CDI
GPS DEMO
OBS 360
Location is blocked on this page, so the VOR has no position to work from.
Test signal: centres on 360 FR or 180 TOCalibrate by moving phone in a figure of 8
Install Pocket VOR on this phone so it opens full screen and works offline in the cockpit.
Simulated stations for teaching. They stay on this phone. Tap one in the list to tune it.
TST on 108.00, here
Puts VOR TST at your exact present position on 108.00 and tunes to it. Press again to move it.
Add a VOR by position
Latitude as N521132 or 52.1922, longitude as W0013652 or -1.6144. Each gets the next free VOR frequency from 108.05 upwards.
Flies a simulated aircraft at 100 kt and 2,500 ft with a 250/15 wind, so you can try the instrument on the ground. Drag across the dial to turn.
Tap one to tune it.
Set the station’s frequency on the drums: MHz, then tenths and hundredths. Press VOR to switch the receiver on. IDENT switches the station audio on and off: the Morse ident on a 1020 Hz tone, every 10 seconds. On a VOR/DME every fourth ident is the DME’s, at a higher pitch.
The HSI button, bottom right of the dial, chooses the display: off for a CDI, on for an HSI. The knob, bottom left, selects the course: turn it, or tap its left or right side for 1°. The course also shows beside the knob.
A red NAV flag means there is no usable signal: the set is off, the frequency has no station, the station is out of range, or you are passing overhead. A red GPS flag means there is no position; HDG (HSI only) means there is no heading.
TEST sends a test signal, like a VOT. Set the course to 360 and the needle centres with FR showing; set 180 and it centres with TO.
Menu holds practice VORs, the display, heading reference, ident volume, a demo flight and the stations near you.
With HSI off, this is a CDI loosely like the Garmin GI 106A. The OBS knob turns the card and sets a course under the top index. The needle shows where that course line lies: fly towards the needle to centre it. Each dot is 2°, so full scale is 10° either side. TO or FR shows whether the course takes you to the station or away from it.
The CDI works from your position alone, so it knows nothing of your heading. Fly against the course you have set and the needle reads the wrong way round (reverse sensing).
With HSI on, the card turns with your heading and the CRS knob sets the yellow course arrow. The centre of the arrow, the course bar, shows where the selected course lies from the aircraft symbol, as if seen from above. Each dot is 5°, so full scale is again 10°. The white triangle points along the arrow when the course takes you to the station, and back towards its tail when it takes you away.
Heading comes from the phone’s compass, or GPS track if you choose it in Menu or the compass gives nothing.
In Menu, Set TST here puts VOR TST on 108.00 at your exact position and tunes to it, so a student can fly away from it and track back. Add a VOR by position takes a three-letter ident and a latitude and longitude, and gives it the next free VOR frequency from 108.05 upwards. WSB at Wellesbourne is there as an example on 108.05. Practice VORs are received anywhere within 200 nm, at any height.
Real stations need line of sight: roughly 1.23 × (√ your altitude + √ station height) nm, with heights in feet, and they are limited to 50, 100 or 200 nm by power. The app uses GPS altitude, or 2,000 ft if the phone gives none. Low down or far away the NAV flag stays in view, and on the ground expect only nearby stations. In Stations near you, stations out of reach at your altitude are greyed out.
Over the station you pass through its cone of confusion: the needle swings, the NAV flag flickers, and TO changes to FR.
If the HSI card seems wrong, calibrate the phone’s compass. Away from metal and electronics, slowly wave the phone through a figure of 8 a few times, tilting and turning it as you go. Android phones often need this, especially after being near magnets, speakers or a phone mount. Then check against the DI and use Align (Menu, Heading reference) if it is still a few degrees out.
Inside a car, or near metal and electrics, a phone compass can be thrown out by 90° or more. When you are moving faster than 20 kt the app checks the compass against GPS track. If they disagree by more than 30°, it uses GPS track instead and shows TRK · GPS, compass disturbed until they agree again. The CDI does not use heading at all, so it is not affected.
Training aid only. This is not a navigation instrument and must not be used as one.
Stations come from the OurAirports database (downloaded November 2026): VORs and VOR/DMEs in the UK, Isle of Man and Channel Islands. TACANs and ILS localisers are not included, so an ILS frequency (108.10 to 111.95 with an odd tenth) leaves the NAV flag in view. Some stations may since have been withdrawn, so check the UK AIP before relying on one. Radials are aligned to magnetic north at each station from the WMM2025 model; a real VOR’s alignment can differ by a degree or two.
Audio: on iPhone, ident audio needs the ringer on unless your iOS supports playback mode. Plug into the intercom or use Bluetooth for headset audio.
Copyright J Wilby 2026