SCLK Required Reading

This required reading document is reproduced from the original NAIF document available at https://naif.jpl.nasa.gov/pub/naif/misc/toolkit_docs_N0067/C/req/sclk.html

Note

These required readings documents were translated from documentation for N67 CSPICE. These pages may not be updated as frequently as the CSPICE version, and so may be out of date. Please consult the changelog for more information.

Important

NOTE any functions postfixed by "_" mentioned below are Fortan-SPICE functions unavailable in SpiceyPy as the NAIF does not officially support these with "_c" function wrappers within the CSPICE API. If these functions are necessary for your work please contact the NAIF to request that they be added to the CSPICE API

Abstract

The SCLK system is the component of SPICE concerned with spacecraft clock correlation data.

Introduction

The spacecraft clock is the onboard time-keeping mechanism that triggers most spacecraft events, such as shuttering of a camera. Since telemetry data are downlinked with this clock's time attached to it, spacecraft clock time (SCLK is pronounced s-clock) is the fundamental time measurement for referencing many spacecraft activities.

It is natural, then, that SCLK have an important role in the CSPICE system. In fact, all C-kernel pointing data are referenced to SCLK. SPICE contains functions to convert between SCLK and other standard time systems, such as Ephemeris Time (ET) and Universal Time Coordinated (UTC).

References

SFOC SIS SFOC-2-SYS-Any-TimeForms, 02/06/90.

Support for New Missions

The suite of SCLK functions has been designed to easily accommodate future missions. A later section describes how the system might be easily expanded to incorporate new spacecraft clocks.

Detection of Non-native Text Files

Starting with the N0057 release of the SPICE Toolkit (March, 2004) the SPICE data loading mechanism detects and prohibits loading text kernel files containing lines terminated with EOF character(s) non-native to the platform on which the Toolkit was compiled. If a non-native EOL terminator is detected in the first 132 characters of a text kernel, the execution is stopped and an error message is displayed. This feature does not work with files that are smaller that 132 bytes or have the first line longer that 132 characters.

The Basics

In this section, we present a minimal subset of facts about the SPICE SCLK system that you can get by with and still use the system successfully.

SCLK rates

Most of the complexity of dealing with SCLK time values arises from the fact that the rate at which any spacecraft clock runs varies over time. As a consequence, the relationship between SCLK and ET or UTC is not accurately described by a linear function; usually, a piecewise linear function is used to model this relationship.

The mapping that models the relationship between SCLK and other time systems is updated as a mission progresses. While the change in the relationship between SCLK and other systems will usually be small, you should be aware that it exists; it may be a cause of discrepancies between results produced by different sets of software.

SCLK kernels

SCLK files conform to a flexible format called NAIF text kernel format. The SPICE file identification word provided by itself on the first line of a text SCLK file is KPL/SCLK. Both the NAIF text kernel format and SPICE file identification word are described in detail in the Kernel Required Reading document, kernel.

To use any of the SCLK conversion functions, your program must first load a SCLK kernel file. The code fragment

furnsh("path to the SCLK kernel file goes here")

accomplishes this. You must supply the actual name of the kernel file you want to load. In addition, you will usually need to load a leapseconds kernel. For some missions, conversions between SCLK and ET will require that both an SCLK and a leapseconds kernel be loaded. The code fragment

furnsh("path to the LEAPSECONDS kernel file goes here")

loads a leapseconds kernel. Leapseconds kernels are described in the TIME required reading document, time.req. Normally, you will load these kernels at just one point in your application program, prior to using any time conversion functions.

Details concerning the kernel pool are covered in the KERNEL required reading document, kernel.

Partitions, briefly

The lifetime of each mission is divided into intervals called partitions. Partitions are time intervals during which the spacecraft clock advances continuously. Every time that a discontinuity in a spacecraft clock's readout values occurs, a new partition is started. Discontinuities may consist of positive jumps, in which the spacecraft clock's readout skips ahead, or negative jumps, in which the spacecraft clock regresses.

The fact that a spacecraft clock may regress raises the possibility that the clock may give the same reading at two or more different times. For this reason, SCLK strings in SPICE are prefaced with partition numbers.

The partition number is a positive integer followed by a forward slash, for example

4 /

Any number of blanks are allowed on either side of the slash. An example of a Galileo SCLK string with a partition number is

1/100007:76:1

Partition numbers serve to ensure that spacecraft clock readings can be interpreted unambiguously.

Converting between SCLK strings and ET or UTC

The time known as spacecraft event time (SCET) is usually UTC. You must verify that this is the case for your spacecraft.

To convert a SCLK string to a double precision ET value, you can use the function call

from spiceypy import *

et = scs2e(sc, clkstr)

To convert a SCLK string to a UTC string, you can use the code fragment

et = scs2e(sc, clkstr)
utc = timout(et, pictur)

where

sc: is the NAIF spacecraft ID code for your spacecraft.
clkstr: is a SCLK string.
et: is an ET time.
pictur: is a format picture for timout().
utc: is the UTC time equivalent to SCLK.

See these functions for details concerning their arguments. The inverse conversion is performed by the code fragment

et = str2et(utc)
clkstr = sce2s(sc, et)

Using encoded SCLK

The CSPICE C kernel (CK) system tags CK data with SCLK times. Within the CK system, these time tags are encoded as double precision numbers. To look up CK data, you will need to supply encoded SCLK time tags to the CK reader functions.

You can obtain encoded SCLK values from SCLK strings via the function scencd(). The code fragment

sclkdp = scencd(sc, clkstr)

encodes the SCLK string 'clkstr' as the double precision value 'sclkdp'. Encoded SCLK values can be converted to strings using the code fragment

clkstr = scdecd(sc, sclkdp)

You can obtain continuous encoded SCLK values from ET via the function sce2c(). The code fragment

clkdp = sce2c(sc, et)

encodes the ephemeris time ET as the double precision value sclkdp. SCLKDP need not be integral; even though non-integral tick values do not represent SCLK readings, they are permitted to avoid truncation error when representing ET as encoded SCLK. A parallel routine sce2t() converts ET to encoded SCLK, rounding the result to the nearest integral tick.

The inverse conversion is provided by the routine sct2e(), which is called as follows:

et = sct2e(sc, sclkdp)

SCT2E handles integral or continuous tick values as inputs. There is a special function that is used for encoding tolerance values for the CK readers. (See the CK Required Reading, ck, document for a discussion of the CK readers.)

The code fragment

ticks = sctiks(sc, clkstr)

produces an encoded tolerance value. sctiks() takes SCLK strings WITHOUT partition numbers as inputs; this is because the strings indicate a delta time rather than an absolute time. All of the concepts used in this section are discussed in greater detail in the following sections of this document.

Encoded SCLK

The fundamental representation of SCLK in the CSPICE system is a double precision numeric encoding of each multi-component count. Encoding SCLK provides the following advantages:

Encoding makes for a more compact representation. Direct (un-encoded) representation of spacecraft clock counts usually requires multiple numbers for the separate components making up the SCLK count.
Having a single numeric equivalent for each count makes it much easier to compare SCLK times (Is time t1 greater than time t2? Is time t1 closer to time t2 than time t3? And so on.)

For these reasons, encoded SCLK is the time representation that is associated with pointing data in the C-kernel. Encoded SCLK is the basis by which conversions are made from SCLK to other time systems. To convert a character representation of an SCLK count sclkch to its double precision encoding sclkdp, use the function scencd() (Encode SCLK):

sclkdp = scencd(sc, sclkch)

The function scdecd() (Decode SCLK) recovers the character representation of spacecraft clock from its double precision encoding.

sclkch = scdecd(sc, sclkdp)

The first argument to both functions, sc, is the NAIF integer ID for the spacecraft whose clock count is being encoded or decoded (for example, --32 for Voyager 2). Each spacecraft may have a different format for its clock counts, so the encoding scheme may be different for each. Later chapters describing clock types give complete details on clock string formats for spacecraft clocks supported by the CSPICE Toolkit.

Ticks

The units of encoded SCLK are ticks since spacecraft clock start, where a tick is defined to be the shortest time increment expressible by a particular spacecraft's clock.

An analogy can be drawn with a standard wall clock, showing hours, minutes, and seconds. One tick for a wall clock would be one second. And a wall clock time of

10:05:50

would represent

10(3600) + 5(60) + 50 = 36350

ticks. As in the case of the wall clock, the length of time associated with a tick varies as the clock rate varies.

Since not all spacecraft clocks are the same, the particular time value for one tick varies from spacecraft to spacecraft. For Mars Global Surveyor, for instance, one tick is equivalent to approximately four milliseconds. For Galileo, it's about 8 1/3 milliseconds.

In addition to representing spacecraft clock readings, ticks can be used to represent arbitrary epochs. In order to minimize discretization error, continuous (non-integral) tick values are supported: ephemeris times may be converted to non-integral ticks via the function sce2c().

Conversion of spacecraft clock strings to ticks always produces integral tick values.

Partitions

One desirable feature of encoded SCLK is that it increases continuously throughout the course of the mission. Unfortunately, real spacecraft clocks do not always behave so nicely. A clock may reset to a lower value, rendering certain counts ambiguous. This might happen if the clock has reached its maximum expression, or because of a power surge. A clock may also jump ahead.

Any time one of these discontinuities occurs, we say that SCLK time has entered a new partition. The partitions must be accounted for when encoding and decoding SCLK.

To continue our analogy, say our wall clock was being used to keep time throughout an entire day. Then 10:05:50 is ambiguous, because we don't know if it falls in the morning or evening partition. So we append the indicators a.m. or p.m. to be clear.

We handle SCLK similarly. Instead of just converting a clock count to ticks (10:05:50 to 36350), we take into account the partition that the count falls in, and compute the number of ticks since clock start (10:05:50 a.m. to 36350; 10:05:50 p.m. to 36350 + 12(60)(60) = 79550).

When you pass a SCLK string to scencd(), it is normally prefixed with a number indicating the partition in which the count falls. Sample SCLK strings for Voyager 2, including partition numbers, are given in an example program later in this document.

The presence of the partition number is not always required. If it is missing, scencd() will assume the partition to be the earliest one possible that contains the clock string being encoded. It's good practice to always include the partition number in SCLK strings.

To convert to ticks since clock start, scencd() processes the partition number. It has to know how many ticks were in all preceding partitions, and what the start and stop clock values were for each. This information is stored in a SCLK kernel file for that spacecraft. The SCLK kernel file is described in detail in a later section.

New partitions may occur at any time throughout the course of active missions. The responsible mission operations team must update the SCLK kernel file to include new partitions as they occur.

In converting encoded SCLK back to an equivalent clock string, scdecd() must also use the SCLK kernel file. Note, however, that you only have to load the SCLK kernel file once in your program, no matter how many calls to scencd() and scdecd() are made afterwards. See the KERNEL required reading file, kernel.req, for information about loading miscellaneous kernel files into the kernel pool.

scdecd() always returns a clock string prefixed by a partition number and the '/' character, for example

2/2000:83:12

If you want to read partition start and stop times for yourself, use the function scpart():

nparts, pstart, pstop = scpart(sc)

SCLK Conversion Functions

In order to correlate data obtained from different components of the CSPICE system, for example pointing and ephemeris data, it is necessary to be able to convert between SCLK time and representations of time in other systems, such as UTC and ephemeris time (also referred to as ET, barycentric dynamical time, and TDB).

CSPICE contains the following functions to convert between encoded and character SCLK, ET and UTC. Note that the names of the functions involving SCLK are all prefixed with sc, for Spacecraft Clock.

et2utc(): Convert ET to a utc string
utc2et(): Convert a utc string to ET
scencd(): Encode SCLK
scdecd(): Decode SCLK
sct2e(): Convert encoded SCLK ticks to ET
scs2e(): Convert SCLK string to ET
sce2c(): Convert ET to continuous ticks
sce2t(): Convert ET to encoded SCLK ticks
sce2s(): Convert ET to SCLK string

It takes at most two function calls to convert between any two of the four representations. CSPICE also contains two functions that can encode and decode relative, or delta SCLK times. These are SCLK strings without partition numbers that represent time increments rather than total time since clock start. Such strings are encoded as tick counts. The functions are:

sctiks(): Convert delta SCLK to ticks
scfmt(): Convert ticks to delta SCLK

Distinguishing Between Different Clocks

The algorithms used to encode and decode SCLK, and convert between SCLK and other time systems are not necessarily the same for each spacecraft.

The differences are handled by the SCLK software at two levels: High-level differences are managed in the code itself through clock types. More detailed spacecraft-specific differences are handled using parameters in a SCLK kernel.

Clock Types

A clock type is a general clock description that may encompass several separate spacecraft clocks. Each clock type is identified in the SCLK functions by an integer code. At the release date of the current revision of this document, all supported missions use spacecraft clock type 1.

A spacecraft clock data type has two components: a format defining the set of acceptable spacecraft clock (SCLK) strings, and a method of converting SCLK strings to a standard time representation, such as ephemeris or UTC seconds past J2000.

For example, a type 1 clock consists of some number of cascading integer counters. An individual counter can increment only when the immediately preceding counter reaches its maximum expression and rolls over. Our wall clock is an example: the counters are hours, minutes and seconds. One tick for a type 1 clock is defined to be the value of the least-significant component increment. Clock type 1 uses a piecewise-linear interpolation process to convert between SCLK and other time systems.

The chapter SLCK01 describes clock type 1 in detail. It includes the specific SCLK string formats for each of the type 1 spacecraft clocks supported by the CSPICE Toolkit.

SCLK functions determine the clock type for a particular spacecraft from the SCLK kernel file (described in the next section).

Clock type-specific functions

Each clock type is supported in the encoding and decoding process by the function sccc_c, where cc is the number of the clock type. sccc_c contains two entry points:

sctkcc_: SCLK string to ticks, type cc
scfmcc_: Ticks to SCLK string, type cc

sctkcc_ and scfmcc_ do not process any partition information; that work is handled at a higher level by scencd() and scdecd(), and is the same for all spacecraft clocks. sctkcc_ and scfmcc_ are called by sctiks() and scfmt(), respectively.

Each clock type is supported in the time conversion process by two functions:

sctecc_: Encoded SCLK ticks to ET, type cc
sceccc_: ET to continuous ticks, type cc

Spacecraft-Specific Parameters

Once the clock type has been determined, SCLK functions need parameters that uniquely distinguish each spacecraft within the same SCLK type. For instance, for type 1, they need to know: How many components make up this particular clock? What are the modulus values for each of the components? What are the coefficients defining the mapping from SCLK to a parallel time system, such as ET? Spacecraft-specific parameters such as these are read from the SCLK kernel file at run-time (see below).

The SCLK Kernel File

NAIF SCLK kernel files supply CSPICE SCLK conversion functions with information required to convert between SCLK values and other representations of time. Typically, a NAIF SCLK kernel will describe the clock of a single spacecraft.

Before calling any of the functions to encode or decode SCLK, or convert between SCLK and other time systems, an application program must load the contents of the SCLK kernel file into the kernel pool, using the function furnsh() (load pool):

furnsh("path_to_SCLK_kernel_file")

An application must also load the leapseconds kernel file if there are any conversions to be performed between ET and UTC. This is typically done in the initialization section of your program. The SCLK kernel file you use should contain values for the particular spacecraft you are dealing with. The variables expected to be found in the file are all prefixed with the string

SCLK_

These variables include partition boundaries, clock type, and several other parameters associated with the clock type. These are described below.

Partition boundaries

The tick values for the beginning and end of each partition are given by:

SCLK_PARTITION_START_ss = ( .....
                            .....
                            .....
                            ..... )

SCLK_PARTITION_END_ss   = ( .....
                            .....
                            .....
                            ..... )

where --ss is the spacecraft ID code. These variables are arrays containing one element per partition. The nth element of .. code-block:: text

SCLK_PARTITITION_END_ss

is considered to be the first tick of the (n+1)st partition. Mathematically speaking, partitions may be thought of as intervals that are closed on the left and open on the right.

Clock type assignment

If --ss is the NAIF ID code of a spacecraft, the associated clock type for that spacecraft is given by the assignment

SCLK_DATA_TYPE_ss = ( cc )

where cc is the clock type. New clock types will be developed as needed. Note that multiple spacecraft ID codes can be associated with the type 1 SCLK data type at one time. Since the spacecraft codes are included in the SCLK variable names, there will be no naming conflicts. (We don't expect this feature to be used much, if at all, but it's there should you need it.)

Clock type-specific parameters

Each spacecraft clock type has its own set of parameters that the CSPICE SCLK functions require in order to convert SCLK values of that type. A complete list and description of these parameters, and their variable names for the kernel pool, is given for type 1 in the chapter SCLK01.

Expanding the system: What NAIF must do

Accommodating new spacecraft clocks may involve no code changes to the SCLK subroutines whatsoever.

If a new clock fits into the framework of clock type 1, then the clock can be accommodated simply by producing a new kernel file for that spacecraft clock. For the new clock, a new set of kernel variables corresponding to those described above, and those in the chapter SCLK01, could be added to an existing SCLK kernel file. Alternatively, an entirely new SCLK kernel file containing the new parameters could be created --- this is the more likely approach. Once this is done, all existing SCLK functions will function, without modification, using the spacecraft ID.

If a new clock does not fit into the clock type 1 framework, then NAIF will design a new clock type. This will involve writing new versions of the four clock type-specific functions described earlier:

sctkcc_
scfmcc_
sctecc_
sceccc_

where cc is the new clock type number. New cases will have to be added to the code of the following higher-level SCxxx conversion functions to call the new, type-specific functions:

It will probably be necessary to design new SCLK kernel file variables to accommodate the new type, and augment the standard variables described above. Adding a new clock type does not change the calling sequence of any of the high-level conversion functions. Thus, once you've learned how to use the SCLK conversion functions, you won't have to re-learn just because a new spacecraft clock has been introduced.

An Example Using SCLK Functions

The following example shows how some of the SCLK functions might be used in a typical application program. This one reads pointing data from a C-kernel file. In this example, a set of four input clock times are hard-coded in the program for the purpose of demonstration: A real application written by you would likely get input times from some external source, such as a file or through interactive user input.

#!/usr/bin/env python
"""
Request pointing from a C-kernel file for a sequence of
pictures obtained from the Voyager 2 narrow angle camera.
Use an array of character spacecraft clock counts as input.

Decode the output clock counts and print the input and
output clock strings. Also print the equivalent UTC time
for each output clock time.

Note that the SCLK kernel file must contain VGR 2 clock
information.
"""
import spiceypy as spice


def main():
    # Instrument, reference frame, and spacecraft ID.
    INST = -32001  # Voyager 2 narrow angle camera.
    REF = "J2000"
    SC = -32  # Spacecraft clock ID for Voyager 2.

    # Clock tolerance string and array of input spacecraft clock counts.
    clktol = "0:01:001"
    sclkin = ["2/20538:39:768", "2/20543:21:768", "2/20550:37", "2/20564:19"]

    # Load the required kernels.
    spice.furnsh("VGR2NA.BC")
    spice.furnsh("SCLK.KER")
    spice.furnsh("LSK.KER")

    # Convert the tolerance string to ticks.
    tol = spice.sctiks(SC, clktol)

    # Process each clock count.
    for clock in sclkin:
        # Convert the input spacecraft clock string to ticks.
        timein = spice.scencd(SC, clock)
        # Retrieve the pointing information.
        # spice.ckgp returns a tuple: (cmat, clkout)
        cmat, timeout = spice.ckgp(INST, timein, tol, REF)
        # Decode the output clock count into a string.
        sclkout = spice.scdecd(SC, timeout)
        # Convert the output spacecraft clock ticks to ephemeris time.
        et = spice.sct2e(SC, timeout)
        # Convert the ephemeris time to a UTC string.
        utc = spice.et2utc(et, "D", 3)
        # Print the results.
        print(f"\nInput  s/c clock count: {clock}")
        print(f"Output s/c clock count: {sclkout}")
        print(f"Output UTC:             {utc}")
        print("Output C-Matrix:")
        for row in cmat:
            # Format each element in the row as a floating point number.
            print("\t".join(f"{elem:f}" for elem in row))
        print()

    # (Optional) Unload the kernels when done.
    spice.kclear()


if __name__ == "__main__":
    main()

The output from this program looks like this:

Input  s/c clock count:  2 / 20538:39:768
Output s/c clock count:  2/20538.39.768
Output UTC:              79-186/21:50:23.000
Output C-Matrix:  <first C-matrix>

Input  s/c clock count:  2 / 20543:21:768
Output s/c clock count:  2/20543.22.768
Output UTC:              79-187/01:35:57.774
Output C-Matrix:  <second C-matrix>

Input  s/c clock count:  2 / 20550:37
Output s/c clock count:  2/20550.36.768
Output UTC:              79-187/07:23:57.774
Output C-Matrix:  <third C-matrix>

Input  s/c clock count:  2 / 20564:19
Output s/c clock count:  2/20564.19.768
Output UTC:              79-187/18:22:21.774
Output C-Matrix:  <fourth C-matrix>

SCLK01

This chapter describes the type 1 SCLK format and conversion algorithms in detail. Also, the SCLK formats for supported spacecraft whose clocks conform to the type 1 specification are described.

Conforming spacecraft clocks

The following spacecraft have SCLK formats that conform to the type 1 specification:

Cassini
Galileo Orbiter
Mars Global Surveyor
Mars Climate Orbiter
Mars Polar Lander
NEAR
Stardust
Voyager 1
Voyager 2

The spacecraft clock encoding and conversion functionality described in this document is fully supported by the CSPICE Toolkit for these spacecraft.

Type 1 SCLK format

The first standard NAIF spacecraft clock data type has two components: a format defining the set of acceptable spacecraft clock (SCLK) strings, and a method of converting SCLK strings to any of a set of standard time systems such as TDT or TDB.

Type 1 SCLK strings have the form

pppp/<time string>

where pppp is a partition number between 1 and 9999 inclusive, and

<time string>

is a time representation that conforms to the type 1 SCLK format. The partition specification (number and slash character) is optional; SCLK strings without partition numbers are assumed to refer to times in the first partition in which the specified clock count occurred. It's good practice to always include the partition number. An example of a type 1 SCLK string (for Galileo) is

3 / 10110007:09:6:1

The number 3 is the partition number, the slash is a delimiter, and the rest of the string is a time string. With this example in hand, we're ready to define the type 1 SCLK format. The partition number is a positive integer followed by a forward slash, for example

4 /

Zero or more blanks are allowed on either side of the slash. A type 1 SCLK time string consists of a series of one or more fields, each of which contains an integer. All fields but the leftmost are optional. The fields of a time string represent modular counts of time units. (A mod n count increments from zero to n-1, and then cycles back to zero.) The values for a given field may be offset by some fixed integer, so that they range from m to m+n, where m is non-negative. The moduli of the various fields are not necessarily the same. The time unit associated with a given field, multiplied by the modulus for that field, gives the time unit for next field to the left.

For each field but the first, values may exceed the modulus for the field. For example, the modulus of the fourth field of a Galileo SCLK string is 8, but the digit 9 is allowed in that field. So

0:0:0:9

is a valid Galileo SCLK string and represents the same time as

0:0:1:1

On input to CSPICE functions, the fields of a type 1 SCLK string may be separated by any of the delimiter characters

-  .  ,  :  <blank>

Consecutive non-blank delimiters containing no intervening digits are treated as if they delimit zero values, consecutive blanks are treated as a single blank, while blanks preceding or following a non-blank delimiter are ignored, as illustrated by this list of eqivalent Galileo SCLK strings:

00000001:00:3:4
0:3:4
:3:4
0 3 4
0   3    4
: 0   3  :  4
: : 3 : 4

On output from CSPICE functions, the delimiter characters will be those defined by a parameter in the SCLK kernel, described later. Note that all fields in time strings represent integers, not decimal fractions. So, the strings

11000687:9

11000687:90

do not represent the same time value: in the former, the second field indicates a count of 9; in the latter, 90.

Galileo SCLK format

An example of a valid time string (without a partition number) for the Galileo spacecraft clock is:

16777214:90:9:7

Numbering the fields from left to right, the time units and moduli of the fields are:

Field              Time unit              Modulus
-----     ---------------------------     --------
  1       60 2/3   sec.                   16777215
  2          2/3   sec.  (666 2/3 ms)           91
  3          1/15  sec.  ( 66 2/3 ms)           10
  4          1/120 sec.  (  8 1/3 ms)            8

Fields 1--4 are known as: Real time image count (RIM), mod 91 count, mod 10 count or real time interrupt count (RTI), and mod 8 count. The values in all fields normally range from zero to the modulus of the field, minus one. The maximum time value that the Galileo spacecraft clock can represent (16777214:90:9:7) is approximately 32 years.

Mars Global Surveyor SCLK format

An example of a valid time string (without a partition number) for the Mars Global Surveyor spacecraft clock is:

4294967295.255

Numbering the fields from left to right, the time units and moduli of the fields are:

Field           Time unit                Modulus
-----     ----------------------       ----------
  1       approximately 1 sec.         4294967296
  2       1/256 sec.                   256

Field 1 is known as the sclk_secs count. Field 2 is known as the sclk_fine word. The values in the first and second fields normally range from zero to the modulus of the field, minus 1. The maximum time value that the Mars Global Surveyor spacecraft clock can represent (4294967295.255) is approximately 136 years.

Voyager SCLK clock format

An example of a valid time string (without a partition number) for both the Voyager 1 and Voyager 2 spacecraft clocks is:

65535.59.800

Numbering the fields from left to right, the time units and moduli of the fields are:

Field        Time unit              Modulus
-----     ------------------       ---------
  1         2880    sec.             65536
  2           48    sec.                60
  3            0.06 sec.               800

Fields 1--3 are known as: Mod 16 count (actually mod 2**16), mod 60 count, and mod 800 count. The values in the first and second fields normally range from zero to the modulus of the field, minus 1. The range of the third field is from 1 to 800. The offset for the third field is 1, so values in this field normally range from 1 to 800 rather than from 0 to 799; values above 800 are allowed and treated as described above. The maximum time value that the Voyager 1 and Voyager 2 spacecraft clocks can represent (65535:59:800) is approximately six years.

Type 1 SCLK conversion

SPICE contains functions that convert between type 1 clock strings and the following representations of time:

ET (TDB)
encoded SCLK

The functions that carry out these conversions are described above in the chapter SCLK Conversion Functions. Since SPICE also contains functions that convert between any of a variety of standard time systems, including ET, UTC, Terrestrial Dynamical Time (TDT), TAI, TDB Julian date, TDT Julian Date, and UTC Julian Date, conversion between SCLK strings and any other time system supported by SPICE requires at most two function calls.

Conversion algorithms

For every type 1 spacecraft clock, encoded SCLK values are converted to ephemeris time (TDB) as follows: first, encoded SCLK values are mapped to equivalent time values in a standard time system such as TDB or TDT. If the standard time system is not TDB, values from this system are mapped to TDB.

The standard time system used for the conversion is referred to here and in the SPICE SCLK functions as the parallel time system. Normally, the SPICE Toolkit will use only one parallel time system for any given spacecraft clock.

Conversion from TDB to encoded SCLK follows the reverse path: first, TDB values are converted, if necessary, to equivalent values in the parallel time system; next, those parallel time values are converted to encoded SCLK.

For each type 1 spacecraft clock, encoded SCLK is related to the parallel time system for that clock by a piecewise linear function. The function is defined by a set of pairs of encoded SCLK values and corresponding values in the parallel time system, and by a set of rate values that apply to the intervals between the pairs of time values. The rate values give the rate at which parallel time increases with respect to encoded SCLK time during the interval over which the rate applies. The rates in a type 1 SCLK kernel have units of

 parallel time system units
----------------------------
most significant clock count

The units of the currently supported parallel time systems TDT and TDB are seconds measured in those respective systems. So clock rates may be TDT or TDB seconds per most significant clock count. For example, for the GLL orbiter spacecraft clock, the rate unit is TDB seconds per RIM. The specific method by which pairs of time values and rates are used to map encoded SCLK to parallel time values is explained in detail below. In the following discussion we'll use the name PARSYS to refer to the parallel time system. We'll use the name MSF to indicate the number of ticks per most significant SCLK field.

We can represent the data that define the SCLK-to-PARSYS mapping as a set of ordered triples of encoded SCLK values (in units of ticks since spacecraft clock start), their equivalents in PARSYS time, and the rates corresponding to each pair of times:

( s/c_clock(1),  parsys(1),  rate(1) )
                  .
                  .
                  .
( s/c_clock(n),  parsys(n),  rate(n) )

The mapping of SCLK values to PARSYS times is carried out as follows: If the sclk time clock satisfies

sclk(i)  <  clock  <  sclk(i+1)
         -

then the corresponding PARSYS time is

parsys(i)  +    ( rate(i)/MSF )  *  ( clock - sclk(i) )

If

clock  >  clock(n)
       -

the formula still applies, with i = n. To convert PARSYS time values to SCLK, we use an analogous method. If time is the value to be converted, and

parsys(i)  <  time  <  parsys(i+1)
           _

then the corresponding continuous encoded SCLK value is

                time -  parsys(i)
sclk(i)    +    ----------------
                  rate(i)/MSF

If

time >  parsys(n)
     -

the formula still applies, with i = n. Note that this method will not handle rate values of 0 parallel time system units per tick.

When the function described by the pairs of time values and rates is continuous, then all rates except for the last one are redundant, since

                 parsys(i+1) -  parsys(i)
rate(i)/MSF  =   ------------------------
                  sclk(i+1)  -   sclk(i)

If the mapping from encoded SCLK to the parallel time system is not continuous, then the mapping will not be strictly invertible: if an encoded SCLK value is mapped to a parallel time value, then mapping that parallel time value to encoded SCLK may not yield the original encoded SCLK value. However, the capability of supporting non-continuous mappings is provided in case it is needed to implement the mapping provided by a flight project. In order for CSPICE SCLK conversion functions to work, the information represented by the ordered triples described above must be loaded via the kernel pool. See the section The spacecraft clock kernel file below for details.

Type 1 SCLK functions

Type 1 SCLK functions are normally called by the higher-level SCLK functions scencd(), scdecd(), scs2e(), sct2e(), sce2c(), sce2t(), sce2s(), sctiks_c, and scfmt(); you should not need to call these functions directly, though direct calls to these functions are not prohibited.

The type 1 SCLK functions are

scfm01_: Convert ticks to a type 1 SCLK string
sctk01_: Convert a type 1 SCLK string to ticks
scec01_: ET to continuous ticks, type 1
scet01_: Convert ET to ticks, type 1
scte01_: Convert ticks to ET, type 1
scld01_: SCLK look up of double precision data, type 1
scli01_: SCLK look up of integer data, type 1
sclu01_: SCLK lookup, type 1
sc01_: SCLK conversion, type 1

The last two functions sc01_ and sclu01_ are umbrella functions which exist for the purpose of allowing their entry points to share data. These functions should not be called directly.

The type 1 SCLK kernel file

Before any CSPICE functions that make use of type 1 SCLK values can be used, a SCLK kernel file must be loaded into the kernel pool. Regardless of the clock type, an SCLK kernel assigns values to variables that define:

The clock type for a spacecraft
The format of SCLK strings, for one or more spacecraft.
The mapping between encoded SCLK values and a parallel time system.

Variables that are used for all clock types have names that start with the string

SCLK_

Variables that are applicable only to type 1 spacecraft clocks start with the string

SCLK01_

An SCLK kernel file makes the following assignments for each spacecraft whose clock values are to be treated as type 1 by the CSPICE SCLK functions:

Kernel ID assignment

Each SCLK kernel must assign a identifier to the kernel variable

SCLK_KERNEL_ID

This identifier is normally a UTC time string, preceded by the character '@', for example,

@04-SEP-1990

If you have loaded multiple SCLK kernels into the kernel pool, the identifiers for these kernels should be distinct.

Parallel time system code assignment

If --ss is the NAIF ID code of a spacecraft, this ID is associated with a parallel time system by the assignment

SCLK01_TIME_SYSTEM_ss  = ( nnn )

where nnn is a numeric code designating the time system that the coefficients in the kernel map encoded SCLK to. The time systems and codes currently in use are:

Barycentric dynamical time (TDB): 1
Terrestrial dynamical time (TDT): 2

This assignment is optional; if absent, the parallel time system is assumed to be barycentric dynamical time.

SCLK type assignment

If --ss is the NAIF ID code of a spacecraft, this ID is associated with a SCLK type by the assignment

SCLK_DATA_TYPE_ss = ( 1 )

Note that multiple mission ID codes can be associated with the type 1 SCLK data type at one time. Since the mission codes are included in the SCLK variable names, there will be no naming conflicts.

Format constant assignments

All of the format constants start with the string

SCLK01

and end with the string

_ss

where --ss is the NAIF mission ID code. This allows the type 1 SCLK functions to find the correct constants for each mission ID associated with the first SCLK data type. The format constants that must be assigned are

SCLK01_N_FIELDS_ss
SCLK01_MODULI_ss
SCLK01_OFFSETS_ss
SCLK01_OUTPUT_DELIM_ss

Here are sample assignments of values to the variables describing the format of type 1 SCLK strings. The values shown apply to the Galileo SCLK format. Number of fields:

SCLK01_N_FIELDS_77  =  ( 4 )

Modulus of each field:

SCLK01_MODULI_77  =  ( 16777215 91 10 8 )

Offsets for field values. Offsets are listed for each field in left-to-right order:

SCLK01_OFFSETS_77 = ( 0 0 0 0 )

Code for delimiter to be used in output strings. The codes and corresponding delimiters are:

Code          Delimiter

             .

             :

             -

             ,

           <space>

For Galileo, the code assignment would be:

SCLK01_OUTPUT_DELIM_77 = ( 2 )

Time coefficients

The data that define the mapping between SCLK and the parallel time system are called time coefficients. This name is used because the data are coefficients of linear polynomials; as a set, they define a piecewise linear function that maps SCLK to the parallel time system.

The time coefficients are assigned to the variable

SCLK01_COEFFICIENTS_ss

where --ss is the spacecraft ID code. The assigned values are triplets of SCLK values, corresponding parallel time values, and rates. The SCLK values are expressed in total ticks since clock start. The parallel time values may be expressed in a variety of units. The rate values have units that depend on the units used for the parallel time values: if we call these units

PARALLEL_TIME_UNITS

then the rate units are

   PARALLEL_TIME_UNITS
----------------------------
most significant clock count

The term most significant clock count shown in the denominator refers to the length of time associated with one count of the most significant (leftmost) field of the formatted spacecraft clock string. For example, for Voyager 2, the most significant field of a formatted SCLK string is the mod 16 field. For Galileo, the most significant field is the RIM count. For Mars Global Surveyor, the most significant field is the sclk_secs count.

Partition boundaries

In order to convert between SCLK strings and their encoded form of ticks since spacecraft clock start, it is necessary to know the initial and final SCLK readouts for each partition. These values are given by:

PARTITION_START_ss
PARTITION_END_ss

where --ss is the spacecraft ID code. These variables are arrays containing one element per partition.

Sample SCLK kernels

The following is a sample SCLK kernel for Galileo:

 KPL/SCLK

 \begindata
 SCLK_KERNEL_ID            = ( @04-SEP-1990//4:23:00 )

 SCLK_DATA_TYPE_77         = ( 1                )
 SCLK01_N_FIELDS_77        = ( 4                )
 SCLK01_MODULI_77          = ( 16777215 91 10 8 )
 SCLK01_OFFSETS_77         = (        0  0  0 0 )
 SCLK01_OUTPUT_DELIM_77    = ( 2                )

 SCLK_PARTITION_START_77   = ( 0.0000000000000E+00
                               2.5465440000000E+07
                               7.2800001000000E+07
                               1.3176800000000E+08 )

 SCLK_PARTITION_END_77      = ( 2.5465440000000E+07
                               7.2800000000000E+07
                               1.3176800000000E+08
                               1.2213812519900E+11 )

 SCLK01_COEFFICIENTS_77    = (

 0.0000000000000E+00  -3.2287591517365E+08  6.0666283888000E+01
 7.2800000000000E+05  -3.2286984854565E+08  6.0666283888000E+01
 1.2365520000000E+06  -3.2286561063865E+08  6.0666283888000E+01
 1.2365600000000E+06  -3.2286558910065E+08  6.0697000438000E+01
 1.2368000000000E+06  -3.2286557090665E+08  6.0666283333000E+01
 1.2962400000000E+06  -3.2286507557565E+08  6.0666283333000E+01
 2.3296480000000E+07  -3.2286507491065E+08  6.0666300000000E+01
 2.3519280000000E+07  -3.2286321825465E+08  5.8238483608000E+02
 2.3519760000000E+07  -3.2286317985565E+08  6.0666272281000E+01
 2.4024000000000E+07  -3.2285897788265E+08  6.0666271175000E+01
 2.5378080000000E+07  -3.2284769395665E+08  6.0808150200000E+01
 2.5421760000000E+07  -3.2284732910765E+08  6.0666628073000E+01
 2.5465440000000E+07  -3.2284696510765E+08  6.0666628073000E+01
 3.6400000000000E+07  -3.2275584383265E+08  6.0666627957000E+01
 7.2800000000000E+07  -3.2245251069264E+08  6.0666628004000E+01
 1.0919999900000E+08  -3.2214917755262E+08  6.0666628004000E+01
 1.2769119900000E+08  -3.2199508431761E+08  6.0665620197000E+01
 1.3085799900000E+08  -3.2196869477261E+08  6.0666892494000E+01
 1.3176799900000E+08  -3.2196111141061E+08  6.0666722113000E+01
 1.3395199900000E+08  -3.2194291139361E+08  6.0666674091000E+01
 1.3613599900000E+08  -3.2192471139161E+08  6.0666590261000E+01
 1.4341599900000E+08  -3.2186404480160E+08  6.0666611658000E+01
 1.5069599900000E+08  -3.2180337818960E+08  6.0666611658000E+01
 1.7253599900000E+08  -3.2162137835458E+08  6.0666783566000E+01
 1.7515679900000E+08  -3.2159953831258E+08  6.0666629213000E+01
 1.7777759900000E+08  -3.2157769832557E+08  6.0666629213000E+01
 3.3451599900000E+08  -3.2027154579839E+08  6.0666505193000E+01
 3.3713679900000E+08  -3.2024970585638E+08  6.0666627480000E+01
 3.3975759900000E+08  -3.2022786587038E+08  6.0666627480000E+01
 5.6601999900000E+08  -3.1834234708794E+08  6.0666396876000E+01
 5.6733039900000E+08  -3.1833142713693E+08  6.0666626282000E+01
 5.6864079900000E+08  -3.1832050714393E+08  6.0666626282000E+01
 8.9797999900000E+08  -3.1557601563707E+08  5.9666626282000E+01
 8.9798727900000E+08  -3.1557595597007E+08  6.0666626282000E+01

8.9799455900000E+08  -3.1557589430307E+08  6.0666626282000E+01 )

 \begintext

Below is a sample SCLK kernel file for Mars Global Surveyor. Note that the text prior to the first

\begindata

directive is treated as a group of comment lines by the SPICELIB kernel readers. The labels shown in this comment area are examples and should not be construed as a correct specification.

   KPL/SCLK

   Status
   -----------------------------------------------


    This file is a SPICE spacecraft clock (SCLK) kernel containing
      information required for Mars Global Surveyor spacecraft
      on-board clock to ET conversion.


   Production/History of this SCLK files
   -----------------------------------------------

      This file was generated by the NAIF utility program MAKCLK,
      version 3.3, from the most recent Mars Global Surveyor
      spacecraft SCLK SCET file.


   Usage
   -----------------------------------------------


     This file must be loaded into the user's program by a call to
      the FURNSH subroutine

            CALL FURNSH( 'this_file_name' )

      in order to use the SPICELIB SCLK family of subroutines to

    convert MGS spacecraft on-board clock to ET and vice versa and
      to use MGS frames defined below as reference frames for
      geometric quantities being returned by high-level SPK and
      CK subroutines.


   References
   -----------------------------------------------


1. SCLK Required Reading file (sclk.req), NAIF document number 222
      1. MAKCLK User's Guide, NAIF document number 267


   Inquiries
   -----------------------------------------------

      If you have any questions regarding this file contact

      MGS Spacecraft Operations Team (SCOPS)
      Lockheed/Martin, Denver

      Boris Semenov - NAIF/JPL
      (818) 354-8136
      bsemenov@spice.jpl.nasa.gov


   SCLK DATA
   -----------------------------------------------

   \begindata

   SCLK_KERNEL_ID           = ( @1999-02-07/03:51:29.00 )

   SCLK_DATA_TYPE_94        = ( 1 )
   SCLK01_TIME_SYSTEM_94    = ( 2 )
   SCLK01_N_FIELDS_94       = ( 2 )
   SCLK01_MODULI_94         = ( 4294967296 256 )
   SCLK01_OFFSETS_94        = ( 0 0 )
   SCLK01_OUTPUT_DELIM_94   = ( 1 )

   SCLK_PARTITION_START_94  = ( 1.3611133440000E+11 )

   SCLK_PARTITION_END_94    = ( 1.0995116277750E+12 )

   SCLK01_COEFFICIENTS_94   = (

    0.0000000000000E+00  -9.9510252675000E+07  9.9999996301748E-01
    8.3066265600000E+08  -9.6265476795000E+07  9.9999994844682E-01
    1.9330583040000E+09  -9.1959244017000E+07  9.9999994927604E-01
    2.7708477440000E+09  -8.8686629183000E+07  9.9999994213351E-01
    4.0538009600000E+09  -8.3675093473000E+07  9.9999993609973E-01
    4.7829370880000E+09  -8.0826905655000E+07  9.9999993275158E-01
    5.2473643520000E+09  -7.9012736777000E+07  9.9999993064539E-01
    5.4909818880000E+09  -7.8061105843000E+07  9.9999992770059E-01
    6.7515176960000E+09  -7.3137138199000E+07  9.9999992410889E-01
    7.9017973760000E+09  -6.8643858540000E+07  9.9999992038548E-01
    8.9854187520000E+09  -6.4410962877000E+07  9.9999991689249E-01
    9.9588085760000E+09  -6.0608659193000E+07  9.9999991330346E-01
    1.1222619136000E+10  -5.5671899621000E+07  9.9999990916047E-01
    1.2448517120000E+10  -5.0883236056000E+07  9.9999990447344E-01
    1.3831336704000E+10  -4.5481597572000E+07  9.9999990051645E-01
    1.5223486464000E+10  -4.0043513113000E+07  9.9999989497162E-01
    1.7390367488000E+10  -3.1579135002000E+07  9.9999988993180E-01

  1.7567130624000E+10  -3.0888654078000E+07  9.9999989100000E-01 )
   \begintext