Tuesday, May 30, 2023

Battery Life of Phones Running Location Services

Phones and tablets are often valuable backup navigation systems, with many excellent navigation and weather programs available such as qtVlm and LuckGrib. It is known that location services, which we must turn on to access the internal GPS, cause an enhanced drain on the battery. Here we look into how much of a drain is this.

The topic came up again as we introduced the new low-cost miniature AIS called dAISy that will not only run in any computer-based nav program, but it will also work plugged into an Android phone or tablet running qtVlm. The battery-life question then extended to how much more of a drain will it be to run this AIS in a phone, which will be on top of the drain from the required location services.  

Below we see the battery drain with with dAISy being fully powered by an Android phone along with qtVlm.

Figure 1. A new Galaxy A03s Android phone running both qtVlm and the dAISy plugged in and monitoring AIS Traffic without external power.

This test ended as shown when we noted that the dAISy had quit working at about 5.5 hr, although qtVlm continued to function.  We suspect that the phone shut off the USB connection at some level to protect battery life — not realizing that in this case the dAISy had in fact almost negligible effect on the drain as we see below.

Note too that this direct plugin arrangement of the dAISy is just for quick observations of traffic, up to, as we know now, four or five hours.  For continuous operation, the dAISy should be connected with an OTG adapter which allows for external power application.

To see how this differs with no dAISy, we have the data below for full navigation functions running in qtVlm but no USB attachments. 

Figure 2.  A new Galaxy A03s Android phone running qtVlm with a continuous GPS fix.

We notice here that the rate of battery life drain is essentially the same as with the dAISy attached. In short, this phone running location services and a large nav app loses battery life at about 10% per hour, with or without the dAISy attached.

This is a new phone with its new battery; older phones might not be as efficient.  Also note that this Galaxy A03s is an economical choice for a backup nav system at about $88 for refurbished unit with both GPS and a barometer... plus it will run an inexpensive AIS system.

Below is the same measurement as Figure 2 using 18-month-old iPhone 11.

Figure 3.  An 18-mo-old iPhone 11 phone running qtVlm with a continuous GPS fix.

So we see a rough generality emerging, namely the battery drain from location services in action is about 10% per hour.  This will of course have to be tested to learn what other factors may influence this observation, but at least we have a starting point.

Sunday, May 21, 2023

Fast Barometer Setting in US Waters

Setting a barometer to the right pressure is not the same as calibrating it, but it is the certainly the first step, and might indeed meet many practical needs. In short, we assume that if we set it to be right at one pressure, we hope it is at least nearly right at other pressures.

Also, if the barometer is in the boat, then we know it is roughly at sea level so we do not have to worry about the corrections for elevation above sea level, which is usually a dominating factor. A barometer that is 6 ft above the water is only reading 0.2 mb lower than what it would read floating on the water.

What we need is just an accurate (official) value of the sea level pressure that we can set our own barometer to read, because our own yacht's barometer is also effectively at sea level. This is not true for a ship, were the instrument could be 80 feet above the waterline.

In the past we taught that you can get accurate pressure from various National Data Buoy Center (NDBC) or lighthouse reports, or from airports (METARS). That still works, but those data takes some steps to access and on top of that they are only updated every hour.  Thus if we are to use one of them, we have to note the time and the given 3-hr pressure tendency to compute the correction to the official reading.

We now have a much faster and easier solution. I say "now," but this source has actually been available for probably over a year now, I just had not discovered it. We get the pressure data now from exactly the same place that all mariners now have to work with to get tide and current data, namely


Go to that site, click the state you are in, and then on the top right, turn Legends on  if needed, and then check Barometric Pressure and signs will pop up at the places were it is known.  These data are updated every 6 minutes, which is ideal for this operation.

Then you can either set your barometer to that pressure, or maybe better still, start a note book with the time and date and the correction you observed.  As you get more of this table filled in over various pressures, then indeed you are calibrating your barometer.  If you just set it, and do not record the correction you do not learn about the pressure dependence of its errors.

Sample barometer data display at tidesandcurrents.noaa.gov when clicking FL.

In this view there is good coverage on the Gulf Coast, but not much north of there. When this happens, try clicking another state.  

Sample barometer display clicking the state of SC

When trying all near by states does not work, then you can turn to  www.starpath.com/barometers, which covers global sources of various kinds.

This could be a great resource when sailing on other vessels and you want to check its barometer. You can read read this pressure data from your phone (tidesandcurrents.nooa.gov—we should know it by heart because it is now the only official source of tide and current data)... but if you have a phone, then you are better off using our barometer app, which you could set using this method as well.