|
JNISpice version 2.0.0 |
||||||||
PREV CLASS NEXT CLASS | FRAMES NO FRAMES | ||||||||
SUMMARY: NESTED | FIELD | CONSTR | METHOD | DETAIL: FIELD | CONSTR | METHOD |
java.lang.Objectspice.basic.AberrationCorrection
public class AberrationCorrection
Class AberrationCorrection represents selections of aberration corrections that can be applied to state vectors.
This class provides methods to enable an application to determine attributes of a correction, for example whether stellar aberration correction is included.
Normally this class is used in association with constructors for any of the classes
Aberration corrections are applied to states of a target relative to an observer. When light time corrections are used, the orientation of a time-dependent reference frame also may depend on the one-way light time between the observer and the ephemeris object acting as the center of the reference frame. Details can be found in the documentation of methods that employ aberration corrections.
About aberration corrections ============================ In space science or engineering applications one frequently wishes to know where to point a remote sensing instrument, such as an optical camera or radio antenna, in order to observe or otherwise receive radiation from a target. This pointing problem is complicated by the finite speed of light: one needs to point to where the target appears to be as opposed to where it actually is at the epoch of observation. We use the adjectives "geometric," "uncorrected," or "true" to refer to an actual position or state of a target at a specified epoch. When a geometric position or state vector is modified to reflect how it appears to an observer, we describe that vector by any of the terms "apparent," "corrected," "aberration corrected," or "light time and stellar aberration corrected." JNISpice can correct for two phenomena affecting the apparent location of an object: one-way light time (also called "planetary aberration") and stellar aberration. One-way light time ------------------ Correcting for one-way light time is done by computing, given an observer and observation epoch, where a target was when the observed photons departed the target's location. The vector from the observer to this computed target location is called a "light time corrected" vector. The light time correction depends on the motion of the target relative to the solar system barycenter, but it is independent of the velocity of the observer relative to the solar system barycenter. Relativistic effects such as light bending and gravitational delay are not accounted for in the light time correction performed by this routine. Stellar aberration ------------------ The velocity of the observer also affects the apparent location of a target: photons arriving at the observer are subject to a "raindrop effect" whereby their velocity relative to the observer is, using a Newtonian approximation, the photons' velocity relative to the solar system barycenter minus the velocity of the observer relative to the solar system barycenter. This effect is called "stellar aberration." Stellar aberration is independent of the velocity of the target. The stellar aberration formula used by this routine does not include (the much smaller) relativistic effects. Stellar aberration corrections are applied after light time corrections: the light time corrected target position vector is used as an input to the stellar aberration correction. When light time and stellar aberration corrections are both applied to a geometric position vector, the resulting position vector indicates where the target "appears to be" from the observer's location. As opposed to computing the apparent position of a target, one may wish to compute the pointing direction required for transmission of photons to the target. This also requires correction of the geometric target position for the effects of light time and stellar aberration, but in this case the corrections are computed for radiation traveling *from* the observer to the target. We will refer to this situation as the "transmission" case. The "transmission" light time correction yields the target's location as it will be when photons emitted from the observer's location at `et' arrive at the target. The transmission stellar aberration correction is the inverse of the traditional stellar aberration correction: it indicates the direction in which radiation should be emitted so that, using a Newtonian approximation, the sum of the velocity of the radiation relative to the observer and of the observer's velocity, relative to the solar system barycenter, yields a velocity vector that points in the direction of the light time corrected position of the target. One may object to using the term "observer" in the transmission case, in which radiation is emitted from the observer's location. The terminology was retained for consistency with earlier documentation. Below, we indicate the aberration corrections to use for some common applications: 1) Find the apparent direction of a target. This is the most common case for a remote-sensing observation. Use "LT+S": apply both light time and stellar aberration corrections. Note that using light time corrections alone ("LT") is generally not a good way to obtain an approximation to an apparent target vector: since light time and stellar aberration corrections often partially cancel each other, it may be more accurate to use no correction at all than to use light time alone. 2) Find the corrected pointing direction to radiate a signal to a target. This computation is often applicable for implementing communications sessions. Use "XLT+S": apply both light time and stellar aberration corrections for transmission. 3) Compute the apparent position of a target body relative to a star or other distant object. Use "LT" or "LT+S" as needed to match the correction applied to the position of the distant object. For example, if a star position is obtained from a catalog, the position vector may not be corrected for stellar aberration. In this case, to find the angular separation of the star and the limb of a planet, the vector from the observer to the planet should be corrected for light time but not stellar aberration. 4) Obtain an uncorrected state vector derived directly from data in an SPK file. Use "NONE". 5) Use a geometric state vector as a low-accuracy estimate of the apparent state for an application where execution speed is critical. Use "NONE". 6) While this routine cannot perform the relativistic aberration corrections required to compute states with the highest possible accuracy, it can supply the geometric states required as inputs to these computations. Use "NONE", then apply relativistic aberration corrections (not available in the SPICE Toolkit).
isGeometric()
.
Constructor Summary | |
---|---|
AberrationCorrection(AberrationCorrection abcorr)
Copy constructor. |
|
AberrationCorrection(java.lang.String abcorr)
Construct AberrationCorrection from a string. |
Method Summary | |
---|---|
boolean |
equals(java.lang.Object obj)
Test two AberrationCorrections for equality. |
java.lang.String |
getName()
Return a string describing the aberration correction. |
int |
hashCode()
Return hash code for an AberrationCorrection object. |
boolean |
hasLightTime()
Indicate whether light time correction (converged Newtonian or not) is included. |
boolean |
hasStellarAberration()
Indicate whether stellar aberration correction is included. |
boolean |
isConvergedNewtonian()
Indicate whether the correction is converged Newtonian. |
boolean |
isGeometric()
Indicate whether the correction is geometric (equivalent to "NONE"). |
boolean |
isReceptionType()
Indicate whether the correction type is "reception." |
java.lang.String |
toString()
Return a string describing the aberration correction. |
Methods inherited from class java.lang.Object |
---|
clone, finalize, getClass, notify, notifyAll, wait, wait, wait |
Constructor Detail |
---|
public AberrationCorrection(AberrationCorrection abcorr)
public AberrationCorrection(java.lang.String abcorr) throws SpiceErrorException
abcorr indicates the aberration corrections to be applied to the state of a target body to account for one-way light time and stellar aberration. See the discussion in the Particulars section for recommendations on how to choose aberration corrections. `abcorr' may be any of the following: "NONE" Apply no correction. Return the geometric state of the target body relative to the observer. The following values of `abcorr' apply to the "reception" case in which photons depart from the target's location at the light-time corrected epoch et-lt and *arrive* at the observer's location at `et': "LT" Correct for one-way light time (also called "planetary aberration") using a Newtonian formulation. This correction yields the state of the target at the moment it emitted photons arriving at the observer at `et'. The light time correction uses an iterative solution of the light time equation (see Particulars for details). The solution invoked by the "LT" option uses one iteration. "LT+S" Correct for one-way light time and stellar aberration using a Newtonian formulation. This option modifies the state obtained with the "LT" option to account for the observer's velocity relative to the solar system barycenter. The result is the apparent state of the target---the position and velocity of the target as seen by the observer. "CN" Converged Newtonian light time correction. In solving the light time equation, the "CN" correction iterates until the solution converges (three iterations on all supported platforms). The "CN" correction typically does not substantially improve accuracy because the errors made by ignoring relativistic effects may be larger than the improvement afforded by obtaining convergence of the light time solution. The "CN" correction computation also requires a significantly greater number of CPU cycles than does the one-iteration light time correction. "CN+S" Converged Newtonian light time and stellar aberration corrections. The following values of `abcorr' apply to the "transmission" case in which photons *depart* from the observer's location at `et' and arrive at the target's location at the light-time corrected epoch et+lt: "XLT" "Transmission" case: correct for one-way light time using a Newtonian formulation. This correction yields the state of the target at the moment it receives photons emitted from the observer's location at `et'. "XLT+S" "Transmission" case: correct for one-way light time and stellar aberration using a Newtonian formulation This option modifies the state obtained with the "XLT" option to account for the observer's velocity relative to the solar system barycenter. The position component of the computed target state indicates the direction that photons emitted from the observer's location must be "aimed" to hit the target. "XCN" "Transmission" case: converged Newtonian light time correction. "XCN+S" "Transmission" case: converged Newtonian light time and stellar aberration corrections. Neither special nor general relativistic effects are accounted for in the aberration corrections applied by this routine. Case and blanks are not significant in the string `abcorr'.
SpiceErrorException
Method Detail |
---|
public boolean equals(java.lang.Object obj)
equals
in class java.lang.Object
public int hashCode()
hashCode
in class java.lang.Object
public java.lang.String getName()
public boolean isGeometric()
public boolean hasStellarAberration()
public boolean hasLightTime()
public boolean isConvergedNewtonian()
public boolean isReceptionType()
public java.lang.String toString()
toString
in class java.lang.Object
|
JNISpice version 2.0.0 |
||||||||
PREV CLASS NEXT CLASS | FRAMES NO FRAMES | ||||||||
SUMMARY: NESTED | FIELD | CONSTR | METHOD | DETAIL: FIELD | CONSTR | METHOD |