SCIAMACHY Mission Planning Concept
The ENVISAT mission planning is a distributed task in the ENVISAT/SCIAMACHY ground segment. Responsibilities are shared between ESA as mission provider and DLR as AO instrument provider, who has agreed to host all operations responsibilities also on behalf of the Dutch and Belgian partners.
The main effort of SCIAMACHY's mission planning starts with the orbit analysis by SOST-DLR/IMF. This allows the investigation of instrument specific orbit characteristics. Sun and moon related events (e.g. Sunrise, moonrise, sub-solar events) can be analysed to provide the basis for the definition of timelines applicable in specific intervals of the orbit. These timelines, combined with the rules established for the implementation of the SCIAMACHY mission scenarios, lead to the regular AOP SCIAMACHY input for the ENVISAT mission planning. The planning effort on ENVISAT side is therefore to determine of the absolute time-tags for the events requested in the SCIAMACHY planning input. Absolute time-tags are dependant on the actual ENVISAT orbit, i.e. they can only be provided by the ENVISAT Flight Operation Control Centre (FOCC) at ESOC.
SCIAMACHY planning documents with relevance for actual instrument operations are listed below.
ROE File: This file contains information about the
orbit parameters of ENVISAT. It lists the
and for each orbit of the ROE interval the
phase and repeat cycle settings
eclipse exit and entry (from ANX)
specific Sun zenith angles
The OSDF specifies which timelines have to run in a pre-defined orbit interval. It lists the status of the timelines loaded on-board at the start of the OSDF period, the required timeline sequences in the orbits of the OSDF period and requests for exchanging timelines on-board with timelines already defined on-ground. In routine operations the OSDF is generated by SOST using the SCIAMACHY Mission Planning (SMP) Tool and provided to ENVISAT for a period of typically 1 or 2 repeat cycles, i.e. 501 or 1002 orbits.
The SOST-operated SCIAMACHY Timeline Scheduler is a tool for the generation of a simulated
DMOP containing only the most relevant SCIAMACHY entries, i.e. times for ANX and timelines start/stop with corresponding auxiliary information. It is
available immediately after availability of the OSDF. Thererfore a simulated
SDMOP (SIM_SDMOP) with the same format as the SDMOP can be found on the SOST
web pages as soon as an OSDF has been generated by SOST and delivered to RGT.
The accuracy of the SIM_SDMOP is about 1-2 sec as compared to the SDMOP.
Having the SIM_SDMOP available, it is possible to derive the expected nadir and limb state coverages (reported via geolocation files) together with the planned sequence of states in an orbit well in advance of DMOP generation by ESA. This turned out to be a big asset for users when planning associated measurement campaigns, e.g. for validation.
The DMOP, generated by the ENVISAT Mission Planning System, describes particular command & control activities for the complete ENVISAT mission. SCIAMACHY timeline related entries in the DMOP are derived by the FOCC using the MPS module SCIACAL. SCIAMACHY DMOP entries are comprised of the instrument status measurement
The DMOP is the final mission planning document provided by ENVISAT to SCIAMACHY.
SCIAMACHY's nominal operations concept requires only few macrocommands to be scheduled by the MPS. Therefore the DMOP is to a large extent unrelated to SCIAMACHY. SOST-DLR/IMF extracts from the ENVISAT DMOP the SCIAMACHY relevant entries, translates the generic DMOP format into a user-friendly format and generates the SDMOP. This plan provides orbit number and time-tags for ENVISAT ANX
The SDMOP is the mission planning document finally provided by SOST to the SCIAMACHY user community and project management. Based on the SDMOP, the sequences of states per orbit are derived. These sequences supplement the SDMOP as they provide detailed information on SCIAMACHY´s measurement activities.
Visualization of SCIAMACHY´s state ground pixels is based on the ESA s/w tool ESOV (Earth Observation Swath and Orbit Visualization). Because of the complex structure of SCIAMACHY timelines it is necessary to process geolocation files which identify individual states. The start/stop times of these state dependant files are a function of the timeline start and the state sequence in the timeline definition files. SOST-DLR/IMF generates from the DMOP and the timeline files the associated geolocation maps (nadir and limb - limb start refers to limb_low at an altitude of 0 km / limb stop refers to limb_high at 100 km), and displays the ground pixel location over the orbit as gif files.
Besides of visual displays, users are also interested in receiving accurate positions of the ground pixels, expressed in geographic longitude/latitude. This is accomplished by SOST through geolocation text files. Such files are generated simultaneously with the swath maps. They provide for each nadir and limb state in alphanumeric form the left/mid/right geographic coordinates at state start/stop, both for wide and small swath state settings.
The times quoted in all planning files refer to the reference orbit as listed in the Reference
Orbit Event (ROE) file. Since the actual orbit differs from the
reference orbit (e.g. orbit drift), the times given w.r.t. the reference orbit
also do not reflect exactly the actual absolute times of events along the
orbit (e.g. ANX, sunrise, sub-solar, moonrise, eclipse). The requirements
for orbit maintenance may result in time differences of usually < ±10 sec.
In same cases this value may even reach ±1 min, however.
FOCC provides the associated time difference via the auxiliary state vector files
While the restituted file is only available after sensing, the file with
the actual predicted state vector can be used prior to operations to derive
the difference D = predicted - reference
time with high accuracy. If the predicted times are delayed w.r.t. the
reference orbit, then D >0 sec; in
the other case D <0 sec.
SOST monitors predicted versus reference time on a weekly basis. The resulting difference - spanning a complete year - is updated with the same frequency. Note that the parameter elapsed time, which denotes the time elapsed since ANX and which is also listed in the planning files, remains generally unaffected by the time difference as the D value applies to all events along the orbit in almost the same way.