RWB

October 12, 2010

Installation of Radar Wave Monitoring Station

Equipment is now available at Romberg Tiburon Center to establish a radar monitoring station near Ocean Beach, with the capability of detecting ocean waves and currents. Data from this installation will be used in the analysis and possible mitigation of erosion threatening the Great Highway. Installation of this system should ideally be during Fall of 2010, in preparation for this season's winter storms.

The system consists of a rotating antenna mounted on a mast, connected to a control unit and power supply. Space nearby is required for a control laptop computer and a separate computer for data acquisition and analysis. The data analysis computer should have an internet connection, for data transfer to a University server for archiving and further analysis.

The antenna must be located at an elevated point, not too far back from the beach. In a first site visit a spot was located, where there are already located a CODAR antenna and a GPS receiver antenna and location-correction antenna used by the USGS. Locating the antenna here will permit measuring waves up to a distance of 500 to 750 m off-shore from the waterline.

Below we give the detailed requirements of each of the components of the system, which we hope can be met at the proposed site.

1. Antenna, Pedestal and Mast. The antenna consists of a bar nine feet long and about five inches by seven inches in cross section. The antenna rotates about once a second. It is mounted on a cast-iron housing, the "pedestal," which contains the drive mechanism, micro-wave generator and control circuits. Total weight is about 60 pounds. This must be supported on a mast which keeps the antenna eight to ten feet above the ground, for a clear view of the beach and to prevent personnel from passing through the horizontally collimated beam. The mast will probably require some guying to support the antenna in high winds. We anticipate purchasing a nine-foot mast from Rohn Products (Guyed Tower 25AG3, for example), and attaching it to the waist-high concrete wall where the CODAR USGS GPS antenna is attached, in a similar way.

Locating the antenna well above head height prevents it from offering a collision hazard to personnel on the roof. The antenna in operation radiates 9.41 GHZ (3-cm-wavelength) pulsed electromagnetic waves. The average power is 5 watts (less if backward angles are blanked out, as we expect will be done). This is comparable to the power from earlier cell phones, in wavelength and power level. While the beaming of the antenna increases the radiance in certain directions, levels of radiation are well below established safe limits. (More details are given in an appendix below.)

2. "Blue Radar Control Box and Power Supply. The pedestal connects to the "radar sensor control box" and its power supply by a heavy weather-proof cable about 100 feet long. The control box and its power supply are each about 9" by 12" and a few inches thick. The power supply requires 120V power, with power consumption with the antenna pulsing and rotating measured at 111 watts. The part of this which is dissipated by the radar control box and power supply should be easily handled by a suitable housing which could be located near the antenna. If there is access to enclosed office or storage space within 100 feet of the antenna, that would be equally good.

3. Laptop and Data Acquisition Computers. The laptop connects to the radar contrl box via a standard cat-5 network cable, which could be up to 200 feet long. There are also four coax cables (50-ohm cables with BNC connectors) connecting the radar control box to the data-acquisition computer. The computers should be in a sheltered workspace of some sort. Perhaps the environmental offices of the waste-treatment plant would be suitable?

4. Internet Access. The data acquisition and processing produces about 300 MB of data each hour. We would like to transfer this data to a computer on the SFSU campus in near real time. Access to the plant's non-secure local area network would be quite satisfactory.

Appendix A. Safe levels of microwave radiation.

The applicable standard limiting exposure of persons to microwave radiation comes from the International Commission on Non-Ionizing Radiation Protection (ICNIRP). (Web site: .) They give limits for exposure, averaged over 6 minutes, for occupational workers and for the general public. There are no legal requirements in force in the US that I found, though, nor legal requirements for warning signs. I did not find any specific standards for X-band radar transmitters either. I did find some calculations done by interested parties, determining Minimum Save Distances (MSD's) based on the ICNIRP limits and manufacturer specs for some radar systems. The fact is that X-band radar systems radiate very little power, comparable to cell phones, and limits for cell phones have not been firmly established.

The ICNIRP limits on irradiance are:

occupational exposure100 W/m^2

public exposure 10 W/m^2

The occupational exposure limit is a factor of 10 below the level at which a 40-minute exposure might cause cataracts. It is worth noting that all hazards taken into consideration are based on effects of heating by the radiation. (The wavelength is close to that used in microwave ovens.) There is some concern, mainly among the general public and non-scientists, about other effects which have not been identified yet, and so for which no save limits can be set.

I have carried out some simple calculations to determine minimum safe distances from the antenna for public exposure. I used equations from an on-line reference, "Marine Radar 2kW-4kW Non-Ionizing Radaition Hazards, by Dr. R.S. Mowbray. I also did my own calculations from first principles as a check. Here are the results.

  • The distance di from the center of the antenna to the point where radiation starts to dominate over near-field radiation is 3.12 m for our antenna.
  • The MSD from the antenna, pulsed and rotating, is d10 W/m2 = 1.44 m, using inverse-square falloff for the power. However, this calculation is not reliable for distances less than di, so the MSD for the general public can be safely taken to be 3.12m, or about 10 feet.

Another on-line source concluded similar calculations by stating, "The power density of a Marine X-Band Radar is not a biologic concern." I think this applies to our antenna, mounted well above head level.