Your expected GNSS 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.
Compare with a 25 meter reflector height:
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.
Designing a GNSS Reflections Site:
- Sampling interval should be commensurate with your reflection target area. You can generally get away with 30 sec for surfaces that are < 10 meters below the antenna, but I urge you to use 15 sec. For reflectors larger than 50 meters, I recommend 1 sec sampling. The bare minimum sampling rate numbers you need can be calculated using the code in Roesler and Larson (2018). I will likely port this to https://gnss-reflections.org.
- Make sure your antenna is surrounded by natural planar surfaces. No crashing waves. No outlet glaciers. No large ships coming and going.
- Use the reflection zone app to make sure that you can sense the surface you want to measure. This is extremely important for water levels, as many groups think seeing the water in a photo means you can measure it. All you need to check this is the position of your site. The app will calculate the geoid correction for the ellipsoidal height. If you are trying to measure an interior water body (where mean sea level is not relevant), there is a manual override.
- If you have flexibility, take into account that sites at mid-latitudes have holes in their sensing zone. In CONUS, don’t face your GNSS receiver to the north. In southern Africa, South America, and Australia, don’t try to use GNSS-IR to measure water levels to the south.
- If you are trying to measure snow accumulation in polar regions, you should ensure that your antenna is always at least 1 meters above the highest snow level. This may mean you need to revisit your site to reset the pole vertically.
Operating a GNSS reflections site:
- Always remove the elevation mask on the receiver.
- Set the sampling interval by evaluating reflection surfaces. The standard GNSS sampling interval of thirty seconds was selected over thirty years ago before the internet existed! Collect (and archive) more data.
- Take photographs of your site.
- If you plan to put your GNSS antenna on a roof, pick the corner that gives you the best view of natural surfaces.
- 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.
- Put SNR data in your RINEX 2.11 file.
- Provide SNR data in the RINEX 3 format to ensure full access to multi-GNSS SNR data.