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 23, 12:03 -0700
From: Peter Monta
Date: 2013 Aug 23, 12:03 -0700
Hi Frank, > You are at the center of the universe: Silicon Valley. Ha! > The time is always the largest uncertainty in this sort of fix, both because > your camera's clock (or whatever you used to time the photo) and the > satellite's mean longitude could both be in error by a second or two. Yes. The camera unfortunately writes its EXIF timestamp only to the nearest second; after taking the photos I took one more photo of my PC's NTP clock as a calibration (the camera turned out to be 14 seconds out). So the error could be as much as two seconds. Better would be a little optical gadget that inserts an LED time display into the actual exposure, or, simpler, an external device that records the shutter signal. > ... you're along an LOP running between 37.54°N, 122.34°W and 37.35°N, > 122.08°W. Picking a middle point, I'll go with 37.43°N, 122.19°W Very close! Attached is a kmz with the GPS position and the estimates you give. About 380 meters from the LOP and about 415 meters from your fix. I think much of this 400 meters must be ISS orbit error. > ... Then I realized that this motion is the composite of the > actual motion and the motion of your camera during the exposure. It would be nice for the camera to remove this ambiguity by interrupting the exposure for a few milliseconds near the end of the 500-ms main exposure. That way there would be a little dot indicating the endpoint, for both stars and satellite. Yet another reason why these cameras need to be fully general-purpose platforms rather than just photo gadgets. Don't get me started on the utter inadequacy of the camera APIs for Android and iOS. > By the way, a digital camera/space station/Internet determination of an > observer's position independent of GPS calls for a press release, wouldn't > you say? I think it can certainly be said to be fully independent of GPS: the ISS orbit would be determined from the space radars. Now, no doubt ISS carries GPS receivers; is that data available anywhere, so that a truly precise orbit can be used? In my opinion the various pieces of space junk (rocket bodies, defunct satellites) should be instrumented with GPS receivers and the real-time orbits disseminated for the benefit of optical observers :-). Cheers, Peter