PDS_VERSION_ID = PDS3 RECORD_TYPE = FIXED_LENGTH RECORD_BYTES = 80 OBJECT = TEXT PUBLICATION_DATE = 1999-05-05 NOTE = "Description of MGS spacecraft antennas, software, and kernels needed to reconstruct their positions and orientations." Formatted with up to 78 constant width characters per line." END_OBJECT = TEXT END This document describes the high- and low-gain antennas on the Mars Global Surveyor (MGS) spacecraft (s/c), how their locations and pointing can be described, and the software and kernel files produced by the Navigation and Ancillary Information Facility (NAIF) at JPL that allow users to reconstruct these quantities. Most of the material which follows has been adapted (copied) from documentation provided by Boris Semenov JPL/NAIF. ============================================================================== MGS Antenna Descriptions ============================================================================== The MGS spacecraft was equipped with one high-gain antenna (HGA) and four low-gain antennas (LGA), two for transmit and two for receive. The HGA was mounted on a boom attached by a hinge to the +X side of the AFT deck of the spacecraft propulsion module. The HGA boresight direction was fixed and co-aligned with the spacecraft +X axis when the antenna was in the stowed configuration (during Cruise and Orbit Insertion phases of the mission, and during the first month of the Mapping phase). After the HGA was deployed for Mapping operations, it was pointed by rotation of the azimuth and elevation gimbals. Both transmitting LGA's were mounted on the TWA Enclosure Box which was itself mounted on the HGA reflector. One of the transmit LGA's (LGT1) was co-aligned with the HGA boresight (nominally in the +X direction while the antenna was stowed), while the other's (LGT2) boresight was oriented approximately 160 degrees away from this axis (near the -X axis while the HGA was stowed). The two receive LGAs were mounted on the -X panel of the equipment module (LGR2) and the +X side of the propulsion module (LGR1). ============================================================================== MGS HGA Boom/Gimbal Diagram ============================================================================== The diagram below illustrates relative locations of the HGA hinge, gimbal, and boresight frame centers (frames will be discussed more extensively below). The antenna is shown as if it were fully extended along the s/c +X axis in the s/c XY plane; dimensions are given in inches (1 inch = 2.54 cm): Top view (+Zsc view): --------------------- * * * * * * * * * * * * * * * * * ---------------------------- * o * ^ * * | * * | 27.33" * * | * * | * * | * *____ v * __| |_____ ------------------------------------ * * o__*___| ___ o| ^ |_____| | | | azimuth _|_|_ | ______ gimbal | | elev. | 14.33" | | | gimbal | |____ |_____| v | |___________________________________| | --------------- | o _____________________________________| | |_____| ------ o | deployment ^ | hinge | 13.00" | | | ________| Side view (-Ysc view): ---------------------- ------------------------------ ___________o___________ ^ | \__ __/ | 12.06" |___ \__ __/____ ___ v / \______________\_________/_|_____|_ / \ -------- _________| o ___________________o______________| o | ^ \ \___/ |_____| \___/ | 6.788" \ | v \ | | | ------ __o_____\ | | 31.655" | | |<-------------->| | | | | | | | | 27.25" | 90.778" | |<--------->|<------------------------------------>| | | | ============================================================================== MGS LGT1/LGT2 Diagram ============================================================================== The diagram below illustrates location of the LGT1 and LGT2 with respect to the HGA boresight frame (dimensions are given in inches; 1 inch = 2.54 cm): Front View (+Zb): ----------------- ^ | +Yb TWA _/\ box _/ \ _/ * * * * * _/ * * LGR2_/ * * * ------------/@ * * * ^ --------\ @ LGR1 * * 15.41"| ^ \_* * * | |15.31" ||* * _ * V V ||* / \ * ------------ ||* | o |* * * * ** --> +Xb ||* \_/ * ||* * * ||* * * || * * * || * * * || * * * || * * || * * * * * || 28.18" | ||<---------->| | | |<----------->| 30.05" Side View (-Xb): ---------------- ~18.6" |<------->| | | ~0" | ->|<----- | _____ | | TWA | | | box |/| | | | | \ |_____| | LGR2 @=| | | \ | |=@ |_____| |LGR1 \ / | | | / | o - - | Sub-reflector | | \ \ | | | / \ | | | / \ | | | / \| ============================================================================== MGS LGR1/LGR2 Diagram ============================================================================== The diagram below illustrates location of the LGR1 and LGR2 with respect to the spacecraft frame (the HGA is shown in stowed configuration; dimensions are given in inches; 1 inch = 2.54 cm): Top View (+Zsc): ---------------- ^ +Ysc | | __ ___________ LGR1/ | | | | / | v | | |=@/ | ----- 0"| | | / | | 5.29" v | || HGA | | ----- @=| o ||__ | ----- ---> ^ LGR2| || | | ^ +Xsc | | || | | | | || | | |___________|| | | | |\__| | |=@ @=|__| LGT2 LGT1 Side View (-Ysc): ----------------- ^ +Zsc | | LGT2 __ LGT1 @=| |=@ | | __ |__|/ | ___________ / | | | / | | | / | | || | LGR2| || | HGA ----- @=| || | ^ | |___________| \ | | | | | \ | | | | | \ | | | | | \__| 49.94"| | | | | | | | LGR1 | | | |=@ --- | | |___________| | ^ | | / \ | | 17.56" | | / \ | | v | / \ | v ------+- +----o----+ -+---- ---> | | | +Xsc |<----->|<----->| 25.81" 31.75" ============================================================================== MGS HGA Hinge Geometry ============================================================================== The MGS HGA deployment hinge was the device attaching the HGA boom to the +X side of the AFT deck of the propulsion module. After one month of Fixed High- Gain Antenna Mapping, the locks holding the antenna in its stowed configuration were released and, driven by a spring, the hinge rotated from stowed to deployed position and locked into the deployed position for the rest of the mission. The hinge rotation axis was parallel to the s/c Y axis. In the stowed configuration the antenna boom was parallel to the s/c Z axis. In the deployed configuration the boom was parallel to the s/c XZ plane and was rotated by 5 degrees from the s/c -Z axis towards the s/c +X axis. The full nominal deployment angle between stowed and deployed positions was 175 degrees. There was no direct measurement of the hinge deployment angle available in the s/c engineering telemetry after the antenna was deployed. The actual deployment angle was estimated from signal strength during HGA calibration tests immediately after deployment. The Hinge CK (HCK) file contains the hinge angle for the deployed HGA, which is taken to be +85.0 degrees. The hinge angle for the stowed HGA (not included in the file) was -90.0 degrees. There was no rotation (angular rate) either before or after deployment. ============================================================================== MGS HGA Gimbal Geometry ============================================================================== The MGS HGA Gimbal assembly connected the HGA reflector to the end of the HGA boom. It consisted of two independent gimbals -- elevation (EL) and azimuth (AZ) -- and was used to achieve antenna pointing in the deployed configuration. The EL gimbal was attached to the antenna boom. The AZ gimbal was attached on one side to the EL gimbal with a "corner-like" fitting and on the opposite side to the antenna reflector. The EL gimbal rotation axis was parallel to the deployment hinge rotation axis. The AZ gimbal rotation axis was perpendicular to the EL gimbal rotation axis and parallel to the s/c XZ and the antenna reflector rim circle planes. The gimbals had the following soft/hard stop positions, determining limits of the rotation ranges: Soft Stops: azimuth: -8 deg ... +81 deg elevation: -153 deg ... +153 deg Hard stops: azimuth: -30 deg ... +190 deg elevation: -158 deg ... +158 deg The "zero" angle position (EL=0, AZ=0) put the HGA boresight vector parallel to the -Z axis of the spacecraft. Other recognized antenna positions included: Stowed Position: azimuth: +180 deg elevation: -95 deg Initial Deployed azimuth: 0 deg Position: elevation: -90 deg Park Position: azimuth: 80 deg elevation: -120 deg The antenna gimbal angle values were available in the spacecraft engineering telemetry channels: F-0190 (HGA_AZ_ANG) Azimuth Angles F-0195 (HGA_EL_ANG) Elevation Angles at the s/c housekeeping medium rate -- i.e., once every 32 seconds. The angle values were downlinked in radians. There were also three additional HGA data telemetry channels: F-0193 (HGA_AZ_TRG) Azimuth Targets F-0198 (HGA_EL_TRG) Elevation Targets F-0200 (HGA_STATS) HGA Status words the first two of which gave the expected final values +/-0.04 degrees of the gimbals when motion was commanded and which are unnecessary for the computation of the gimbal rotations. During mapping, movement of the azimuth gimbal was, in general, fixed for a given orbit. For a fixed azimuth position, the elevation gimbal angle was varied to provide proper pointing throughout the orbit. ============================================================================== MGS Antenna Frames ============================================================================== The following MGS Antenna frames are defined in the Text Frames Kernel file (NAIF antenna ID codes are of the form -9407x): Name Relative to Type NAIF ID ====================== =================== ============ ======= MGS_HGA_HINGE MGS_SPACECRAFT CK -94070 MGS_HGA_EL_GIMBAL MGS_HGA_HINGE CK -94071 MGS_HGA_AZ_GIMBAL MGS_HGA_EL_GIMBAL CK -94072 MGS_HGA MGS_HGA_AZ_GIMBAL FIXED -94073 MGS_LGT1 MGS_HGA FIXED -94074 MGS_LGT2 MGS_HGA FIXED -94075 MGS_LGR1 MGS_SPACECRAFT FIXED -94076 MGS_LGR2 MGS_SPACECRAFT FIXED -94077 In the list above "CK" means "CK kernel based frame" and "FIXED" means "fixed offset frame". The "Frames Required Reading" file in the general NAIF documentation package contains more information on supported frame types. The NAIF ID code of the MGS s/c fixed frame origin is -94000 (located at the center of the bottom of the main engine). The MGS s/c center of mass has NAIF ID code -94; this is the point for which an OD (orbit determination) solution is done and the position of which is stored in a normal s/c SPK file. ============================================================================== Coordinates ============================================================================== This table contains coordinates of various antenna structures with respect to each other. These data can also be found in the Antenna SPK (ASP) file. ID CENTER FRAME X,m Y,m Z,m ------- ------- -------------------- ------- ------- ------- -94000 -94 MGS_SPACECRAFT 0.000 0.000 0.000 -94070 -94000 MGS_SPACECRAFT 0.692 0.330 0.172 -94071 -94070 MGS_HGA_HINGE 2.306 0.000 0.364 -94072 -94071 MGS_HGA_EL_GIMBAL 0.804 0.000 0.000 -94073 -94072 MGS_HGA_AZ_GIMBAL 0.694 -0.306 0.000 -94074 -94073 MGS_HGA -0.716 0.389 0.000 -94075 -94073 MGS_HGA -0.763 0.391 -0.472 -94076 -94000 MGS_SPACECRAFT 0.806 0.134 0.446 -94077 -94000 MGS_SPACECRAFT -0.656 0.000 1.268 -94078 -94073 MGS_HGA 0.000 0.000 0.000 ============================================================================== MGS Antenna Frames Hierarchy ============================================================================== The diagram below shows the MGS antenna frames hierarchy: "IAU_MARS" "IAU_EARTH" MARS BFR(*) EARTH BFR(*) ------------ ------------- ^ ^ | | | <--pck | <--pck | "J2000" INERTIAL(*) | +-----------------------------------------------+ | | <--ck | V "MGS_SPACECRAFT"(**) +-----------------------------------------------+ | | | | <--fixed | <--ck | <--fixed | | | V V V "MGS_LGR1" "MGS_HGA_HINGE" "MGS_LGR2" ---------- --------------- ---------- | | <--ck | V "MGS_HGA_EL_GIMBAL" ------------------- | | <--ck | V "MGS_HGA_AZ_GIMBAL" ------------------- | | <--fixed | V "MGS_HGA" +-----------------------------------------------+ | | | <--fixed | <--fixed | | V V "MGS_LGT1" "MGS_LGT2" ---------- ---------- (*) Inertial and body-fixed rotation (BFR) frames are standard frames supported by the NAIF SPICE system and, therefore, don't require custom definitions here. (**) for historical reasons the MGS_SPACECRAFT frame is defined in an MGS SCLK file. ============================================================================== MGS HGA Frame Definitions ============================================================================== The MGS HGA deploy hinge frame is defined as follows: - Z axis is along deploy hinge rotation axis, and is parallel to and points in the same direction as the s/c frame +Y axis; - X is perpendicular to the hinge rotation axis, parallel to the HGA boom central axis and points along it from the deploy hinge side towards the elevation gimbal side; - Y completes the right hand frame; - the origin of this frame is located at the intersection of the hinge rotation axis and a plane perpendicular to the rotation axis and containing the central axis of the boom. The MGS HGA elevation gimbal frame is defined as follows: - Z axis is along the elevation gimbal rotation axis and points from the HGA boom side towards the azimuth gimbal side; - X is perpendicular to the elevation gimbal rotation axis, parallel to the azimuth gimbal rotation axis and points from the elevation gimbal side towards the HGA reflector mounting side; - Y completes the right hand frame; - the origin of this frame is located at the intersection of the elevation gimbal rotation axis and a plane perpendicular to this rotation axis and containing the azimuth gimbal rotation axis. The MGS HGA azimuth gimbal frame is defined as follows: - Z axis is along the azimuth gimbal rotation axis and points from the elevation gimbal side towards the HGA reflector mounting side; - X is perpendicular to the azimuth gimbal rotation axis, parallel to a plane containing the HGA reflector rim circle and points from the azimuth gimbal towards the HGA rim circle center; - Y completes the right hand frame; - the origin of this frame is located at the intersection of the azimuth gimbal rotation axis and a plane perpendicular to this rotation axis and containing the HGA reflector central symmetry axis (boresight axis). The MGS HGA boresight frame is defined as follows: - Z axis is along the HGA reflector central symmetry axis (boresight axis) and points from the reflector surface towards the feed horn; - X is perpendicular to the boresight direction, perpendicular to azimuth gimbal rotation axis and points from the antenna symmetry axis towards the side of the reflector where the azimuth gimbal is attached; - Y completes the right hand frame; - the origin of this frame is located at the intersection of the antenna reflector symmetry axis and a plane containing the HGA reflector rim circle. The diagram below illustrates the HGA hinge/gimbal/boresight frame definitions (the antenna is shown as if boom were fully extended along the s/c +X axis; elevation and azimuth gimbal axes are in the s/c plane and the antenna boresight is along the s/c +Z axis): Top view (+Zsc view): --------------------- * * * * * * * * * * * * * * * * * * +Zb o----> +Yb * * | * * | * * v +Xb * * * * ^ * ^ +Zel * |+Xaz *____ | * | __| |____|_ <----x__*___| +Xel<----o|+Yel +Zaz +Yaz |_____| | | azimuth _|_|_ ____________ +Zh gimbal | | elev. | ^ | | gimbal +Ysc |_|__ |_____| ^ +Yh| |___________________________________| | | | x---->___________________________________| | +Xsc |_____| +Xh o----> | deployment +Zsc | hinge | | | ____________| Side view (-Ysc view): ---------------------- ^ +Zb | | | +Yb | ___________o---->______ +Yel | \__ +Xb __/ ^ |___ \__ __/____ _|_ /+Zh\_______________\__+Xaz __/_| |_/ | \ ____________| x---->____________<----x______| +Xel<---x +Zel ^+Zsc \_|_/ +Xh +Zaz | |_____| \___/ | \ | | | +Xsc V +Yh v +Yaz ___x--->_\ +Ysc In the diagram: +Xsc,+Ysc,+Zsc -- axes of the s/c frame; +Xh, +Yh, +Zh -- axes of the hinge frame; +Xel,+Yel,+Zel -- axes of the elevation gimbal frame; +Xaz,+Yaz,+Zaz -- axes of the azimuth gimbal frame; +Xb, +Yb, +Zb -- axes of the HGA boresight frame; "o" shows axes pointing "out of the page", "x" shows axes pointing "into the page" As follows from the definitions, the HGA boresight frame is rotated from the azimuth gimbal frame by two rotations -- first by +90 degrees about the +X axis and second by +180 degrees about the +Z axis. Actual frame definition keyword sets are contained in the Text Frame Kernel file (note opposite sign/order of rotations in the MGS_HGA definition because the definition contains a transformation from antenna to reference frame). ============================================================================== MGS Transmit LGA Frame Definitions ============================================================================== The MGS LGT1 boresight frame is defined as follows: - Z axis is perpendicular to the antenna "patch" surface and points away from the surface; - X axis is parallel to the line connecting the "patch" center with the "patch" corner farthest from both antenna connectors attached to the bottom side of the patch and points from the center towards the corner; - Y axis completes the right hand frame; - the origin of this frame is located at the geometric center of the antenna "patch" square. The MGS LGT2 boresight frame is defined as follows: - Z axis is perpendicular to the antenna "patch" surface and points away from the surface; - X axis is parallel to the line connecting the "patch" center with the "patch" corner farthest from both antenna connectors attached to the bottom side of the patch and points from the center towards the corner; - Y axis completes the right hand frame; - the origin of this frame is located at the geometric center of the antenna "patch" square. The LGT1 frame is rotated from the HGA boresight frame by +214 degrees about the +Z axis (34 degrees due to the TWA box mounting on the HGA reflector plus 180 degrees due to patch orientation with respect to the bracket on which it's mounted). The LGT2 frame is first rotated +34 degrees about the +Z axis (due to the TWA box mounting on the HGA reflector), then by -130.88 degrees about the new direction of the +Y axis, and finally by -10.4 degrees about the new direction of the +X axis (the last two rotations are due to the "sophisticated" geometry of the lower LGT2 mounting bracket). Actual frame definition keyword sets for the LGT1 and LGT2 frames are given in the Text Frames Kernel file (note opposite sign/order of rotations because the definitions contain the transformation from antenna to reference frame). ============================================================================== MGS Receive LGA Frame Definitions ============================================================================== The MGS LGR1 boresight frame is defined as follows: - Z axis is perpendicular to the antenna "patch" surface, and is parallel to and points in the same direction as the s/c +X axis; - Y axis is parallel to and points in the same direction as the s/c +Y axis; - X axis completes the right hand frame; - the origin of this frame is located at the geometric center of the antenna "patch" square. The MGS LGR2 boresight frame is defined as follows: - Z axis is perpendicular to the antenna "patch" surface, and is parallel to and points in the same direction as the s/c -X axis; - Y axis is parallel to and points in the same direction as the s/c +Y axis; - X axis completes the right hand frame; - the origin of this frame is located at the geometric center of the antenna "patch" square. The LGR1 frame is rotated from the spacecraft frame by +90 degrees about the +Y axis and the LRG2 frame is rotated from the spacecraft frame by -90 degrees about +Y axis. The diagram below illustrates LGR1 and LGR2 frame definitions: Side View (-Ysc): ----------------- LGT2 ____ LGT1 @=| |=@ | |_ |__ / | ___________ / | +Xlgr2 | | / | ^ | | / | | | || | +Zlgr2 | | || | HGA <----x=| || | +Ylgr2|___________| \ | | | \ | | | \ | | | \__| | | | +Ylgr1 | |=x----> |___________| | +Zlgr1 +Zsc \ | / ^ \ v / | \ +Xlgr1 / | \ /_____x----> +Ysc +Xsc In the diagram: +Xsc, +Ysc, +Zsc -- axes of the s/c frame; +Xlgr1, +Ylgr1, +Zlgr1 -- axes of the LGR1 frame; +Xlgr2, +Ylgr2, +Zlgr2 -- axes of the LGR2 frame; "x" shows axes pointing "into the page" Actual frame definition keyword sets for the LGR1 and LGR2 frames, which incorporate these rotations, are in the Text Frames Kernel file (note opposite sign/order of rotations because the definitions contain the transformation from antenna to reference frame). ============================================================================== MGS Antenna Gimbals Kernel (AGK) Files ============================================================================== An AGK file contains orientation and angular rate data for the MGS HGA elevation and azimuth gimbal frames. The orientation of the 'MGS_HGA_EL_GIMBAL' is given with respect to the 'MGS_HGA_HINGE' frame; orientation of the 'MGS_HGA_AZ_GIMBAL' is given with respect to the 'MGS_HGA_EL_GIMBAL' frame. AGK files were reconstructed C-Kernel files created by NAIF. They could be used for spacecraft operations and radio science data processing support by all project science and engineering teams. Each file was generated automatically. The process extracted HGA EL and AZ gimbal angle values from the MGS Telemetry Data System, computed corresponding rotations and wrote them to a CK file using the NAIF MSOPCK program. Since MGS s/c engineering telemetry didn't contain angular rate data for the gimbals, the angular rates stored in each file were computed from the orientation information with the assumption that each gimbal was rotating with a constant rate between any two adjacent angle values extracted from telemetry. Each file contains two type 2 CK segments which provide linear interpolation between orientation data points extracted from telemetry. Such interpolation is not applied to the whole coverage of a segment but only inside intervals where enough orientation telemetry data is available and orientation data points are close enough to each other in time for such interpolation to make sense. A table containing the complete list of valid interpolation intervals in each segment of the file is provided in a comment field at the end of each file. The start time and stop times of the total coverage for every segment in the file are given in the header of the intervals table for that segment. ============================================================================== How to Use SPICE to Get Antenna Position and Orientation ============================================================================== Antenna position and orientation can be obtained using SPICE tools as follows: (1) Load the following SPICE kernels file into your program (file names are names as assigned by NAIF): * SPICE leapseconds file (e.g., naif0007.tls) * SPICE PCK file (e.g., pck00006.tpc) * MGS on-board clock SCLK file (e.g., MGS_SCLKSCET.000xx.tsc) * MGA HGA Frame Definitions kernel (e.g., hga.tf) * MGS Antenna Structures SPK file (e.g., hga.bsp) * MGS s/c CK file (e.g., mgs_spice_c_kernel_YYYY-DOY.bc) (*) * MGS HGA Hinge CK file (e.g., mgs_hga_hinge.bc) * MGS HGA Gimbal CK file (e.g., mgs_hga_ck_YYYY-MM-DD.bc) (*) (*) these CK files should contain enough data to cover time (or interval of time) of your interest. (2) To get position and velocity of the center of a particular antenna with respect to the spacecraft frame, call SPKEZ routine as follows: CALL SPKEZ ( ANTID, ET, 'MGS_SPACECRAFT', 'NONE', -94000, STATE, LT ) where ANTID is the NAIF ID code of the antenna of your interest (see above for complete list of the IDs); For present purposes, the phase center of the HGA is assumed to be on the axis of the HGA reflector at a distance "d" from the plane defined by the rim of the physical reflector. "d" is presently unknown, but it could be the distance of the feed horn mouth from the planes of the reflector rim. Since the location of the phase center is just a fixed offset with respect to the antenna boresight frame (defined by the plane formed by the rim of the reflector and the axis of the dish) it can be easily updated at any time. Phase center of each LGA is assumed to be the geometric center of the antenna "patch". Interpolation between gimbal readings in the engineering data stream will be linear except that no value will be returned if the gimbal data spacing is larger than a preset TBD limit. (3) to get orientation of a particular antenna with respect to the spacecraft, inertial, or Mars or Earth body fixed frame call SXFORM as follows: CALL SXFORM( FROM, TO, ET, XFORM ) where XFORM is an output 6x6 matrix that rotates state vectors from the frame with the name FROM to the frame with the name TO at ephemeris time ET. For example a call CALL SXFORM ( 'MGS_HGA', 'J2000', ET, XFORM ) will give you the transformation from HGA boresight frame to the inertial J2000 frame at time ET.