Monday, February 18, 2019

Moving Files Around on an iPhone and iPad

In a recent post I discussed a slick way to transfer files among iOS, Android, PC, and Mac wirelessly at sea (away from the internet)—or at home as needed using your normal wifi connection. The process works great, and away from the internet you can use any local network of your own. The Iridium Go device creates such a network, as does the RedPort Optimizer and the Ocens SideKick—the latter two being devices used to wirelessly share files downloaded from a sat phone to apps on other devices.

We also show (in a video linked in the post) that the $20 HooToo portable router does this job fine if you do not have any of those satcom instruments, and it is literally plug and play. Just plug it in with the USB cable provided into your computer or a phone charger, and you will see in the wifi setup of all devices in the room a new network (TripMateNano-xxx) you can use to share files.

At this point, if you only care about Apple products (iOS devices and Mac Computers), you are done.  Connect all devices to your local network and use Airdrop to move them around.

The bigger challenge is connecting PCs and Android devices into the Apple devices, and that takes a couple extra east steps.

We also need a free app from the App Store to carry out the transfers. There are several free options. Such things are also numerous on an Android system. Transfers then work among iOS, Android, PC, and Mac.... or whatever else you have with a wireless connection option.

The only snag in this system, it turns out, is handling the files within the iPhone or iPad itself. This step is tied to the app you choose to do the transfer.  The first free app we tried (described in the post) worked fine for a while, then failed.  Now we have found a better one called, fairly enough, File Transfer App.  Also free, and so far I have not seen any ads.

Having now attempted several videos to illustrate moving files around in the phone, I realize we need an outline of what it involved and a review of how the phones work with files.

• Phones deal with photos and videos in a special way. Generally if they see anything with an image or video extension they try to force it to the Photos directory on the phone.  That is not a problem, because all of the transfer apps we are dealing with assume the main thing you want to transfer is photos, music, and videos so they have this all dialed in. Usually with special buttons for each of these and then another button for "files."  The navigator can need to transfer images, but we are mostly involved with files: GRB, GPS, TXT, PDF (ie share a manual).

• The primary app for moving files on an iOS device is called Files from Apple. I believe this is a stock app that comes with the iOS, but not sure since this one can be deleted, and most stock apps cannot be deleted. If Files is not there (a blue folder icon) then it can be downloaded from the App Store.  The attached video shows the use of this app.

• We also must be aware that there are only certain iOS apps that can work with external files. Any GRIB viewer app is an example as they download and display GRIB files and then save them and some  offer the opportunity to share these files.  A navigation program is another example as they can create and then share GPX files of routes, tracks, and waypoints. Likewise some can import a GPX file created elsewhere.  And the Mail program is another one that can send and receive attached files

• Some but not all apps that work with external files provide access to them through the Files app, but many others do not, i.e., ebooks apps Kindle, iBooks keep books in their own private libraries.

• Email attachments such as GRIB files from Saildocs can be transferred without using the Files app.

• In fact, any file in the phone that can be shared can usually be done without the Files app, but the Files app is often a convenient holding tank.

• In many cases the extension of the file is associated with an appropriate app on your phone. This facilitates some aspects of moving files around.

If we get for example an email attachment with extension .pdf, then a long press on the attachment will reveal a list of all the apps that can read a pdf.  When you choose to open the pdf in say Adobe Digital Editions (ADE) will copy that file into its own library. On an iPhone—in contrast to an Android phone—that then is a dead end for the file. You can access it from the ADE app but can't move it from there, only delete it.  If you had subsequently deleted the email that had it as an attachment, then that file is gone.

What you can do is, instead of immediately loading into a specific app, save the file in the Files app, just putting it any convenient folder.  Then from there you can later open in any of several apps.

The contrast with Android is, we can go into the Android file directory and see where ADE is storing their library files, and so on, which is not doable in an iOS device.

• The other two videos show how the transfers work, the main step left to cover is the preparation of the files in the phone so the app can send them. Here are some scenarios to be demo-ed.

1) Import a GRIB file as an email attachment and move it LuckGrib folder or open it directly from the email.

2) Transfer a LuckGrib GRIB file transfer app so it can be sent to a PC.

3) Move Bad Elf GPX track to Files app then share it to MotionX GPS

4) Take a screencap or cell photo and send it to the Transfer app via Files folder (only option).

A video demo of the above 4 operations.

Monday, February 11, 2019

Brightness of Stars and Planets

In our cel nav course we provide two textbooks: Celestial Navigation and The Star Finder Book.  A question came up in class the other day that reminded us that we have many folks learning cel nav on their own using our main textbook who may not have the second book. These folks were then missing our discussion of star brightness. This note corrects that. In the next printing of the book, we will replace Section 11.20 with this new section of star brightness. Sadly enough, Section 11.20 was the one covering time tics and storm warnings from the NIST on the HF broadcasts from WWV and WWVH. These were both discontinued on Jan 31, 2019. I will have a post on that topic later. 

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The brightness of a star is often a valuable aid to its identification. Brightness is specified by the body's magnitude. Star magnitudes are given in the star list at the back of the Nautical Almanac, not on the daily pages; star magnitudes change very little, if at all, throughout the year. The magnitude of the selected "navigational stars" are also reproduced in the Index to Selected Stars at the back of the Almanac (for years on page xxxiii). The latter is often reproduced on a yellow card bookmark.

Planet magnitudes are listed at the head of the planet columns on each daily page, because their brightness changes slowly throughout the year. The same magnitude scale applies to stars and planets. Samples are shown in Figure 11.20-1.

 Figure 11.20-1 Star and planet magnitudes listed in the Nautical Almanac. Planet values, i.e., -3.4 for Venus, are on the daily pages; star values are in the star list at the back of the Almanac (see below). The yellow card insert shows the values for the selected navigational stars. The list at the back includes many more stars.

Figure 11.20-1a. Star list sample pages from back of the Almanac, showing facing pages with different star name conventions. 

The Figure above shows a nuance in the star lists at the back of the Nautical Almanac. They list the star data by month, but they only show the star's proper name in the second half of the year.  Thus we learn that sigma Puppis does not have a proper name, whereas alpha Geminorum (of Gemini) is the alternative name to Castor.   We also learn that Sirius is the alpha star of the constellation Canis Major.  An "alpha star" is just what you think it is—the dominant star in the constellation.

There is not a simple correspondence between the numerical magnitude of a star and the visual brightness that we perceive. Each magnitude difference of 1.0 implies a brightness difference of 2.5. The magnitude scale is logarithmic, which means we need special tables, such as Table 11.20-1, to figure the actual brightness difference between two stars, or between a star and planet. And to complicate things even further, the scale is inverted; the lower the magnitude, the brighter the star.

(The system dates to Ptolemy in about 150 AD, who decided that the brightest stars we see are 100 times brighter than the faintest we can see, and then choose to divide the range into 5 magnitudes, so we end up with each being a factor of the fifth root of 100 (2.511) brighter than the next.)

The faintest stars we might navigate by would have a magnitude of about 3.0 although it would be rare to use such a faint star. A typical bright star has magnitude 1.0, which we could say is "two magnitudes brighter" than a faint magnitude 3 star. But the actual brightness difference between the two would not be a factor of 2.0; the bright one would appear just over 6 times brighter than the faint one.

The magnitude scale can also go negative for very bright objects. Venus, for example, at magnitude -4.0 would be 5.5 magnitudes brighter than a star with magnitude 1.5. Only two stars, the southern stars Sirius (-1.5) and Canopus (-0.7), are bright enough to have negative magnitudes. Venus and Jupiter are always negative, meaning always very bright, but Mars and Saturn are only rarely negative.

The sign of the magnitude difference is not important; the object with the lower magnitude is always the brighter object. Remember -1 is less than +1; and -3 is less than -2, and so forth. Objects with the same magnitude are equally bright—in Table 11.20-1 this is indicated by showing that a zero magnitude difference means an object is 1.0 times brighter than another object with the same magnitude.

For all practical star identification it is not necessary to be very technical about brightness and magnitudes. It is sufficient to classify stars in three rough categories:

Magnitude-one stars are the 20 or so brightest ones—pick a favorite and use it as your standard.

Magnitude-two stars are stars about as bright as the Big Dipper stars. There are only about 70 of these, each two to three times fainter than magnitude-one stars. And finally the

Magnitude-three stars  are like Pherkad, which is the lesser of the two Guards on the cup edge of the Little Dipper. Kochab, nearest the Pole is a magnitude-two star; Pherkad below it is a perfect 3.0 magnitude-three star. The two trailing stars of Cassiopeia, Ruchbah (2.65) and Segin (3.35), are both magnitude-three stars. There are only about 200 of these in all of the sky. The vast majority of celestial navigation is done with magnitude-one stars, and magnitude-three stars are hardly ever used.

Tip on star ID

It is rare to see stars below 10º or so (a hand width) on the horizon, because there we view them through the thickest layer of the earth’s atmosphere, where much of their light intensity is lost to scattering. Even the brightest stars fade as they descend toward the horizon, as shown in Fig. 11.20-2. Consequently, if you see an isolated star low on the horizon, you can bet it is a bright one, even if it appears faint. Since bright stars are well known stars, this observation alone often identifies the star for you.

Figure 11.20-2. How star brightness changes with the height of the star.  All stars fade as they descend toward the horizon because more of their light is lost to scattering. Polaris, for example, can rarely be seen at latitudes lower than about 5º  to maybe 10º N.

Or, an isolated low “star” could be Venus or Jupiter. But this confusion is unlikely since navigators tend to keep pretty close track of where these guys are, and even low on the horizon they remain notably bright. On a clear night, a low, bright Venus can startle a weary helmsman who sees it for the first time.

For more sophisticated star ID, it helps to know that several stars are distinctly reddish. These are in a class of stars called the Red Giants, and knowing these can be a valuable aid to their identification. See The Star Finder Book for more details on star and planet ID.

Practice with magnitudes

(1) Arcturus has magnitude 0.2 and Dubhe has magnitude 2.0. The magnitude difference is 2.0 - 0.2 = 1.8, and from Table 11.20-1, Arcturus is 5.2 times brighter than Dubhe.

(2) Sirius has magnitude -1.6 and Antares has magnitude 1.2 The magnitude difference is 1.2 - (-1.6) = 1.2 + 1.6 = 2.8, and from Table 11.20-1, Sirius is 13 times brighter than Antares.

(3) Jupiter, on some date, has magnitude -2.1 and Canopus has magnitude -0.9. The magnitude difference is -2.1 - (-0.9) = -2.1 + 0.9 = -1.2, and from Table 11.20-1, Jupiter is 3 times brighter than Canopus.

(4) Venus can be routinely as bright as magnitude -4.3 and the North Star, Polaris, has magnitude 2.1. The magnitude difference is 2.1 - (-4.3) = 6.4. From Table 11.20-1 we can estimate that Venus is roughly 400 times brighter than Polaris. Venus can be as bright as -4.8.

Tips on Planet Identification

The planet Mercury can be seen with the naked eye, and it can even be quite bright. But it is only rarely visible, and when it is, it will be low on the horizon, just before sunrise or just after sunset, and very near the sun. Since it is rare to be seen and always very low on the horizon it is not used for navigation. Its Dec and SHA are not listed in the Nautical Almanac.

Venus and Jupiter always stand out nicely among the stars. When either of these two are visible, they are always much brighter than any stars around them. Mars and Saturn, on the other hand, appear only as bright or medium bright stars. The main function of Mars and Saturn is to confuse the navigator by appearing as stars where no stars should be. Mars can sometimes appear reddish, and most planets will appear as tiny disks, rather than points, when viewed through 10-power binoculars.

Another identifying characteristic of planets is their lack of twinkle. Stars twinkle, planets do not. The reason can be traced to the apparent size of the light source—distant stars are point sources of light; the much closer planets are disk sources. A patch of warm air can momentarily refract all of the star light out of our eye, causing it to twinkle; but such transient refraction cannot remove all of the light from a planet since it comes from slightly different angles depending on its origin on the disk.

The relative location of the planets can also sometimes confirm or assist in their identification. The sun, moon, and all planets always lie along the same arc across the sky. On those occasions when 3 or more of these objects are visible (say, moon and two planets), this alignment can sometimes aid in their identification.

A consequence of the above, which comes about because the orbits of all of these bodies lie within the same plane (± 9º or so), is that planets are always found within a Zodiac constellation. With that said, the concept of constellation, let alone the Zodiac,  does not come up much in cel nav as we do not need it for anything. We do refer to groups of stars, but these are groups we make up from stars of neighboring constellations, such as the Summer Triangle.