Re: [time-nuts] Another Trimble Tbolt question....
From: Darrell Robinson [EMAIL PROTECTED] To: Discussion of precise time and frequency measurement time-nuts@febo.com Sent: Sunday, March 30, 2008 3:19 AM Subject: Re: [time-nuts] Another Trimble Tbolt question More guessing. I wonder if the altitude you see is from the difference between Mean Sea Level and Height Above Ellipsoid? This document has a map on page 7 - http://www.avionicswest.com/PDFiles/alt2.pdf Still no expert Darrell - Original Message - From: Michael Baker [EMAIL PROTECTED] To: time-nuts@febo.com Sent: Saturday, March 29, 2008 5:49 AM Subject: [time-nuts] Another Trimble Tbolt question Hello, All-- Thanks to the collective savvy of this list and the patient assistance of Ken Winterling wa2lbi I was finally able to get my Tbolt to stop requiring me to configure the COM-4 channel every time I ran the monitoring software. So-- I had such good luck with that question, I'm going to pester the group with one more Tbolt question: In the Position area of the Tbolt monitoring display my altitude is given as 1.4 meters. The USGS topo map and a recent survey both say that the ground level at my house is 84 feet above MSL. The antenna for the T-bolt is on the top of my house 19 feet above the ground. If I open SETUP / POSITION and SET ACCURATE POSITION to, say: 28 meters and enter that value, the indicated altitude changes to 28 meters, but the next day or so, I always find that it has reverted to 1.4 meters. Admittedly, having a (relatively) accurate altitude display is of minor importance to me since my primary reason for having the Tbolt is for a decent 10MHz reference. However, as a matter of principle it would be nice to have the altitude display be a little closer! Any suggestions as to how to get the Tbolt to display a more correct altitude? Thanks! Mike Baker WA4HFR Micanopy, FL For what it's worth, I have two of these units and they are horrible on altitude but otherwise seem to be ok. Just a thought ! Phil ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Another Trimble Tbolt question....
In a message dated 30/03/2008 07:47:57 GMT Daylight Time, [EMAIL PROTECTED] 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 ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Another Trimble Tbolt question....
From: Darrell Robinson [EMAIL PROTECTED] Subject: Re: [time-nuts] Another Trimble Tbolt question Date: Sat, 29 Mar 2008 23:41:36 -0700 Message-ID: [EMAIL PROTECTED] I am not an expert. 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. No, that cancels out. The geographical position comes from the time differances between the signals and that is how they are perceived by the antenna. The cable will maintain these time-delays differances fairly accurate (only the doppler frequency offsets skews the delay) but the receiver will receive them at a later time due to cable delay. Therewill be a systematic offset of the time solution thought, due to the cable delay. The reason for that is that the receiver actually finds a position in [X Y Z T]t for the antenna, not the receiver, so the cable delay on the antenna can be seen as the additional output delay on the PPS. So no, the height is not that greatly affected by the cabel delay. I would look at what types of ellipsoid-geoid compensations is performed, and use of a local datum. 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. Don't bother. The receivers position is irrelevant, as it is the antenna position that is important. Cheers, Magnus ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Another Trimble Tbolt question....
From: [EMAIL PROTECTED] Subject: Re: [time-nuts] Another Trimble Tbolt question Date: Sun, 30 Mar 2008 05:46:10 EDT Message-ID: [EMAIL PROTECTED] 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 ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Another Trimble Tbolt question....
On Sun, 2008-03-30 at 05:46 -0400, [EMAIL PROTECTED] 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 ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Another Trimble Tbolt question....
Mike, A quick answer to this - two reasons why... 1) Your Thunderbolt is using WGS84 coordinate system and not USGS, and.. 2) The altitude fix is usually the most inaccurate parameter, as it is very unlikely that you have a GPS bird directly overhead. Think of your position fix as you sitting inside a sphere of probability. In the ideal world with perfect GPS geometry it would be a sphere, but in reality it is an ellipsoid due to less accuracy in the Z plane. Why, after setting (and assuming storing) a known accurate position it changes again, I don't know, unless the unit is reverting to a survey mode. I don't know this product intimately, so I'm sure others will proffer better suggestions. Rob Kimberley -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Michael Baker Sent: 29 March 2008 12:50 To: time-nuts@febo.com Subject: [time-nuts] Another Trimble Tbolt question Hello, All-- Thanks to the collective savvy of this list and the patient assistance of Ken Winterling wa2lbi I was finally able to get my Tbolt to stop requiring me to configure the COM-4 channel every time I ran the monitoring software. So-- I had such good luck with that question, I'm going to pester the group with one more Tbolt question: In the Position area of the Tbolt monitoring display my altitude is given as 1.4 meters. The USGS topo map and a recent survey both say that the ground level at my house is 84 feet above MSL. The antenna for the T-bolt is on the top of my house 19 feet above the ground. If I open SETUP / POSITION and SET ACCURATE POSITION to, say: 28 meters and enter that value, the indicated altitude changes to 28 meters, but the next day or so, I always find that it has reverted to 1.4 meters. Admittedly, having a (relatively) accurate altitude display is of minor importance to me since my primary reason for having the Tbolt is for a decent 10MHz reference. However, as a matter of principle it would be nice to have the altitude display be a little closer! Any suggestions as to how to get the Tbolt to display a more correct altitude? Thanks! Mike Baker WA4HFR Micanopy, FL - ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
