Addis Ababa, Ethiopia, 02 May 2011 1

AFREFIGS Guidelines / Station Requirements

Rui FernandesDI-UBI, Univ. Beira Interior, Covilhã, Portugal.

Instituto Geofísico D. Luíz, Lisboa, Portugal.

DEOS, Delft Univ. of Technology, The Netherlands.

Addis Ababa, Ethiopia, 02 May 2011 2

Requirements – Site Selection The Big Five (criteria)

2Km

- VisibilityIdeally: No obstruction above horizon in any direction.Acceptable: No significant obstructions above 10º in any direction.

- SecurityProtection of antenna against robbery or vandalism acts.Reserved access to the receiver.

- Electric PowerIdeally: Access to reliable public electrical network.Alternative: Autonomous systems (e.g., solar panels).

- InternetIdeally: Continuous and reliable access

- Monument StabilityIdeally: rigidly connected to the ground

REQUIRED BY ALL APPLICATIONS

DEPENDENT OF THE APPLICATION

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Monumentation and mount

Monuments and Site Stability

• Bedrock foundation preferred, but rooftops sometimes the only option

• Short and deep drilled braced monuments are preferred by many

• Pillars are easier to build and good under the right conditions – top of pillar effects on GNSS signal

• Reference marks and site / monument stability surveys are desired at core stations

Antenna Mounts• Securely attach antenna to

monument/tamper resistant• Ability to centre, level and orient

antenna in azimuth• Reduce potential for multipath by

minimizing surface area • Current best practice: SCIGN mount

or similar

Addis Ababa, Ethiopia, 02 May 2011 4

Location

Site security, ownership and permission

• Location must be viable over long term

Horizon mask• Rule of thumb: minimize obstructions

above 10°

Multipath• Rule of thumb: site should be at least 15

metres from reflective sources• Antenna height: at ground seems good,

but observed multipath high at some sites with ~0.5 m height

• Avoid creating cavity between ground plane and monument top

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Requirements Monument Stability

2Km

Application Level

Tectonic Motions and Vertical MonitoringSub-

centimeter

Early Warning Systems (Tsunamis, Volcanoes)

Sub-centimeterCentimeter

Real-Time Products (Orbits, Surveying) Centimeter

Reference FramesSub-

centimeterCentimeter

Meteorological Applications Decimeter

Ionospheric Monitoring MetersProper Self-Center Mounting Device is fundamental

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Power and protection

Power Systems• AC power with battery back-up is preferred• DC solar systems are feasible where AC is

unavailable• Power budget is typically 6-20 W,

depending on receiver model and communications

Lightning protection• Options for antenna, power and

communication connections

Enclosures• Climate controlled building is best• Securely house all station equipment

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Communication and ancillary

Communications • Communications solution depends on

data requirement• Options range from mobile networks to

broadband satellite• Use of public Internet (DSL) is generally

preferred

Ancillary Sensors• Meteorological – surface pressure,

humidity and temperature at the GNSS antenna are required for water vapour measurements

Addis Ababa, Ethiopia, 02 May 2011 8

Requirements Permanent access to Internet

2Km

ApplicationRequired?

*

Tectonic Motions and Vertical Monitoring No (Local Storage)

Ionospheric MonitoringYes

No (Local Storage)

Reference Frames Yes

Early Warning Systems (Tsunamis, Volcanoes)

Yes

Real-Time Products (Orbits, Surveying) Yes* In order to properly monitor the station remotely and

automatically, the internet access is always desirable.

Addis Ababa, Ethiopia, 02 May 2011

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Communication requirement

Level 1: •15-30 second sampling, daily to hourly retrieval, many hours latency•Precise orbits, reference frame, tectonics, ground based PWV/climate

Level 2: •15-30 second sampling, 10-60 minute retrieval, 10-60 minute latency•Ground based PWV/forecasting, ionosphere, space based PWV

Level 3: •10 Hz to 1 second sampling, streamed or small batch retrieval, several minutes latency, guaranteed delivery•Seismology (ground shaking and large event detection), tsunami warning, photogrammetry/airborne LIDAR•Real-time kinematic

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GNSS Receivers: Tracking Performance

General: Dual frequency, many independent channels, up to 50Hz sampling, ~ 1 mm phase

precision

GNSS observables Current:

GPS L1 C/A, L1&L2 P, L1&L2 phase (mandatory)GPS L2C, L5 (most modern receivers)

GLONASS L1 C/A Code, L1&L2 P, L1&L2 phase (most modern receivers)

Galileo – 2012+ (expected)

Performance measures99%+ of expected data

Cycle slips/observations <0.1%

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GNSS Receivers: Features• Power consumption 3-10 W• Memory up to many GB• Multiple I/O ports• Log and output multiple formats simultaneously• Raw, RINEX, BINEX, RTCM SC104, etc. • Command and control interface• Built in server technology supports http and ftp over TCP• Configuration over network by uploading configuration file• Serial commands and custom interface applications• Environmental specifications: -40 to +60 C, humidity

sealed• Power management• Ability to cycle power remotely and automatic restart in same

configuration after power loss • Ability to log and stream data from external sensors

(met, tilt)• Code and carrier multipath rejection and ability to

disable• External timing frequency input• Reliability: Mean Time Between Failure (MTBF) ~60,000

hours

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GNSS Receivers: Commonly used models

Commonly Used Models

Trimble NetR8/NetR9Topcon GB-1000/G3-A

Leica GRX1200Septentrio PolaRx

Costs (US$)Street Price: $15k-30kAcademic: $10k-15k

Large Quantity: $10k-15k

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GNSS Antennas

AntennasStable, well defined phase pattern – consistent between like modelsBackplane that rejects multipathAbsolute calibrations of antenna and radome pairCurrent best practice: Choke Ring design

Radomes – avoid use if possible!Material should be homogeneous and of uniform dimensionHemispherical shape with centre of curvature at absolute L1/L2 phase centre Radome should be calibrated along with antennaCurrent best practice: do not use radome unless required for weather, debris or vandal protection

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General AFREF station requirements

• Continuously operating• no end in operation expected• Communications• continuous internet access• Very stable monument• Availability of the data• data distributed freely in near real-time• Safe and secure instrument housing• Multipath-free installation site• Clear horizon to low elevation angles• Reliable electricity supply or UPS

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Issues concerning data availability

Not Public Available

e.g., Algeria, Angola

No Internete.g., EthiopiaMozambique Channel

Unreliable Internete.g.,

Many locations (but the situation is improving)

No IGS Conformale.g.,

SCINDA