Homework 2
Prerequisite: You need to make sure the software has been properly installed and you have successfully completed the “homework 0” assignment.
Purpose: Learn how to measure snow surface variations with GNSS-IR
Station: gls1
gls1 was installed at Dye2 on the Greenland Ice Sheet in 2011. The antenna is mounted on a long pole; approximately 3.5-meter of the pole was above the ice at the time of installation. A detailed discussion of the monument and data from the station can be found in Larson, MacFerrin, and Nylen (2020). The latest position time series for gls1 can be retrieved from the Nevada Geodetic Laboratory.
As gls1 is on an ice sheet and the ice surface is relatively smooth in all directions, it is unlikely that a complicated azimuth mask will be required.
gls1 was originally installed with an elevation mask of 7 degrees, which is suboptimal for reflections research. Even though the mask was later removed, we will use 7 degrees as the minimum elevation angle for all our analysis. Similarly, even though the site managers later changed to enable L2C tracking, this was not the case in 2012, so here we will only use L1 data.
Use the geoid tab at the gnss-reflections webapp to gather more i nformation about the site. You can enter the station coordinates by hand if you know them, but since gls1 is part of a public archive provided by UNR, coordinates have been stored in the webapp. Just type in gls1 for the station name.
Using gnssrefl
Our ultimate goal in this use case is to analyze one year of data. We have chosen the year 2012
because there was a large melt event on the ice sheet. In order to set the proper quality control parameters, we will use
quickLook
for one day.
First you need to translate one day of RINEX data. Use the year 2012 and day of year 100.
Now run quickLook
.
If you don’t remember how to run these modules.
Looking at the QC metrics plots created by quickLook
, do you have some ideas on how to change the azimuth mask angles?
Now make SNR files for gls1 for the all of 2012. Use the -weekly True
setting to save time.
We will next analyze a year of L1 GPS reflection data from gls1. We will use the default minimum and maximum
reflector height values. But for the reasons previously stated, you will want to
set a minimum elevation angle of 7 degrees. We also specify that we only want to use the L1 data.
Use the utility gnssir_input
to set and store the analysis settings.
(Hint: we recommend the argument -azlist2 40 330
.)
Now that you have SNR files and analysis inputs, you can go ahead and estimate reflector
heights for the year 2012 using gnssir
.
Note that it is normal to see ‘Could not read the first SNR file:’ because we only created SNR files once a week.
Now you can use the daily_avg
tool to compute a daily average reflector height for gls1.
Try setting the median filter to 0.25 meters and individual tracks to 30. These numbers are used
by daily_avg
to set QC in order to recover a trustworthy daily average (there is more information here on these parameters). A plain txt file
with the RH outputs are created as well as several plots.
The data in the main RH plot show you long-term accumulation as well as relatively small snow
accumulation events. The overall plot is dominated by the large melt event in the summer of 2012.
Note that RH is plotted on the y-axis with RH decreasing rather than increasing. Why do you think we
did that?