Date  File  Comment by  Comment  Rating 

11 Aug 2014  Convert Azimuth and Elevation to Right Ascension and Declination Algorithm will convert Az/El angles to RA /DEC using a topocentric coordinate frame  Darin Koblick  You should reference Vallado pg. 162 for additional information on "topocentric coordinate frame". It is a Geocentric equatorial coordinate system with the origin translated from the center of the Earth to the origin of the observer. 

23 Apr 2014  Efficient Vector Rotation Matrix Routine Efficient Method to Determine a Matrix Needed to Rotate One Vector to Another  Darin Koblick  Alexander, you are correct. I went ahead and changed the vector angle check to be closer to 1 instead of 0.99. Also, I found that I needed to add the case where vectors where close to parallel (i.e. abs(c) > 11e13) and two or more components had the same values. Thank you for pointing out the problem to me. 

01 Sep 2013  Convert ECI to ECEF Coordinates Transform ECI position, velocity and acceleration to ECEF position, velocity & acceleration.  Darin Koblick  Chad, The two code lines you are referencing are trying to find the midnight that occurs before the provided Julian Date. As written, the first check will be true if the Julian date occurs after midnight and 12+ hours before the time. As written, the second check will be true if the Julian date occurs after midnight and less than 12 hours before the time. If both checks are true, the code will use the last midnight that occurred. This is the desired functionality. Because I am rounding down on the Julian date and subtracting a half day, there is no possible way that jd could be less than jdMin. Darin 

20 Oct 2012  Convert UTC to Solar Apparent Time Convert a time given in UTC to Solar Apparent or Solar Mean Time  Darin Koblick  Brandon, Also, if you are trying to compare the Solar Apparent Time, (SAT) be sure to input the longitude in degrees (W < 0, E > 0). You can see that this function will match the solutions (almost exactly ... depending on the year) to those solutions in Example 4 of the ASTRONOMICAL INFORMATION SHEET No. 58 http://astro.ukho.gov.uk/nao/services/ais58.pdf 

20 Oct 2012  Convert UTC to Solar Apparent Time Convert a time given in UTC to Solar Apparent or Solar Mean Time  Darin Koblick  Brandon, Latitude and Longitude are needed to determine the input time value to the equation of time (UT). They aren't needed to determine the equation of time ... it has nothing to do with latitude and longitude. Perhaps you should compare to a location with a 0hr local time offset. See this document for a description of what the equation of time is, you can also compare results to this as an "example". I don't guarantee any specific accuracy with EoT as it depends on the mean anomaly of the sun which is obviously a first/second order approximation. http://www.ips.gov.au/Educational/2/1/14 

11 Aug 2012  Convert ECEF to ECI Coordinates Take any vector or series of vectors in the ECEF Coordinate frame and convert them to ECI.  Darin Koblick  John, 35 leap seconds since 1972 sounds like that is indeed what you are seeing in STK. As you know, ECEFtoECI.m takes in a Julian date, so the correction would not be internal to this particular conversion routine. In fact, I don't think Mathworks attempts to address this issue with their juliandate.m routine. You would more than likely want to make the leap second correction when converting UTC to Julian date. Are you able to check for the estimated position differences using Julian date in STK instead of UTC? Perhaps this should make the comparisons a little more accurate as well. Truthfully, unless you are trying to use this routine in highprecision satellite mission operations, as long as you are consistent with your coordinate frame conversions (i.e. always using these conversion routines), it really shouldn't matter if there are minor discrepancies with STK (it is all relative motion anyway). Darin 

28 Jun 2012  Convert ECI to ECEF Coordinates Transform ECI position, velocity and acceleration to ECEF position, velocity & acceleration.  Darin Koblick  Drake, J2000 is factored into the ECI to ECEF conversion routine through the call to JD2GAST.m (converts Julian Date to Greenwhich Apparent Sidereal Time). In order to do this, the effects of nutation are considered, and depend on the correct number of Julian centuries since J2000. 

28 May 2012  Convert ECI to ECEF Coordinates Transform ECI position, velocity and acceleration to ECEF position, velocity & acceleration.  Darin Koblick  Johan, You are correct, thank you for spotting that mistake. I should have left everything in degrees since the mean obliquity of the ecliptic is referenced directly in the final computation.
EPSILONm = 23.4392910.0130111.*T  1.64E07.*(T.^2) + 5.04E07.*(T.^3); This corresponds to the estimates contained in Vallado, Fundamentals of Astrodynamics and Applications, Second edition, (EQ 353). 2001. Thank you again for double checking my work. 

11 Apr 2012  Vectorized Analytic Two Body Propagator (Kepler Universal Variables) Analytic propagation routine uses universal variables to solve a single formula for all orbit types  Darin Koblick  John, The zip file seems to unzip without problems. Try using either the default windows zip utility, the MATLAB command unzip(), or the free 7zip utility. If all else fails, you can view the source code here: http://www.mathworks.com/matlabcentral/fileexchange/35566vectorizedanalytictwobodypropagatorkepleruniversalvariables/content/keplerUniversal.m 

08 Apr 2012  Convert ECI to ECEF Coordinates Transform ECI position, velocity and acceleration to ECEF position, velocity & acceleration.  Darin Koblick  The following ECI coordinate information rECI = [2392.11241452386
vECI = [3.27682054057320
t = 2453937.82777778; Will yield an ECEF position and velocity of rECEF = [15615.6915464865
vECEF = [0.253618011215938
This was obtained using the routines provided by CelesTrak found from http://www.celestrak.com/software/valladosw.asp The ECItoECEF.m routine I posted on the FileExchange comes very close to this with the following vectors: rECF = [ 15618.7911316673
vECF = [0.253618011215938
The problem is your ECEF position and velocity vectors do not correspond to each other. A very easy way to check this is to compute the magnitude of your position vectors. The magnitude of the first ECI position vector you have provided is: 26,366.726420485 km The magnitude of the ECEF solution should be exactly the same as the magnitude in the ECI reference frame. The magnitude of your ECEF position vector is 31,124.2960748149 km. Hopefully this solves your problem. 

07 Apr 2012  Convert ECI to ECEF Coordinates Transform ECI position, velocity and acceleration to ECEF position, velocity & acceleration.  Darin Koblick  The MATLAB function juliandate.m should be fine. Can you please post the epoch, the position/velocity vector in ECI coordinates, and your ECF solution? 

05 Apr 2012  Convert ECI to ECEF Coordinates Transform ECI position, velocity and acceleration to ECEF position, velocity & acceleration.  Darin Koblick  Check to make sure your time vector is in J2000 format, not Unix Time (epoch should be starting 01/01/2000 12:00:00). 

09 Jan 2012  Convert ECI to ECEF Coordinates Transform ECI position, velocity and acceleration to ECEF position, velocity & acceleration.  Darin Koblick  Syed, Send me your test case and I will take a look at it. Thanks,


28 Oct 2010  Convert ECI to ECEF Coordinates Transform ECI position, velocity and acceleration to ECEF position, velocity & acceleration.  Darin Koblick  Ismail, You are more than likely using the wrong dimensions for r_ECI.
Try:
In other words, transpose the r_ECI input for the correct answer. 
