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# 20010917: spheroid model used in McIDAS

• Subject: 20010917: spheroid model used in McIDAS
• Date: Mon, 17 Sep 2001 15:19:17 -0600
• >from: Peter Romanov <address@hidden>
• >keywords: 200109172001.f8HK1R125056
• >organization: UCAR/Unidata

```------- 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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