Experimentation

The fieldwork done as part of this thesis is presented in this section.The experiments includes the survey of a metal pole upon which a Locata antenna is mounted at the University of New South Wales (UNSW).Also, a light tower was measured at the Sydney Cricket Ground to determine the nature and magnitude of its daily movements.

 

Locata antenna pole at UNSW

This experiment was performed to study behaviour of an antenna pole when known loads are applied at different distances along its length. The pole is used for Locata (http://www.locatacorp.com/) experiments on the UNSW campus. The results from the experiment were used to make recommendations on material type, shape and the positioning of the pole relative to its support structure to minimise wind induced deformations.

 

A reference mark was made on the steel pole and the rope was tied at a known distance above the mark. The pole was measured to be 2.53m long and has a 32mm diameter. Using a spring balance the rope was pulled thus applying a known load on the steel pole to simulate the effect of wind force as shown in Figure 7 below. This was repeated by attaching the rope at different lengths along the steel pole.

 

Figure 7 - Locata antenna pole, UNSW © A. Shahapurkar, 2011

 

The movement was measured by observing the mark using the theodolite mounted parallel plate micrometer. The parallel plate micrometer can measure to a high level of accuracy and is a direct method of measurement thereby minimising systematic errors. The measurements taken from 3 trials are shown in Table 1 below ,

 

Table 1 - Movement vs. force applied for different distances along the pole

 

 

It can be seen that the magnitude of movement is greater for the same force at a higher point along the length of the pole. This means that the wind induced displacement will be greatest at the apex of the antenna pole where most receivers are mounted. Also, the movement is a function of the magnitude & direction of the wind load and height at which force acts. For a receiver is mounted near the highest point along the pole it will be under constant motion due to wind pressure.

 

Therefore, it may be beneficial to calculate wind force (N) along different points and on different days to select the material, best configuration and mount assembly for such mounting poles.

 

Light tower at the Sydney Cricket Ground

This experiment was performed to determine the nature and magnitude of daily movements undergone by free standing tower structures in Sydney. The object monitored is Light Tower No.5 which is situated in the Sydney Cricket Ground premises and provides lights to players and spectators during night games on the playing field.

 

The light tower is predominantly a steel structure approximately 55 metres high with a head frame further 10 metres higher as shown in Figure 8 below. The slender shape of this free standing tower leads to significant movements under wind loads.

 

Figure 8 - SCG Light Tower No.5, Sydney © A. Shahapurkar, 2011

 

The survey involved setting up a total station instrument at a sighting distance of about 85 metres from the base of the tower which gave a clear view of the structure. The measurements were taken in 15 minute intervals for a period of two hours starting at noon.The readings were taken to three points located at the top of the tower using reflectorless electronic distance measurement (EDM). (See fig. 9)

 

Figure 9 - Steel bolt on the Light tower © A. Shahapurkar, 2011

 

The daily movements observed for the SCG light tower were found to be significant with the largest movement being around 150mm. Also,these movements depend on the weather conditions and position of the sun during the day i.e the steel tower moves in an anti-clockwise direction (away from the sun) under solar effects.

 

Therefore, it is important that such movements be considered when mounting a GNSS antenna or using similar tower structures as survey reference points.