AirTalk.org Aviation discussions newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  GPS accuracy          AirTalk.org Forum Index -> Sailplanes View previous topic :: View next topic   Author Message googlebart2000@yahoo.com Guest Posted: Thu Jan 13, 2005 5:09 pm    Post subject: GPS accuracy This question is motivated by the recent discussion about deriving polars from flight logs. We know that GPS fixes have some amount of (absolute) error. What is their accuracy relative to each other? In other words, when I'm analyzing flight logs, how accurately can I calculate something like sink rate using just the GPS fixes ("B" records)? I'm thinking about taking an early morning tow sometime this winter when the pressure is high and the air is stable (at least vertically) so I can calculate my actual polar, but I don't want to waste time and money on this if the flight log isn't good enough. (And a few cockpit notes of course -- IAS at various times, etc). Back to top boblepp@aci.on.ca Guest Posted: Thu Jan 13, 2005 9:57 pm    Post subject: Re: GPS accuracy Prove the altitude variance to yourself, stand still and watch the altitude change slowly up and down. It can really move around, maybe enough to make a GPS polar useless. 1 meter per second change while dead still is seen frequently on my GPS. Bob [email]googlebart2000 (AT) yahoo (DOT) com[/email] wrote: Quote: This question is motivated by the recent discussion about deriving polars from flight logs. We know that GPS fixes have some amount of (absolute) error. What is their accuracy relative to each other? In other words, when I'm analyzing flight logs, how accurately can I calculate something like sink rate using just the GPS fixes ("B" records)? I'm thinking about taking an early morning tow sometime this winter when the pressure is high and the air is stable (at least vertically) so I can calculate my actual polar, but I don't want to waste time and money on this if the flight log isn't good enough. (And a few cockpit notes of course -- IAS at various times, etc). Back to top googlebart2000@yahoo.com Guest Posted: Thu Jan 13, 2005 11:53 pm    Post subject: Re: GPS accuracy Ah. I knew that. I'd have to use the pressure altitudes from the B records. But these have to be adjusted .. the OAT must be recorded at the various altitudes and used to adjust the pressure altitudes to SL/standard atmosphere. Can anyone fill me in on how to do that? Back to top Papa3 Guest Posted: Fri Jan 14, 2005 2:35 pm    Post subject: Re: GPS accuracy "But these have to be adjusted .. the OAT must be recorded at the various altitudes and used to adjust the pressure altitudes to SL/standard atmosphere. Can anyone fill me in on how to do that?" Why? You're not concerned with absolute (True) altitude; just a delta between contiguous points. As long as you're flying through a sharp frontal boundary during your measurements, I can't see how this will matter. It's been a while since I actually did these manually, but Rho (density) cancels out in a ratio of Lift to Drag, no? L = 1/2RhoV2 CL S D = 1/2 Rho V2 CD S In any case, there are a number of places to get the gas law equations to convert to ISA. Google on ISA conversion formula or somesuch - I got several. P3 D= Back to top Papa3 Guest Posted: Fri Jan 14, 2005 2:40 pm    Post subject: Re: GPS accuracy Should have writtent "NOT flying through a sharp frontal boundary..." Back to top prep@prep.synonet.com Guest Posted: Tue Jan 18, 2005 6:29 pm    Post subject: Re: GPS accuracy [email]googlebart2000 (AT) yahoo (DOT) com[/email] writes: Quote: This question is motivated by the recent discussion about deriving polars from flight logs. We know that GPS fixes have some amount of (absolute) error. What is their accuracy relative to each other? In other words, when I'm analyzing flight logs, how accurately can I calculate something like sink rate using just the GPS fixes ("B" records)? A standard logger is not much chop. You really need to record raw pseudo-range data, and have a unit at a fixed place as well. Then with post processing you can get to a few feet with good single frequency units, and to a hand full of cm with a dual frequency one. I've heard mutterings of mm accuracy with P-code over a few meters. Get a reference on GPS surveying and look at the tollerence they quote. Quote: I'm thinking about taking an early morning tow sometime this winter when the pressure is high and the air is stable (at least vertically) so I can calculate my actual polar, but I don't want to waste time and money on this if the flight log isn't good enough. (And a few cockpit notes of course -- IAS at various times, etc). -- Paul Repacholi 1 Crescent Rd., +61 (0 9257-1001 Kalamunda. West Australia 6076 comp.os.vms,- The Older, Grumpier Slashdot Raw, Cooked or Well-done, it's all half baked. EPIC, The Architecture of the future, always has been, always will be. Back to top Display posts from previous: All Posts1 Day7 Days2 Weeks1 Month3 Months6 Months1 Year Oldest FirstNewest First         AirTalk.org Forum Index -> Sailplanes All times are GMT Page 1 of 1   Jump to: Select a forum Aviation----------------alt.aviationAviation safety EnhancementPlanes, gliders and flying in AustraliaAviation et pilotage (Français)rec.aviationAviation (Aerobatics)Hot air, gas balloons and airshipsHang-gliding, paragliding, foot-launched flightAircraft Design & BuildingInstrument Flight RulesAviation (Classifieds)Military AircraftNaval AviationAviation (Miscellaneous Topics)Owning AirplanesAviators (General Discussions)Aviation related productsAircraft RestorationHelicopters and rotary wing aircraftsFlight SimulationSailplanesAviation (Students)Light aircraftFlying related activities in the UK Airports----------------alt.airportsEdinburgh Airport Glasgow Airport Humberside Airport Luton Airport Aberdeen Airport Glasgow Prestwick Airport  You cannot post new topics in this forum You cannot reply to topics in this forum You cannot edit your posts in this forum You cannot delete your posts in this forum You cannot vote in polls in this forum Powered by phpBB © 2001, 2006 phpBB Group SEO toolkit © 2004-2006 webmedic.