Once the other satellites were launched in September 2003, AlSat had been brought to within 300m in sma of the rest of the constellation. On 27th October 2003 the satellite parameters were as follows:
Since AlSat had already expended a considerable fraction of its fuel budget, it was decided to change the reference orbit to that of the AlSat satellite. This meant phasing the other 3 satellites in sma. The evolution is shown below:
The AlSat satellite starts the third one up. We see from the data that the semi-major axis is decreasing due to atmospheric drag. Nominally this is the same for all the satellites, but it is clear from this plot that there were variations. The NigeriaSat started with the highest sma and UKDMC the lowest in this plot. BilSat starts below AlSat but rises above it in the first set of firings. The slope of the decay in sma is shallower for BilSat as this satellite was more massive that the others and so the effect of drag was reduced.For NigeriaSat and AlSat there were cross-track firings to correct the inclination differences. These appear as a set of bunched changes in the sma plot at MJD 53020 and 53040. AlSat still had to make cross track firings as otherwise the difference in LTAN would lead to AlSat drifting in ascending node away from the rest of the constellation. This is shown in the figure below. The mission was designed to last 5 years and the requirement was that all the satellites would be in a 10AM sun-synchronous orbit with a 30 minute margin. The plot below shows AlSat leaving this window 3 months before the end of the mission.
Problems were encountered in the groundstation software that interrupted the second stage of firings on NigeriaSat and UKDMC. This can be seen where UKDMC starts to increase its sma in 53040 (6th February 2004) and then stops. Similarly NigeriaSat starts to drop its sma and stops just after the inclination corrections.
The inclination changes are shown here. The consortium decided not to correct for the differences.
In inclination apart from AlSat that can be seen to join the others on day 53020 (16 January 2004). The spikes in the data are GPS errors rather than estimation errors as no estimator was being run at the time. The periodic oscillation and general decline of the inclination is due to the gravitational attractions of the Sun and Moon upon the satellites.
The cost in terms of ΔV for the satellites for these phasing manoeuvres was
- AlSat 65.5 cm/sec
- UKDMC 150.9 cm/sec
- NigeriaSat 59.6 cm/sec
- BilSat 47.9 cm/sec