Receiving PLB homing signal at 121.5MHz

[st599]

It does not really matter if 121.5MHz is remotely monitored, what matters is that if I loose a crew member overboard 100nm from land, and he has a PLB I would like to get to the MOB first and ASAP and an homing signal receiver would be essential.

So how do I get hold of one?.

Doubt you can.

One reply suggested the use of a directional Yagi-uda antenna tunned to 12.5 MHz,:
1) Multipath would be a nightmare at sea
2) How would the average user, in a panic, know he wasn't peaking on the reverse lobe
3) You'd need quite a few directors on the Yagi-Uda

You'd need a phased array DF - like you see on the lifeboats, but at 121NHz - it'd be quite large.

 

[MASH]

But what Capt. Submarine has not pointed out is that although 121.5 is routinely monitored by commercial aircraft (it is the airborne emergency frequency) reporting beacon activity to ATC is pointless as no ATC unit in Europe is the least bit interested, there are so many spurious transmissions.

121.5 just might be useful in the open ocean beyond VHF range from land, but within range of an airway as it would be far less likely to be yet another dumb yachtie who'd accidentally triggered his SARBE in the marina. (Probably a dozen a day on the S coast.)
As to who can DF it, I expect the Navy can, probably big lifeboats too (range very limited indeed if you are in the water). Airborne assets? I doubt SAR helos can (Violet Picture Homing wasn't it, Capt?), Nimrod would, but they're not exactly commonplace.

ie, not much use, I fear.

 

[rob2]

What does receive only have to do with licensing problems?

If you haven't got a licence for a particular frequency you can't legally monitor either.

It's news to me that you would need a license to detect a transmission! I was just thinking that as at night you could be within two boats lebgths of a casualty and not see them, then a directional antenna might allow you to trace the PLB. Yes the reverse lobe could be problematic, but then the aerial is usually used to detect a null (90 deg from the signal) and the correct orientation will be that closest to the reciprocal track of the vessel. to seek a null I believe the efficiency of the aerial can be much reduced, making it much smaller.

If you're inclined to panic under stress, then there is no such thing as foolproof equipment.

Rob.

 

[john86uk]

True, but neither is the PLB a replacement for a DSC set as the yacht can't get a position fix from the PLB, whereas a couple of DSC transmissions could get a boat quickly back to the location of the MOB if they had a DSC handheld with them that they could transmit regularly from - that could get them back on board within minutes without needing to tie up rescure services. At the moment you really need both to cover all the bases.

I never once suggested that a PLB could replace DSC. Both are very important parts of GMDSS.

What your saying sounds good in theory but suppose for example the person gets knocked over the side by the boom and is unconcious in the water, how then do they operate their DSC radio.

DSC is used to raise the alarm to the shore, and a PLB is used to home in on the MOB.

As far as i am concerned every boat commercial and leisure should be carrying VHF DSC, unfortunatly this is not the case.

PLB's make our job even easier should we be searching for a MOB. The 121.5Mhz signal would take us right to the casualty, concious or unconcious. The same cannot be said about DSC, however without it/VHF we wouldnt have been alerted in the first place.

 

[grumpy_o_g]

I never once suggested that a PLB could replace DSC. Both are very important parts of GMDSS.

What your saying sounds good in theory but suppose for example the person gets knocked over the side by the boom and is unconcious in the water, how then do they operate their DSC radio.

DSC is used to raise the alarm to the shore, and a PLB is used to home in on the MOB.

As far as i am concerned every boat commercial and leisure should be carrying VHF DSC, unfortunatly this is not the case.

PLB's make our job even easier should we be searching for a MOB. The 121.5Mhz signal would take us right to the casualty, concious or unconcious. The same cannot be said about DSC, however without it/VHF we wouldnt have been alerted in the first place.

Not suggesting you did at all - I think we're agreeing but surely a PLB is no use either if the MOB is unconscious? Don't they have to be manually deployed?

The thing about the PLB is that the ONLY people that can respond to it are the rescue services and it takes a while to get the system moving, say an hour at the very best for a recovery. The DSC gives all the boats in the area the position of the MOB and a chance to get him/her out of the water within a few minutes if they've only just gone overboard, not to mention the psychological boost of knowing someone has your position and is on the way and the crew knowing the MOB is OK. As you say, it doesn't replace a PLB but it augments it massively if you sail in very busy waters like the Solent.

 

[William_H]

Locating PLB

Most beacons in addition to transmitting the new signal directly a satelite on 406 mhz also transmit on the old 121.5mhz AM. This is because aviation is firmly set in VHF AM transmissions so all aircraft can receive 121.5. They have techniques where you detune the receiver to 121.55 or 121.45 after receiving the beacon. This detuning gives an aircraft a better chance of locating where the beacon seems loudest. a pass at rightangles to the original track starts to give an estimate of location. In addition high flying commercial jets will report the point on their track where the beacon sounds loudest.
An aircraft properly set up for SAR will have a direction finding capability on the VHF receiver. This will give a direction so making finding much quicker.
Receivers are around to receive this 121.5 mhz beacon and if it has a gain control and is used with a YAGI antenna you can get a direction to victim fairly quickly. A Yagi is a bit like a TV antenna unfortunately it would be about 34 inches wide and perhaps as long so not easily stowed.
So as already said it might be a bit too much trouble to set up for finding a beacon by receiving 121.5 though I am sure if you were set up and lost a MOB who had a beacon you would be very grateful.
No the old DF sets were for frequencies of 200 to 400 khz so no use at all for VHF.

The original question is very valid and indeed you may reasonably choose to set up for receiving 121.5.
Incidentally they also transmit on 243mhz the military UHF channel and a receiver and antenna on this frequency might be more stowable. olewill

 

[RestlessL]

I never once suggested that a PLB could replace DSC. Both are very important parts of GMDSS.

What your saying sounds good in theory but suppose for example the person gets knocked over the side by the boom and is unconcious in the water, how then do they operate their DSC radio.

Whilst there are PLBs which will automatically activate in the water, the budget ones such as the McMurdo Fastfind do not, and need to be set off by the casualty. The reason I have a HH DSC (SH HX531) is because it can be activated by the crew on the boat by sending a DSC position request. The MMSI number of the HH is programmed into the ship's main VHF, and a position request is automatically answered by the HX531, and a waypoint displayed in the chartplotter. This can be repeated as many times as needed. The HX531 is set to automatically respond to position requests, and this cannot be disabled.

So even if the casualty is unconcious, there is a good chance that he will be found when the boat crew realise that he is missing. They also have the ability to talk to the casualty. With a (non-automatic) PLB all the boat crew can do is raise a MayDay, start searching, and hope that the casualty is able to set off the PLB.

J

 

[Juan Twothree]

Airborne assets? I doubt SAR helos can

ie, not much use, I fear.

Rest assured, they can, along with all-weather lifeboats and the latest generation of rigid inshore lifeboats (the Atlantic 85).