This is an archive of CompuServe forum messages from 1985 to 1995, as saved in transcripts by John Foust.
Search Results (35 messages)
This is correct, except that you need *four* satellites. Your receiver clocks are not perfectly synchronised with the satellite clocks, so you have four unknowns, XYZ and T (time). With 4 observations, you can solve for the user receiver clock offset to GPS time. With more satellites in view, you…
Greg: See my previous post ( and the ones of last week, if they haven't scrolled yet). The "accuracy" with which you can measure a baseline is dependent on the "precision" ( spread of a number of repeat readings, expressed in Standard Deviations, or in error ellipse size/ orientations, or…
This is not the case ( my previous posts on Differential GPS must have scrolled). In differential mode, you occupy each end of a baseline ( points A and B ) *simultaneously* with 2 GPS receivers. Over baselines of " a few hundred" km, if you are tracking the same…
What do you mean by this, Al? "within inches of the true position" has very limited meaning to a surveyor. Your coordinates depend on the datum used, as well as the projection. Your position on the topography is related to the geoid, whch in turn is related to any arbitrary…
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As a surveyor, I would always make a site plan of any location that I planned to return to: even if I was locating myself with GPS. This need only be a sketch with major features: maybe for you, 3 rough compass bearings to prominent outcrops would be enough for…
Don, sorry for any confusion, the "Denise Greenwood" msg is from me. Fatal log-on cock-up! Rob
Lanny, You probably just got info from either vested interests or GPS "newbies". Until about 2 years ago the number of people in the whole world who could speak authoritatively on GPS was very small. Those working in each specialism had generally known each other for years. Now the salesmen,…
Sorry about spoiling your day, Lanny, but just look t it this way: Why would I, as a surveyor, spend L 16,000 plus on a GPS receiver if I can get the same accuracy from a $800 unit? Rob
Hi, Rob. I've been using GPS and DGPS in oil-exploration activities since 1985. I've seen a few changes since waiting up all night in Brazil to catch a 25 minute period of good Block I geometry!! It's a while since I spent any time in this Section, but I see…
GPS Error
Message #160666
This is not directly the case. The corrected pseudorange for the "bad satellite" broadcast from the differential reference station(s) would compensate for the error. The DGPS technique assumes that range errors for each satellite at the reference and remote sites will be very similar. Thus, SA and some types of…
There are probably hundreds of establishements around the world researching GIS. Two periodicals available ( in the UK at least) are: Mapping Awareness (I don't have a copy handy) GIM (Geodetical Info Magazine) USA Subscription Agent: JC Hescock, SISAT Ic, PO Box 487, North Salem, NY 10560 tel: 914 237…
Trimble has a huge product range. They've been most active in the surveying markets, which they've got practically sewn up. Most of their "economy " model units are built on the back of work that was originally done for the survey market. They have a good level of support, very…
Bill… yes, they do have a GPS on board, for both time and position! Rob
High Bill: I've been away for a while, and probably missed a lot on this forum. To answer your question about differential corrections: yes, the corrections would still be valid at the accuracies you're talking about. The height difference would probably introduce more errors than the 50 km horizontal difference.…
Detailed reply to follow, Bill ( along with part 2 of the lesson!). I've been tied up with work pressure for the last few days. However, you are on the right track. The WGS 84 ellipsoid can vary by up to +/- 100m and then some from the geoid at…
Don't worry about it… part 2 should clarify things, with a short precis at the end in "plain English" to isolate the main "datum facts". I'll probably write it tonight. Rob
* Secrets*: Can't give you a proper reply in time for this trip, but on the "fixed height" thing: Generally, 4 satellites are required to give a 3D fix, because you have 4 unknowns: Lat, Long, Height and User Clock Offset from GPS system time…ie , how far away from…
I will… give me a little while to think of one . Rob
Righy, Guy. Every satellite carries a full set of ephemerides for the whole constellation, so once you track one, you've got the orbits of them all Rob
No, you're very close: the mask angle just cuts out those satellites which the rx *knows* ( from the orbit ephemerides transmitted) will be below the selected angle at the approximate location of operation. There *is* a health flag whereby the satellites flag themselves as sick ( or are told…
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…
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…
Second message: remember, the satellites are orbiting very high, so it's not physically impossible to have 12 above the horizon at all, even at a relatively low height. Trimble sells some PC software for producing maps of satellites in view/ geometries etc… quite a useful planning tool if you're doing…
see my post to Bill… I've seen 12 satellites often, but the low ones were not so good! The shift you saw may not have been due to SA… it could also be due to multipath ( reflections), especially with canyon walls around you. With 3 satellites, working in a…
You can see up to 12 satellites at once with a 12 channel receiver, which the Scout ain't!! Depends on the mask angle you set… if it's at 10 or 15 degrees there won't often be 12 satellitees above the view horizon at once, but if you set it to…
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…
Just downloaded my mail.. no time for detailed responses today, but I'll get back to you tomorrow, wnen I've had some sleep ( gruelling 48 hours!) Rob
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…
Didn't see your reply before I made pretty much the same point. I've been using Pulse/8 ( the Loran survey derivative, which is more accurate than Loran itself) since 1978, and I've never had better than 20-30 m at the 2 sigma level from it. I've used GPS since 1984…
Loran… 10 feet accuracy: no way: the crossover points of the pulse transmissions cannot be determined to better than 20ns, never mind the tracking point location in the pulse. Then there's propagation path effects etc. I've used Pulse/8 ( the survey derivative of Loran, which is *more* accurate) for many…
There are 0nly 24 block 2 satellites in orbit. There are still a couple of old satellites left over from the block 1 ( concept testing) launches which still work. There are no satellites which sit around the equator: they are all in orbits inclined at 55 degrees to the…
Yes: I've been in offshre seismic for about 15 years. I'm actually an Astrophysicist turned Surveyor, but you do't work alongside snake wrestlers and grease pencil wavers for that long without picking up something. Rob