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Which mechanisms exist for the artificial degradation of the GPS signals?
Selective Availability (SA): The accuracy of the positioning is degraded. This works with the so called dithering. There the satellite clock is manipulated in the range of 2 s, witch correspond to 60m. With relative positioning the SA wasn’t a problem. The SA was switched off at May 2, 2000. Anti-Spoofing (AS): The P-Code is encrypted by superposing a additional W-code. => Y-code. The AS was implemented to avoid that position quality can be degraded by a “wrong” artificial GPS-signal. Because of the AS there is an increased noise level in the code measurements ans especially in the carrier phase measurements.
Which mechanisms exist for the artificial degradation of the GLNASS signals?
no signal degradation mechanisms
Which terms are present in the observation equation of GPS?
receiver position at reception time, satellite position at emission time, tropospheric delay, ionospheric delay, relativistic correction, influence of multi-path, light travel time, light velocity
What is the meaning of the terms in equation (2.43)
re(tE): Receiver position at reception time (tE), rs(tS): =rs(tE-tES): Satellite position at the time of emission tS, trp: signal delay in troposphere, ion, signla delay in ionosphere, rel: relativistic correction, mul: infuence of multi-path
Differences between the phase and code observation equations
Vorzeichen für Ionosphären-Korrektur (pase advane, group delay), andere Wert für Multipath-Korrektur.
Why are often differences formed in GPS analyses (especially double-differences)?
to improve the accuracy: get rid of the satellite clock error, receiver clock error and other effects
Explain single Diff in GPS measurements?
2 Reveivers observe simultanous the same satellite // calculate the difference of the two observation equations // eliminated satellite clock error (it’s not totally eliminated - but nearly) // other errors are reduced as well, if baseline is not too long (sallite orbit errors, atmospherical and relativistic effects) // system noise increases by a factor of sqrt(2)
Explain double Diff in GPS measurements?
2 receivers, 2 satellites // differences of the observation eqations // eliminates receiver clock error (you still need to know the clock error to calculate the satellite positions in the correct epoch!) // system noise increases by a factor of 2
Explain tripple Diff in GPS measurements?
2 reveivers, 2 satellites measured in two epochs // eliminates the ambiguity // first robust solution for relativ-coordinates // system noise increases by a factor of 2*sqrt(2)
What type of instrumental effects have to be considered in GPS? What can be done to reduce their adverse effects?
a) multipath-effects b)phasecenter of GPS antenna: actual point of reception. Depending on azimuth and elevation of the GPS satellites. // 1) combining different antenna types: mainly affecting the height component (up to 10cm), horizontal position maximally a few millimeters if tropospheric zenith delays are estimated in the adjustment // 2) long baselines, errors even present if identical antenna types are used! due to different elevtion angle to the same satellite. //
ant(z,a) = -(r0e+(a,z)),
e = unit vector pointing from receiver to satellite,
Corrections are also necessary for the satellite antenna phase center!
What differences exist between GLONASS and GPS?
(GPS/GLONASS), altitude (26'600km, 25'500km), Period (11h58', 11h16'), Inclination (55°, 65°), orbital planes (6, 60°spacing, 3, 120° spacing), CodeDivisionMultipleAccess <=> FrequencyDMA
Which Meteo-Parameters have to be taken in account when you are measuring with GPS?
Air pressure, temperature, humidity
What is the influence of water vapour to the accuracy of GPS?
5-40cm (wet part)
Formular 4.23, explain. How is it for GPS? Other Effects in the Atmosphere?
v_ph: kann grösser als c sein und ist grösser als v_gr,
phase advance und group delay,
Ionosphäre ist Orts und Zeitabhängig (TEC)
Tell something about the Era before GPS?
Optical measurements (with Photos) to the satellites (strobe-light and balloon-satellites) with relative coordinates to the stars
Compare the relativistic effects of GPS and VLBI?
Lorentz-Transformation nur bei VLBI, Unterschiede der Uhren bei VLBI viel kleiner (vernachlässigbar), bei GPS nicht (Sender und Empfänger), Signale im vergleichbaren Wellenlägenbereich, relativistische Effekte (Allgemeine Relativität) auf Signale gleich.
Was entspricht bei GPS der Quellstruktur von VLBI?
Variationen des Antennenphasezentrums, Ungenauigkeit der GPS-Bahnen
Why aren't there multi-path effects for VLBI?
GPS antennas are omnidirectional, VLBI antennas are directed.
Simplest form of the observation equation of SLR/LLR?
rho = c * delta_t / 2
Advantages of SLR/LLR?
favorable propagation characteristics of visible light in the atmopshere (water vapor has no influence) // direct distance measurement are performed // satellites with a very long life-time // simple instrumentation on the satellite (nothing but a retro-reflector)
Disadvantages of SLR/LLR?
depending on weather conditions (clouds, fog) // expensive infrastructure and instrumentation at the ground station // very heavy equipment (transport)
Main components of an SLR/LLR system
retro-reflectors // generator of the laser pulses // optical telescope // receiving system: detector and analyzer of the returning pulse (echo) // timing equipment
Which parameters can be determined by SLR, which ones by LLR?
Nutation (LLR), Polar Motion (both), Length of Day (both), Ocean tide amplitude (SLR), Coord. + Velocities (both), Geocenter (SLR), etc. see Table 2.1
Simplest form ot the observtion eqation of satellite altimetry?
rho = h* - h, with rho = c * delta_t / 2
What are altimetry data used for? What can be determined with altimetry data?
sea level also possible for bigger lakes and Iceshields. // Geoid over the oceans very accurately and with hight spatial resolution // gravitiy anomalies (computed out of the altimeter geoid) => new earth models // information about the dynamics of the oceans (currents, ocean tides)
What is a cross-over point?
A Point where a descending and a ascending groundtrack cross-over eachother
What is the use of cross-over points?
Two corrected measurements have to be equal at a cross-over point. // The differences between those measurements cam be used for the orbit determination/correction // the height of the mean sea level can be corrected and improved
What is the meaning of the terms in equation (2.82)? Sketch?
h*: height of the satellite above reference ellipsoid, N: geoidundulation, Zeta_p: permanent part of ocean currents, Zeta_v: variable part of ocean currents, htrp: Delay caused by the tropospheric refration, hion: Delay caused by the ionospheric refraction, hwav: Correction of the wind waves on the sea surface. It is computed based on the “signifant wave height (SWH), that can be reconstructed from the temporal distribution of the reflected pulse (shape of pulse), hsys: instrumental corrections, e: error of the altimeter observation
Planung einer Satellitenmission um Meeresspiegeländerungen von 3mm zu detektieren, mögliche Varianten
Satelliten-Altimetrie, Crossover-Points, Modellierungen von Geoid, Strömungen, etc.
Kalibrierung mit GPS-Bojen / SLR-Stationen. Ev mit InSAR?
Table 2.4, why are there groups with the same latitudes?
to reduce systematic errors (due to orbits), following satellite missions are placed on orbits with the same parameters. The measurements can be performed under the same conditions for long time.