MISSION ANALYSIS FOR METOP-B&C Jose Maria de Juana Gamo(1), Pier Luigi Righetti(2) (1)EUMETSAT, Eumetsat Alle 1, 64295 Darmstadt, Germany, +49 61518077352, [email protected] (2)EUMETSAT, Eumetsat Alle 1, 64295 Darmstadt, Germany, +49 6151807767, [email protected] ABSTRACT The EUMETSAT Polar System (EPS) is the European contribution to a joint European-US satellite system, called the Initial Joint Polar-Orbiting Satellite System (IJPS). To develop EPS there were also cooperative agreements with the European Space Agency (ESA) and with Centre National d’Etudes Spatiales (CNES). The prime objective of the EPS Metop mission series is to provide continuous, long-term data sets in support of operational meteorological and environment forecasting and global climate monitoring. The EPS programme consists of a series of Meteorological Operational (Metop) satellites, to be flown successively for more than 14 years from 2006, together with the ground facilities. The first launch, the launch of Metop-2, was on 19 October 2006, from Baikonur cosmodrome using a Soyuz launcher. Once in orbit, satellites are alphabetically ordered, so the first satellite that was launched was called Metop-A in operation. In the frame of the EPS programme, and in preparation activities for subsequent launches of Metop- B and Metop-C, a dedicated mission analysis and related studies had to be carried on in order to: - Find the best in-orbit configuration for concurrent operations of more than one Metop satellite in-orbit while still satisfying all operational constraints and maximizing user benefits. - In combination with the above, and in preparation for the definition of launch and LEOP services, select a favourable launch strategy (injection altitude) allowing a robust LEOP and drift phase implementation while complying with given operational constraints (collision risks and RF interferences with in-orbit satellites, namely Metop-A for Metop-B case, impact on NOAA provided support, impact on propellant budgets…) 1. MISSION REQUIREMENTS AND CONSTRAINTS 1.1 EPS Programme The EPS system is designed to support full space and ground operations for a period of at least 14 years. Three satellites form part of the programme, each designed with a nominal lifetime of 5 years and with a planned 6 month overlap between operations of Metop-A and Metop-B and between Metop-B and Metop-C. The specified satellite commissioning duration is 3 months for Metop-B and Metop-C. All satellite operational orbits shall be based on the same reference orbit, which is defined as follows: Reference Local Time at Descending Node 09:30:00 Repeat cycle 29 days Cycle length 412 orbits Orbital period 6081.553 sec / 101.3592 min Flying over a given reference node (or ground track “anchor” point) is also needed for complying with specific constraints deriving from a given active instrument on-board (ASCAT). All satellites provide manoeuvring capabilities in order to maintain MLST and Ground Track within prescribed control dead bands. Control dead bands Requirement Current Metop-A control Local Time at Descending Node +/-120 seconds +/-90 sec with +/- 30 sec margin Ground Track +/-5 km +/-10 km (5 + 5 margins) Position on orbit +/- 5 km N/A Inclination N/A +/-0.045 degree Two mission data downlink antennas (CDAs) at Svalbard form also part of the system (one currently be used as prime Metop-A antenna, the other one used as back-up and NOAA support antenna). There are no blind orbits, and mission data is to be downloaded at each pass once per orbit. An important requirement imposed at system level is that the system shall not prevent further evolutions to support the parallel operation of up to three Metop satellites and one NOAA satellite. To this extend, and although continued parallel operations of two Metop satellites is beyond the initial EPS programme baseline, continued parallel satellite operations can be proposed in view of the inherent flexibility and within the given capabilities of the system. Metop-A, the first of three satellites of the EPS program, was launched on 19 October 2006. On 15 May 2007, after about 7 months of commissioning, the satellite was officially declared operational. Nominal launch date of Metop-B is currently foreseen April 2012. 1.2 Scope of Mission Analysis The scope of initial studies was to aid in the final selection of the in-orbit separation (phasing) of Metop satellites (final selection discussed later in Section 2) as well as in the launch strategy selection (final analyses/decisions on injection altitude and launch dates are presented in Section 3 and 4 respectively). Important factors to consider/analyze are the impact on fuel budgets, the impact on NOAA provided support and the potential cases of interferences between both Metop during LEOP and drift phase (including characterization and fuel penalty associated with avoiding interferences). Many other factors are also to be analyzed whenever appropriate and convenient (user needs/desires, safety issues, operational robustness issues, as well as other system aspects as discussed in sections 2, 3 and 4). 1.3 Metop-B Mission Requirements and Constraints The following are the main mission requirements and constraints for Metop-B mission: - Metop-B launch is to be performed on same launcher as Metop-A (Soyuz/ST) from Baikonur - same launch program as for Metop-A can be assumed (due to same launch site, injection orbit, launcher and s/c). This implies same launcher trajectory and in turn same in-orbit position (PSO) at separation and separation time (relative to lift-off time) as for Metop-A launch (although most probably a somehow lower altitude will be targeted for Metop-B separation). The same trajectory also applies for each day (since launch program is the same for all days). - similarly, the following launcher dispersion can be assumed 3-σ uncertainty Semi-major axis +/- 7.5 km Eccentricity +/-0.001 Inclination to equator +/-0.1 deg Longitude of ascending node +/-0.12 deg1 Argument of perigee +/-12 deg - as per Metop-A, first manoeuvre is assumed not to occur until 48 hours after separation (time needed for post-separation operations) - it can be further assumed from Metop-A experience and commissioning/user needs that Metop- B(&C) shall reach its final position in-orbit within 16 days after separation - Metop-B shall follow the same ground-track of Metop-A (on a different reference date). - both, Metop-A and Metop-B, are to be supported with the same polar antenna (CDA) during routine phase. Currently, a single CDA can perform two consecutive passes as long as they are separated by 24 minutes or more (separation here is measured as AOS to AOS) assuming sufficiently wide operational margins. - the two polar antennas at Svalbard (CDA1 and CDA2) can be used during LEOP and drift phase of Metop-B in case of common visibility (or not enough separation) of both s/c, with the side effect of potential impact on NOAA operations support (NB: as part of the NOAA- EUMETSAT cross-operational support, NOAA18 and NOAA19 blind orbits from Fairbanks are to be supported as much as possible by a Metop antenna at Svalbard). Impact to this NOAA support is therefore to be minimized to the maximum extend possible. - all Metop satellites are identical. Interference in-orbit between Metop-A and Metop-B is to be minimized if not completely avoided, especially during the first hours after Metop-B separation. For the purposes of this mission analysis, interference is assumed whenever the two spacecraft are within 0.3 degree as seen from the ground antenna. - at least 3 or 4 consecutive days/opportunities are needed. The larger number of valid consecutive days, the better. 2. PRELIMINARY ANALYSES Preliminary analyses are performed in order to study the sensitivity of a series of key performance factors (such as fuel budget, flexibility to launch day selection, Metop interferences and NOAA support impact) to in-orbit phasing and injection altitudes. 2.1 Separation conditions and phase margin budgets at separation Given the fixed launcher trajectory for all days, the fixed launch time for all days (to achieve desired MSLT of the orbital plane) and given the orbit repetition cycle of 29 days, Metop-B launcher separation conditions repeat every 29 days wrt Metop-A position in-orbit. Analyzing 29 days is therefore enough to analyze all possible cases since conditions any other day can be obtained by adding/subtracting an integer number of repeat cycles (29 days) to the initial 29 days being studied. A given Metop-B launch day will nominally induce a given in-orbit separation (phasing) between Metop-A and Metop-B at Metop-B injection in orbit. Figure 1 shows this nominal Metop-A/B separation at Metop-B injection for a given Metop-B launch date within a 29 day repeat cycle. Given however launcher dispersion, post-separation conditions may vary. In addition to this, Metop-A orbit also differs from its reference orbit (within control dead bands). The table below shows the total required margin in in-orbit position between both Metop satellites at separation in order to guarantee no interference between the two satellites during the first 48 hours of Metop-B (time to first manoeuvre opportunity) and assuming Metop-B nominal injection at the Metop-A orbit altitude. 1 0.12 degree in longitude of ascending node translates in 28.8 seconds in LTAN Metop-B injection dispersion: Uncertainty Contribution to PSO margin Semimajor axis 7.5 km 3-σ 8 deg/day => 16 deg Eccentricity 0.001 3-σ <0.01 deg RAAN 0.12 deg 3-σ 1.7 deg Argument of perigee 12 deg 3-σ <0.01 deg TOTAL DUE TO INJECTION ERRORS (RMS) 16.1 deg Metop-A orbit margins: LTAN from nominal Metop-B LTAN 195 seconds2 11.54 deg to keep GT (120+75) GT Metop-A control margin 5 km = 9.55 sec tran.