In Part 1, we discussed what makes up a GPS fix and what the errors are. We also discussed how AV Assistant (AVA) calculates a Position Fix (PF) using HA, CA, and SE.
In the diagram above,
HA = Horizontal Accuracy (also known as EPE) – generally 50% confidence
2DRMS = 95% Confidence circle
CA = Closest Approach (distance from GPS to aid)
SE = Speed Error (speed in passing aid)
PF = Position Fix
AP = Assigned Position (Light List)
We also covered the following diagram showing you could be as close as 23.8 ft or as much as 109.8 ft away from the AP since your actual position could be anywhere in that 43 ft radius circle.
Let’s now look at the positioning tolerance (PT) for an aid and how AVA uses its position fix (PF) to calculate whether an aid might be off-station.
Positioning Tolerance
Positioning tolerance (PT) is the area around the assigned position (AP) in which the fixed or floating aid can reside. The AP is the aid’s position (latitude/longitude) published in the Light List. Just as your actual location might be anywhere in the PF circle around your GPS latitude and longitude readout, the actual location of the aid may be anywhere in the PT circle around the aid’s AP.
The Office of Navigation Systems (CG-NAV) has defined the following positioning tolerances for private ATONs (PATONs):
- Fixed – 60 feet, or 20 yards
- Floating – 120 feet, or 40 yards
Off Station
As soon as a position fix is taken, AVA calculates the distance betweeen the PF and the AP. Let’s look at how AVA calculates off-station.
Mathematically, it is very simple. If the distance from the PF to the AP is greater than PF + PT (circle radii), then the aid may be off-station.
It may be easier to visualize this. Let’s show the PF and PT circles well apart (d > PF + PT). This shows the aid may be off-station – there is no location within your PF circle that could be in the PT circle.
Now let’s see the case where you are closer to the aid (d <= PF + PT). Now you cannot be off-station since there are areas where your actual location could be in the positioning tolerance circle (overlapping area).
So, let’s say you arrive at the aid, start your report, and take a position fix. AVA immediately computes this distance, and a dialog pops up when you go to Preview that informs you the aid may be off-station from its assigned position.
Is it? Maybe, maybe not. AVA does the calculation. As the Aids Verifier, you have to make that determination. You, and only you, are responsible for your report. Answering Yes causes AVA to set the Not on Assigned Position switch.
Marking Best Water
There is another factor to consider before reporting an aid off-station – is it marking best water? If it is a lateral aid, is it on the edge of the channel it marks (and not 100 ft into the shoals)? Sometimes you may need to do some depth sounding to verify this. If you’re a local and familiar with the channel, you’ll probably already know the answer to this question.
The aid may be in the right position but the AP is incorrect – in this case, the aid is Not on Assigned Position but IS marking best water, and you need to make it clear in the comments that the AP is wrong and needs to be updated.
If the aid is Not Marking Best Water, there is a switch for this also.
This is covered in the manual as follows:
Reporting Not on Assigned Position
While AVA will notify you that an aid is off station, keep in mind that you also need to observe if the aid is marking best water. This may require checking and documenting the depth profile across the channel.
From COMDINST 16500.1D, Positioning & Range Surveying, the manual states:
However, placing an ATON at a specific geographic location is secondary to ensuring the ATON’s actual location best marks the waterway and serves the purpose for which it is intended.
Based on your observations, including depth, you should make a recommendation as to whether the owner should update the location on their CG-2554 (aid is marking best water), or the owner should be contacted to move the aid (not marking best water) and update the location on their CG-2554. If you are not sure of best water, don’t report it as off-station (Not on AP) – just report on the other characteristics of the aid. Please note the Coast Guard ANT and Waterways (dpw) are responsible for making this determination and notifying owners – you can only make a recommendation.
In general,
Not on Assigned Position AND Not Marking Best Water
—> Owner should move the aid and update their CG-2554
Not on Assigned Position AND is marking best water
—> Owner should update their CG-2554
Conclusion
AVA automatically computes the PF-AP distance when you take a position fix and alerts you that the aid may be off-station. That does not mean you need to report it as such. Remember you are responsible for your report. If you are not certain of your findings, you can choose to ignore the off-station alert and come back later for another survey.
As pointed out in Part 1, your best position fix will come from coming to a dead stop and putting the GPS in contact with the aid. An external GPS on a pole makes this easy without endangering the boat or crew.
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