[Spice_discussion] Solar zenith angle and azimuth
Nat Bachman
nathaniel.bachman at jpl.nasa.gov
Thu Jul 1 12:41:02 PDT 2010
Hi Michael and Joe,
My colleague Boris Semenov pointed out that I didn't comment on
cspice_ilumin. Yes, that routine gives you the solar zenith
angle; unfortunately it doesn't compute the solar azimuth angle.
-Nat
Nathaniel.Bachman at jpl.nasa.gov
K. -Michael Aye wrote:
> Yes, a very nice complete (it seems, didn't try everything yet) description, many thanks, Nat!
>
> However, we found an alternative way, and are wondering if it's really equivalent or if we will have errors doing it this way, that we don't see yet.
> Here is what we found:
>
> First assumption: Local time of a surface point is analog to the sun's azimuth.
> (We need this to calculate the solar irradiation times on elevated topographical features)
>
> Then, cspice_et2lst provides the local time for any surface point of a body.
>
> Furthermore, cspice_ilumin provides the illumination angles for a surface point on a body and we just pick any observer and from the output we only use the incidence angle for the sun's elevation.
> The inputs we are using for ilumin is IAU_MARS for fixref, and srfrec to convert the given surface point into rectangular coordinates as required for the 'spoint' input.
>
> Does that sound ok or are we doing some big mistakes here?
>
> Best regards,
> Michael
>
>
> On 1-Jul-2010, at 13:19 , Joe Knapp wrote:
>
>> Excellent description--thanks. I was working on the same problem.
>>
>> Question: to get the apparent elevation (zenith angle), is a parallax
>> correction to the elevation needed? Refraction?
>>
>> Joe
>>
>> On Wed, Jun 30, 2010 at 6:46 PM, Nat Bachman
>> <nathaniel.bachman at jpl.nasa.gov> wrote:
>>> Dear Dr. Aye,
>>>
>>> Hello, this is from Nat Bachman. I'm a member of
>>> the NAIF team headed by Chuck Acton.
>>>
>>> SPICE does not contain a subroutine to
>>> compute solar azimuth and elevation (zenith angle
>>> is the complement of elevation). There are two basic
>>> approaches to the solution:
>>>
>>> 1) If you have a "small" number of surface points
>>> of interest, use the SPICE utility PINPOINT
>>> to create an SPK file providing locations of
>>> these points and a frame kernel providing
>>> topocentric frame specifications. Then
>>> proceed as you've described.
>>>
>>> PINPOINT is available from the utilities area
>>> of the NAIF web site at URL
>>>
>>> http://naif.jpl.nasa.gov/naif/utilities.html
>>>
>>> I've also included below some notes on running PINPOINT
>>> to generate SPICE kernels for surface points on
>>> the *Earth*. Most of the discussion applies to Mars,
>>> but for Mars you won't need the high-precision
>>> Earth PCK; instead you'd use the either the generic
>>> NAIF text PCK to provide Mars orientation, or you'd
>>> use a PCK of your own choosing (perhaps a standard
>>> one used by your project).
>>>
>>> 2) If you have too many surface points for (1)
>>> to be practical, create at run time the
>>> necessary surface-point-to-sun vectors
>>> and topocentric frames for each surface point.
>>>
>>> The surface point to sun vectors can be created by
>>> subtracting the respective Mars-centered surface
>>> vectors from the Mars to sun vector; this is valid
>>> as long as the aberration corrections don't have to
>>> exact (and for most purposes they need not be).
>>> Reception-style light time and stellar aberration
>>> corrections should be used for a realistic approximation
>>> of the apparent direction to the Sun. Here Mars is
>>> the observer, the Sun is the target, and the epoch
>>> is that at which photons are received at the surface
>>> point. Note that if the illumination at the surface point is
>>> being observed by a remote sensing instrument, this epoch
>>> may be the epoch of photon reception at the instrument minus
>>> one way light time between the instrument and the surface
>>> point.
>>>
>>> Each topocentric frame can be represented by a
>>> 3x3 matrix that converts vectors from the body-fixed
>>> frame associated with the central body
>>> to the topocentric frame associated
>>> with the surface point of interest.
>>>
>>> For a given surface location, the desired
>>> transformation matrix has as its rows the basis vectors
>>> of the associated topocentric frame. For all locations
>>> except at or extremely near the poles, these vectors
>>> are (listed in the order of computation):
>>>
>>> [Here x denotes the cross product]
>>>
>>> Row 3: (Z-axis) the unit zenith direction vector
>>> expressed in the central body's body-fixed,
>>> body centered reference frame (IAU_MARS, in
>>> your case).
>>> Row 2: (Y-axis) the zenith direction x the body's north
>>> pole direction (0,0,1), scaled to unit length.
>>> Row 1: (X-axis) Y x Z, scaled to unit length.
>>>
>>> At the poles this definition goes singular.
>>>
>>> It's usually convenient to call the SPICE routine UCRSS to
>>> obtain a unit-length cross product.
>>>
>>> Usually (and it is done this way in PINPOINT) the zenith direction
>>> is taken as the direction of increasing altitude in the planetodetic
>>> coordinate system; this direction is parallel to the normal to
>>> the central body's reference spheroid that passes through the
>>> surface point.
>>>
>>> One has to be a bit careful here because the surface
>>> point might not lie on the reference spheroid. If it does, call
>>> SURFNM to obtain the surface normal direction. If it doesn't,
>>> call RECGEO to find the planetodetic coordinates of
>>> the surface point, next find the point at the same planetodetic
>>> lon/lat but at zero altitude; convert those coordinates back to
>>> the rectangular system, and finally call SURFNM to obtain
>>> the surface normal, which is also the desired zenith direction.
>>> Note that none of the above works for a truly triaxial reference
>>> ellipsoid.
>>>
>>> The radii of the reference spheroid can be obtained by calling
>>> either of the SPICE routines BODVCD or BODVRD; one must first
>>> have loaded a PCK file that contains the radii.
>>>
>>> With the matrix in hand, left-multiplying the surface-sun vector
>>> transforms it to the corresponding topocentric frame.
>>>
>>> Please let us know if you have further questions.
>>>
>>> Best regards,
>>>
>>> -Nat
>>>
>>>
>>>
>>>
>>> Using an Earth surface site as an ephemeris object
>>> ==================================================
>>>
>>> Note: a list of references is included at the bottom of this
>>> discussion.
>>>
>>> Before creating new kernels, it may be worthwhile
>>> to check the NAIF web site to see whether
>>> the location of interest is covered by kernels
>>> already provided by NAIF:
>>>
>>> http://naif.jpl.nasa.gov/naif/data.html
>>>
>>> If the data you need are not available, you can create
>>> the kernels you need by following these steps:
>>>
>>> 1) Decide on a name and ID code for the site of
>>> interest. In order to enable use of the name in SPICE
>>> API calls, create a text kernel that establishes the
>>> name/ID code mapping. Applications that require this
>>> mapping should load this kernel at run time.
>>> This is done by kernel variable definitions of the
>>> form
>>>
>>> NAIF_BODY_NAME += 'MY_OBJECT_NAME'
>>> NAIF_BODY_CODE += nnnnnn
>>>
>>> where 'MY_OBJECT_NAME' is replaced by the actual
>>> name enclosed in single quotes, and nnnnnn is replaced
>>> by an integer ID code that doesn't conflict with any
>>> other you or other consumers of your new kernels
>>> will be using. The kernel must start with the \begindata
>>> control sequence (this is supposed to read "backslash
>>> begindata" but might not appear that way, depending on
>>> your e-mail client).
>>>
>>> The kernel including these assignments must conform
>>> to the SPICE text kernel format; see the "Intro to Kernels"
>>> tutorial cited under "References" for details.
>>>
>>> See the NAIF_IDS Required Reading or tutorial for further details.
>>>
>>>
>>> 2) Use the SPICE utility PINPOINT to create an SPK file
>>> for the site. PINPOINT is available from the NAIF
>>> web site. The PINPOINT user's guide is available from
>>> the web site as well; also, you can make PINPOINT
>>> print out the user's guide text by typing
>>>
>>> pinpoint -help
>>>
>>> See the URL
>>>
>>> http://naif.jpl.nasa.gov/naif/utilities.html
>>>
>>> to obtain a PINPOINT executable for your platform.
>>>
>>> Create a "definitions file" for your site (see PINPOINT User's Guide).
>>> Normally the reference frame for your site location data should
>>> be ITRF93. For *very* low-accuracy work, the frame IAU_EARTH
>>> can be used. Note that inertially referenced site locations
>>> can be in error by as much as ~6 km if IAU_EARTH is used.
>>> If your site location data are in a frame that is substantially
>>> different from ITRF93, you may need to transform them to the
>>> ITRF93 frame before providing them to PINPOINT.
>>>
>>> Run PINPOINT to create an SPK file for the site.
>>>
>>> Note that PINPOINT can create one SPK file providing data for
>>> multiple sites.
>>>
>>>
>>> 3) If you need to compute geometry relative to a topocentric
>>> (local level) reference frame centered at the surface site, you'll
>>> also need to create a topocentric frame kernel for that site.
>>> The easiest way to do this is to use PINPOINT's topocentric
>>> frame kernel generation feature. See the PINPOINT User's guide for
>>> details.
>>>
>>> To see examples topocentric frame specifications, see
>>> the NAIF topocentric frame kernel for the DSN stations:
>>>
>>> ftp://naif.jpl.nasa.gov/pub/naif/generic_kernels/
>>> fk/stations/earth_topo_050714.tf
>>>
>>> See the comments in this frame kernel (it's a text file) for details
>>> concerning its contents.
>>>
>>> Having defined a topocentric frame centered at the surface site,
>>> you can use the name of this frame in calls to any SPICE routine
>>> that requires a frame name as input.
>>>
>>>
>>> 4) Obtain one or more binary Earth PCK kernels to provide
>>> Earth orientation data. These kernels can be obtained from
>>> the NAIF ftp site:
>>>
>>> ftp://naif.jpl.nasa.gov/pub/naif/generic_kernels/pck
>>>
>>> See the aareadme.txt file there for a discussion of the types
>>> of binary Earth PCKs that are available.
>>>
>>> Once you have the SPK kernel, name/ID code mapping kernel,
>>> and binary Earth PCK file loaded these into your application,
>>> your application can call the SPK API routines (such as SPKEZR
>>> or SPKPOS) to compute position or state vectors using the surface
>>> site as an observer or target.
>>>
>>>
>>> 5) Validate your kernels.
>>>
>>> - Look up the Earth-surface site vector in the ITRF93 frame using
>>> SPKEZR. Check position and velocity against your input data.
>>>
>>> - Check the topocentric frame kernel, if you've created one. If this
>>> frame has been specified to be consistent with the frames NAIF
>>> defines for the DSN, the +X axis should point north, the +Y axis
>>> west, and the +Z axis "up."
>>>
>>> Note that the axes of the topocentric frame, expressed relative to
>>> ITRF93, can be picked off from the rows of the ITRF93-to-topocentric
>>> frame transformation matrix returned by the SPICE routine PXFORM.
>>> The SPICE routines UCRSS (unit cross product) and VSEP (3-d vector
>>> angular separation) can be helpful for vector comparisons.
>>>
>>> - You also can obtain the outward normal vector on the Earth reference
>>> ellipsoid at your site's longitude and geodetic latitude; this vector
>>> should be parallel to your topocentric frame's +Z axis. To do this,
>>> convert the site location to geodetic coordinates using RECGEO; then
>>> use the resulting latitude and longitude and an altitude value of 0
>>> as inputs to GEOREC to produce a point on the reference ellipsoid at
>>> the same latitude and longitude of your site. Call the SPICE routine
>>> SURFNM to obtain the outward normal vector at this point. **Note: unless
>>> your site has an altitude of 0, don't input its Cartesian coordinates
>>> directly into SURFNM; the resulting computation will produce a normal
>>> vector for a different ellipsoid than the one you want to use.**
>>>
>>> After following these steps, you can get the state of a target ephemeris
>>> object,
>>> as seen from the surface site, using just one SPICE call: SPKEZR returns the
>>> observer-target state vector, expressed in a user-specified reference frame,
>>> at a
>>> specified observation time. (If velocity is not needed, SPKPOS can be called
>>> instead.)
>>>
>>> Target azimuth and elevation can be obtained using a second SPICE call:
>>> after a call to SPKEZR or SPKPOS, where the reference frame used is the
>>> topocentric
>>> frame associated with the surface site, a subsequent call to RECLAT yields
>>> the elevation and longitude of the target in that frame. Negating longitude
>>> yields azimuth.
>>>
>>> One note of caution: a variety of "local level" and "azimuth"
>>> definitions exist. The instructions above are compatible with
>>> definitions used by NAIF. Check these against your requirements;
>>> some adjustments might be needed for your application.
>>>
>>>
>>> References
>>> ==========
>>>
>>> PINPOINT User's Guide
>>> http://naif.jpl.nasa.gov/naif/utilities.html
>>> (Follow links for your platform)
>>>
>>>
>>> NAIF_IDs tutorial
>>>
>>> ftp://naif.jpl.nasa.gov/pub/naif/toolkit_docs/Tutorials/
>>> pdf/individual_docs/06_naif_ids.pdf
>>>
>>>
>>> Text kernel tutorial
>>> ftp://naif.jpl.nasa.gov/pub/naif/toolkit_docs/Tutorials/
>>> pdf/individual_docs/08_intro_to_kernels.pdf
>>>
>>>
>>> PCK tutorials
>>>
>>> ftp://naif.jpl.nasa.gov/pub/naif/toolkit_docs/Tutorials/
>>> pdf/individual_docs/20_pck.pdf
>>>
>>> ftp://naif.jpl.nasa.gov/pub/naif/toolkit_docs/Tutorials/
>>> pdf/individual_docs/25_lunar-earth_pck-fk.pdf
>>>
>>>
>>> SPICE Toolkit Required Reading
>>>
>>> http://naif.jpl.nasa.gov/naif/documentation.html
>>> (Follow links for your platform)
>>>
>>> Also included in your SPICE Toolkit under /doc
>>>
>>>
>>> SPICE binary PCK programming Lesson
>>>
>>> ftp://naif.jpl.nasa.gov/pub/naif/toolkit_docs/Lessons/
>>>
>>>
>>> PCK file aareadme.txt file on NAIF ftp site:
>>>
>>> ftp://naif.jpl.nasa.gov/pub/naif/generic_kernels/pck/aareadme.txt
>>>
>>>
>>> DSN station topocentric frame kernel on NAIF ftp site:
>>>
>>> ftp://naif.jpl.nasa.gov/pub/naif/generic_kernels/fk/stations/
>>> earth_topo_050714.tf
>>>
>>>
>>> Comments from DSN station SPK file on NAIF ftp site
>>>
>>> ftp://naif.jpl.nasa.gov/pub/naif/generic_kernels/spk/stations/
>>> earthstns_itrf93_050714.cmt
>>>
>>> Dear all,
>>>
>>> is it possible with SPICE to calculate on an arbitrary coordinate on the
>>> Mars surface at an arbitrary epoch to calculate the local sun zenith angle
>>> and the azimuth of the sun?
>>>
>>> I searched around a lot in the SPICE docs, but it seems that one always
>>> needs an observer frame, but I was hoping that this calculations are
>>> possible without having to write my own frame kernel.
>>> I am aware that I need to define my local frame at my surface point on Mars,
>>> but I am not sure how.
>>> Any help or hint for the right spot in the docs would be appreciated.
>>>
>>> Best regards,
>>> Michael
>>> _______________________________________________
>>> Spice_discussion mailing list
>>> Spice_discussion at naif.jpl.nasa.gov
>>> http://naif.jpl.nasa.gov/mailman/listinfo/spice_discussion
>>> _______________________________________________
>>> Spice_discussion mailing list
>>> Spice_discussion at naif.jpl.nasa.gov
>>> http://naif.jpl.nasa.gov/mailman/listinfo/spice_discussion
>>>
>
More information about the Spice_discussion
mailing list