Johnathan Wojok said: >If the difference in the various geoids is < 50m, and if the error range >in TLE's is already 500m, then I think the errors in the various geoids >can basically be ignored. Leaving aside the semantic issue of whether the differences between national datums constitutes an "error", I think it is worth noting that such differences tend to be more than 50 metres rather than less. WGS84 itself is more than a hundred metres of Longitude from the Prime Meridian at Greenwich and many national datums are also more than 50 metres either from the old Prime Meridian system or from WGS84. Note too that these differences can become cumulative when two or more datums are involved in measurement of the same celestial object from different places on Earth. As for the error range of the TLEs being 500 metres, I'm surprised if it is true that precise elsets are no better than half a kilometer. Perhaps Dr TS Kelso and others are wasting their time when they produce excellent computational software which outputs to an arc-second of declination and to a metre of orbital height? Will you tell, them? Are you suggesting that SpaceCom somehow deliberately degrades the orbital parameters of satellites such as NavStar and Transit to no better than 500 metres? If so, it seems a rather pointless degradation as it is so easily corrected by the user community. The IGEX programme for example, tracks Glonass and NavStar (and occasionally a few other) satellites to an accuracy of well under five metres and cumulatively to sub-metric precision, albeit in relation to the rather wacky PZ-90 datum. >Clock tolerance level: 10 to 30 seconds< Take a look at the SabObs on this list and you will see observation times listed to a precision of a hundredth of a second and declinations listed to an arc-second or even to a hundredth of an arc-minute. Note that there are many different time datums, such as terrestrial Time, GPS time, GMT, UT, UT1, and UTC, but by common consent, most people have either adopted UTC or some specifically declared variant, such as EDT, which is easily understood by other observers. As for datum differences (or "errors", if you insist) being less than 50 metres: you are simply wrong. The local datum co-ords (AS58 on the International spheroid) of the USAF Space Command tracking station on Ascension Island are known to great precision. That position is 68 metres from the same location when expressed on WGS84. That is clearly more than 50 metres, but there are much greater differences elsewhere. For example; the USAF GPS tracking station on Diego Garcia in the local datum (ISTS072 on the International spheroid) is also known to great accuracy. That position is more than 413 metres from the WGS84 figure for the same location. There are even greater differences elsewhere in the world, if you care to look for them. That is why it is important to state the geodetic basis of apparently precise co-ordinates. >the various geoids can basically be ignored. I think you mean datums and spheroids, rather than geoids. Yes, you can ignore the ellipticity of the Earth, but you cannot ignore its three dimension-ness. Any position, whether a location on board Planet Earth or that of an orbiting satellite, is a 3-D entity and therefore needs to be described in three dimensional terms. You could, I suppose, replace the ellipse of rotation with a simple sphere, but you would still have to declare a dimension for that sphere because we still live in a three (at least!) dimensional universe. You could make the radius of your simple sphere equivalent to the radius of a sphere which has approximately the same volume as the WGS84 spheroid and round off your chosen radius to 6,371 Kilometres, for example. What would you actually achieve by such a dumbing down of real-world geodesy? Nothing, I would suggest. All you would do is add yet another "standard" to be converted to the figure of the Earth which other people choose to use. The Cospar standard of precision for observational location happens to be four places of decimal degrees. I have no opinion whether that should be improved or not. I would prefer that standards are not intentionally and unnecessarily degraded, that's all. All I have suggested is that we should have some common Lingua Franca when expressing that position. WGS84 is one widely known standard, but there are others which have equal legitimacy. Simply stating the geodetic basis of a stated position is enough. I can express my height as either "Five Eleven" or as "One Eighty". Each is a reasonable description of my height which is approximately 5' 11" or 1.8m. To make sense of my declared measurement, I need only label it with the measurement basis of my declared ordinate. Similarly, if I declare a temperature to be 123°, I really ought to declare whether that is °K, °C, or °F. There is an old joke amongst air pilots that a suitable response to an air trafic controller who asks "What is your height and position?" is:- "Five feet eleven and sitting in the front". The joke has a serious point. There needs to be some common ground between the writer and the reader of expressed co-ordinates, otherwise disorder displaces order. If I give a Lat/Long to an apparent precision of ±8m (eg to four places of decimal degrees of Lat/Long), then I really ought to declare the basis of that measurement if I have that information available to me. If I say that something happened sometime last Thursday, when I really don't know when on last Thursday it actually happened, I would might be misleading people if I declared that the occurrence was at 01:23:45.67. Similarly, if a geodetic position is expressed to an apparent precision of a tenth of an arc-second of Lat/Long, as I've seen legitimately shown on SeeSat-L, then the geodetic basis of those co-ordinates really ought to be mentioned too. I should immediately say that in four out of four cases of SeeSat-L posters whose co-ords I have looked into closely, all have used very well-sourced data and all have used perfectly justifiable levels of precision in their expression, despite having come from three different countries with three completely different national mapping datums and spheroids. Two of them have since posted the geodetic basis of their co-ords and a third has privately confirmed the suspected national datum basis of his co-ords. Several others have written privately to express a positive interest in the subject matter, only one has been unpleasantly negative. Yes, there are those who will exhort us to dumb down our co-ords to two places of decimal degrees (or worse) and there might even be some who feel that mere 2-D cords are a suitable expression of position in their flat-Earth world. I suggest that those who care about precision; and those who understand the difference between accuracy and precision: give a thought to what those concepts actually mean and present suitable labels upon their stated co-ords. Since I have broached the subject on the List. I have noticed that several List-members have admirably done so and I have also received a remarkable number of non-hostile private communications from others who are also interested in making good use of the data which they have at their disposal. I think it was Albert Einstein who once said that: The Truth ought to be expressed as simply as possible, but never more so. Cheers, Chris Olsson ----------------------------------------------------------------- Unsubscribe from SeeSat-L by sending a message with 'unsubscribe' in the SUBJECT to SeeSat-L-request@lists.satellite.eu.org http://www2.satellite.eu.org/seesat/seesatindex.html
This archive was generated by hypermail 2b29 : Mon Aug 20 2001 - 17:12:07 PDT