GPS Global Positioning System Satellites in high orbit

GPS = Global Positioning System • Satellites in high orbit (24+, ~20000 km) • Microwave frequency • Constantly sending signals with very accurate timestamps, satellite location details • Time of travel from satellite to handheld receiver (corrected for relativity) gives distance to satellite • Satellite positions known

Trilateration 3 satellites needed for x-y position on surface of earth. 4 needed for elevation (z) BUT may use up to 20 if visible

Visibility of Satellites

Errors from obstruction and reflections in “urban canyon” More sophisticated units can correct some multipath errors– antenna design helps

Error in “real” canyons Cleaver Crack route on Mt Wilson – took same route up and down, but up to 150’ difference in narrow slot portion.

What stops microwaves? Ovens work because liquid water (in food) absorbs microwaves. Your head and body are 70%+ water! So try to keep GPS high on body. What about clouds and rain? Not big problem, ~2 cm error, slightly weaker signal. Drops are small and only small fraction of atmosphere.

Smartphone vs. Dedicated GPS • Smartphones NOW have very good GPS chipsets– sophisticated signal processing, almost as good as new dedicated units • Good, cheap apps for smartphones • Smartphone antenna design not so good • Smartphones tend to be more fragile (but Galaxy s 7 active…) • Battery life usually worse in smartphone • Dedicated GPS usually has transreflective display; MUCH easier to read in sunlight. VERY important. (Newer phones much better. )

Nexus Android vs. Dedicated Garmin 64 In house, right at the window in this room!

When NOT to use a phone GPS

“Go. TO” use on GPS Summerlin Peak visible from front window Notice I have customized the map fields to show elevation and GPS “accuracy”

Two ways to use a GPS • Read your location – good for SAR! (Important to know datum for your unit – WGS 84 by default). Match datum and find location on paper map. Not done much anymore. OR: • Preplan a trip with waypoints and tracks you load to GPS from computer, internet - Can draw tracks on maps on computer (Google Earth allows), upload to GPS - Can download tracks from (e. g. ) peakbagger. com (Demo with Android if time)

About that datum stuff • The GPS unit calculates surface (x, y) position by approximating the earth as an oblate ellipsoid (a squashed, stretched sphere). The origin of the ellipsoid is related to the datum. • Old way: different parts of the world use different origins for the ellipsoid to get the “best fit” in their region. A lot of paper topo maps assume NAV 27, a datum that is meant to give the most accurate “fit” to the earth in North America. This datum made the ellipsoid go through a point on the surface of Kansas. • Newer: there is an earth-centered ellipsoid, WGS 84; improvements in calculation speed allow for accurate calculations, especially for the elevation (z) via geoid approximations.

More datum stuff Old way (like NAD 27) New way (like WGS 84) The “geoid” is truer representation of sea level – the “equipotential” surface. If you could dig thin, deep canals in a grid across the surface of the earth, water in the canals would settle to the geoid. All modern GPS have a pretty good geoid approximation as a “correction” to the ellipsoid.

What you should have in addition to GPS • Always, always, carry extra batteries. If you have a handheld GPS that takes AA: consider carrying a pair of Li metal batteries, wrapped individually in plastic. They are light, have long shelf life, act well in temperature extremes. For a smartphone, carry a backup battery, perhaps built into the case. • Carry a small, but good compass, and paper map if you can. You may be in a deep, winding canyon, where the compass will still work. GPS doesn’t work when there is no view of the sky. • Most GPS (handheld or smartphone) have a barometric altimeter; if yours does not, consider an altimeter watch.