Apps for Hidden Transmitter Hunting

by Joe Moell KØOV

Most of the time, I'm not an early adopter of consumer electronics.  When I'm spending my own money, I tend to sit back while new technologies work their early bugs out, features improve, and prices become reasonable.  Sometimes that's smart.  I never got stuck with a Betamax or a 300 bps modem.

Desktops and laptops met all my computing needs for many years, but I followed the proliferation of mobile devices and apps with interest.  When Bob Iannucci W6EI first told me about his Foxhunt app, I had to agree that it was useful and "cool."  At the time, I was experimenting with automatic mobile bearing-taking and triangulation on my laptop.  I wasn't interested in using a cell phone for triangulation because the screen is far too small to display maps well.  Then the Map-n-Compass radio-orienteering course layout app for Apple devices came along and I knew that it was time to spring for an iPad Mini that would run it and all the other radio direction finding (RDF) apps on a display of useful size.

Here are my reviews of three bearing-taking and triangulation apps, all designed by ham radio transmitter hunters for transmitter hunters.

Foxhunt App Gets Better

From my Homing In column in the July 2014 issue of CQ-Plus Digital Magazine

I reviewed the first version of W6EI's Foxhunt in Homing In for the Spring 2010 issue of CQ-VHF.  Bob is a MIT-trained engineer and computer scientist who got the idea for the app when he and his wife Susan W6SJI went with friends on their first mobile transmitter hunt with the San Francisco Bay T-hunt group.  He concluded that his iPhone 3GS could do a better job of keeping track of all the bearings than he could with paper maps.

At present, Bob's program doesn't interface directly with radios or RDF antennas.  It's just a very simple way for anyone to record bearings from multiple locations and display them and their intersections on Google Maps.  After getting a bearing with an RDF system, the user looks in that direction while holding the iPhone directly in front.  The phone's GPS engine and internal compass determine location and azimuth and then the program puts a bearing ray on the display map.  When multiple bearings have been entered, the best triangulation can be computed and displayed.

Map view in Foxhunt Pro.  A green dot on the upper left of the screen indicates that FHP is in contact with the cloud bearing server. The map turns as it orients to the internal compass reading.  The slider at right zooms the map in and out, replacing the traditional "pinch" method.

The accuracy of your transmitter location predictions with this app will depend on the directivity pattern of your RDF antenna, signal reflections, and how well you can sight your bearings from the antenna boom while looking at the screen of the phone.  But you won't have to worry about bearings extending beyond the edge of a paper map or making an error by not correcting for magnetic declination.

The original Foxhunt app works on iPhone 3GS and later models, as well as iPad.  It is still available for free, supported by advertising.  Dr. Iannucci soon had several ideas for a more advanced version, but for a while that had to take a back seat to new demands on his time as he became Associate Dean of the College of Engineering at Carnegie Mellon University and Director of the Silicon Valley campus.  With help from Bob's son Peter K6PAI, Foxhunt Pro (FHP) finally became a reality in February of this year.

FHP doesn't display ads and is available in the App Store for $6.99.  Like the original Foxhunt app, you must search for it under iPhone apps, even if you plan to use it on an iPad.  FHP's primary new feature is bearing sharing via the cloud.  Although it's "every team for itself" in most competitive transmitter hunts, there are some situations, such as a search and rescue mission, where it's important for everyone to be aware of all the hunters' locations and bearings.

Collaborating foxhunters must have a data connection (usually cellular) and must log on with their unique user ID and password before the hunt starts.  One hunter on the team creates a hunt ID on the server and invites team members to join in sharing bearings during that hunt.  Whenever someone logged into that hunt ID takes a bearing, it goes to the server and all others logged into that hunt ID see it on their screens.

The Analysis Mode shows a list of all the hunt's bearings.  Edit it by deleting ones you don't want and adding stars to the ones that you think are best.  With that done, a tap of the Intersections button puts a green pin on the map at the estimated location of the transmitter and an "ellipse of uncertainty" around it.  FHP determines the pin location and ellipse size by triangulating all the bearings, giving greatest weight to the ones that were given the most stars.  With a couple more taps, FHP displays directions from your location to the green pin.

Result of the triangulation of three bearings in Foxhunt Pro.  The purple ellipse is the uncertainty of the triangulation, based on the confidence level given to each bearing by the assigned number of stars.

When I installed and tested version of FHP, I liked some additional subtle improvements over the first version that I found, including the constant display of current GPS coordinates and compass reading at the bottom of the screen.  This is a true heading, corrected for magnetic declination.  The map view shows your position as a red dot and a red dashed line extending in the direction that the device is pointed.  The dashed line is convenient if you want to use terrain features on the satellite view to sight your bearings.

In addition to receiving bearings from your friends via the cloud, you can manually add bearings that they send you by changing the data source to Manual and then pinching and sliding the map to place the red dot over the location from which the bearing is to be entered.  A slider at the bottom of the display rotates the map and the bar at bottom shows the true bearing to be entered.  When location and heading are right, press the plus-sign button.  You can also tap the GPS coordinates bar to bring up a data entry form to type in GPS latitude/longitude or UTM coordinates plus numeric true or magnetic headings.

FHP has upgraded the triangulation algorithm to use great-circle calculations.  That's important because flat maps have large inaccuracies at a distance, due to curvature of the earth.  A flat map would lead you to believe that Atlanta is directly east of Los Angeles, but the actual bearing from LAX to ATL is just over 80 degrees true.

A long-distance triangulation on Foxhunt Pro, showing how the bearings take into account the curvature of the earth.

Great-circle triangulation is a boon when skywave interference on HF bands must be found.  I can imagine a network of HF-RDF stations around the USA exchanging bearings from their homes via the cloud or e-mail, with each participant being able to triangulate them with FHP.  Accuracy of the bearings wouldn't be good enough to pinpoint the interference sources from afar, but it could be good enough to determine where mobile foxhunters could go to receive the interference via ground-wave.

SigTrax for Apple and Android

From my Homing In column in the July 2014 issue of CQ-Plus Digital Magazine

In January 2014, I received e-mail from Jim McCullers WA4CWI, who is a computer software engineer and entrepreneur.  In telling me about his new app, he wrote, "I was co-winner in 1982 of the first foxhunt conducted by Birmingham Amateur Radio Club and have participated in many hunts since then, both as fox and hound.  I have also been involved with several near-space balloon launches and I succeeded in recovering one where all telemetry was lost and we depended on a 30 milliwatt two-meter beacon to locate the payload.

"I also help in finding interference and have tracked down a pirate broadcaster," Jim continued.  "With SigTrax, I combined my enjoyment of RDF and programming to create a solution that eliminates a lot of the burden of carrying maps and drawing bearing lines while attempting to locate a signal."

An important goal for Jim was for his app to work on either Apple or Android devices.  He wrote, "I believe an app that is general purpose in nature should not limit itself to a single platform.  I have given it an attractive price that, combined with the web site, should encourage people to purchase."

SigTrax iOS version is $3.79 in the Apple Store and the Android version is $3.99 at Google Play.  If you cannot find the program, verify that your device has built-in GPS.  Upon loading, the program displays map or satellite view, whichever you have selected, with a blue dot indicating your position.  GPS coordinates are shown in the upper right.

Like FHP, SigTrax has an internal Compass Rotation mode that allows you to put a bearing on the map by simply rotating your device until the incoming signal indication is dead ahead.  Then tap the blue dot that indicates your GPS position.  You will get a dialog box that asks if you want to "Insert Fix Bearing?" Tap Yes and the bearing will be added to the list.

SigTrax Map View always shows all bearings at once, drawn in the color selected by Settings.  Wherever any two bearings intersect, the program places a red pin.  (SigTrax calls the intersections "crosspoints.") Tapping on a red pin pops up a dialogue box with the GPS coordinates of the crosspoint.  The box also has an icon which, if tapped, takes you out of the program to Google Maps, where a blue-lined map and directions from your location to the crosspoint are shown.

SigTrax puts a red pin at every bearing crosspoint.  White bearings show up best on Google satellite view.  The traditional "pinch" method zooms the map in and out to show exact locations of the crosspoints.

When I first tried SigTrax, our county RACES organization had begun a monthly cooperative two-meter transmitter hunt to help newcomers gain RDF skills that they could use for search and rescue.  During the hunt, participants communicate on a UHF repeater with announcements such as, "I'm at Third and Elm and the signal is due east." I told Jim that it would be nice to be able to enter such a bearing quickly by sliding the map to Third and Elm, tapping it, and typing 90 degrees into a pop-up dialogue box.

When Apple Version 1.3.0 of SigTrax was released in March, that feature was included.  Jim told me that it may not be possible in Android because only the Apple OS can distinguish between pressure on the screen and a tap.  Also there are differences among Android devices in their responses to varying touch forces.

An extensive Settings Menu allows selection of bearing color, bearing length, measurement units, etc.  The maximum bearing length is 10,000 kilometers (6214 miles).  SigTrax uses great-circle algorithms for bearing traces, so it can be used for long-distance triangulation like FHP.  The red pin is slightly offset from the crosspoint when the triangulation is distant, but it isn't important because of the inherent uncertainty of skywave bearings.

SigTrax includes a method of entering bearings from your location without relying on the internal compass.  "The compass technology in mobile devices is not well developed and impossible to keep in calibration," WA4CWI opines.  "I tell people that if lost in a forest, I would place the device against a tree and let moss grow on the north side rather than trust its internal compass heading.  So I added the Rotation Ring."

If you set Map Rotation to Manual in the Settings menu, the Map View includes a ring near the bottom of the screen and a true heading at the top that corresponds to the map orientation.  Tap the right side of the ring to rotate the map counterclockwise and increase the straight-ahead true heading, or tap the left side to do the opposite.  The map moves in ten-degree steps, but you can toggle that to one degree or 0.1 degree steps by pressing the center of the ring for about a second.  When the map is oriented such that the signal is toward the top, press your blue location dot as before to insert a bearing from your location.

To take SigTrax bearings without using the internal compass, rotate the map in 10, 1, or 0.1 degree steps with the rotation ring at the bottom.  Red bearings show up best on Google map view.

I have a higher opinion of the internal compass than Jim does.  When I first tried out my iPad with a dedicated compass app (Digital Compass), I agreed with him because the compass heading was inaccurate and would often "stick" as I turned the device, even after performing Apple's "wave in a figure-eight" compass calibration routine.  Then I removed the iPad from its Belkin folio case/stand and the compass began working fine.  The folio has a magnet closure, but even cutting off the magnet wouldn't get the iPad compass to work while inside it.  So now the iPad goes "naked" or in a zip-lock bag when I'm transmitter hunting.  Compass performance is excellent most of the time, but it works best when I hold the device parallel to the ground, not "standing up."

Apple makes two iPad Mini models, one with cellular data plus WiFi, and one with only WiFi.  Only the cellular-equipped iPads include GPS, because the iPad GPS depends on cellular system data for rapid startup.  When I was away from cellular service on the high seas recently, I couldn't get the iPad GPS to acquire my location.

Cellular data plans for iPad are available on a month-to-month basis with no contract from the four major carriers.  Even before I signed up, my Verizon version acquired GPS in seconds because it picked up LTE location data from the towers.  But for streaming Google Maps in the boonies for the triangulation programs, a cellular plan is a necessity.

SigTrax Plus, DataTrax and TraxTeam

From my Homing In column in the the August 2019 issue of CQ Amateur Radio Magazine.

SigTrax Plus by Jim McCullers WA4CWI has an add-on interface for automatic display and plotting of Doppler bearings on the move.

With no RS-232 or USB ports to work with, getting bearing data into a phone or tablet was a challenge that Jim took on in 2014.  He looked into an AFSK interface box to go between the Doppler and the audio input jack, like the Square credit card reader.  He also considered feeding data audio into the device's microphone, as is done in some medical accessories.  In the end, he decided to transfer the Doppler data via Bluetooth LE.  That scheme works equally well with Apple and Android devices.  It has the advantage of a wire-free connection from Doppler to SigTrax Pro.  The Navigator can sit anywhere in the vehicle.

Commercial RS-232-to-Bluetooth adapters that Jim tested proved to be unsuitable for this project, so he produces and sells his own proprietary DataTrax "dongle" at his Web site.  It plugs into the Doppler's DB9 RS-232 port and gets power via USB.  (Your new vehicle has USB charging ports, right?)

The DataTrax dongle attaches to the DB9 RS-232 connector of the Doppler set and transmits serial data to the phone or tablet via Bluetooth LE.  Power comes from a USB connection.

Over the years, several Doppler RDF products have incorporated the Agrelo serial bearing interface including the DFjr by Joe Agrelo, N2OOC (Agrelo Engineering), the DSP-RDF by Dan Welch, W6DFW (SK) and the MicroFinder by Rich Harrington, KN6FW (AHHA! Solutions).  None of them are currently in production, but if you find a working used unit, it should be compatible with DataTrax.  They all combine the GPS and RDF data into a single serial input to the dongle, although there are differences in the number of positions and bearings sent out per minute.

My favorite of these Dopplers is MicroFinder because it includes second-harmonic detection of the Doppler tone to help filter out reflected bearings.  It also can be set up to send single bearings to the mapping program at the touch of a button.  My first test of the SigTrax Pro serial interface was with the Microfinder and a Magellan Meridian Gold GPS.  That was successful, so the next step was to test with a Doppler that readers can readily purchase.

The only Doppler sets with Agrelo-format output now being marketed are designs of WB6EYV.  Bob sells his PicoDopp through his company, Doppler DF Instruments.  It comes to you as a set of assembled and tested circuit boards that you will need to wire together and mount in the enclosure of your choice.  If you have built electronic kits and can handle the packaging details, Bob's extensive documentation should get you the rest of the way.

In 2011, WB6EYV licensed his PicoDopp design to Global TSCM Group, which sells it as the DF2020T kit.  It's not quite plug-and-play because some minor assembly is required.  All you will need is a screwdriver.  Simply mount the processor, display and connector boards into the pre-drilled box, attach four whip antennas of appropriate length to the magnetic-mount bases, and you're ready to go mobile.  MFJ sells the same Doppler set as model MFJ-5005.

In order to plot bearings relative to true north, SigTrax needs to know the vehicle heading.  The most accurate way to determine the vehicle's direction of travel is with GPS, but the vehicle must be moving at least 3.5 MPH.  When the vehicle is stopped or moving more slowly than that, "No Course" appears at the bottom of the screen.

To map Doppler bearings when the vehicle is stopped (at the hilltop starting point of a T-hunt, for instance), switch to the STATIC mode in the SETTINGS menu and enter the vehicle heading manually.  SigTrax will make the appropriate heading correction.  Switch back to the MOBILE mode for bearings as you drive.

When Doppler bearings are being received by the program and the vehicle is moving, they are displayed in the map view at a rate determined by the SAMPLE INTERVAL setting.  When RECORDING is off, bearings are displayed but not saved.  When RECORDING is turned on (red dot flashing), these bearings disappear and a new set of bearings begin to appear.  The details of these bearings go into a list that can be saved in an archive file.  Later, the file can be recalled, edited, and added to newer bearings.

The list of bearings can be viewed and edited.  Bearings can be added to and deleted from the list.  They can be saved in archives and shared with other foxhunters.

It's important to spend some time optimizing the three DataTrax adjustments in the SETTINGS menu: SAMPLE INTERVAL, SMOOTHING LEVEL and COURSE DELTA.  SAMPLE INTERVAL selects the bearing plotting rate over the range of every second to every 60 seconds.  SMOOTHING LEVEL and COURSE DATA control how the bearings are averaged and how sudden changes of vehicle direction affect the displayed bearings.  When you're starting out and tracking a steady signal, the sample interval should be 30 to 60 seconds, unless you think you are very close to the transmitter.

In some situations, continuous plotting will cause confusion.  Examples are a foxhunt with multiple transmitters on the same frequency or when hunting on a repeater input with numerous users.  It may be better to plot bearings on the target by turning RECORDING on and off.  To minimize screen clutter and to get the best bearings for triangulation, try to map them only from high locations when the signal is clear with a minimum of multipath flutter.

This discussion of Doppler sets and bearing mapping wouldn't be complete without a few words about accuracy.  A well constructed and installed mobile VHF/UHF Doppler set will give good bearings most of the time, especially when the target signal is only a few blocks away.  But just because your Doppler's serial output is in decimal degrees or even tenths of a degree, don't fool yourself into thinking that your bearings with it are that accurate, especially when the target isn't line-of-sight.  Hills, buildings and other large objects in the path can easily cause significant bearing errors.  Bearings to targets across bodies of water are subject to error due to refraction effects.

A good rule of thumb is to assume that Doppler bearings taken under ideal conditions are accurate to five degrees, plus or minus.  A five-degree error at 11.5 miles distance causes a line-of-bearing to miss the target by one mile.

For a near-worst-case example, the screen below shows bearings on the output of a two-meter repeater that was 45 miles away with intervening hills, as I drove freeways and surface streets.  The inconsistencies in the plotted bearings are easy to see.  There are even a few "wild" bearings that could have been caused by large trailer trucks passing by.  It's clear that the signal source is far away and the crosspoint icons are not triangulated fixes. 

First test of the SigTrax and MicroFinder combination on the road map view.  The sample interval is short, resulting in a large number of orange bearing lines that show the normal variations in Doppler bearings from a moving vehicle.

The screen below shows mobile bearings under better conditions.

A series of red mobile bearings taken over flat terrain en route to a hilltop fox.  Bearings were recorded in clear locations at key intersections as the transmitter was approached from the southeast, south, and then west.  Most of the crosspoints are in or near the park where the transmitter is located.

SigTrax Plus is available for Apple and Android devices.  It works with iPads of Generation 2 and later.  Internal GPS and an Internet connection for map updating are required.  Also available as a SigTrax add-on is TraxTeam.  Registered users of TraxTeam can form teams to share bearings in real time as they privately collaborate to find the sources of signals.


From my Homing In column in the July 2014 issue of CQ-Plus Digital Magazine

Charles Scharlau NZ0I discovered hidden transmitter hunting in the Puget Sound area of Washington State in the 1980s.  He went on to compete in the first USA Amateur Radio Direction Finding (ARDF) Championships in 2001, which led to a trip to the Czech Republic in 2004 as a member of USA's team for the World Championships.  Charles and his wife Nadia have continued their involvement in ARDF as much as family obligations have allowed.  They were the course planners and setters for the 2013 USA ARDF Championships near their home in North Carolina.

Charles is a software engineer and embedded systems specialist who has formed his own company, Digital Confections.  He is the proud author of the new Map-n-Compass app (MnC).  It sells for $9.99 and is only available for Apple devices.  MnC is so feature-rich that you will spend a lot of time learning how to use it.  Charles must have realized this, because upon loading, the first dialogue box asks if you want to go to the map or to the 44-page help file.

Like the other two apps, MnC will go along on a hunt with you, keeping track of your position, plotting your bearings and providing estimated transmitter locations based on triangulations.  Charles intended the app to be used for all-on-foot hunts under international rules, but there's no reason why it can't also be used on area-wide mobile hunts, provided that you go into the settings and increase the maximum bearing length as appropriate.

An advantage of MnC is that it is designed for multiple-transmitter hunts and will keep track of bearings for up to five different foxes at a time.  In the screen shot of a sample mobile hunt, the timing is stopped and all bearings are displayed with different colors for each fox.  When the hunt clock is running, bearings for only one transmitter at a time are shown.  The program draws a purple trace as "bread crumbs" showing your route.

Map-n-Compass in use during a mobile transmitter hunt, starting from Coyote Hills.  After 14 minutes, the first fox (MOE) has just been found with three bearings and two additional bearings have triangulated the possible location of the second fox (MOI).

MnC is intended to be used with orienteering maps that have been created in accordance with International Orienteering Federation (IOF) standards.  However, it also works with open-source street, aerial and topographical maps, either online or stored on-device.  For this illustration, the base map is open-source 4U Outdoor Map.  4U also has a street map version which would be a better choice in urban areas.

MnC not only helps you win transmitter hunts, it creates them.  In as little as five minutes, the program can lay out a complete five-transmitter course for ARDF with GPS coordinates for each fox.  According to Charles, "I wouldn't want to try designing and field checking courses without it.  We carry all of Nadia's courses and Backwoods Orienteering Klub maps in the app, and we use it to help us navigate quickly and easily to all the transmitter locations when we put them out."

I think the coolest MnC feature is its ability to provide a virtual on-foot foxhunt experience with all of the fitness benefits and none of the transmitting hardware.  Have a friend create an ARDF course map in the device for you and tell MnC to hide the foxes, so you won't know where they are.  Then take the app, plug in your earbuds, go to the start point and switch to navigate mode.  Slide the navigation arrow to display "#1-MOE."

You will hear the first virtual transmitter.  Hold the device level to the ground and turn in azimuth for the strongest audio signal, just as you would with a two-meter beam.  Tap the navigation icon to put a bearing on the screen and move in that direction.  Move on while taking bearings until you get within 20 meters of the hidden fox icon, whereupon the program will automatically declare that you found it.

You can then switch to "#2-MOI" and continue on to find that one with virtual RDF, and so on.  After finding #5-MO5, you can navigate to the finish by selecting "F" and homing in on the MO signal, or just use map navigation to the double circle.  Tap "STOP" and the app will create a record of your course with your GPS track and time to find each fox.

ARDF course-setters can quickly design courses like this within the Map-n-Compass app and then use the device to locate and set the transmitters in the field.  The map is oriented to magnetic north, which is standard for ARDF.  The five transmitter icons can be hidden for assisted or virtual tracking.

What a great way this is to practice your bearing-taking and navigation skills in between your local ARDF events.  As in regular ARDF, your results will be better if you check bearings on all foxes at the start, then choose the order of finding the five foxes to minimize your travel time.  MnC's virtual ARDF is easier than actual ARDF in the Beginner mode because all five foxes are continuously transmitting on separate virtual "frequencies" so you can easily tune among them.  When you have mastered Beginner mode, switch to Intermediate and Expert, where the foxes are on for 60 seconds each in sequence, just like championship courses.

According to Charles, "Menus should be left in restaurants," so there are no pull-down menus.  All functions are accomplished by single-tap, double-tap, or tap-and-slide.  This is faster and easier when you are out in the field, but it requires practice and experience to memorize the function commands before you go.

I have become a big fan of this app, but for me it had a long learning curve because of the number of features and the lack of menus.  I spent a lot of time switching back and forth between the app and the help files to figure out what to tap, hold or swipe to make it do what I wanted.

There are still many features that I haven't tried yet, such as the ability to create an ARDF course on PC or Mac and send it to MnC by e-mail or iTunes file sharing.  Similarly, courses created within MnC can be shared and viewed on PC or Mac.  Hunt results files can be displayed and animated to "relive" the hunt experience.  You can choose the display orientation, portrait or landscape.  Settings can be customized for the number of transmitters, on-air time for each, antenna beamwidth, signal tone pitch, peak versus null signal tracking, arrival range and much more.  This app is going to keep me busy for a very long time.

Final Thoughts

International rules forbid the use of GPS positioning and electronic mapping by participants in formal ARDF championship contests.  National and world-class radio-orienteers are expected to do it the old-fashioned way, with paper maps supplied by the course-setters and their own hand-held compasses.  Don't be surprised if the organizers of championship ARDF events forbid the carrying of any mobile devices on courses from now on.

In addition to these Apple/Android apps, there is PC software for mobile RDF.  The GoogleHunt program by Bob Simmons WB6EYV automatically plots bearings from Doppler RDF sets that have serial output in Agrelo format.  I reviewed GoogleHunt, the Agrelo format and compatible Doppler sets in Homing In for the Winter 2012 issue of CQ-VHF Magazine.

I look forward to hearing from you about your transmitter hunts, including your experiences with new tools such as these apps.  When you experiment with computer mapping and tracking, keep safety in mind at all times.  On mobile hunts, get a helper to handle the computer and RDF gear when you drive, or get someone else to drive so you can concentrate on the RDF task.  Have solid mountings for your computer, GPS and RDF gear so they can't fly about during sudden stops.  Minimize distractions and pay full attention to the road while driving.  Take your time and have fun!

© 2014, 2019 Joseph D. Moell. All rights reserved.

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This page updated 18 January 2020