Sequencing problems

Sequencing: ground commanding vs$,$ autonomy for a LEO with a dedicated ground station

A low$-$earth$-$orbiting spacecraft in an almost polar orbit has an orbital period of $90$

minutes. The spacecraft collects $12$ megabits of data per orbit and automatically

stores the data for later retrieval. There is sufficient storage for two days worth of

data.

It makes contact with a single dedicated equatorial ground station every $16$ hours. It

does not use any other communications networks. Passes over the ground station range

in time depending on how much of the circular field of view the spacecraft cuts

across, and average $13$ minutes. Lockup occurs on the first frame, and no wasted

downlink frames are required to establish contact.

The maximum data transmission rate is $300,000$ bit$/$sec$.$ A backup low$-$rate speed of

$100,000bi\frac{t}{sec}$ is also available, but is notSequencing problems$$

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Sequencing: ground commanding vs$.$ autonomy for a LEO with a dedicated ground station$$

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A low$-$earth$-$orbiting spacecraft in an almost polar orbit has an orbital period of $90$

minutes. The spacecraft collects $12$ megabits of data per orbit and automatically

stores the data for later retrieval. There is sufficient storage for two days worth of

data.$$

$$

It makes contact with a single dedicated equatorial ground station every $16$ hours. It

does not use any other communications networks. Passes over the ground station range

in time depending on how much of the circular field of view the spacecraft cuts

across, and average $13$ minutes. Lockup occurs on the first frame, and no wasted

downlink frames are required to establish contact.$$

$$

The maximum data transmission rate is $300,000$ bit$/$sec$.$ A backup low$-$rate speed of

$100,000$ bit$/$sec is also available, but is not nominally used.$$

$$

The framing overhead is $10\%($i$.$e$.$ transmitting $10,000$ bits of data requires $1000$ bits

of framing information$).$ This overhead is the same regardless of the speed.$$

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The following commands are necessary to initiate the transponder downlink signal:$$

telecom.dwn powerOn$$

telecom transmit speed: $300000$

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The following commands feed the science data to the downlink:$$

dwn send apid: $10$

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If real$-$time engineering data at $10000$ bit$/$sec is desired, the following additional

command is needed:$$

dwn send apid: $9$

$$

The following command stops science data being from fed to the downlink:$$

dwn halt apid: $10$

$$

The following command stops real$-$time engineering data from being fed to the downlink:$$

dwn halt apid: $9$

$$

The following command turns off transmission:$$

telecom.dwn powerOff$$

$$

Uplink is sufficiently fast that any transmitted command will execute within $0\mathrm{.}1$

seconds of dispatch, unless the command is somehow corrupted in transit, in which case

it will be ignored by the flight software.$$

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$1.$ How much time is necessary to downlink $16$ orbits of data with real$-$time engineering

telemetry turned off under nominal conditions?$$

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$2.$ How much time is necessary to downlink $16$ orbits of data under nominal conditions

with real$-$time engineering telemetry turned on the entire contact? $[$hint: subtract off

the capacity of the downlink used up by the real$-$time data stream and figure out how

long it takes to downlink the science data on the reduced stream$]$

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$3.$ Should the spacecraft operations be designed around on$-$board sequencing, or will

ground$-$based automation suffice? List the factors driving the decision.$$

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$4.$ Assuming the spacecraft uses ground$-$based commanding, describe the operations for

commanding the spacecraft to send its science and engineering data, and to terminate

the transmission at the end of the contact.$$

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$5.$ Assuming the spacecraft uses onboard sequencing for autonomous commanding, describe

the operations for commanding the spacecraft to send its science and engineering data,

and to terminate the transmission at the end of the contact.$$

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$6.$ For the sequencing case, a VML reusable block to start transmission is defined as

follows:$$

block startDownlink$$

input bool sendRtEng$$

body$$

telecom.dwn powerOn ;command on transmission$$

$$

waitUntil tlmTelecomState $=$ #on ;wait to stabilize$$

$$

telecom transmit speed: $300000$ ;set to high speed$$

$$

r$10\mathrm{.}0$ dwn send apid: $10$ ;send collected science data$$

$$

if sendRtEng then ;check to send real$-$time engineering data$$

dwn send apid: $9$

endIf$$

endBody$$

$$

In the master sequence, a typical call to the block to include real$-$time engineering

telemetry appears as:$$

A$2022-030$T$12$:$22$:$34\mathrm{.}5$ call startDownlink sendRtEng: true$$

$$

Update the block to accept another input parameter, and select the downlink rate to be

either the nominal $300000$ or the low$-$rate $100000$ based on that parameter. Then write a

master sequence invocation of the block for each case, issuing the call to the block.