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Optical communication as a driver for a data-centric

ground network service

SpaceOps Workshop 2013-06-11

Petrus Hyvönen, SSC, Sweden

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SSC as ground service provider

• SSC has been providing ground services to companies and agencies for 50 years

• Today a global network, PrioraNet is located at 10 sites with 34 operational antennas,

of which 14 are utilized as multi-mission

• PrioraNet is used by companies and agencies, including ESA, NASA, CNES, DLR,

CTLC, JAXA,…

• The PrioraNet network serves both polar and equatorial missions in Routine and

LEOP phases

• Today the service is RF based

• S-band for TT&C

• X-band for payload data acquisition

• Basic concept: Sharing of Resources between many missions

• Designed to maximize the use of ground resources

• EO pass is about 11 minutes each 90 minutes

• LEOP using the network during a limited time

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PrioraNet

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The TESLA optical link project

• SSC involved as a reference operator in

the RUAG-SZ led TESLA optical link

project

• Engineering Model Development ongoing

for space terminal and optical ground

station

• Flight model ready in 2016

• 1550 nm laser, on-off keying gives robust

link

• Space Terminal for small LEO satellites

• 5 kg mass

• 45 W power consumption during downlink

• 2 Gbps user downlink data rate

• Ground Terminal Prototype

• 0.6 m diameter telescope

• Bidirectional asymmetrical link needed for

link signaling but can also upload

commands

• Minimum elevation 20 deg above horizon

for prototype

TESLA Ground station protype at RUAG-SZ

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From prototype to operational

Left: „Double Tube“ Right: „Fork Mount“, dual instrument

• Future system upgrades achieved by adding secondary (telescope) payload to same mount

• Today 80 cm telescopes available at similar price as 60 cm prototype telescope

• 80 cm telescope allows lower elevation angle down to 15 degrees (about 1.5x longer downlink time)

OB + Elec 1 OB + Elec. 2

OB + Elec 2

OB + Elec 1

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Baseline Scenario for discussion

• Earth Observation satellites in

Sun Synchronous orbit (polar

orbit)

• Onboard memory for data

storage

• Optical communication for high

data rate payload downlink

• Limited but existing uplink

capabilities on optical link, such

as the TESLA link

• RF for TT&C, Emergency and

LEOP

• Optical ground network of

several stations, serving several

users

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General overview of operational concept in RF

• Service Level Agreement defined in contract with satellite owner / operator

• Rough planning of supported passes 1-2 weeks ahead

• Detailed schedule and pass confirmations made 2 days ahead

• For payload data, spacecraft is often pre-programmed to start data

transmission by time tagged commands

• Very high pass success rate on RF, outages are mainly technical failures,

environmental factors hardly influence successful pass rate

The process is mature, very well defined, and repeated literally hundreds of

times per day. The downside is that it is not easily changed.

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Main operational Difference optical – current RF

• Sensitivity to Weather

• No / Part / Full communications

during pass

• Communications intermittent

during pass

• Higher elevation than RF needed

• 15-25 deg compared with 5 deg

for RF

• Local weather, season, can strongly

influence link probability

These factors have profound effects on

the needed operational principles..

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Historical Cloud Coverage Analysis results in a

distribution of daily contact durations

• Clear sky availability analysis

performed in TESLA project by GMV,

SA, Spain, based on CDFS-II cloud

data base

• Statistical evaluation of clear sky

conditions at 7 selected sites including

PrioraNet sites for the years 2002-

2010

• Reference parameters • 700km SSO

• Min elevation 25 deg

• Solar exclusion angle +-10 deg

• Histogram shape allows Gaussian Fit

for network; fits well to Binomial

distribution model ~33% cloud

probability per single site 10 20 30 40 50 60 70 80 90

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04

[min] contact per day

De

ns

ity

7 sites PDF and Gaussian fit, =44.3841 sig=11.1482

7 sites data

Gauss fit

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PrioraNet Station Cloud Statistics

Esrange, Northern

Sweden

Santiago de Chile Western Australia

Space Center

Altitude 341 m 520 m 34 m

Day Mean Cloud

Coverage

59%

34%

34%

Night Mean Cloud

Coverage

55% 33% 28%

Max Contact Minutes

per day*

25.5 min

10.8 min

11.2 min

Mean Contact

Minutes per day incl.

Cloud coverage*

9.9 min

6.2 min

6.3 min

Mean Tbit/day* 1.2 0.9 1.0

*System estimates with a 700 km SSO, TESLA prototype performance OGS (60 cm telescope, 25 deg min

elevation).

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How to operate within this limitation

• Forget passes

• Forget predicted scheduling

• Focus on data delivered not individual stations

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Passes

• The concept of operations by passes as the

main quantifier needs to go away • Pass can be Completely cloudy

• A pass can be partly cloudy

• Full pass

• No blind downlinks by time tagged commands,

all data downloads must be triggered

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Ground Network Metrics

• Data Capacity (TB / day)

• Optical strength

• Latency (minutes or hours)

• RF strength due to robust reception

• Can be partly overcome by using

priority on data level

• Direct coverage (Area)

• Coverage area depending on

elevation angle

• RF has larger coverage than optical

• Optical higher bitrate

Latency map for RF network

Capacity:

Punta Arenas 4.67 Pass/day

Esrange 9.50 Pass/day

Inuvik 9.67 Pass/day

WASC 3.00 Pass/day

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Service Level Agreement for Optical

630 km SSO, 7x 80cm OGS, 2x 1Gbit/s downlink, 33% cloud probability per site

6 Tbit/24 hrs at 95% probability

This would be the starting point for a SLA for a optical ground service.

0.0%

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90.0%

100.0%

0.00

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Pro

bab

ilit

y

Tb

it p

er

day

Sites with clear sky

Tb per 24 hours Probability of clear sky

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Focus on Data

• Focus on the data as

files / group of

packets

• Provide a service

based on priority of

data, can differ on

single mission and/or

between missions

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Scheduling of Optical Ground Stations

• Constant scheduling needed

(including what we today call

’rescheduling’)

• Knowledge at Network Management

Center (NMC) of current weather systems

and forecasts at the ground stations

• Flexible and adaptable priority scheme in

order to deliver latency sensitive data first

• Scheduling rules need to be established

based on some TBD data quantifier, file /

group of packets to ensure lowest latency

of usable products

• Ground network capabilities part of

scheduling algorithm

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Data handling & Dissemination

RF System

• Bent pipe data to customer

• For high data rates data is

temporarily stored at the receiving

ground station

• Pass based quantification of data /

file

• High bitrate communications from

ground station to customer

Optical System

• RF architecture based on pass will

not work

• Fragments of data spread among

different stations

• Received data is assembled from

multiple locations to produce a

deliverable product

• Data distribution either through

central repository or in bittorrent-

type

• Large datasets – don’t transfer

data you don’t need

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Satellite Impacts

RF System

• Satellite owner performs all

commanding, ground network

as bent-pipe

• Often time tagged commands

for data dumps

• Distinct separation of customer

/ ground network

Optical System

• Owner performs satellite

tasking, ground network

schedules its resources

• Satellite owner prioritizes

which data to downlink

• Ground network signals to

satellite when a resource is

available for downlink

• Onboard memory sized for

acceptable service level

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Service Formulation

• The service formulation should be clear to the satellite owners in order to

design the on-board memory accordingly

• The ground network provider needs clearly set requirements on how to

operate the system in terms of allocating ground resources to the different

satellite users.

• Detailed simulations are needed to find the quantitative expression but a

starting point should be that the system delivers for example 12 Tb of data

to a certain network location, over 48 hours, with a probability of 95%.

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What about upcoming RF

• High bitrates also coming for Ka-,

K-band (26 GHz) LEO

• K-band (from ESA EUCLID info):

• Full loss (with any stronger rain,

possibly with any rain, depending

on the link margin), expected for

1% to 2% of the time.

• Intermediate/scintillation range

with cut off of the scintillation

frequency at ~10 Hz. (i.e. link

conditions can be assumed as

constant over 0.1 s).

• Good range (here the K-band link

works as well as any other radio

link), expected for 90% of the

time.

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Ka-band properties

• Ka band usage in LEO is different from high elevation GEO applications

• LEO tests with the Australian FedSat mission (20 GHz)

• Atmospheric channel distortions at low elevation angles are pronounced in

Ka band, requiring higher elevation angles (>10° ?) • significant increase in high-frequency fading

• terrestrial multipathing limits the spectral efficiency

• Weather dependency

From: ”Ka Band Propagation Experiments on the Australian Low Earth Orbit

Microsatellite ‘FedSat’”

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Optical as a driver for next generation

ground network service?

• The characteristics of weather dependency is not only

for optical – also higher RF frequencies

• The current S- and X-band operational pass based

concept might not be suitable for high frequency RF or

optical

• Although less sensitive than optical, the proposed

service type would be suitable for RF and potentially

advantageous

• Optical can be a driver of this transformation with the

necessity of new operative concepts, new protocols,

and architectures

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Questions

• New operational concepts, but one operative system

that supports both optical and future RF: • Division of functionality between network provider and satellite operator?

• Service Management Interfaces?

• Service Delivery Interfaces?

• How do we design these interfaces and what protocols

are needed / selected? (DTN, CFDP, Bittorrent…)

• How do we ensure that the two segments space and

ground are designed and optimized as one system? • Downlink requirements

• Onboard storage

• Near reception processing

• …