Light is slightly slower in a medium, the "vacuum" value should be divided by the index of refraction. Air is about 1.0003. If Pat wants the next digit we need dry bulb, dew point, and pressure.
________________________________ From: Michael Payne <metricmik...@gmail.com> To: U.S. Metric Association <usma@colostate.edu> Cc: U.S. Metric Association <usma@colostate.edu> Sent: Thu, April 22, 2010 8:53:16 AM Subject: [USMA:47234] Re: Air flight altitudes in meters Where on earth did this come from? Radar is just radio and travels at the speed of light which if memory serves me is 300 000 km/s (exactly 299 792 458 m/s) regardless of the distance unit used. So the distance travelled would be (1852 x 2) m or 3704 m for Nautical miles and 2000 m for Kilometers. I think this is 12,3 µs for nautical miles and 6,67 µs for kilometers. Mike ----- Original Message ----- From: Pat Naughtin >To: Michael Payne >Cc: U.S. Metric Association >Sent: Wednesday, 21 April 2010 22:00 >Subject: Re: [USMA:47211] Re: Air flight altitudes in meters > >Dear Mike, > > >Can you help me with the places where I have xxxx? > > ># >Miles of radar in seconds > > >The time required for a radar signal to travel from the transmitter a distance >of one mile to an object, and then return to the receiver. Both ordinary >(statute) and nautical miles are used: the radar statute mile is about 10.8 >microseconds (µs) and the radar nautical mile is about xxzx microseconds. An >equivalent SI unit would be the radar kilometre, which is about XXZX >microseconds (µs). ># > > >Cheers, >Pat Naughtin >Author of the ebook, Metrication Leaders Guide, that you can obtain >from http://metricationmatters.com/MetricationLeadersGuideInfo.html >PO Box 305 Belmont 3216, >Geelong, Australia >Phone: 61 3 5241 2008 > > >Metric system consultant, writer, and speaker, Pat Naughtin, has helped >thousands of people and hundreds of companies upgrade to the modern metric >system smoothly, quickly, and so economically that they now save thousands >each year when buying, processing, or selling for their businesses. Pat >provides services and resources for many different trades, crafts, and >professions for commercial, industrial and government metrication leaders in >Asia, Europe, and in the USA. Pat's clients include the Australian Government, >Google, NASA, NIST, and the metric associations of Canada, the UK, and the >USA. See http://www.metricationmatters.com/ to subscribe. > >On 2010/04/22, at 00:21 , Michael Payne wrote: > > >>All large modern aircraft built since around 1975 have Inertial Reference >>Systems (IRS) for Navigation. Nowadays IRS is a Laser Ring Gyro with no >>moving parts. Since around 1995 GPS came on the scene and the IRS get its >>initial position from a GPS or pilot input source, once aligned, the IRS >>measures motion and acceleration in 3 axis (plural?) and will determine its >>own position. This position information is fed into 3 Flight Management >>Computers (FMS) that store the route of flight as Latitude/Longitude >>coordinates. The flight management computers receive input from 3 IRS and 2 >>GPS receivers and 2 VHF Distance Measuring Equipment receivers (DME), >>updating using the distance from 2 or more known DME sources on the ground to >>triangulate position using lateral and altitude information. The FMS will >>give preference to GPS position, then FMS position then DME position. >>Additionally each FMS compares its position to that of the other two, if one >>goes beyond a certain distance from the other two it's voted out of the computation and the other 2 FMS are used only. The theory is that if GPS and DME are lost over the oceans the aircraft position would be known to the FMS based only on IRS information which has a known drift rate. Before GPS came along IRS was the only source of position over the oceans until ground stations could be picked up and the position updated based on DME/DME. It's a very sophisticated and expensive piece(s) of equipment. >> >>You could say that nowadays GPS is the primary source of navigation data with >>IRS as a backup. Large aircraft also use the IRS as the sole source of >>attitude information for the pilots (there are 3 of them), there is no >>separate attitude or directional gyro, there is no longer a magnetic compass >>either, the FMS computes position and required track based on True North, >>then adds (or subtracts) the local magnetic variation to give a magnetic >>track, all of this from the database used for navigation. >> >>It is possible to look up the GPS altitude but it's not used for any purpose. >>We still use the same pressure level converted to a Flight Level (1013 hPa) >>or an altitude MSL based on a local altimeter setting (QNH). The change from >>Flight Level to Altitude varies worldwide by country, in the North America >>it's 18000 ft, Mexico 20000 ft. Europe 3-5000 ft above the airport, Australia >>15000 ft, Africa 3-5000 ft above the airport. >> >>It's been my experience that GPS is normally accurate to around 5 m >>laterally, there are times when there are insufficient satellites to use for >>a GPS approach and the pilot is warned. The navigation is so accurate that >>ICAO recommends aircraft offset to the right of track when out of radar >>contact in case someone is coming the other way on the same track at the same >>altitude. This happened in Brazil about 4 years ago when 2 aircraft coming in >>opposite >>directions collided, the winglet of one apparently sliced the wing of the >>other aircraft which crashed, one landed OK. >> >>Mike Payne >> >> >>----- Original Message ----- From: <mech...@illinois.edu> >>To: "U.S. Metric Association" <usma@colostate.edu> >>Sent: Monday, 19 April 2010 03:32 >>Subject: [USMA:47172] Re: Air flight altitudes in meters >> >> >> >> >>> >>What is the contribution of GPS data to the navigation of large aircraft? >>> >>Do GPS data dominate barometric data? >>> >> >>> >>---- Original message ---- >>> >>Date: Sun, 18 Apr 2010 18:10:10 -0700 (PDT) >>>> >>From: "John M. Steele" <jmsteele9...@sbcglobal.net> >>>> >>Subject: [USMA:47170] Re: Air flight altitudes in meters >>>> >>To: "U.S. Metric Association" <usma@colostate.edu> >>>> >>Cc: "U.S. Metric Association" <usma@colostate.edu> >>>> >> >>>> >> Actually, it is nominally based on height above >>>> >> sea level. Sitting on the runway, with altimeter >>>> >> correction dialed in, it will read the published >>>> >> height of the runway above sea level. >>>> >> >>>> >> At cruise levels, no altimeter correction is used >>>> >> and reading is called flight level. It is the >>>> >> height above sea level IF sea level were at 15 °C, >>>> >> 101.325 kPa, and a lapse rate of -6.5 K/km to the >>>> >> stratosphere (11 km), and zero lapse rate above that >>>> >> to 20 km. Further it it uses a height variable >>>> >> called geopotential height, the height that would be >>>> >> true if gravity were constant with height. There is >>>> >> a transformation between that and geometric height >>>> >> in the standard. >>>> >> >>>> >> ------------------------------------------------ >>>> >> >>>> >> From: James R. Frysinger <j...@metricmethods.com> >>>> >> To: U.S. Metric Association <usma@colostate.edu> >>>> >> Cc: U.S. Metric Association <usma@colostate.edu> >>>> >> Sent: Sun, April 18, 2010 8:47:07 PM >>>> >> Subject: [USMA:47168] Re: Air flight altitudes in >>>> >> meters >>>> >> >>>> >> Altimeters work off of atmospheric pressure >>>> >> readings, Pat. But the readout is in terms of height >>>> >> above terrain. So assignments and reports are always >>>> >> in length units. No human pressure to altitude >>>> >> correlation procedures are used. >>>> >> >>>> >> Jim >>>> >> >>> >> >
