The azimuthal angle of the source is determined by observing the modulation of the signal as the spacecraft spins. The URAP instrument takes a series of measurements during each spin. We use the high frequency part of the Radio Astronomy Receiver of URAP which has 12 frequencies in the range 52 to 940 kHz. Not all of the frequencies are recorded on each spin but the instrument cycles through the 12 frequencies (this actually depends on the instrument mode and during parts of the mission not all of the 12 frequencies may be available). For a single spin at one freqency the data are fit to an equation of the form:
p(1)+p(2)*cos(2*phi)+p(3)*sin(2*phi)
where p(1), p(2), and p(3) are the parameters fit and phi is the rotation angle of the spacecraft. The factor 2 arises from the fact that the X antenna is a dipole perpendicular to the spin axis that has the same aspect when rotated 180 degrees about the Z axis.
After the fit to the above equation has been performed, three quantities are computed:
| Intensity | p(1) |
| Modulation Index | sqrt(p(2)**2+p(3)**2) |
| Azimuthal Angle | arctan(p(2)/p(3)) |
The coordinate system used for direction finding is non-rotating and spacecraft centered. The Z axis is defined as being parallel to the spacecraft rotation axis and pointing towards Earth from the spacecraft. This axis is always closely aligned (within about 1 degree) with Earth since the high gain antenna is aligned on this axis as well. This axis must be periodically re-aligned with Earth as Ulysses follows its trajectory. This is done by using the spacecraft thrusters. The X axis of the coordinate system is defined as being in the plane defined by the Z axis and a vector pointing from the spacecraft to the Sun. The Y axis is defined so that X, Y, Z define an orthogonal right-handed coordinate system. When the direction finding procedure is carried out it provides x and y, the components of the source direction along X and Y. The azimuthal angle is defined in the usual manner - it is the arctangent of y/x. Note that if the Sun were to be on the Z axis, then the X axis would be indeterminate. During some parts of the mission, the Sun was very close to the Z axis. This makes the X axis very sensitive to any changes in the position of the Z axis and this phenomena can be observed, particularly when a thruster firing causes the Z axis to change abruptly.
Due to the symmetry of the antenna, there is an inherent ambiguity of 180 degrees in the azimuthal angle. For this reason, the azimuthal angle (A) is further processed to place it in the range -90 ≤ A ≤ 90 degrees. This is done via the following algorithm: if A < -90, A ← A+180 and if a > 90, A ← A-180. When comparing the source direction to determine which celestial body from which it might have come, the azimuthal angle of the planet should also be "folded" in this way.
Plots have been created of the results of the direction finding for the entire mission. Each plot has two graphs showing the results at a number of frequencies. The frequencies are color coded and the frequencies are at the bottom of the plots with each frequency in its color. The lower plot is the intensity of signal and the upper plot gives the azimuthal angle of the source direction as determined by the method explained above. The azimuthal angle of the planets Jupiter and Saturn are also shown. All of these angles have been adjusted so that they fall into the range -90..90 degrees.
The results of the URAP direction finding are recorded in an ASCII text file. The file has one line for each fit solution. All lines of the file are the same length and the contents are described below. Each line is terminated by a line-feed character (ASCII 10). If a fit to the model is very poor, the result is not recorded in the output file. The results are listed in chronological order except that, because of the way that the instrument cycles through the frequencies, some frequencies may precede others that follow them chronologically. However, the results of each spin of the spacecraft (which has a period of 12 seconds) follows previous spins. The "Format" column is the Fortran format used when creating the file.
| Format of Direction Finding File | |||
|---|---|---|---|
| Name | Type | Format | Description |
| Date | Integer | I8 | Year, month, and day-of-month as YYYYMMDD. |
| Time | Integer | I7.6 | Hour, minute, and second as HHMMSS. |
| Day of year | Real | F10.5 | Day of year and time as a fraction. |
| Frequency | Integer | I4 | Frequency of radio signal in kHz. |
| Intensity | Real | ES10.3 | Average intensity of signal (volt/m)^2. |
| Modulation Index | Real | F6.3 | After the signal is fitted to a sine wave, the modulation index is calculated - it is the amplitude of the sine wave divided by the average signal level. It will be in the range 0..1. |
| Azimuth angle | Real | F7.2 | The azimuth angle of the source in the spacecraft coordinate system. It is in degrees and is in the range -90..90. |
| Error in azimuth | Real | F6.2 | The estimated error in the azimuth angle in degrees. |
| RAR Mode | Integer | I3 | Indicates the mode of the RAR high-band signal. If 0, the instrument was in separate mode. If 1, the instrument was in summed mode. |
| Bit Rate | Integer | I2 | Indicates the bit rate the instrument was operating
at when the data for the current record was recorded. The values are:
1 = 128 bps |
| Jupiter Azimuth | Real | F8.2 | Azimuthal position of Jupiter in spacecraft coordinate system. This is in degrees and in the range -180 to 180 degrees. |
| Saturn Azimuth | Real | F8.2 | Azimuthal position of Saturn in spacecraft coordinate system. This is in degrees and in the ragne -180 to 180 degrees. |
2005-04-15
Content copied from http://ufa.esac.esa.int/ufa/#data URAP and df on 2013-04-19