Errors in Positioning Matt King, Newcastle University, UK

“Static” Issues

Propagation of Sub-daily signal Propagation of mis/un-modelled periodic signals (e.g., ocean tide loading displacements) in 24h solutions Well described by Penna & Stewart (GRL, 2003) and Penna et al., JGR, Admittances in float ambiguity PPP solutions up to 120% in worst case (S2 north component into local up)‏  Depends on coordinate component of mismodelled signal & frequency & “geometry” Output frequencies depend on input frequency  Annual, semi-annual and fortnightly, amongst many others

Periodic Signals Penna et al., JGR, 2007 mm

Effect in real data King et al, GRL, 2008

Local Site Issues

Kinematic Issues

Day Boundary Jumps Day boundary jumps regularly seen in kinematic positioning Filter edge effects and different parameter (trop, real valued ambiguities) estimates either side of day boundary -> different coordinate estimates “Continuous” filtering not possible in track (yet)‏ Mitigation strategy -> process 2+h overlap, then crop back

Multipath (Near-) Sidereally repeating sampling of site (baseline) MP ( s)‏ Choi et al. showed mean is closer to s, but varying in time with satellite constellation Mitigation strategy: sidereal filtering Coordinate domain Observation domain Ragheb et al suggested little difference Caution: for glacio applications, unmodified approach would remove tidal signal After “sidereal” filtering Raw positions <10m baselineRagheb et al., cm 2cm

Response to tidal forcing – how much is real? Rutford Ice Stream (W Antarctica) experiences tidal modulation of its flow How much of this signal is real? Rutford Ice Stream Window considered here

Response to tidal forcing – how much is real? Two processing approaches Precise point positioning (GIPSY)‏ Relative to a base station (Track), 30km away Tidal decomposition of de- trended along-flow velocity PPP – very large response at high frequencies from little downstream vertical forcing e.g., M2 vertical tide ~1.5m; 2SK5 probably <0.05m GIPSY (PPP)‏

Response to tidal forcing – how much is real? Relative vs PPP LF (fortnightly) terms in good agreement Relative processing – HF terms not sig. GIPSY (PPP) and Track (relative)‏

Response to tidal forcing – how much is real? Relative vs PPP In relative processing, smaller diurnal and semi-diurnal vertical tide terms not significant Same data Why the differences? GIPSY (PPP) and Track (relative)‏

Response to tidal forcing – how much is real? Relative processing is rover minus base (TOLL)‏ How much signal is being differenced by the base? Gives “tidal error spectrum” for SEI1 HF signal evident at base station on rock Common GPS satellite position biases? Care needed in interpreting HF velocity signals in glaciological GPS time series LF velocity signals are reliable in all solutions

Solution Precision/Accuracy Convincing yourself the solution is robust Examine track stats – Average RMS should be <20mm Pay attention to track warnings (particularly lots of clock edits – likely a prior coordinates are bad)‏ Vary elevation cut-off angle to examine effects of systematics Examine residuals (may give idea on where low elevation effects increase and hence better cutoff angle)‏ Coordinate time series looks sensible! Precision of solutions Sigmas in track output files are relative to the precisions you specify in DATA_NOISE Hence are likely unreliable Include 2nd static site similar distance away Examine linear sections of time series and use RMS to scale track sigmas

Other Issues Incorrect Ambiguity Fixing Leads to drift in coordinates of metres Don’t force fixing unless you are sure! Cut-off angle No right single choice, but probably some wrong ones Relative PCVs only down to 10 degrees (including converted to Absolute PCV)‏ Track Tropo MF is not as advanced as GAMIT’s