Title: Detection of GPS station position errors due to instrumentals and environmental effects
1Detection of GPS station position errors due to
instrumentals and environmental effects
- Thomas Herring, tah_at_mit.edu,
- Department of Earth Atmospheric and Planetary
Sciences, MIT
2Overview
- 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
3Diagnostics
- 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.
4Multipath 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?)
5Site NDAP Site had shown annual signal in North
for a long period of time In July 2006, antenna
replaced.
6(No Transcript)
7NDAP North/MP1
The MP1 values before antenna switch are
correlated with temperature. Old antenna had
failed patch element.
8LTUT (another example)
Antenna change
9SNR 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
10P708 near Yellowstone
Site notes indicate that so much snow during
visit that they could not get to monument
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12P708 L1 SNR (no snow)
13P708 L1 SNR (snow)
Note Low elevations not that effected by snow
14P708 L2 (no snow)
15P708 L2 SNR (snow)
As average Use fit value at zenith
16P708 Height and SNR
17P708 Height/MP1 overlay
Generated online from interactive
plotshttp//pboweb.unavco.org/plotting/src/publi
c/gps_qc_soh_page.php?stationP708
East effects here
18P708 East MP1
Jump in east detected in multipath RMS
19P708 Height/Phase RMS
Phase RMS also catches early event but others
also.
20Summary 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.
21Other examples
- Cases generated using on-line tools at
http//pboweb.unavco.org - Most of these are probably snow events (cases
based on site visit notes and height of site)
22P665
23P665 North MP1
24Alaska Volcano (AV04)
25AV04 time series
26AV04 Height and MP
MP1/2 indicates that most height signal due to
snow coverSome interesting hysteresis after snow
melt
27Summary
- 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 dont
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).