****************************************************************************** DORIS Electronic Mail Mon Apr 20 14:43:24 WETDST 1998 Message Number 0040 ****************************************************************************** Author: Gilles Tavernier (CNES) Subject: SPOT-4: News from CNES Thanks to DORIS and its navigation software DIODE, the Earth observation satellite SPOT4 knows its own position in real-time with an accuracy of a few meters, for the first time. The DIODE navigation software on board SPOT4 was initialised on March 26th at 21:25 UTC during a pass over Toulouse (France). A few minutes later, using the measurements received from the DORIS master beacon in Toulouse, DIODE performed the first real time determination of its position and transmitted it to the ground. The assessed accuracy was with in 10 to 20 meters. After one revolution, during the second pass over Toulouse, the error level of the real-time on board orbit was observed to be less than 5 meters when compared with a ground based solution. When a ground station receives the scenes taken by the satellite, their precise locations have to be determined. Moreover, they have to be corrected to ensure maximum quality. DIODE's role is to calculate the satellite's position in real time, on board the satellite. It is then possible to automatically "tag " the scenes at the time they are acquired. The imagery and position information are available together, with no further processing, when the ground station receives them. The geographical location of SPOT 4 scenes must be known to within approximately 100 metres: DIODE, using the precise measurements provided by DORIS, determines SPOT 4's position to within a few metres. Traditionally the Operations Control Centre (according to the previous day's trajectory and using satellite orbit determination programmes) predetermi ned the positions of Earth observation satellites. These were periodically sent to the Direct Receiving Stations. The DIODE procedure is thus: - more efficient (the information is up to date and the data are accurate), - automatic, hence easier to use, - used for accurate position determination. The first SPOT 4 image was taken at 9 h 38 min 26 sec (UTC) on 27 March 1998. The scene came from imagery acquired during orbit 113 by the HRVIR instru ment no.2 with an oblique-viewing incidence of 18.9 deg. The satellite acquired the scene and DIODE gave the satellite's position at that instant. The scene is geographically centred over the Baja (Hungary) site, south of Budapest on the Danube River (see http://www.spotimage.fr/anglaise/spot4/entiere.html). The " passenger " instruments can use this information as well: POAM and the Vegetation payload also use the Navigator data for their own needs. The last pair of manoeuvres for the acquisition of the nominal mission orbit was performed on Tuesday, March 31st: - a first manoeuvre at 20:42:34 : 2,062m/s, duration 224,8 seconds - a second manoeuvre at 21:33:00 : 2,062m/s, duration 236,8 seconds The semi major axis was thus increased by 8 km with an accuracy of 30 metres. The DIODE navigation software on board SPOT4 allowed to monitor the pair of manoeuvres thanks to the real time estimation of the position of the satellite as shown on the figure. The estimation of the position was not interrupted by the manoeuvres and the Kalman filter rapidly converged after each thrust. SPOT 4 is now on its final position in orbit, properly phased with SPOT1 and SPOT2. [Mailed From: Pascal Willis ]