and which remains a cost-effective choice. It combines an 8-bit micro-processor with a 1024-byte EPROM (eras-able, programmable read-only memory).The program for the 8748 is written in as-sembly language.

Timing

The once-a-second timing pulse from theGPS receiver is used to interrupt the micro-processor so it can add 1 to its internal clockand initiate a transmission when appropri-ate. The time packets from the GPS receiverare used to set or correct the clock’s value. Inaddition to validating the parity on the char-acters from the GPS receiver, the controllerrequires two time packets in a row that agreeas to the correct time before it sets or resetsits internal clock. In practice, the time pack-ets and pulses from the GPS receiver haverarely been garbled, but this arrangementmeans that a time drift from a lost or extratiming pulse will normally be correctedwithin 10 minutes and that nonsensical timepackets will be ignored.

When the beacon is first powered up, theGPS receiver does not know where it is orwhat satellites to look for. It takes a fewminutes for the GPS receiver to orient itself.The controller knows this. When first pow-ered up, the controller waits until the GPSreceiver declares itself “healthy,” and thenwaits until it has received two time packetsthat agree. This process can take as much as15 minutes.

Decoding the time messages is somewhattricky. The program first assembles indi-vidual characters by watching an input con-nected to the serial-data line from the GPSreceiver. It accumulates characters until ithas a valid packet. If the packet is not a timepacket, it is ignored. If a time packet arrivestoo close to the beginning of a second it isignored, since there is an ambiguity as towhether it refers to the second just ending orto the second just starting. Unfortunately,the time-of-day information in the packet isprovided as a floating-point number ratherthan as an integer. Converting the 48-bitfloating-point number to an integer using 8-bit arithmetic is lots of fun. The entire algo-rithm for extracting the time of day from thecharacter stream sent by the GPS receiverwas written and tested as a C program on aPC before it was coded in assembly lan-guage.

Frequency Control

Setting the frequency of the transceiver

Beacon Hunting from Northwest Connecticut“Sure—I can monitor 14.100 MHz and log some beacons over the weekend

for W6ISQ’s article.” It sounded like a piece of cake: log some beacons, write upa little blurb, and get back to my planned weekend fun of replacing the water linefrom the house to the road. But actually hearing the beacons proved to be moredifficult than digging the 55-foot-long, 4-foot-deep trench for the water line.

It’s not that I was trying to monitor the NCDXF beacons with a direct-conver-sion receiver hooked up to an unwound 10-foot reel of hook-up wire. Thoughperhaps modest to some, the equipment consisted of a Kenwood TS-440S/ATloaded with primo IRCI crystal IF filters, and the choice of a rotatable MosleyTA-53-M at 73 feet or a Cushcraft A3, fixed south, at 36 feet. The Yagis areswitch-selectable.

What became immediately apparent was that packet stations—strong packetstations—on both sides of the supposedly guarded beacon frequency, made fortough going. I could hear 4U1UN, W6WX, CT3B and LU4AA regularly, aspropagation supported the paths, of course. But copying the other five stations inthe existing beacon network proved fruitless.

Amateurs with bare-bones stations trying to log beacons on 14.1 MHz will nodoubt experience varying degrees of frustration. I believe that users of stock HFtransceivers with only a 2.1-kHz IF filter are going to get eaten alive by packetstations on or near the beacon frequency. This isn’t to say that basic HF stationswon’t be able to hear any of the beacons. But in order to make the most out ofthe NCDXF beacon system, here is a list of things that might help:

• A good receiver—robust front-end design that doesn’t overload easily;quality IF crystal filters with steep skirts and negligible ringing; IF shift orpassband tuning; audio filtering or tailoring.

• Rotatable, directional antenna or antennas. One Yagi or quad is nice; two ofthem is at least three times as nice as one. Trust me on this.

• A high degree of patience.—Jeff Bauer, WA1MBK, Chief Operator, W1AW

(much better than we need!) and a serial-data line on which ASCII-character packetsare sent.

The GPS receiver sends many differenttypes of packets over the serial data line.Normally, a GPS receiver is used to providelocation, as in the recent QST article on anautomated direction finder2 and the recentQST product review of the Trimble ScoutGPS receiver that automatically displaysMaidenhead grid locators.3 Unlike thoseGPS applications, we ignore the packets thattell us our latitude and longitude, and useonly the packets that specify the time (inUTC) and those that tell us that the GPS unitis “healthy.”

Art Lange, W6RXQ, who works forTrimble, has provided technical guidance inthe use of GPS timing. Within Trimble, Artchampioned the marketing of a self-con-tained stand-alone timing product. (Art wasalso the person who pushed internally tohave the hand-held Scout unit display Maid-enhead grid locators, a feature whose onlyknown application is ham radio!)

Beacon ControllerA controller serves as the brain of each

Phase III beacon (Figure 1). The other com-ponents are a Kenwood TS-140S trans-ceiver, the Trimble Accutime GPS receiverand an RF detector that produces a voltage

Figure 1—The block diagram of a Phase III beacon, showing the control-signal andRF paths.

Copyright © 1998 ARRL

The W6WX/B housing on Mt Umunum. K6GSJ (l) and N6EK (r) are working on some ofthe coaxial cabling. The GPS unit is mounted on the pole at the far left corner of thehousing.

is relatively easy; the program sends the riga string of ASCII characters. In the case ofthe Kenwood TS-140S, for example, onecan set the frequency to 14.1 MHz by send-ing to the transceiver the characters“FA00014100000;” (the “;” is one of thecharacters that must be sent to the trans-ceiver—it is not an ordinary punctuationmark). Each character must be bracketedwith start and stop bits and the bits must havea specific duration on the wire, just as withcomputer serial ports and telephonemodems. In this case, the program sets thevalue for the wire into one of the output bitsand then loops, doing nothing until it is timeto set the next value. (We might have usedspecialized UART chips for sending charac-ters to the TS-140S and for assembling char-acters from the GPS receiver, but there wasno need to add such hardware cost in thiscase.)

Output Power Control

The output power level is controlled byadjusting the ALC input to the transceiver,using analog circuitry. A diode detector at-tached to the RF output of the transceiverproduces a voltage proportional to the RFoutput. This voltage goes through a voltagedivider, the ratio of which is controlled bythe microprocessor. The output of the volt-age divider is compared with a fixed refer-ence voltage in an operational amplifierstring that produces an ALC voltage for thetransceiver to force the output of the voltagedivider to match the reference voltage.

The tricky part of all this is to design thecontrol circuitry so the keying envelope issmooth at all power levels and so the powerlevel is accurate at all frequencies. It wasnecessary to provide a microprocessor-con-trolled ALC bias voltage so the ALC voltageis approximately correct for the desiredpower level before the transceiver starts totransmit.

Keying

The keying is accomplished electricallywith one section of a 7417 open-collectordriver chip. The program for sending Morsecode, written by Jack Curtis, K6KU, wascarried over from the original beacon. Themessages to be sent by the beacon are storedin the memory as a string of ASCII charac-ters, rather than as a string of dots anddashes. To send a character, the character islooked up in a table that specifies the dot anddash representation of the character. As onemight imagine, a 1 bit in the specificationstands for a dot and a 0 bit stands for a dash;the tricky part is to find some simple way tospecify how many dots and dashes a particu-lar character requires.

The solution is useful for any programthat sends Morse code. The unused bitpositions are filled with a 1 followed byenough 0s to fill the byte: A is encoded as1010000 for dot-dash; B is encoded as

01111000 for dash-dot-dot-dot; and so on. Ifyou have written any bit-twiddling pro-grams, you can probably imagine the algo-rithm for sending the character specified insuch a way: If the high-order bit of the speci-fication is a 1, send a dot, otherwise send adash. Shift the specification left one bit po-sition, throwing away the old high-order bitand filling the low-order bit with a 0. If thespecification is now 10000000, exit; other-wise, go back to send the next dot or dash.

Further Ideas

One can imagine building a computer-controlled beacon monitoring station andattaching it to the packet networks used byDXers. Such a device could include a com-puter-controlled transceiver for listening tothe various frequencies and a GPS receiverfor accurate timing. Perhaps DXers couldcall up the actual propagation history for thepath from their location to particular bea-cons for the past hour, day, month or year.

The value of knowing that you can heara particular power level is much more thanthe information provided by existing propa-gation prediction programs, because thoseprograms do not fully account for atmo-spheric noise. Perhaps a systematic historyof actual propagation, such as might beavailable with automated monitoringstations, could provide the raw data for re-fining these prediction programs so theycould forecast signal-to-noise ratios insteadof merely signal levels.

Another interesting possibility arises

from the extremely precise timing of the bea-con transmissions. Since the GPS receiversprovide synchronization to the nearest mi-crosecond, one could easily calculate thetravel time for the radio signal from the bea-con to the monitoring station to within a fewmicroseconds. Would this allow one to de-termine the actual pattern of ionospheric re-flections that occurred? Could this informa-tion shed light on unusual propagationmodes such as skew paths? Fascinating pos-sibilities exist, and need only be tested anddeveloped by the users!

Conclusion

When complete, the Phase III beacon net-work will allow you to check for band open-ings on a particular band (any of the fivebands from 20 to 10 meters) in3 minutes. Or, you will be able to track thesame beacon through five bands to determinethe band that has the best propagation to thatarea. We are in for some interesting propa-gation experiences in the next several yearsas the sunspot count begins to move up fromthe approaching minimum. We can hardlywait!

Notes1Trimble Navigation, OEM Sales, Post Office

Box 3642, Sunnyvale, CA 94088, 408-481-8000.

2R. Flanagan and L. Calabrese, “An AutomatedMobile Radio-Direction-Finding System,”QST, Dec 1993, pp 51-55.

3M. Wilson, “Product Review: Trimble ScoutGPS Hand-Held Global Positioning SystemReceiver,” QST, Mar 1994, pp 77-ff.

Copyright © 1998 ARRL