GBAS - VHF Data Broadcast antenna type
Thread Starter
Join Date: Dec 2015
Location: West Midlands UK
Posts: 38
Likes: 0
Received 0 Likes
on
0 Posts
GBAS - VHF Data Broadcast antenna type
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?
Join Date: Apr 2009
Location: Hotel Gypsy
Posts: 2,821
Likes: 0
Received 0 Likes
on
0 Posts
I work on a specialist aircraft which utilises GBAS technology, giving us a positional accuracy of 15-20mm. In our circumstance, we use a separate antenna on the aircraft to receive the correction signal from our ground unit; this signal is vertically polarised.
Join Date: Mar 2008
Location: London
Posts: 76
Likes: 0
Received 0 Likes
on
0 Posts
Try to google Murphy and Imrich, IEEE in I think 2008 for I have their paper somewhere if I knew where to look.
VApilot may be onto something. Military machines do things other than GBAS approaches and may fly extraordinary attitudes hence circular or rather elliptical polarisation. Years ago now [and Cows Getting Bigger may know this], HMS Argyll sounded out masses of data about the turbulence close to the proposed landing sites on Prosperous Plain. There was a lady Boffin whom the Brits called Professor Peabody [female sidekick of Dan Dare]. I still have her business card from a major USA supplier. That places the winds survey in Jan-June 2001. In 2004 I was engaged in trials on Norfolk Island where there were high hopes that the Honeywell Pelorus system would be the answer. It wasn't. I met the Thomson CSF team who were on the competing team and they gave a convincing account of why the Pelorus system had systemic weaknesses. They later became Thales.
I do not have any books now as I am quitting my job, moving back to Oz and closing this chapter with a resounding thud.
Is this the Farnborough Year? Why not visit a Trade Day? You simply show the passport, pay something like £30 and ask away.
VApilot may be onto something. Military machines do things other than GBAS approaches and may fly extraordinary attitudes hence circular or rather elliptical polarisation. Years ago now [and Cows Getting Bigger may know this], HMS Argyll sounded out masses of data about the turbulence close to the proposed landing sites on Prosperous Plain. There was a lady Boffin whom the Brits called Professor Peabody [female sidekick of Dan Dare]. I still have her business card from a major USA supplier. That places the winds survey in Jan-June 2001. In 2004 I was engaged in trials on Norfolk Island where there were high hopes that the Honeywell Pelorus system would be the answer. It wasn't. I met the Thomson CSF team who were on the competing team and they gave a convincing account of why the Pelorus system had systemic weaknesses. They later became Thales.
I do not have any books now as I am quitting my job, moving back to Oz and closing this chapter with a resounding thud.
Is this the Farnborough Year? Why not visit a Trade Day? You simply show the passport, pay something like £30 and ask away.
Last edited by mathy; 17th Jun 2016 at 06:56. Reason: added a suggestion or two
