Site Foundation

• Local geology – consider proximity to faults, landslides, subsidence areas, etc.
• Bedrock foundation is preferred
• Soil or alluvium options require more extensive monumnetation
• 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.orgfor 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 Characteristics

• 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 Monuments from the UNAVCO web site
• More Information on Monuments 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 reliabiable: Mean Time Between Failure (MTBF) ~60,000 hours

Commonly Used Models

• Ashtech MicroZ iCGRS
• Trimble NetRS
• Topcon GB-1000 or Odyssey-RS
• Leica GRX1200
• Septentrio PolaRx

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

• 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 available antennas

• 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

• 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

• 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

• 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

• Information on options for antenna, communications and power connections

• Securely house all station equipment
• Many options are available

• 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
• 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