Module 5/6

Question 1

Pseudo Random Noise Code Difference

Answer 1

Typical Time Difference 3*105/3*108 = .001

L1 Carrier frequency 1.57542*109 (GHz)

~1.6 *106 cycles in offset waveform, adequate for PRN calculation.

Question 2

Fundamental Idea of Differential GPS

Answer 2

Receiver

Unknown coordinates = xi,yi,zi

DGPS correction= xi+(xj-.‐error),

yi+(yj-.‐error), and

zi+(zj-.‐error)

WAAS Station DGPSBase

Known coordinates = xj+0, yj+0, zj+0

Correction=xj-.‐error, yj-.‐error, zj-.‐error

Question 3

WAAS

Answer 3

Wide Area Augmentation System

Question 4

How good is WAAS

Answer 4

With Selective Availability set to zero, and under ideal

conditions, a GPS receiver without WAAS can achieve

fifteen meter accuracy most of the time.*

Under ideal conditions a WAAS equipped GPS

receiver can achieve three meter accuracy 95% of the

time.*

Precision depends on good satellite geometry, open sky, and atmospheric distortion errors.

Question 5

Ionosphere and Tropospheric Errors Can Be Corrected in Two Ways

Answer 5

• The use of dual frequency reception
• Differential corrections

* All other errors cannot be reconciled through these techniques

Question 6

Dual frequency L1/L2 phase shift

Answer 6

(relative time delay) due to

Ionosphere results from differential bending of the UHF

Radiowave.

Question 7

Cesium atomic clocks aboard the NAVSTAR satellite

Answer 7

produces the fundamental frequency, 10.23 Mhz. The

L1and L2 carrier frequencies are generated by multiplying the fundamental frequency. Each GPS satellite transmits data on two L-Band (low microwave) frequencies, L1 (1575.42 Mhz 154•10.23MHz) and L2 (1227.60 MHz 120•10.23MHz).

Question 8

GDOP

Answer 8

GPS Error is measured By ‘Dilution of Precision’.

GDOP is the Geometric Dilution of Precision

1/V(tetrahedron)

Large Tetrahedron volume = higher precision

Question 9

Ideal best satellite configuration

Answer 9

is an Isoceles tetrahedron

v=1/v(tetrahidrum)

V = a3/6√2 = √3 3/6√2

Largest Tetrahedron volume of 0.612

= highest ideal GDOP precision ~1.6

Actual best GDOP is ~1.73

Question 10

GPS Accuracy:

Answer 10

Dilution of Precision DOP

Sources of User Equivalent Range Errors

Computing DOP, consider the unit vectors from the receiver to satellite i

x,y,z denote the position of the receiver and xi, yi, zi denote the position of satellite i. Formulate the matrix, A, as:

The first three elements of a row of A are the components of a unit vector from the receiver to the satellite.

The elements in the fourth column are c , the speed of light. Formulate the matrix, Q, as:

d’s represent geometric equivalents of

variance and covariance. DOP uses the variance diagonal.

Question 11

DOP Values, and what they mean

Answer 11

1.7 Ideal This is the highest possible confidence level to be used for

applications demanding the highest possible precision at all times.

2-3 Excellent At this confidence level, positional measurements are

considered accurate enough to meet all but the most sensitive applications.

4-5 Good Represents a level that marks the minimum appropriate for

making business decisions. Positional measurements could be used to make

reliable in-route navigation suggestions to the user.

6-8 Moderate Positional measurements could be used for

calculations, but the fix quality could still be improved. A more open view of the sky

is recommended.

9-20+ Fair-POOR Represents a low confidence level. Positional

measurements should be discarded or used only to indicate a very rough estimate

of the current location. At this level, measurements can be as inaccurate as 50

m and should be discarded.

Question 12

Causes of Precision Error

Answer 12

• Satellite 1’s radio signal travels through less of the atmosphere, resulting in less distortion. Satellite 2 is low on the horizon, however, resulting in significant atmosphericdistortion.
• Deviations in a satellite’sactual orbit path can also cause loss in precision
• A receiver is confused by “multipath”, where several

reflected signals are received (red) along with the direct radio

signal (green).

Question 13

Real Time Kinematic DGPS

Answer 13

Rover Receiver

DGPS Coordinates=xRover +ΔxBased, yRover+ ΔyBase

Base Receiver

True coordinates = xBase, yBase

Correction = ΔxBased, ΔyBase

Question 14

Real Time Kinematic (RTK) & Network RTK

Answer 14

Real time error correction from one or more base stations (e.g. Continuously Operating Reference

stations - CORS)

Question 15

Differentials Transmitted Real-Time,

Rover accuracy

Answer 15

1-3 cm

Question 16

Post-processed differential GPS corrections

to a 15 minute – 48 hour data gather.

Answer 16

DGPS correction = xi+(xj-error),

yi+(yj-error) and zi+(zj-error)

3 or more CORS Stations

True coordinates = xj+0, yj+0 , zj+0

Correction = xj-error, yj-error, zj-error

Question 17

CORS

Question 18

How does OPUS compute your position?

Answer 18

OPUS – using CORS

3 single baselines computed

3 positions averaged —simple mean (equal weights)

Question 19

OPUS Guidelines

Answer 19

• L1/L2 data
• 2-48hrs GPS (OPUS-S)
• 15min-4hrs GPS (OPUS-RS)
• Static GPS observations only
• 30-second epochs processed
• Correct vertical requires:
• antenna type/phase model
• mark-to-ARP vertical height

Question 20

How are OPUS-S Positions Computed?

Answer 20

• NGS PAGES processing software
• Ionospheric/Troposphere-free solution (solar

interference & weather)

• Fixed ambiguities
• Average of 3 unique CORS ties
• ITRF & NAD83 coordinates/errors

Question 21

Antenna Height

Answer 21

• true vertical distance
• measured in meters

Question 22

Is your Opus-S solution good?

Answer 22

check ephemeris type

• > 90% observations used
• > 50% ambiguities fixed
• < 3 cm overall RMS

Check antenna info

• < 5 cm peak-topeak errors

and which CORS were used? …resubmit later for better CORS scenario & ephemeris

Question 23

HOW GOOD ARE OPUS ORTHOMETRIC HEIGHTS?

Answer 23

IT DEPENDS!

ORTHOMETRIC HEIGHT ~ 0.02 – 0.04 m

GEOID03 ~ 0.048 m (2 sigma – 95% confidence)

Error ~ 0.03 + 0.05

~ 0.08 m

Question 24

To enhance vertical accuracy use rapid orbits available in 24 hours

Answer 24

Ultrarapid Orbits ~ 0.02- 0. 04 m (12 hours)

Rapid Orbits ~ 0.01 – 0.02 m (24 hours)

Precise Orbits ~ 0.005 – 0.01 m (two weeks)

Question 25

OPUS Accuracy is

Answer 25

time dependent and NOT distance dependent

Question 26

OPUS-Rapid Static (OPUS-RS)

Answer 26

• 15-minute to 2-hour sessions
• ties to 3 – 9 CORS (< 250km)
• P1/P2 code & L1/L2 phase observations
• resolves all ambiguities
• similar to Real-Time Network computations

Question 27

OPUS-RS Output

Answer 27

“Overall RMS” replaced by “Normalized RMS”

• unitless quantity, “expected” = 1
• aka standard deviation of unit weight
• if > 1, noisy data somewhere
• typically <1, meaning noise less than expected

Peak-to-Peak replaced by Est. Standard Deviations

• approximately 95% confidence
• derived from scatter of single baseline solutions
• formal standard deviations (optimistic) available in Extended Output

Question 28

CORS PARTNERS

Answer 28

>98% VOLUNTEER (NON-NGS)