The following topics are presented in view graph form and include the following: scheduling application; the motivation for the Flexible Envelope Request Notation (FERN); characteristics of FERN; types of information needed in requests; where information is stored in requests; FERN structures; generic requests; resource availability for pooled resources; expressive notation; temporal constraints; time formats; changes to FERN; sample FERN requests; the temporal relationship between two steps; maximum activity length to limit step delays; alternative requests; the temporal relationship between two activities; and idle resource usage between steps

Lavallee, David

Weinstein, Stuart

Zoch, David R.

English

NASA Technical Reports Server

N92"11050
FLEXIBLE ENVELOPE REQUEST NOTATION
( FERN )
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December 13, 1990
David Zoch
David LaVallee
Stuart Welnsteln
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•, n, ....
Agenda
• Background
• FERN Language Concepts
• FERN Syntax Examples
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https://ntrs.nasa.gov/search.jsp?R=19920001832 2020-03-19T16:29:05+00:00Z
Scheduling Application
schldullng
S_mn
Reque_lSchoduJ _ "_L%_.%Req uem I Schedu.
• Science users send requests to the Resource Scheduling System.
• Requests are requirements for planned instrument operations and are written
in FERN.
• The Resource Scheduling System, which may reside in a POCC, processes
the requests and generates a schedule.
• The schedule specifies the timeline of user activities and is distributed to the
science users.
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Motivation for FERN
• Science users must represent their resource requirements and constraint
relationships in a format that can be interpreted by computers.
If their initial resource requests cannot be satisfied, science users need to
propose reduced resource amounts or alternative experiments for their
=nstrument operations. Thus, some of the science user requests may be
flexible and complex rather than simple.
FERN uses a language format. For example, "TAPE DUMP for 5 minutes to.
10 minutes" is more user-friendly than "TAPE_DUM_5,10." This format
allows users to state their requirements in a more direct and natural manner.
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Characteristics of FERN
• ROBUST
- Supports a variety of user resource requirements and constraints.
- Supports alternative resource amounts and requests.
- Supports repetitive requests ("generic requests") based on orbital events
rather than specific start times.
• READABLE
- Keyword based, not positional. For example, avoids "ROB1,2-4,60,200-300."
• FLEXIBLE
-Time durations and relaxable constraints
• OBJECT-ORIENTED
- Data abstraction
- Reusable data objects
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Types of Information Needed in Requests
• Flexible resource requirements
• Flexible request durations
• Flexible experiment timing / coordination requirements between activities
• Scheduling information for repetitive activities
° Alternativeactivities
• Relative importance of each requirement
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Where Information is stored in Requests
JREQUEST FOR RESOURCES !
ACTIVITY
Is,.p_._.o I Resources ] Constraints
Activity1 _ _ Activity1 _ _Activity2D'-
Step Activity Acitvtt 7
Gap Gap
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FERN Structures
GENERIC REQUEST
Pattern of replication of activities
Alternative activities
Rules
ACTIVITY
- Sequence of steps that comprise the activity
Duration of steps
- Constraints common to whole activity
- Defined in database, then referenced by name in GENERIC REQUEST
STEP
- Amounts of resources
- Constraints
- Defined in database, then referenced by name in ACTIVITIES
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FERN Structures (cont'd)
RESOURCES
Support user operations.
- Are represented as scalars that vary over time.
CONSTRAINTS
Restrict the times when a request can be scheduled.
Are specified with respect to timegraphs, activities, steps, or other requests.
TIMEGRAPHS
Are used to specify time windows, view periods, preferable scheduling times,
spacecraft events, calendar events, etc.
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Generic Request
Generic GENERIC NAME is
3 to AS MANY A-S POSSIBLE activities per Sun in view
With default min start time separation 5 minutes,
With default max start time separation 10 minutes,
With summed duration 4 hours, -- sum of multiple activity durations ts 4 hours
With priority 2,
With strategy Maximizing_Separation
Schedule
ACTIVITY1 and ACTIVITY2
Or schedule
ACTIVITY3
Or schedule
ACTIVITY4 With rain start time separation 4 minutes
End generic
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....... - ,,, ' I ..... , ..........
Activity
Activity ACTIVITYNAME is
Steps
STEP1 for 1 to 8 minutes,
idle STEP2 for 2 to 5 minutes,
STEP3 for 5 minutes,
interruptable STEP4 for AS LONG AS POSSIBLE,
STEP5 for 5 minutes
With activity duration 30 minutes
6nd activity
Interruptable Step - resources of step can be re-allocated without
disrupting activity.
Idle Step - same as interruptable, but not displayed on timeline. Used
to represent idle periods.
I step J/S ie-P iilstep3Jlstep4
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Step
Step STEP NAME is
Resources
INSTRUMENT_X,
POWER 5 watts,
TDRSS SA 1,
ll=
Constraints
Occurs entirely during ORBIT_DA YLIGHT,
Starts at the same time as ACTIVITY_X,
End step
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Resources
• Initial amount may vary over time in discrete steps
Pooled resources contain equivalent or nearly equivalent items:
- TDRSS is (TDRSS_E, TDRSS_W)
- Crew member is (commander, pilot, mission_specialist)
- Redundant equipment is (line_recorder_l, line_recorder_2)
• Some resources are available at different times to different users
(e.g., TDRS)
• Resources may be either durable or consumable
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Pooled Resources
Resource TDRSS SA is
(Forever (TDRS_ E SA1,
TDRSS W SA 1,
End resource
TDRSS E SA2,
TDRSS W SA2))
TDRSS E SA1
TDRSS E SA2.
TDRSS W SA1
TDRSS W SA2
TDRSS_SA Allocation
10:00 10;30 11:O0
Even though some TDRSS_SA is available at every point,
no single antenna is continuously available. Thus, a
request for 50 minutes of "rDRSS_SA is NOT satisfied.
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Resource Availability for Pooled Resources
Some resources are available at different times
to different users
For example, TDRSS communication resources are available
st different times to different satellites, depending on tim
poslUon of the satellite with respect to TDRS,_
Step DATA_ LINK is
Resources
TDRSS E
Constraints
Occurs entirely during TDRSS IN VIEW
End step
TDRSS E
TDRSS IN VIEW _ .
Availability ___ .
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Expressive Notation
Supports non-specific durations:
VIEW STAR STEP for AS LONG AS POSSIBLE
RECk, LIB STEP for 2 to 8 minutes
Supports flexible requests where the resource amounts and
duration of the request are selected by alternative relaxation
levels. This capability allows the scheduling algorithm to
reduce resource amounts or shorten the duration of the request
in order to fit the request on the schedule:
RESOURCE1
RESOURCE2
15 units,
(25 units, 23 units AT RELAXATION 4,
19 units AT RELAXATION 8,
15 units AT RELAXATION 12)
STEP1 for (30 minutes, 28 to 30 minutes AT RELAXATION 5,
25 to 30 minutes AT RELAXATION 15)
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Temporal Constraints
• Temporal Constraints specify when a request can be scheduled with
respect to:
Calendar Events, Orbital Events, Requests, or User Defined Events
• Allow for precise activity sequencing and coordinated activity
dependencies.
• Sample temporal relationships between request A and object B are:
I--_B t
I BIAI
IA rBTI
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Occurs before B
Occurs after B
Ends 5 minutes after the start of B
Occurs right after B
Overlaps all of B
Does not overlap B
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Profiles
Things that vary over time
FOREVER
CREATE
CREATE PERIODIC
INVERT
UNION
INTERSECT
MODIFY
With start earlier by 5,
With end later by 5
SELECT 1,3,4
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Time Formats
Representation of Absolute Time:
• 1990/120/09:00:15.12 April 30, 1990, 9:00:15.12 am
• 1990/120-09:00:15.12 April 30, 1990, 9:00:15.12 am
• 1990/4/30-09:00:15.12 April 30, 1990, 9:00:15.12 am
Representation of Relative Time:
• 3/2:30
• 2.5
• 2.5 hours
• :24.25
• 24.25 minutes
3 days, 2 hours, and 30 minutes
2 hours, 30 minutes
2 hours, 30 minutes
24 minutes, 15 seconds
24 minutes, 15 seconds
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Changes to FERN
New FERN
• UIL like - keywords
• Generic repetition by
iteration or user-
defined windows
• Direct support of
alternatives
• Flexible duration
• Pooled resources
• Database of steps
Old FERN
• LISP like- ( )
• Generic repetition by
iteration
• Alternatives by mutual
exclusion
• Fixed duration only
• No pooled resources
• Unnamed phases
G G
$1 $2 $3 R1 R2 R3
R1 R2 R3
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Sample FERN Requests
Support the following features:
-- Temporal relationships between steps or activities
-- Maximum activity length to limit step delays
-- Alternative requests
-- Idle resource usage between steps of the same activity
-- Flexible request durations
-- Relaxable constraints
-- Event driven planning/scheduling concepts
-- ESP and UIL time formats
-- Step oriented (generics -> activities -> steps)
-- Min and max delays between steps and activities
-- User priorities
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Temporal Relationship between Two Steps
Problem: The steps ERBS TR DUMP and ERBS_RANGING occur
concurrently when command uplink and telemetry downlink are available
(coherent transponder mode). This example shows how to specify
relationships between steps by using a constraint expression.
Step ERBS TR DUMP is
Resources
TDRSS I CHANNEL FORWARD_LINK,
TDRSS_I_CHANNEL-RETURN_LINK ,
TDRSS_Q_CHANNEL_RETURN_LINK
End step
-- mode 1.0 kbps
-- mode 1.6 kbps
-- mode 32 kbps
Step ERBS_RANGING is
Resource
TWO_ WA Y_RANGING_AND_DOPPLER 1,
Constraint
Occurs entirely during ERBS_ TR_DUMP
End step
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- , .... L'................. &.........
Maximum Activity Length to Limit Step Delays
Problem: The transition between steps is flexible and does not need to
occur at a specific time. Switching from command uplink only mode to
command uplink and telemetry downlink mode may begin from 5 to 7.5
minutes after the ERBS activity start time.
Activity ERBS_NORMAL_CASE is
Steps
ERBS CMD [.OAD AND DOPPLER for 5 minutes to 7.5 minutes,
ERBS-CMD-LOAD-for 2.5 minutes to 5 minutes,
ERBS-TR DUMP AND RANGING for 13 minutes,
ERBS- TR-DUMP-for 10minutes
With ac'tivi{'y duration for 33 minutes
Constraint
Starts during ERBS_ WINDOW
End activity
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Alternative Requests
Problem: In some cases, all of the activities (instances) belonging to a
generic request cannot be scheduled. Alternative requests are backup
requests which tell the scheduling system how to resolve conflicts. In
this example, the last alternative request applies only to those activities
(instances) that remain unscheduled after the nominal request and first
alternative request were processed.
Generic ERBS SUPPORTis
1 activity per-EVERY_TWOERBS_ORBITS
Schedule - schedule nominal first
ERBS_NORMAL__ CASE
Or schedule - move ranging step to try to resolve resource conflict
ERBS RETURNand ERBS_SMALL_ WINDOW_ TRACKING
Or schedule - if one of the ERBS activities cannel be scheduled, p_ace it within the next 3 orbits
ERBS_BIG_ WINDOW_RETURN and ERBS_BIG_W/NDOW_ TRACKING
End generic
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Temporal Relationship between Two Activities
Problem: The CLAES instrument normally views for three days on and
three days off. However, during a spacecraft yaw manuever, the
science user wants to interrupt the normal view activity to close the
instrument's aperature door. The normal view activity resumes after the
spacecraft yaw manuever.
Activity CLAES_CLOSED_DOOR_ VlEW_A C T is
Steps
CLAES_CLOSE_APERATURE_ STEP for 1 minute,
CLAES DOOR CLOSED VIEW STEP for as long as possible,
CLAES-OPEN-_APERA TURE_STEP for 1 minute,
Constraints
Overlaps exactly UARS YAW MANUEVER
Occurs entirely during CLAES_NORMAL_ VIEW_ACT
End activity
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Idle Resource Usage between Steps
Problem: The HALOE instrument alternately views the sunrise and
sunset. In between, it is stowed. The idle step is used to maintain the
minimum resources required for stowing between viewing.
Activity HALOE_NORMAL_ACT is
Steps
HALOE SUNRISE VIEW STEPfor 15 minutes,
HALOE_SUNRISE_SLEHF_TO_STOW_STEP for 20 seconds,
idle HALOE STOW STEP for as long as possible, ,,,,,_ _oahoy,2sre,n=.,
HALOE SUNSET VIEW STEPfor 15 minutes,
HALOE-SUNSET-SLEVV TO STOW_STEP for 15 seconds,
Idle HALOE_STO-W_STEPfo-ras long as possible- ,o,,_,_a,o,o_,
End activity
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Flexible Envelope Request Notation (FERN)

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