Mission Analysis for Metop-B&C

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. 0.6 deg if GT error trans. TOTAL DUE TO METOP-A ORBIT CONTROL 12.14 degree TOTAL REQUIRED MARGIN 12.14+16.1 = 28.24 degree

Hence, by imposing the condition of no interference during the first 48 hours of LEOP under +/-3σ launcher performances, and making no assumptions on Metop-A orbit (in order to make the analysis valid for any launch epoch), a number of launch days every 29 days would/could need to be avoided (dates lying in between the two red lines in Figure 1).

(METOP-A PSO - METOP-B PSO) at METOP-B injection METOP-A PSO minus METOP-B PSO (deg) METOP-A/B in-orbit separation at inj. (min) 180.0 49.0 165.0 150.0 135.0 39.0 120.0 105.0 29.0 90.0 75.0 19.0 60.0 45.0 30.0 9.0 15.0 0.0 -1.0 -15.0 -30.0 -45.0 -11.0 -60.0

METOP-B Injection (deg) Injection METOP-B -75.0 -21.0 -90.0 -105.0 -31.0

METOP-A PSO minus METOP-B PSO at PSO at METOP-B minus PSO METOP-A -120.0 -135.0

-150.0 -41.0 (min) inj. sep. METOP-B at in-orbit METOP-A/B -165.0 -180.0 -51.0 2011/10/01 2011/10/06 2011/10/11 2011/10/16 2011/10/21 2011/10/26 Epoch

Fig. 1 Nominal separation conditions over a repeat cycle

The above applies in the case of selecting a nominal injection target altitude equal to that of the reference Metop orbit. However, injecting Metop-B at a slightly lower altitude can be beneficial for safety as well as interferences issues, as it will be shown later. For injections at lower altitudes, there is a direct fuel penalty proportional to the difference between the nominal altitude and the reached altitude, since propellant will be needed not only to correct launcher dispersion but to correct a lower than nominal orbit altitude. Additionally, the induced nominal PSO drift at separation (due to the lower semi-major axis) needs to be accounted for.

2 195 seconds is slightly conservative. 120 seconds is the control requirement but 90 seconds seems to be the actual implemented control (including margins). +/-75 seconds account for launch window (TBC) Altitude lower by Delta-V penalty Approximate fuel lifetime Induced PSO drift penalty3 5 km 2.59 m/s 0.37 years 5.34 deg/day 10 km 5.17 m/s 0.74 years 10.68 deg/day 15 km 7.77 m/s 1.11 years 16.04 deg/day

Assuming a 10 km lower than nominal altitude at injection, the PSO margins at separation due to the lower altitude (and semi-major axis uncertainty, 7.5 km 3-σ) need to be the following:

Altitude Lower bound Upper bound Total required margin Nominal altitude -16 deg in 48 hours +16 deg in 48 hours [-28.2,+28.2] Lower by 10 km +0 deg in 48 hours4 +37.4 deg in 48 hours [-12.1,+49.6]

Hence, by imposing the condition of no interference during the first 48 hours of LEOP under +/-3σ launcher performances (around a nominal 10 km lower injection orbit) and making no assumptions on Metop-A orbit (in order to make the analysis valid for any launch epoch), again five days every 29 seem forbidden (the two red lines in Fig. 1 need to be shifted upwards, widening up a little bit too).

These budget analyses are however very preliminary. Interferences for instance need to be better characterized, since some of them are easy to handle (if sufficiently short and/or far away from separation) while others can also appear and be significant say after the first 48 hours after separation. The impact on fuel budget when injecting at a lower altitude differs also from the one computed above when launcher dispersions is taken into account (and also if drift reversions for avoiding interferences are also included). Other factors are also to be analyzed such as potential impacts on NOAA provided support.

2.2 A Monte Carlo approach

In order to better characterize the sensitivity of some performance parameters to in-orbit phasing, injection altitude and launch day, Monte Carlo simulations are performed on a relatively simple (but still representative) model of the LEOP and early orbit drift operations. These simulations are performed on a relatively few number of reference cases, from which general conclusions can be easily drawn and extrapolations to other cases can be done. The following is done/selected: - Metop-A following reference orbit - Metop-B injection altitude subject to launcher performance as per launcher service provided data (Gaussian distribution assumed). Two cases are analyzed: nominal target injection altitude at the nominal orbit reference altitude, and nominal target injection altitude at 10 km below the nominal orbit reference altitude - for each target injection altitude above (nominal or 10 km lower altitude) 29 consecutive launch dates are considered - for each considered case (i.e. target injection altitude and launch day), 500 Monte Carlo simulations are performed using launcher performance based on Gaussian distributions. - initially a target phasing for routine operations between Metop-A & Metop-B of 120 degrees is arbitrarily assumed (Metop-A leading Metop-B by 120 degree). As it will be shown, results depend very much on the difference between the relative in-orbit separation at Metop-B injection and the one that needs to be achieved for routine operations. Thanks to this, results for some other values of relative phasing are reasonably easy to extrapolate from this case - the LEOP and drift orbit phase operations of Metop-B is also simplified as follows: o out-of-plane and in-plane problems decoupled

3 By default, 7 m/s per year (approx. 14 kg of fuel) is used in this note for computing lifetime penalties 4 Minimum value between 0 and 5.4 (drift for -3σ in 48 hours) o eccentricity corrections done by drift manoeuvre corrections without additional penalties o costs associated to pre- and post-manoeuvre slews disregarded (out-of-plane problem) o mean performance based on Metop-A flight data o a first impulsive and single in-plane manoeuvre happens at exactly 48 hours after separation in order to adjust the phase drift that permits reaching the final position in- orbit within the prescribed period of 14 days after this manoeuvre. The possibility is left to revert the drift if this prevents potential interferences between Metop-A and Metop-B to occur o a second final impulsive and single in-plane manoeuvre occurs 14 days after the first in- plane manoeuvre at the target relative phasing in order to stop the drift The following is computed as output of each simulation: - total incurred cost associated with in-plane manoeuvres in the case of: o no interference between Metop-A & Metop-B are allowed (cost with no interference) o interference between Metop-A & Metop-B are permitted 48 hours after Metop-B separation (minimum cost) - in case of interference, number of days after the first 48 hours that the interference can happen and duration of interference - number of days in which two stations are needed due to Metop-A & Metop-B not keeping sufficient separation (implying potential impact on the NOAA cross-operational support). This is computed for the same two cases as before, i.e.: o no interference between Metop-A &Metop-B are allowed o interference between Metop-A & Metop-B are permitted 48 hours after Metop-B separation (minimum cost case) The following table shows some of the results corresponding to the nominal injection altitude case. For propellant lifetime costs, percentiles 50% and 90% are shown. For interferences, the day after the first 48 hours when interference can occur is shown (mean +/- standard deviation). The duration of these potential interferences is also shown in hours (just mean, since the variance happens to be always very small). For potential impact to NOAA support, the number of days during which the in-orbit separation is below 107 degree (30 minutes) is shown (percentiles 50% and 90%).

Propellant lifetime cost Interference Potential NOAA impact [

Note should be taken that these results are still repeatable every 29 days, i.e. results obtained for a given launch epoch apply also for epochs shifted in time backwards or forwards by an integer number of repeat cycles (29 days). A number of conclusions, most of them obvious, can already be extracted from the results above: - compared to Metop-A (first launched Metop satellite), there is an additional cost incurred due to the fact that a given relative in-orbit position relative to Metop-A is now to be achieved (since Metop-A position in-orbit was arbitrarily selected, just determined by the launch date and to a lesser extend by the performance of the launcher) - the closer to the final position at separation, the lower the extra cost for phasing operations - interference can occur only for negative phase separations (being the target separation positive). In these cases, the closer the relative separation at injection, the sooner the interference may happen - in most of the days, interferences can be avoided at no extra cost. For other dates, avoiding interference implies a fuel-cost penalty but also a reduction in the potential impact on the NOAA support - the case of -120 degree target for routine operations (i.e. Metop-B leading Metop-A) shall be symmetric to the one exposed above. - potential impact to NOAA support exist when the relative phase separation at injection is below 107 degree (30 minutes), existing from injection onwards. - as known, relative position in-orbit between both Metop at Metop-B injection changes significantly from one day to the next. This makes the selection of an optimal day not straightforward (since optimal days are not consecutive and can even be followed by undesirable days).

Since it presents a significant advantage if the actual sign of the relative phasing can be selected for each launch day (i.e. if given a selected Metop-A/B in-orbit separation for routine operations, which of the two is leading and which one is trailing is irrelevant and can be freely selected), the table below combines the two cases of +120 degree and -120 degree relative PSO for routine operations. A tentative selection of 3-day launch opportunities is also shown.

Target PSO 120 or -120. Nominal injection altitude 90% confidence levels Launch Equivalent Nominal injection cost Interference 2 CDAs date launch on5 1/10/2011 Risk of early interference 2/10/2011 First day opportunity <6.9m. no <11 days 3/10/2011 Second day opportunity <8.1m. no no 4/10/2011 17/10/2011 Third day opportunity <4.3m. no no 5/10/2011 16/10/2011 Fourth day opportunity <11.6m. no <13 days 6/10/2011 Risk of early interference 7/10/2011 First day opportunity <5.4m. no <10 days 8/10/2011 Second day opportunity <9.4m. no no 9/10/2011 12/10/2011 Third day opportunity <4.4m. no < 3 days 10/10/2011 11/10/2011 Fourth day opportunity <12.9m. no <13 days 11/10/2011 Fifth day opportunity? <12.9m. no <13 days

5 For these cases, a relative PSO target of -120 is selected. Equivalent launch date refers to the symmetric case with relative target PSO of 120 degree (previous table) 12/10/2011 First day opportunity <4.4m. no < 3 days 13/10/2011 8/10/2011 Second day opportunity <9.4m. no no 14/10/2011 7/10/2011 Third day opportunity <5.4m. no <10 days 15/10/2011 Risk of early interference 16/10/2011 Missed day <11.6m. no <13 days 17/10/2011 First day opportunity <4.3m no no 18/10/2011 3/10/2011 Second day opportunity <8.1m. no no 19/10/2011 2/10/2011 Third day opportunity <6.9m. no <11 days 20/10/2011 Risk of early interference no 21/10/2011 Missed day <9.9m no <13 days 22/10/2011 First day opportunity <5.0m no no 23/10/2011 27/10/2011 Second day opportunity <6.5m. no no 24/10/2011 26/10/2011 Third day opportunity <8.5m. no <12 days 25/10/2011 Collision risk 26/10/2011 First day opportunity <8.5m. no <12 days 27/10/2011 Second day opportunity <6.5m. no no 28/10/2011 22/10/2011 Third day opportunity <5.0m. no no 29/10/2011 21/10/2011 Fourth day opportunity <9.9m no <13 days

So far we have been assuming that early interference are to be avoided at all cost. That’s the reason why some days are not even considered. This constraint however will be relaxed in the final launch day selection as it will be shown in Section 4.

The same results are obtained for a nominal injection altitude 10 km below the nominal Metop reference orbit altitude, and for two different relative PSO for routine operations, 120 degree and - 120 degree (in this case the symmetry observed previously disappears and full simulations are needed for the two cases separately). Full results are here omitted for the sake of keeping this paper within its prescribed maximum number of pages. The table below just shows the equivalent summary table for the case of targeting injection at 10 km lower altitude and assuming a PSO separation between Metop-A and Metop-B of 120 degree, irrespective of which one is leading and which one is trailing.

Target PSO 120 or -120. 10km lower injection alt. 90% confidence levels Interf. from Launch Selected Nominal injection cost Interference 2 CDAs injection at date target 1/10/2011 Risk of early interference 2/10/2011 120 Missed day <19.4m yes <11 days 5th day 3/10/2011 120 First day opportunity <12.0m no no 4/10/2011 120 Second day opportunity <12.0m no no 5/10/2011 -120 Third day opportunity <12.0m. no <6 days 6/10/2011 Risk of early interference 7/10/2011 120 Missed day <17.6m. no <10 days 8/10/2011 120 First day opportunity <11.9m. no no 9/10/2011 120 Second day opportunity <12.0m. no no 10/10/2011 -120 Third day opportunity <12.2m. no <8 days 11/10/2011 Risk of early interference 12/10/2011 120 Missed day <16.1m. no < 9 days 13/10/2011 120 First day opportunity <11.9m. no no 14/10/2011 120 Second day opportunity <11.9m. no no 15/10/2011 -120 Third day opportunity <11,8m no < 10 days 16/10/2011 Risk of early interference 17/10/2011 120 First day opportunity <14.9m no < 7 days 18/10/2011 120 Second day opportunity <12.3m. no no 19/10/2011 -120 Third day opportunity <12.0m. no < 1 days 20/10/2011 -120 Fourth day opportunity <12.0m no < 12 days 21/10/2011 120 Missed day <20.9m yes < 12 days 4th day 22/10/2011 120 First day opportunity <13.5m no no 23/10/2011 120 Second day opportunity <11.6m. no no 24/10/2011 -120 Third day opportunity <12.6m. no < 3 days 25/10/2011 Collision risk 26/10/2011 120 Missed day <20.5m. yes <12 days 4th day 27/10/2011 120 First day opportunity <11.9m. no no 28/10/2011 120 Second day opportunity <12.2m. no no 29/10/2011 -120 Third day opportunity <12.0m no < 4 days

There are a few interesting conclusions that can be derived from the results above: - although cost is lower if target injection is at the nominal reference orbit altitude, the differences in cost is not as much as the one would have thought thanks to the inherent cost existing in phasing operations. When considering for instance the worst performance of the best 3 consecutive days for each case (nominal injection or 10 km lower injection), the difference is relatively small (8.1m if targeting at the nominal altitude versus 11.9m if targeting at 10 km lower, i.e. only 0.3 yrs penalty as opposed to the 0.74 yrs penalty we would have thought for just having to raise the altitude by 10 km) - costs are also more flat (they change less from one day to the next) if injection is targeted at a 10 km lower altitude - interferences seem also to be more favourable if injection at 10 km below the nominal altitude is targeted. Either the duration of these interferences are shorter, due to the larger relative drift (and due to this, also less likely to occur), or avoiding them represent an extra cost relatively low (not shown in data above).

2.3 Common costs

For comparison purposes, the additional propellant cost associated with the out-of-plane dispersion of the launcher (not shown in the figures before, that show only in-plane costs) is 5 months in half the number of cases (i.e. percentile 50%) and 12 months in 90% of the cases (percentile 90%) with the metrics used in this note (i.e. 7 m/s of fuel lifetime per year) and for all launch days (this cost does not depend on selected launch date).

2.4 Sensitivity analysis to drift phase duration

So far we have assumed 14 days as the time required for Metop-B to reach its final position in-orbit (and final altitude) from its initial position 2 days after injection in orbit. In order to provide additional help in the decision-taking around the required duration for drift operations (mainly driven by in-orbit validation activities, instrument decontamination constraints and start of commissioning plans), it results of interest to know how the presented costs change with the duration of this drift.

For injection targeted at nominal orbit altitude, at first order of approximation part of the presented cost is inversely proportional to the duration of the drift phase, while part of the cost is fixed and due to the launcher dispersion (~4 months for confidence level of 90%). As example, a cost of 8 months with 14 days of phase drift operations could be reduced to 6 months with 28 days of phase drift operations (=(8-4)*14/28 + 4). No much gain for having a twice as long drift phase duration.

For injection targeted at 10 km below the nominal altitude, again part of the cost is inversely proportional to the duration of the drift phase, while part of the cost is fixed and due to the launcher dispersion and nominal lower altitude (~12 months). As example, a cost of 16 months with 14 days of phase drift duration could be reduced to 14 months with 28 days of phase drift duration (=(16- 12)*14/28 + 12). Again no much gain for having a twice as long drift phase duration.

2. FINAL SELECTION OF METOP-A/B IN-ORBIT CONFIGURATION (PHASING)

In selecting the relative PSO separation (phasing) between Metop-A and Metop-B for routine operations, there are a number of constraints and points to consider, namely: user needs/wishes as well as constraints deriving from space segment, ground segment and operational concepts. A detailed view of all the constraints will not be presented here since it involves introducing too many details at system level. One should note that even though parallel exploitation of data from two Metop satellites was investigated and considered as part of the trade-offs, continued parallel operations of two Metop satellites was not part of the initial baseline of the EPS programme. The following points were comprehensively looked at: - Orbit definition constraints - Ground stations constraints (polar ground stations, link from polar ground stations to central site, local reception stations) - Data processing and dissemination constraints (sizing of the processing and dissemination chains, calibration and validation, “split mission”6 scenarios) - operations (routine operations, maintenance operations for instance of antennas) - space segment constraints (such as satellite interferences) - accommodation of a third Metop satellite (Metop-C) in-orbit

In the next subsections follow the most significant and driving constraints.

2.1 Polar ground stations operations

With the current operational concept, one of the two stations (CDA) in Svalbard is used nominally for the monitoring and control of the Metop-A satellite, the other ground station for the blind orbit support for NOAA-19. The separate use of the two ground stations for Metop and NOAA satellites has been implemented as the NOAA satellites are drifting and there are periods of overlap of NOAA passes with Metop passes. This concept will remain in case of operations of two Metop satellites, i.e. Metop-A and Metop-B would use one CDA, NOAA-18 and NOAA-19 the other when needed. The combined time needed to set up the CDA for a pass, the complete duration of the pass itself and the time needed to reset the CDA after the pass, lead to a minimum separation constraint compatible with single-CDA operations of about 24 minutes. However, as orbit control deadband for each satellite is +/- 2 min., the minimum acceptable nominal phasing would be 24 min. plus 2 times 2 min., i.e. 28 minutes separation between reference orbits, to ensure that the actual separation is at no time smaller than 24 min (no synchronization of orbit control is assumed between the two Metop satellites).

2.2 Calibration and validation

Due to calibration and validation constraints imposed by some instruments (mainly by ASCAT, instrument calibrated by means of ground-based transponders, optimized for the Metop-A ground- track), an identical ground track should be ideally implemented for Metop-B to have the same viewing geometry of the ASCAT instrument with respect to the on-ground transponders. This reduces the possible positions in-orbit to only 29 (given the repeat cycle of 29 days of the reference orbit). Within the previous constraint of minimum separation of 28 minutes, and assuming which s/c is leading and which one is trailing is irrelevant, the possible relative in-orbit positions (phasing) reduces to the following list (first column indicates the ground-track revisiting times between the two satellites):

6 split mission refers to the situation in which, due to instrument failures or degradations, different instruments on different satellites need to be operated routinely (i.e. two satellites are needed to complete a full Metop mission) Revisit Interval Phasing Phasing (Days Within the 29-Days Cycle) (Minutes) (Degrees) 11 / 18 +/- 27.96 +/- 99.31 13 / 16 +/- 31.46 +/- 111.72 8 / 21 +/- 34.95 +/- 124.14 3 / 26 +/- 38.45 +/- 136.55 2 / 27 +/- 41.94 +/- 148.97 7 / 22 +/- 45.44 +/- 161.38 12 / 17 +/- 48.93 +/- 173.79

2.3 Exploitation of data from two satellites

Both the optimization of the spatial coverage (on a short time scale as well as for longer term to achieve full global coverage) and the short-term revisit time for the same point on the Earth are factors that are relevant for the exploitation of data from two phased Metop satellites.

The short-term revisit time is given by the phasing. The shorter the phasing the more quickly a point on Earth is revisited provided the point is within the overlap region of the instrument swaths for the two satellites. On the other hand, the most regular spread in time for the observations would be achieved with a maximum phasing between the satellites.

Regarding short-term coverage of the Earth, this would be maximized for the prime instruments for NWP (Numerical Weather Prediction) by the largest possible phasing value due to the related largest longitudinal displacement between the swaths of the two satellites.

The feedback received by the Users showed so far also a preference for the larger phasing in view of the use of Metop data for NWP, with the desire to maximise the spatial coverage by the relevant instruments.

The value of 48.93 minutes separation, irrespective of which satellite is leading was finally proposed as being in line with all identified constraints and with the preferences expressed by the Users.

3. FINAL SELECTION OF METOP-B INJECTION ALTITUDE

A separate dedicated analysis was carried out in order to exhaustively study all pros and cons of the possible injection strategies, i.e. injecting at the nominal reference orbit altitude or injecting at a lower altitude (say by 10 km). The following performance factors were considered: - safety - flexibility - fuel saving optimization - interference constraints - robustness in the event of contingency scenarios o large launcher underperformance (launcher shortfall) o LEOP contingency (manoeuvre still not possible after 48 hours)

Nominal scenario Injection altitude at Score card Metop orbit altitude 10 Km lower altitude Safety -- ++ Flexibility -- + Fuel-saving + - Interference constraint - -

Injecting at the nominal reference orbit altitude presents the following advantages: - it is fuel optimal (by less than 10 kg, which seems to be anyway not very significant given the available propellant budget) - could potentially prevent interferences (but at the cost of limiting the possible launch dates and always assuming nominal launch performance and LEOP scenarios) Injecting at a lower altitude could clearly be the preferred solution, since: - prevents Fregat upper stage or Metop-B/C s/c to get stranded in the operational Metop orbit in the case of a Fregat de-orbiting failure or the loss of Metop-B/C right after separation (safety reasons) - allows to launch virtually any day (flexibility reasons) - limits the duration of interference periods to 1 (one) pass of the currently operational Metop s/c (interference constraints) - provides additional safety margins in the case of LEOP contingency scenarios

Mainly in view of the safety aspects, but also considering the flexibility in launch date selection, it was therefore recommended to pursue a launch strategy with an injection below the target orbit (approach still to be discussed with STARSEM to consolidate the technical assumptions and assess also the potential cost implications of this strategy).

4. LAUNCH DATE SELECTION AS PER CONFIRMED CURRENT BASELINE

Launch date of Metop-B is currently foreseen for April 2012 from Baikonur. A launch injection at about 10 km below the nominal Metop altitude is expected to be targeted and a final in-orbit separation between Metop-A and Metop-B of 48.932 minutes (between reference orbits) is also to be pursued for routine operations (irrespective of which satellite is leading and which one is trailing).

A dedicated analysis was performed in order to identify in detail the potential interferences to be expected between Metop-A and Metop-B. This analysis also listed the operational recommendations and open issues related to each type of interference. This analysis shows that interferences are easy to handle, but it is still to be confirmed to which extend they should be avoided (since they may represent undesired mission outages). Currently, it was found useful not to impose any strong constraint on RF interferences between Metop-A and Metop-B (as opposed to initial studies) and to leave open until a later stage/decision whether interferences need to be absolutely avoided or not, for instance during the first hours of Metop-B in orbit.

Fig. 2 shows an overview (or snapshot) of the relative position between Metop-A and Metop-B for each launch date in April 2012 and covering the period from Metop-B injection in orbit until Metop-B arriving at its final position. This form of representation is found to be the most useful so far for easily seeing the implications of a given launch day and even a given launcher under- or over-performance.

No launch window is assumed (i.e. just one opportunity each day at a given fixed time of the day for each day), although a launch window of up to +/-15 seconds could be implemented. The same target inclination and local time as for Metop-A is assumed to be implemented (+35 mdeg wrt SSO inclination and -60 seconds wrt nominal MLST respectively and in order to start an 18 months local time station keeping cycle).

(METOP-A PSO - METOP-B PSO)

separation at inj. 2 days after inj. (overperformance) 2 days after inj. (underperformance) Target routine ops Metop-A control margin 180.0 49.0 165.0 150.0 135.0 39.0 120.0 105.0 29.0 90.0 75.0 19.0 60.0 45.0 30.0 9.0 15.0 0.0 -1.0 -15.0 -30.0 -45.0 -11.0 -60.0 -75.0 -21.0 -90.0 -105.0 -31.0 -120.0 (min) separation in-orbit METOP-A/B

METOP-APSO minus METOP-BPSO (deg) -135.0 -150.0 -41.0 -165.0 -180.0 -51.0 2012/04/01 2012/04/02 2012/04/03 2012/04/04 2012/04/05 2012/04/06 2012/04/07 2012/04/08 2012/04/09 2012/04/10 2012/04/11 2012/04/12 2012/04/13 2012/04/14 2012/04/15 2012/04/16 2012/04/17 2012/04/18 2012/04/19 2012/04/20 2012/04/21 2012/04/22 2012/04/23 2012/04/24 2012/04/25 2012/04/26 2012/04/27 2012/04/28 2012/04/29 2012/04/30 Epoch

Fig. 2 Metop-B launch, early orbit phase and drift operations overview

Fig. 2 shows, among other things, the relative position of Metop-B wrt Metop-A reference position in-orbit at the instant of Metop-B injection (separation from launcher). From one day to the next, the change of about 72 degree in relative position can be observed (corresponding to the ~14.2 orbit per day completed by Metop-A, i.e. ~0.2 x 360 = 72 degree).

The actual position of Metop-A may also vary from its reference position due to the natural orbit control margins (by +/-7.1 degree corresponding to the 2 minute control margin in LTDN and the 5 km in ground track control). Metop-A potential range of actual positions in-orbit wrt its reference position is shown in Fig. 2 by the two thick green lines at +/-7.1 degree.

After separation from launcher, Metop-B will drift freely until the first manoeuvre opportunity (assumed 48 hours after separation). The phase drift induced during these first 48 hours is mainly driven by the launcher performance and the initial lower targeted altitude. The range of total Metop- B phase drifts in two days for a range of +/-3σ launcher performances is from -37.2 degree to -5.3 degree (wrt the in-orbit position of Metop-A). Fig. 2 shows these expected ranges of Metop-B relative positions (wrt Metop-A) during the first 48 hours (the vertical ~32 degree long traces).

After these first 48 hours, Metop-B shall be manoeuvred in order to be left with a residual phase drift in order to reach its final position in-orbit within a given number of days after handover (TBC if 14 days). A phase drift stop manoeuvre shall then need to be implemented at its final position in- orbit in order to stop this drift. Fig. 2 shows the target PSO to be achieved by Metop-B at the end of its drift phase. These are the thick lines at +/-173.8 degree (+/-48.932 minutes separation). A preliminary estimate of the size of manoeuvres to be performed can be easily derived from the figure for each day knowing the actual launcher performance (to be known post-launch), the number of days within which Metop-B needs to be at its final position in-orbit (TBC if 14 days or different) and the need or not to avoid interferences during the phasing operations (since this may change the actual target to be reached and the need to revert the drift).

Potential impact to the support provided to NOAA can also be easily identified. Whenever Metop-B relative position is below say 24 minutes (+/-86 degree), the 2 CDAs are needed to provide support to Metop-A and Metop-B, implying the back-up antenna is no longer free for providing support to NOAA-19 or NOAA-18 blind orbits.

4.1 Launch day selection

In principle, and assuming interferences between Metop-A and Metop-B can be handled appropriately and do not represent an issue, every launch day is possible. The risk of collision between Metop-A and Metop-B can be considered negligible since injection occurs at a sufficiently lower altitude.

In the event of preferring to avoid potential interferences between Metop-A and Metop-B during the first 48 hours, then a number of launch days shall be avoided every 29 days, namely days 2, 16, 21 and 26 of April 2012 (and all days distant apart from these a multiple of 29 days). In this case, the following days maximize the number of consecutive days with low or no risk of interference between Metop-A and Metop-B: • 3 April 2012 (13 consecutive potentially interference-free launch day scenarios) • 17, 22 & 27 April (4 consecutive potentially interference-free launch day scenarios)

The following table shows all possible launch dates from 1 April 2012 until 17 October 2012. Those with identified high potential interferences between Metop-A and Metop-B are shaded.

Cases Equivalent launch dates. Covered period: 01/04/2012 to 17/12/2012 studied 01/04/2012 30/04/2012 29/05/2012 27/06/2012 26/07/2012 24/08/2012 22/09/2012 21/10/2012 19/11/2012 02/04/2012 01/05/2012 30/05/2012 28/06/2012 27/07/2012 25/08/2012 23/09/2012 22/10/2012 20/11/2012 03/04/2012 02/05/2012 31/05/2012 29/06/2012 28/07/2012 26/08/2012 24/09/2012 23/10/2012 21/11/2012 04/04/2012 03/05/2012 01/06/2012 30/06/2012 29/07/2012 27/08/2012 25/09/2012 24/10/2012 22/11/2012 05/04/2012 04/05/2012 02/06/2012 01/07/2012 30/07/2012 28/08/2012 26/09/2012 25/10/2012 23/11/2012 06/04/2012 05/05/2012 03/06/2012 02/07/2012 31/07/2012 29/08/2012 27/09/2012 26/10/2012 24/11/2012 07/04/2012 06/05/2012 04/06/2012 03/07/2012 01/08/2012 30/08/2012 28/09/2012 27/10/2012 25/11/2012 08/04/2012 07/05/2012 05/06/2012 04/07/2012 02/08/2012 31/08/2012 29/09/2012 28/10/2012 26/11/2012 09/04/2012 08/05/2012 06/06/2012 05/07/2012 03/08/2012 01/09/2012 30/09/2012 29/10/2012 27/11/2012 10/04/2012 09/05/2012 07/06/2012 06/07/2012 04/08/2012 02/09/2012 01/10/2012 30/10/2012 28/11/2012 11/04/2012 10/05/2012 08/06/2012 07/07/2012 05/08/2012 03/09/2012 02/10/2012 31/10/2012 29/11/2012 12/04/2012 11/05/2012 09/06/2012 08/07/2012 06/08/2012 04/09/2012 03/10/2012 01/11/2012 30/11/2012 13/04/2012 12/05/2012 10/06/2012 09/07/2012 07/08/2012 05/09/2012 04/10/2012 02/11/2012 01/12/2012 14/04/2012 13/05/2012 11/06/2012 10/07/2012 08/08/2012 06/09/2012 05/10/2012 03/11/2012 02/12/2012 15/04/2012 14/05/2012 12/06/2012 11/07/2012 09/08/2012 07/09/2012 06/10/2012 04/11/2012 03/12/2012 16/04/2012 15/05/2012 13/06/2012 12/07/2012 10/08/2012 08/09/2012 07/10/2012 05/11/2012 04/12/2012 17/04/2012 16/05/2012 14/06/2012 13/07/2012 11/08/2012 09/09/2012 08/10/2012 06/11/2012 05/12/2012 18/04/2012 17/05/2012 15/06/2012 14/07/2012 12/08/2012 10/09/2012 09/10/2012 07/11/2012 06/12/2012 19/04/2012 18/05/2012 16/06/2012 15/07/2012 13/08/2012 11/09/2012 10/10/2012 08/11/2012 07/12/2012 20/04/2012 19/05/2012 17/06/2012 16/07/2012 14/08/2012 12/09/2012 11/10/2012 09/11/2012 08/12/2012 21/04/2012 20/05/2012 18/06/2012 17/07/2012 15/08/2012 13/09/2012 12/10/2012 10/11/2012 09/12/2012 22/04/2012 21/05/2012 19/06/2012 18/07/2012 16/08/2012 14/09/2012 13/10/2012 11/11/2012 10/12/2012 23/04/2012 22/05/2012 20/06/2012 19/07/2012 17/08/2012 15/09/2012 14/10/2012 12/11/2012 11/12/2012 24/04/2012 23/05/2012 21/06/2012 20/07/2012 18/08/2012 16/09/2012 15/10/2012 13/11/2012 12/12/2012 25/04/2012 24/05/2012 22/06/2012 21/07/2012 19/08/2012 17/09/2012 16/10/2012 14/11/2012 13/12/2012 26/04/2012 25/05/2012 23/06/2012 22/07/2012 20/08/2012 18/09/2012 17/10/2012 15/11/2012 14/12/2012 27/04/2012 26/05/2012 24/06/2012 23/07/2012 21/08/2012 19/09/2012 18/10/2012 16/11/2012 15/12/2012 28/04/2012 27/05/2012 25/06/2012 24/07/2012 22/08/2012 20/09/2012 19/10/2012 17/11/2012 16/12/2012 29/04/2012 28/05/2012 26/06/2012 25/07/2012 23/08/2012 21/09/2012 20/10/2012 18/11/2012 17/12/2012

5. CONCLUSIONS

The present paper shows the internal mission analysis studies and trade-offs being carried out internally at EUMETSAT in preparation for the upcoming launches and operations of the next Metop satellites (first being Metop-B). This includes the analysis, trade-offs and final decisions taken regarding final in-orbit configuration of the satellites in-orbit, injection altitude of second satellites into orbit and preliminary selections of launch dates. The work here presented may be found useful for any LEO satellite family flying on the same orbital plane requiring optimization of data exploitation from all satellites (mainly maximization of short term spatial coverage) while keeping the same Ground Track and compliance with a number of classical Earth Observation mission constraints (such as limitation of on-ground resources, avoidance/minimization of satellite interferences or maximization of operations robustness and safety).

6. REFERENCES

[1] EPS System Requirements Document, EUM.EPS.SYS.REQ.93001, Issue 4, Rev. 4. 6 November 2003 (available at http://www.eumetsat.int)

[2] EPS Programme System Design Document, EUM.EPS.SYS.TEN.01.007, Issue 2, Rev. 2. 17 September 2004 (available at http://www.eumetsat.int)

[3] EPS Operations Service Specification, EUM/OPS-EPS/SPE/04/0019, v1C, 17 January 2007 (available at http://www.eumetsat.int)

[4] Initial Joint Polar Agreement (http://projects.osd.noaa.gov/IJPS)

[5] Perlik F. et al, MetOp-A Operations Concept, AIAA 2008-3362 (SpaceOps conference 2008)

[6] Montero D. and van Holtz R., MetOp-A Launch Early Orbit Phase: Managing a LEOP service for a Starsem launch from Baikonur, AIAA-2008-3509 (SpaceOps conference 2008)

NB: References to internal EUMETSAT documentation not directly available to the public has been avoided. This includes dedicated mission analysis, hand-over plans and operations concepts (in line with some of the upper level documentation cited above and freely available through the EUMETSAT web page)

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