# Presentation: 669256

## Detection of GPS station position errors due to instrumentals and environmental effects 

- Thomas Herring, _[tah@mit.edu](mailto:tah@mit.edu)__,_
- Department of Earth Atmospheric and Planetary Sciences, MIT

## Overview

- Examine origin and diagnostics of systematic deviations in GPS time series due to instrumental and environmental effects (not monument stability)
- Cases:
- Antenna failures
- Environmental effects: Snow but also growing vegetation.
- Diagnostics:
- TEQC MP1 and MP2: Estimates of pseudorange multipath.
- Signal to noise ratio
- Phase residual RMS Scatter

- 06/04/08

- IGS ACWS TAH Errors

## Diagnostics

- Pseudorange multipath at L1 and L2: MP1 and MP2
- Difference between L1 and L2 pseudorange computed from L1 and L2 phase measurements
- Values generated by TEQC using an algorithm that accounts for cycle slips and offset between phase and range.
- Values shown are an RMS over the duration of the rinex file (24-hours)
- Phase RMS scatter
- RMS scatter of the ionospheric free phase residuals from post-processing.
- Signal-to-noise ratio
- Commonly available now in rinex files although quality is variable
- Here we use an “average” value for the day
- MP1/2 and SNR can be used before processing data.

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- IGS ACWS TAH Errors

## Multipath diagnostic

- Example of antenna failure:
- Site NDAP (Nucleus site)
- Site had strong annual signal in north but not in height (rather strange if loading effect)
- Effect of antenna change on position and multipath values
- Example a LTUT (no MP1 in this case: Should be available but is not?)

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- IGS ACWS TAH Errors

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- Site NDAP
- Site had shown annual signal in North for a long period of time
- In July 2006, antenna replaced.

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## NDAP North/MP1

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- The MP1 values before antenna switch are correlated with temperature.  Old antenna had failed patch element.

## LTUT (another example)

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- Antenna change

## SNR analysis

- Many RINEX files now contain S1 and S2 and the units are now better defined
- Analysis here:
- Fit elevation angle dependent model to SNR at L1 and L2
- Extract the fitted values at zenith and 20 degrees elevation
- Example on following slides

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## P708 near Yellowstone

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- Site notes indicate that so much snow during visit that they could not get to monument

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## P708 L1 SNR (no snow)

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## P708 L1 SNR (snow)

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- Note: Low elevations not that effected by snow

## P708 L2 (no snow)

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## P708 L2 SNR (snow)

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- As average: Use fit value at zenith

## P708 Height and SNR

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## P708 Height/MP1 overlay

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- Generated online from interactive plots:http://pboweb.unavco.org/plotting/src/public/gps_qc_soh_page.php?station=P708

- East effects here

## P708 East MP1

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- Jump in east detected in multipath RMS

## P708 Height/Phase RMS

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- Phase RMS also catches early event but others also.

## Summary for snow events

- Both SNR and MP1/2 work well as indicators of snow
- Low elevation angle SNR may indicate horizontal position error (not tested)
- Phase noise RMS has problems in that atmospheric “noise” that might not effect position very much can increase noise
- SNR and MP1/2 can be computed before data analysis as a quality check.

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## Other examples

- Cases generated using on-line tools at _[http://pboweb.unavco.org](http://pboweb.unavco.org/)_
- Most of these are probably snow events (cases based on site visit notes and height of site)

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## P665

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## P665 North MP1

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## Alaska Volcano (AV04)

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## AV04 time series

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## AV04 Height and MP

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- MP1/2 indicates that most height signal due to snow coverSome interesting hysteresis after snow melt

## Summary

- Assessment of data quality through use of pseudorange multipath measures and SNR analysis looks promising. (Available for PBO sites at UNAVCO: current UNAVCO summary files do not have enough significant digits.)
- Detection of antenna problems and snow effects possible.   Vegetation effects also probably can be detected.
- Phase RMS scatter is probably not a good detector in that it can be disturbed by effects that don’t greatly change the station positions.
- Not discussed but needed: Detection of sites with corrupt data and how these should be treated  in archives.  Important in re-processing so that station numbers are not wasted in bad sites (i.e., most ACs use fixed number of stations).

- 06/04/08

- IGS ACWS TAH Errors