This program augments the nominals file by appending the nominal inertial space position differences between the current image and up to four images before and after. This information is used to constrain the solution so that the spacecraft position solution does not deviate too much from a dynamically realistic state. This program also allows for different mission phases to have different "sigmas" via DYNAMICS.TXT.
In version 3.1, the number of images that is used before/after is set in INIT_LITHOS.TXT with keyword DYNO.
Dynamics will useINIT_LITHOS.TXT to set the parameters for SPICE. This is set with keyword PCK. Typically, we set the leap second kernel and the PCK.
If you want to have these the images before/after impact the location of the spacecraft, then the PICWTS option 5 (WT) must be set to 1 (or a non-zero value) in INIT_LITHOS.TXT
- Updated NOMINALS file
Input for the program in DYNAMICS.TXT includes values for 3 parameters and lists of nominals to which they apply. For additional information, see DYNAMICS.TXT.
The following sample is based on excerpts from a DYNAMICS.TXT file for DAWN at Vesta:
FRAME='BOD_FRAME' APPROACH ETLM= 1800, 1.D-6 VSIG= 0.200, 0.200, 0.200 PSIG= 0.0001, 0.0001, 0.0001 F11A0001225 F11A0001241 ... F21A0003895 F21A0003910 FRAME='dxR_FRAME' SURVEY ETLM= 1800, 1.D-6 VSIG= 0.100, 0.100, 0.040 PSIG= 0.0001, 0.0001, 0.0001 F21A0003931 F21A0003932 ... F21A0032347 F21A0032348 END
A typical ETLIM= record is:
ETLM= 1800, 1.D-6
ETLIM= gives a maximum time difference in seconds that limits the neighboring images to be used. The images that are included in dynamics are limited by both this number, as well as number of images. Dynamics will stop including images when either of these limits are reached. The second number in ETLM is an estimate of the velocity uncertainty that goes into an uncertainty estimate included in the added records. This velocity uncertainty is used to determine the sigma in position associated with dynamics. A typical value is 1 mm/s; the same as in the example. A larger value will weight the dynamics solution less, while I a smaller value will weight the dynamics solution more.
FRAME= has these three choices:
FRAME= ['dxR_FRAME', 'Dxr_FRAME', 'BOD_FRAME']
FRAME= specifies how to interpret the sigmas in spacecraft position found in the nominals file.
dxR_FRAME uses the radial direction (R) as prime for the third component with the second component being the cross track direction (X) and the first component, roughly in the downtrack direction completing the right-handed coordinate system. This is used in orbital operations around large bodies, where the radial component is well known from the Doppler data.
Dxr_FRAME is used during approach where the downtrack velocity is best known. The "radial" (impact parameter) direction is crossed with the downtrack to give the crosstrack direction.
BOD_FRAME simply uses the components in the body-fixed frame, generally with equal uncertainties in each direction.
The VSIG= record has three components:
VSIG= 0.100, 0.100, 0.040
VSIG= allows the user to change the position sigmas from those in the original nominals file. The record in the sample, sets the uncertainties in the three components to 100 m, 100 m and 40 m respectively. If, during another mission phase, these sigmas change, another VSIG= record would be added to affect subsequent nominals files.
The PSIG= record has three components:
PSIG= 0.0003, 0.0003, 0.0005
PSIG= allows the user to change the pointing sigmas in the nominal files, which are in rad. The three components refer to rotations about the three camera axes. The sample shows the approximate values expected for OSIRIS_REx with a larger twist uncertainty since there is only one star tracker.
(Compiled by TC)