Permanent GNSS/GPS Station Planning: Technology, Equipment, Costs

Site Foundation

• Local geology – consider proximity to faults, landslides, subsidence areas, etc.

• Bedrock foundation is preferred

• Soil or alluvium options require more extensive monumentation

• Roof tops are sometimes the only option -- near support structures on bearing wall buildings is best, if roof top location is required

Horizon mask

• Rule of thumb: minimize obstructions above 15°

Site security, ownership and permission

• Location must be secure and viable over long term

Multipath

• Rule of thumb: site should be at least 15 meters from reflective sources

• Antenna height: at ground seems good, but observed multipath high at some sites with ~.5 m height

• Avoid creating cavity between backplane and monument top

• Researchers are working on methods for calibrating site multipath -- one example is the AMCS project

Radio Frequency Interference (RFI)

• Rule of thumb: site should be at least 1 km from powerful microwave sources, independent of the frequency that they operate at

Co locations with other instrumentation

• Co-location with VLBI or SLR systems is desired at core global reference stations

  • High precision tie survey essential, but is difficult and requires specialized knowledge and capabilities – contact support@unavco.org for more information

• Site infrastructure (site location, power and communications) can be shared with nearby seismic stations

  • Requires coordination with operator such as IRIS

Data quality assessment can be accomplished using the UNAVCO TEQC toolkit

More Information on Site Reconnaissance and Security from the UNAVCO Knowledgebase

• Short and deep drilled braced monuments are preferred by many

• Pillars are easier to build and good under the right conditions – avoid top of pillar effects on GNSS signal from rebar or metal mounting plates embedded in the pillar

• Reference marks and site/monument stability surveys are desired at core stations, such as IGS global stations

• More information on monumentation from the UNAVCO Knowledgebase

• More information on monumentation from the IGS web site

Tracking Performance

• General:

  • Dual frequency, many independent channels, up to 20Hz sampling, ~ 1 mm phase precision

• GNSS observables

  • Current:

    • GPS L1 C/A, L1and L2 P, L1 and L2 phase (also under AS)

    • GLONASS L1 C/A Code, L1 P and L2 P, L1 and L2 phase

  • Coming soon:

    • GPS L2C, L5 – first launches in 2005-2007, IOC/FOC in 2012/2015

    • Galileo – IOC/FOC 2010+ (anticipated)

• Performance measures

  • 99%+ of expected data

  • Cycle slips/observations <0.1%

  • MP1 and MP2 <0.5 m at elev. >10°

  • Zero baseline phase precision < 1 mm

  • Short baseline precision 2 mm horizontal/4 mm vertical

Features and Specifications

• 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

• Highly reliable: Mean Time Between Failure (MTBF) ~60,000 hours

Commonly Used Models

• Link to UNAVCO Member equipment purchasing information 
• Link to UNAVCO receiver test reports 
• More information on receivers available from UNAVCO

• Stable, well defined phase pattern -- consistent between like models

• Backplane that rejects multipath

• Absolute calibrations of antenna and radome pair are now considered standard by IGS

• Current best practice: Gold standard is still the choke ring design with D&M element, which is produced by several manufacturers, but many question whether extra cost is worth it and are going with less expensive models

• More information on antennas available from UNAVCO

• Securely attach antenna to monument/tamper resistant

• Ability to center, level and orient antenna in azimuth

• Reduce potential for multipath by minimizing surface area (do not create resonant chamber behind antenna ground plane)

• Current best practice: SCIGN mount or similar

• More information on antenna mounts used by UNAVCO

• Material should be homogeneous and of uniform dimension

• Hemispherical shape with center of curvature at average (absolute) L1/L2 phase center Radome should be calibrated along with antenna

• Current best practice: Do not use radome unless required for weather, debris or vandal protection

• If used with a choke ring antenna, UNAVCO typically uses the SCIGN tall model

• More information on antenna radomes used by UNAVCO

• Met – surface pressure and temperature at the GNSS antenna is required for water vapor applications

• Tilt – site and monument stability, slope monitoring, volcano deformation

• Data are typically logged within the GNSS receiver

• More information on meteorological sensors used by UNAVCO

• Power budget is typically 6-20 W, depending on receiver model and communications

• Options include:

  • AC power with battery back up

  • DC with solar or wind generation systems

• More information on power systems and lightning protection used by UNAVCO

• Securely house all station equipment

• More information on equipment enclosures used by UNAVCO
  

• Communications solution depends on data requirement

• Options range from analog modems over circuit switched (telephone) networks to broadband satellite

• Use of public Internet is generally preferred

• More information on data communications equipment used by UNAVCO
  

• The following tables list data communications requirements for different GNSS applications and technology options to satisfy requirements

• Data Retrievals

  • Method depends on receiver type and communications

  • IP scripting for Internet capable receivers like NetRS

  • Batched and streamed protocols and data formats

• Performance Monitoring and Verification

  • Outage notification via Internet

  • For batched retrievals, file size check before retyring

  • Verification using TEQC

  • More sophisticated approach for streamed data (RAIM or other)

  • Metrics include expected vs. observed data, cycle slips, multipath, position residuals, retreival latency

• Data Available at Public Archives

  • UNAVCO, SOPAC, CDDIS