------- Forwarded Message To: address@hidden Subject: spheroid model Hi, I have a question about the global coordinate system (or the Earth's spheroid model) used in mcidas to navigate satellite data. In geolat.for function as well as in the code used to navigate GOES Imager data (in particular in NVINIgoes.for function) I have found values of 6378.388 km for a semimajor axis (equatorial radius, Re), 6356.912 km for a semiminor axis (polar radius, Rp) and Rm=6371.221 for the Earth's mean radius. In the function NV1INImsat (Is this a part of a Meteosat navigation code ?) a different ellipsoid model is used with Re=6378.155 and 1/297 for the ellipsoid flattening, hence Rp=6356.680 (Ellipsoid flattening, (f) is related to semimagor (Re) and semiminor (Rp) axes as f=(Rp-Re)/Rp). Lastly, in NV1INIgvar function (Is this a part of GOES Sounder navigation software ?) Re and Rp are taken equal to 6378.137 and 6356.753 respectively. Thus ellipsoid flattening is 1/298.26. It is worth mentioning that only the latter model corresponds to the one accepted as standard in the World Geodetic System of 1984 (WGS 84). The question is whether my conclusion is really true, that different ellipsoid models are used to navigate data from different satellite sensors ? What model is used to navigate NOAA AVHRR data ? What is the source for the first two models mentioned above ? Thanks, Peter ================================================ Peter Romanov NOAA/NESDIS/ORA, WWB 712 5200 Auth Road Camp Springs, MD 20746 USA tel: (301) 763 8042 fax: (301) 763 8108 ================================================ ------- End of Forwarded Message From: Dave Santek <address@hidden> Date: Mon, 17 Sep 2001 15:52:29 -0500 Subject: Re: spheroid model Peter Romanov wrote: For Meteosat & GOES, the radius was included in the code provided to us from the respective agencies that operate the satellites. For the geographic projections, we use: Equatorial radius: 6378.388 km Polar radius: 6356.912 km According to the USGS, this is the 1924 International Standard. For the NOAA polar satellites, check the source module: nvxtiro.dlm If found these couple of lines in nvxtiro.dlm: PARAMETER (ERAD = 6378.388) AE=6378.135D0 I don't know where the 6378.135 comes from. dave From: John Benson <address@hidden> Subject: Re: spheroid model In each case, it's important that the model we use to compute navigation transforms conforms to the model used to construct the data sent to us. That is especially true for Meteosat, which is remapped by ESA, using their constants. If we wish to know where they put a certain point on the earth, we have to run their inverse transform. --johnb
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