[Spice_discussion] Re: Spice_discussion Digest, Vol 44, Issue 1

Henry DeWitt hdewitt at dewitt-assoc.com
Fri Jun 11 15:25:56 PDT 2010


Bill:

Could the Stereo kernel be in a different reference frame from the time 
used as an earth reference?   If one used J1950 and the other used 
J2000, that could explain the difference.  I know that when I worked 
with JHU/APL to tracked STEREO on their 18 Meter antenna, a .1 degree 
difference would not have been noticed

Without doing the math, I would say that the plot is consistent with a 
fixed time offset.  The elevation error would be minimized at noon and 
maximized at STEREO rise/set.  The azimuth error would be maximized at 
noon and minimized at STEREO rise/set.

You can get the time offset (if that is the problem ) by dividing the 
error (about .0967 degrees) at noon by the earths rotational rate (360 
degrees per 23.9344 hours).  The answer is about 23 seconds.

I am not sure exactly what one needs to do in SPICE to compensate for 
different time reference frames.

Henry

William Thompson wrote:
> Henry:
>
> All calculations are done in double precision.
>
> That was a good suggestion about looking for patterns in the error.  
> The error has a very definite pattern to it, as shown in the attached 
> plot.  This is probably a valuable clue as to what is going on, but I 
> don't know how to interpret it.  The dominant error is in the azimuth 
> angle, and this is largest at 180 degrees azimuth.
>
> I thought about the possibility of a timing error, but I don't really 
> see how that is possible.  The ASCII UTC timestamps that appear in the 
> file are converted into ephemeris time via cspice_str2et to pass into 
> the SPICE routines.  This command includes all leapsecond factors via 
> the naif0009.tls kernel.  No further manipulations are applied to the 
> time beyond that point.
>
> Bill
>
>
>
> Henry DeWitt wrote:
>> Bill:
>>
>> A few comments/questions:
>>
>> Is the error fairly constant, show some kind of pattern, or is it 
>> random?
>> Are some of the calculations being done in single precision floating 
>> point?  This might lead to random errors.
>> Is there some subtle time error?  A time error would lead to a fairly 
>> constant error term (combined Az and El).  I think it looks like a 23 
>> second error from the few points you provide, this quite close to the 
>> number of leap seconds in the leap seconds kernel.
>>
>> Henry DeWitt
>> DeWitt & Associates, Inc.
>>
>> spice_discussion-request at naif.jpl.nasa.gov wrote:
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>>> Today's Topics:
>>>
>>>    1. Problem generating station ephemerides (William Thompson)
>>>    2. Re: Problem generating station ephemerides (Bogdan Nicula)
>>>    3. Re: Problem generating station ephemerides (William Thompson)
>>>
>>>
>>> ----------------------------------------------------------------------
>>>
>>> Message: 1
>>> Date: Fri, 11 Jun 2010 12:04:30 -0400
>>> From: William Thompson <William.T.Thompson at nasa.gov>
>>> Subject: [Spice_discussion] Problem generating station ephemerides
>>> To: "spice_discussion at naif.jpl.nasa.gov"
>>>     <spice_discussion at naif.jpl.nasa.gov>
>>> Message-ID: <4C125E8E.8040804 at nasa.gov>
>>> Content-Type: text/plain; charset="ISO-8859-1"; format=flowed
>>>
>>> SPICE experts:
>>>
>>> I generate ephemerides of the two STEREO spacecraft for volunteer 
>>> antenna stations collecting low-rate beacon telemetry outside of the 
>>> normal DSN contacts.  Recently it's come to my attention that the 
>>> ephemerides that I generate may be off by as much as 0.1 degrees, 
>>> mainly in the direction of ecliptic longitude.  I would like to go 
>>> over the procedure that I use to see if anybody can suggest what the 
>>> problem might be.
>>>
>>> I will use as an example a station in Bochum, Germany, because this 
>>> can be directly compared to ephemerides generated by the JPL 
>>> Horizons website at
>>>
>>>     http://ssd.jpl.nasa.gov/horizons.cgi
>>>
>>> This website generates the following ephemeris entries for the 
>>> STEREO Behind spacecraft:
>>>
>>>   Date__(UT)__HR:MN     Azi_(a-appr)_Elev            delta      deldot
>>>
>>>   2010-Jun-18 15:24 *m  179.5797  52.2818 1.17021276739671   0.8852056
>>>   2010-Jun-18 15:25 *t  179.9768  52.2822 1.17021312270827   0.8864513
>>>   2010-Jun-18 15:26 *m  180.3738  52.2815 1.17021347851951   0.8876971
>>>
>>> However, the ephemeris entries that I generate are slightly different.
>>>
>>> Date/Time                      Azimuth        Elevation        Doppler
>>> (UTC)                           (deg.)          (deg.)          (km/s)
>>>
>>> 2010-06-18T15:24:00.000       179.48305       52.282704      0.88506974
>>> 2010-06-18T15:25:00.000       179.88011       52.283347      0.88631548
>>> 2010-06-18T15:26:00.000       180.27718       52.282909      0.88756124
>>>
>>> You can see that the azimuth values are about 0.1 degrees smaller in 
>>> my calculation than in the Horizons calculation, while the azimuth 
>>> values are about the same.  Values near maximum elevation are shown 
>>> to highlight the direction of the error.
>>>
>>> The first question to ask is whether the difference is due to 
>>> different spacecraft ephemerides being used in the two 
>>> calculations.  In fact, there is a slight difference in the 
>>> predictive ephemerides being used, but this seems to only change the 
>>> calculation in the last signficant digit.  Here's a complete list of 
>>> all the loaded kernels in my calculation.
>>>
>>> /solarsoft/stereo/gen/data/spice/gen/naif0009.tls
>>> /solarsoft/stereo/gen/data/spice/gen/de405.bsp
>>> /solarsoft/stereo/gen/data/spice/gen/pck00008.tpc
>>> /solarsoft/stereo/gen/data/spice/gen/heliospheric.tf
>>> /solarsoft/stereo/gen/data/spice/gen/stereo_rtn.tf
>>> /solarsoft/stereo/gen/data/spice/sclk/ahead/ahead_science_06.sclk
>>> /solarsoft/stereo/gen/data/spice/sclk/behind/behind_science_06.sclk
>>> /solarsoft/stereo/gen/data/spice/epm/ahead/ahead_2009_050_definitive_predict.epm.bsp 
>>>
>>> /solarsoft/stereo/gen/data/spice/epm/ahead/ahead_2010_155_01.epm.bsp
>>> /solarsoft/stereo/gen/data/spice/epm/behind/behind_2009_049_definitive_predict.epm.bsp 
>>>
>>> /solarsoft/stereo/gen/data/spice/epm/behind/behind_2010_148_01.epm.bsp
>>> /solarsoft/stereo/gen/data/spice/depm/ahead/ahead_2006_350_01.depm.bsp
>>> /solarsoft/stereo/gen/data/spice/depm/ahead/ahead_2008_037_01.depm.bsp
>>> /solarsoft/stereo/gen/data/spice/depm/ahead/ahead_2008_078_01.depm.bsp
>>> /solarsoft/stereo/gen/data/spice/depm/ahead/ahead_2010_155_01.depm.bsp
>>> /solarsoft/stereo/gen/data/spice/depm/behind/behind_2007_021_01.depm.bsp 
>>>
>>> /solarsoft/stereo/gen/data/spice/depm/behind/behind_2007_053_01.depm.bsp 
>>>
>>> /solarsoft/stereo/gen/data/spice/depm/behind/behind_2008_037_01.depm.bsp 
>>>
>>> /solarsoft/stereo/gen/data/spice/depm/behind/behind_2008_078_01.depm.bsp 
>>>
>>> /solarsoft/stereo/gen/data/spice/depm/behind/behind_2010_148_01.depm.bsp 
>>>
>>>
>>> The .depm.bsp files are definitive ephemeris files based on 
>>> measurements, and do not cover future dates such as 18-June-2010.
>>>
>>> The procedure I use to generate the station ephemerides is as 
>>> follows.  First, I generate a frame definition kernel from the 
>>> geodetic coordinates of the station.   The coordinates of the Bochum 
>>> station are 7°11'33.4''E, 51°25'37.0''N, and with an altitude of 
>>> 207.33 m.  My frame definition file is as follows:
>>>
>>> \begindata
>>>
>>>     FRAME_BOCHUM_TOPO                     = 1451007
>>>     FRAME_1451007_NAME                    = 'BOCHUM_TOPO'
>>>     FRAME_1451007_CLASS                   = 4
>>>     FRAME_1451007_CLASS_ID                = 1451007
>>>     FRAME_1451007_CENTER                  = 399
>>>
>>>     TKFRAME_1451007_RELATIVE          = 'IAU_EARTH'
>>>     TKFRAME_1451007_SPEC              = 'ANGLES'
>>>     TKFRAME_1451007_UNITS             = 'DEGREES'
>>>     TKFRAME_1451007_AXES              = ( 3, 2, 3)
>>>     TKFRAME_1451007_ANGLES            = ( -7.192566, -38.57301, 
>>> 180.00 )
>>>
>>> \begintext
>>>
>>> Therefore, my first question is whether there's something subtly 
>>> wrong about the way I define the frame file.
>>>
>>> Next, I calculate the position of the spacecraft in the Earth's 
>>> reference frame using the following command:
>>>
>>>      cspice_spkezr, sc, et, 'IAU_EARTH', 'lt+s', 'Earth', state, ltime
>>>
>>> I considered whether that should be 'xlt+s', but that didn't seem to 
>>> make much difference.
>>>
>>> Next, I calculate the position of the spacecraft relative to the 
>>> station via the following commands:
>>>
>>>      cspice_bodvar, 399, 'RADII' , radii
>>>      flat = (radii[0]-radii[2]) / radii[0]
>>>      cspice_georec, lon_r, lat_r, alt, radii[0], flat, origin
>>>      origin = [origin, 0, 0, 0]
>>>      state = state - origin
>>>
>>> Next, I convert to the station frame
>>>
>>>      cspice_sxform, 'IAU_EARTH', strupcase(station) + '_TOPO', et, 
>>> xform
>>>      state = transpose(xform) # state
>>>
>>> Finally, I convert into azimuth and elevation
>>>
>>>      cspice_reclat, state[0:2], distance, azimuth, elevation
>>>      azimuth   = -azimuth  * (180.d0 / !dpi)
>>>      elevation = elevation * (180.d0 / !dpi)
>>>      if azimuth lt 0 then azimuth = azimuth + 360.d0
>>>
>>> The complete code can be found at
>>>
>>> http://sohowww.nascom.nasa.gov/solarsoft/stereo/ssc/idl/spice/ssc_write_station.pro 
>>>
>>>
>>> I'd appreciate any suggestions that might help explain the 0.1 
>>> degree discrepancy.
>>>
>>> Thank you,
>>>
>>> Bill Thompson
>>>
>>>   
>>
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