Broken Biscuits
15th Jun 2016, 20:49
This might seem a bit esoteric, but I haven't been able to find the answer anywhere on the web (yet!). I've been trying to get up to speed on GBAS - Ground Based Augmentation System - which is being installed as a replacement for ILS systems in many parts of the world.
As you might know, this is a differential GNSS (i.e. GPS/Glonass,Galileo etc) arrangement where reference receivers local to an airport produce correction signals which are linked via a data broadcast to aircraft sat nav receivers to improve the accuracy of the position solution. By using several reference receivers the integrity of the satellite signals can also be monitored and faulty signals can be excluded from the position calculations.
The data corrections (and other information) are sent to the aircraft over a VDB (VHF Data Broadcast) transmitter, usually located on the airport or close by. The frequency band used is from 108 to 118 MHz i.e. the same band occupied by ILS and VOR equipment.
Now because aircraft equipped for ILS and VOR reception use horizontally polarized antennas, the VDB transmission also needs to be horizontally polarized so that existing aircraft antennas can be used. However, the ICAO recommendation is for elliptical polarization, which has both horizontal and vertical components in the radiated signals.
I have seen an FAA website which states that the vertically polarized component is required to cater for certain military aircraft which cannot use horizontally polarized antennas for some reason.
Does anyone know what sort of aircraft these are? (I wonder if they are helicopters?) Do these aircraft not already have ILS or VOR receivers which require horizontally polarized antennas?
It seems to me to be rather unnecessary to specify elliptical polarized antennas for the VDB when nearly all aircraft will be using horizontally polaraized antennas. Use of elliptical polarization makes the VDB antenna system more complicated and thus more expensive than it needs to be. Or is this just a typical manifestion of the "money no object" attitude prevalent in many avionics systems specifications?
As you might know, this is a differential GNSS (i.e. GPS/Glonass,Galileo etc) arrangement where reference receivers local to an airport produce correction signals which are linked via a data broadcast to aircraft sat nav receivers to improve the accuracy of the position solution. By using several reference receivers the integrity of the satellite signals can also be monitored and faulty signals can be excluded from the position calculations.
The data corrections (and other information) are sent to the aircraft over a VDB (VHF Data Broadcast) transmitter, usually located on the airport or close by. The frequency band used is from 108 to 118 MHz i.e. the same band occupied by ILS and VOR equipment.
Now because aircraft equipped for ILS and VOR reception use horizontally polarized antennas, the VDB transmission also needs to be horizontally polarized so that existing aircraft antennas can be used. However, the ICAO recommendation is for elliptical polarization, which has both horizontal and vertical components in the radiated signals.
I have seen an FAA website which states that the vertically polarized component is required to cater for certain military aircraft which cannot use horizontally polarized antennas for some reason.
Does anyone know what sort of aircraft these are? (I wonder if they are helicopters?) Do these aircraft not already have ILS or VOR receivers which require horizontally polarized antennas?
It seems to me to be rather unnecessary to specify elliptical polarized antennas for the VDB when nearly all aircraft will be using horizontally polaraized antennas. Use of elliptical polarization makes the VDB antenna system more complicated and thus more expensive than it needs to be. Or is this just a typical manifestion of the "money no object" attitude prevalent in many avionics systems specifications?