Showing what VOR and ILS Aviation Signals Look like in SDR#
Over on YouTube user RedWhiteandPew has uploaded two videos showing what VOR and ILS signals look like in SDR# with an RTL-SDR dongle. VOR and ILS are both radio signals used for navigation in aviation.
VOR stands for VHF Omnidirectional Range and is a way to help aircraft navigate by using fixed ground based beacons. The beacons are specially designed in such a way that the aircraft can use the beacon to determine a bearing towards the VOR transmitter. VOR beacons are found between 108 MHz and 117.95 MHz.
Here I am picking up the VOR beacon from KSJC. The coolest part is at the end of the video. I believe the signal moving back and forth is caused by the Doppler effect, because VORs transmit their signals in a circular pattern. The VOR wiki article has a GIF that shows how it works here https://en.wikipedia.org/wiki/VHF_omn…. If you play and pause the video at different points before I zoom in, you can see that the two signals on the side are the opposite phase.
ILS stands for Instrument Landing System and is a radio system that enables aircraft to land on a runway safely even without visual contact. It works by using highly directional antennas to create four directional lobes (two in the horizontal plane, two in the vertical) that are used to try and ensure the aircraft is centered and leveled on the approach correctly. The ILS frequencies are at 108.1 – 111.95 MHz for the horizontal ‘localizer’, and at 329.15-335.0 MHz for the vertical ‘glide slope’.
Here I have tuned into one of KSJC’s ILS frequencies. You are able to hear the faint identifier beeping transmitting its ISL ID code which is ISJC. For comparison, I used to morse code translator website.
The reason I am hearing ISJC and not ISLV even though they are on the same frequency is because the localizers transmitting the signal are directional along the length of the runway. Since I am located to the south east of the airport, and I am within its transmitting beam, I am able to listen to it on a scanner.
If you’re interested in these signals then this previous post about actually decoding them might be of interest to you.
Thank you for this explanation… I tried but did not find anything else about the signal modulation (may be I should have tried harder).
And the most funny is that I live in São José dos Campos / Brazil where the ILS morse is ISJ at 110.3. It was so familiar that took me a while to figure out the video was about ISJC morse.
I have tried a github project called vortrack for VOR (115.4 SCP here) and it really worked well. I’m now looking to decode ILS signals.
Thank you for your time writing this article.
I am an HVAC and Solar Energy guy, and also a new pilot with an RV12. I recently built a Stratux ADS-B for my airplane which I use with my iPad. I have been amazed at the capabilities and performance of the Stratux. I’m now taking an ‘Instrument Navigation’ groundschool class, but have no VORs or an ILS in my airplane to practice with……..but why not? It seems to me that with the same Raspberry Pi3 and SDRs, it should be possible to have both VOR and an ILS in the airplane using my iPad. It would be great to have the software to program a Stratux box to do VORs and ILS instead of ADS-B and have that WiFi signal in the cockpit for a second iPad. What do you think?
There are VHF Omnidirectional Ranges (VOR) and Doppler VHF Omnidirectional Ranges (DVOR). Their signal consist of a fixed reference signal and a variable signal that periodically rotates over the full 360°, and does not move back and forth. Only Microvave Landing Systems (MLS) employ a scanning beam.
Aircraft VOR receiver compare the reference with the variable signal to determine their bearing to or from a (D)VOR) beacon.
The Id is transmitted in Morse code repeatedly every 30s to 40s.
While not used much any more (D)VOR have a voice channel (300 to 3000 Hz) that can be used e.g. for ATIS.
The reference and the variable signal are transmitted via separate antennas, in case of DVOR using a large Doppler base, with the reference antenna in the center and 30 or more antenna around the edge of the counterpoise. Several different methods exist to generate a DVOR, but independent which one is used, aircraft VOR receiver operate with DVOR and VOR signals.
When a voice signal is transmitted by a DVOR you will hear the burr.
Instrument Landing Systems (ILS) provide course (Localizer or LLZ) and clearance information,sometimes still omnidirectional clearance. Frequency rangeis 108.1 to 111.95 MHz and odd x.1, x.3, x.5, x.7 and x.9 MHz and x.15, x.35, x.55, x.75 and x.95 MHz. Even frequencies are used by VOR, also between 112 to 117.95 MHz. ILS-LLZ are frequency paired to ILS GP (glidepath) frequencies in the 330 MHz range.
The signals are generated by 2 transmitter, fed through an elaborate phasing network to feed large ILS antenna bases of 6 to more than 21 antennas. The difference in the Depth of Modulation for the 90 Hz and 150 Hz AM signal is only an only detectable in the far field, far removed from the ILS antenna.
On course center the Difference in Depth of Modulation (DDM) is 0. Depending where your receiver antenna is located in reference to the Runway (RWY) center line the DDM will vary.
To mitigate multipath ILS-LLZ since about 1955 so called 2 frequency ILS-LLZ are used which require an additional 2 transmitter, to generate separate set of signals one for the course and one for the clearance. 2 f ILS therefore require to frequencies for the 2 AM signals about 7 to 14 kHz removed from each other, but centered on the ILS channel center.
ILS-LLZ transmit their Id in Morse code repeatedly every 30s to 40s. While not used to often ILS also have a voice channel (300 to 3000 Hz) that can be used e.g. for the RWY information as was done in Frankfurt for decades e.g. “This is Frankfurt 25 Left”