Keep on the good work buddy. Another comment or question, we know that SBO is modulated by the 90Hz added to the inverse of 150Hz tone, so if you may explain why you are presenting SBO as -90 + 150 !!? I really thank you for the great efforts 🙏
Good question! I had to do a deep dive on that very issue. Here's the answer: I initially learned, as most of us likely did, that SBO was + 90 - 150 at the transmitter. But later, I found this to be contradicted by other sources (1970's Wilcox Technical Manuals, and some FAA theory course material) saying SBO is - 90 + 150 from the 90/150Hz generator in the transmitter. It was driving me nuts - which is it?! It also seems impossible to discern on an o'scope because we can't determine the start of the signal and it goes through an absolute value circuit where we see the (fingers pattern) audio waveform. So, what convinced me that it must be -90 + 150 (besides various texts and opinions) is the resultant 150 tone below the first SBO null (glide path) on the Glide Slope. Electromagnetic theory states the (SBO) lobe above of the first null in an image array glide slope is an inverse lobe. So, the SBO lobe below the null is positive (relatively speaking). Thus, the SBO audio phase from the transmitter must be -90 + 150 for 150 to predominate below glide path - since CSB +90 +150 is also considered to be a positive lobe, the 90's cancel, leaving 150 to vectorily add. I also talked to Roger Holms, the author of ILS fundamentals and owner of NANCO, a European navaid consultant company on this very topic who had this to say: "Since ICAO never defined any sign to DDM, it has been a worldwide confusion what is plus or minus. Manufacturers and book writers simply had to make their own definition. In the 50s and maybe the 60s you could only say fly right or fly left after the DDM value, but in modern flight inspection equipment and monitors it will be difficult not have a sign included. I think most cases just used 90 and 150 in that order, so I believe DDM = 90 - 150 is the most used local definition worldwide. But I found no particular mathematical nor vectorial reasons to go for that version. On the contrary, using 150 was much easier to explain and demonstrate for starters. I clearly see that that this discussion should be included in the book, and I have started to prepare an extra page. I hope it explains the reasons why we used to 150 instead of 90 to indicate positive DDM."
Keep on the good work buddy.
Another comment or question, we know that SBO is modulated by the 90Hz added to the inverse of 150Hz tone, so if you may explain why you are presenting SBO as -90 + 150 !!?
I really thank you for the great efforts 🙏
Good question! I had to do a deep dive on that very issue. Here's the answer:
I initially learned, as most of us likely did, that SBO was + 90 - 150 at the transmitter. But later, I found this to be contradicted by other sources (1970's Wilcox Technical Manuals, and some FAA theory course material) saying SBO is - 90 + 150 from the 90/150Hz generator in the transmitter. It was driving me nuts - which is it?! It also seems impossible to discern on an o'scope because we can't determine the start of the signal and it goes through an absolute value circuit where we see the (fingers pattern) audio waveform. So, what convinced me that it must be -90 + 150 (besides various texts and opinions) is the resultant 150 tone below the first SBO null (glide path) on the Glide Slope. Electromagnetic theory states the (SBO) lobe above of the first null in an image array glide slope is an inverse lobe. So, the SBO lobe below the null is positive (relatively speaking). Thus, the SBO audio phase from the transmitter must be -90 + 150 for 150 to predominate below glide path - since CSB +90 +150 is also considered to be a positive lobe, the 90's cancel, leaving 150 to vectorily add.
I also talked to Roger Holms, the author of ILS fundamentals and owner of NANCO, a European navaid consultant company on this very topic who had this to say:
"Since ICAO never defined any sign to DDM, it has been a worldwide confusion what is plus or minus. Manufacturers and book writers simply had to make their own definition. In the 50s and maybe the 60s you could only say fly right or fly left after the DDM value, but in modern flight inspection equipment and monitors it will be difficult not have a sign included. I think most cases just used 90 and 150 in that order, so I believe DDM = 90 - 150 is the most used local definition worldwide. But I found no particular mathematical nor vectorial reasons to go for that version. On the contrary, using 150 was much easier to explain and demonstrate for starters. I clearly see that that this discussion should be included in the book, and I have started to prepare an extra page. I hope it explains the reasons why we used to 150 instead of 90 to indicate positive DDM."
Great job with the graphics and signals. Very clear explanations. Thanks
Thank you. Can you please tel me why they choose 40% depth of modulation in GS instead of 20% used on localizer?
This is for technicians not for engineers.