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## Where do precise orbits and clocks come from?

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**Where do precise orbits and clocks come from?**Kristine M. Larson ASEN 6090 Spring 2010**Starter question: where do broadcast orbits and clocks come**from? • Always harder to predict orbits than to calculate them. Broadcast orbits and clocks are predictions. • DoD tracking network (with real-time telemetry). Positions of sites known “well.” • Real-time data analysis • Simple models (note e.g. clock t and t^2 terms) • Pseudorange data? • No mandate for cm-level precision.**Which broadcast ephemeris?**• Modifications in the 1990s included updating solar radiation pressure models, satellite mass, ground station coordinates, and process noise covariance values.**How do we evaluate orbit accuracy?**• How do we evaluate orbit precision? • The same questions exist for clocks.**The code you are using**• Need Keplerian elements and fudge factors from Nav message. • The code solves Kepler’s equations. • Officially, only GM and constant Earth rotation are used. • Applies multiple corrections (delta terms). Remember: the message only has to be good for 2 hours. • Last bit: Rotates from Earth-centered Inertial to Earth-centered Earth-fixed.**Ephemeris Code**u = phi + cuc*cos(2*phi)+cus*sin(2*phi); r = A*(1-ecc*cos(E)) + crc*cos(2*phi)+crs*sin(2*phi); i = i0+idot*tk + cic*cos(2*phi)+cis*sin(2*phi); x1 = cos(u)*r; y1 = sin(u)*r; satp(1,1) = x1*cos(Omega)-y1*cos(i)*sin(Omega); satp(2,1) = x1*sin(Omega)+y1*cos(i)*cos(Omega); satp(3,1) = y1*sin(i);**For highest accuracy**• Most of what was discussed in the previous page is wrong or sub-optimal in some sense. • More terms of the Earth’s gravity field are needed. • Tracking site positions are much better known. • Better models are needed for the space and ground segments. • Earth rotation is not constant. • Earth’s pole of rotation changes. • Use carrier phase data (5 mm precision instead of 50 cm precision).**How do you know what they did to produce your precise orbits**(i.e. the sp3 file)? Look at the IGS website to find out! http://igscb.jpl.nasa.gov/igscb/center/analysis/esa.acn http://igscb.jpl.nasa.gov/igscb/center/analysis/jpl.acn**Draw the frame on the board**(last time we talked about the “ruler”)**One year of broadcast orbits**+- 40 m +- 20 m +2/-6 m Igs orbits being treated as truth**radial/cross/along**0 - 10 m**Things to keep in mind**• Evaluating orbit precision (and accuracy) is a moving target on both sides - broadcast orbits are changing (for reasons that may have nothing to do with improvements to physical models) • Civilian (IGS) precise orbits are also changing for a variety of reasons.**Broadcast-precise model differences**• J2 (FF) • Other harmonics (FF) • Sun/Moon (FF) • Earth tides (FF) • Ocean tides • Solar pressure (big effect!) • Albedo pressure We’ll talk about these model changes on Monday**Keep in mind**• There is more than one kind of IGS orbit • Precise (available after ~week) • Rapid (within 24 hours) • Ultra-rapid (real-time)**IGS (precise) orbits**• Implement these model changes. • Conventions are required (as we talked about last time) • Individual centers can do research to evaluate the efficacy of other models. • When significant model changes are made, what happens to the IGS precise orbit products?**The IGS also produces Clocks**What is a ns?**Variability by analysis**center**Monday, January 25**• OH: M1-2:30 and F 2-3:30 and MWF by appt. • Talk about homework 1 • Questions about homework 2 • Precise reference frames - which leads us into precise orbit determination**ECI to ECEF**Earth Centered Inertial to Earth Centered Earth Fixed And vice versa**Ways to think about the frame**• We are really talking about XYZ. And goals are all relative to 1mm. • Rotation axis in space • Rotation axis in the “Earth” • How fast does the Earth rotate. • Complication: tectonic plates moving on the surface of the Earth. • How are we going to realize the “Earth?” (we’ll talk about the link to space as well).**Realizing the Frame**• Measurements on the Earth to: • Satellites using lasers • Quasars using radio telescopes • Satellites, radio freq., ranges (GPS, Glonass) • Satellites, doppler • Some laser measurements to the moon - but not very important • Once we know where “things” are, we fix the coordinates of these sites, and estimate other things (such as orbits of satellites)**The frame**• Is updated regularly (every 2-3 years). • It attempts to combine the “best” of all techniques. • ITRF (International Terrestrial Reference Frame) is the ECEF part. • It is not a frame for purposes of comparison- its goal is to correctly define the Earth’s crust wrt to XYZ.**How is the frame distributed?**• Positions at an epoch and velocities. Is this adequate? What kind of discontinuities exist in time series, and why?**http://pboweb.unavco.org/shared/scripts/stations/?checkkey=TPW2**http://pboweb.unavco.org/shared/scripts/stations/?checkkey=TPW2**Discontinuities & non-linearities are**an ongoing problem in frame realization**Why SLR ?**Why not GPS? Is this a problem?**Colocation**• Multiple measurement systems don’t do much for you unless you have some of them in the same place.