Appl. Sci. 2023, 13, 1872 
70f17 
angles, which most likely are sky-waves [18]. The transmitter antenna pattern is generally 
optimized to transmit the signals horizontally. Therefore, the sky-wave is negligible in 
the vicinity of the transmitter, due to the transmitter antenna pattern, even if the receiver 
antenna is assumed to be isotropic. The sky-wave becomes stronger at larger distances 
where the reflected wave, generated by the transmitter antenna, presents a dependency on 
the elevation angle. Thus, having a non-isotropic receiving antenna with a specific pattern 
could help in mitigating the influence of the sky-wave. 
3.3. Transmitter Instability 
MF radio beacons were not initially designed to be used as a stand-alone positioning 
system. Therefore, some adaptations and modifications are needed for the hardware on 
the transmitter side. In principle, these adjustments, needed to enable R-Mode signals 
transmission, could be easily implemented at the sites. Figure 4 presents the generalized 
difference in architecture between a standard DGNSS beacon and an R-Mode-enabled radio 
beacon. First of all, the MSK modulator must be replaced with an R-Mode modulator, then, 
two additional components are required: an R-Mode message generator and a stable and 
accurate timing source synchronized with the R-Mode system time. The last component 
required is an R-Mode signal monitor. This solution is extremely cost-efficient when 
compared to the deployment of a new GNSS satellite. 
V 
DGNSS 
Message 
Generator 
' RTCM2 MSK 
Modulator 
—— —— - 
Radio Beacon Monitoring 
DGNSS 
Message 
Zenerator 
' RTCM2 
—— 
R-Mode 
Modulator 
R-Mode Tem? ı 
Message 
Generator ‘ \10 MHz, 
1 PPS 
4 Time 
R-Mode System Time: UTC(n), GNSS 
wm — ] = 
N z nn 
R-Mode Signal Monitoring 
Signal 
Data 
Signal 
—— — 
Data, 
Ranging 
Amplifier 
Amplifier 
Antenna 
Tunning Unit 
Standard 
radio beacon 
V 
Antenna 
Tunning Unit 
R-Mode enabled 
radio beacon_ 
Figure 4. Difference between a standard MF radio beacon architecture (above) and the R-Mode- 
enabled radio beacon (below). 
Nevertheless, the hardware components in the standard radio beacons are not identical. 
They differ depending on the country. In general, different manufacturer and different 
hardware specifications might be used in different countries. For this reason, transmitter- 
side instability effects can be observed for some radio beacons but not for others, even in 
che same countrv. As an example, we show in Figure 5 an issue that was identified at the