# What EM spectrum can stars be most cheaply observed using Remote Sensing during the day?

I would like to augment my GPS navigation with remote sensing of 'the sky.' I am looking for any astronomical body that can be perceived during the day.

Maybe Pulsars emit something easily detected, seems IR is obvious..

Any input is appreciated.

• I'd recommend you un-accept the currently accepted answer until it has been augmented with some facts. See for example Venus and Jupiter Dance in the Daylight, and 10 Surprising Things to See in the Daytime Sky and Weekend Plans? Try Spotting Jupiter in Daylight. – uhoh Nov 11 '17 at 2:36
• How Bright is the Daytime Sky? shows an image of Venus taken during the day with an 3.5 inch telescope. The original image is from here. Sky brightness is roughly magnitude 3 per square arcsecond in the visible, and about 5.5 in the near IR. It drops farther if you go to say 12 microns, but of course the brightness of objects reflecting sunlight also drops (sunlight peaks in the visible). – uhoh Nov 11 '17 at 2:40
• Since the sky brightness is essentially flat/uniform within a telescope's field of view (and can often be reduced relative to the object of interest using a polarizer) it's easy to subtract the uniform background from the image and locate objects with surface brightness well below that of sky light. Using the red channel of your image, or a red filter with a monochrome sensor will further suppress the predominantly blue skylight which is mostly from Rayleigh scattering ($1 / \lambda^4$). – uhoh Nov 11 '17 at 2:44
• I'd question how stellar observation can be easily used to augment GPS for navigation purposes. If GPS isn't good enough for navigation then I think you have a bigger problem. – StephenG Nov 11 '17 at 12:15
• The idea is to detect GPS spoofing.. Thats it.. I wan't to take my presumed location from GPS, and verify that an astronomical object is where it should be.. Or, If I have n GPS signals, and I remove any 1, does my location change significantly.. If so, the above would help isolate the 'jamming' signal. Out of pure curiosity, the same technique could be applied anywhere, even on a spaceship to mars.. Just a curiosity. Sailors use sextants at night.. Seems like a sextant with a filter' of some sort is feasible. – ttugates Nov 11 '17 at 13:03