High Data Volume SMOV requirements and
TDRS downlink capability
Alan Patterson
1.0 Introduction
The forthcoming SMOV activities that will immediately follow servicing mission 3B will
include the checkout of a new instrument, the Advanced Camera for Surveys (ACS), that
will routinely produce more data per image and more data per orbit than any previous
instrument. During SMOV the currently active instruments and the re-activated NICMOS
also will be undergoing their checkout at the same time. This report reviews the ability of
the HST systems to manage the on-board science and engineering data and to transfer the
large quantities of data expected during SMOV through the TDRSS links available to
HST.
1.1 Data Storage
The data stream from the science instruments is buffered in a Solid State Recorder (SSR)
before transmission via TDRSS to the ground. The SSR is partitioned for use between
engineering and science data with the science partition size set at 8.65 Gbits. There is a
second SSR on HST which is presently not in use. The ground system PASS software and
the on-board Flight Software are being upgraded (OPR 42484) to allow use of both SSRs
at the same time i.e. as a single Logical Science Recorder (LSR). When that is completed
it will be possible to devote the entire second SSR to science data storage, giving a total
science storage capacity of over 19 Gbits.
1.2 Transmitter Availability
There are two operable high rate transmitters on HST, each one configured to transmit
through a separate high gain antenna (HGA). Only transmitter TR-1 is routinely used
because of concerns about the lifetime of the transmitters related to thermal cycling. The
conservative policy is to defer usage of the second transmitter as a contingency against
the failure of the one in use.
1.3 TDRS downlink requests
The requests for science data downlink services through TDRS can be made in two ways,
generically or non-generically. The Science Planning and Scheduling Team (SPST) at
STScI has recently changed to the generic method because it will permit significant
operational process improvements. However, it does have a potential disadvantage. It
High Data Volume SMOV requirements and TDRS downlink capability – January 17, 2001
does not use the pointing profile of HST in determining the request time for TDRS
contacts. While the request will know when the TDRS satellites are visible from HST, it
will not know when the TDRS satellites are visible from the HGAs. Each HGA can see
only one hemisphere. The TDRS contacts granted to SPST by NCC are a subset of the
ones that SPST requests. Some moderately small proportion of the requests are either
trimmed or denied. STScI matches the granted requests with the contact needs of the
flight Mission Schedule (MS) during the week before execution of the MS starts.
1.4 Limits on downlink capacity
In a previous report ITM-2000-01 “High Data Volume SMSes and TDRS Contact Usage”
SPST determined practical limits for the downlink capacity in various circumstances. For
single transmitter operation with generic TDRS requests the conservative limit is 120
Gbits per week. For single transmitter operation with non-generic TDRS requests the conservative limit is 126 Gbits per week. Because the tests were performed on only a limited
number of representative weeks, this difference is not very significant.
At least during the first few weeks of SMOV SPST expects to return to non-generic
TDRS requests because so much of the SMOV activity will be pre-planned. The
information about the HST pointing profile will be known well ahead of the TDRS
request deadline. Therefore SPST should make use of the pointing profile because it will
be available. Later weeks which will have progressively fewer SMOV activities and rely
less on early planning of the calendars. SPST can then return to using generic TDRS
requests
The 126 Gbits per week limit converts to an average value of 1.2 Gbits per orbit or 18
Gbits per day.
2.0 Data Volumes during SMOV
The total data volume for SMOV activities required by each instrument is given in the
table. The raw data volume does not include engineering overhead.
The total volume of 331.6 Gbits would require almost 3 weeks at the maximum capacity
of the TDRS links. As the SMOV activities are expected to spread over more than 3
weeks it would seem that SPST could handle the data volume. This is confounded by four
aspects: some programs produce very large amounts of data in a short period of time, the
programs need to occur in a well defined sequence, the single SSR can only buffer a
moderate amount of data and single transmitter operation creates a typical 3 orbit gap in
possible data dumps through the TDRS links using the HGAs.
High Data Volume SMOV requirements and TDRS downlink capability – January 17, 2001
TABLE 1. SMOV data volumes
Instrument
Raw Data volume (Gbits)
ACS Total
119.3
Data volume with engineering
overhead (Gbits)
155
NICMOS
90.8
118
STIS
19.1
24.8
WFPC2
27.0
33.8
2.1 SMOV program data volumes
Accommodating two high data rate activities with the moderate 8.65 Gbit single SSR data
buffer will require planning several orbits of low data rate activities between them. This
must be done to ensure enough TDRS contact time to bring the stored data volume to a
very low level prior to the start of the later high rate observations. The 3 orbit TDRS coverage gap will induce an additional spacing requirement.
2.1.1 WFPC2
The peak in data volume for WFPC2 activities occurs on day 12/24/01 (from a tentative
SMOV plan assuming a 29 Nov 01 SM3B launch) with a value of 8 Gbits. This is less
than the SSR science capacity. So considered alone there is no difficulty in handling this
data volume. However, this activity does occur during the time span designated for the
high data rate ACS fine alignment. The ACS fine alignment and WFPC2 activities must
be scheduled on different days to avoid overflow of the single SSR and to permit the
WFPC2 observations to be downlinked promptly.
The week beginning 12/25/01 has 12 Gbits of WFPC2 data volume and the remainder of
SMOV has 13.8 Gbits spread thinly. No difficulties are expected here.
2.1.2 STIS
The peak data rates for STIS are 0.5 Gbits per orbit. The only issue will be to avoid the
times of high data rates from other instruments. The STIS sensitivity and geometry program has a total data volume of 2.5 Gbits at the 0.5 Gbit per orbit rate and is planned for
12/25/01 where it does not collide with high data rate visits from other instruments.
2.1.3 NICMOS
Very few NICMOS SMOV activities have a data rate per orbit above 0.5 Gbits. Of these,
High Data Volume SMOV requirements and TDRS downlink capability – January 17, 2001
the few early visits have a total data volume of less than 1.5 Gbits which is well under the
single SSR capacity of 8.65 Gbits. Almost the last SMOV activities, the grism sensitivity
and wavelength calibration will each produce 4 Gbits at a rate of 2 Gbits per orbit. These
occur on 1/8/02 and are not expected to collide with high rate activities on other instruments.
The NICMOS cool down activity requires continuous monitoring every orbit for about a
week producing 8.7 Gbits per day. This is almost 50% of the downlinkable capacity.
Therefore only low data volume producing activities on other instruments will be possible
during this time. No rapid access visits for other instruments will be possible during this
time.
2.1.4 ACS
ACS has by far the largest data rates and data volumes of any SMOV program group.
There are seven SMOV programs with data rates higher than the nominal average downlink mark of 1.2 Gbits per orbit (9005, 9013, 9014, 9015, 9018, 9028, 9029). To attempt
the scheduling of most of the 15 or more visits of these programs will at least require
ensuring that the single SSR is empty before the start of each visit. The preceding orbits
will need to be devoid of science recording activity and the spacecraft orientation must
permit TDRS downlinks through the HGA associated with transmitter TR-1. Therefore
the SMOV activities of the other science instruments must avoid these ACS visits and
their immediate neighborhood. There must be no parallel science record activities during
or around any of these high data volume visits. This will impact even the low level
monitoring programs such as once per day monitors of all instruments.
The high data rate and volume ACS SMOV programs 9015, 9018 and 9029 are all earmarked for dates 12/30/01 through 12/31/01. The data volume for visits of all three
cannot be accommodated in 2 days. A conservative approach would be to separate these
activities by more than a day, especially considering that some level of activities on the
other science instruments should be allowed.
3.0 Discussion
The baseline for considering TDRS usage during SMOV is single transmitter usage with
1 SSR and, at least for the first few weeks of SMOV, using non-generic TDRS requests.
This would ensure the best possible match between requested services and those granted
by NCC.
SMOV is anticipated to occur at a planned time after release of the serviced HST which
is dependent upon the date of Servicing Mission launch, currently planned for 11/29/01.
Changes in launch date and consequent slips in SMOV activities will inevitably change
where the pattern of possible TDRS to HGA visibilities fall against SAA free zones and
SMOV program target visibilities. Therefore it is prudent to prepare for SMOV activities
High Data Volume SMOV requirements and TDRS downlink capability – January 17, 2001
to occur against the most inconvenient arrangements of TDRS to HGA visibilities, SAA
patterns and target visibilities.
The weekly data transfer capacity is assumed to be 126 Gbits. This was determined from
tests using a model of the NCC trimming and denying action that is now 2 years old.
There are newer satellites (the manned International Space Station as well as the high data
volume Earth observing TERRA) that have higher priority with NCC than HST.
Therefore it should not be expected that additional contacts will be easily obtained. Under
the 126 Gbit per week assumption, the daily average is 18 Gbits and the rate per orbit is
1.2 Gbits. There is a 3 orbit gap in TDRS coverage implicit in single transmitter
operation. Outside of this gap the average transfer rate would be 1.5 Gbits per orbit.
Any visit that approaches the average 18 Gbit per day of 1.2 Gbit per orbit capabilities of
single SSR single transmitter operation necessarily prevents or severely limits the
scheduling of any other visits at the same time or close in time.
The data volume associated with most of the visits of the ACS high data rate programs
approaches or exceeds the full science capacity of the single SSR. For programs 9005 and
9018 in particular the data volume and rates are sufficient that overflow of the SSR
should be expected for each visit. There are two very high data volume parts of program
9005. Part 1 produces 17.4 Gbits in 10.4 hours or 6.5 orbits for a data rate of 2.7 Gbits
per orbit. Part 2 produces 41.1 Gbits in 22.5 hours or 14 orbits at a rate of 3 Gbits per
orbit.
As currently specified the data from part 2 of ACS program 9005 will overwhelm
single SSR single transmitter operation. The actions of part 2 could be broken into
many pieces and scheduled well separated in time. Then single SSR single transmitter
operation would be possible. The data volume from part 1 of ACS program 9005 equals
the daily downlink capacity of single transmitter operation. Therefore part 1 may be
schedulable if the SSR is empty at the start of the ACS activity and no other recording
actions are allowed for an entire day (i.e. there must be 9 empty orbits). The data rate
from Part 1 will cause overflow of the single SSR if part 1 is scheduled through the 3
orbit TDRS coverage gap. Therefore scheduling of Parts 1 and 2 of program 9005 is only
possible under single SSR single transmitter operation if modifications are made to the
structure of Part 2 (allowing splitting into pieces), no other instruments are permitted to
record during or aound their scheduled times and the scheduling of Part 1 avoids the 3
orbit TDRS coverage gap.
As currently specified the data from part 2 of ACS program 9005 will overwhelm
dual SSR single transmitter operation. The data volume of 41.1 Gbits is greater than
the total of the dual SSR capacity (19 Gbits) and the single transmitter daily downlinkable
volume (18 Gbits). Keeping part 2 of program 9005 as a contiguous data visit
requires 2 transmitter operation. Retaining single SSR single transmitter operation is
only possible if part 2 of program 9005 is broken into many pieces and scheduled in a
well spaced fashion. There will also be significant limits on the data volume produced by
the other instrument activities scheduled through the entire time that parts 1 and 2 are
High Data Volume SMOV requirements and TDRS downlink capability – January 17, 2001
scheduled. If these high data volume parts of ACS program 9005 need to be complete
before many other ACS SMOV activities then these later activities would be delayed
(probably by at least a week). The only visits of other instruments schedulable during the
split and spread ACS program 9005 will be those that have low data volume.
The data volume of ACS program 9018 is 12 Gbits in 7 orbits for a rate of 1.7 Gbits per
orbit. This volume and rate is just managable under single SSR single transmitter
operation if the SSR is empty at the start of the visit, no other activities are permitted
during the visit and in neighboring orbits and the visit is scheduled to avoid the 3 orbit
TDRS coverage gap.
There is an implicit assumption that SPST will not be able to choose execution times
convenient for assuring TDRS to HGA visibility because of the need to satisfy the
observational constraints (targeting) of the SMOV activity (including SAA impacts) and
the need for them to occur in a specific time sequence. Placing visits to ensure science
data downlink possibilities is not considered by any of the software or processes that
SPST uses. Only in the most fortuitous arrangement of SAA pattern with TDRS to HGA
visibilities would it be possible to avoid the SSR overflowing, and the requirement for no
science record activities by the other instruments would still need to be enforced.
All the visits from these high data volume and data rate programs could be split and
scheduled with some reasonable gap. Logically this gap would be 1 day. For the fifteen
visits, this would extend SMOV by more than 2 weeks beyond the baseline plan, because
all activities are tied together in a time sequence. This arrangement would allow most of
the lower volume monitoring activities of all instruments to occur. The delay to general
science availability would vary with the instrument but would affect the ACS the most. It
is expected that a minimum 3 week extension to SMOV would be required.
Using both SSRs for science data buffering will avoid the possibility of overflowing
the data storage capacity and permit the SMOV activities to be executed in the short
time span planned. This assumes that part 2 of ACS program 9005 will be split and
spread reasonably.
The expected use of a single transmitter during SMOV will impact the time that some
data will be available following execution. This will affect those programs that require
access to the data promptly. Whether one or two SSRs are in use there will be times when
at least 8 Gbits of data are buffered for downlink. In a worst case situation SPST may
encounter the 3 orbit TDRS coverage gap at the time when at least 8 Gbits are in the
SSR(s). Data will be held for these 3 orbits before the opportunities for downlinking
recur. It may take an additional 4 orbits to get the full 8 Gbits down, giving a total latency
of 7 orbits or almost 12 hours for the last piece of data to reach the ground. This is the
quoted delivery time for the WFPC2 programs that require rapid access. The extra steps
required to process the data after receipt from HST will produce an additional delay.
Therefore it cannot be guaranteed that the routine downlinking process will give access to
the data in the desired 12 hour timeframe when the SSR storage level is high. The rapid
access WFPC2 program visits only half fill the single SSR. Their data will be accessible
High Data Volume SMOV requirements and TDRS downlink capability – January 17, 2001
rapidly if the SSR is empty at the start of their observations and the time of their
execution avoids the 3 orbit TDRS coverage gap.
Operationally prompt downlinks have rarely been requested for rapid access to data.
These will require additional downlink opportunities over and above those needed for
standard downlinking if the particular data must be dumped ahead of other data already
on the SSR(s). In this case SPST does not remove the urgently needed data from the chain
of data buffered in the SSR. This data would then be downlinked again as part of the
standard process because the data on the SSR is regarded as a continuous stream for the
purposes of data dumping. If SPST were to use an immediately available dump
opportunity for the rapid downlinking of particular data then the complete emptying of
the SSR(s) would be delayed by the number of orbits used for the rapid downlink. In
addition, because these orbits would not be used for the routine emptying of the SSR,
SPST must avoid all significant recording activity to the SSR(s) to avoid overflows.
Therefore single transmitter, single SSR operation requires that rapid access SMOV
activities be preceded by enough orbits of no activity to allow the SSR to be emptied and
followed by several orbits of low science recording activity.
Prompt downlinks may also be forcible as part of the standard procedure for determining
dumps. In this case no double dumping of the data would occur, but the SSR(s) must be
empty of data prior to acquisition of the rapid access data. The forced use of all possible
TDRS contacts, even very short ones, will produce a higher rate of transmitter TR-1 turnons and less efficient usage of TDRS contact time. This effect would be limited to a few
specific programs and only during SMOV.
Only a few WFPC2 programs planned for 12/24/01 specifically mention rapid access. If
there are significant numbers of other programs that also have this requirement, the
planned sequence of SMOV activities should specifically include orbits of low record
activity both preceding and following the rapid access visits, and this may extend the time
needed for SMOV. The extra time needed will depend on the data volume in these
particular visits and their spacing. Should any of the high data volume ACS visits require
rapid access they would produce most of the extra burden on downlink throughput. For
these high data volume ACS visits the burden would be about 12 hours of forced low or
no science record activity per visit. The need to separate the high data volume ACS visits
planned for 12/30/01 to 12/31/01 has already been noted for the case of single SSR operation. If the data from these visits needs to be accessed rapidly, then the separation requirement still applies even in the case of two SSR operation.
If there are high data volume ACS rapid access visits then using both transmitters would
certainly avoid extending SMOV and delaying the start of science operations. This situation is different from routine science operations where there is no urgency in getting any
particular data to the ground rapidly. It is the confluence of the desires to have high data
volume observations accessible rapidly without delaying the sequence of SMOV
activities that indicate the desirability of 2 transmitter operation.
High Data Volume SMOV requirements and TDRS downlink capability – January 17, 2001
3.1 Summary
3.1.1 Single SSR Single Transmitter Operation
ACS program 9005 parts 1 and 2 must be split into many separate visits and scheduled in
a spaced fashion. Other instrument activities will be limited during the times and vicinity
of all ACS high data rate and volume visits. Other instrument activities will not be
possible during or nearby to rapid access visits. No rapid access or more than moderate
data rate visits will be possible during the NCS cool down monitoring or the post BEA
NICMOS monitoring. Rapid access visits will require an empty SSR at the start of the
activity and no other instrument activities can occur at the same time.The number of these
rapid access visits will affect the duration of SMOV.
The duration of SMOV activities will extend in general 3 weeks longer than the baseline
plan.
3.1.2 Dual SSR Single Transmitter Operation
ACS program 9005 parts 1 and 2 must be split into many separate visits and scheduled in
a spaced fashion. Other instrument activities will be possible during the times and vicinity
of the modified 9005 ACS program and all other ACS visits without causing a delay in
the overall SMOV timeline. The other high data volume high data rate visits will not
cause an extension in the SMOV duration. Rapid access visits will require an empty SSR
at the start of the activity and no other instrument activities can occur at the same
time.The number of these rapid access visits will affect the duration of SMOV.
3.1.3 Single SSR Dual Transmitter Operation
ACS program 9005 parts 1 and 2 must be split into many separate visits and scheduled in
a spaced fashion. Other instrument activities will be limited somewhat during the times
and vicinity of all ACS high data rate and volume visits. No rapid access or more than
moderate data rate visits will be possible during the NCS cool down monitoring or the
post BEA NICMOS monitoring. No extension in the SMOV duration is expected.
3.1.4 Dual SSR Dual Transmitter Operation
If ACS program 9005 is unmodified then other instrument activities will need to avoid
the times that parts 1 and 2 occur. Rapid access visits may be possible during the NCS
cool down monitoring or the post BEA NICMOS monitoring if an extra delay of 1 to 2
hours is acceptable. No extension in the SMOV duration is expected.
High Data Volume SMOV requirements and TDRS downlink capability – January 17, 2001
4.0 Recommendations
It is recommended that both SSRs be in use at the same time during SMOV. This will
avoid a forced extension in SMOV activities.
If there are very few SMOV activities that require rapid access and these do not include
any high data volume ACS programs then the careful placement of the known visits can
be handled with single transmitter operation. If there are more than a handful of high data
rate and volume ACS visits requiring rapid access it is recommended that both transmitters be in operation during the first few, critical weeks of SMOV.
If part 2 of ACS program 9005 must be executed contiguously then 2 transmitter
operation is necessary.
It is recommended high data rate high data volume SMOV visits that do not require rapid
access be split into pieces and scheduled separately.
High Data Volume SMOV requirements and TDRS downlink capability – January 17, 2001