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#21
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"Derrick Steed" wrote in message ... Isn't there some form of flux gate compass which can be used? The GPS's we use originated in the maritime market (after NMEA is a maritime standard), surely there is a flux gate compass with NMEA output. Rgds, Derrick. A flux gate compass is just another kind of magnetic compass. It eliminates the jiggling of a pivoting compass card but it is still subject to installation, variation and dip errors. The dip error means that it only provides useful data when the wings are level. Still, they are probably cheaper than GPS compasses. GPS compasses are elegant in that they point to true north. Bill Daniels |
#22
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Dave,
Posting to the group in hopes you answer might serve others... My SN-10B serves me well. However, I am having a small problem based on airspeed error. I have noted on ridge flights that after prolonged high-speed flight without turns, the instrument starts counting up the wind speed. On one leg, I saw a 25 knot increase. Clearly I need to fix the asi error table, but if the SN-10 is counting up, which way do I need to go in the table? OC |
#23
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Marine users have been using electronic compasses for some time. These are
fluxgate compasses and most have NEMA output so what you want is not impossible. The marine ones are expensive circa $400 but there are a new bread of digital compasses some of which are included in GPS handhelds but I cannot see if any have NEMA outputs. In addition there ae some aviation sensors coming n the market (MEMS) (see http://www.willow.co.uk/html/heading.html) You will need a NEMA sentance output of HDM or better HDG from such a device and a NEMA concatenator to meld the NEMA output into the IPAQ datastream. On the Plus side You would have a compass that had little lag - easier to roll out of thermals onto heading You would get your wind data On the downside Expensive as one-off Yet more battery drain Device would need A solid state memory for an electronic version of deviation card Device would have to be installed in tail as they are V sensitive You only get Magnetic heading from device - GPS is essentially True and if flying in areas of local magnetic anomalies you will get strange results. Given most gliders fly Track as opposed to bearing it would be easier simply to note magnetic heading & speed once on track and make sure that the two remain constant if they diverge then your wind direction has changed and you can either do the mental maths (or use a flight computer) or put in a 360. What we are all really waiting for is a Touch Single panel display system that would include something like this along with a Head-Up display. But the market is just too small to make something like this a real economic proposition at this time along with the associated battery drain. You would be lucky to sell more than 500 units. rgds stephen "Bill Daniels" wrote in message news:_y8Ac.53766$0y.15362@attbi_s03... Dave, can it give highly accurate, real-time wind data, second by second on a straight one hour glide? My last flight showed this problem during a long glide. For 30 miles the wind data showed SW winds at 20 knots but then I noticed the glider drifting to the west. Two 360 turns and the computer showed wind at 090 at 10 knots. When I crossed the wind shift line I had a chance to change strategy. 30 miles later when the wind error became obvious it was too late. More and more I think we need very accurate real-time wind data with no requirement to be constantly changing heading. A cheap, reliable heading sensor would make this possible. Bill Daniels "Dave Nadler YO" wrote in message m... Bill - The heading is not required. In the ILEC SN10 we do this without heading, and it normally has the wind by the time you're off tow. Pilot feedback and measurements indicate that we do it quite accurately. We just use TAS and GPS ground track over multiple observations (TAS requiring accurate IAS, press, temp). Best Regards, Dave "YO" "Bill Daniels" wrote in message news:gw%zc.59643$HG.25574@attbi_s53... I've been analyzing the IGC files from my flights and comparing them to others. Generally, in good conditions, I'm circling less than 25% of the time. Other, far better pilots than I, are circling less than 15% of the time. This means that wind data that depends on the glider flying circles gets way out of date during the long glides. OK, "S" turns can give the computer a chance to compute wind if you remember to fly them but there are still a lot of long, straight glides. To compute real-time wind data we need Groundspeed, Ground Track (any GPS gives these)True Airspeed (Some computers do this if they have OAT, Pressure Altitude and IAS) and accurate heading data. Heading data it the tough nut to crack. So, how do we get Heading? One way might be to put a GPS antenna in each wing and look at the carrier phase difference. If the computer saw a constant track, it could assume the wings are level and determine a heading value. This heading value might update an aerospace quality MEMS heading gyro that would maintain a flow of heading data to the glide computer during times when the wings were banked. With good heading data we could get uninterrupted high quality wind data regardless of the gliders' maneuvering or lack of it. OK, RAS gageteers, how about it? Bill Daniels |
#24
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Hi Chris - Clearly you need our compass option ;-)
Give a call over the weekend to discuss your issue, and also to tell me where you've located such a long ridge, Best Regards, Dave "YO" (Chris OCallaghan) wrote in message . com... Dave, Posting to the group in hopes you answer might serve others... My SN-10B serves me well. However, I am having a small problem based on airspeed error. I have noted on ridge flights that after prolonged high-speed flight without turns, the instrument starts counting up the wind speed. On one leg, I saw a 25 knot increase. Clearly I need to fix the asi error table, but if the SN-10 is counting up, which way do I need to go in the table? OC |
#25
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One thing that a lot of the UAV guys are doing, is to use a magnetic
heading sensor coupled with a MEMS gyro, or if more money is available, a FOG. The magnetic sensor can be used to periodically determine heading and to correct for signal drift from the gyro. For instance, the gyro can be used as the primary source for heading info, while the magnetic sensor is used during long, stable glides. Very similar to how the DG and compass are used in power planes. Of course, the gyro would have to be either mechanically gimbaled to remain parallel to the horizon, or, alternatively, several gyros on different axes can be used together to determine actual heading change, rather than yaw rate. Using some basic Kalman filtering, it should be possible to get fairly accurate (5-8 degree) heading info. This will, of course, be magnetic heading, and subject to the usual accuracy issues. However, it might be possible to correct the data to true heading by loading a map of lines of deviation and magnetic anomalies (which can also be fed into the Kalman filter). Simply feed the wind component data from the GPS, the raw heading info from the magnetic sensor, and the stabilized data from the gyro into a Kalman filter, and pretty stable and accurate heading data should be available. Definitely do-able. Let's do it! :O) BTW, check out www.rotomotion.com. These guys have several fairly cheap AHRS', as well as several cheap IMU's. They've also published a bunch of open source code on Sourceforge under the Autopilot project http://autopilot.sourceforge.net/ with an effective Kalman filter to integrate GPS, IMU, and magnetic sensor data. - Robert |
#26
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Derrick Steed wrote:
Isn't there some form of flux gate compass which can be used? The GPS's we use originated in the maritime market (after NMEA is a maritime standard), surely there is a flux gate compass with NMEA output. Rgds, Derrick. Hi, Bill. The Garmin eTrex Vista includes GPS, barometric altitude, and electronic compass. IIRC all the above data are output on the serial interface as NMEA or Garmin sentences. Can you get by with +- 2 degrees heading accuracy? No idea how big the dip error is when wings banked, but maybe your flight computer could be smart enough to use the compass when straight & level and switch to turning-base wind calculation when banked? http://www.garmin.com/products/etrexVista/spec.html Dave |
#27
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"Dave Houlton" wrote in message ... Derrick Steed wrote: Isn't there some form of flux gate compass which can be used? The GPS's we use originated in the maritime market (after NMEA is a maritime standard), surely there is a flux gate compass with NMEA output. Rgds, Derrick. Hi, Bill. The Garmin eTrex Vista includes GPS, barometric altitude, and electronic compass. IIRC all the above data are output on the serial interface as NMEA or Garmin sentences. Can you get by with +- 2 degrees heading accuracy? No idea how big the dip error is when wings banked, but maybe your flight computer could be smart enough to use the compass when straight & level and switch to turning-base wind calculation when banked? http://www.garmin.com/products/etrexVista/spec.html Dave The "compass" on many handheld GPS units really shows ground track although I think some of them have electronic flux gate compasses. To compute true wind data you need four inputs: True Heading, True Airspeed, Ground Speed and Ground Track. We only need to add heading to get true wind info. There are fluxgate and other magnetoresistive sensors that use the earth magnetic field to determine heading. The $500 NMEA marine units would probably work but they suffer from all the problems that any magnetic compass has. There are other clever devices that detect the magnetic field in three axes and, with the GPS position data and a look-up table, subtract the variation and dip errors to output true north. I still like the idea of a double-headed GPS receiver with two antennas a meter apart on the fuselage centerline. These should be good to a half degree accuracy. If you add MEMS gyros for a "coast through" in the event of a GPS signal interruption, you have an elegant heading sensor. Bill Daniels |
#28
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"Dave Houlton" wrote in message ... Derrick Steed wrote: Isn't there some form of flux gate compass which can be used? The GPS's we use originated in the maritime market (after NMEA is a maritime standard), surely there is a flux gate compass with NMEA output. Rgds, Derrick. Hi, Bill. The Garmin eTrex Vista includes GPS, barometric altitude, and electronic compass. IIRC all the above data are output on the serial interface as NMEA or Garmin sentences. Can you get by with +- 2 degrees heading accuracy? No idea how big the dip error is when wings banked, but maybe your flight computer could be smart enough to use the compass when straight & level and switch to turning-base wind calculation when banked? http://www.garmin.com/products/etrexVista/spec.html Dave The "compass" on many handheld GPS units really shows ground track although I think some of them have electronic flux gate compasses. To compute true wind data you need four inputs: True Heading, True Airspeed, Ground Speed and Ground Track. We only need to add heading to get true wind info. There are fluxgate and other magnetoresistive sensors that use the earth magnetic field to determine heading. The $500 NMEA marine units would probably work but they suffer from all the problems that any magnetic compass has. There are other clever devices that detect the magnetic field in three axes and, with the GPS position data and a look-up table, subtract the variation and dip errors to output true north. I still like the idea of a double-headed GPS receiver with two antennas a meter apart on the fuselage centerline. These should be good to a half degree accuracy. If you add MEMS gyros for a "coast through" in the event of a GPS signal interruption, you have an elegant heading sensor. Bill Daniels |
#29
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"Dave Houlton" wrote in message ... Derrick Steed wrote: Isn't there some form of flux gate compass which can be used? The GPS's we use originated in the maritime market (after NMEA is a maritime standard), surely there is a flux gate compass with NMEA output. Rgds, Derrick. Hi, Bill. The Garmin eTrex Vista includes GPS, barometric altitude, and electronic compass. IIRC all the above data are output on the serial interface as NMEA or Garmin sentences. Can you get by with +- 2 degrees heading accuracy? No idea how big the dip error is when wings banked, but maybe your flight computer could be smart enough to use the compass when straight & level and switch to turning-base wind calculation when banked? http://www.garmin.com/products/etrexVista/spec.html Dave The "compass" on many handheld GPS units really shows ground track although I think some of them have electronic flux gate compasses. To compute true wind data you need four inputs: True Heading, True Airspeed, Ground Speed and Ground Track. We only need to add heading to get true wind info. There are fluxgate and other magnetoresistive sensors that use the earth magnetic field to determine heading. The $500 NMEA marine units would probably work but they suffer from all the problems that any magnetic compass has. There are other clever devices that detect the magnetic field in three axes and, with the GPS position data and a look-up table, subtract the variation and dip errors to output true north. I still like the idea of a double-headed GPS receiver with two antennas a meter apart on the fuselage centerline. These should be good to a half degree accuracy. If you add MEMS gyros for a "coast through" in the event of a GPS signal interruption, you have an elegant heading sensor. Bill Daniels |
#30
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I have no idea how three copies of the previous post got sent. I swear I
hit the send button only once. Bill Daniels |
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