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HIGH GAIN ANTENNA DEPLOYMENT SEQUENCE OF EVENTS

 

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Animation of HGA Deployment - Large 570 KB

HGA Deployment - Small 147 KB

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 Deployment Timeline in PDF Format

Overview of Activities

There are two stages to the HGA deployment and several subsequent steps in checkout and calibration. Stage 1 is the deployment itself (HGADPLY1), stage 2 is the azimuth unstow (HGADPLY2). After successful deployment and azimuth unstow, the elevation is moved to +X-axis alignment, and the downlink is configured for HGA two-way non-coherent mode (HGACHK). Following acquisition of the HGA downlink carrier there is a coarse calibration in elevation only, to correct for latch angle uncertainty (HGACAL1). A fine calibration in azimuth and elevation may be required depending on the KaBLE pointing information obtained during HGACHK and HGACAL1.

Figure 1 shows the HGA configuration throughout HGADPLY1. Figure 2 shows the HGA configuration at the start and end of HGADPLY2.

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Figure 1. HGA Deploy Cmds Part 1; a) ANS; b) ISH; c) Partially Deployed; d) Fully Deployed; e) ISH 1

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Figure 2. HGA Deploy Cmds Part 2; a) Gimbals after Hinge Deployment; b) Gimbals after "Unstow"

The first stage consists of the deployment itself (HGADPLY1 is added to a background sequence), during which body rate and HGA boom hinge potentiometer data are recorded on two recorders, SSR_2A and SSR_1B Partition 1. The first stage is expected to take no more than ten minutes to complete, and fifteen minutes is scheduled. The go ANS commands on the deploy orbit and the following two orbits will be edited out of the background GravCal-type sequence. For contingency purposes another set of three Earth-pointed ISH-only orbits will follow after two ISH-ANS orbits. The AEM scripts will have been disabled by the final Fixed HGA Mapping sequence and appropriately timed replacement commands will be added to the GravCal background sequence. This background sequence will be edited to deconflict automatic Telecom activities with designed deployment-related Telecom activities.

At the end of the first stage the downlink and the uplink remain configured for the LGA and recorder 1B is stopped while 1A continues to record. In the case of a nominal deployment the downlink is lost due to the new pointing of the LGA transmit antenna some time during the post-deploy ISH. Ground personnel will monitor the downlink carrier during deployment to make an assessment of deployment success. After the post-deploy ISH is completed, downlink is switched by the onboard sequence to the alternate low gain transmit antenna LGT2, which is mounted askew on the back of the TWTA box. In the case of an early loss of carrier lock due to deployment perturbation ground personnel will rely on retrieval of 10 bps Emergency Mode telemetry from LGT2, and the DSN stations will have the rest of the deploy orbit to lock up on this telemetry. The spacecraft will remain in the post-deploy Earth-pointed ISH attitude until the fourth orbit.

If either the expected carrier behavior is seen through deployment OR post-deploy 10 bps telemetry verifies the expected potentiometer values then the deployment will be considered a success and the command to start the HGADPLY2 sequence will be sent at the next uplink window. If upon entry into the first post-deploy occultation neither of these validations are available, the second orbit will provide another opportunity for the DSN to lock up on the 10 bps telemetry. If there are indications that a partial deployment has occurred, then the +X Earth-pointed attitude may preclude even carrier lock over LGT2, in which case the second orbit will be used to execute a Blind Deployment Contingency Sequence. This sequence switches back to LGT1 and slews the Earth toward -Z in the X-Z plane at the solar array panel movement rate, thus providing power while giving the ground another opportunity to see the carrier and distinguish between an LGT2 problem and a partial deployment. If carrier lock is established and maintained throughout the course of this manuever, correlation of signal strength with attitude will provide rough indication of boom deployment angle. This contingency sequence may be executed several times on consecutive orbits, as needed.

After analysis has verified that the boom hinge latched and HGA gimbal movement may commence safely the Mission Phase Relays are set for Mapping and the second deploy stage (HGADPLY2) is initiated. It consists of post-deploy parameter updates, activation of HGA gimbal drive electronics and related REDMAN monitoring, movement of the azimuth gimbal in such a way as to distance the HGA from the boom and move it to within soft stop-controlled angular ranges, and switch back to LGT1 at 45° of AZ movement (to prevent LGT2 from irradiating the -Y SA). The initial (stowed) gimbal angles are AZ=180°, EL=-95°. To avoid contact with the boom, the HGA is first moved only in the negative azimuth direction (through +90°) to AZ=0°,EL=-95°.

In the nominal case the carrier strength will drop and eventually lock will be lost as the azimuthal motion carries Earth out of the LGT2 beam and the sequence switches to LGT1. Carrier and eventually telemetry should be seen again as azimuthal motion moves Earth into the center of the LGT1 beam. The 10 bps telemetry will be used to verify the position of the azimuth gimbal. In that case, a command will be radiated (on the same orbit, if possible) to move the EL gimbal from -95° to -90°, aligning the HGA beam with the spacecraft +X axis. (To avoid boom impingement, elevation movement can only take place after verification of AZ unstow.) On the following orbit the HGACHK sequence will be initiated by ground command.

If the azimuth gimbal becomes stuck during unstow there will be indications in the downlink carrier signal. If little or no drop-off of LGT2 carrier is seen until the expected switch to LGT1, it can be concluded that the azimuth gimbal did not move or moved less than about 40°, in which case a contingency command will be sent to switch back to LGT2 for anomaly assessment. If LGT2 carrier drop off and loss are as expected but there is no or low return of carrier over LGT1 then it is likely that the azimuth gimbal became stuck somewhere around the halfway point (~80°±40°). In this case neither low-gain transmit antenna is a viable source of 10 bps telemetry and a Partial Unstow Contingency Sequence is initiated on a No-ANS orbit. This sequence asserts carrier-only downlink on LGT1 and moves the solar arrays through optimal positions while the spacecraft slews in such a way as to move the Earth across the LGT1 beam. Correlation of carrier signal strength with attitude will provide indication of azimuth gimbal position. This contingency sequence may be executed several times, as needed, but not on consecutive orbits due to probable battery discharge .

After stage 2 is complete, the HGACHK sequence is initiated. This sequence configures for 2 kbps ENG downlink over the HGA but keeps uplink over the LGA.

If HGA downlink has been satisfactorily reacquired at +X alignment then the coarse (elevation) calibration (HGACAL1) will not be necessary. Otherwise, the HGACAL1 sequence is initiated by ground command. This sequence puts HGA gimbal positions into high rate telemetry and moves the HGA slowly through 10° elevation twice to allow ground calibration of elevation bias, recording the movement on SSR_1B Partition 2. If there is a problem with the HGA downlink, a real-time command is sent to switch back to LGT1 at 10 bps for anomaly assessment.

Once the coarse calibration is complete, a real-time command is sent which sets the elevation bias in Flight Software. Ten bps telemetry is then used to verify that the elevation bias update worked properly, at which point a configuration and playback load-and-go script is uplinked. This script selects HGA for both uplink and downlink and performs two contiguous playbacks of the recorded deployment events on SSR_1Bp1 and also the recorded CAL1 events on SSR_1Bp2. The DSN then performs an uplink acquisition into the HGA and the Spacecraft Team uplinks the fine calibration GravCal sequence and rests in preparation for fine calibration activities the next day.

The three fine calibration activities consist of a slew to point principle axes toward Earth and very slow movement of the HGA across a grid pattern for calibration of 3-axis biases, recording each cal slew on a separate partition of SSR_1A. Each activity is hand-edited into a GravCal type background sequence, taking the place of ISH-ANS activities for their respective orbits. The +Y calibration is likely to result in battery discharge, so it is performed last. On the orbit following the +Y cal the three recorded partitions on SSR_1A are played back twice. In case of ground problems these calibrations may be repeated during any pass in which Ka-band support is available.

After fine calibration of the HGA, a sequence is executed which tests HGA auto-tracking and calibrates MHSA performance in Mapping style orbits while in CSA backup control mode. Once this test and calibration is complete MGS is ready for instrument turn-on (and possible calibrations ) and primary mode mapping.

 

HGA COARSE AND FINE CALIBRATION GOALS AND PROCEDURE

Deployment of the HGA and subsequent calibration can be accomplished in approximately two days if all goes well. It will probably take several days to sort out the misalignment survey data and to prepare an uplink of the HGA gimbal misalignment matrix. Estimates of HGA misalignments in cruise are in agreement with pre-launch measurements, so there are no indications of any change to the HGA system alignments so far in flight. HGA deployment is expected to proceed nominally without introducing any significant new misalignments into the system.

Calibration of the actual pointing of the HGA is split up into two phases. Phase 1 performs an elevation alignment calibration to return HGA downlink bit rate capability to 2 kbps or better. Note that when using the low gain downlink antenna with a significant angle off of the Earth, a 70m station is necessary to provide adequate tracking of the spacecraft. Phase 2 evaluates the HGA pointing at three different pointing angles in the nominal range of the elevation and azimuth gimbals. This data will be used, if necessary, to build and uplink corrections for the remaining pointing errors.

The largest known error source in the alignment of the deployed HGA is the final hinge position. The HGA deploy hinge axis is very nearly parallel to the elevation gimbal, such that an error in the hinge position can be corrected by updating the gimbal encoder bias parameter. The initial HGA pointing calibration, then, is designed to provide an elevation encoder bias that will correct for the true hinge position.

The purpose of Phase 2 is to evaluate lower magnitude errors in gimbal axis alignment and orthogonality as well as any hinge or boom distortion. Using the misalignments measured at each of three pointing vectors aligned with a cardinal axis, a misalignment transformation matrix can be calculated. When uploaded, the misalignment matrix should provide the best overall fit at all commanded gimbal positions. Note that if the results of all test points are inconsistent (and hopefully below specified pointing requirements for the HGA) then the identity matrix should remain loaded in the misalignment matrix, and the remaining errors will simply be tolerated during mapping operations.

Initial Elevation Calibration

The HGA Check sequence switches the downlink to the HGA and sets the data rate to 2 kbps on the 21.333 kHz subcarrier with the modulation index set for 59.9°. The USO is enabled on MOT 2 (disabled on MOT 1) and the transponder will remain in two-way non-coherent mode to ensure no carrier mode changes during the calibration. The uplink is left configured via LGR1 and the KaBLE oscillator select switch is commanded to position C (USO). This configuration will remain in force until the next sequence execution is started by ground commanding.

On the ground, DSS 25 will attempt to acquire both the X and Ka band downlink signals, sending both carrier power measurements back to JPL and to LMA in monitor channel telemetry. From this data a rough estimate of pointing can be made. If the Ka-Band is in lock, then the elevation misalignment is almost certainly less than 0.5°. If not, the X-Band carrier power level can be used to determine the total misalignment of the HGA out to about 1.5° with a precision of approximately 0.25°. No information will be available at this time regarding the direction and orientation of the misalignment.

Next the HGACAL1 sequence is started. It turns off telemetry modulation for extra signal strength, commands the HGA EL gimbal first to -85°, -95° and then back to the nominal ANS position of -90°. The desired rate of gimbal motion is 2 steps per second or about 0.02° per second. At this rate the 3dB beamwidth of the HGA will take more than a minute to cross the Earth. Over DSS 25, the Ka-Band beam should appear in the center of the HGA main beam for about 15 seconds, and is narrow enough such that detecting it can be taken for near-perfect alignment of the HGA boresight. HGACAL1 ends by commanding telemetry modulation on. The sequence should be complete in approximately 20 minutes.

The time of the peak amplitude in X-Band and hopefully Ka-Band will be recorded and an elevation gimbal bias angle will be calculated and uplinked to the spacecraft. This should result in nearly perfect pointing of the HGA and most of the desired X-Band uplink and downlink capability while in ANS or in the automatic Earth tracking mode. Further calibration will likely be required to meet the 0.25° pointing requirement for mapping data playbacks.

Final HGA Alignment Survey and Correction

In order to determine remaining boom and gimbal axis misalignments, a survey of the exact location of the HGA X and Ka Band beam peaks will be performed for each of the principal axes. Unlike the other HGA deploy and calibration sequences, the HGACAL2? sequences are designed to be used at any convenient time after the initial calibration has been completed. Since perturbation of the test by a go-ANS command is unacceptable, these sequences will have to be integrated into a background sequence which places these events on the ISH orbits of an alternating ANS-ISH sequence. Also, during each fine calibration ISH period the HGA AZ and EL gimbal angles will be temporarily patched into the high rate deck of SCP Engineering Telemetry Alternate Map 33.

First the spacecraft is slewed to an attitude that points the +X spacecraft axis towards the Earth. Then the HGA gimbals are commanded over a grid pattern covering ±0.5° in elevation and azimuth angles about the nominal boresight position (see Figure 3 below). The rate at which the gimbals are commanded is 1 step per second or about 0.01° per second. At this rate, one grid scanning pattern will take approximately 40 minutes.

Next the sequence will slew the spacecraft such that the Earth is along the -Z axis, and the grid pattern will be repeated with respect to that axis. Finally the grid scan will be performed about the +Y spacecraft axis.

Data from the three grid scans can then be used to analyze the misalignment of each gimbal axis and of the HGA system about the other three axes. If significant and systematic misalignment errors are found in the three axis survey, an uplink will be prepared to correct the errors. A simple azimuth error can be corrected using an azimuth encoder bias. Three-axis misalignments can be corrected via the HGA gimbal transformation matrix in flight software.

During this time period the spacecraft will be occulted from the Earth for approximately 40 minutes out of each orbit, leaving about an hour of useful calibration time.

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Figure 3, HGA Alignment Survey Grid Pattern, showing Misalignment the HGA Beam with respect to a Spacecraft Axis


DEPLOYMENT ASSESSMENT

Ground personnel will observe the downlink carrier as deployment occurs, assuming carrier lock is retained through the perturbation of partially undamped deployment.

If carrier lock is retained, the signal level will first increase by about 5 dB as the peak of the low gain antenna beam passes through the Earth. Then the level will drop to a minimum of -170 dBm (carrier only, 70m DSS) or until the BVR looses carrier lock. If signal is not lost and the downlink carrier power is above -166 dBm, then the boom has probably not completely deployed and steps must be taken to ascertain the actual deployment angle. Part 1 of HGADPLY finishes by slewing the spacecraft back to an ISH attitude with the spacecraft +X axis pointed at the Earth. See Section 2, Figure 1e.

If carrier lock is lost early, ground personnel will rely on the activation of low-gain transmit antenna #2, which is located on the back of the TWTA box. Assuming LGT2 functions properly and the boom has deployed at least 90°, the DSN stations should have time to lock onto 10 bps Emergency Mode telemetry before the next occultation.

In the case of carrier-only information, the criteria for commanding Part 2 of HGADPLY are two-fold; 1) 70m downlink carrier power of less than -166 dBm at the end of hinge motion and 2) Loss of signal as the spacecraft slews back to post deployment attitude. If 10 bps telemetry is available, HGA boom hinge potentiometer data should indicate to within 5° whether or not the hinge is fully deployed.

The second part of HGADPLY "unwinds" the AZ (azimuth) gimbal to 0°. When the signal from LGT2 is expected to be lost, downlink is switched back to LGT1. Low Gain Transmitter 1 should support 10 bps downlink over a range of low gimbal azimuth angles. Downlink carrier power can be used to tell whether the hinge is within 20° of the latched position.

If the carrier over LGT2 does not drop off until the switch to LGT1, or LGT1 carrier does not reappear then it is likely that the azimuth gimbal became stuck during the unstow. In this case the pre-loaded Partial Unstow Contingency Sequence will be initiated. See section 2.0 for details. The purpose of this sequence is to determine by an alternate method the position of the azimuth gimbal.

If azimuth unstow is successful and the deployment angle is determined to be large enough, a "go" will be given to initiate the first coarse elevation calibration of HGA pointing. The first step is to establish uplink through the primary string (LGR1/MOT1/CDU1) and command the initiation of the preliminary calibration sequence. Note that the secondary string (HGA/MOT2/CDU2) may also achieve lock on the uplink signal, if the hinge position is close to nominal.

 

FINAL CALIBRATION

After the HGACAL1 sequence completes, Telecom and AACS will determine the appropriate HGA Elevation encoder bias to uplink to the spacecraft. A real-time command will then be built containing this encoder bias. Once the bias is successfully updated a Load&Go script is sent to switch to HGA uplink and play back the recorded deployment and coarse cal events on SSR_1B.

Following successful acquisition of HGA uplink, the fine calibration sequences are initiated within a new GravCal type background sequence, requiring a minimum of three orbits to complete. There are no real-time assessments to be made during the fine calibrations other than spacecraft monitoring.

Upon completion of the three-axis calibration analysis, AACS and Telecom will specify the body-to-baseplate matrix elements to be uplinked, if necessary. This completes the calibration.


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