From Analog to Digital Radar
What AIS does to radar
To comply with IMO and USCG
requirements, operators are installing AIS on their ships and rather than
setting up a transponder as a little black box somewhere on the bridge and
letting it do its own thing, most feel it is worthwhile to connect it to the
radar or the electronic chart or both. Connecting a transponder to radar takes
it another step to becoming pure digital.
Radar has gradually become more and more digital. Its analog
part isn’t going away but over the years it has constantly been enhanced with
more and more digital information. Pure old-fashioned analog radar gave us a
picture of what is ahead. Then radar was enhanced with an automatic radar
plotting aid (ARPA) that calculates the relative course and speed of a radar
target and displays them as a vector. This information, in turn, was used to
predict the closest point of approach (CPA) and the time of the closest point
of approach (TCPA). Later GPS was connected to radar. Doing so allowed showing
the traffic situation in absolute rather than relative terms by calculating a
target’s ‘absolute’ course over ground (COG) and speed over ground (SOG). Next
it became possible to enhance echoes of fixed features by overlaying them with
an electronic chart.
Now ships are starting to exchange navigation information
with each other directly. So rather than having to derive a target’s COG and
SOG from own ship GPS data and the range and bearing to a target, a target
sends you its GPS information (and then some) directly even if it is hiding
behind a cape or an island. Ships can do that because VHF, too, is going
digital. First came digital selective calling (DSC) on channel 70 that made it
possible to broadcast your position in an emergency (via GMDSS) and establish
contact with a particular station using its MMSI number. Now AIS takes that a
step further and turns a VHF radio into a regular albeit somewhat slow modem.
It sends and receives data just like any other modem.
Apart from GPS information, what other information could
ships exchange with each other that would enhance the Officer of the Watch (OOW)’s grasp of the
traffic situation without causing information overload? Transponders are
programmed to exchange lots of information but only some of it fits the above
criterion. First lets look at how AIS can be used to further enhance radar.
There are five major areas where AIS improves radar:
- It
helps radar find targets ‘around’ a cape (useful in rocky coastal waters)
- It
translates radar echoes into ship names (useful when hailing them on VHF)
- It
improves prediction of a target’s path by taking into account its rate of
turn (ROT)
- It
extends radar’s range
- It
clarifies a target’s intentions
AIS can identify targets on the other side of a cape
The range between REGAL PRINCESS and SEA PRINCESS was only 6.3 NM during a test of AIS in 2000 near Juneau Alaska, or well within radar range, except that there was a 2,500-foot tall island (Douglas Island) between them… AIS easily scaled that mountain thus confirming that it can help the OOW to anticipate a traffic situation earlier.
AIS translates radar echoes into ship names
Being able to hail “NORDIC VENTURE” on VHF by its name avoids potentially dangerous confusion about which ship is responding to “Ship off my starboard bow”. AIS allows radar (and ECDIS) manufacturers to label a target with (an abbreviation) of its name or show its name when it is selected.
AIS improves prediction of a target’s path
Knowing a target’s ROT improves the OOW’s ability to anticipate a traffic situation more accurately:
1. Straight-line path prediction
In the crossing situation shown above, the blue line represents the straight-line predicted path of the own ship making 12 knots on a true course of 110° and a target ship (red line) making15 knots on a true course of 240°. The target initially is on a true bearing of 70° with a range of 2 NM (green line). With straight-line projection, the CPA of .4 NM will be reached in 4.8 minutes (light blue line).
2. ROT-corrected path prediction
In the same crossing situation shown here, the blue arc represents the predicted path of the own ship again making 12 knots on a true course of 110° and a ROT of +5° per minute (starboard). Again the target is initially on a bearing of 70° at a range of 2 NM (green line) making 15 Knots on a true course of 240° with a ROT of –10° per minute (portside). The red arc shows the its predicted path. The CPA is now only 380 feet after 7.9 minutes (light blue line).
The above clearly shows the value to the OOW of taking ROT into account when predicting a target’s path.
AIS extends radar’s range
This is useful when approaching features like a bridge, a point or a narrows. The extended range and target identification that AIS provides, allows the OOW to identify other ships with a similar ETA to the same feature, even if they are approaching it from the opposite side and are still some 40 NM away. This allows the OOW to anticipate a traffic situation earlier and make passing arrangements well over an hour in advance via VHF, or, if the other ship is still beyond VHF range, via an AIS instant message.
AIS clarifies a target’s intentions
If the OOW, for whatever reason, scrutinizes a target, AIS will provide not only its destination but also its intended route. This route can be temporarily displayed to anticipate a target’s intentions. If a target is has limited maneuvering capabilities, for instance due to its draft, AIS will provide such details too.
Connecting AIS to radar also complicates matters:
- An
AIS target’s GPS information may, for various reasons, not be accurate
- A
target’s AIS position may be different from the position of its echo on
the radar screen
- ROT-corrected
(T)CPA calculations are hard.
Accuracy of AIS positions
The OOW will know when an AIS target is no longer sending
position updates because its icon will be crossed-off with a flashing line. If
a target continues to send out AIS position updates but its GPS is not working
properly then the OOW can only find that out by comparing AIS and radar (ARPA)
positions (assuming it is clear which radar and AIS icons are associated with
each other).
Target Icon Consolidation
If a nearby ship has both an AIS and a radar icon on the screen and the two do not overlap then the software in the radar (or ECDIS) will try to decide whether the two separate icons should be replaced with a single consolidated icon and where to place that icon on the screen. The software bases this decision on a comparison of each icon’s range, bearing, relative course and relative speed. If they pass this test then the target’s radar icon is dropped from the screen.
ROT-corrected (T)CPA predictions are hard
While (T)CPA predictions that take ROT into account are preferable there is no readily available algorithm to make such predictions. Fourier-LaPlace transformations of the necessary equations have to be used to solve them for CPA. Until such an algorithm has been developed, once per second re-calculation of ROT-corrected (T)CPA of all selected AIS targets remains impractical.
Nevertheless, even without (T)CPA predictions, the OOW will have a more complete representation of the traffic situation if the predicted path of a target is shown on-screen as an arc that reflects its ROT.
Conclusion
It will take some time for all these benefits to become readily available, but connecting AIS to radar will significantly improve the OOW’s ability to anticipate and avoid potential collisions. Anticipation will be improved primarily by AIS’ extended range, its capability to see around a point and by improving target’s path predictions. Avoidance will be improved by clearly identifying targets for hailing purposes via VHF or via instant messaging.