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Notes: SOAP/TNO/001/CRA
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…02…SOAP/TNO/001/CRA
…02…PEH/810302…02……02…#
TNO1: SAR PROCESSING SYSTEM
…02……02…SOAP
T̲A̲B̲L̲E̲ ̲O̲F̲ ̲C̲O̲N̲T̲E̲N̲T̲S̲
1. SCOPE ....................................4
2. APPLICABLE LITTERATURE....................5
3. THE SAR GROUND SEGMENT....................6
4. SPACE SEGMENT CHARACTERISTICS ............9
5. REQUIREMENTS TO SAR GROUND SEGMENT ......10
6. DISCUSSION ON REQUIREMENTS ..............16
7. SAR PROCESSING SYSTEM CONFIGURATION .....21
7.1 Description of System Elements .......21
7.2 Requirements to System Elements ......25
1̲ ̲ ̲S̲C̲O̲P̲E̲
This document gives a functional description of the
antire SAR processing system. This shall be considered
as the baseline for the study of SAR operational Aspects.
2̲ ̲ ̲A̲P̲P̲L̲I̲C̲A̲B̲L̲E̲ ̲L̲I̲T̲T̲E̲R̲A̲T̲U̲R̲E̲
1. LASS Phase A Study
ESA Contract 3638/78/F/CG
Dornier Systems
Matra-Espace
Christian Rovsing A/S April 1979
2. Phase A Study of Earth Resources Synthetic Aperture
Radar ESA Contract 3524/78/F
Marconi Research Laboratory
December 1978
3. Phase A Study of Earth Resources Synthetic Aperture
Radar ESA Contract 3523/78/F
Thomson-CSF
January 1979
4. Final Report on the Study of the Definition of
a Ground SAR Processor for Seasat-A
ESA Contract 3350/77 ESA CR (P) 1199
Thomson-CSF
May 1978
5. Introduction to discussion on Ground Segment and
Image Quality Specifications for LASS and COMSS
ESA Contract 3397/79
J.P. Antikidis - ESA
December 1979
6. FAMS Study Report
7. Orbit restitution, Study Report
8. COMSS - SAR baseline definition
PB-RS (80), 24 rev. 2
9. Survey of technology available for very high speed
data recording and handling of very large data
archives
Final Report, September 1980
ESA Contract No. 4.031/79/F/FC (SC)
Christian Rovsing A/S
10 Future Earthnet dissemenation System (FEDS)
Phase II report, September 1980
ESA Contract 4241/80/F/FC(SC)
Logica, Christian Rovsing A/S
3̲ ̲ ̲T̲H̲E̲ ̲S̲A̲R̲ ̲G̲R̲O̲U̲N̲D̲ ̲S̲E̲G̲M̲E̲N̲T̲
The complete SAR Systems is made up of several physically
widely separated elements.
- satellite
- TT&C Stations
- Mission Control Center
- SAR Signal Acquisition Facility
- " " Processing "
- Archiving Facility
- Dissemination Facility
3.1 G̲E̲O̲G̲R̲A̲P̲H̲I̲C̲A̲L̲ ̲D̲I̲S̲T̲R̲I̲B̲U̲T̲I̲O̲N̲
The Geographical locations of service of these elements
are quite obvious, while a considerable freedom exists
for the locations of other.
There is heavy constraints and non-trivial trade-offs
in the mutual location of
- SAR acquisition facility
- SAR processing "
- Archiving "
- Dissemination "
Due to the huge amount of data, and the requirements
of fast delivery to users, it is very desirable to
have these four activities on the same site.
However, the acquisition facility must be spread over
at least two widely separated stations in order to
archieve total coverage for all European countries.
One back-up station should be considered. Even the
presence at an On Board High Density recorder would
not substantially change this due to e.g. downlink
data rate considerations. From a geographical coverage
point of view, preferred locations of the two stations
would be one site in the northern Sweden (e.g. Kiruna)
and one in Mid and/or Southern Europe. Considering
the interest in open-sea measurements would obviously
favour the mid-west part of Europe.
Such a location would still cover all European land
areas.
For this study we will just assume two sites, widely
geographically separated, within Europe and they will
be called Northern Site (NS) and Southern Site (SS),
respectively.
3.2 I̲N̲T̲E̲R̲F̲A̲C̲E̲ ̲T̲O̲ ̲E̲A̲R̲T̲H̲N̲E̲T̲
The interface within Earthnet is depicted in fig. 3.1.
The SAR Ground Station interfaces with
- Satellite. SAR data and some auxiliary data are
received via the x-band link.
- Mission Management Centre.
All relevant scheduling information together with
attitude, orbit and other information collected
from Earthnet facilities as received from the MMC.
Product quality information is reported back.
- Users.
Quick look poroducts from recent acquisitions are
disseminated directly to the users.
Very short term (3 - 6 hrs) delivery are made via
high speed data links.
Short term deliveries (few days) of CCT's and films,
prints are by mail.
- Central Archive and precision processing facility.
The raw SAR data are delivered on HDDT'S to the
Central Archive together with the available Quick
products.
4̲.̲ ̲ ̲S̲P̲A̲C̲E̲ ̲S̲E̲G̲M̲E̲N̲T̲ ̲C̲H̲A̲R̲A̲C̲T̲E̲R̲I̲S̲T̲I̲C̲S̲
For the purpose of this note, the following has been
used (ref. to PB-RSC80)/24 rev. 2:
Satellite Altitude: 650 km
Satellite Velocity (Nadir point): 7 km/sec.
Orbit time : 100 minutes
Swath width : 75 km
Data rate : 100 Mbit/Sec.
S/C pointing : 0.15 degrees roll,
Pitch Yaw
Visibility time, Nadir pass,
5…0e…o…0f… elevation : 12 minutes
SAR duty cycle, max. : 10 mins/pass
The SAR signal is a range compressed, digitized and
time-expanded SAR Video signal. The signal is transmitted
in the X-band on a separate channel (not time-multiplexed
with Optical Payload Data).
Relevant S/C telemetry data are time-multiplexed with
the SAR Video.
5̲.̲ ̲ ̲R̲E̲Q̲U̲I̲R̲E̲M̲E̲N̲T̲S̲ ̲T̲O̲ ̲T̲H̲E̲ ̲S̲A̲R̲ ̲G̲R̲O̲U̲N̲D̲ ̲S̲E̲G̲M̲E̲N̲T̲
The SAR Ground Segment comprises the entire ground
based system for SAR signal acquisition, processing,
archiving, retrieval and dissemination to users.
5.1 B̲A̲S̲I̲C̲ ̲R̲E̲Q̲U̲I̲R̲E̲M̲E̲N̲T̲S̲
The Ground Segment shall perform
- SAR signal acquisition
- Acquisition of auxiliary data
- Processing of auxiliary data
- Processing of SAR data for Quick-Look
- Evaluation based on Q.L.
- Precision SAR processing
- Updating of Catalogue
- Archiving
- Retrieval
- Dissemination
The user products to be generated are
- Quick-Look image (CCT, film, print)
- System corrected image (CCT, film print)
- Precision processed image (CCT, film, print)
5.1.1 Q̲u̲i̲c̲k̲-̲L̲o̲o̲k̲ ̲I̲m̲a̲g̲e̲
The Q.L. image is characterized by
- Useability for Quick-Look purposes (print)
- The goal is a spatial resolution similar to
that of the precision processed image
- Reduced radiometric resolution (no look-summation)
- Geometric accuracy not important
There is an urgent requirement for having Q.L. products
within a few hours after reception. This poses heavy
constraints on the degree of processing possible for
this product. This problem will be discussed later
during the study.
The Q.L. image shall be available as
- CCT
- Film, print
5.1.2 S̲y̲s̲t̲e̲m̲ ̲C̲o̲r̲r̲e̲c̲t̲e̲d̲ ̲I̲m̲a̲g̲e̲
The System Corrected image (SC) has been subject to:
Migration Compensation, Azimuth Correlation, Look Summation
and first step of the post-processing.
It is envisaged that the SC product shall be available
for users within a few days after the acquisition.
Economical and technological constraints necessitates
a careful sorting-out of all time-consuming processing
which is not strictly rquired.
The aim of the SC image is to provide:
- A product for users who requires the high spatial
resolution, but not requiring the ultimate
in time-consuming registration accuracy, Geometric
Accuracy and radiometric accuracy.
- An intermediate product for generation of an
optimized System corrected image (OSC) where
radiometric, Geometric and registration accuracy
has been improved by further processing.
- A product which is standardised in the sense
that it has a format compatible with images
from other SAR instruments, and even with optical
SC products.
The SC and OSC images shall be available as
- CCT
- Film
- Print
5.1.3 P̲r̲e̲c̲i̲s̲i̲o̲n̲ ̲P̲r̲o̲c̲e̲s̲s̲i̲n̲g̲
The Precision Processed (PP) SAR image represents the
ultimate obtainable in terms of:
- spatial accuracy
- radiometric accuracy
- geometric accuracy
- registration accuracy
In order to reach this, additional post acquisition
data have to be collected and processed.
The additional information concerns
- Improvements of Mean doppler estimates by refined
orbit and attitude calculations or by examination
of the SAR data
- Improvement of radiometric accuracy by statistical
analysis of data
- Improvement of geometric and registration accuracy
by Ground Control Points.
The Precision processing should also include a mapping
into a standard projection. (E.g. UTM).
The input to this processing will be
- Raw SAR data
- Misc. Aux. data.
The PP image shall be available as
- CCT
- Film
- Print.
5.2 T̲H̲R̲O̲U̲G̲H̲P̲U̲T̲ ̲R̲E̲Q̲U̲I̲R̲E̲M̲E̲N̲T̲S̲
According to the RFQ, the following requirements applies:
- Quick-Look
Throughput: 200 scenes/day (75 x 75 km…0e…2…0f…)
CCT's Films
Quality: Spatial resolution app. 50 m,
reduced radiometric resolution
Delay: 3 - 6 hours.
- Precision processed
Throughput: 50 scenes/day CCT'S, Films
Quality: System corrected or better, to
be
further defined
Delay: 3 days from Quick-Look generation.
5.2.1 S̲t̲o̲r̲a̲g̲e̲ ̲R̲e̲q̲u̲i̲r̲e̲m̲e̲n̲t̲s̲
With the envisaged orbit and two ground stations, a
total at approx. 16 contacts per day with average duration
of 8 minutes gives
- 9% contact time
- 7,700 seconds of reception time/day
- 770 Gbit/day or
- approx. 100 Gbytes/day or
- 720 scenes/day.
One pass gives a maximum of
- 10 mins of data or
- 60 Gbit or
- 7.5 Gbyte or
- 56 scenes
with 1 scene = 1,07 Gbit = 134 Gbyte (75 x 75 km…0e…2…0f…)
For the Northern station, the maximum acquisition load
is approx.
- 480 Gbit/day or
- 60 Gbytes/day or
- 450 scenes/day
The corresponding Archive load is estimated to 66%
of received data (overlap between stations, non-useful
data or non-useable data):
- 320 Gbit/day or
. 40 Gbytes/day or
- 300 scenes/day
The Quick-Look production for this station amounts
to
- 150 Gbit/day or
- 19 Gbytes/day or
- 140 scenes/day
The corresponding Output amounts to
- 560 Mbytes/day
Using 42 track HDAT for the Archive type, this corresponds
to 4 - 5 tapes /day for one station or approx. 7 tapes/day
for both.
6̲.̲ ̲ ̲D̲I̲S̲C̲U̲S̲S̲I̲O̲N̲ ̲O̲N̲ ̲R̲E̲Q̲U̲I̲R̲E̲M̲E̲N̲T̲S̲
Having summarized the characteristics of the input
to and output from the SAR processing sytem, the following
remarks can be made:
1. The high downlink bit rate does not leave time
for any kind of actual processing during downlink,
unless very expensive (mission) dedicated hardware
is considered.
Even if such hardware was available, the processing
would not be reversible and hence ultimate quality
would to a large extent be limited to that obtainable
from pre-acquisition information.
This is obviously not acceptable, unless a FAMS
system is included in the payload. The potential
benefit in bit rate is a factor of appr. eight
for the azimuth correction.
2. The only device which can cope with such rates
and amounts of data is an HDDR.
Both European and US Suppliers exists which can
deliver suitable equipment.
3. The Auxiliary data, embedded in the downlink have
to be extracted before the data can be processed.
It is desireable to have this kind of data separated
onto a CCT or disk prior to the SAR processing
for the reasons of processing speed.
4. The Format of the source-HDDT for Quick Look processing
could be either:
- As received with time code track for search,
without any kind of re-formatting or
- Frame synchronized with some degree of
re-formatting.
The first approach is straight forward and poses
no serious technological problems. The disadvantages
are:
- It is time consuming to extract auxiliary
data.
- Search for specific data is not straight
forward.
- All retrieval requires a (mission specific)
frame synchronizer.
- The format will probably not be very convenient
for subsequent SAR processing.
The second concept can be accomplished in two ways.
a) Frame synchronization and re-formatting during
acquisition.
b) Frame synchronization and recording re-formatted
data onto a separate HDDR during replay.
Concept a will require a very expensive frame synchronizer
and the reformatting must be kept to a very simple
scheme.
Concept b allows the frame synchronization to be
performed using normal TTL technology and more
time is available for generating a convenient format.
Another important aspect is that non-useful data
(from geographical or other reasons) can be discarded.
The disadvantages are
- Need for additional HDDT'S
- The additional replay introduces an additional
delay of approx. 1 hour for the Quick-Look
generation.
Concept b is chosen as the baseline for this study.
5. The basic Archive tape contains the raw data.
No other products are considered acceptable since
non-reversible processing has been performed which
could seriously limit the useability.
On the opposite, taking the Archive product to
be a system corrected image, using all available,
also post-acquisition information, for the Azimuth
compression and look-summation, would mean a very
heavy processing load which could presumeably not
justify the advantages.
6. It is proposed that all auxiliary data, pertaining
to an Archive tape is stored on a CCT, including
all additional information gained during QL generation,
screeing, from post-acquisition calculations on
attitude and orbit etc. This is to be considered
against having the data stored on the HDDT together
with the SAR data.
Some, more or less obvious, remarks on this are:
- It is not possible to erase part of an
HDDT, so updating is not directly possible.
- It is very difficult or impossible to recover
from bit errors in such kind of data.
- Many compatibility problems (different
missions, different users) can be alleviated
by proper information on the CCT
- Catalogue information can conveniently
and safely be conveyed on the CCT to the
Central Archive.
7. The acquisition of auxiliary data, comprising the
reception from various sources, storage into a
suitable form onto a suitable media in due time,
is one of the functions which could strongly influence
the delay of SAR products. Taking into account
the envisaged organisation (fig. 3.1) of Earthnet,
it seems adequate to have all Earthnet gathered
information of this kind routed through the Mission
Management Centre, rather than having direct link
to the source.
One disadvantage is the inherent additional delay
caused by an added link in the communication path.
However, there is no doubt that it is technically
feasible to keep this added delay down to a non-significant
value, due to the moderate amount of data.
The advantages gained are:
- Number of acquisition station interfaces
is reduced.
- Homogeneity of data for the processing
for the different stations is assured.
- Part of the aux. data processing could
be done centrally rather than duplicated
on each station.
- Since the first period will be pre-operational,
a considerable amount of monitoring and
adjustments is expected.
Such activities are most efficiently conducted
in a centralized approach.
It is believed that the advantages gained by a
centralized approach makes this the best solution.
This leaves a separate task of setting up adequate
procedures for assuring low delays.
8. The output from the Acquisition Station is primarily
- Quick-Look products (CCT, Film)
- Archive tapes (HDDT, CCT)
The dissemination of both types of products poses serious
problems of keeping up with the desired maximum delays.
According to the FEDs study, phase II report (litt.ref.
10), Hybrid Solution, the cost of transmission links,
whether terrestrial or satellite, is a significant
fraction of the total running costs for SAR products.
However, the only way of meeting the delay goal for
QL products is by transmitting the data over expensive
high bit rate data links, whenever high resolution
imagery is required within a few hours.
But for the Archive tapes, there exists the possibility
of sending the tapes by courier, using normal flight
connections.
Whether this method is reliable enough (time-wise)
has still to be investigated.
Considering a daily courier service, the normal delay
could probably be kept less than 36 hours.
7̲.̲ ̲ ̲S̲A̲R̲ ̲P̲R̲O̲C̲E̲S̲S̲I̲N̲G̲ ̲S̲Y̲S̲T̲E̲M̲ ̲C̲O̲N̲F̲I̲G̲U̲R̲A̲T̲I̲O̲N̲
Based on the requirements, previous discussion and
general knowledge on presently or near-future available
equipment, the configuration of figs. 7.1 and 7.2 was
selected as the baseline for this study.
The large amount of data and the very high bit rates
pose serious constraints on the design.
It would clearly be desireable to have the flexibility
offered by a disk-based buffer memory between the acquisition
and QL processing element, but the high bit rates and
large amounts of data means that `this flexibility
cannot actually be utilized with realistic disk configurations.
The concept has, therefore, been based on a smooth
data stream, sourced from an HDDT containing re-formatted
downline data (an archive tape), with a semiconductor
memory buffer between the source and the Quick-Look
processor.
7.1 D̲E̲S̲C̲R̲I̲P̲T̲I̲O̲N̲ ̲O̲F̲ ̲S̲Y̲S̲T̲E̲M̲ ̲E̲L̲E̲M̲E̲N̲T̲S̲
7.1.1 C̲e̲n̲t̲r̲a̲l̲ ̲C̲o̲n̲t̲r̲o̲l̲ ̲a̲n̲d̲ ̲P̲r̲o̲c̲e̲s̲s̲i̲n̲g̲ ̲S̲y̲s̲t̲e̲m̲
This system performs the following tasks
- Processing of auxiliary data (Front-End processing)
- SAR data processing
- Cataloque updating
- High-level control, monitoring and timing of
acquisition, archiving and dissemination subsystems
- Coordination and scheduling of activities for
performing the tasks.
It is our experience that the advantages offered by
a centralized and distributed system can be combined
if the system is made up of subsystems which are self-contained
in the sence that they can be operated and used locally
and independently to perform all important functions,
while the normal operation is controlled centrally.
The HOST computer which may turn out to be divided
into two separate computers, interfaces to
- SAR Processing Unit and performs all control,
monitoring, supply of processing parameters
etc. necessary for the SAR processing.
The processed output is conveyed through the
HOST to the Dissemination subsystem.
- Acquisition subsystem for setting up of timing
and orbit information and synchronisation of
replay function with the Archiving subsystem.
- Archiving subsystem.
Selection of scenes to be archived is made
by the HOST computer and status on bit error
rate etc. is collected.
The HOST computer supplies also the identification
labels to be used. The replay for QL processing
is initiated and synchronized through this
interface.
- Dissemination Subsystem.
The Dissemination subsystem, being organized
around a file processor, receives the processed
SAR data and high level commands for output
generation.
The link is also used for storage and retrieval
of data from the disk file.
- Data link interface to MMC for informations
exchange.
- Disk and CCT, used for e.g. program storage.
- Operators Console
The operator interface includes a CRT colour display
of current activities and configurations.
The Host Computer shall, to the extent possible, aid
in the timing and coordination of tasks in order to
efficiently utilize the resources available.
7.1.2 A̲c̲q̲u̲i̲s̲i̲t̲i̲o̲n̲ ̲S̲u̲b̲s̲y̲s̲t̲e̲m̲
The acquisition subsystem shall perform
- Antenna pointing control
- Receiver monitoring and control
- HDDR monitoring and control
All functions can be controlled from a local console
for maintenance and degraded-mode operations.
7.1.3 A̲r̲c̲h̲i̲v̲i̲n̲g̲ ̲S̲u̲b̲s̲y̲s̲t̲e̲m̲
THe Archiving Subsystem performs the two tasks in parallel:
- Recording of Archive tapes
- Retrieval
7.1.3.1 R̲e̲c̲o̲r̲d̲i̲n̲g̲
During recording, data are simultaneously replayed
from the Acquisition HDDT and recorded on the Archive
HDDT.
The SAR data are frame synchronized and de-multiplexed
into a file and block oriented format.
Each file representing one scene, divided into blocks,
each representing one SAR return.
The first file is a label, identifying the tape.
All blocks are supplied with consecutive block number
and file identifier.
This identification is duplicated onto a low density
search track for fast search. Non-usable or non-useful
data are rejected scene-wise at this stage. Auxiliary
data are extracted and transferred to the Host computer
via the direct link.
7.1.3.2 R̲e̲t̲r̲i̲e̲v̲a̲l̲
During retrieval, data are re-played selectively, scene
by scene at constant speed.
A semiconducter memory acts as buffer memory to the
Quick-Look processor.
7.1.4 D̲i̲s̲s̲e̲m̲i̲n̲a̲t̲i̲o̲n̲ ̲S̲u̲b̲s̲y̲s̲t̲e̲m̲
The Dissemination Subsystem generates the output products.
The QL image is intermediately stored on a disk.
At this stage, data rates and volumes have been reduced
such that it can be handled by available I/O oriented
mini-computers and normal peripherals.
The functions to be performed are:
- Generation of CCT's with QL imagery
- Generation of CCT's with Aux. data
- Generation of high resolution film
- Supply of data to the high speed data link
- Supply of data to the Quick-Look display.
The CCT's could be normal 1600 bpi, 9 track devices.
The photoplotter should be a medium-resolution (10
lines/mm laserplotter, able to generate 200 x 200 mm…0e…2…0f…
film.
The Quick-Look display should be a high resolution
colour display, augmented with processing and memory
capacity for performing:
- False colour enhancement
- Histogram analysis and display
- Overlag
- Etc.
7.2 R̲E̲Q̲U̲I̲R̲E̲M̲E̲N̲T̲S̲ ̲T̲O̲ ̲S̲Y̲S̲T̲E̲M̲ ̲E̲L̲E̲M̲E̲N̲T̲S̲
The requirement for Quick-Look delay of 3-6 hours leads
to a requirement for 24 hours/day operation for the
Acquisition station. Thus, the average over one day
gives a rough, yet indicative figure for the requirements
to the Input/output and processing speeds of the subsystems,
taken for the most demanding station, the Northern
station.
- Acquisition subsystem
Input peak: 100 Mbit/sec.
Mean: 9 mbit/sec.
Output Peak: 1000 Kbytes/sec.
Mean: 700 Kbytes/sec.
- Archiving subsystem
Input peak: 1000 Kbytes/sec.
mean: 510 Kbytes/sec.
Output (HDDR) peak: 300 Kbytes/sec.
mean: 220 Kbytes/sec.
- Quick-Look SAR processor
Input peak: TBD
mean: 220 Kbytes/sec.
Output peak: TBD
mean: 6 Kbytes/sec.
- Dissemination Subsystem
Host computer
link mean: appr. 10 Kbytes/sec
CCT mean: 7 Kbytes/sec.
10 min/scene
Photoplatter mean: 7 Kbytes/sec.
10 minutes/image
High speed data link mean: 2.3 Kbytes/sec.
peak: 5 Mbit/sec.
As a working assumption, it is estimated that one third
of the QL products are transmitted directly to the
users via data link.
