NICMOS Instrument Science Report 95-004
NICMOS Design Reference Mission and
Command Volume Estimate
Wayne E. Baggett
November 21, 1995
ABSTRACT
We present an estimate of the uplink command volume associated with the NICMOS exposure commanding using the NICMOS Design Reference Mission developed by MacKenty
and Kutina (ISR NICMOS-001). Based upon this model of NICMOS usage and a few additional assumptions, it appears that the daily mean command volume will be approximately
412 uplink words/orbit for NICMOS commanding, and the highest command volume will
occur for observations of background-limited targets with NICMOS as the prime instrument. In this worst case (120 ACCUM exposures per orbit), we expect a command volume
of about 1014 words/orbit, effectively the same as the typically available command volume of ~1100 words/orbit. However, this maximum command volume is expected for only
about one orbit per day on average, so it should not represent a serious concern operationally. We find no serious impediments to the HST science program with this level of
command volume, even when factoring in the effects of the other SI’s. We also discuss
three possible methods of raising the typically available command volume. Overall, these
results suggest that the effort expended early in the NICMOS macro activation request
development process to minimize the size of the uplinked data structures was successful.
1. Introduction
This ISR will derive an estimate of the NICMOS command volume based upon the
Design Reference Mission described by MacKenty and Kutina (1995). As with the data
volume prediction of MacKenty and Kutina, this command volume estimate is intended to
be used as a guide for the development of the HST Ground System and Flight Software
elements which generate and process the NICMOS commands. As of this writing, the
Flight Software design is essentially fixed (with respect to the commanding interface, at
least), so the purpose of this ISR is to determine if there are potential problems which may
need to be addressed from an operational standpoint.
The remainder of this document will describe the essential aspects of the Design Reference Mission and the analysis performed in making the command volume estimate. Section 2 details the exposure characteristics described in the Design Reference Mission,
Section 3 summarizes the command volume each type of exposure will require, and Section 4 will present the consolidated command volume calculation. Finally, Section 5 will
discuss the results with respect to the ground system limitations on uplink command volume.
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NICMOS Instrument Science Report 95-004
2. Exposure Characteristics
The Design Reference Mission ISR (MacKenty and Kutina, 1995) details the assumptions
and caveats associated with its analysis, and it should be studied for that information. We
are presenting here only the “final” distribution of exposures and are using them at face
value. It should be noted that the Design Reference Mission does not include any target
acquisition exposures nor any bright object mode exposures; however, these exposure
types are approximately equivalent to an ACCUM exposure in command volume, and
their exclusion does not adversely affect our estimate here.
The Design Reference Mission assumes that the NICMOS will be the prime science
instrument one-third of the time (5 orbits/day) and that it will be a parallel instrument the
remainder of the time (10 orbits/day). Tables 1 and 2, taken from MacKenty and Kutina
(1995), show the anticipated distribution of NICMOS exposures when it is the prime
instrument and when it is the parallel instrument, respectively.
Table 1: NICMOS as Prime Instrument (from MacKenty and Kutina, 1995)
Science Type
Percentage
of Orbits
Camera(s)
Exposures per
Orbit
Orbits
per Day
Datasets
per Day
Images
per Day
Bright Targets
25%
Prime (1, 2, or 3)
Parallel Cameras
40 ACCUM
4 RAMP
1.25
50
5
50
5
Special Targets
25%
Prime (1, 2, or 3)
Parallel Cameras
5 ACCUM
4 RAMP
1.25
6
5
6
5
Faint Targets
35%
Prime (1, 2, or 3)
Parallel Cameras
2 MULTI-A
4 RAMP
1.75
3.5
7
70
7
Bkgd Limited
Faint Targets
15%
Prime Camera (3)
40 ACCUM
(1 SAM/EXP)
80 ACCUM
0.75
30
30
60
60
Parallel Cameras
Table 2: NICMOS in Parallel with other SIs (from MacKenty and Kutina, 1995)
Science Type
Percentage
of Orbits
Camera(s)
Exposures per
Orbit
Orbits
per Day
Datasets
per Day
Images
per Day
NonBackground
Limited
67%
Prime (3)
Parallel Cameras
2 MULTI-A
4 RAMP
6.7
13.4
27
268
27
Background
Limited
33%
Prime (3)
(No Parallels)
40 ACCUM
(1 FOM/Exp)
3.3
132
132
We want to point out two areas where we disagree with MacKenty and Kutina (1995):
firstly, we don’t believe that the NICMOS will be the parallel instrument for 10 orbits per
day. Because of the scheduling of other instruments on targets which preclude the use of
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NICMOS Instrument Science Report 95-004
parallel instruments (primarily on moving targets) and because we have no reason to think
that the WFPC-II parallel programs will completely disappear, we feel that 10 orbits per
day of NICMOS parallels is somewhat overestimated. We think an average of 8 orbits per
day may be more realistic. However, this is not a serious overestimation for either the data
volume or the command volume estimates, so we will continue the use of this estimate in
order to remain consistent with the Design Reference Mission.
Secondly, MacKenty and Kutina use RAMP mode for the parallel exposures when the
NICMOS is the prime instrument and when it is the parallel instrument on the non-background limited exposures. Our Phase 2 development decision was that RAMP mode
would not be used in parallel with ACCUM mode exposures due to conflicts in the NICMOS CPU having adverse effects on the ACCUM mode exposure timing. As of this writing, Ball Aerospace is investigating whether a solution to this conflict can be implemented
in the flight software, but we are assuming for our development that the combination of
ACCUM and RAMP will not be allowed. Again, however, we will simply use the Design
Reference Mission in our command volume estimate in order to be consistent, but will
discuss the effect of this difference in Section 5.
3. Command Volume Components
Now that we have described the distribution of the exposures which will be commanded,
we will detail the commanding associated with each exposure. The NICMOS Flight Software/Ground System interface has been defined by Macro Activation Requests (MAR’s),
which are uplinked to HST via PDB table loads and are processed in the NSSC-1 by the
NICMOS Macro Application Processor (Macro AP). The Macro AP passes the MAR’s
and their data to the NICMOS Control Section Flight Software, which then acts upon the
request to provide the necessary activity in the instrument. Our concern here is restricted
only to the uplinked tables themselves, as they are the components of the command volume.
The MAR’s involved in commanding the exposures outlined in Section 2 are listed in
Table 3, which also specifies the number of words in each MAR and the number of words
required to uplink each MAR. The number of MAR words has been taken from System
Engineering Report OPS-002 (Blue, 1995); the number of uplink words is larger than this
value because of some overhead words needed by the NSSC-1. Also, note that there is
more commanding to dump the science data to the STR than just sending up the
NDUMPSD MAR. The total number of uplink words needed to perform a dump is actually 48 words (12 words for the SCIHDR data block, 6 words for the format selection, 21
words for the STR commanding, 3 words for dumping the NSSC-1 Standard Header
Packet, and 6 words for the NDUMPSD MAR).
It is expected (in fact, required) by the Design Reference Mission that the NICMOS will
be in a high reconfiguration state (OPERATE or OBSERVE) at all times, and the commanding involved to move from OPERATE to OBSERVE and back down should be
included in the command volume estimate. It should be understood that commanding from
OBSERVE to OPERATE and from OPERATE to OBSERVE will occur at the beginning
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NICMOS Instrument Science Report 95-004
Table 3: MAR Uplink Size
Macro Activation
Request
# of MAR
words
# of Uplinked
words
NPROGSx
4
8
NFILTERx
2
6
NACCUMxx
3
7
NRAMPxx
6
10
NMULTIxx
40
52
NDUMPSD
2
6
NFOM
2
6
NFPATMP
2
6
NDTCALIB
2
6
and end of every occultation period, respectively, so it represents a single additive number
to the exposure-derived estimate. The transition from OPERATE to OBSERVE consists of
disabling the focal plane assembly (FPA) temperature sensors via a single execution of the
NFPATMP MAR, followed by a request to calibrate the analog to digital converters
(NDTCALIB MAR), all of which requires a total of 12 uplink words. The downward
reconfiguration is accomplished by placing each filter wheel in its BLANK position
(NFILTERx MAR) and enabling the FPA temperature sensors, thus requiring 24 uplink
words. Hence, we need to add 36 words to the exposure commanding uplink volume estimates to properly account for the instrument reconfigurations.
The standard exposure commanding sequence consists of uplinking the programmatic
information (PROG_ID and OBSET_ID) for the first exposure on a given detector in the
obset (NPROGSx, x=1,2,3), requesting a filter wheel motion (NFILTERx), and starting
the science exposure (NACCUMxx, NRAMPxx, or NMULTIxx). In addition, if the Field
Offset Mechanism (FOM) is used for moving the image on the detector, there will be a
FOM move commanded after each exposure in the series (NFOM) — this will only occur
when NICMOS is the parallel instrument and the science is background limited. Finally,
once the science data are collected, they need to be dumped to the Science Tape Recorder
(STR). This will usually happen only once per visibility period, but will sometimes happen 2 or 3 times per visibility period (such as observing background limited faint targets
when NICMOS is the primary instrument).
4. Command Volume Estimation
The Design Reference Mission clearly specifies the distribution of exposures among the
various modes for each orbit of the day. This allows us to compute the command volume
due to the exposure start MAR’s, but we need to make some additional assumptions in
order to estimate the contribution of the programmatic information loads and filter wheel
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NICMOS Instrument Science Report 95-004
movement. We therefore make the following assumptions:
• For NICMOS as the prime instrument, we assume that one obset of each science type
(see Table 1) is executed per day;
• For NICMOS as the parallel instrument, we assume that 2 out of every three orbits will
start a new obset;
• We assume the following filter wheel move scenarios for each orbit:
a — Background limited observations (prime or parallel) will all be made on a single filter/camera;
b — Non-background limited parallel exposures will be made in two filters/camera, meaning that every such exposure described in Table 2 will move the filter
wheel;
c — Prime exposures on bright targets, faint targets and special targets will move
the filter wheel with every other exposure, resulting in a 50% duty cycle.
Using these assumptions, we can now compute the command volume per orbit which they
imply. Table 4 summarizes the command volume expected when NICMOS is the prime
instrument; this table gives, for each type of science, the number of exposures per orbit,
the number of loads of the NPROGSx and NFILTERx MAR’s, the number of command
words needed for instrument reconfigurations and science data dumps to the STR, and the
command volume per orbit. As is easily seen, the worst case occurs when observing backTable 4: Command Volume, NICMOS as Prime SI
Exposures per
Orbit
NPROGSx
Loads
NFILTERx
Loads
Recon
Words
Data
Dump
Words
Command
Words per
Orbit
Bright Targets
40 ACCUM
4 RAMP
3
22
36
48
560
Special Targets
5 ACCUM
4 RAMP
3
5
36
48
213
Faint Targets
2 MULTI-A
4 RAMP
3
3
36
48
270
Bkgd Limited
Faint Targets
40 ACCUM
80 ACCUM
3
3
36
96
1014
Science Type
ground limited targets and is about 1014 words per orbit. The average command load per
orbit while NICMOS is the prime instrument is simply the sum of the values in the last
column of Table 4, weighted by the number of orbits on each target type (from Table 1),
and divided by the 5 orbits/day in which NICMOS is prime; this works out to 440 words/
orbit on average.
Table 5 provides the same analysis for the orbits in which the NICMOS is the parallel SI.
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NICMOS Instrument Science Report 95-004
Table 5: Command Volume, NICMOS as Parallel SI
Science Type
Exposures
per Orbit
NPROGSx
Loads
NFILTERx
Loads
NFOM
Loads
Recon
Words
Data
Dump
Words
Command
Words per
Orbit
NonBackground
Limited
2 MULTI-A
4 RAMP
3
6
0
36
48
288
Bkgd Limited
40 ACCUM
1
1
40
36
48
618
These conditions produce a much lower command volume than when NICMOS is the
prime instrument, with a worst case of 618 words/orbit. The daily average command load
volume for the time in which the NICMOS is the parallel instrument works out to be 397
words/orbit.
5. Discussion
We have presented an estimate of the NICMOS uplink command volume expected from
the definition of the Design Reference Mission (MacKenty and Kutina, 1995). The worst
case command volume of about 1014 words/orbit occurs when the NICMOS is the prime
science instrument and is observing background limited faint targets. The daily NICMOS
command uplink size is predicted to be 6169 words, resulting in a daily average of 412
words/orbit.
There are a number of factors which would cause these command volume estimates to
change significantly, and most of them are outlined in MacKenty and Kutina (1995). The
most important factor which could cause an increase in the predicted command volume is
a higher use of the MULTIACCUM mode; with its much larger uplink size (52 words/
NMULTI MAR), underestimating its use on a per orbit basis could raise the estimate by a
considerable amount. If RAMP mode is not used in parallel with ACCUM mode as discussed in Section 2, then it is reasonable to assume that MULTIACCUM mode would be
used instead. If this is the case, then we get the command volume estimates given in tables
6 and 7, which were derived in a fashion identical to the estimates above. These alternative estimates lead to an average command volume of 583 words/orbit while NICMOS is
the prime instrument, 510 words/orbit while the parallel instrument, and a combined daily
average of 534 words/orbit. Thus, the use of MULTIACCUM mode in place of RAMP
mode does, indeed, increase the command volume estimates by a significant amount.
Note, that the worst case (faint background limited targets when NICMOS is prime) is
unaffected by this alternative estimate.
The typical available command volume is estimated in the following fashion. The total
amount of memory currently available to the absolute time processor (ATP) is 8500 words
(Balzano, 1995), and the rule of thumb is that each command load has available to it about
2/3 of the total, or about 5667 words (this allows unexecuted commands to remain in the
ATP stored command memory while new commands are uplinked). With the current practice of having three loads per day, each load must accommodate about 5 orbits, leading us
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NICMOS Instrument Science Report 95-004
to an available command volume/orbit of about 1133 words for the ATP.
For reference purposes, an analysis of 25 weeks of SMS’s in 1995 shows a median command load size of about 5600 words, which includes both ATP and RTCS loads. A different set of command loads indicates that the loads are distributed in an ATP/RTCS ratio of
about 5/4, implying that HST currently needs about 3000 words/command load for average ATP use. Hence, the current suite of SI’s averages approximately 600 words/orbit of
ATP command volume, assuming three loads/day. It is important to realize that the typical
available command volume of 1100 words/orbit is occasionally exceeded by the current
set of SI’s, and operational procedures are in place to handle them as routine occurrences
without any undue effort or deleterious effects.
Table 6: Command Volume, NICMOS as Prime SI with MULTIACCUM
Exposures per
Orbit
NPROGSx
Loads
NFILTERx
Loads
Recon
Words
Data
Dump
Words
Command
Words per
Orbit
Bright Targets
40 ACCUM
4 MULTI-A
3
22
36
48
728
Special Targets
5 ACCUM
4 MULTI-A
3
5
36
48
381
Faint Targets
2 MULTI-A
4 MULTI-A
3
3
36
48
438
Bkgd Limited
Faint Targets
40 ACCUM
80 ACCUM
3
3
36
96
1014
Science Type
Table 7: Command Volume, NICMOS as Parallel SI with MULTIACCUM
Science Type
Exposures
per Orbit
NPROGSx
Loads
NFILTERx
Loads
NFOM
Loads
Recon
Words
Data
Dump
Words
Command
Words per
Orbit
NonBackground
Limited
2 MULTI-A
4 MULTI-A
3
6
0
36
48
456
Bkgd Limited
40 ACCUM
1
1
40
36
48
618
There are several possible actions which could increase the typical available command
volume. First, if we can double the number of daily command loads to 6 per day, then the
value is raised to about 2132 words/orbit. Second, the amount of NSSC-1 memory allocated to the ATP may be changed in order to increase the available command volume/
orbit. With the current set of instruments, the HRS and FOS use the largest amount of
RTCS memory, and therefore are driving the amount of memory allocated to RTCS’s.
With the removal of these two SI’s during the second servicing mission, some of this
RTCS memory could be reallocated (“move the fence”) and made available to the ATP.
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NICMOS Instrument Science Report 95-004
Third, the HRS and FOS NSSC-1 flight software occupies a considerable amount of the
NSSC-1 memory, while the NICMOS and STIS NSSC-1 flight software will be significantly smaller (Bacinski, 1995). Hence, if some of the memory currently used by the
NSSC-1 flight software can be made available for stored commanding, the available command volume per orbit can be raised accordingly. It is expected that changes to the NSSC
memory allocation will be investigated as the new flight software memory requirements
are finalized and understood.
The daily average command load of 412 to 534 words/orbit is well below the typically
available command volume, assuming that NICMOS is the only user of the ATP. We
know, however, that other SI’s will be using some portion of this command capacity in
parallel with the NICMOS, so we need to factor in this total usage in order to ascertain
whether we are in a problem situation. It is worth noting that among the current suite of
SI’s on HST, the FOS and HRS are the biggest users of command volume, and these two
SI’s will not be present on HST during the NICMOS era. Furthermore, the FOC is currently by far the least-used instrument, so the only reasonable command load issues are
expected to be between the NICMOS, STIS, and WFPC2. The WFPC2 command volume
is estimated currently to be about 150 words/orbit (based on the first 48 SMS’s of 1995,
we find 224 WFPC2 exposures/SMS at about 67 words/exposure), so we don’t expect any
difficulties to arise with NICMOS in parallel with WFPC2.
The STIS presents a computational problem because its command volume is as uncertain
as is the NICMOS’. If we naively assume that the STIS will require as much command
volume as the NICMOS, then there are potential periods of high command volume if the
STIS exposures are parallel to NICMOS observations on background-limited targets for
extended periods of time and we have done nothing to increase the available command
volume. Modifications to the NSSC-1 memory allocation will probably relieve the situation, while increasing the number of command loads to accommodate the additional command volume will certainly allow the additional commanding. This latter procedure is
what is done today when we experience higher than normal command volume situations.
It thus appears that we will be able to support all combinations of instruments for the
majority of the time without resorting to any special processing considerations, and there
are reasonable procedures available which should allow us to deal with the high volume
situations as they arise.
The worst case command volume of 1014 words/orbit is near the current typically available command volume of ~1100 words/orbit. However, since this command volume is
projected to occur for only about one orbit per day on average, we don’t expect serious
difficulties supporting it. Programs which use this mode of observing (NICMOS as the
prime instrument, observing background-limited faint targets) for extended periods of
time may limit the possible parallel programs unless one of the methods of increasing the
typically available command volume/orbit is implemented. Note, though, that it is precisely this observing scenario where parallels will be very restricted anyway since the
background observations will most likely be made using small angle maneuvers to reposition the telescope between exposures — all parallel exposures on other SI’s would require
exposure times and overhead times such that they will schedule within the (probably)
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NICMOS Instrument Science Report 95-004
short time period between the spacecraft motions.This coincidence is not likely to occur
with pure parallel exposures, but it may happen with coordinated parallels.
These findings suggest that the effort spent in the early development phases to minimize
the NICMOS command volume was prudent and has been successful in preventing serious impacts to the science program due to command volume limitations. There will almost
certainly be specific scenarios which will not be supportable with the available command
volume, but we expect this to be a rare occurrence which can be resolved on a case-bycase basis, as is done today for both data volume and command volume considerations.
6. Acknowledgments
I received many useful suggestions from Vicki Balzano while preparing this ISR.
References
Bacinski, J. 1995, private communication
Balzano, V. 1995, private communication
Blue, G. 1995, NICMOS System Engineering Report OPS-002, Ball Aerospace Corporation, Boulder, Colorado
MacKenty, John W. and Kutina, Ray 1995, Instrument Science Report NICMOS-001,
STScI
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