A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Position-Finding
From: Frank Reed
Date: 2018 Jan 8, 12:00 -0800
David Pike, you wrote:
"I managed to download your free “GPS Anti-Spoof” app into my hand-me-down (or hand-me-up from my son in my case!) Galaxy III"
Oh good! I'm assuming the "Galaxy III" is a "Samsung Galaxy S3" (on its boot-up screen, they use a Roman numeral III and an "S" that is so stylized that you might think it's supposed to be a lightning bolt, but yeah, it's an S, and everyone refers to it as an "S3"). That's a very fine smartphone and still my primary phone and primary Internet access device. Ninety-nine percent of my NavList messages in the past five years, including this message, have passed through my Galaxy S3 serving as an Internet hotspot. I have needed two inexpensive battery replacements in those years, but otherwise it's doing fine.
"I must say it’s a great sextant training or aircraft sextant calibration aid."
I'm glad you think so. When spoofing is not an issue, it works great in these roles. Instant feedback is a wonderful thing.
What does the “5 second delay” caption mean? Does that mean the Hs given on the screen is what it should have been 5 seconds ago? That would be quite good because it takes you that long to look away from the sextant and onto the screen."
Yes, that's exactly the idea. Note that you can set the delay to zero and also to 10, 15 seconds, etc., as suits you. If you have an assistant and call out "mark" when you take the sight, I still recommend setting the delay to 5 or 10 seconds. The assistant can then potentially compensate a bit for the reaction time delay and count to five before hitting pause. Note also that the seconds display of UTC is still rolling along until you hit pause, so counting seconds off can be quite accurate. If you're working alone, when you like the sight, you immediately think "zero" and count to five or ten, tapping the pause exactly when you reach the final count. Note also that this is how I teach sight-timing for celestial navigation generally. I count to ten and then record the time. It's easy to subtract off ten if I don't have an app handy.
"Also, what does “T adjust: 1.2s"(this figure varies) mean?"
Aha. That's rather important and especially so if you're at sea or otherwise off-network. The device has its own internal clock. It resets this regularly, especially when it boots up, by accessing the accurate time from the wireless network. Devices without telephone/wireless service may need access to WiFi to reset the time. But that internal clock is not much of a clock, and it drifts. If you're off-network for a day, an error of a few seconds is not unusual. And a device at sea might be wrong by a minute or more. My app checks the internal time and then compares with the time stamps from location services (GPS normally but sometimes other lower accuracy services). Those time stamps arrive intermittently but they determine the error of the device's system clock. In traditional terms, it's just the "watch error" of the device. This value will bounce around a little when the app first starts up. It will also change over the course of some hours.
One way to reduce the system time error, if you want to do so for other apps, is to reboot regularly. If you haven't done this on a smartphone, press and hold the power button for five seconds. A dialog comes up with a variety of options including shutting down, setting "airplane mode", and restarting. That last option is a good old-fashioned soft reboot. If you select that, the device will do a clean reset and it should also reset the internal time. I also recommend resetting a smart device once every day or two for other reasons. Most people never reset and deal with problems they shouldn't have to worry about. On the other hand, if you normally power the phone down on a daily basis, then you're all set.
PS: get the pro version of the app here. Includes all navigational stars and planets, plus Moon sights, and also operates off-network, e.g. at sea, for months at a time (the free version can go for 3-5 days without network access).