Radio navigation¶
Overview¶
The mod completely replaces the stock sim system. It reads the currently active nav database (NavBlue, Navigraph, whatever) from the sim, optionally adds decommissioned NDB stations, and calculates all signals in custom code.
The code was initially heavily inspired by libradio but is not based on it. The implementation is based on ITU publications and data created using publicly available tools and data sources.
In order to provide performant terrain lookups over long ranges, the mod ships a global terrain database in a custom, optimized format that allows for efficient lookups and only contains exactly the details we need (terrain elevation and land classes).
All gauges, the autopilot and the TAWS implementation exclusively use the signals from the custom system.
All calculations use the physical effects appropriate for the specific frequency ranges. If you're interested, the Radio tab in the EFB shows many of the details and the code is open source. In this manual, I will focus on the differences you might see compared to the stock sim system.
Note
There is some (defensible) cheating going on:
The regulations are pretty clear that stations must reach their published range, but they're not allowed to use more power than necessary to achieve that range, in order to avoid interference.
The actual transmitter wattage is not published. So I try to recreate that regulatory decision for every station individually and pick the least powerful equipment that can satisfy the range based on the location and terrain around the station.
LF/MF¶
An NDB signal is low or medium frequency and mostly travels along the ground, following the terrain. It doesn't depend on line-of-sight. The range varies massively based on the conductivity of the terrain the signal traverses.
A coastal NDB that reaches its published range inland will have a much higher range when you approach it over water.
NDB signals are much less reliable than you're used to from the stock system. They tend to become more reliable as you get closer, but even up close, the signal can be skewed by coastal refraction, terrain "bending", or reflection by buildings in urban areas.
For me, this makes ADF navigation a lot more fun. You can't just follow the needle when it comes alive, you have to consider whether the direction it's pointing to is plausible, maybe wait a bit for the needle to settle before you trust it.
Known issue: the ADF radio should react to lightning, but the sim doesn't provide the necessary data that would be needed to bend the needle toward a storm cell.
VHF¶
The Very High frequency (VHF) band is used by VOR and localizers. They reach their optimal range when you have line-of-sight to the station.
If terrain is blocking LOS, you can still receive the signal refracted by the terrain, but it will lower its range.
Localizers are shaped beams that, by regulation, must cover a specific cone and range.
UHF¶
ILS glideslopes and DME signals use ultra-high frequencies.
The DME frequency (VHF) you set in your radio is not actually the real DME frequency (UHF). To keep pilots from having to dial in yet another frequency, the DME interrogator uses a frequency that is paired with a specific VHF frequency.
Example
If you set your NAV radio to a VOR on 117.40, there are two paths to get a distance:
Garmin avionics will listen for the station ID (the Morse code) to positively identify the VOR station, then they look up the station (based on ID and frequency) in their database, get the station’s GPS location, and calculate the ground distance based only on the GPS coordinates.
A DME interrogator is a piece of hardware that is part of your radio stack. It determines the actual DME frequencies from the NAV radio’s frequency: in our example, 117.40 MHz (VHF) results in a request frequency of 1135 Mhz (UHF), the response is received on 1198 MHz (UHF).
The DME interrogator then sends signals to the ground station (the transponder) which responds after a fixed delay. Slant distance is then calculated based on the round-trip time.
The Twotter does not simulate that handshake, that would be pointless, but it does determine the correct UHF frequency and uses it to simulate the propagation of the DME signal, because the frequency determines how well the signal traverses terrain, which affects the range.
Main difference to the stock sim: VOR signals and their paired DME signals have different ranges.
The same applies pretty much to the ILS glideslope. Glideslopes have other fun properties like false lobes that appear above the actual glideslopes, making a "slam dunk" capture from above way more exciting.
Tip
The false glideslope lobes for a standard 3° approach sit at 6° and 9°, and the one at 6° produces reversed guidance.
Reviewed 2026-08-18.