Anchor Alarms?

[noelex 77]

Originally Posted by sailorboy1

I didn't find it on an app search

I think it might be apple only. It is still in the apple store:

[IslandHopper]

Originally Posted by sailorboy1

I didn't find it on an app search

https://itunes.apple.com/vc/app/idri...480452196?mt=8

[sailorboy1]

Originally Posted by noelex 77

I think it might be apple only. It is still in the apple store:

found it, (no dash after the i)

$8.99. Since I don't really have a real need I'm going to need to hear from users that it works first . Afterall that is a 6-pack of beer and I normally have that need.

[kas_1611]

I use Sailsafe on my Sony Xperia Tipo phone. There is a "pro" version with extra bells and whistles but I have found the basic version to work well.

Some tips on best use if I may:

Make sure your GPS is active before you drop your anchor, it can take time to get that fix
Whenever possible update your "assisted" datafile as this will improve the accuracy
Activate the alarm app as soon as you start dropping the anchor not when it hits the seabed
Calculate the maximum circle including the length of your boat then add a little extra for any accuracy degradation (not too much though)
Put your phone in standby including Airplane mode and switch off any data or wifi to improve battery life. Like this my phone will watch over my anchor for 3 days. My laptop dies after 6 hours and I have never got the alarm to work on my chartplotter.


I also have a Bluetooth GPS device with an associated (free) app so that I can leave the BT GPS on the roof sending a stronger signal to my phone in the cabin. Not normally necessary but if your anchorage is heavily wooded you can lose the signal from the satellites as they get closer to the horizon. Extra signal strength from the few satellites overhead can improve accuracy or at least minimise the degradation.


aGPS is "assisted" in the same way that dGPS is. Differential GPS uses known signals from known radio towers to improve the accuracy, assisted does it from cell towers. It's the same basic system we use in chartplotters that use sGPS or WAAS which is Satellite Differential GPS using geostationary GPS satellites to improve the triangulation.

Why do I use an anchor alarm? Not because I don't trust my ability to set my anchor well or for it to hold but because it is an extra arrow in my arsenal to ensure a calm, restful nights sleep.

Keiron

[noelex 77]

Originally Posted by kas_1611

aGPS is "assisted" in the same way that dGPS is. Differential GPS uses known signals from known radio towers to improve the accuracy, assisted does it from cell towers. It's the same basic system we use in chartplotters that use sGPS or WAAS which is Satellite Differential GPS using geostationary GPS satellites to improve the triangulation.

Keiron

My understanding is that aGPS and dGPS work in very different ways.

aGPS uses the cell phone towers to "triangulate" a fix. Like we would use a 3 point fix taken with a hand bearing compass. Although more or less than three points can be used.

dGPS uses a ground station that with a known fixed position that can determine the error in the satellite signal. This error is then uploaded to another satellite (if it a satellite rather than a ground based differential signal) and broadcast. A GPS receiver can then make adjustments for the error in the satellite signal and compute a more accurate fix. Virtually all phones and tablets should have a GPS chip that will decode the satellite differential signal if it is available in your area. Marine GPS units are strange many of the slightly older units have very poor quality GPS chips and require considerable fiddling in the menus to enable dGPS to function especially with EGNOS. Check the display and see if you really are getting a differential fix.

The practical difference is that aGPS can give a more rapid fix it will also work where the GPS satellite signal is completely lost or poor. However it is not very accurate so does not add to the quality of the fix if a reasonable GPS signal can be obtained. dGPS considerably improves the accuracy of the GPS fix, but will not work if there is no GPS signal.

I think one of the reasons people report false alarms when using an anchor alarm is that they are getting a poor quality GPS signal. You need a high quality fix for a reliable alarm. If your phone is using phone tower triangulation to improve the fix it is getting a very poor quality GPS signal and I would not expect the fix to be adequate for an anchor alarm.

[estarzinger]

Originally Posted by kas_1611

Put your phone in standby including Airplane mode and switch off any data or wifi to improve battery life.

Mmmm . . . Does not airplane mode turn off the GPS? You may not know it because a number of apps, including several of the anchor alarms I just tested, use the last position, but they don't appear to be updating the position. For instance, the XL app mentioned above has a timer for age of fix, and it just counts up when in airplane mode. And my celestrial programs say "no GPS".

So, that may be why you are not getting false positives

Edit: Apple support confirms airplane mode turns GPS off:"When you enable airplane mode from the Settings screen, an airplane icon () appears in the status bar at the top of the screen and the following wireless connections and services are turned off:Cellular (voice and data), Wi-Fi, Bluetooth, GPS, Location services"

[transmitterdan]

On the iPhones we have checked if airplane mode is ON then GPS receiver is OFF. And it is a bit insidious in that it seems to only report to the app the accuracy is degraded to 65 meters which is a hint something is wrong but not really correct. The accuracy is meaningless in airplane mode.

[jackdale]

Airplane mode on my Android phone and Windows tablet shuts off the GPS.

[noelex 77]

It is one concern with the apps that it seems reasonably easy to disable the alarm whether through operator error, software conflicts, or battery problems. Combine this with what seems a much higher instance of false alarms and a good marine anchor alarm with an external aerial is worth considering.

Hopefully, the apps have at least generated more interest in anchor alarms, but I wonder if their limitations are putting people off whole idea.

I have not used the apps much and maybe my impression of their performance is wrong, but don't give up. A sensitive, reliable anchor alarm that gives almost no false alarms and only has low power consumption is achievable.

[estarzinger]

Originally Posted by noelex 77

A sensitive, reliable anchor alarm that gives almost no false alarms and only has low power consumption is achievable.

How/where/which/what specifically are you thinking of?

[noelex 77]

The requirements are simple. A marine GPS (therefore there is no danger of software conflicts). The ability to specify a remote point as the centre of the alarm.

A 12v battery supply (so there is no danger of battery problems)

An external aerial placed with a good view of the sky.

A loud alarm.

A B&W screen. For low power consumption although small LED LCD screens are OK.

Adjustable so the brightness can be set very low (preferably red) so night vision is OK.

Differential ability.

Optional: a fast (5Hz) modern GPS chip with GLONASS etc.

I have a Furuno GP-32. It meets most of those requirements, but there are alternatives. The chip is quite poor and my HH gives a more stable position.

Power consumption is a bit under 0.2 A. It is on 24 hours a day when we are not sailing.

[estarzinger]

Originally Posted by noelex 77

The ability to specify a remote point as the centre of the alarm.

OK, looking at the GP-32 manual . . . .it says "Before setting the anchor watch alarm, set current position as destination waypoint".

Can you in fact set another position, not the current position, as the anchor waypoint?

If it can I think this is quite rare - does anyone know of any other plotters that can in fact do it?

We know the vesper cannot do it. I myself have a small furuno plotter (GP7000) that meets most (except it's a color screen) of your other requirements. But it cannot set the anchor except at the boat's current position. So, this is not uniform across the furuno plotters.

[noelex 77]

Originally Posted by estarzinger

Can you in fact set another position, not the current position, as the anchor waypoint?

Yes this is what I always do.

There are other plotters that will do this. It is just the opposite of an arrival alarm. It is a bit hard to determine from the handbook. Most manufacturers assume you want an anchor alarm centred on the current position. So the handbook lists instructions for this even if other options are possible.

I think they have probably never anchored

[estarzinger]

Originally Posted by noelex 77

There are other plotters that will do this.

well if anyone can cite some others, I would love to learn.

I am quite sure my furuno cannot. The pictures in my furuno manual are identical to yours, but the text is different, and there is no text/step at all to 'set the waypoint' for the anchor alarm. On mine you can in fact have both an 'arrival' and the 'anchor' alarm set at the same time.

[noelex 77]

Originally Posted by estarzinger

On mine you can in fact have both an 'arrival' and the 'anchor' alarm set at the same time.

Yes it sounds like they are different. I cannot set an arrival and an anchor alarm at the same time on my unit.

I suspect the software treats the arrival alarm as almost the same as the anchor alarm.

One is just an alarm if the distance to the waypoint is less than the selected number and the other an alarm if the distance is greater than the selected number.