Sources of GPS Error Ionospheric Delays
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Sources of GPS Error Ionospheric Delays
Sources of GPS Error • Clock Errors • Satellite Geometry • Ionospheric and Tropospheric Delays • Selective Availability • Atmospheric Delays • Multipath • Ephemeris Errors Ionospheric Delays • Ionosphere - blanket of charged particles 80-120 miles above the earth – Charged particles affect the speed of light – Charged particles slow down radio signals – Charged particles also refract radio waves 1 2 Tropospheric Delays • Troposphere – we usually think of this as the atmosphere – from earth surface to about 10 miles above earth – Troposphere refract radio waves – Water vapor slows down radio waves (but at different rate than ionosphere) Ionospheric Delays • Effects on radio signals – when electromagnetic radiation travels through ionosphere, it slows down at a rate inversely proportional to (1/freq)22 – so lower frequencies slow down more 3 Ionospheric Delays • Dual frequency receivers (survey grade) receive both L1 and L2 frequencies • If we compare the arrival times of signals from both frequencies we can compute the reduction in speed of light (and signal speed) • By knowing this speed reduction, the dual frequency receivers are able to calculate a more accurate position Tropospheric Delays • Although they are similar to ionoshperic delays, the water vapor delay is not frequency dependant – Dual frequency receivers can’t correct for tropospheric delays 4 Ionospheric / Tropospheric Delays • One way to limit ionospheric and tropospheric errors is to set a mask angle – Mask angle is a setting in the GPS receiver that tells it to not use satellites that are low on the horizon – Satellites low on the horizon means that their signal will travel through a larger portion of the troposphere / ionosphere – therefore more errors – 15 degree is a typical mask angle setting Multipath • Satellite signals can be reflected off buildings, water, trees, etc. and then travel to the receiver antenna – this reflected travel path is longer than it should have been and gives a longer travel time – longer travel times yield incorrect positions 5 Ephemeris Errors • Ephemeris errors are due to slight deviations in the orbital paths of the satellites from their predicted paths Satellite Geometry • Satellite geometry affects accuracy of GPS positions – Geometric Dilution of Precision (GDOP) – GDOP refers to where the satellites are in relation to each other 6 Satellite Geometry Effects • Satellites spread far apart – bands of uncertainty shown Possible Locations Satellite Geometry Effects • Satellites clustered close together – our position will be in a larger “box” • i.e., greater uncertainty in position 7 Satellite Geometry • Satellite geometry classified by GDOP • We usually only quantify PDOP – Position Dilution of Precision (PDOP) – PDOP includes • VDOP (Vertical DOP) • HDOP (Horizontal DOP) • GDOP also includes TDOP (Time DOP) which quantifies clock offset errors Satellite Geometry Effects • Satellites spread far apart – LOW PDOP 8 Satellite Geometry Effects • Satellites clustered close together – HIGH PDOP Satellite Geometry • Most receivers allow you to set a PDOP mask – PDOP Mask determines a level of PDOP above which the receiver will not collect data – low PDOP (less than 4) means low position error – high PDOP (greater than 8) means high error • PDOP is predictable - mission planning allows you to select times when you can work with less error 9 GPS Elevation Accuracy • GPS estimates of elevation typically have errors that are 2 to 5 times worse than the horizontal position error – strictly based on geometry – VDOP (Vertical DOP) is lower if more satellites are overhead • we usually don’t have that many satellites directly overhead Lower Vertical DOP (smaller elevation error) 10 High Vertical DOP (greater elevation error) Satellite Geometry • Most receivers allow you to set an elevation angle mask – Elevation Angle is the angle above the horizon • when satellites are close to horizon, there is more atmosphere, troposphere, and ionosphere for the signals to go through - ie. more noise • when satellites are close to the horizon, the VDOP is greater 11 Satellite Geometry • Most receivers allow you to set an elevation angle mask – Elevation Angle Mask determines the minimum elevation angle below which the receiver will no longer use a satellite in its computations • a typical value for Elevation Angle Mask is 15oo • the lower limit for Elevation Angle Mask is 5oo Selective Availability • The US Department of Defense has the ability to degrade the accuracy of GPS SPS for civilian users – Selective Availability (SA) – SA is the process of introducing artificial clock errors or ephemeris errors – typical GPS errors with SA are 100 meters horizontal, up to 300 meters vertical 12 Selective Availability • SA was active throughout most of the time GPS has been operating • SA was turned off temporarily during the Gulf War • SA was turned off May 1, 2000 (hopefully permanently) Other Error Sources • Sunspots distort GPS signals • Electronic noise distorts signals – when we configure the GPS receiver, we can set a mask for Signal-to-Noise Ratio – we want the SNR to be as high as possible – a value of SNR for relatively high accuracy data collection is 6.0 – a lower limit value for SNR mask is 4 13