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GPS

Reflection zones at Summit Camp, Greenland, where the GPS antenna is ~16 meters above the surface.

Your expected GPS reflection zone is not a mystery. You can calculate it before you go in the field. In the animation above I have calculated the rising/setting reflection zones for all visible GPS satellites at Summit Camp, Greenland on January 1, 2017. The Fresnel (reflection) zones for satellites with elevation angles between 5-20 degrees are calculated and updated every minute, and the map view is saved at 30 minute intervals. The only inputs needed are

  • the approximate position of the GPS site
  • the positions of the GPS satellites
  • the height of the antenna above the reflecting surface
  • GPS signal wavelength (~0.19 or 0.244 meters for L1 vs. L2)

In this example I used the GPS broadcast message for the satellite orbits and assumed that the antenna at Summit Camp is 16 meters above the ice. The equations you need for a Fresnel zone are given in the appendix for Larson and Nievinski (2013). Here are static examples for a 2 meter reflector height for L1 and L2.

L1 first Fresnel zones for 2 meter reflector ht. Elevation angles are defined in the legend.

L2 first Fresnel zones for 2 meter reflector ht. Elevation angles are defined in the legend.

 

 

 

 

 

 

 

Compare with a 25 meter reflector height:

25 meter reflector height Fresnel zones.

 

 

 

 

 

 

 

Similarly, the sampling rate you need to use is not unknown – you just need to understand how the Nyquist frequency is defined for the SNR observations.

Operating a GPS reflections site:

  • Sampling interval should be commensurate with your reflection target area. 30 sec is fine for many sites, but not all. For typical cryosphere sites (2-3 meters above the snow), I would suggest a sampling rate of 15 seconds. For tower sites, I recommend you read Roesler and Larson (GPS Solutions, 2018), which is posted on the publications page.
  • Always remove the elevation mask.
  • Track all GPS signals! (L1 and L1C, L2P and L2C, L5). If you can track GLONASS, Galileo, Beidou without costing a lot of money, I strongly recommend it.
  • It doesn’t matter if you turn on multipath suppression algorithms or buy a fancy antenna. They don’t stop multipath.