SAR MISSION PLANNING FOR ERS-1 AND ERS-2
S. D'Elia & S. Jutz
ESA / ESRIN - RS/EU, via G. Galilei, 00044 Frascati, Rome (Italy)
Fax: 39 6 94180652, E-mail: firstname.lastname@example.org, email@example.com
ABSTRACT. The Mission Planning activities performed at ESRIN to schedule data acquisitions for the Synthetic Aperture Radar (SAR) instruments on board of ERS-1 and ERS-2 (alone and in Tandem) are described as initially organised and as they evolved with experience and changed requirements, after a short description of the ERS Ground Segment and an introduction, which lists other key mission factors and summarises ESRIN responsibilities in Earth Observation (ESA and Third Party Missions).
The major topics discussed are user interface, User Requests and their conflicts, Baseline Plans, Data Policy, Mission guidelines, and platform, sensors, ground segment and exploitation constraints, as well as planning tools, manpower needs and interfaces with ground stations. The experience gained can be used for future missions in the identification of the really achievable objectives, the definition of the offer to the user and the design of the Mission Planning system, in particular for user interface, mission planning tools and preparation of agreements with ground stations.
1. ERS MISSION & GROUND SEGMENT
The ERS-1 and ERS-2 satellites, launched respectively on July 17, 1991 and April 21, 1995, represent so far the unique case of a dual Remote Sensing mission and have provided results beyond expectations, in particular for Tandem data acquisition with one day difference, serving with data many user categories like real-time operators involved in meteorological, oceanographic and environmental applications, long-term research groups working off-line, commercial users, etc. The two satellites, carrying on board the set of instruments listed in Table 1, have been exploited in the different mission phases listed in Table 2.
The ERS Payload Data Ground Segment, sketched in Figure 1 and providing SAR coverage as per Figure 2, is managed by ESRIN, via its Earth Remote Sensing Exploitation Division (RS/E), and is composed of:
* The ESRIN ERS Central Facility (EECF), located in Frascati,
in charge of:
· user interface and user support (training, promotion, documentation, tools, etc.)
· monitoring of investigations and transfer of technology
· mission planning in conjunction with the Mission Management and Control Centre (MMCC) at ESOC
· ground stations' interface & coordination of National & Foreign Stations (NFSs: National = stations of countries participating in the ERS programme, Foreign = stations of non participating countries)
· planning and monitoring of production and delivery of near real-time and off-line products
· generation and maintenance of a world wide inventory of acquired data
· coordination of the commercial Eurimage, Spotimage and Radarsat International Consortium (ERSC)
· assessment of instrument behavior and of related margins
· monitoring and control of ERS data and product quality
· management of the Ground Segment facilities and monitoring / control of related files' routing
· maintenance of the "Reference System" for the High- and Low-Rate Fast Delivery Processing chains
· maintenance of data-processing software for the entire Ground Segment
* The ESA Ground Stations: Kiruna (Salmijaervi, Sweden), Fucino (Italy), Maspalomas (Canary Islands), Tromsoe (Norway), and Gatineau and Prince Albert (Canada). All stations but Kiruna, operated by ESOC and fully dedicated to ERS operations (including telemetry, tracking and control activities), are multi-mission and operate under ESRIN contracts. This network ensures acquisition of regional ERS SAR data and acquisition, processing and delivery of global ERS LBR data within three hours from sensing.
* A network of 26 NFSs which acquire ERS SAR data around the world (no on-board tape recorder for SAR) under the terms and conditions of a standard Memorandum of Understanding (MoU).
* Four Processing and Archiving Facilities (PAFs), which are joint national / ESA endeavors to support / expand the applications of ERS data (SAR and/or LBR) through data archiving and off-line generation of precision products, in Brest (France, operated by IFREMER), Farnborough (UK, operated by NRSCL), Oberpfaffenhofen (Germany, operated by DLR) and Matera (Italy, operated by the Italian Space Agency).
In this paper, the term "Mission Planning" indicates (is limited to) only the activities performed at ESRIN for planning SAR acquisitions. Other linked activities, like planning of LBR instruments (mostly performed by default at ESOC) or production planning are not discussed.
SAR acquisition planning, even if a complex task, has not caused
bottlenecks or problems for the ERS mission. The overall effectiveness
of the ERS SAR mission was or is affected much more by:
* Experimental nature of radar data:
· off-the-shelf tools to manipulate data are just emerging on the market
· application potentialities are still being demonstrated (research and promotion to be funded)
· users are not yet familiar with radar image interpretation (training required)
* Data coverage and continuity:
· possibility to plan at short notice for special events (24 h service depends on funds)
· high revisiting frequency is essential for operational applications
· data should be systematically acquired and long term archived for future use
· NFSs should be encouraged to build and maintain a long term data archive (Data Policy)
· reasonable quota should be defined for investigations (and budget allocated)
· commitments for data from foreign stations should be carefully taken (contract vs. MoU?)
· formats should be homogeneous across all generation facilities and, possibly, missions
· products should be tailored to user needs (e.g.: fast delivery low resolution, terrain corrected, etc.)
ESRIN is in charge of the planning and handling of the Earth Observation data from ESA and Third Party Missions (TPMs). For TPMs (e.g.: Landsat and JERS-1), planning is limited to the collection of user needs, the definition of other potentially relevant acquisitions, the transmission of the resulting plan to the satellite operator, and the scheduling of ESA stations. ERS-1&2 SAR activity planning is instead by far much more complex because, besides the number of on-board instruments and the parallel activities of two satellites, it must match user requirements (USER REQUESTS), gathered through the USER INTERFACE, and the BASELINE PLAN (derived from Mission / Data Policies, anticipated user needs and contingency planning) with the system CONSTRAINTS. This activity is supported by a balanced combination of dedicated PLANNING TOOLS & MANPOWER and relies on INTERFACES with the ground stations. All these elements are discussed below, describing their initial implementation and their evolution in line with the experience made and the changing requirements.
3. USER INTERFACE
The User Interface has been organised around three Desks: the ESA Help Desk (for information, documentation, tools, etc. to all users), the ERSC Customer Service (for commercial users) and the ESA Order Desk (for non commercial users), with the possibility to exchange correspondence through fax, telephone, letter, E-mail, etc., on user choice.
Currently 3068 users are registered, of which only 760 have a kind of on-line access (mainly E-mail) and 735 have submitted at least one User Request (of these only 263 have E-mail). It is evident that normal correspondence media are still widely used. Their use will continue in the coming years also in view of the opening of Eastern and African markets (telecommunication links to be set-up).
The ERS User Interface was designed to serve a variety of user
categories (see Table 3):
· Investigators participating to ESA Announcements of Opportunity (AO) or Pilot Projects (PP)
· National and Foreign Stations
· Commercial / Operational users or Institutional Organisations
· Others (calibration / validation, training, promotion, public relation, etc.).
The service was set-up to treat all users on identical footing, but some users feel "more identical" than others, following strange routes instead of the standard ones, causing overhead, confusion and possibly bad service. Any service should be set-up to deal with such exceptions and be ready to dig out detailed history even after a few months, in order to face possible complaints.
After mission start, it was evident that interaction with users was more difficult and demanding than expected, despite that some key documents describing the system had been prepared and widely distributed (some users had the impression that we could move the satellite wherever necessary). It became essential, particularly for planning, to improve the user "visual" knowledge of the mission and to have him and our Desks to speak the same language. Therefore the graphic, simple and powerful Display ERS-1 SAR Coverage (DESC) tool, running on PCs, was developed and distributed. It was enhanced over time through valuable user feedback, up to the most recent Display ERS Swath Coverage for Windows (DESCW), which is multi-mission, supports quick-look display, provides on-line help, etc.
DESCW shows graphically the coverage of the various sensors in the future and/or in the past (through inventory search and filter). It is based on visibility files for possible future acquisitions and on compressed inventory files for past and planned acquisitions. The inventory files are either historical (past years) or updated weekly and are available online for free-of-charge downloading via FTP or Internet, together with the software and all supporting data. The entire software, the basic files, the Help text, the inventory files (about 20 years of inventory data in total for ERS-1, ERS-2, JERS-1 and Landsat) take less than 2 Mbytes and therefore are also distributed on two PC diskettes on user request.
Over time DESCW has been more and more used by our Desks and also by the mission planner, particularly to identify possible acquisition conflicts with other missions (the ERS mission planning system is not multi-mission), to derive rough indications useful for detailed mission planning and to quickly check future planning over small areas.
4. USER REQUESTS
The system was designed to permit formalisation of user needs through User Requests, which, for acquisition planning, mainly define the area and time period of interest and can equally well identify single frames and very large acquisitions (e.g. the full station visibility area for some months).
Large sensing requirements must be submitted to planning about one month ahead of acquisition, limited ones up to five working days ahead and exceptional cases have been handled up to two-three working days ahead (uplinking of the spacecraft telecommands is done one day before the acquisition).
User Requests can be submitted and their status verified on-line through dedicated forms, via X.25 and VT200 terminals. Users are also actively informed via e-mail or fax at major status changes.
Figure 3 shows the total number of
User Requests per User Category since mission start and Figure 4
shows their variation over time. It must be noted that:
· Commercial requests are increasing (even if absolute value is still much below other categories)
· Investigator requests are lined up with the number of accepted projects
· National stations and Foreign stations with no-exchange of funds agreements request large amounts of acquisitions (even with few User Requests, since area and time range are wide)
· Foreign stations ("pay per frame") limit data requirements to the minimum (limited area & time)
· the complexity of the Baseline Planning is increasing (more specialised User Requests)
A few months after exploitation start, its was realised that some NFSs and most of the Investigators were submitting large acquisition requests, causing overhead in mission planning and possible waste of satellite resources. Since most of the investigations had a production quota defined, the Investigators were asked to limit their acquisition requirements to those to be associated in future to a product. This simple measure permitted to drastically reduce not the number of User Requests, but their size. However, when justified, the excess acquisitions were accepted within the Baseline Plan (see below).
The need emerged to speed up provision of information to users in case of sensor unavailability (some users take in situ measurements during satellite over-passes). Therefore an automatic procedure was added to inform via fax all affected users, immediately after reception of a sensor unavailability information. Many times this information is available only after the event, as in the case of arching (it can be imagined the reaction of a group of Japanese scientists, taking in situ measurements out on the cold Antarctic pack, while the SAR overflying the site did not acquire the data: complaints were flowing in all directions, hyperspace included).
5. BASELINE PLAN
Shortly before ERS-1 launch, when starting to handle User Requests, it was realised the need for an ESA Baseline Plan (a set of mission planner User Requests), implementing Data Policy and Mission Guidelines (see Table 4 for the most relevant ones) and collecting data of potential commercial, operational or scientific interest. In particular, the Mission Guidelines, defined for each Mission Phase in the High Level Operations Plan, influence planning over selected areas depending on the phase, while the Data Policy has large impacts on data requests from NFSs.
The Baseline Plan was more and more defined and complex. Currently
it is centered on acquisitions for:
· a mapping mission (build up consistent thematic data archives; anticipate future user needs; collect data for exceptional events and natural disasters; etc.)
· phase / season dependent targets (monitor seasonal changes such as ice, ice boundaries and vegetation growth; collect full data sets over selected areas for applications like interferometry, change detection; etc.)
· system related objectives (optimise instrument and ground segment utilisation; plan instrument calibrations; optimise acquisition over stations working on campaigns; etc.)
· large Investigators' requests (follow moving targets like icebergs or ships; scan large areas for oil pollutions or special phenomena; etc.)
· anticipated user requirements (that is not yet formalised)
During the Tandem Phase, data acquisition from both satellites
was implemented through a special Baseline Planning considering:
· areas as large as station visibility for descending passes
· small areas around steep slopes for ascending passes
· stations' availability (linked also to signature of MoUs, which for ERS-2 were normally late)
· conflicts among stations due to SAR acquisition limits (in practice only one station can be in full tandem at any time along the same meridian)
· orbit maintenance manoeuvres (for best Tandem data, the orbits of the two satellites were made to cross around equatorial regions during Winter and over the poles during Summer)
· segments linked to user requests on one satellite and unavailable for Tandem on both satellites
Some of the system constraints (the major ones in Table 4) are imposed by the physical characteristics of the instruments, spacecraft or orbit, while other derive from ground segment and exploitation possibilities (of course more detailed constraints are taken into account at ESRIN and ESOC). Even a few of the listed constraints make the planning process complex, also because their relative emphasis changes over time in relation for example to day/night, season, mission phase, etc. (likely, some of the most complex constraints were avoided, with the excuse that we were having already enough fun).
The constraints marked with an asterisk in Table 4 were defined a few months before ERS-1 launch, after a pre-release of the ESRIN mission planning system was delivered, and therefore induced late changes. Those with a slash were defined around the same period but were not implemented. The constraints marked with a plus have been encountered during exploitation.
It is evident that, a part from a few technical issues, the major constrains influencing mission planning have been drastically changing during the real mission exploitation.
7. PLANNING TOOLS & MANPOWER
The basic implementation of the ESRIN Mission Planning System was embedded in the development of the Central User Service (CUS) by MacDonald Dettwiler. In such core sub-system the planning is based on User Requests, shared with other sub-systems (User Request Handling, Order Handling, Production Planning, etc.), while a specific set of tools (forms, graphics and reports) assists the planner in his activities.
Before mission start, an attempt was made at ESTEC to include planning rules into an expert system based on Key / Lisp and running on a SUN workstation. Its use was successful in analysing LBR dump strategies, but it was judged not efficient and flexible enough for the complex and changing SAR mission. Moreover, it required additional expertise on the application package and it was difficult to interface it with CUS. It was therefore decided not to use it for acquisition planning.
ESRIN had developed with Advanced Computer Systems a mission analysis tool used to verify possible use of SAR sensing in various mission scenarios (different launch dates and cycles). Since the major concern was related to the probability of acquisition conflicts, this tool was upgraded to test a simple algorithm for possible conflict resolution, reduction or at least identification. The problem was extremely simplified, generating for three key types of User Requests all the visible (by a ground station) orbit and frame combinations, with all their possible alternatives. The algorithm was designed to allocate acquisitions starting from the less critical orbits (those with more frames available and less requests) and propagating the effects to all involved User Requests (the algorithm was designed to minimise conflicts and not to optimise planning, allocating the minimum number of sensing segments).
The results were promising, since, feeding the tool (which could have also been easily interfaced with CUS) with the available AO User Requests, practically no conflict was detected.
It was decided to verify CUS planning in practice before connecting this algorithm to CUS.
An analysis was made on the real conflicts experienced among User Requests. From Table 5, related to Phase C, it is evident that the limited commercial (top priority) requirements could not cause conflicts, while Investigators have larger conflict probabilities, even if, with a share of only 1/5 of the total allocation, other resources could have been freed for them if necessary. But this was not the case, since only 0.25 % (5 out of 2024) of the requests were in conflict and therefore marginally descoped.
During Phase D the requests in conflict grew to 1.75 % (13 over 743), because the short repetition cycle (3 days) and Phase (3 months) forced the grouping of the large requirements from the ice scientists over much less orbits (43 against 501).
Currently the orbit configuration for both satellites is the one of phase C and large Investigator requests tend to decrease. Therefore even less conflicts are being experienced among User Requests.
Acquisition planning is currently performed at ESRIN by one contract staff supervised by 50 % of an ESA staff, who ensures back-up during working days, but also contributes to the preparation of planning documents, defines detailed acquisition strategy in line with mission guidelines, sets-up the baseline plan, follows specific cases, contacts the stations for special arrangements, ensures correct reporting, etc. This manpower level is just adequate and in periods of particular load, such as Tandem Mission, some low priority activities are descoped, deferred or canceled (e.g.: internal reporting, analysis of station reports, etc.). The use of an expert system would have not reduced the manpower requirements below this limit, since, in addition to the planner, there would have been the need for an expert of the expert system for changing the rules according to the constantly varying mission needs (and possibly additional manpower for corrections and tuning).
To play fair, no real conflict can exist with such type of (politically sensitive) missions. In fact, even before a problem can be anticipated, users let us know about it, not plainly contacting the mission planner or our Desks, but with the proper emphasis through high level links (no push from the top of the hierarchy so far, unless the recent lightning at ESRIN is a sign of it).
Besides the internal ESA interface between EECF and MMCC for mission planning, the EECF has planning Interfaces with acquisition stations, mainly for sending Acquisition Schedules and spacecraft ephemerids, and receiving Acquisition Reports. This loop is essential for the User Request satisfaction, since, in case of lost acquisitions, the sensing must be replanned, if it is still acceptable to the user. This interface was defined in two documents, one for ESA stations and a simplified one for NFSs, both based on files exchanged through telecommunication links using two file transfer protocols (FTAM and FTSV) over X.25.
When NFSs started to join the ground segment, it became evident that only a few of them had prepared their interfaces in line with the specifications. Therefore new interfaces and procedures had to be quickly defined and implemented based on faxes. Even these simple procedures were some times not applied (only telex working; requested report provided irregularly and after solicitations; reports not containing all required information or not providing adequate visibility; etc.). Slowly over years some of the stations started to migrate towards online connections, but unfortunately using their preferred protocols (in some cases also changing over time). We had to progressively add new protocols to our system, in order to simplify our operations, but at the expense of complexity.
The stations can submit User Requests like any user, except that they should indicate whether the request is for general data acquisition with lower priority or coming from an end user with higher priority. Table 6 shows the number of SAR frames acquired over all stations for both missions.
Due to the experimental nature of the mission, the complexity of the ground segment and the political drive and sensitivity, the ERS SAR Mission Planning, like many other activities related to the ERS mission, has been dealt with in many cases more by exceptions than through stable rules, since it was not possible to anticipate many of the ground segment constraints and the requirements' evolution. Therefore the planning system of similar missions should be designed starting from simple and adding complexity over time, when the real constraints, requirements and possibilities are known. The initial system should be flexible and enough resources should be foreseen for this expansion / adaptation.
Since the user is an integral part of the system, large information exchange is necessary. This should happen through the user's preferred methods and possibly supported by a graphic, simple, powerful and friendly tool, running at least in the most popular environment. This tool should be enough precise and complete to be also used internally, in order to talk with the user on the same ground. The resources (facilities and manpower) necessary for a proper interaction should be carefully evaluated and allocated, since they are essential to reduce problems and workload and to improve overall service quality.
The initial forecast that practically no conflict would exist with 12 minutes of SAR per orbit was confirmed by the experience, reassuring that the decision not to implement neither an expert system nor a special conflict resolution tool was correct. The absence of conflicts and the variability of constraints and rules, make flexibility more important than plan optimisation, with mission planning better based on natural more than on artificial intelligence (supported by powerful tools). A smart mission planner can anticipate and resolve conflicts before they are formalised, can judge new requirements against his knowledge of the constraints and his mental representation of the already performed planning, can learn from past and dynamically adapt procedures to the changing environment (missions with inconstant pattern represent still an area of revenge for the natural intelligence, if available, over the artificial one).
In conclusion, SAR Mission Planning has never been a limiting
factor for the ERS mission. Other factors had and have much larger
overall impacts, like:
· data policy
· experimental nature of the radar missions
· product types and formats
· behavior of stations in developing countries (such data is essential also for the rest of the planet)
· data availability (revisiting frequency) which would require either a cluster of satellites or coordinated and homogeneous access to data from all available remote sensing satellites
Table 1 : ERS-1 and ERS-2 On-board Instruments
|Active Microwave Instrument||.||.|
|SAR Image Mode||X||X|
|SAR Wave Mode||X||X|
|Along Track Scanning Radiometer-1||X||.|
|Along Track Scanning Radiometer-2||.||X|
|Global Ozone Monitoring Experiment||.||X|
|Precise Range and Range-rate Equipment||(not active)||X|
Table 2 : ERS-1 and ERS-2 Mission Phases
|Mission Phases||Start||Cycle||SAR Mission Objectives|
|- Payload switch-on & verif.||17-Jul-91||.||.|
|A Commissioning||25-Jul-91||3 days||all instruments; until 10-Dec-91|
|B Ice||28-Dec-91||3 days||ice & pollution; interferometry possibil.|
|R Roll-tilt (Experimental)||02-Apr-92||35 days||Different SAR incidence angle (35 deg)|
|C Multi-disciplinary||14-Apr-92||35 days||AO; land & ice mapping; consistent set|
|D 2nd Ice||23-Dec-93||3 days||see Phase B|
|E Geodetic||10-Apr-94||168 days||radar-altimetric mission; SAR as C|
|F Shifted Geodetic||28-Sep-94||168 days||8 km shift vs. phase E for denser grid|
|G 2nd Multi-disciplinary||21-Mar-95||35 days||see Phase C|
|G Tandem||17-Aug-95||35 days||Interferometry & mapping|
|G Back-up||2-Jun-96||35 days||.|
|- Payload switch-on & verif.||21-Apr-95||35 days||.|
|A Commissioning||02-May-95||35 days||SAR commissioning|
|A Tandem||17-Aug-95||35 days||see ERS-1 Tandem Phase G|
|A Multi-disciplinary||3-Jun-96||35 days||see ERS-1 Phase C|
Note: 3 days = 43 orbits; 35 days = 501 orbits; 168 days = 2411 orbits
Table 3 : Current number of Users per User Category
|USER CATEGORY||No. of USERS|
|AO / PP||1004|
|NA / FO||28|
Table 4 : System Constraints
(- = initial; * = close to ERS-1 Launch; / = * but not implemented; + = during exploitation)
- national stations (with signed a MoU) can acquire data in a non interference basis
- stations with approved MoU can request data acquisition (at no cost for national stations and for foreign stations with no-exchange of funds agreements)
- adhere to Mission objectives (phase dependent)
- SAR has priority in descending passes, Wave and Scatterometer in ascending passes (night)
- solve conflicts applying priorities to the user categories and then to users according to past allocation
/ allocate acquisitions for AOs within the assigned quota, in a 6 months moving window, varying their priority according to remaining time, past allocation for country and application category
* LBR activity has priority over SAR in ascending passes every other cycle
PLATFORM AND SENSOR'S CONSTRAINTS:
- SAR can be activated only in visibility of a ground station (no HR tape recorder on-board)
- in each 100 min. orbit, SAR can be activated < 12 minutes in total, < 10 minutes per segment on descending passes, < 4 minutes in eclipse (in addition, merge gaps <30 seconds)
- max. number of SAR on/off switches = 6 per orbit
- SAR imaging mode of AMI mutually incompatible with SAR Wave mode and Windscatterometer
- Windscatterometer must be switched on 128.2 seconds (850 km) before and after the site of interest
GROUND SEGMENT CONSTRAINTS:
- take into account the real station visibility mask in planning SAR sensing
- instrument planning and Kiruna station scheduling must follow defined time constraints
* schedule SAR sensing from 5 to 2 degrees above horizon
* handle station unavailability at major subsystem level
+ adhere to ground station specific operational constraints, like: working hours (depending on campaign, country or religion), conflicts, available tapes, interval between adjacent passes, etc.
+ schedule all stations in visibility of planned segments, unless no MoU exists, unless it is national or there is a commercial request or for a natural disaster, unless there is no hope to serve the user
+ schedule overlapping stations depending on reliability and on station or PAF processing capability
+ some stations report on acquisitions with a variable delay (even of months, causing loss of replanning opportunities) and occasionally their reports are discovered to be incorrect
+ some MoUs signed later than expected or signature proceeding with hiccups
+ reduce number of HDDTs avoiding overlapping acquisitions and minimising night passes
* avoid bridging of specific segments (precise start flag)
/ monitor and control energy and thermal balances over and across orbits
/ handle SAR gain setting at User Request level
/ permit planning of sensor modes (e.g.: OGRC / OBRG)
/ handle solar panel occultation of downlink antenna (changing over the year and with latitude)
+ 12 SAR minutes not per orbit, but from eclipse start to eclipse start (changing with seasons)
+ apply `common sense' (strict application of HLOP rules prevents optimised use of resources)
+ assign higher priority to `production requests' requiring new planning over `acquisition only' ones
+ assign higher priority to requests over stations working in campaigns
+ change confirmed requests only in case of natural disasters or calibration
+ ensure proper and complete tandem planning (no multimisson planning tool, user requirements might conflict with tandem mission, ground stations operational constraints more difficult to match, etc.)
+ keep to a minimum the number of IDHT on/off switches (from June 1996, to extend lifetime)
+ "keep alive" scenario requires planning of at least two segments per day, about 12 hours apart
Table 5 : frames allocated during ERS-1 Phase C vs. User Category
|USER CATEGORY||No. of FRAMES|
|AO / PP||66331|
|NA / FO||80028|
Table 6 : Total SAR frames acquired worldwide for both missions
|TOTAL ESA STATIONS||384497||69830||454327|
|TOTAL NATIONAL STATIONS||572937||84748||657685|
|TOTAL FOREIGN STATIONS||385413||63139||448552|
|Distinct/Grand Total (%)||61.94%||70.19%||63.09%|