NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Satellite photo for navigation
From: Peter Monta
Date: 2013 Aug 25, 01:15 -0700
From: Peter Monta
Date: 2013 Aug 25, 01:15 -0700
Frank writes: > Peter: last January I bought myself a new digital camera. While the camera > in my cell phone (Samsung Galaxy S3) is stunningly good, it has no zoom and > no controllable time exposure function. The camera I bought is the Canon > SX260. This is basically identical to the one you've been using, right? I've > taken some very nice handheld Moon photos with it, but no other astronomical > photos yet. I will have to try for an ISS photo soon. After a quick glance, the SX260 does seem very similar, yes. It has a bit more zoom than the SX160 and, of course, the crucial image stabilization. Just a few random notes on camera settings: - use manual focus, and if your camera has a feature called "Safety MF", turn that off: you don't want the camera to touch up your focus setting when the shutter button is pressed, since with a black sky it will have nothing to work with. In general it's probably a good idea to leaf through the user manual's section on manual mode. I use the infinity setting, but it could be that the truly optimal setting is a few button clicks shy of infinity. I'll have to do a focus series at some point. At f/5.9 it's not too critical. - turn down the LCD brightness: it is blindingly bright at night. It really does help to see the target in the LCD; in fact the LCD can see objects that I cannot with the naked eye - set JPEG compression to superfine if the camera doesn't have a raw mode - set ISO to 1600 or whatever your maximum is, provided you don't come close to saturating the 8-bit dynamic range with 0.5-second exposures or whatever you choose - and, of course, use the smallest focal ratio (maximum aperture) available at max zoom. Clear aperture on my camera is about 10 mm. > Also a little jitter might actually be useful. I remember a few years ago > reading a description of the star-finding software aboard a little > spacecraft navigating in the asteroid belt. The images of the stars were > jittery, looking like melted pretzels. But this was considered a benefit > since it distinguished actual star and asteroid images from noise and cosmic > ray impacts on the detector array, and it also provided a simple means for > determining the average center of each star image. That makes sense. I'm a little worried about JPEG noise, even at superfine quality; sometimes the edges of star images seem a little blocky. I suppose it will average out. > ... While most of their satellite data is derived from radar > observations, they may well be using the position data supplied by NASA for > the ISS and a handful of other satellites. If they are using better source data than is available from radar, that's good, but it's still limited by the quality of the orbit model, which has only a dozen parameters or so. > If we could improve the accuracy of this system by just a factor of five or > ten, which seems within reach, we would run into an interesting problem with > ISS observations: it's 100 meters across! The orbital data probably refers > to a location close to the physical center, but the visual center could > easily be fifty meters away. Yes, that occurred to me too. It might be interesting to check for any waviness in the optical track as the glints move around (assuming most of the light is specular). Of course the people who do this sort of imaging for real can resolve exceptional detail, so they could compute the center of the object directly from the image. > [ Iridium ] Iridium. Interesting idea. You mention the flares: those would give LOPs against the Sun or Moon or Venus (or the horizon, I suppose) during the day, provided one is very, *very* quick with the sextant, or has a suitable camera, perhaps set to video-recording mode. My only worry is that they're too low for good orbital predictability long-term. I didn't know they were that bright, though (mag 6.0). I'll have to look for one in the binoculars. > They do maneuver but very rarely. I guess if multiple satellites were used for any given fix, for extra robustness, a satellite that maneuvered would be an outlier. It could then be ignored until new orbit data is available. Oh, speaking of fixes: I believe this came up already, but using the along-track data will require millisecond shutter-time accuracy. Certainly possible. An alternative is to use only the cross-track data, giving an LOP, as you describe, and then use additional objects for additional cross-track LOPs. Cheers, Peter