CompuServe Thread

Hi, Bill > I find it a little disconcerting when standing still and the Scout is in > the navigation mode, it decides to start "moving" at about 5 or 6 mph > until it gets to some new location. How far does it 'go'? Mine rarely exceeds 2.5 mph, and usually settles down after 100 feet or so. OTOH my Loran is constantly on the 'move'. 🙂 GPS trivia time: Radio station WWV in Fort Collins, CO transmits status information on the GPS 'constellation' every hour at 14 minutes after the hour. You can call (703) 313-5900 to request information about the system, or ask to be transferred to the recording (same info that WWV carries). I haven't tried it yet, but they supposedly have a BBS at (703) 313-5910, 300 to 14.4k bps, 8 None & 1. Cheers, Guy

Bill When your Scout moves like that just tie its legs together. Yeah, that is real pain. When that type of thing happens I just guessed it was the dithered signal being changed so that the error is changing. For that reason you can get a fix then watch it change, sometimes it will be stable for several minutes then start changing. For me last Sunday evening on the Salmon river, I got a reading, wrote it down, then just as I was going to turn it off both the lat and lon started decreasing and ended up with both nearly .2 seconds lower in a couple minutes (its a narrow canyon with only 3 satellites being read). That was a gross change equal to about 1200 and 800 feet difference from the first reading! The 55 degree positions and passing over anyone point twice a day would explain a few things. Although, is it really physically possible for 12 satellites to be in view and readable by some units as they claim in their ads, even 8 seems like a physical impossibiltiy unless you were high in the Rockies (or Mt. Everest). Lanny

I didn't see the original qustion re the " 2 week delay thing" but I presume that what is being discussed here is the availability of Precise Ephemeris data for enhanced processing. The delay there is about 2 weeks. What you get in real time is a position derived from satellite orbit parameters which the satellites themselves are broadcasting as part of the navigation message. This data is based on the last orbital parameter correction from the ground control segment, and is therefore based on extrapolated predictions from orbital measurements made up to the upload time. The tracking stations are continuously measuring the *actual * orbits of the satellites as they pass over. This data is logged and refined to produce a set of parameters which describe where the satellites *were* at various times, as opposed to where they * thought* they were at those times. You're quite right in saying that us surveyors can often wait a couple of weeks for refined ephemeris parameters. However, in real time using differential techniques plus 16,000 dollar multi-channel geodetic GPS receivers we routinely achieve better than the 30 feet ( at the 2 sigma level) which you guys seem to crave. There's nothing to stop you setting up a differential base station on a geodetic point to cover a local area where you might be doing a lot of work. All you need is another Scout and a telemetry system! I'll revert on the errors soon…BTW, I hope you guys appreciate this: I normally charge 600 dollars per day for teaching basic surveying and GPS techniques to geologists and geophysicists in the seismic industry <bg>. Rob

The position movement seen when standing still may be the result of either SA or of picking up a reflected signal from a particular satellite. If the Scout is in "navigation" mode, it probably "expects" you to be moving, so the jumps that you get within the normal spot accuracy of the system may serve to set it off tracking a false course. I'm not familiar with the Scout in any detail, but most Trimble receivers incorporate a Kalman filter (predictive) model into the positioning computation. You also have to bear in mind that the system has a particular quoted "accuracy". To get a "proper" fix on a static location you should really occupy it for a period of time, taking positions at frequent intervals. These positions will jump around a little. If you plot them out you'll get ( in 2 dimensions) a scatter plot which will usually have an elliptical shape, with size and orientation dependent on current satellite geometry. This set of positions will usually have a normal ( gaussian ) distribution. It is possible to compute the magnitude of the standard error at any orientation around the point from the size of the semi major and semi minor axes and their orientation. Unlike the standard error of a one dimensional set of observations, which identifies the limits within which around 68 percent of observations will fall, the SE value of a 2 dimensional data set indicates the limits within which 39 percent of observations will fall: ie, 61 percent of your fixes will fall *outside* the one standard error ellipse. ie, over 6/10s of your fixes taken will fall outside this ellipse, *but they are still perfectly valid fixes, since the observations falling in the *tails* of a gaussian distribution are just as valid as those falling towards the mean value. To get the size of ellipse within which 95 percent of your fixes will fall ( a fairly standard figure used as a confidence level in many applications), you need to scale up the axes of your original ellipse by a little under 2.5. This is a very empirical ( rather than theoretically rigorous) illustration, but basically, once you have established the size of your error ellipse, the fix can jump about within those ellipse boundaries as much as * it likes*… even from one end of the major axis to the other on subsequent fixes, and the receiver has still given you a *statistically valid* position. Even the odd *valid* fix ( about 5 percent of the total!) which falls outside the 95 percent ellipse boundaries must be allowed for. What this gives us, though, is not an *accuracy* for the fix… it is the *precision* of the fix…. ie, a measure of the spread of random errors within the observed system. This *precision* only accounts for random errors. Non-random errors such as biases in observation will not contribute. ie, we could have a very tight gaussian curve for a set of positions who's mean is nevertheless a long way from the "true" position due to an uncorrected bias ( such as observing a reflected range from one satellite). On the other hand, poor satellite geometry could give us a very low, wide gaussian curve who's mean is very close to the "true" position of a spot. Consequently, we have to add a factor called *reliability* to the factor called *precision* in order to obtain an estimate of the thing called *accuracy*. I don't plan to go into the statistics here, but reliability is basically a statistical determination of the likelyhood of being able to detect a gross (non-random) error of a certain size within a set of observed values. Many (most?) adverts and other literature actually quote the precision achievable with a particular instrument, rather than it's accuracy. In any event, precisions should always be quoted with a confidence figure attached ( ie, 95 percent, 2 sigma, 39 percent etc). Sorry about the lecture <g>. Most of the above may not be new to you, but I would say that 30 percent of my time is now spent explaining the above to people within the oil industry when they've been "sold a lemon" on positioning of a survey. You guys live or die by your own efforts, it seems, but in oil companies the advent of GPS has led to explorationists deciding that they no longer need surveyors, since they now have black boxes to replace them. I'm not complaining too much, though, since I'm now making more money rectifying geophysicist's screw-ups than I ever made in gathering and processing the initial data for them<bg>. Next installment will be on datums. Rob

Here begineth the lesson on datums: In the beginning was the *topography*, which was up and down all over the place, and not describable mathematically. And the topography was without form and mightily vexatious for surveyors, because it interfered with their measurements. And lo! the surveyors invented the *geoid*. The geoid took the form of the earth from which all the topography had been removed. And the *geoid* was an imaginary equipotential surface corresponding to the location of mean sea level if the earth was covered with a frictionless, limitless, tideless sea. And the surveyors called the geoid *geoid* because this word means *earth-shaped*. And the surveyors enlisted the assistance of the tribe of the rockhounds, to realise the dream of the geoid by chipping little bits of the topography away until, in the fullness of time, the geoid will become a real surface. And the shape of the geoid is like unto a pear, if one stretches the imagination a bit. But the surveyors found that lo! although this surface contained fewer lumps than the topography, it was still a bitch to calculate on. And it came to pass that eventually the great prophets at the DMA constructed a gravitational model of the geoid with 18,000 coefficients which was a representation of the geoid of that type known as *pretty fair*. This model was constructed to feed the appetite of the arcane beast known as SDI, but the great prophet in the White House forbade the release to the surveyors of more than the first few coefficients in this series, fearful as he was that any other prophet may be able to pinpoint the true location of the White House to a few metres and thus destroy his abode with accurately targeted fire and brimstone. Lacking enough of the wisdom of IBM, Amdahl and Cray, the surveyors were mightily vexed to find that a model containing even the first few coefficients despatched unto the people by the miserly and secretive prophets of the DMA was still a bitch to calculate, and so returned to a surface used for computational purposes by many previous generations of surveyors. And the name of this surface was *ellipsoid* also known unto certain brethren as *spheroid*, and many of the sons and daughters of the surveyors used the two terms interchangeably without perceptible problems. And the ellipsoid was without form and void, until some smart cookie created it by rotating the form of an ellipse about it's minor axis. Oh joy! This surface was found to be easy to calculate with in the grade of *dead cinch*. And the ellipsoid was completely defined by an axis of the type called *semi-major*, and a value called *eccentricity*. And the distance between two points on the ellipsoid was found to be calculatable using only the most basic spherical trigonometry of doubly curved surfaces. And the surveyors surveyed it, and were glad, for the mysteries of div, grad and curl, not to mention the calculus of tensors, were well beyond the capacity of most of them, for otherwise they would have been of the tribe of astrophysicists. And the joy of the surveyors, at first as sweet as the good wines of Lebanon, turned to dust in their mouths when they realised that the shape of this ellipsoid bore only passing resemblance to the shape of the geoid, and it's facility of calculation was therefore of the type known as *effectively useless*. And so the tribe of the surveyors fragmented across the globe. Prophets arose in each location. And each prophet measured the shape of the earth in his own location and derived an ellipsoid which closely modelled the shape of the geoid around the location of that prophet. And the sizes of these ellipsoids, and their eccentricities, were all different. And none of the ellipsoids had the same location in space, for all were designed to fit the geoid at a particular location and nowhere else. ….. To be continued!!! Rob

>As the GPS accuracy is only supposed to be +- 100 meters, my best >guess is that I had lucked out and the DOD had temporarily turned off SA >for the occasion. 🙂 Guy — I think they turned it off for me too a few weeks ago. I had stored a gold mine location using "over & up" and when I finally found it, the Scout said I had 74' to go. — Bill

SA is only applied to 1 or 2 satellites at a time: they just don't tell you which ones, and they jump it about. It's quite probable that you positioned yourself using uncorrupted data. NB: I've only just started looking at this thread (pity, really), but I see you all talking about accuracies, the need for good maps etc. Are you guys aware when using your maps that the GPS units, as a default, generally output positions in WGS 84 datum, and that your maps, in the US, are likely to be in NAD 27 datum, still, in most places? Thus plotting your position on a NAD map without selecting NAD output on your rx will put you out of position by around 150 metres. Rob

Guy One can get lucky. The geologist I was with during this last trip (just returned today) has a Garmin 50 that he has had for 3 years mounted on his dashboard. He was taking me to a new agate locality in Central Idaho and as we drove up to the spot he read off the coordinates and that stupid Garmin ("stupid" being a smartass comment meaning it was showing us up) came up reading exactly the same coordinates (to me hundredth second, it doesn't go to the thousandth). I stepped out of his Cherokee with my Scout and got readings that were different by about .03 and .05. I asked him to turn his back on and check and the reading came up similar to mine, but different by a few hundredths. Guess we were lucky when we drove up, but very impressive! As I understand the accuracy, it is supposed to be accurate to 10 m except when SA is screwing it up so that it is only within 100 m. Thus we never really know. I see that Campmoor has the Magellan Trailblzer down to $400 now. Lanny