###### Abstract  
The advent of modern geodetic satellite techniques (GNSS, including GPS) permitted to observe dynamic deflections of bridges, initially of long flexible ones, and more recently of short, essentially stiff bridges with modal frequencies > 1 Hz, and with small SNR (signal-to-noise ratio), even SNR < 1. This was an enormous progress, but not without problems. Apart from monitoring results consistent with structural models, experimental data and serviceability criteria, there exist some apparently unexplained cases of stiff bridges for which there have been claimed apparent dynamic deflections too large for common healthy structures. Summarizing previous experience, this article: (i) discusses structural constraints, experimental evidence, and serviceability limits of bridges as constraints to GNSS monitoring; (ii) examines a representative case of careful monitoring of a reinforced concrete road bridge with reported excessive dynamic deflections; and (iii) explains such deflections as a result of a double process generated by large reflective surfaces of passing vehicles near the antenna; first corruption/distortion of the satellite signal because of high-frequency dynamic multipath, and second, shadowing of some satellites; this last effect leads to a modified observations system and to instantaneously changed coordinates and deflections. In order to recognize and avoid such bias in GNSS monitoring, a strategy based on practical rules and structural constraints is presented. structural health monitoring; concrete bridge; GNSS/GPS; dynamic deflection/displacement; natural/modal frequency; serviceability limit; multipath; resonance +
Footnote †: journal: 17 November 2020  
19 January 2021  
10.11.11.11.11.11.11.11.11.11.11.11.11.1.11.11.  
on scaffoldings at the bottom of the bridge deck [5; 6]). This last technique was, however, possible for bridges in favorable conditions, for example, over a dry riverbed and with short clearance (distance of the deck from the ground bottom). For this reason, deflections were mainly based on model predictions (e.g., [7]). Concerning long, flexible bridges, the problem was more complex because wind tends to produce large deflections (Figure 1). In such cases, analysis of scaled models in wind tunnels is a solution, but detailed analysis of wind effects on long bridges requires high-pressure, high Reynolds number wind tunnels, are very few (for instance, in Princeton University, USA; [8]). For these reasons, deflections of bridges of different types due to dynamic loads remained rather unknown, and regulations for bridge performance all around the world relied on model predictions and on certain empirical criteria (e.g., AASHTO 1997 [9]). In the last 20 years, however, GPS, known as GNSS after the advent of GLONASS and of other satellite positioning systems in the last decade, has been used to measure dynamic deflections of long span bridges [4; 10; 11] (Figure 1); in some cases permanent monitoring systems including geodetic sensors have been adopted [12]. Geodetic techniques, usually combined with accelerometers, have been recently used to cover short span, stiff bridges (main natural frequency f > 1 Hz) with deflections of a few mm also [13; 14; 15; 16; 17]. In a few cases, however, combination of GNSS with RTS (robotic theodolite or robotic total station) have permitted to recognize that the GPS (currently GNSS) signal may be corrupted, usually beyond repair [18]. In addition, as is explained in Section 4 below, there has been reported some cases of short, stiff bridges for which excessive, non-realistic dynamic deflections based on GNSS monitoring have been claimed. This indicates that geodetic monitoring of bridges, especially of short, stiff bridges has certain limitations, and ignoring these limitations may lead to pitfalls.