-----Original Message----- From: Ron Rogers [mailto:rcrogers@annapolis.net] Subject: Re: GPS & autopilots (Long) I had the first model of the Micrologic GPS. During the Persian Gulf Crisis Now I have an old Garmin and some questions. Why does Leica claim greater accuracy for its GPS as opposed to WAAS GPSs? In fact, why do they charge over $600 to add DGPS input to these "wonderful" Leicas? Thanks in advance, Ron Rogers Arild replies: One thing we tend to forget in these protracted discussions on GPS. Most of us use cheap consumer product we can readily get, whereas the military, Coast Guard and commercial airlines use models that cost thousands of dollars more. Do you actually believe there is absolutely no difference between a $200 Garmin sold at west Marine and the FAA certified Garmin porducts sold for aircraft use? The topic of filtering was mentioned. Some receivers can adjust the rate, others cannot. Raw GPS data displayed at the fastest possible speed would look like a jumble. Mask angle also plays a part. An aircraft at 40,000 feet is miles away from any interference and much of the atmospheric disturbance that affects a receiver on the ground. Military grade receivers use two discrete frequencies, consumer grade units use one. The software for a high end commercial/industrial unit is much more sophisticated than the cheap consumer grade products. Error detection and self correction is a very high priority in such softeware. There are numerous factors that differentiate our low end consumer products from high end commercial/survey/military grade units. Lastly, this thread began when someone asserted that the compass was an anachronism and would soon be replaced with a purely GPS based device for heading information. Any autopilot must have a positional reference which is many times faster than what most consumer GPS displays can manage. Furthermore, in the present world, all marine instruments are linked via a NMEA data port; which operates at one sentence per second. Most if not all data is filtered to give a rolling average which is many seconds long. We are now seeing rate gyros in combination with fluxgate compasses replace floating card compasses, BUT these are still not pure GPS data sources. These hybrid compasses have a data rate about ten times as fast as a normal NMEA data protocol. While the display for human use can use a once persecond update, the direct link to an autopilot is faster. GPS compasses have been experimented with using survey grade GPS ( at $60,000 each ) and requires two antenna located 10 meters apart. The installation cost on a vessel amounts to several thousands of dollars. This not practical or even possible on small recreational vessels. Nor are these suitable for driving an autopilot. They were used for establishing offshore baselines during hydrographic surveys lasting many days. So; while all sorts of wonderful things are possible with GPS, ( no restriction on cost ) the reality is, cheap consumer models are not capable of doing much of it. Comparing a $200 West Marine special with the latest MIL spec WAAS - GPS used by commercial airlines or the military is utterly futile. Cheers Arild

At 07:38 PM 4/29/01 -0700, you wrote: >. Why does Leica claim greater GPS does not know where it is going and never will, even when it is "going". It only knows where it was going and to know that it had to be "going" somewhere. If it ain't going, then... If ground speed is zero, GPS has NO direction. If you hit current strong enough to neutralize your ground speed to zero... the GPS will not have a direction! But, the compass may not be usable someday, when the magnetic field drops to zero. Which it might do before it switches polarity. The North Pole becomes the South. Remember what the the Joker (in Batman the movie) told Lt. Ekhart, "think about the future". And then he put him out of his misery. Capt. Mike Maurice Near Portland Oregon.

. Why does Leica claim greater > accuracy for its GPS as opposed to WAAS GPSs? In fact, why do they charge > over $600 to add DGPS input to these "wonderful" Leicas? > > Thanks in advance, > Ron Rogers Leica makes a premium GPS. They use a combination of a good antenna, very good rf, and some fast and interesting electronics to perhaps double the accuracy of a $100. Garmin. For this they charge a kilobuck or so. So if you want 40 cms instead of a meter or so go to Leica. Is this really relevant in the recreational boating world? > Arild replies: > One thing we tend to forget in these protracted discussions on GPS. > Most of us use cheap consumer product we can readily get, whereas the > military, Coast Guard and commercial airlines use models that > cost thousands > of dollars more. Do you actually believe there is absolutely no difference > between a $200 Garmin sold at west Marine and the FAA certified Garmin > porducts sold for aircraft use? I certainly believe there is a difference. One unit is STCd and the other is not. That alone is worth a grand or more. Does it improve performance? Nahh Does it have better human factors? Nahh. But it is certified for aircraft use and does have the liablility insurance that goes with such a use. To work the general question - Is the normal commercial gear better than the consumer stuff? Sure it is. Is it better by a factor that reflects the price multiplier? Not a chance. You can double the accuracy by paying 5 to 10 times as much. > > The topic of filtering was mentioned. Some receivers can adjust > the rate, > others cannot. Raw GPS data displayed at the fastest possible speed would > look like a jumble. Mask angle also plays a part. An aircraft at 40,000 > feet is miles away from any interference and much of the atmospheric > disturbance that affects a receiver on the ground. Military grade > receivers use two discrete frequencies, consumer grade units use one. The > software for a high end commercial/industrial unit is much more > sophisticated than the cheap consumer grade products. Error detection and > self correction is a very high priority in such softeware. > There are numerous factors that differentiate our low end > consumer products > from high end commercial/survey/military grade units. This is getting a little far fetched. It is true that two frequencies are better than one. As one gets to sub-meter accuracy two frequencies may well be required. As this relevant to recreational boating? Virtually all commercial units for non-military and non-surveying use single frequency receivers. Fancy antennas can help a lot by avoiding multi-path. Better rf can substantially improve performance. There are some games that can be played with the processing electronics. All of this gets you from 4 or 5 meters down to less than one. A number of experimenters in this field use certain Motorola units that have all of the bells and whistles for the clever stuff and cost less than $250. Virtually all of these techniques can be applied to consumer equipment and likely will be over the next years. Error detection is pretty close to the simplest thing going with WAAS. Available to even the cheapest consumer set. The question is whether or not a boater would really care. So you can get from 4 or 5 meters down to 1 with good commercial gear. Is this relevant? > > Lastly, this thread began when someone asserted that the compass was an > anachronism and would soon be replaced with a purely GPS based > device for > heading information. Any autopilot must have a positional > reference which > is many times faster than what most consumer GPS displays can manage. > Furthermore, in the present world, all marine instruments are > linked via a > NMEA data port; which operates at one sentence per second. Most > if not all > data is filtered to give a rolling average which is many seconds long. > We are now seeing rate gyros in combination with fluxgate compasses > replace floating card compasses, BUT these are still not pure GPS data > sources. > These hybrid compasses have a data rate about ten times as fast > as a normal > NMEA data protocol. While the display for human use can use a once > persecond update, the direct link to an autopilot is faster. GPS > compasses have been experimented with using survey grade GPS ( at $60,000 > each ) and requires two antenna located 10 meters apart. The installation > cost on a vessel amounts to several thousands of dollars. This > not practical > or even possible on small recreational vessels. > Nor are these suitable for driving an autopilot. They were used for > establishing offshore baselines during hydrographic surveys lasting many > days. Boy Arild you really are shilling for the buggy whip manufacturers. Yup them automobiles will never be as cheap or reliable as a good horse - and the horse can find its way home if you pass out drunk. The "someone" asserted that the magnetic compass will soon become an anachronism and that children born in the present will have no knowledge of the compass as a real tool. Let's see - how many years ago was it that 5 meter GPS was science fiction? In this case I think the art is easy - but the market for accurate compasses is relatively small. I suspect they will end up doing it in automobiles before boats. GPS is presently too slow for the inner loop in an autopilot - a fact I have repeatedly noted. There is however nothing inherent in GPS that makes this true. Just wait a while. Protocols are phased out as they reach the end of their usefulness - current NEMA is about there. There really are a number of others that will suffice for this application. > > So; while all sorts of wonderful things are possible with GPS, ( no > restriction on cost ) the reality is, cheap consumer models are not > capable of doing much of it. > Comparing a $200 West Marine special with the latest MIL spec WAAS - GPS > used by commercial airlines or the military is utterly futile. > > Cheers > > Arild Again Arild remember the buggy whips. I personally enjoyed the merry chase when we went after core memory with semiconductors - there were a number of experts who testified we could never reach the required reliability. I remember the idiot who suggested that there was nothing useful we could do with semiconductors with one million transistors - a level now exceeded by household appliances. I remember trying to replace the mechanical copy counter in a copier with an electronic one. Took 5 years as the powers that be pointed out that there was no way an electronic counter could be as reliable as the tried and true mechanical. The outcome of all this is remarkably clear and certain. Only the time scale is in doubt. The doubters may build and defend numereous fortifications - it will have no impact on the outcome. Jim

Mike Maurice wrote: > GPS does not know where it is going and never will, even when it is > "going". It only knows where it was going and to know that it had to be > "going" somewhere. If it ain't going, then... > If ground speed is zero, GPS has NO direction. If you hit current strong > enough to neutralize your ground speed to zero... the GPS will not have a > direction! I agree completely, Mike. The problem here is that engineers and navigators speak different languages. "Heading" is only loosely correlated with "track angle" or "course over ground" in vehicles that move in the air or on the water. Now, navigating a tank in the Iraqi desert is a different matter - perhaps that is the application where this confusion arose. Without a mag or gyro heading sensor and a log, we could not use the GPS data to compute set and drift. My magnetic compass is the last instrument I would dispense with, though it is true that you could get from A to B with a GPS alone by hand steering so as to maintain a constant bearing to waypoint B. But when everything else fails, the old reliable compass will still work. Tony Marshall K39 "Growler" Lying Hilton Head, SC