Giblets
Well-Known Member
Can anyone tell me if the speed indicated by Class A vessels is through the water or over the ground?
AIS Indicated Speed
- Thread starter Giblets
- Start date
Allan
Well-Known Member
As the information is derived from GPS I'm sure it is SOG.
Allan
Allan
jackh
Active Member
Wikipedia says SOG...
(certainly Class B must do, as they only have access to GPS -- and it would seem to me a bizarre system if the speed definition was different between classes)
(certainly Class B must do, as they only have access to GPS -- and it would seem to me a bizarre system if the speed definition was different between classes)
marklucas
Well-Known Member
And if it was speed through the water one would have to know the current at their position and their leeway to work out CPA!
Piers
Well-Known Member
AIS is ground stabilised since it uses GPS for SOG (speed over ground) and COG (course over the ground). MARPA detection-wise can be very eroneous if there's any tide or wind blowing. Take care....
matelot
Well-Known Member
?? The GPS course data takes tide and wind into account so why would the MARPA data ( I assume you mean CPA) be dodgy?
Jamesuk
Well-Known Member
Its SOG on our unit
wadget
Well-Known Member
When using AIS both your data and the targets data is from the GPS so unless there is a difference in tide or wind between the 2 positions there is no problem.
However with MARPA your data is GPS so speed and course over the ground where as the radar data is relative and does not take wind and tide into account so the data is incompatible.
PeteCooper
Well-Known Member
If AIS data is fed from GPS why is it I have seen parked ferries showing 15knots?
I.e. I can see that the ferry is parked, but his AIS signal says 15 knots?
I.e. I can see that the ferry is parked, but his AIS signal says 15 knots?
wadget
Well-Known Member
Because GPS is susceptible to error. Hence why it should not be relied on for navigation or collision avoidance.
PeteCooper
Well-Known Member
Yes I know that - but I don't believe that a ferry's GPS is showing 15 knots whilst it's clearly stopped.
wadget
Well-Known Member
I've seen my GPS jump up to 150 knots while doing about 12.
yesod
Well-Known Member
in order to minimise the issues with collision detection using ais, the sog and cog are derived from the gps unit that the ais transmitter must have in order to do its transmission slot timing.
typically, heading and rate of turn come from a data feed and are more likely to be incorrect. or receivers interpret the data wrong (classic case being a compass heading of 511 degrees - which means 'no heading data', but some badly written receivers claim 151degrees (ie. remove 360).
to show an incorrect sog or cog is very uncommon.
Allan
Well-Known Member
Surely as the target data is relative (to your position) it is also derived from GPS? It's SOG and COG also take the affects of wind and tide into account.
Allan
Allan
Well-Known Member
I believe this is normally a problem with the interface between GPS and AIS units or the AIS unit itself. I have seen a ship appear to stay in the same place for 10 days when it was obviously not there. In this case SOG and COG appeared to be correct but the position had frozen.
Allan
estarzinger
Well-Known Member
LadyInBed
Well-Known Member
MARPA :
- Modification And Replacement Parts Association
- Mid Anglia Rail Passengers Association
- or Marpa Lotsawa, Tibetan Buddhist teacher?
Allan
Well-Known Member
I took it refer to MARPA (Mini automatic radar plotting aid), which does a similar job to the CPA and TCPA on an AIS/chartplotter system. I prefer the Buddhist teacher idea, I'm sure you would be safer with him onboard.
Allan
Allan
Buck Turgidson
Well-Known Member
Need to be a little more specific with this as almost every commercial airliner you see fly overhead is flying GPS primary and many will be flying GPS final approaches too.
The cheap receivers some boats may have fitted might be prone to large errors and poorly mounted or shielded antenna may induce errors but GPS is not only suitable for navigation but EGNOS offers safety of life service with differential GPS giving guaranteed accuracy to less than 5 metres 95% of the time.
Look for the term RAIM and SBAS or DGPS or WAAS when next buying a GPS receiver. RAIM is a receiver based integrity monitor which will discard erroneous satellite data. SBAS/DGPS/WAAS are all names for augmentation systems which increase the accuracy from the standard 15 meters down to 5 or less.
wadget
Well-Known Member
I am not familiar with the airline industry but the current training and advice in the maritime world is that a GNSS should not be relied as a sole means of position fixing or for collision avoidance.
