NASA ESA Meeting 012301

1
GLAST Time Presentation
Thomas B. Jennings GPO Systems Engineer
2
Time Systems used for GLAST Mission

• Time Systems
• Coordinated Universal Time (UTC)
• Global Positioning System (GPS) Time
• Terrestrial Time (TT)
• GLAST Time (GT)

3
Coordinated Universal Time (UTC)

• Definition
• Coordinated Universal Time (UTC) is synched to
  the International Atomic Time (TAI) Scale,
  however it is offset by an integer number of
  seconds. UTC is kept within .9 seconds of
  Universal Time 1 (UT1) through the use of
  leap-seconds. UTC is essentially a bridge
  between TAI and UT1.
• Areas of Use
• GSRD requirements specify the use of UTC for all
  operations activities
• Ground system data transfer time tagging
• S/C command requests
• Time tagging of GCN messages???

4
GPS Time

• Definition
• GPS time is synchronized with UTC (within 1
  microsecond), but does not contain leap-seconds.
  GPS Time is currently ahead of UTC by N seconds
  due to the leap-seconds that have been inserted
  into UTC. The GPS epoch is identified as the
  number of seconds that have elapse since the
  previous Saturday/Sunday midnight. GPS weeks
  start with week 0 on January 6, 1980.
• GPS satellites are equipped with atomic clocks
  and contribute to the TAI average. Parameters
  are uploaded to each GPS satellite to allow the
  satellite to convert the reading of its atomic
  clock to GPS Time.
• Areas of Use
• Timing source for adjusting the Spacecraft 1PPS
  signal
• GNC Software for performing consistency checks???

5
Terrestrial Time (TT)

• Definition
• Terrestrial Time (TT) is a dynamic time scale
  based upon the orbital motions of the Earth,
  Moon, and planets. It is defined by clocks using
  SI seconds on the surface of the Earth.
• Epoch is 000000 January 1, 1977
• TT TAI 32.18 sec
• Areas of Use
• Time system for processed GLAST science data
• (Note seconds in SI units is defined as the
  duration of 9,192,631,770 cycles of radiation of
  cesium 133)

6
GLAST Time

• Definition
• The elapsed seconds (SI units) since the
  reference epoch of 000000 January 1, 2001 UTC.
• Areas of Use
• Time distributed within the GLAST Observatory
• Used as time tag for housekeeping, diagnostic,
  and science data packets
• (Note seconds in SI units is defined as the
  duration of 9,192,631,770 cycles of radiation of
  cesium 133)

7
Diagram of S/C Time Handling
8
GNC Time Handling

• GNC parameters will need to be updated
  periodically to properly convert GLAST time into
  other time systems.
• One parameter represents the offset from UTC to
  UT1
• GNC_PARM_DEL_T_UT1
• This parameter will need to be updated
  periodically to account for the change in delta
  between these two time systems.
• Time scale is yet to be determined expected to
  be once every 2 weeks
• Needs to be updated for a leap-second event
• A second parameter represents the offset from UTC
  to TAI. (This is the number of leap-seconds)
• GNC_PARM_DEL_T_AI
• Needs to be updated for a leap-second event
• GNC Software used to correlate subseconds
• When the Spacecraft Time is set to support the
  GLAST Launch the subseconds field will be
  uncorrelated.
• Following GPS initialization, an algorithm in the
  GNC software will be employed to modify the
  number of OCXO cycles that represent a second
  over a period of time to correlate the Spacecraft
  Time subseconds to GPS Time subseconds.

9
FSW Time Modification Impacts

• The sequence processing task (ATS and RTS) of the
  flight software may be impacted by modifying the
  seconds field of the Spacecraft Time.
• ATS execution may hang if the time is set while
  an ATS sequence is being processed.

10
Potential MOC Impacts

• As a result of a 1st order analysis of the
  impacts to the MOC for handling of Leap-seonds in
  the ground system, it was determined that the
  following systems would be impacted.
• ITOS
• MPS (Mission Planning System)
• Frame Accounting
• ITPS (Trending Software)
• FDS (Flight Dynamics System
• Simulator Support
• PSS (Portable Spacecraft Simulator)
• MOC Training Simulator - Display and Control
  Software
• Approach would be to add a leap-second table
  accessed by the MOC applications to convert
  between Spacecraft Time and UTC.
• More detailed analysis is being performed.

11
GLAST Time Requirements
12
Time Requirements (Level-1)

• Mission System Specification
• 3.3.1.12 Time Accuracy
• The observatory time accuracy shall be maintained
  within 10 µsec (with a goal of within 3 µsec)
  relative to Universal Time Coordinated (1 sigma
  rms).
• Science Requirements Document

11 Relative to spacecraft time.
7 Relative to spacecraft time.
10 Relative to Universal Time, 1 sigma r.m.s.
13
Ground System Time Requirements (Level-2)

• Ground System Requirements Document
• SYS0040 The ground system shall use Universal
  Time Coordinated (UTC) time as the time base for
  all operations activities.
• MOC0290 The MOC facility shall provide a master
  time signal for the MOC systems.
• MOC4210 The MOC shall use UTC time for planning
  and generation of commands.
• MOC4640 The MOC shall ensure 1 second accuracy
  for a minimum of 3 days for Absolute Time
  Commands.
• MOC6400 The MOC shall monitor accuracy and
  performance of the S/C clock as it compares to
  UTC.
• GCM0290 The GCM facility shall provide a master
  time signal for the GCM systems.
• GCM4210 The GCM shall use UTC time for planning
  and generation of commands.

14
S/C Time Requirements (Level-2)

• Spacecraft Performance Specification
• 3.7.3.1.3 GPS Data
• The CDH subsystem shall acquire time, velocity,
  and position data from the GPS receiver.
• 3.7.3.2.1.1 Signal Distribution
• The CDH subsystem shall distribute a
  pulse-per-second signal as defined in the
  SC-instrument IRDs.
• 3.7.3.2.1.2 Signal Accuracy
• The CDH subsystem pulse per second signal
  accuracy shall be as defined in the SC-instrument
  IRDs.
• 3.9.6.2 Time
• The FSW subsystem shall generate for distribution
  GPS Time messages correlated to the hard line
  pulse-per-second signal as documented in the
  SC-instrument IRDs.

15
S/C Time Requirements (Level-3)

• Observatory Detailed Requirements (SC CDRL-29)
• CDHDR21 The CDH subsystem shall acquire time,
  velocity, and position data from the GPS
  receiver.
• CDHDR89 The CDH subsystem shall provide a
  Pulse-Per-Second (PPS) signal to the GBM at a
  nominal frequency of 1 Hz continuously during
  normal operations.
• CDHDR90 The CDH subsystem shall use negative
  logic (falling edge) in generating the PPS signal
  with a minimum low of 1 msec.
• CDHDR91 The CDH subsystem shall provide the
  PPS signal accurate to 1.5 µsec when the SC is
  receiving Global Positioning System (GPS)
  updates.
• CDHDR92 The CDH subsystem shall not let the
  PPS signal drift more than 1 µsec in any given
  100 second period when GPS signals are
  unavailable.
• CDHDR93 The CDH subsystem shall use LVDS
  drivers for the PPS.
• CDHDR125 The CDH subsystem shall provide a Pulse
  Per Second (PPS) signal to the LAT at a nominal
  frequency of 1 Hz continuously during normal
  operations.

16
S/C Time Requirements (Level-3) cont.

• Observatory Detailed Requirements (SC CDRL-29)
  cont.
• FSW31 The FSW subsystem shall generate for
  distribution GPS Time messages correlated to the
  hard line pulse-per-second signal as documented
  in the 1553 Bus Protocol ICD.
• EPSDR97 The EPS subsystem shall provide a wire
  harness that reverses the signal to the GBM such
  that the falling edge generated by the SC will
  look like a rising edge to the GBM.

17
LAT Time Requirements (Level-2)

• LAT Performance Specification
• 5.2.11 Time Accuracy
• The time accuracy of event time measurements
  shall be lt 10 microseconds relative to spacecraft
  time.
• GOAL - The goal is to achieve time accuracy of
  better than 2 µs relative to spacecraft time.
• 5.4.9.3.2 LAT-SC CTDB Data Content
• LAT-SC CTDB communications shall include LAT
  housekeeping data, PPS time mark signal, time
  distribution, analog monitoring signals, discrete
  control signals, configuration commands, memory
  and table loads, real-time pointing commands,
  instrument mode set
• 5.5.1.2.5.1.1 CTDB Specification
• Commands, telemetry, time messages, and ancillary
  data shall be transferred between the LAT and the
  SC via a serial CTDB compliant with MIL-STD-1553B.

18
LAT Time Requirements (Level-2) cont.

• LAT Performance Specification (cont.)
• 5.5.1.2.5.2.1 One Pulse Per Second (1PPS) Bus
• The LAT shall receive from the spacecraft a 1 PPS
  signal. The SC provided 1PPS signal will be
  accurate to 1.5 µsecs when the GPS timing signal
  is available.
• 5.5.1.2.5.2.2 GPS Receiver Time Dropout
• The PPS signal will be provided without
  interruption to the LAT in the event of a loss of
  the time signal provided by the GPS receiver.
• 5.5.1.2.5.2.3 PPS Signal Drift
• The LAT shall operate as specified herein with a
  PPS signal drift of up to 1 µsec over any 100
  second period.
• 5.5.1.2.6.3.1 Distribution Format
• The LAT shall receive from the SC a Time Message
  that gives a time at the tone will be message
  in Global Positioning System (GPS) format.
• 5.5.1.2.6.3.2 Distribution Timing
• The LAT shall receive Time Messages from the SC,
  issued between 500 ms and 800 ms before the
  transition of the 1 PPS time mark signal, and
  operate as specified herein.

19
LAT Time Requirements (Level-2) cont.

• LAT Performance Specification (cont.)
• 5.5.2.9.1.7 CDH
• The SC will perform the following functions with
  the LAT instrument.
• Receive and store science data Transfer the data
  to the communications subsystem for data
  transmissions
• Communicate telecommands and telemetry over 1553.
• Provide a timing pulse
• 5.5.2.9.1.7.3 Discrete Electrical Interfaces
• The LAT is redundantly allocated the following
  discrete command and telemetry interfaces
• 1 Discrete Time Pulse.
• 16 Discrete Control Commands.
• 8 Discrete Monitor Telemetry.
• 96 Passive Analog Telemetry.

20
LAT Time Requirements (Level-2) cont.

• LAT Performance Specification (cont.)
• 5.5.2.9.1.7.3.1 Discrete Time Pulse
• The LAT shall receive the SC provided One Pulse
  Per Second (1PPS) signal at a nominal frequency
  of 1 Hz continuously during normal operations.
• The 1PPS signal falling edges will be accurate to
  1.5 µsec when the SC is receiving Global
  Positioning System (GPS) updates.
• The 1PPS signal will not drift more than 1 µsec
  in any given 100 second period when GPS signals
  are unavailable.
• The 1PPS signal characteristics shall be of LVDS
  type, negative logic (falling edge) with a
  minimum low duration of 1 msec as shown in 1196
  EI-Y46311-000C Figure 6-15. The LAT shall have a
  100 Ohm 10 Ohm terminator on the differential
  input signals as shown in 1196 EI-Y46311-00C
  Figure 6-15.
• 5.6.1 Digital 1553 Messages
• Messages to the LAT shall contain any one of the
  following data types
• d. Ancillary data packets and Time tone messages
  generated by the SC.

21
LAT Time Requirements (Level-2) cont.

• SC to LAT Interface Requirements Document
• 3.2.5.2.1 Pulse Per Second (PPS) Bus
• The SC shall provide to the LAT, across an LVDS
  interface, a 1PPS signal accurate to 1.5 µsec
  when GPS provides the timing signal.
• 3.2.5.2.2 GPS Receiver Time Dropout
• The PPS signal shall be provided without
  interruption to the LAT in the event of a loss of
  the time signal provided by the GPS receiver.
• 3.2.5.2.3 PPS Signal Drift
• The 1 PPS signal shall not drift more than 1
  µsec over 100 seconds.
• 3.2.6.3.1 Distribution Format
• The SC shall issue to the LAT, a time message
  that gives a time at the tone will be message
  in Global Positioning System (GPS) format.
• 3.2.6.3.2 Distribution Timing
• The Time Mark Message shall be issued between 500
  ms and 800 ms before the transition of the 1 PPS
  time mark signal.

22
S/C-LAT Time Requirements (Level-2)

• SC to LAT Interface Control Document
• Section 6.4.3.1
• The SC shall provide a Pulse Per Second (PPS)
  signal to the LAT at a nominal frequency of 1 Hz
  continuously during normal operations
• The PPS signal falling edges shall be accurate to
  1.5 µsec when the SC is receiving Global
  Positioning System (GPS) updates
• The PPS signal shall not drift more than 1 µsec
  in any given 100 second period when GPS signals
  are unavailable
• The LAT shall use LVDS for the PPS signal
• The SC shall use LVDS for the PPS signal
• The LAT shall use negative logic (falling edge)
  for the PPS signal
• The SC shall use negative logic (falling edge)
  for the PPS signal
• The PPS signal shall have a minimum low duration
  of 1 msec

23
GBM Time Requirements (Level-2)

• GBM Requirements Document
• 3.1.1.2 Time Resolved Spectra
• The GBM shall provide time-resolved spectra to
  correlate the Low-to-Medium gamma ray emission
  with emissions detected by LAT accurate to 10µs
  (2 µs goal) with respect to the spacecraft clock.
• 3.4.2.5 Spacecraft Command and Data Handling
  Interfaces
• The DPU shall include a redundant digital
  crossed-strapped serial interface to receive
  commands, telemetry, time mark messages, and
  ancillary data from the spacecraft Command,
  Telemetry, and Data Bus (CTDB), and to send
  selected telemetry data back to the SC.
• 3.4.2.5.2.6 Time Mark Messages
• The DPU-CTDB interface shall receive time mark
  messages from the spacecraft CDH system. Time
  mark messages will contain the time in GPS time
  format referenced to a time mark signal from the
  pulse per second bus. The time mark message will
  be issued by the spacecraft no less than 500
  milliseconds before the transition of the
  corresponding pulse per second time mark signal
  (see 3.4.2.5.3). Time mark messages will be
  accurate to -/ 1.5 microseconds referenced to
  the SC GPS receiver.
• 3.4.2.5.3 Pulse per Second Bus Interface
• The DPU shall include a redundant cross-strapped
  asynchronous LVDS interface to receive time mark
  signals from the SC pulse per second (PPS) bus.
  The timing of these signals will be accurate to
  -/ 1.5 microseconds referenced to the SC GPS
  receiver.

24
GBM Time Requirements (Level-2) cont.

• GBM Requirements Document (cont.)
• 3.4.3.2.2 Data Types
• The DPU shall accumulate and handle the following
  digital data types, as described herein (see
  Table 3.4-1 and Figure 3.4-2). Under software
  control, the DPU shall be capable of
  simultaneously starting the accumulation time of
  the ISPEC, ITIME, CSPEC and CTIME data types.
  The accumulation time boundaries of the ISPEC,
  CSPEC and CTIME data types shall always match an
  ITIME time boundary.
• 3.4.3.2.2.1 Internal Data Types
• The DPU shall generate two basic types of digital
  detector data for internal use by the flight data
  processing software ITIME and ISPEC. These data
  types shall be continuously accessible to the
  flight software during normal in-flight
  operation.
• 3.4.3.2.2.1.1 ITIME Data
• The ITIME (Internal TIME) data type shall consist
  of the counting rates from each GBM detector with
  an integration time of 16 ms, and shall use the
  same energy channel definitions as those used for
  the generation of CTIME continuous data (see
  3.4.3.2.2.2.1.1).
• 3.4.3.2.2.2.1 Continuous Data
• Continuous data shall be initiated or terminated
  (normally activated whenever the instrument is
  operational) upon command received from the CTDB,
  and shall consist of three sub-types CSPEC,
  CTIME, and CHK. Each continuous data sub-type
  shall be controlled by independent commands.

25
GBM Time Requirements (Level-2) cont.

• GBM Requirements Document (cont.)
• 3.4.3.2.2.2.1.1 CTIME Data
• The CTIME (Continuous TIME) data type shall
  acquire the counts from each GBM detector with 8
  pulse height channels and 0.256-second time
  resolution.
• The CTIME pulse height channels shall be rebinned
  from the linear ADC channels based on
  programmable lookup tables stored in memory.
  There shall be one lookup table for the NaI
  detectors and one for the BGO detectors.
• Each CTIME energy channel shall have 16 bits for
  counting the number of counts. The counter shall
  roll-over rather than locking at its top value.
  This will permit ground-based analysis to
  reconstruct the true number of counts, in some
  cases even if multiple roll-overs have occurred.
  
• The CTIME integration time boundaries shall be
  recorded with an accuracy of at least 10
  microseconds (goal 2 microseconds) relative to
  the UTC time provided by the spacecraft. CTIME
  data shall be transferred to the spacecraft via
  the HSSDB.
• Goal the CTIME time resolution shall be
  adjustable via spacecraft commands from 64 ms to
  1024 ms, with step sizes being integer multiples
  of 64 ms.
• 3.4.3.2.2.2.2.1 TTE Data
• The TTE (Time-Tagged Events) data type shall
  acquire individually digitized pulse height
  events from the GBM detectors.
• 3.4.3.2.2.2.2.1.1 TTE Data Contents
• Each TTE event shall be tagged with 32 bits of
  data, consisting of
• (a) Detector identifier.
• (b) Digitized pulse height, with 128-channel
  resolution.
• (c) Time of occurrence, relative to GPS with at
  least 10 microsecond (2 microsecond goal)
  accuracy relative to the GPS time provided by the
  spacecraft.

26
GBM Time Requirements (Level-2) cont.

• SC to GBM Interface Requirements Document
• 3.2.5.1.1 CTDB Specification
• Commands, telemetry, time messages, and ancillary
  data shall be transferred between the GBM and the
  SC CDH via a serial CTDB compliant with
  MIL-STD-1553B.
• 3.2.5.2.1 Pulse Per Second (PPS) Bus
• The SC shall provide the GBM a 1 PPS signal
  accurate to 1.5 microseconds across an LVDS
  interface.
• 3.2.5.2.2 GPS Receiver Time Dropout
• The PPS signal shall be provided without
  interruption to the GBM in the event of a loss of
  the time signal provided by the GPS receiver.
• 3.2.5.2.3 PPS Signal Drift
• The 1 PPS signal shall not drift more than 1
  µsec over 100 seconds.
• 3.2.6.3.1 Distribution Format
• The SC shall issue a time message that gives a
  time at the tone will be message in GPS time
  format.
• 3.2.6.3.2 Distribution Timing
• The Time Mark Message shall be issued 500
  milliseconds or more before the transition of the
  1 PPS time mark signal.

27
S/C-GBM Time Requirements (Level-2)

• SC to GBM Interface Control Document
• 6.4 CDH
• The SC shall provide a timing pulse.
• 6.4.3.1 Discrete Time Pulse
• The SC shall provide a Pulse-Per-Second (PPS)
  signal to the GBM at a nominal frequency of 1 Hz
  continuously during normal operations.
• The SC shall use negative logic (falling edge) in
  generating the signal with a minimum low of 1
  msec.
• The SC harness shall be wired to reverse the
  signal to the GBM such that the falling edge
  generated by the SC will look like a rising edge
  to the GBM as shown in Figure 6-16 of the GBM to
  SC IDC.
• The PPS signal shall be accurate to 1.5 µsec
  when the SC is receiving Global Positioning
  System (GPS) updates.
• The PPS signal shall not drift more than 1 µsec
  in any given 100 second period when GPS signals
  are unavailable.
• The PPS signal characteristics shall be of LVDS
  type.