#flight instruction

So, GPS works by having whats called a constellation of satellites, there's about 31 operational ones as of right now, and usually you can see around 5 at once. What a satilite really honest to god does is just go "hey. its 4:45:54343243 zulu time right now, and I'm exactly right here in space." and then sends that signal down to a GPS receiver which goes "okay well i think its 4:45:54343563 so that means im PROBABLY around Here."

so the receiver gets a big 3d circle that it goes "im anywhere in this specific area" but thats a BIGASS AREA right? so.. how do we narrow this down?

well fuck it we know where this satilite is tellig us we are. so we're probably around in this area. but thats still a big wide area, not super useful right now

okay well theres like 6 in view most of the time (minimum of 5 in view at one time by design, but 6 is an average)

Now we're fuckin talking.

so we have this teeeny tiny little area (of like. 10-20 meters) that All sattilites agree we're in, so that must be in this little area here, right? of course!

we have about 6 satellites CURRENTLY talking to the receiver, which means that we have a 3d position of where we are, we need at minimum 4 satellites in reception to get 3d positioning (longitude, latitude, and altitude), and a minimum of 3 satellites for 2d positioning (all but altitude)

Next up, is RAIM, Receiver Autonomous Integrity Monitoring, which is a fancy way of saying "I watch all satellites in view and make sure they all make sense" and Exclusion, which does exactly what you think. Says "this satillte is saying something weird, i'm ignoring it." you need a minimum of 5 satellites for RAIM, or 4 + an altimeter that talks to the receiver, and 6 (5 + a baro altimeter) for both fault detection And exclusion

next up is my favorite bitch of all, WAAS, or Wide Area Augmentation System. WAAS works by having a bunch of little mini receivers scattered across the US, which all know exactly where they are coordinate wise, and then send the info they get from the satellites to a big building which uses PFM to figure out how far off the satellites signals are from actuality, and how to correct this data. This correction data is then sent to a WAAS Satellite, (there are 3 being used in the US, one over the east side, one over the west side, and one over Hawaii) which sends that data to a WAAS receiver, which corrects the original data, to give you about 3-5 meters of accuracy.