My Anchor Watch

'My Anchor Watch' is a program for smartphones that enables you to monitor that your boat is not moving away from its anchored position. The program is using the phone's built in GPS to constantly monitor the boat's (or actually the phones) position.

The accuracy depends on the built in GPS accuracy. This accuracy is as good as any GPS receivers accuracy. The major determining factor is how many satellites are received at any given time. The number of satellites that are received depends on how the satellites happens to appear, but also on how unobstructed the signal is reaching the telephone.

If the telephone is inside a plastic (GRP) or wooden boat, the reception is usually very good. Inside steel boats the reception needs to be verified.

Normally the "jitter" (error when the phone is kept stationary) is less than 10 meters. The problem for an anchor watch application is that we can not depend on what is 'normally' the case. If we have the receiver  on during the the whole night we are bound to receive several readings that are outside of what is called 'normal' and we might trigger an alarm.

Generally GPS is specfied to be accurate to 3m 95% of the time. Wikipedia has a good summary of GPS errors. In practice, the accuracy has in most cases proven to be far better than needed, but the alarm circle might need to be put 20-30m out in some cases in order to avoid spurious alarms. Hence, under normal conditions 10 meters distance should be added from the cable  length (yellow circle) to the alarm distance (red circle) to ensure that no erroneous alarms occur.

'Cable length' is the length of your chain or rope that you have put out. The boat can in normal conditions move around inside a circle with a radius of the cable length. The cable length is plotted in yellow on the screen.

'Alarm length' is the distance from the anchor point that you want the boat to be allowed to move before sounding the alarm. Due to inaccuracies in the GPS reception the 'alarm length' should be 10 meters or more larger than the 'cable length'. The 'alarm length' is indicated by a red circle (alarm circle) and the area inside the alarm circle is colored green to indicate that it is an allowed area.

'Outside space' is only used for presentation purposes. It is the distance outside the alarm circle that as a minimum is available for plotting the boats position.

'Blocking angles' is a feature that you can choose to use if you do not want to allow the boat to swing around in all directions. If the wind or current is constant from one direction, you can choose to anchor close to shore or close to a fixed buoy. However, if the wind changes you then risk running aground or hitting the buoy. The program can then block these sectors and thus warn you if the conditions have changed so that you ar at risk.

The program ensures that it atleast can store 48 hours of positions. When reaching the storage limit, the program automatically discards some positions and continues. Hence, there is no limit to how long the program can run and monitor your position.

As long as the phone has enough power, the program will continue to operate. If the phone is not charging, there is naturally a limit to how long it can operate. This will differ depending on what phone model you are using, how big battery you have and how fresh the battery is. 'My Anchor Watch' uses the GPS, which normally causes the phone to comsume slightly more power than if the GPS is switched off. For long term anchor supervision it is advisable to have a charger plugged in to the phone.

The anchor watch only uses the GPS to determine the position. Hence the program works in the middle of the ocean if need be (hmmm, a little difficult to set anchor though....).

The only limitation when out of cell tower reach is that the phone can not send its alarms via SMS and not receive commands via SMS. The rest of the program will work as usual.

An alarm can be signaled via a dialog box on the screen, via a sound alarm, via vibrations on the phone or by sending a SMS to another phone.

There is no other requirement than that the phone running the 'My Anchor Watch' can send SMS and the phone receiving the alarm can receive SMS. Hence almost all phones will work for receiving alarms via SMS.

The SMSes for alarms, status and commands incur normal SMS charges from your network operator.

From any phone that can send and receive SMS, you can control 'My Anchor Watch' through simple commands. The program replies to the phone with a status message or information.

You can choose any codeword that have to begin all SMS commands that are send to the phone. Only if this codeword is present does the program accept the changes.

To further increase the security, you can configure the phone to send responses to only a predifined number, to only the sender or to both sender and the predifined number (if different).

Examples are:
GETPOS to get the position in Latitude and Longitude as well as distance from the anchor.
GETSTAT to get general information about settings and alarms.
ZOOMOUT or ZOOMIN to change zoom.
Commands to change alarm settings, angle blocking etc.