# Pointing Accuracy of an Operational Polarimetric Weather Radar
## 2 Antenna Pointing  
Mechanical drive systems steer a weather radar antenna that is mounted on a pedestal. The reading of the azimuth and elevation angles is relative to the rotation plane. During a radar installation, the rotation plane has to be leveled horizontally. This leveling must consider possible tilts of the radar tower, pedestal and as well as for non-zero angles between the fixed part of the pedestal and the bearing for the antenna head. This is illustrated in Figure 1.  
In order to determine the static leveling, we have installed an electronic level parallel to the elevation axis above the azimuth rotation plane. The electronic level must show the same value at all azimuth angles after the proper placement of shims at the interface between the pedestal and radar tower. For our system, the level readings can be accessed remotely and they are stored to keep track on the pedestal leveling. With this method an absolute accuracy of better than \(\pm\)0.01\({}^{\circ}\) can be achieved. During operation, there will be additional time varying tilts, e.g., due to wind load on the tower. Also differential heating of the tower by the sun may result in tilts (we have measured a tilt of >0.1\({}^{\circ}\) for a 50 m concrete tower). A method to monitor the leveling of the pedestal by using a sequence of solar boxscans throughout a day is illustrated in this contribution. Other methods are discussed in [3]. Our radar system uses a typical drive to steer the radar antenna. The angle positions are sampled by standard encoders (Figure 2). The drive train has normal gears with some inherent degree of backlash. The absolute encoder itself has anti-backlash gears. Since the drive train consists of normal gears, the adjustment of the pinion gear on the bull gear needs some backlash in order to avoid heavy mechanical stress during operation. Backlash may vary as a function of azimuth and elevation since gears are not precisely circular. The absolute high point of the bull gear in relation to the pinion gear is the reference for the adjustment of minimal backlash. Backlash can be quantified by commanding small angle steps to the motor and then noting the total motor angle step once the the absolute encoder reading changes. This is called the \"manual backlash determination\". Since the cogs are not exactly equally distributed over the gear ring, there may be also an angle-dependence of backlash. This is also shown to be true for the anti-backlash gears of the absolute encoder. There are in principle two angle sources in our system: motor shaft encoder and an absolute encoder (see Figure 2). If the antenna is commanded to a new azimuth position the drive moves the antenna to its new azimuth angle and the motor encoder checks the new angle with the commanded angle. The actual angle value that is used to tag a pulse is recorded by the so called absolute encoder (Figure 3). The absolute encoder samples angles at a rate of 6000 Hz with a theoretical angle resolution of 0.011\({}^{\circ}\). The angle readings from the absolute encoder are used to tag each pulse individually. This is done when the received data is digitized. The angle data from the absolute encoder are transmitted through a dedicated serial line to the Intermediate Frequency Digitizer (IFD) where each pulse is tagged with a time stamp and the corresponding angle. This is illustrated in Figure 3. Due to the finite angle sampling, an uncertainty of the angle tagging on the order of \(\approx\)0.014\({}^{\circ}\) at 18\({}^{\circ}\)s\({}^{-1}\) antenna speed can be expected. In case there is a failure of the absolute encoder we optionally can use the motor encoder angles to tag the pulses with elevation and azimuth angles. Then, the angles are sent to the signal processor via Ethernet. With Ethernet communication an exclusive timely delivery of the angle data cannot be guaranteed, but tests show that this connection can be used as an alternative with sufficient reliability.