PDS_VERSION_ID = PDS3 RECORD_TYPE = STREAM OBJECT = TEXT PUBLICATION_DATE = 2020-07-01 NOTE = "Description of the 2001 Mars Odyssey spacecraft orientation and pointing modes." END_OBJECT = TEXT END The labels of all THEMIS standard and special data products contain four keywords which describe the elements of the 2001 Mars Odyssey (ODY) spacecraft attitude orientation and pointing mode at the time of image acquisition. This description is independent of the operational mode for maintaining the spacecraft attitude. The Odyssey spacecraft uses one of two modes to control and maintain the spacecraft attitude, commonly referred to as Gyro and All-Stellar. GYRO based attitude control employs the Inertial Measurement Unit (IMU) to determine when attitude adjustments need to be made, while the All-Stellar mode employs star camera image analysis for the same purpose. The transition between the two modes frequently requries an INERTIAL_HOLD event. Intentional off-nadir maneuvers can only be executed while under Gyro based attitude control. The stability of the spacecraft orbit during imaging is documented in the The GEOMETRY_QUALITY History object, described in the GEOMETRY.pdf document, Appendix A.8. SPACECRAFT_ORIENTATION ====================== The ODY spacecraft orientation is defined by the two keywords and value pairs: SPACECRAFT_ORIENTATION_DESC = (PITCH,ROLL,YAW) SPACECRAFT_ORIENTATION = (#,#,#) where # is the angle in degrees of the rotation around the corresponding axis listed in the SPACECRAFT_ORIENTATION_DESC. For all ODY orientations, the geometry values (latitude, longitude, etc) provided for the THEMIS images correctly account for the position of the spacecraft. If an image does not fully intersect the Mars surface, the latitude and longitude parameters will record the predicted sub-spacecraft coordinates and the remaining geometry parameters will be null. For the ODY spacecraft, a positive angle indicates a right hand rotation around the axes as follows: PITCH is a rotation about the orbit normal direction (Z). ROLL is a rotation about the velocity vector (Y). YAW is a rotation about the nadir vector (X). From the THEMIS frame of reference, a negative pitch maneuver corresponds to a downtrack rotation; the Martian limb is visible with a 60 degree pitch. A positive roll maneuver corresponds to shifting the THEMIS images crosstrack from true nadir, which is farther east for the descending orbital node images and farther west for the ascending orbital node images. The THEMIS boresight is along the nadir vector (-X), therefore, a positive yaw maneuver corresponds to a counterclockwise rotation around the THEMIS boresight. Notes about YAW rotations ------------------------- YAW orientation effects are most obvious in geometrically projected visible (VIS-GEO) images, as each framelet appears to be rotated relative to the image length. This is the only effect on the visible images; there is no change to the quality of the calibrated radiance. While less obvious in the projected infrared images, the effect of YAW maneuvers on the infrared data is more severe. In the nominal mapping orientation (0,0,0), each line of an infrared image is collected perpendicular to the flight direction, and Time-Delay Integration (TDI) can be used to improve the signal-to-noise ratio of each pixel by co-adding 16 measurements of each image line. However, the use of TDI during the YAW orientation has the effect of "blurring" together adjacent pixels in each line. When TDI is enabled throughout the YAW periods, the cross-track spatial resolution is compromised in all infrared images; the values for infrared pixel resolution (LINE_RES_KM and SAMPLE_RES_KM) calculated by ISIS and available in the THMIDX_IR.TAB are incorrect. Until further notice, the THEMIS team recommends that IR-RDR data collected during an HGA Mitigation with YAW rotation be used with caution. These data should not be used for compositional analysis, as there are known problems resulting from using the standard infrared calibration algorithms. SPACECRAFT_POINTING =================== The ODY spacecraft pointing mode is defined in the keyword and value pairs: SPACECRAFT_POINTING_MODE = "[pointing_mode]" ^SPACECRAFT_POINTING_MODE_DESC = "[this_text_file]" where pointing mode is one of the modes listed below, in this text file. SPACECRAFT_POINTING_MODE = "NADIR" ---------------------------------- The NADIR pointing mode is the nominal mode for imaging during the Mapping and Extended-Mission phases of the Odyssey Mission. The nominal SPACECRAFT_ORIENTATION is (0,0,0) for this mode. If the SPACECRAFT_ORIENTATION is not (0,0,0), then the spacecraft experienced a notable change in pointing even though it was expected to remain NADIR pointed. SPACECRAFT_POINTING_MODE = "ODY_MANEUVER" ----------------------------------------- The ODY_MANEUVER pointing mode is used to describe the approximate orientation of the Odyssey spacecraft during off-nadir maneuvers for mission testing purposes or special events. The value of the SPACECRAFT_ORIENTATION keyword was derived using the NAIF tool (frmdiff) to calculate the difference between the orientation measured during the maneuver (available in CK kernels) relative to an ideal nadir orientation. THEMIS images were collected during the following Odyssey maneuvers: ODY_TEST/EVENT ORBIT_RANGE ODY SCAM Test (00837-00838) GRS Boom Deployment (02071-02097) ODY UHF Test (07561-07572) ODY OTM2 Maneuver (08602-08602) PHX-EDL Support (28597-28598) ODY Safe Mode Entry (46499-46501) ODY Side-swap (48418-48466) Comet Siding Spring (56994-57018) Phobos Image Slew #1 (70056-70057) Phobos Image Slew #2 (71747) Phobos Image Slew #3 (76994-76995) Phobos Image Slew #4 (79780-79781) Phobos Image Slew #5 (80733-80734) Phobos Image Slew #6 (81105-81107) SPACECRAFT_POINTING_MODE = "OFF-NADIR" -------------------------------------- The OFF-NADIR pointing mode is invoked when the ODY spacecraft rotates away from nadir to perform a Requested Off-nadir Targeted Observation (ROTO). Routine ROTO observations started in orbit 32214; the value of the SPACECRAFT_ORIENTATION keyword describes the ROTO rotation. SPACECRAFT_POINTING_MODE = "HGA_MITIGATION_[x+#]" -------------------------------------------------- In order to mitigate potential communication problems, the ODY spacecraft has performed several rotation maneuvers to reorient the High Gain Antenna (HGA). The last few characters [x+#] of the HGA_MITIGATION pointing_mode value describe both the sense and the amount of rotation, where x is P, R, or Y respectively for the Pitch, Roll, or Yaw rotation axes + is + for positive rotations, or - for negative rotations # is the angle of rotation in degrees. The value of the SPACECRAFT_ORIENTATION usually corresponds with the [x+#] description; for example, when SPACECRAFT_POINTING_MODE = "HGA_MITIGATION_Y+5", usually SPACECRAFT_ORIENTATION = (0,0,5). However, when ROTOs are performed during an HGA mitigation maneuver, these two values will appear to show a discrepancy. For example, a ROTO which returns the spacecraft to true nadir during a HGA mitigation maneuver would be represented by the keywords SPACECRAFT_POINTING_MODE = "HGA_MITIGATION_R-10" and SPACECRAFT_ORIENTATION = (0,0,0). The HGA mitigation solution usually involves a symmetrical series of discrete rotations, first stepping away from true nadir, then returning. The following table summarizes the HGA mitigation maneuvers throughout the ODY mission: SPACECRAFT_POINTING_MODE AXIS ANGLE ORBIT_RANGE HGA_MITIGATION_Y+5 YAW +5 (15621-15791) HGA_MITIGATION_Y+12 YAW +12 (15791-16411) HGA_MITIGATION_Y+5 YAW +5 (16411-16559) HGA_MITIGATION_R-10 ROLL -10 (32867-33295) HGA_MITIGATION_R-20 ROLL -20 (33295-33997) HGA_MITIGATION_R-25 ROLL -25 (33997-35092) HGA_MITIGATION_R-20 ROLL -20 (35092-35492) HGA_MITIGATION_R-10 ROLL -10 (35492-35758) HGA_MITIGATION_R-10 ROLL -10 (42247-43265) HGA_MITIGATION_R-15 ROLL -15 (43266-44702) HGA_MITIGATION_R-10 ROLL -10 (44703-44957) SPACECRAFT_POINTING_MODE = "INERTIAL_HOLD" ------------------------------------------ The INERTIAL_HOLD pointing mode is used to identify images collected during the periods during and after the Odyssey spacecraft was commanded to maintain a position of "inertial hold". Although this is not technically a spacecraft maneuver, when the hold is released the spacecraft tends to wobble on all axes while attempting to regain NADIR pointing. The value stored in the SPACECRAFT_ORIENTATION keyword was derived using the same NAIF tools described above, and, in most cases, records the average orientation of the spacecraft during image collection. Many THEMIS images collected during an INERTIAL_HOLD event actually show a significant change in pointing (0.5 to 2.0 degrees). Any instability will be documented in the GEOMETRY_QUALITY History object of the calibrated (RDR) and projected (GEO) image products. Additional pointing modes will be defined as they are needed for THEMIS documentation.