<HTML><BODY style="word-wrap: break-word; -khtml-nbsp-mode: space; -khtml-line-break: after-white-space; ">Hello, <DIV><BR class="khtml-block-placeholder"></DIV><DIV>I'm using spkezr to get the position of a spacecraft (target) in the J2000 reference frame with the observer as the Sun. What coordinate system is this based upon? Is it Heliocentric Aries Ecliptic HAEJ2000? </DIV><DIV><BR class="khtml-block-placeholder"></DIV><DIV>Ultimately, I need to get the lat and lon of the spacecraft based on the Heliocentric of Date (HCD) coordinate system so that I can convert to Heliocentric RTN (HGRTN). I thought I could either get the position of the spacecraft in HAEJ2000 and convert to HCD from there or get the position in GEIJ2000 and convert from there. However, when I tried the GEIJ2000 route, my calculations were more off than they were when I did it from scratch. I tried the algorithm in the SPK Required Reading but still didn't get a good answer. Any suggestions? <BR><DIV> <P style="margin: 0.0px 0.0px 0.0px 0.0px; font: 12.0px Helvetica; min-height: 14.0px"><BR></P><P style="margin: 0.0px 0.0px 0.0px 0.0px; font: 12.0px Helvetica; min-height: 14.0px">Thanks</P> <P style="margin: 0.0px 0.0px 0.0px 0.0px; font: 12.0px Helvetica; min-height: 14.0px"><BR></P> <P style="margin: 0.0px 0.0px 0.0px 0.0px"><FONT face="Helvetica" size="3" style="font: 12.0px Helvetica">~~~~~~~~~~~~~~~~~~~~~~~</FONT></P> <P style="margin: 0.0px 0.0px 0.0px 0.0px"><FONT face="Helvetica" size="3" style="font: 12.0px Helvetica">Aimee Mostella<SPAN class="Apple-converted-space"> </SPAN></FONT></P> <P style="margin: 0.0px 0.0px 0.0px 0.0px"><FONT face="Helvetica" size="3" style="font: 12.0px Helvetica">Research Analyst</FONT></P> <P style="margin: 0.0px 0.0px 0.0px 0.0px"><FONT face="Helvetica" size="3" style="font: 12.0px Helvetica">Southwest Research Institute</FONT></P> <P style="margin: 0.0px 0.0px 0.0px 0.0px"><FONT face="Helvetica" size="3" style="font: 12.0px Helvetica">Space Science Division</FONT></P> <P style="margin: 0.0px 0.0px 0.0px 0.0px"><FONT face="Helvetica" size="3" style="font: 12.0px Helvetica">(210) 522-6735</FONT></P> </DIV><BR></DIV></BODY></HTML>