Use LEFT and RIGHT arrow keys to navigate between flashcards;
Use UP and DOWN arrow keys to flip the card;
H to show hint;
A reads text to speech;
56 Cards in this Set
- Front
- Back
|
Two parts of selective availability. |
Epsilon: modifying satellite ephemeris (position info) Dither: manipulation of satellite clock. Has the effect of changing the PRN code resolution |
|
|
How is antispoofing done
|
P-code is encrypted with another PRM code (W code) to create the Y-code. |
|
|
atmosphere layers |
IONOSPHERE STRATOSPHERE TROPOSPHERE |
|
|
What is deflection of the path of frequencies and change in velocity due to? |
The number of free electrons in the ionosphere. Frequencies > 30Ghz are dispersed. |
|
|
are frequencies below 30Ghz non dispersive?
|
yes
|
|
|
What affects the refractive index of gases. (and so also affects the phase velocity) |
- Gas composition of the atmosphere - amount of water vapour - temperature of the gas - pressure of the gas composition - frequency of the radiated signal |
|
|
What contribute to the path delay caused by tropospheric refraction/pathdelay |
- Gas composition of the atmosphere - amount of water vapour - temperature of the gas - pressure of the gas composition |
|
|
the propogation delay of the neutral atmosphere is commonly known as what? |
tropospheric path delay or tropospheric refraction. (neutral because <30Ghz frequencies are nondispersive) |
|
|
What are the two parts of tropospheric propogation dealy? |
Wet (0.5-20% of error, not easily estimated) and Dry (Larger error easily modelled) |
|
|
What causes wet dealy, for tropospheric refraction? |
radiowaves cause the vibration of water molecules which delay the signals transmission. Water vapour cannot be measured accurately as it changes all over the place all the time. |
|
|
Is troposphere path delay larger at low or high elevations |
Low |
|
|
How is troposphere delay reduced in GPOS positioning models. |
A standard model is used for pseudorange measurements. Carrierphase positioning with two receivers can get rid of the troposphere delay through differencing. (a common mode error) |
|
|
Where is the ionoshpere? |
~50km - >2000km. Peak electron count at 350km. |
|
|
What happens to frequencies >30Ghz and <30Ghz in the ionoshpere. |
>30Ghz : path delay. <30Ghz : reflected. |
|
|
Is the effect of path delay in the ionoshpere increased or decreased with a higher frequency |
decreased. smaller path delay. |
|
|
TEC |
Total Electron Content |
|
|
What time of the day does the ionoshper epeak at? |
2pm |
|
|
What do you need to compute DOP |
satellite positions and approx receiver positino |
|
|
What errors etc does differential GPS remove |
-removes biases in satellite orbit - satellite clock error - troposphere/ionoshpere |
|
|
PRC |
Pseudorangecorrection |
|
|
Common mode errors include |
Satellite,atmosphere biases |
|
|
What are the steps to Carrier phase processing |
Step 1: Triple differencing. To detect cycle slips and bad data. Step 2: Double Differ float solution. station coords estimated with carrier phase ambiguity terms. Satellite/receiver clock terms are canceled. atmosphere cancelled if baseline 10-20km. Step 3: Ambiguity Resolution. ambiguity term estimated. Step4 : Double diff Fixed solution. With ambiguity terms fixed. the number of unkown parameters decreases, and as such the covariancematrix of parameters is more precise. |
|
|
what is ambiguity resolution. |
When the ambiguity term is close to a whole number and the precision estimate ( ambiguity standard error) is small, the term is rounded to the whole number. |
|
|
Two reasons for differencing carrier phase observations. |
1: Reduces number of parameters that need to be soled ( satellite & receiver clock errors. 2: Reduces common mode biases. (sat orbit and ephemeris, tropo, iono errors. |
|
|
What is a nuisance parameter. |
A bias that is well understood / can be estimated. Eg: carrierphase ambiguity, troposphere delay. |
|
|
What is the primary reason to have two carrier phase frequencies. |
To remove ionoshpere delay through Ionosphere-free LC |
|
|
2 things to consider with Ionosphere-free LC |
1: the combined ambiguity term is not an integer. 2: Noise level has increased by a factor of 3. Its good for long baselines, but not worth it for short baselines (where ionoshpere bias will cancel with differencing anyway) |
|
|
Another name for ionoshperic LC |
Geometry-free LC |
|
|
What does ionospheric LC remove. what is its primary use? |
position term is elimated. (geometry-free LC). Primary use is to model the ionoshpere. |
|
|
How much does Wide-lane LC (l5) increase ionosphere bias term by? |
2x |
|
|
How much does Narrow-lane LC (l6) increase ionosphere bias term by? |
4x |
|
|
Current GNSS deficienies |
Integrity not guaranteed not all satellites monitered always. no indication of quality of sevice Accuracy not sufficient. vertical accuracy >10m. Availability and conitinuity Integity. Monitor stations |
|
|
Augmentation is to |
use netowr solutions to help wth GNSS deficiencies. |
|
|
what is the weighting strategy fo rpseudorange stochastic models based on? |
satellite elevation |
|
|
WAAS |
Wide Area Augmentation System |
|
|
Estimated parameters for PPP |
receiver coordinates receiver clock errors tropo delay carrier phase ambiguity |
|
|
PPP Benefits |
Single GPS receiver No local base station needed Not limited by a spatial range No need for simultaneous obs Position in terms of global ref system |
|
|
PPP limitations |
Long initialisation time >20mins Access to precise orbit and clock products Systematic biases : phase wind-up, satellite antenna offsets. |
|
|
Accuracy of double difference crarier pase measurement |
5mm + 1mm/km |
|
|
Single diff, double diff, triple diff. what biases are eliminated / reduced |
SINGLE: satellite clock error. atmosphere DOUBLE: sat/receiver clock errors. atmosphere TRIPLE: sat/receiver clock errors. integer ambiguities. atmosphere. |
|
|
what is needed to resolve ambiguities |
SV geometry chane. ie; time |
|
|
Ionoshpere free L3 is used for what |
Long baselines where ionoshpere delay will muck things up |
|
|
Ionospheric-free is also known as what, and is used for what |
Geometry-free. To model the ionoshpere. Doesnt give any positional information |
|
|
What are wide and narrow lane for? |
Ambiguity resolution |
|
|
What does DOP not indicate |
Accuracy / reliability. It just shows good geometry |
|
|
How are distance indepoendednt errors controlled? |
Distance independent errors: multipath. fiz with >30min SV geometry change. |
|
|
how are distance dependent errors controlled? |
distance dependent errors: atmosphere, tropo iono. Fixed with independent baselines. double ties, second base station, totalstation measurements. |
|
|
Benefits of Netowrk RTK |
No basestation setup: security, time etc.. Netowrk integrity: improved reliability, detect problem reference stations, Data modelling: imprioved accuracy, reduces distance dependent errors. Models common moe errors. |
|
|
Limitation of Network RTK |
High cost Subscription fees Limited by cellphone coverage Availability dependent on netowrk extent accuracy affected by network density. |
|
|
horizontal transformation scale factor needs what? |
a range of distances. Is dependent on the number of mark tied to |
|
|
If all distances ina hz tranformation are similar, what happens to the scale |
scale change and coordinate errors are not distinguishable. |
|
|
How do you push errors into the coorindate residuals in a hz tranformatino? |
Only use 3 parameters N E Rot. Scale will be set to 1.00 |
|
|
what is a trivial baseline |
a not independent baseline. |
|
|
The 2 layers of the lithosphere |
SCHIZOSPHERE - brittle PLASTOSPHERE - ductile |
|
|
OTF |
On the Fly use a searching technique to check various combinations within a predefined 'ambiguity space' to resolve the ambiguities. Main advantage is fast resolu8tion and capability to be able to recover from cycle slips and get ambiguities while antenna is moving. Need MINIMUM 5 SVs |
|
|
How is the shuler mean calcd |
N = 0.5( ( o.5(u1+u3) + u2) |
