1

IAC-12-B4.1.6 MULTISTATIC SMALL SAR SATELLITE NETWORK FOR OIL MONITORING IN

NIGERIA

Abdul Duane Lawal

Space Advanced Research Team, Aerospace Department University of Glasgow, G12 8QQ,

a.lawal.1@research.gla.ac.uk

Radice, G

Space Advanced Research Team, Aerospace Department University of Glasgow, G128QQ

Gianmarco.Radice@glasgow.ac.uk

The requirement for more sophisticated methods for monitoring the widespread oil resources within Nigeria has

been evident. The use of Synthetic Aperture Radar (SAR) systems is particularly appealing since this payload is

unaffected by adverse weather conditions and independent of sunlight for its operation. The all-weather

capability of SAR makes it the most suitable remote sensing platform for monitoring environmental disasters

such as oil spillage which could be highly elusive to optical sensors such as those onboard NigeriaSat-2. The

application of interest for this proposed mission is the detection of oil spillage and assistance in monitoring

illegal oil bunkering activities within and around the Gulf of Guinea region. In this paper, we present aspects of

the mission design, including the mission objectives and a preliminary satellite design. The user requirement of

high resolution SAR images, near-real-time data, low revisit time and low mission cost are the main drivers of

the mission. The paper also describes the analysis for the selection of SAR frequency, consistent with providing

high resolution images and low spacecraft mass. A discussion of the trade-off analysis between a near polar

orbit and near equatorial orbit for reduced revisit time is presented. Altitude selection as a function of available

pulse repetition frequency (PRF), consistent with range and azimuth ambiguities is also highlighted.

Furthermore, to meet coverage and revisit time requirements with due consideration for cost of the mission, a

system consisting of a few number of small SAR satellites is described. Satellites within the constellation will

fly in close formation and maintain fixed across track and radial baseline distances that enable possible

interferometric applications. Finally, the paper will conclude with a brief description of the small SAR network

system operating solely within the equatorial region, for monitoring oil spillage disaster and oil bunkering

activities within and around the Gulf of Guinea.

mailto:a.lawal.1@research.gla.ac.uk

2

INTRODUCTION

The benefits of space generated data has seen wide

spread application areas [1, 2, 3], and has benefited

mankind for over five decades. Space missions

have provided support in areas such as disaster

monitoring and mitigation, telecommunications,

meteorological forecasting and Earth Observations.

However, even with the apparent contributions of

space science and technology in driving state-of-

the-art advancements, several nations are yet to

embrace grass-root space science and technology

as a possible solution for economic growth and

urban development.

Furthermore, the net contribution of space science

to education, offers a different dimension in regards

to the application of fundamental studies acquired

from educational institutions. The breadth of space

science spans across multi-disciplinary aspects of

education, encompassing also law, business studies

and medicine.

As a source of revenue, space business has become

an alternative to terrestrial solutions by providing

service packages over a plethora of business

models. The most common being the use of

satellite communications for the mass market

services such as television, telephony and

broadband. Others include the creation of niche

markets for radio, messaging (Little LEOs) and

imaging (Remote Sensing), as well as infrastructure

business by way of capacity leasing,

manufacturing, launch services and ground

equipment [5].

From a security point of view, the most developed

nations depend on their space resources to protect

their territorial integrity. This is epitomised by the

rate at which acquired satellite data is processed

into information and disseminated to the

appropriate authorities, thereby supporting the

process of quick and timely decision making.

Decisions taken are then relayed to commanders on

the battle field for onward implementation. In

addition to reducing troops causality rate, it also

increases their chances of victory [6].

The scientific community has continued to make

remarkable progress by using space research

programs in protecting humanity. Such examples

include the contributions of the European space

systems for monitoring climate changes and its

effects [7]. Others include the Global Monitoring

for Environment and Security (GMES) programme

used for managing natural resources and

biodiversity, with a recent mission adapted for real-

time fire monitoring using satellite imagery as one

of the sources of data into a GIS platform [8].

The high cost of space missions has been a key

factor to determining the nations that partake in

space activities. The Space Shuttle Endeavour, the

orbiter built to replace the Space Shuttle

Challenger, cost approximately 1.7 billion US

dollars [10], while a typical space mission program

can costs hundreds of millions of dollars [11].

Although the launch phase plays a huge role in the

determination of the mission cost as highlighted on

Figure 1, recent approaches have been taken to

reduce spacecraft mass. The design, launch and

operations of low cost, high data space missions

has recently become a niche market with

recognised companies like Surrey Satellite

Technology Limited (SSTL), UK considered as a

major stake-holder. With over 20 years of

experience in launching and operating highly

capable and cost effective small satellite missions,

with 39 missions recorded, SSTL provides access

to space for a fractional price normally associated

with space missions [14, 15].

Most nations yet to embrace the relevance of space

missions as a multi-dimensional avenue for

development, due to the relatively unaffordable

cost, are mainly classified as developing

countries. There are 156 countries classified as

developing nations according to the International

Monetary Funds World Economic Outlook Report

given April 2012 [17], with Cuba and North Korea

not included. According to Sheffrin et al (2003),

these countries are known as less developed

countries (LDC), with low living standards,

undeveloped industrial base, and low Human

Development index (HDI) relative to other

countries[16].

3

Figure 1: Spacecraft mass vs mission cost [11]

For the purpose of this paper, the Equatorial

Region (ER), is defined as the area geographically

located within 10 degrees of the Equator.

Subsequently, there are 49 countries located within

the ER with only Singapore classified as a

developed country [18]. Furthermore, in this

region, only Brazil, Indonesia, Malaysia and

Nigeria have space capabilities at different levels

defined under the framework of four major

technology categories as defined by Wood, D.

(2011).

To address the highlighted issues preventing most

developing nations from acquiring space

capabilities, this paper proposes an approach that

encourages the collaboration between countries

within the ER. The approach suggests the

establishment of a network of space systems

dedicated and operated by member states within the

ER. To define the overall mission concept, this

paper uses Nigeria as a case-study.

To this end, baseline mission objectives for

Master_SAR_01 (M01) will be outlined, leading to

a discussion on its mission design. A brief

description of pulse repetition frequency (PRF) and

orbit selection process is presented. Coverage

analysis comparing the use of a near polar orbit

(NigeriaSat-2) and a near equatorial orbit (M01) for

the proposed mission is conducted. Furthermore the

proposed sites for locating the ground segments for

the network will be highlighted.

Finally a system for a network of satellites

operating within the ER will be described.

MISSION OBJECTIVES

The recent environmental disaster experienced in

New Zealand as a result of a cargo ship running

aground, with over 340 tonnes of oil spilled is

regarded as New Zealands worst in decades [20].

In 2010, the Gulf of Mexico oil spill, also regarded

as the worst US environment disaster saw at least

20 million gallons flow into the Gulf of Mexico

and affecting more than 70 miles of the Louisiana

coastline. The accident that led to explosion and

fire which killed 11 people, caused wide spread oil

pollution in the Gulf of Mexico [21, 22, 23].

It is estimated that between 100,000 and 130,000

barrels per day (bbl/d) of crude oil, worth an

estimate of $3billion is stolen from Nigeria via

illegal oil bunkering activities within and around

the Gulf of Guinea [24]. The Gulf of Guinea

stretches through Central and West Africa and is

increasingly identified as one of the worlds most

poorly governed maritime stretches [25]. It has

been reported to be used for various forms of

nefarious activities such as human trafficking from

Senegal to Europe, drug trafficking and oil related

crimes [25, 26, 27, 28].

The capacity to monitor the local maritime traffic

and infrastructures involving oil exploitation rigs

and platforms provides addition benefits which

include: information on geographical location of

ships, ship size (resolution dependent), ship route

and speed.

With the stage set for a low-cost sophisticated

approach to addressing the aforementioned issues,

the use of a spaceborne remote sensing system for

complimenting terrestrial efforts has become

paramount. Although Nigeria current has 3

spaceborne optical remote sensing platforms

(NigeriaSat-1, NigeriaSat-2 and NigeriaSat-X),

their operation is limited by daylight and adverse

weather conditions. Additionally, to ensure rapid

response in the event of oil spills, and to maintain

continuous vigilance over the Gulf of Guinea, 24

hour surveillance is required.

The primary mission objective is the use of

interferometric operation for generation of high

resolution data for the Equatorial Region. The

following are the mission design drivers:

Low cost space mission

24 hour surveillance of Gulf of Guinea

4

Monitor oil resources/ detection of oil

spill

High resolution data

Near-real-time data acquisition

Low temporal resolution

M01 MISSION DESIGN

The baseline operational orbit is a circular LEO at

an altitude of 700km and inclination of 10 degrees.

The orbit and system parameters are listed in Table

1 [41].

Orbit and System Parameters

Altitude (m) 700

Revolutions/day 14.5

Inclination 10

Period (mins) 98.6

RAAN (deg) 0 Table 1: Orbit and System Parameters

The choice of PRF for a spaceborne SAR is an

important parameter that influences many other

system parameters. PRF selection is affected by

several factors which include antenna length,

incidence angle, swath width, platform velocity,

platform altitude and transmitter pulse length. The

system parameters affected by PRF include duty

factor, peak transmit power and raw data rate.

Ultimately, the selected PRF constraints must be

satisfied to ensure the combination of swath width

and antenna length are compatible by ensuring the

Nyquist requirement and ambiguity constraints do

not conflict [39, 40].

The baseline mode of operation for M01 is the

stripmap mode. To meet the Nyquist criteria, the

selected PRF must be greater than the Doppler

bandwidth of the imaged scene. The Doppler

bandwidth is given by:

[ ]

Where Vsc is the platform velocity, az is the half

power beamwidth angle and is wavelength.

The beamwidth is related to the antenna length by

[2]

Substituting [2] into [1] yields a minimum PRF

given by:

[ ]

where laz, is the antenna length in the along track

direction.

The maximum PRF dictates that the maximum

echo duration must be less than the interpulse

period. A system with pulse length p, and scene

echo duration s, the maximum PRF is calculated as

follows:

[4]

where s is given by

[5]

and, SWsr is the slant range extent calculated from

the difference Rf Rn [39]

Rf - is the slant range to the far end of the swath

Rn - is the slant range to near end of swath.

Although the selected PRF will between the

PRFmin and PRFmax, other factors such as eclipsing

and nadir returns must be considered. To avoid

eclipsing (blind ranges) resulting from isolation

issues inherent in radar systems, such that returning

echoes coincide with transmit times, the following

inequality must be satisfied [39].

[6]

where:

B are whole numbers (1,2)

corresponding to pulses.

The other issue that must be avoided are echoes

from nadir returns, since for every transmit pulse,

there is a nadir echo. These time of occurrence

nadir commences from the beginning of the

transmit pulse. nadir is calculated from

[ ]

Where h is the platform altitude above the nadir

point.

The PRF that coincides with returning scene echoes

is described by the inequality [39].

where:

5

N are whole numbers (1,2)

corresponding to pulses.

[8]

There are other approaches to avoiding nadir

echoes such as:

Changing the slope of the chirp waveform

polarity from positive to negative on an

interpulse basis.

Designing antenna to have low gain in the

nadir direction

Phase coding the transmit signal to ensure

filtering of energy from different transmit

pulses.

M01 SYSTEM PARAMETERS

Oil spill modifies surface tension and therefore it

has a strong impact on radar backscatter level [32].

The effect on the sea surface is the reduction of the

surface roughness by damping the waves. This

results in decreased radar backscatter which appear

as dark patches on SAR images when compared to

the surroundings [28, 29].

Several factors such as oil characteristics and

environmental factors play key roles in oil spill

detection in addition to the radar properties. The

choice of the operational frequency is informed by

the results obtained from studies which conducted

multi-frequency scatterometer measurements. The

results showed that as frequency increases, the sea

contrast ration of oily/clean sea also increases.

Therefore, for applications such as oil spill

detection, higher radar frequencies are expected to

provide better contrast ratio [30, 31].

Previous SAR missions like the Radarsat-2, ALOS

and Envisat missions have used the C-band

frequency for operations. However, with the recent

emergence of several X-band Earth observation

missions notably TerraSAR, COSMO SkyMed and

Tandem-X, it can be argued that better contrast

ration is amongst the factors influencing the

paradigm shift. For the chosen application area, this

paper selects X-band for its operational frequency.

Table 2 summarizes the characteristics and

corresponding values of the M01 system parameters.

System Parameters

Frequency (GHz) 10

Band X

PRF (kHz) 2.7

Bandwidth (MHz) 44

Polarization VV

Look direction Left

Antenna width (m)

Antenna length (m) 6

Incidence angle range (deg) 45

Resolution (m) 3 Table 2: M01 System Parameters

COVERAGE ANALYSIS FOR M01

Coverage analysis of the ER was first conducted

using one master satellite (M01) and then using two

master satellites (M01 & M 11). The simulation was

carried out over 7 days period, using Satellite Tool

Kit (STK), with granularity set to 2 degrees. Table

3 summarizes the result of the coverage analysis of

M01 over the ER. An average time of 12% is spent

covering each latitude range within the defined

region, thereby increasing the possibility of

increasing the number of looks at any area of

interest in addition to the benefits of short revisit

time.

Latitude

(deg)

Time covered

(%)

Total time covered

(mins)

-9 12.76 183.75

-7 13.20 190.15

-4 15.51 194.54

-2 13.69 197.14

0 13.76 198.08

2 13.71 197.38

4 13.54 195.02

7 13.26 190.89

9 12.83 184.78

Table 3: Summary of coverage analysis for M01 over the ER

Figure 2 show that M01 potentially has access to

100 percentage of the ER on a daily basis

depending on the sensor swath.

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Figure 2: Coverage analysis of ER using M01

Coverage over Nigeria

With Nigeria as the primary focus of this paper, a

coverage assessment was conducted over 7 days.

Figure 3 show that M01 makes contact with the

existing ground station in Abuja on each of its

passes. The average duration of each pass is 13

minutes. This is in contrast to the average of 4

passes made by existing missions such NigeriaSat-

2, whose mission involves global coverage.

NigeriaSat-2 is a near polar orbit at an altitude of

700km [37, 38].

Figure 3: Typical M01 daily access to Abuja groundstation

Therefore, for applications that do not require the

acquisition of global coverage data, a near

equatorial orbit is better suited when compared to a

near polar orbit. Table 4 compares the difference

between the access times to the Abuja

groundstation in Nigeria, between NigeriaSat-2

(near polar orbit) and M01 (near equatorial orbit).

Parameter NigeriaSat-2 M01

Payload MRI/VHRI SAR

Classification Passive Active

Period of operation Daylight 24 hours

Altitude (Km) 700 700

Inclination (deg) 98 10

Orbital period 98 98

Average number of daily passes 4 14

Average duration of pass (min) 10 13.5

Total average daily access (min) 40 189

Table 4: Comparing daily access to groundstation in Abuja, between NigeriaSat-2 (near polar orbit) and M01 (near equatorial

orbit)

Improving Coverage of Nigeria

The busy nature of the Gulf of Guinea and the

territorial waters around Nigeria owing to seafaring

oil related activities prompts the need of frequent

surveillance. The approach taken to meet this

requirement involves the introduction of M11, an

identical monostatic SAR satellite to M01 in

configuration. M11 also shares the same orbital

elements as M01, but are separated by mean

anomaly of 180 degrees.

Figure 4: Coverage analysis of ER using M01 and M11.

Figure 4 shows that the introduction of M11, flying

in the same orbit as M01, doubles the percentage of

time available for covering the ER when compared

to Figure 2.

Furthermore, the access time to the Abuja

groundstation in Nigeria also increases two-folds

with a total of 28 passes and a total average time of

364 minutes daily. Figure 5 outlines a typical

access to Abuja groundstation for both M01 and M11

on a daily basis. Each pair of passes per orbit is

separated by approximately 49 minutes.

Figure 5: Typical daily access to Abuja groundstation for M01

and M11

CONSTELLATION DESIGN

7

The requirement for a low cost mission capable of

providing all-weather imaging and high revisit time

while exploring the possibility of collaboration

with other developing nations prompted the choice

of using a constellation of several small SAR

satellites flying in close formation. The

implementation of a semi-active SAR configuration

provides an avenue to reduce launch cost, by using

receiver-only platforms within the constellation. It

is envisaged that the receiver-only (slave) platforms

will invariably weight less in mass.

Another benefit of proposing a constellation is the

opportunity to promote cooperation between

several nations within the ER. This could

potentially involve pooling together various

resources such as: personnel, ground segment

location and equipment as well as space segment.

The overall cost of mission is then shared with

maximum benefit of various forms of data for

several application areas leading to a Data-Full

consortium.

The constellation includes two groups of multi-

static satellites in a pendulum configuration. The

pendulum configuration has been reported to be

well suited for providing multiple baselines at fixed

baseline ratio along the whole orbit cycle [34, 35,

36, 42]. The configuration allows the independent

selection of along-track baselines and is well suited

for velocity measurements. The pendulum

configuration consists of three slave satellites with

the same inclination but different values of right

ascension of ascending nodes (RAAN). For

operational safety, it is typical to apply slight

offsets to the eccentricity vectors when flying near

polar orbits, as this ensures vertical separation at

the poles. For near equatorial region applications,

this eccentricity difference may not be necessary,

as the spacecraft maintain their relative distance

and orientation through the orbit cycle when flying

in close formation. Figure 6 highlights the

pendulum configuration.

Figure 6: Pendulum configuration showing Group 0 flying over

the Equatorial region

Each group consists of one Master satellite (Mnm)

and three Slave satellites (Snm). Where n is the

satellite number within a group and m is the

group within the constellation. Table 5 summarizes

the satellite constellation orbit parameters for group

0. However, group 1will have the same parameters

but with true anomaly offset by 180 deg.

Orbit & System

Parameters

M01/11 S01/S11 S02/12 S03/12

Altitude (km) 700 700 700 700

Revs/day 14.5 14.5 14.5 14.5

Inclination () 10 10 10 10

Period (mins) 98.6 98.6 98.6 98.6

RAAN () 0 0.00752 0.00182 0.00282

Mean Anomaly () 0 5 5 5

Table 5: Constellation orbit parameters

The satellites in each group of the pendulum

formation are evenly distributed along two 2 major

orbits. One orbit consists of the master satellites

M01 and M11 separated by 180 degrees mean

anomaly. The second orbit contains six slave

satellites, with S01, S02 and S03 separated from S11,

S12 and S13 by 180 degrees mean anomaly

respectively. Figure 6 shows a simulation of group

0 satellites flying in pendulum configuration over

the ER.

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Figure 7: Simulation of in-orbit pendulum configuration of group 0, showing M01, S01, S02 and S03

GROUND SEGMENT SITES

As mentioned in section 1, there are over 45

countries within the ER, with only 5 selected as

possible locations for ground segment sites. These

locations are subject to flexibility although certain

criteria such as having existing space agency, and

proximity to Equator were considered. Figure 8

shows a Northerly oriented view of the locations of

the selected groundstations.

Figure 8: Selected locations for Ground Segment View from the North Pole

The distance between subsequent locations varies;

however, the maximum time between successive

contacts with groundstation must be less than 10

minutes. Table 6 details the location of the selected

ground station sites. Sites can be flexibly selected

provided the out of communication time with

satellites is less than 10 minutes.

Country Latitude Longitude

Accra 5 30' N 0 10' W

Cayenne 4 56' N 52 20'W

Mogadishu 2 04' N 45 22'E

Singapore 1 17' N 103 51'E

Tarawa 1 19' N 172 58'E Table 6: Location of selected ground segment sites with ER

Figure 9 shows a screen capture of the simulation

conducted over groundstation locations.

Figure 9: Ground segment location showing M01 making contact with groundstation in Accra

Access to selected Ground Segment Sites

The access to the selected ground segment location

allows each group of the satellite constellation to

always be in view of one ground station.

Figure 10: Access report of M01 to selected ground sites within

ER

Figure 10 summarizes a daily access report for M10

to each ground segment. A total of 70 accesses

daily are available to download captured data, or

upload telecommand to each spacecraft as required.

This configuration serves to usher a new approach

in data generation.

9

A line of sight constraint of a minimum of 6

degrees elevation angle is applied to each

groundstation.

CONCLUSION

The enormous benefits of oil to Nigeria were

highlighted, and so were the potential hazards if not

properly monitored. The current loses incurred due

to inadequate surveillance of Nigerias oil resource

were also mentioned. The approach adopted to

meet a series of developed mission objectives, was

the use of a SAR satellite for surveillance of illegal

oil activities, and the detection of oil spills within

the territorial waters.

The benefits include over 20% increase access time

to the existing ground station in Nigeria when

compare to NigeriaSat-2 mission, as well as the

possibility for 24 hours surveillance divulge of

lighting or bad weather conditions. Furthermore,

the potential to cover the territorial waters where

oil resources are located in every pass exists.

Other neighbouring countries in similar

circumstances were identified using classification

by latitude. An approach to solving Nigerias

pending problem as well as meeting several other

mission objectives was suggested. This is the use of

a Multisatic SAR system dedicated to monitoring

the equatorial region, which Nigeria lays herein.

An approach suggesting the formation of a

consortium of developing nations within the ER

was also proposed. Advantages of the consortium

include further reduction in cost of space mission,

due to shared cost, availability of cheaper data,

availability of a variety of data, and opportunity to

apply the data for various form of development.

Five sites were selected as locations for ground

segments, which will facilitate a nearly 24 hours

availability of data with a total of 70 access passes

to the ground segments.

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http://www.sstl.co.uk/news---events/2012-news-archive?story=2054http://www.sstl.co.uk/news---events/2012-news-archive?story=2054http://www.sstl.co.uk/news-and-events?story=2046http://www.sstl.co.uk/news-and-events?story=2046http://www.bbc.co.uk/news/world-asia-pacific-15251319http://www.bbc.co.uk/news/world-asia-pacific-15251319http://www.whitehouse.gov/blog/2010/05/05/ongoing-administration-wide-response-deepwater-bp-oil-spillhttp://www.whitehouse.gov/blog/2010/05/05/ongoing-administration-wide-response-deepwater-bp-oil-spillhttp://www.whitehouse.gov/blog/2010/05/05/ongoing-administration-wide-response-deepwater-bp-oil-spillhttp://www.bbc.co.uk/news/10194335

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