In a message dated 30/03/2008 07:47:57 GMT Daylight Time, darrell@shaw.ca writes: My guess is that at least some of the difference that you see in your altitude is from the propagation delay in the cable between your antenna and the GPS module. Guessing still further, I would think that the position as reported would be some distance immediately below the antenna. How long is your cable? If the antenna is stationary, you could move the GPS module to any position within the sphere allowed by your length of cable, and the reported position would stay the same. ----------------------------- I think that's all rather optimistic, as mentioned previously height estimation is the most innacurate parameter to be reported by GPS units. It's also important to remember that the "mean" sea level reference is not necessarily the same as "local" sea level. I am running a Thunderbolt from an antenna at most 10 feet away from the unit with a cable not much longer, so not too much propagation delay. The antenna is quite low but has a good view of the sky and at the moment is tracking seven satellites. Altitude will always vary by at least a few metres whenever it makes a survey, it's currently showing 58.8 metres but previous survey was around 64 metres. That's not bad when you consider that I live in the middle of the Clyde estuary in Scotland, with the water's edge somewhere around 50 feet away from the antenna, and that even with allowance for tidal variations the antenna is, at the most, 3 or 4 metres above the physical water level:-) regards Nigel GM8PZR

From: GandalfG8@aol.com Subject: Re: [time-nuts] Another Trimble Tbolt question.... Date: Sun, 30 Mar 2008 05:46:10 EDT Message-ID: <d1d.1f0e59bf.3520bb62@aol.com> Nigel, > I think that's all rather optimistic, as mentioned previously height > estimation is the most innacurate parameter to be reported by GPS units. > > It's also important to remember that the "mean" sea level reference is not > necessarily the same as "local" sea level. > > I am running a Thunderbolt from an antenna at most 10 feet away from the > unit with a cable not much longer, so not too much propagation delay. > The antenna is quite low but has a good view of the sky and at the moment is > tracking seven satellites. > Altitude will always vary by at least a few metres whenever it makes a > survey, it's currently showing 58.8 metres but previous survey was around 64 > metres. > > That's not bad when you consider that I live in the middle of the Clyde > estuary in Scotland, with the water's edge somewhere around 50 feet away from the > antenna, and that even with allowance for tidal variations the antenna is, > at the most, 3 or 4 metres above the physical water level:-) > > regards Considering your position, it seems to agree with modelled differences, just look briefly at the second coluourfull map of the earth on this link: http://cddis.gsfc.nasa.gov/926/egm96/egm96.html and you see that what you have observed does indeed match the deviation in readings that you see. Infact you have provided a nice example of the disagreement between the WGS-84 ellipsoid reading (default readout) and that of the actual geoid. There exists models with which the geoid position can be calculated from the ellipsoid position. In the mean while, the 30,072 m I am reading out of my receivers survey-state right now could be close to a geoid reading. But then I suspect that Novatel have made some form of compensation. I should expect some 25 m or so above the ellipsoid. But then, I expect my GPS antenna to have a poor position in terms of multipath and a move is upcomming. Cheers, Magnus

On Sun, 2008-03-30 at 05:46 -0400, GandalfG8@aol.com wrote: > ----------------------------- > I think that's all rather optimistic, as mentioned previously height > estimation is the most innacurate parameter to be reported by GPS units. Height is about 1.6 to 1.8 times worse than horizontal position, due to geometry reasons and the need to solve for the local clock. Due to our flat earth, its much easier to have an independant idea of you site height than xy-coordinates. Lake heights are often printed on common maps. Anyone here that would spot a 30m horizontal offset of your favorite walk trail? (watching lat/lon without a map overlay) With most low end OEM receivers qouting 1us accuracy of the PPS output -- I wote for "time" as the worst accuracy measurement reported by GPS units. > It's also important to remember that the "mean" sea level reference is not > necessarily the same as "local" sea level. Its very important to know the difference between ellipsoid height and geoid or Mean Sea Level (MSL). The ellipsoid is the idealized earth model used in WGS84 to approximate the geoid (zero MSL). Because the earth is no perfect ellipsoid (or sphere...). The "normal" height for a GPS receiver would be ellipsoid height, but with a model of the geoid imperfections it might also tell you geoid (MSL) height. -- Björn