Reliable long-term GNSS deformation monitoring starts with three site conditions: the monitoring point must move faithfully with the target, the reference point must remain independent of the monitored body, and the observing environment must remain usable and maintainable over time.
A site assessment determines whether those conditions can be maintained and identifies suitable equipment, installation work and field validation.
For satellite signals and error sources, see GNSS Systems, Observables and Positioning Principles. Reference-station distance is covered in Baseline Length and GNSS Relative Positioning.
The short answer: three conditions must hold together
Preliminary site categories
| Preliminary status | Typical situation | Recommended next step |
|---|---|---|
| Ready for planning | A stable-area candidate exists, the monitoring objective is clear, and sky view and maintenance appear workable | Compare monitoring and reference candidates, then plan trial observations and acceptance evidence |
| Needs mitigation | Partial obstruction, reflectors, height difference, communications or power constraints exist, but alternatives are available | Confirm the affected area on site and assess relocation, an additional reference, communications or power backup |
| Site visit required | No trusted stable area is known, the point may be disturbed, construction will continue, or the signal environment is unknown | Use a site visit, short trial or another sensor to establish the design conditions and cross-checks |
These categories route each site toward planning, mitigation or field confirmation. The project then defines acceptance criteria from the required displacement direction, time scale, data availability and external verification.
1. Define the measurand first
Before a visit, identify the target: a slope, dam, bridge, retaining structure or another deforming object. Is the project concerned with horizontal movement, settlement, uplift or a three-dimensional trajectory? Does the project need to capture sudden changes, day-scale change or a long-term trend?
Those answers determine where a monitoring monument must be connected, which part it should move with and which environmental changes could look like real deformation. Without a defined measurand, even high-quality equipment produces coordinates that are difficult to interpret.
2. Sky view is more than “some visible sky”
A receiver needs satellites distributed in different directions. Trees, terrain, buildings, steelwork and temporary machinery reduce usable sky and can make satellite geometry vary strongly over time.
The less obvious problem is multipath. A signal can reflect from a metal roof, guardrail, retaining wall, water or the ground before reaching the antenna. Reflections may create periodic or directional bias. More satellites do not automatically remove the reflection environment around the site.
IGS station guidance likewise highlights a clear horizon, pre-installation multipath and radio-frequency testing, and minimizing metal near the antenna [1].
3. The monitoring point must represent the target
An antenna mounted to a loose railing, a thin thermally expanding member or a location exposed to vehicles and construction may record mount motion instead of ground or structural deformation.
A site visit should check:
- the mechanical relationship between the monument and the monitored target;
- whether maintenance, vegetation control, lifting work or vehicles may disturb it;
- whether buildings, metal equipment, trees or temporary works may appear later;
- whether an antenna replacement can reproduce position, orientation and height;
- whether every change can be recorded with photographs, equipment metadata and time.
NOAA and IGS station resources also emphasize firm anchoring, low multipath, long-term survivability and repeatable installation [1][2].
4. The reference point must remain outside the affected area
Relative positioning measures the monitoring point against the reference. If the reference is on the same moving mass, subsiding area or unstable structure, its motion enters the result with the opposite sign.
A reference candidate should therefore be assessed for:
- location outside the expected deformation area;
- long-term foundation and geological stability;
- distance, height difference and observing environment relative to the monitoring points;
- the ability to check it independently through control survey, redundant reference or long-term series;
- the need for renewed verification after an earthquake, construction or antenna replacement.
Visual appearance alone cannot establish reference stability. The project needs a predefined method for detecting possible reference movement.
5. Power, communications, access and security are measurement conditions
A long-term station without reliable power, transmission, data retention or maintenance access can create persistent data gaps even when the signal environment is good.
Check:
- available power, backup and lightning protection;
- communications coverage and local retention during an outage;
- safe access for maintenance staff;
- exposure to disturbance, vandalism, water, falling rock or maintenance machinery;
- seasonal vegetation, snow, construction machinery or future structures that may alter sky view.
These are not secondary facilities questions. They are part of data availability and traceability.
What to prepare before contacting us
| Suggested information | Why it helps | If it is unknown |
|---|---|---|
| Site location and area map | Shows the monitored body, stable-area candidates and surrounding terrain | A schematic area without precise coordinates is enough for an initial discussion |
| Target components and expected movement direction | Tests whether monitoring points can represent the actual measurand | Mark the area and possible direction on photographs |
| Panoramic and close-up site photographs | Reveals obstruction, reflectors, construction and access risk | List it as an item to confirm during a site visit |
| Existing monitoring or control points | Assesses potential independent references and cross-checks | A description of the type and period is sufficient initially; raw data is not required |
| Power, communications and access limits | Assesses continuity and maintenance | Mark unknowns explicitly rather than guessing |
| Events and response time of concern | Separates long-term trend, day-scale change and sudden-event needs | Use operational language; no technical threshold is needed initially |
What an initial assessment should produce
A preliminary assessment turns the available information into a testable next step. A reasonable result should identify:
- monitoring and reference candidates with their principal risks;
- obstruction, multipath, height difference and future-environment risks;
- power, communications, access and equipment-security unknowns;
- recommended site visit, trial observation or cross-sensor approach;
- questions answerable during planning and questions requiring field data.
Later field validation and project design establish the final acceptance values, processing parameters and alert rules.
Project items to define after site assessment
Once site suitability is understood, field tests and the acceptance process define:
- Achievable absolute accuracy: establish a component-specific evaluation method from field tests and reference checks;
- Minimum detectable displacement: combine time-series variability, temporal correlation and the project’s false-alarm and missed-event risks;
- Near-real-time result timing: include the observation period, transfer, processing and quality review in a defined delivery condition;
- Alert and quality-decision rules: connect the monitored object, data states and decision needs;
- Final acceptance procedure: document trial conditions, evidence, decision steps and delivery format.
The defined measurand, field tests and acceptance evidence provide the basis for these five project items.
GNSS monitoring series
This article covers site suitability; the adjacent articles address baseline design and project acceptance.
Previous: Baseline length and data quality | Series 4 of 7 | Next: GNSS monitoring acceptance
References
- International GNSS Service. Station Operator Resources and Monumentation Recommendations. https://www.igs.org/station-resources/
- National Geodetic Survey. An Overview of Global Positioning System Continuously Operating Reference Stations. https://geodesy.noaa.gov/library/pdfs/GPS_CORS.pdf
- National Geodetic Survey. Guidelines for Establishing and Operating CORS. https://geodesy.noaa.gov/CORS/Establish_Operate_CORS.shtml
Frequently asked questions
Q: How should a site with trees or buildings be assessed? Assess the obstruction direction, time variation, reflection environment and alternative mounting points. When photographs leave a condition uncertain, a site visit or trial provides the needed evidence.
Q: How can a roof be assessed for a reference station? Check the foundation, load-bearing member, thermal motion, reflective surfaces, future construction and whether the antenna can be reinstalled repeatably. Compare another location when the roof cannot demonstrate the required long-term stability.
Q: How should a project choose between an external network and its own reference point? Compare them against the measurand, coordinate frame, tolerance for service interruption and reference traceability. An external network can be one reference source, while the site and acceptance assessment establishes whether it meets the project need.
Q: How can better equipment and more constellations support a difficult site? They can improve observation capability and redundancy. Site design must still stabilize the monument, address reflectors and establish a trusted reference because equipment and additional constellations do not remove those conditions.
Preparing to assess a permanent GNSS monitoring site? Contact RST Ltd. with the site area, points of concern and site photographs, or learn about our near-real-time differential GNSS monitoring service.