****************************************************************************** DORIS Electronic Mail Tue Oct 27 9:24:05 WET 1998 Message Number 0057 ****************************************************************************** Author: Jean-Paul Berthias Subject: SPOT-4 data availability + SPOT-4 models Dear colleagues, We are pleased to announce that the DORIS SPOT 4 data is now available both at the IGN and CDDIS data centers. The SPOT 4 French Earth Observation satellite is the fourth one in the SPOT series. It was launched last March on an Ariane 4 rocket into the same sun-synchronous orbit as the previous SPOTs. The DORIS instrument on-board of SPOT 4 is nearly identical to those already flying on SPOT 2 and TOPEX/Poseidon. The only difference is that a real-time orbit determination capability (DIODE) has been added to produce real-time estimate of the spacecraft position and velocity. This has no impact on the quality and frequency of the DORIS measurements. For further information on SPOT 4 (and DORIS on SPOT 4) you can visit the following web site http://spot4.cnes.fr As for all DORIS instrument, the key to the quality of the measurements is the stability of the on-board Ultra-Stable quartz Oscillator. That stability is only achieved after about a month. This is the reason why the DORIS data set now available starts on May 1, 1998, even though the DORIS instrument was on for about 5 weeks by that time. In order to initialize you orbit determinations, here are the initial conditions around that time DATE APRIL 30, 1998 (days from 1950.0: 17651) seconds in the day: 64560.0 TAI initial conditions given in TERRESTRIEL REFERENCE FRAME POSITION(X): 2759348.52 meter POSITION(Y): 6558734.85 POSITION(Z): 1132602.14 VELOCITY(X): 1080.430 meter/second VELOCITY(Y): -1718.043 VELOCITY(Z): 7258.637 The SPOT 4 satellite has the same geocentric attitude control as SPOT 2 and 3. However, the spacecraft itself is different from the previous ones. In order to accommodate new instruments, the platform was extended, and therefore areas and mass have new values. In addition, the solar panel is much larger. Because of the added mass its rotation axis could not be oriented to compensate for the difference between local time and local noon. The macro-model that we use for SPOT 4 is limited in accuracy. For the previous SPOTs we had a model generated from a rather detailed micro-model with the help of the University of Colorado/CCAR. For SPOT 4 we have generated the model ourselves, from the limited set of data that was available. In addition, so far, we have not tried an in-flight tuning of the model. The characteristics of our macro-model are as follows mass: 2759 kg surfaces X+ X- Y+ Y- Z+ Z- SA+ SA- area 3.5 3.5 7.7 7.7 9.0 9.0 24.8 24.8 specular ref. 1.00 0.63 0.54 0.56 0.47 0.47 0.10 0.24 diffuse ref. -0.38 0.81 0.50 0.38 0.11 0.25 0.15 0.24 The spacecraft axes are defined by X = - pitch axis Y = - roll axis Z = yaw axis (the relation between these axes and the spacecraft shape is given at the above Web site or on the attached picture) The rotation axis of the solar array is obtained by rotating the +X axis by 5 degrees around the Y axis in the direction of the +Z axis. Its components in the spacecraft frame are therefore [cos(5) 0.0 sin(5)]. The rotation angle of the array around this axis is such that the angle between the normal and the direction of the Sun is minimal. If you do not want to use the value of the center of phase correction provided with the data, but would rather compute it yourself, here is the offset of the phase center of the 2 GHz DORIS antenna with respect to the center of gravity X = 1.263 m Y = -0.338 m Z = -1.342 m If you experience any problem processing the SPOT 4 data or understanding the spacecraft description, please send us an E-mail and we will do our best to help you. Jean-Paul Berthias (E-mail: jean-paul.berthias@cnes.fr) Alfred Piuzzi (E-mail: alfred.piuzzi@cnes.fr) CNES Precise Orbit Determination Group DTS/MPI/MS/MO 18 avenue E. Belin 31401 TOULOUSE Cedex 4 France [Mailed From: Pascal Willis ]