Spice

Introduction to SPICE
Jose Luis Vázquez
European Space Astronomy Centre
European Space Agency

Introduction to SPICEIntroduction to SPICE
Jose Luis Vázquez (ESAC/ESA)Jose Luis Vázquez (ESAC/ESA)
A general problem:
An image from Mars is sent to
the Earth, but... whereabouts in
Mars?

A general problem (step
1):
Calculate the position and
orientation of the S/C with
respect to some frame.

A general problem (step
2):
Calculate the intersection
of the camera field of view
with the Mars surface.

A general (big) problem:

I only have the S/C clock information about the time the image was taken.
How do I know the UTC time?

How do I calculate the position and orientation of the S/C for that UTC
time?

How do I know the field of view of the camera and its intersection with
the Mars surface?

Even if I knew that, Mars rotates with time. How do I know the position
of Mars for the time the image was taken?
The solution is...

SPICE!

A few generalities

SPICE is a library (the toolkit) and a data format that will help you when
it comes to geometry and time calculations.

Developed by NAIF, at the Jet Propulsion Laboratory, under a contract
with NASA.

In can be used for data analysis, but also for planning.

The toolkit is available in Fortran, C and IDL. There also is a beta Matlab
version.

It is freely downloadable from the official SPICE web site at
http://naif.nasa.jpl.gov.

Components of SPICE

The SPICE toolkit: available in several programming languages. It
contains a comprehensible html documentation.

Data files, aka kernels. They contain all the data SPICE needs in order to
do its calculations. They are usually provided by NAIF, ESA or other
institutions.

Utility programs to handle and create kernels.

Documentation: Tutorials and Required Readings, also downloadable
from the NAIF web page. You are encouraged to read them!

NAIF organizes regular workshops on SPICE, open to the
communty. They usually are held in USA.
Next SPICE workshop: ESAC (Madrid), Obtober 2007
Contact: Jose Luis Vázquez
Still a few places left...
Training in SPICE

SPICE kernels

The SPICE kernels are data files that contain the information the
toolkit needs for the calculations.

They are several kernel types. Each type contains a different kind of
information (ephemeris, attitude, time, etc.).

A kernel can be a binary or text file.

Types of SPICE kernels (I)
A kernel can be of the following types:

SPK: Spacecraft and planetary kernel with
ephemeris data.

IK: Instrument kernel characteristics like
field of view, or number of pixels in a
CCD.

CK: C-matrix kernel with attitude of
spacecrafts and subsructures.

EK: Events kernel.

Types of SPICE kernels (II)

FK: Frames kernel with information about
different reference frames.

PCK: Planetary constants kernel, with
information like mass, radius, etc. for
Solar System bodies.

LSK: Leapseconds kernel.

SCLK: S/C clock coefficients kernel.

SPICE kernels. Why do they exsist?

Accuracy: Motion of bodies are far from ideal. The ephemeris and
attitude kernels contain actual data from measurements or predictions.

Economy: Not all the information is needed at the same time. You
can use the kernels you need for your application.

Flexibility: Information can be updated/improved. You don't need to
update the toolkit or recompile the application; just get the new
kernels.

SPICE concepts: Time (I)
Time is important, since SPICE does almost all the computations as a
function of time.
Two different ways of keeping track of time:

Based on the Earth rotation: 1 day is the time between two consecutive
passes of the Sun above Greenwich: UT1.

Based on atomic clocks: based on the frequency of atomic oscilations:
TAI (International Atomic Time). TAI is the count of atomic seconds
since a particular epoch.

SPICE concepts: Time (II)
UTC: gives a calendar name to every TAI second.
Problem: the Earth does not always rotate at the same speed. A
complete rotation does not always take 86400 atomic seconds: UT1
and UTC drift apart.
Solution: leapseconds.
3600
s
hour
×24
hours
day
=86400
s
day

SPICE concepts: Time (III)
Leapseconds (i):

UTC ahead UT1 more than 0.7 seconds: a positive leapsecond is added to
UTC:
... DECEMBER 31 23:59:58
... DECEMBER 31 23:59:59
... DECEMBER 31 23:59:60
... JANUARY 1 00:00:00

UT1 ahead UTC more than 0.7 seconds: a negative leapsecond is extrated
to UTC:
... DECEMBER 31 23:59:58
... JANUARY 1 00:00:00

SPICE concepts: Time (IV)
Leapseconds (ii):

The LSK kernel is a text kernel that keeps track of the leapseconds that
have occurred so far:
DELTET/DELTA_AT = ( 10, @1972-JAN-1
11, @1972-JUL-1
12, @1973-JAN-1

Only one leapseconds kernel exists (naif0008.tls). It is updated any time
a new leapsecond is announced.

Leapseconds are announced by IERS. They are typically added at
December 31 or June 30.

SPICE concepts: Time (V)
ET (Ephemeris Time).
ET is the independent variable in the differential equations that describe
the motions of the bodies of the Solar System.
As far as the measurements can detect, ET and TAI advance at the same
rate.
ET is measured in seconds past the J2000 epoch (roughly noon, January
1st, 2000).
If you want to translate from ET to UTC or the other way around, you need
information about the leapseconds.

SPICE concepts: Time (VI)
S/C time (i).
Spacecrafts do not have a watch. They have an on-board counter, which
counts ticks instead of seconds.
The duration of a tick depends on the particular spacecraft. Moreover, it can
change during the mission due to different facts. It can even jump back and
forward, or suffer a reset.
Information about the spacecraft clock rate is gathered on ground, and stored
in the SCLK kernel.

SPICE concepts: Time (VII)
S/C time (ii).

SPICE concepts: Time (VII)
Spacecraft clock kernel (SCLK).
Stores information about:

Nominal rate of the clock (e.g., ticks per second).

How the nominal rate varies during the mission.

How many resets or jumps happened in the past.
The information on the two last points can not be predicted. It is
reconstructed on ground.
The spacecraft clock is the only time information available in the telemetry.
The SPICE toolkit needs the SCLK kernel to translate the S/C ticks to
ET.
There is one SCLK kernel per mission.

SPICE concepts: Time (VIII)
Cookbook (i):
Convert from ET to UTC and the other way around:

Load the leapseconds kernel:
furnsh_c( “naif0008.tls” );

Call the appropiate function:

ET -> UTC:
et2utc_c( ... );

UTC -> ET:
utc2et_c( ... );

SPICE concepts: Time (VIII)
Cookbook (ii):
Convert from S/C ticks to ET and the other way around:

Load the SCLK kernel:
furnsh_c( “VEX_070719_STEP.TSC” );

Call the appropiate function:

ET -> S/C ticks:
et2utc_c( ... );

S/C ticks -> ET:
utc2et_c( ... );

SPICE concepts: Time (IX)
Cookbook (iii):
Convert from S/C ticks to UTC and the other way around: there is no direct
conversion:

S/C ticks -> UTC:

S/C ticks -> ET -> UTC

UTC to S/C ticks:

UTC -> ET -> S/C ticks
You'll need both the SCLK and the LSK kernels. Do not forget to load
them!

SPICE concepts: Frames (I)
A reference frame in SPICE is a particular realization of a Cartesian
coordinate system. A frame is usually attached to a body, spacecraft,
barycenter, etc.

SPICE concepts: Frames (II)
Two kinds of frames:

Inertial Frame: the Newton Laws can be applied.

Non Inertial Frame: the Newton Laws don't apply. Any frame that
rotates with respect to the starts background is non inertial.
The most important frame in SPICE is J2000. It is an inertial frame.

SPICE concepts: Frames (III)

SPICE concepts: Frames (IV)
Frames in SPICE can be built in (J2000), or provided to the SPICE toolkit via
an FK or PCK kernel. They can be:

Inertial (J2000).

Body-fixed frames (IAU_MARS). They need a PCK kernel to work.

Fixed offset frames. Defined in text FK kernels.

CK-based frames. Defined in text FK kernels, with orientation provided
in a CK kernel.

Dynamic frames. Orientation based on dynamic directions computed by
SPICE based on kernel data or mathematical models.

SPICE concepts: Frames (V)
Frames for spacecrafts are usually CK based frames.
Frames for spacecraft substructures are usually fixed offset frames, defined
with respect to the spacecraft frame.

SPICE concepts: Frames (VI)

SPICE concepts: Frames (VII)

SPICE concepts: Frames (VIII)
Rotations (i)
Vectors in SPICE are given in a specific frame. Very often their components
in other frame have to be calculated.
How? Rotations.

SPICE concepts: Frames (IX)
Rotations (ii)
Rotating a frame A turns it into a different one B.
By specifying how to rotate A to get to B, SPICE can figure out how to
transform vectors from the frame A to the frame B. You do that via a
frames kernel (FK).
Three different ways of specifying rotations in SPICE:

Rotation matrix.

Euler angles.

Quaternions.

SPICE concepts: Frames (X)
Rotations (iii)
Euler Angles:

SPICE concepts: Frames (XI)
Rotations (iv)
Rotation matrix: way to transform vectors in SPICE.
vA: Coordinates of v reference frame A
vB: Coordinates of v reference frame B
[T ]: Rotation Matrix .
vB=[T ]×vA

SPICE concepts: Frames (XII)
Cookbook (i)
Transform vectors from frame A to Frame B:

Calculate a rotation matrix:
pxform_c( “J2000”, “IAU_MARS”, et, matrix);

Get the new coordinates via matrix-vector multiplication:
mxv_c( matrix, v, w );
v -> vector in J2000 frame
w -> vector in IAU MARS frame

SPICE concepts: Frames (XII)
Cookbook (ii)
Get the position of a body:
spkpos_c( “MARS”, et, “J2000”, “LT+S”, “EARTH”,
&position, &light_time );
Get the state of a body:
spkezr_c( “MARS”, et, “J2000”, “LT+S”, “EARTH”,
&state, &light_time );
position -> 3-dimensions vector with the position of the
body
state -> 6-dimensions vector with the position and
velocity of the body

This afternoon:

How to find the needed kernels.

How to get information about the kernels.

A few exercices.

Spice

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