4 Final Report - Emits - ESA

4 Final
Report
Alternative solutions which have been investigated are:
• geostationary orbit with limited station keeping,
• inclined geosynchronous orbit,
• Molniya orbit (highly elliptical inclined orbit).
The appealing features of these alternatives are on one hand potential propellant savings for station
keeping and on the other hand improved viewing conditions over Europe.
The maximum propellant saving may be achieved for a geostationary orbit with 7.5 deg inclination and
0 deg initial right angle of ascending node. This orbit is rather stable such that no orbit corrections are
needed. However, as it is the case for all inclined geosynchronous orbits, the orbit trace projected onto
the Earth surface is a figure of eight which the satellite passes through once per orbit. This yields that
the satellite is half an orbit over Northern latitudes (with better viewing conditions over Europe) and the
other half orbit it is over Southern latitudes. If no corrections of the orbital plane are performed, the
local time of the equator crossing will change over the year. This means that the satellite is at the most
Northern position e.g. at 12:00 at a certain day of the year but half a year later this position is achieved
at midnight. Hence, the good viewing conditions at daytime are achieved for a certain part of the year
only and become even worse for the other part of the year. Since this feature is a significant constraint
for the mission flexibility, all alternatives with inclined orbits have currently been dropped.
Orbit determination performance
The achievable precision for the position of the spacecraft is important since it contributes to the
overall pointing budget which is rather stringent. The preferred solution with the best performance is a
ranging technique based on 2 or 3 ground stations and using a spread spectrum method for the
ranging signal. The orbit determination accuracy is 100 m to 150 m (3σ) along track and 10 m to 20 m
(3σ) across track. The interesting feature is that this technology shows the same performance right
after a manoeuvre when using 3 ground stations. Moreover, this technique is routinely used by SES
Astra which gives strong evidence that it can be successfully applied to Geo-Oculus.
The following alternative technologies have also been investigated:
• single ground station ranging + line of sight measurements,
• dual ranging,
• DARTS,
• short baseline interferometry,
• long baseline interferometry,
• optical telescope,
• use of landmarks,
• GPS.
The GPS option is currently being investigated for geostationary orbits. It may provide a similar
nominal performance as the ground based spread spectrum method but a significant degradation is
expected for a couple of hours after a manoeuvre. All performance data for above listed options are
found in [RD 7].
Orbit transfer and launcher
A geostationary transfer orbit strategy is suggested as baseline. The initial elliptical orbit provided
by the launcher is changed to the final circular orbit with a liquid apogee engine installed in the
spacecraft. The achievable maximum mass of the spacecraft in the final orbit is slightly higher
Page 4-28 Doc. No: GOC-ASG-RP-002
Issue: 2
Astrium GmbH Date: 13.05.2009