The installation of a GBAS ground subsystem involves special considerations in choosing prospective sites for the reference receiver antennas and the VDB antenna(s). The following should be considered as part of the siting analysis:

• Proximity to existing power and communications cabling in the area
• Soil stability and height of the surrounding terrain
• Proximity to threshold/s served as well as to aircraft and vehicle movement areas
• Potential impact of aircraft and vehicle movements on GBAS performance
• Site accessibility for equipment maintenance team
• Unauthorized access to the GBAS ground facility, minimized when the GBAS elements are installed within the airport boundaries. 
• Proximity to existing infrastructure on and in the vicinity of the aerodrome and their potential impacts on GBAS performance including GNSS signal reception (obstacles over a given elevation mask angle, that may advice increasing the antenna height), multipath (avoidance of reflection surfaces) and LOCA (Local Object Consideration Area) considerations, and VHF Data Broadcast transmission 
• Location of environmental sensitive areas (e.g. critical wildlife habitat, wetlands, contaminated soil)
• Minimum antenna baselines and specific antenna layouts for spatial ionospheric gradient monitoring, depending on the supported GBAS approach service/s types
• Airport layout (including runway, taxiway and terminal configuration)
• Future developments on the aerodrome and their potential impact on GBAS performance and location
• Obstacle limitation surfaces, intended to restrict the siting of objects in areas on and around the runway region
• GBAS building restricted areas, to avoid unacceptable interference to the signal-in-space
• Potential infringements of the Airport Height Restriction Surfaces surrounding the airport
• Line of sight from the VDB antenna to all operational areas inside the service volume
• Likelihood of intentional or unintentional Radio Frequency Interference impacting GNSS signal reception, including GNSS repeaters used for aircraft maintenance. RFI considerations advice planning distance to public areas. 
• Weather conditions, requiring specific materials, overvoltage protection or antenna design. 

The siting process typically involves site selection, site qualification, installation, survey of the reference points and site acceptance. 

A GNSS data collection campaign should be performed at each site identified for installation of GBAS ground facility, in order to assess the satellite signal performance as part of the technical feasibility analysis, to quantify: 

• Satellite signal reception at each location and at all azimuths. Vegetation at low elevations may cause signal blockage.  
• The magnitude of satellite multipath and noise at the location. Multipath correlated between GNSS reference receiver antennas needs to be avoided. 
• The magnitude of radio frequency interference in the environment and potentiall impact on satellite signal reception

There are two effects of the ionosphere on GBAS, which are the ionospheric group delay and scintillation. Resulted characteristics and severity of both impacts are different in locations, mainly latitude-dependant, and seasons of the year and time of day. It should be noted that they change depending on the solar activity, which has an approximately 11‐year cycle:

• Although user’s range error component of the ionospheric delay is almost removed under nominal condition using the GBAS correction messages, it could increase if there is a large spatial gradient between the GBAS ground subsystem and the user.
• The ionospheric scintillation effect on ranging signals, which is frequent loss of lock, is caused by ionospheric irregularities with ionospheric disturbances. It could result in lower availability and continuity of the GBAS service. 

The different GBAS approach service types apply specific ionosphere threat mitigation schemes. In GAST C, the GBAS ground subsystem is responsible for mitigating the potential impact of ionospheric anomalies. With GAST D, the mitigation of the threat for anomalous ionospheric conditions is shared between the airborne and the ground subsystems, and the ground subsystem must monitor spatial and temporal ionospheric gradients that may not be detectable to the approaching aircraft. 

The GBAS threat model for ionospheric gradients is also dependent on the supported GBAS approach service type. 

In the GBAS safety assessment, it should be defined what is considered “nominal” and “anomalous” ionosphere conditions, together with system architecture and safety design of GBAS ground subsystem. Under the nominal condition, user is protected by the GBAS differential correction messages with an integrity parameter for ionospheric error. In contrast, user is not bounded by the parameter under the anomalous condition. Therefore, it is necessary to detect and exclude such erroneous ranging sources with appropriate integrity monitors at the GBAS ground subsystem. Note that the targeted horizontal scale of the disturbances is within a few tens of kilometres.

• Concerning the nominal condition, the Equatorial anomaly is a dominant factor to determine background spatial gradient of the ionospheric delay in the low magnetic latitude region. It has seasonal variation and dependency on the solar activity. Therefore, the vertical ionospheric gradient, which is an integrity parameter included in the GBAS messages to calculate the user’s protection levels, should be determined, considering these characteristics based on sufficient period of observational data. 
• One example of an ionospheric anomalous condition is plasma bubble, which is a disturbance that produces steep ionospheric spatial gradients and scintillation on GNSS signals. It frequently occurs after sunset in high solar activity periods. For the safety system design of the GBAS ground subsystem, it is necessary to define a ionospheric threat model which describes the ranges of parameters to assess the ionospheric effects on the GBAS. Regarding the range definition, underestimation exposes users to unsafe condition whereas overestimation significantly degrades system availability.

Tropospheric delay is another crucial error source for GBAS, where high‐integrity positioning is required. The tropospheric delay is a function of the air pressure, temperature and humidity that varies depending on time and location:

• As the air pressure decreases rapidly by height, the tropospheric delay decreases by height. It is corrected by an exponential function of the height differences between the aircraft and GBAS ground station with parameters broadcast in the Message Type 2 which should be determined by the GBAS provider based on local meteorological data or empirical models.
• Horizontal decorrelation of the tropospheric delay is assumed to be negligible in the local area of interest for GBAS. When the horizontal decorrelation is not considered negligible, the effect can be bounded by adding tropospheric decorrelation sigma to the uncertainty in the nominal ionospheric delay.

As part of the technical feasibility assessment, a theoretical coverage analysis should be performed to determine the likelihood that VDB coverage requirements will be met. The coverage can be performed using commercially available radio coverage modelling software. The minimum service volume for GBAS approach and GBAS positioning services is defined in the ICAO Annex 10, Volume 1, Standards and Recommended Practices. 

As part of the technical feasibility assessment, it should be analysed the impact that the implementation of GBAS would have in the airline fleets that operates at the airport, as well as any future additional airline. Several documents states the requirements applicable to the airborne equipment, including ICAO Annex 10, Volume 1, Standards and Recommended Practices or the RTCA DO-246 and DO-253, among others.