ferroboat
Well-Known Member
Just read the article on AIS in the new Sailing Today, could someone please explain the difference between it and MARPA.
MARPA ?
- Thread starter ferroboat
- Start date
steve6367
Well-Known Member
MARPA (Mini Automatic Radar Plotting Aid) uses radar - once you have targets on the radar you can tag them and the display with work out closest point of approach, distance, heading etc for you. Works well in conjunction with AIS as smaller boats don’t always transmit AIS information.
Used both on my Raymarine C80 over the summer - visibility was good, but was interesting and reassuring to see both AIS and MARPA gave good accurate information.
Used both on my Raymarine C80 over the summer - visibility was good, but was interesting and reassuring to see both AIS and MARPA gave good accurate information.
pvb
Well-Known Member
Dependency...
Main difference is dependency - with AIS, you're dependent on other vessels transmitting information; with MARPA, you're only dependent on your own radar.
Simple AIS systems (eg the inappropriately named Nasa "AIS Radar") just display basic information on vessels transmitting AIS data. More complex systems (eg chartplotters with AIS input) will also calculate CPA (closest point of approach) and TCPA (time to CPA) - but are still dependent on the other vessels transmitting the correct data.
MARPA uses the changing positions of radar target echoes to calculate CPA and TCPA, and hence is independent of other vessels.
Main difference is dependency - with AIS, you're dependent on other vessels transmitting information; with MARPA, you're only dependent on your own radar.
Simple AIS systems (eg the inappropriately named Nasa "AIS Radar") just display basic information on vessels transmitting AIS data. More complex systems (eg chartplotters with AIS input) will also calculate CPA (closest point of approach) and TCPA (time to CPA) - but are still dependent on the other vessels transmitting the correct data.
MARPA uses the changing positions of radar target echoes to calculate CPA and TCPA, and hence is independent of other vessels.
Piers
Well-Known Member
Re: Dependency...
With AIS you also depend on the target transmitting the correct information.
So, if the target is not transmitting, and not transmitting correctly, it's useless. Far better to use your own radar's MARPA rather rely on AIS.
With AIS you also depend on the target transmitting the correct information.
So, if the target is not transmitting, and not transmitting correctly, it's useless. Far better to use your own radar's MARPA rather rely on AIS.
pvb
Well-Known Member
Re: Dependency...
[ QUOTE ]
With AIS you also depend on the target transmitting the correct information.
[/ QUOTE ]Not sure that's significantly different from my comment "but are still dependent on the other vessels transmitting the correct data", apart from the italics!
[ QUOTE ]
With AIS you also depend on the target transmitting the correct information.
[/ QUOTE ]Not sure that's significantly different from my comment "but are still dependent on the other vessels transmitting the correct data", apart from the italics!
Piers
Well-Known Member
Re: Dependency...
Hi pvb,
I was being pedantic in as much that if a ship is transmitting its info, the info must be correct. Often it isn't.
Hi pvb,
I was being pedantic in as much that if a ship is transmitting its info, the info must be correct. Often it isn't.
AJB43
Well-Known Member
I have MARPA on my Raymarine C80. Excellent feature - have used it in anger several times crossing the North Sea and once caught in thick fog off Brighton. Very reassuring but you do have to be careful as small targets can drop out of the system if their reflection is weak (altho there is an alarm to let you know the target has been lost)
vyv_cox
Well-Known Member
I agree. It still requires that someone mans the radar more or less full time but gives useful information once each ship is logged into MARPA. It can be set up to disply course, speed, closest point and alarm within desired radius. It takes a bit of practice which pays off when visibility is poor.
boatmike
Well-Known Member
AIS as others have said gives you warning of collision from information transmitted by the other party. It therefore follows that it will be a ship of some size and are actually transmitting. If it isn't you won't see it. However it is based upon the premise that if the ship is on a steady bearing you are on a collision course and is therefore fairly accurate because a steady bearing is still a steady bearing even if that bearing is not very accurate.
MARPA takes information from your compass and log to calculate the relative track of your own vessels course and speed to compare with that of others seen by your radar. It is therefore only as accurate as your compass is which can be a problem with cheap fluxgate compasses. If you have a good compass and its on board deviation has been properly corrected it's good. A proper gyro-compass is better. Unless you have a very accurate compass treat MARPA with caution and use it as a warning of possible collision. Also don't assume you are not on a collision course with another vessel because MARPA says so. Remember both are aids to navigation only and mark 1 eyeball is still best!
MARPA takes information from your compass and log to calculate the relative track of your own vessels course and speed to compare with that of others seen by your radar. It is therefore only as accurate as your compass is which can be a problem with cheap fluxgate compasses. If you have a good compass and its on board deviation has been properly corrected it's good. A proper gyro-compass is better. Unless you have a very accurate compass treat MARPA with caution and use it as a warning of possible collision. Also don't assume you are not on a collision course with another vessel because MARPA says so. Remember both are aids to navigation only and mark 1 eyeball is still best!
johnalison
Well-Known Member
Obviously what has been said is correct. I would just like to add that all these systems, though perfectly simple to use, do require a lot of practice to be effective. SWMBO used to call it "playing" until we were caught in a real fog and then she saw why I'd done a lot of playing.
ferroboat
Well-Known Member
Thanks to all, I am going to fit a new chart plotter this winter ( probably the new Garmin 4008 ) but wait till next lay up to buy the radar dome ( to spread the cost). am I right in thinking get the AIS with the chart plotter now and get the MARPA next year with the radar.
pvb
Well-Known Member
Agree in parts....
Fully agree with you that AIS and MARPA should only be aids to navigation. But I'm wondering about some of the other things.
When you talk about AIS, you say "However it is based upon the premise that if the ship is on a steady bearing you are on a collision course and is therefore fairly accurate because a steady bearing is still a steady bearing even if that bearing is not very accurate." You can only see the bearing of an AIS target by using some sort of software (usually via a chartplotter). The software uses GPS data in order to calculate the bearing, so it would surely be accurate?
MARPA is a different matter. Although you said "MARPA takes information from your compass and log to calculate the relative track of your own vessels course and speed to compare with that of others seen by your radar", I don't think MARPA actually needs your own boat's course and speed in order to calculate the relative track of targets. MARPA does work with ordinary fluxgate compasses, but in difficult conditions the faster heading input of a "rate gyro" system is admittedly better.
Fully agree with you that AIS and MARPA should only be aids to navigation. But I'm wondering about some of the other things.
When you talk about AIS, you say "However it is based upon the premise that if the ship is on a steady bearing you are on a collision course and is therefore fairly accurate because a steady bearing is still a steady bearing even if that bearing is not very accurate." You can only see the bearing of an AIS target by using some sort of software (usually via a chartplotter). The software uses GPS data in order to calculate the bearing, so it would surely be accurate?
MARPA is a different matter. Although you said "MARPA takes information from your compass and log to calculate the relative track of your own vessels course and speed to compare with that of others seen by your radar", I don't think MARPA actually needs your own boat's course and speed in order to calculate the relative track of targets. MARPA does work with ordinary fluxgate compasses, but in difficult conditions the faster heading input of a "rate gyro" system is admittedly better.
Oldhand
Well-Known Member
Re: Dependency...
[ QUOTE ]
Main difference is dependency - with AIS, you're dependent on other vessels transmitting information; with MARPA, you're only dependent on your own radar.
[/ QUOTE ]
...and the accuracy of the speed and heading inputs to the radar and the accuracy and speed of the MARPA processing within the radar. In a small craft these inputs are likely to be less accurate than the GPS outputs transmitted as part of the AIS message by ships. Furthermore, AIS CPA and TCPA being based on GPS information are calculated from velocity over the earth's surface, whereas MARPA should correctly use speed through the water and comapss heading, as all vessels are travelling in the water body (I hope!).
[ QUOTE ]
Main difference is dependency - with AIS, you're dependent on other vessels transmitting information; with MARPA, you're only dependent on your own radar.
[/ QUOTE ]
...and the accuracy of the speed and heading inputs to the radar and the accuracy and speed of the MARPA processing within the radar. In a small craft these inputs are likely to be less accurate than the GPS outputs transmitted as part of the AIS message by ships. Furthermore, AIS CPA and TCPA being based on GPS information are calculated from velocity over the earth's surface, whereas MARPA should correctly use speed through the water and comapss heading, as all vessels are travelling in the water body (I hope!).
pvb
Well-Known Member
Re: Dependency...
Maybe I'm missing something here, so what is the point you're trying to make?
Maybe I'm missing something here, so what is the point you're trying to make?
Piers
Well-Known Member
Re: Dependency...
Stand by - contentious post coming up....
IMHO, the most important instruments on a boat are (in order of priority) Compass, Depth sounder and Radar. Then comes chart plotter etc.
And when you use your radar's MARPA, please do not rely on it. All it's doing is something you should be able to do yourself, namely knowing if a collision potential exists and taking the correct action to avoid it. And MARPA is probably not doing it correctly - but that's another story.
Colregs change in restricted vis, most large vessels use a min clearance of 2 miles, some more, so just take care when in fog. Know what to do, know what others require, and act accordingly. Don't become another statistic. MARPA is not a get out jail free card.
So, radar, radar, radar, and a radar course. Radar is an excellent navigation tool as well as showing you what's around. Then chart plotter, then AIS.
Stand by - contentious post coming up....
IMHO, the most important instruments on a boat are (in order of priority) Compass, Depth sounder and Radar. Then comes chart plotter etc.
And when you use your radar's MARPA, please do not rely on it. All it's doing is something you should be able to do yourself, namely knowing if a collision potential exists and taking the correct action to avoid it. And MARPA is probably not doing it correctly - but that's another story.
Colregs change in restricted vis, most large vessels use a min clearance of 2 miles, some more, so just take care when in fog. Know what to do, know what others require, and act accordingly. Don't become another statistic. MARPA is not a get out jail free card.
So, radar, radar, radar, and a radar course. Radar is an excellent navigation tool as well as showing you what's around. Then chart plotter, then AIS.
boatmike
Well-Known Member
Re: Agree in parts....
MARPA does need input from compass and either log (best) or GPS. If you use GPS it is "ground based" the log is "water based" and IMHO better because tide (which affects both vessels) is constant. If you have ever used radar to plot other vessels you will know that the direction and speed of your own vessel affects the apparent track of other vessels. (Pretty obvious really) Using "head up" display it is certainly possible and IMHO still best to use VBM to visually see the relative tracks of your vessels even with ARPA, but even this requires a constant speed and course to be held to be accurate. ARPA or MARPA still requires the same but can be misleading if the course or speed input is wrong or (often the case with small craft) not constant.
As I understand it AIS computes the relative positions of both vessels from GPS and does not rely on calculations of actual course and speed which makes it ground based and the only inaccuracies are positional GPS variations which in the same locality should be similar for both vessels (according to my logic anyway) Therefore AIS is probably more accurate, especially if your course is a bit erratic.
MARPA does need input from compass and either log (best) or GPS. If you use GPS it is "ground based" the log is "water based" and IMHO better because tide (which affects both vessels) is constant. If you have ever used radar to plot other vessels you will know that the direction and speed of your own vessel affects the apparent track of other vessels. (Pretty obvious really) Using "head up" display it is certainly possible and IMHO still best to use VBM to visually see the relative tracks of your vessels even with ARPA, but even this requires a constant speed and course to be held to be accurate. ARPA or MARPA still requires the same but can be misleading if the course or speed input is wrong or (often the case with small craft) not constant.
As I understand it AIS computes the relative positions of both vessels from GPS and does not rely on calculations of actual course and speed which makes it ground based and the only inaccuracies are positional GPS variations which in the same locality should be similar for both vessels (according to my logic anyway) Therefore AIS is probably more accurate, especially if your course is a bit erratic.
Piers
Well-Known Member
Re: Agree in parts....
Agreed. Ground-based (aka ground-stabilised) computations for MARPA/ARPA whenever there is a tide or wind, will be inaccurate, whereas water-based (aka sea-stabilised) is accurate. Using ground stabilised MARPA/ARPA has been heavily criticised by the MAIB & CG since it has contributed to well known collisions.
I agree, boatmike, VBMs are great, although I always keep my screen 'North Up' (my preference).
Agreed. Ground-based (aka ground-stabilised) computations for MARPA/ARPA whenever there is a tide or wind, will be inaccurate, whereas water-based (aka sea-stabilised) is accurate. Using ground stabilised MARPA/ARPA has been heavily criticised by the MAIB & CG since it has contributed to well known collisions.
I agree, boatmike, VBMs are great, although I always keep my screen 'North Up' (my preference).
pvb
Well-Known Member
Raymarine\'s view....
[ QUOTE ]
MARPA does need input from compass and either log (best) or GPS.
[/ QUOTE ]Raymarine's view differs from yours. In their hsb2 radar manual, they say "MARPA functions without SOG and COG data but only relative vector, CPA and TCPA are shown; target course and speed cannot be calculated."
[ QUOTE ]
MARPA does need input from compass and either log (best) or GPS.
[/ QUOTE ]Raymarine's view differs from yours. In their hsb2 radar manual, they say "MARPA functions without SOG and COG data but only relative vector, CPA and TCPA are shown; target course and speed cannot be calculated."
Piers
Well-Known Member
Re: Raymarine\'s view....
MARPA needs boat heading, not COG for collision avoidance info. Without boat speed through the water (not SOG), you cannot calculate the aspect of the target vessel. If you are unsure of the target's aspect, you may take the wrong avoiding action.
When I tested the Raymarine radar on MBM's Calm Voyager in 2005, the crazy thing was that whilst the radar screen showed water based info, the MARPA List box displayed ground based information. Confusing or what!
The article that was subsequently published in MBM showed this test to check whether your radar is water or ground based. It needs to be carried out on a day when there is no wind.
Step 1 Choose somewhere where the tide is running. you know what the tide is doing.
Step 2 Target a fixed object, such as a buoy.
Step 3 Stop the boat and drift with the water become stationary in the water (i.e. stop not making way) .
Step 4 Locate the buoy on the radar, and choose a range scale to fit.
Step 5 Target MARPA, and wait.
If MARPA shows the buoy is stationary (or virtually stationary given instrumentation errors), while the radar shows an apparent speed for your vessel, the radar is ground stabilised.
If MARPA shows that the buoy appears to be moving at the speed of the tide, but in the opposite direction to the tide, then you are sea stabilised, which is the correct setting to run MARPA.
----------------
Moral of the story?
Know whether your radar is water or ground based.
Regardless of which it is, know how to use your radar without relying on MARPA. Use VBLs etc instead. Practice in good weather.
A final comment from the MAIB when I was interviewing thm on the subject. “There is a very real chance that someone will misinterpret the (ground based) information and make an incorrect assessment of the situation which may prove very dangerous.”
MARPA needs boat heading, not COG for collision avoidance info. Without boat speed through the water (not SOG), you cannot calculate the aspect of the target vessel. If you are unsure of the target's aspect, you may take the wrong avoiding action.
When I tested the Raymarine radar on MBM's Calm Voyager in 2005, the crazy thing was that whilst the radar screen showed water based info, the MARPA List box displayed ground based information. Confusing or what!
The article that was subsequently published in MBM showed this test to check whether your radar is water or ground based. It needs to be carried out on a day when there is no wind.
Step 1 Choose somewhere where the tide is running. you know what the tide is doing.
Step 2 Target a fixed object, such as a buoy.
Step 3 Stop the boat and drift with the water become stationary in the water (i.e. stop not making way) .
Step 4 Locate the buoy on the radar, and choose a range scale to fit.
Step 5 Target MARPA, and wait.
If MARPA shows the buoy is stationary (or virtually stationary given instrumentation errors), while the radar shows an apparent speed for your vessel, the radar is ground stabilised.
If MARPA shows that the buoy appears to be moving at the speed of the tide, but in the opposite direction to the tide, then you are sea stabilised, which is the correct setting to run MARPA.
----------------
Moral of the story?
Know whether your radar is water or ground based.
Regardless of which it is, know how to use your radar without relying on MARPA. Use VBLs etc instead. Practice in good weather.
A final comment from the MAIB when I was interviewing thm on the subject. “There is a very real chance that someone will misinterpret the (ground based) information and make an incorrect assessment of the situation which may prove very dangerous.”
boatmike
Well-Known Member
Re: Raymarine\'s view....
Totally agree, and I hope this is reaching a wide audience Piers. I am losing the will to live trying to convince people that MARPA is not magic and you just can't plug it in and go. It clearly isn't and is, IMHO, in the hands of a novice who does not really understand how it works, just something else to lull them into a false sense of security. It can lie badly, especially on a little yacht yawing about in a rough sea. There used to be something widly called a radar assisted collision long before ARPA. Now there is an ARPA assisted collision too!
My real worry though is not the yachts system, it is all those watch officers on Liberian registered ships who just sit and wait for an ARPA warning of collision rather than looking at the screen, or even out of the window. That's why I think its about time I bought myself an AIS as well.....
Totally agree, and I hope this is reaching a wide audience Piers. I am losing the will to live trying to convince people that MARPA is not magic and you just can't plug it in and go. It clearly isn't and is, IMHO, in the hands of a novice who does not really understand how it works, just something else to lull them into a false sense of security. It can lie badly, especially on a little yacht yawing about in a rough sea. There used to be something widly called a radar assisted collision long before ARPA. Now there is an ARPA assisted collision too!
My real worry though is not the yachts system, it is all those watch officers on Liberian registered ships who just sit and wait for an ARPA warning of collision rather than looking at the screen, or even out of the window. That's why I think its about time I bought myself an AIS as well.....
