GPS Technical Overview N5TWP NOV08. How Can GPS Mislead

Transcription

1 GPS Technical Overview How Can GPS Mislead 1

2 Objectives Components of GPS Satellite Acquisition Process Position Determination How can GPS Mislead 2

3 Components of GPS Control Segment Series of monitoring stations and uplink stations Controls and uplinks time and orbital information Satellite Segment Constellation of satellites in geosynchronous orbits (not geostationary) Satellites broadcast time and satellite orbital information User Segment Receiver that receives the satellite broadcast information Calculates the user time and position Typically provides navigation and mapping capability 3

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6 Position Determination Four unknowns: Latitude, Longitude, Altitude, and Time Four unknowns four equations four satellites - four range measurements Range is determined from the difference of the time of the broadcast and the time of reception Range-rate (velocity) Differenced range-rate (acceleration used to aid the tracking loops) The receiver will apply corrections to the measurements for ionospheric and tropospheric errors based on models 6

7 Position Determination One Satellite (SV) We know time but can not estimate range Time + Position within a Solid Sphere Two SV We can now derive a range measurement Time + Position on the Surface of a sphere Three SV We now have two range measurements Time + Position on a ring Intersection of the surfaces of two spheres ring 2D navigation possible assuming we are on the surface of the earth or using barometric altitude as 4 th SV. Four SV 3D navigation + time 7

8 Measurement Errors Ionospheric Tropospheric Multipath Satellite 8

9 Satellite Acquisition Process The GPS receiver will use the last known position, current time and almanac to determine what satellites are overhead to attempt to acquire the first satellite If any of these are significantly in error the acquisition process will be slowed > 150 Km, > 15 minutes, >??? almanac age The first satellite will correct the receiver time and also the almanac Each satellite broadcasts the almanac for the complete constellation Time will be corrected to within the ranging error to the satellite 9

10 Satellite Acquisition Process (continued) Using the revised time and almanac the receiver will continue to acquire satellites Acquisition Problems If the position or time error is significant the receiver will initially look for satellites that are not in view If the anticipated satellites are not found, the receiver will then look for the remainder of the constellation Some receivers may ask for assistance if they can not acquire the expected satellites Modern receivers typically do not need initialization if you are stationary GPS 60CSx Example: GPS last known position Albuquerque GPS next turned on in Thailand No initialization Position determination < 6 minutes 10

11 Earth Centered Earth Fixed The actual GPS computed position is an x-y-z earth centered earth fixed coordinate system (ECEF) Independent of the shape of the earth Simplifies navigation calculations to vector operations The GPS applies a datum to calculate user coordinates Latitude / Longitude / Altitude 11

12 User Coordinates The first step is to convert ECEF to Lat/Lon/Alt using the five terms of the datum To convert to grid coordinates can be a very complex calculation based on the type of projection, the regularity of the grid and the anomalies of the grid Example: ECEF -> Lat/Lon Lat/Lon -> UTM UTM -> MGRS or UPS as a function of latitude 12

13 How Can GPS Mislead Multipath A reflected multipath signal misleads the GPS range calculation Satellite Geometry The errors can be grossly elliptical 2D Navigation Three satellite solution assuming an altitude GPS set to wrong Datum 13

14 Multipath 14

15 Satellite Geometry Poor Geometry Highly Elliptical Error Good Geometry Circular Error 15

16 2D Navigation (3 satellite) True Position on earth surface With no 4 th satellite GPS assumes you are on the ellipsoid surface GPS calculated position on ellipsoid surface. The ranging error causes an offset in Lat/Lon/Alt 16

17 Datum A Datum is composed of five terms 2 terms define the ellipsoid 3 define the x, y, z shift of the center of the ellipsoid relative to the GPS coordinates (WGS 84) The algorithms for conversion from Lat/Lon to grid coordinates are also tied to the Datum A datum may be global or optimized for a specific local area Accidentally using a local datum outside of the intended area can lead to substantial errors Using the wrong datum also can create substantial errors in the grid coordinates 17

18 Datum (continued) Optimized area for local Datum WGS 84 Local Datum 18

19 Satellite Status Page 19

20 Satellite Status Page (Continued) Estimated Position Error Geometry Bad in a line Bad one sided Bad does not make sense Poor Signal Strength 20

21 Remember Check your Datum Check the position error Check the satellite status Number of satellites Satellite geometry Signal strength 21

22 Recommended Reading GPS Satellite Surveying A. Leick Radionavigation Systems B. Forssell Wireless Digital Communications: Design and Theory McDermott N5EG [TAPR] Modern Electronic Communications Beasley & Miller Introduction to Random Signals and Applied Kalman Filtering Grover Brown Applied Optimal Estimation A. Gelb 22