Nasa AIS Radar questions answered

Re: Nasa AIS \"Radar\" questions answered

you mentioned it ..... I have Raytheon MARPA that does some of what........... so i searched raytheon......
 
Re: Nasa AIS \"Radar\" questions answered

...radar with AIS but it was called something else

SSR (Secondary Surveillance Radar) which does not bounce anything of the target but is still called a radar in acceptable technical terms even though at variance to the everyman's dictionary type definition. It relies on a transponder on the aircraft and fixes position (very roughly speaking) by the time that elapses between the interrogation of the aircraft transponder by the ground station and the response, and the rotational position of the ground station's antenna. There is no reflected signal and it is all at low power (sort of 100-200 watts at the ground end and obviously less on the aircraft).

An AIS transponder may be interrogated by a shore station but obviously does not do anything itself to determine the positions of other vessels, just receives what they transmit without any interrogation. So I agree it is stretching it a bit to call the AIS on a yacht a "radar" but that not because there are no reflected signals. And as others are also saying it could be misleading to many.

John
 
The NASA address for AIS is here

The AIS Radar retails for £259 but ranges are quoted as 1, 2, 4, 6, 8, 16 & 32 nm. These are more sensible ranges than those given by Tech-help so hopefully marketing haven't got beyond themselves ....

Don't give a monkey's what they call it if it does the job .....
 
Re: Nasa AIS \"Radar\" questions answered

It requires a big antenna to get a precise gain pattern to allow the position fix in azimuth to be accurate - the aviation ones are those you can often see at airports that look like a rotating long beam rather than being more dished shaped (the more dish shaped ones are primary radars, the conventional kind - sometimes an SSR antenna is mounted on top of the primary radar one and is normally roughly the same span size wise). There are other radars on airports, often on the tower, with much smaller antennas - those are not the ones I mean (they are for ground control), the SSR ones are many times bigger.

They look the same (and work similarly to) the IFF - Identification Friend or Foe - antennas used in the military eg on ships, if you are familiar with those.

They are also quite complex signal processing wise, working on sum and difference antenna gain patterns and also phase information; more complex than a conventional radar.

If Ships Woofy is around he may have come across them and know more about the technical side - I have just managed projects using them.

John
 
OK Sorry be be daft here but this is how I understand it....
RADAR tells me some-one is there
AIS tells me who is there, where they are going etc.....
 
AIS

It's very different to SSR or IFF as the data string includes full position and it only requires a cheap omni-directional comms antenna to receive reports.

Significance of the position is that you can filter unwanted (outside range) targets by simple algorithms knowing your own pos via GPS feed. Saves a packet on fancy narrow-beam antennas alone.

Ships rarely have what our US cousins would call a 'real estate' problem with displays on the bridge. As you know, they go big and they duplicate. For us lesser mortals who don't sail a Tardis 66 there's going to be a bunfight for screen space in the near future.
 
Yes -

- Radar tells you someone and something is there assuming that the someone or something provides a radar reflective target of sufficient size. But small pleasure radars are not always very good with small targets because of the limitation on the size of antenna that can be fitted to a small yacht.

- AIS tells you who and where they are (plus speed, etc), providing they also have AIS.

As Tom points out AIS is just a broadcast system, unlike the SSR you saw which directly interrogates a transponder on the aircraft which then replies. Each vessel's AIS just transmits every fixed time period the vessel's position, MMSI, etc automatically - it does not transmit in response to requests from other vessels ie they do not "talk" to each other in the way that an SSR does.

As all the vessels in an area have their AIS's transmitting and listening without any overall control or "talking" between them, interference from multiple AIS's transmitting at once has to be avoided somehow. So, roughly speaking, each AIS listens for the others in range and from the known time periods between transmissions works out where it can "slot" in its own transmission so it less likely to occur at the same time as that of another's.

John
 
Re: Nasa AIS \"Radar\" questions answered

The Air traffic system you describe is called Secondary Survailance Radar (SSR) and works by the ground RADAR requesting identity and altitude information from the aircraft, The aircraft has a transponder which replies to the request. It's been around for years and is needed because a conventional RADAR can not resolve altitude.
 
Re: Nasa AIS \"Radar\" questions answered

Raymarine did not take over the Raytheon name. Originally the company was Autohelm - Portsmouth, which was bought by the large US elecronics conglomerate and defence contractor, Raytheon. The liesure boat market products were then subject to a management buyout from Raytheon, this resulted in the creation of Raymarine. By doing the latter, Raymarine inherited Raytheon radar expertise, fishfinder technology form Apelco, who had also been bought by Raytheon and radio tecnology from both Raytheon and Apelco.

At least that is how I see it from following the history of the people that started with a tiller pilot as their first product.
 
Re: Nasa AIS \"Radar\" questions answered

As somebody who doesn't have RADAR on board at all, then for me this AIS receiver would be a significant improvement in my perception of the traffic about me. True, its only mandatory for vessels of certain sizes and types to broadcast AIS information - but in my ignorance this covers a significant amount of the traffic I would consider dangerous in my area (ie high speed ferries, container vessels, fast moving but large shipping etc). While running into a smaller coaster would be just as significant event as hitting a large container ship (being hit!) - I percieve it as easier to avoid the smaller vessels. It is probably going slower and its a good deal smaller.

But I understand the comments about what the NASA device actualy does, this is where NASA are perhaps being missleading. Anybody who thinks thay can obtain the functionality of a true RADAR for this money must be dreaming.

I think it is a good tool, well priced. Especially if, as indicated, a PC oriented one become available. I will be finding space for one or the other as soon as they become available - but still havent actually seen one for sale yet (mind you, I've not been looking too hard).

Regards, Jeff.
 
Re: Nasa AIS \"Radar\" questions answered

[ QUOTE ]
So I guess NASA is going for a re-definition of RADAR then.....



[/ QUOTE ]
...No, I think you and your pedantic dictionaries are. RADAR was an acronym invented in the 2nd World War to describe RAdio Direction And Range. I agree it usually does have an active element (the transmission and detection of the reflected signals), but it wasn't in the original definition, was it? The early radars didn't have movable Aerials or Plan-Position Indicators (PPIs) either, but they were still radars.

If it walks like a duck, quacks like a duck, swims like a duck, eats weeds and bread, the chances are it's a DUCK. And if it presents target position and lots of other data on a PPI display, can change the ranges displayed, etc., and uses radio to do it, the chances are it's a RADAR. Why confuse things with quibbles?
 
Re: Nasa AIS \"Radar\" questions answered

[ QUOTE ]
If it walks like a duck, quacks like a duck, swims like a duck, eats weeds and bread, the chances are it's a DUCK. And if it presents target position and lots of other data on a PPI display, can change the ranges displayed, etc., and uses radio to do it, the chances are it's a RADAR. Why confuse things with quibbles?

[/ QUOTE ]It shouldn't be called 'radar' because it isn't radar - and people might be confused. Indeed, if you believe that this thing 'quacks' like a true radar then you are confused. Read back over the whole thread and you'll see what I mean.
 
A good analogy but not complete

Unfortunately this duck quacks only when instructed to do so by receipt of transmitted data from an AIS equipped vessel. It will not quack for vessels that are not AIS equipped, or for buoys, or for rocks, land and so on whereas proper radar will display these on screen also.

The point that some people are trying to make is that this device, useful as it may be especially in ADDITION to proper radar, is only ever going to give partial information and is NOT a substitute for Radar that comes in a cheaper box with lower power consumption. I raised this in a previous post and was told by some to give people credit for understanding the limitations, unfortunately it is clear by some response that there is still confusion as to what AIS is and can do, not helped IMO by Nasa calling it 'AIS Radar'.
 
Re: A good analogy but not complete

I wouldn't mind betting NASA thought about calling it a WIreless, Non-TRansmitting Ais Position display (a WINTRAP) or maybe a Non-transmitting Ais-Showing Radar-thingy (a NASA-NASR), a Direction Information And Position Electronic Recorder (a DIAPER), a NASA Ais Plan-Position indicator for Yachties (a NAPPY for the UK market), and dozens more /forums/images/graemlins/wink.gif.

But honestly, who's going to be misled into thinking they've bought a high-power magnetron-equipped conventional radar with a twirly bit on top when it says "AIS" in the name? NASA's gadget still seems to show a radar-like PPI display, PLUS a full listing of all the MMSIs, Lat & Long, and vessel type of all the AIS-equipped shipping in the receiver's range. Also for the nearest 24 ships on the PPI display, the MMSIs, the range, vessel type, COG, SOG, HDG, and name of each. Does a boring old radar do all that too? And most small radars can't always see all the nearby hazards such as small craft, either.

As with all navigational aids, it's up to the driver to use all the data available for achieving safe passage - you mustn't rely on just one source.
 
Re: A good analogy but not complete

[ QUOTE ]
But honestly, who's going to be misled into thinking they've bought a high-power magnetron-equipped conventional radar with a twirly bit on top when it says "AIS" in the name?

[/ QUOTE ]Why not just call it an AIS - which is what it is? Just because it uses radio doesn't mean that you have to call it 'radar', a term that is already in use to mean an apparatus that transmits pulses and receives echoes - a totally different concept, and different usage. GPS works via radio and doesn't have the term 'radio' in the acronym, either, for example.

Going back to your 'duck' analogy, isn't it rather like naming a newly-discovered creature 'duck' on the only grounds that it has feathers?
 
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