Week 8: GNSS II Flashcards
(124 cards)
1
Q
Three types of GPS error sources
A
- Satellite based errors
- Atmospheric/transmission errors
- Receiver and environment errors
2
Q
Five satellite (and receiver) based errors
A
- Satellite (and receiver) clocks
- Satellite orbits
- Satellite geometry
- Check satellite availability
- Selective availability
3
Q
Satellite clock errors (predicted - not measured offsets) are broadcast in the
A
Navigation code
4
Q
Receiver clock error determined by observations to at least
A
4 SVs
5
Q
Both satellite and receiver clock errors can be eliminated by relative positioning with GNSS is known as
A
Common mode errors
6
Q
The ephemeris broadcast is updated every
A
Two hours (orbital data never exact)
7
Q
Differential positioning involves how many receivers
A
2+ receivers
8
Q
For differential positioning, the relative error is greater for
A
Longer baselines (e.g 5mm for a 100km baseline, 25mm for a 500km baseline)
9
Q
The broadcast ephemeris for differential positioning
A
+-1m -> 0.05 ppm baseline error
10
Q
For precise work (e.g a countrys geodetic network), what may be obtained and used
A
Post-computed orbital data (precise orbits)
11
Q
Precise ephemeris of precise orbits
A
Suitable for long baselines and post processing (+- 0.025m)
12
Q
For total station intersections and resections, the geometry of the fix should be
A
Well conditioned otherwise positioning accuracy will be poor
13
Q
For GNSS, if SVs are close to each, or not well spread around the sky, there will be
A
Poor position fix (high DOP)
14
Q
A measure of geometry strength is
A
DOP (dilution of precision)
15
Q
Low DOP means ____, high DOP means ____
A
Low DOP means good geometry, Hight DOP means poor geometry
16
Q
HDOP is
A
Horizontal dilution of precision, 2D
17
Q
VDOP is
A
Vertical dilution of precision, 1D
18
Q
PDOP is
A
Positional dilution of precision, 3D
19
Q
GDOP is
A
Geometrical dilution of precision, 3D + time
20
Q
Users can usually set a precision in the receiver and where there is high DOP
A
It may take longer to achieve good precision
21
Q
If you overide the set precision and save the result regardless of precision you will need to
A
Make a note of this and consider it in your positioning analysis
22
Q
US DoD intended C/A code measurements to give an accuracy of around ____ for civilians
A
150m
23
Q
US DoD intended C/A code measurements to give an accuracy of around ____ for US defence and allies using encrypted P code
A
15m
24
Q
Better accuracies were possible, so, until switched off in May 2000, what was implemented
A
Selective availability (SA)
25
Two implementations that modified the satellite's transmission
1. Dither was applied to SV clocks to bias the range measurement
2. Epsilon manipulated the ephemeris to degrade orbit information
- Military receivers could undo these effects
26
If SA was on (it could still be switched on at any time, e.g in times of conflict) accuracy would again degrade to
100m for civilian users
27
28
Two layers of atmospheric errors
Ionosphere and troposphere
29
GNSS signals are in the electromagnetic part of the spectrum, and are therefore
Affected (delayed) by the atmosphere
30
The ionosphere extends from
50 - 1000km above the earth
31
Ionosphere comprises free electrons, whose density changes according to
Ionisation of gas molecules - usually by solar activity
32
What do charged free electrons do to the signals that travel at less than the speed of light in a vacuum
Slow them down
33
The density of free elctrons in the ionosphere depends on
Magnetic activity, sunspot activity, time of day, location, direction and elevation of GNSS observation, etc (2024-2026 high activity years)
34
Positional error due to range error typically
10m, but up to 150m
35
Dual frequency receivers (L1 and L2) can measure different signal arrival times, hence can estimate/eliminate
Ionospheric delay
36
Single frequency receivers (L1 only) are less expansive but have to use what to estimate the delay
A model (delay will never be as accurate as dual frequency receivers)
37
Ionospheric delay is mostly cancelled how in single-frequency receivers
Relative positioning between 2 singlke frequency receivers
38
The troposphere extends from
0-10km above the earth
39
The stratosphere extends from
10-50km above the earth
40
The atmospheric zone where weather is experienced
Troposphere
41
For GNSS, tropospheric corrections and errors include the stratosphere. True or false
True
42
Total range errors are up to
2.5m
43
Range errors are not frequency dependent in the troposphere, meaning
It cannot be removed using dual frequency
44
Range errors in the troposphere are modelled as
Dry delay and wet delay
45
The dry component takes up what percentage of tropospheric errors
90% (10% for wet component)
46
What results in the dry component being easier to model
Ground temperature and pressure (the ground humidity does not reflect the humidity at higher altitudes or nearby regions
47
What atmospheric errors are easier to model
Tropospheric, due to smaller error
48
Modelling and differencing can remove most tropospheric delay for baseline (2 receiver relative positioning) up to
10-20km for stations
49
Low elevation satellites are more prone to range delays: an elevation mask of ____ is usually imposed to exclude low elevation satellites
10-15 degrees
50
Two most common ground/environmental errors
Multipath and Antenna height error
51
Antenna are designed to receive
Satellite signals over large areas of sky visibility
52
Multipath errors in GNSS receivers result from
Receiving reflected signal paths
53
How do reflected signal paths cause errors
The signal takes a longer path -> satellite to receiver distance is longer, especially from flat surfaces, e.g water, wet grass, snow, metal, glass, buildings, etc
54
Multipath can give errors of a magnitude of
Several meters, particularly in urban canyon environments
55
Multipath errors can be reduced but
Not completely eliminated
56
What can be used to reduce multipath
Ground plane and choke ring antennae
57
Multipath changes over a few minutes as the satellite moves in relation to
Receiver and reflector (can be averaged out with a stationary setup, 2 or more minutes)
58
Multipath problems can be identified with
Repeated position fixes at different times
59
Cadastral regulations: need a second fix at least ____ later
30 minutes later
60
Critical component of antenna set up, usually mounted upon tripods
Centring and levelling
61
A GNSS measurement is taken to the
Antenna phase centre (APC)
62
Antenna height equation
Vertical height + antenna phase variation
63
The correct mark on the antenna must be used to accurately measure
To the ground
64
One of the biggest sources of vertical inaccuracies is
Measuring or setting the wrong antennal height
65
Differential GNSS positioning is relative positioning with
Code observables
66
Precise relative positioning is relative positioning with
Carrier phase measurements
67
Point positioning accuracy using C/A (1-5m) that can sometimes be degraded also applies to both
Consumer grade (handheld) and geodetic receivers (latter a bit better owing to better antennae and software)
68
Using either code-ranging (SPP) or carrier phase observables in relative positioning mode, accuracy can be improved by
1. Using two (or more) receivers operating simultaneously
2. Either using data from your own base station
3. Or using data provided by a third party (e.g positioNZ, omnistar or other commercial services)
69
A reference receiver or receivers is/are set up at a
Base station or reference stations
70
One or more roving receivers move around the survey area doing what
Collecting data
71
If the extent of the survey is relatively small, say < 20km what occurs
Similar errors are experienced at the base stations and rovers and errors mostly cancel e.g ionospheric / tropospheric delays, clock biases, orbit errors, etc
72
Do the errors in the receiver, antenna or multipath mostly cancel
No, they accumulate
73
Although the observables at both base and rover are subject to a variety of errors, the coordinate differences or baseline vectors between the receivers are
Typically <20km distance and relatively free from error
74
What accumulates between coordinate differences between the receivers
Multipath errors
75
3D baseline vector between the two points (geocentric)
X, Y, Z
76
Distance, azimuth and height difference between two sites can be known as a
Topocentric join
77
Usually the term differential or DGNSS is applied only to
Code observables (not on carrier phase observables
78
DGNSS is termed DGPS when
Only the GPS constellation is used
79
DGNSS usually gives what accuracy rather than 1-5m but can be better depending on which constllations or observables are used etc
0.5-2m
80
Either users establish their own base stations, or else
DGNSS real time correction services are used
81
DGNSS corrections may be provided by
Augmentation services (ground based (GBAS) or satellite based (SBAS)
82
Satellite based augmentation services (SBAS) use a network of
Ground tracking stations that collect GNSS data and compute corrections
83
SBAS corrections are transmitted to
SBAS communication satellites and then to users with adapted receivers
84
The NZ and AUS SBAS technology for both countries is known as
SouthPAN
85
SouthPAN contains
1. Legacy L1 DGPS
2. Multi-constellation DGNSS - GPS, Galileo, QZSS
3. PPP (precise point positioning) - L1 and L5 carrier phase frequencies
86
Is DGNSS, with accuracies of about a metre, suffieciently accurate for surveying applications such as construction engineering, cadastral and topographical surveys
No
87
Precise relative GNSS use
Carrier phase measurements
88
Precise relative GNSS may have what located at a known point
A base station
89
Precise relative GNSS may measure what between survey marks
3D baseline vectors
90
Relative positioning using differencing with two static receivers components
1. Each receiver counts the incoming whole carrier cycles and measures the fractional carrier phase
2. Can derive the difference in position between two receivers (relative positioning)
3. Determine the vector or baseline between two points
4. Thus, compute the coordinates of an unknon point (B) with reference to a known point (A)
91
When using GNSS for surveying, a survey grade GNSS used in relative mode involves what
A baseline is measured and computed, between two receivers
92
Classical method for long baselines and high accuracy
1. 3mm + 0.5 ppm
2. Observing session of 30 mins, 2 hours or 24 hours
2. Create a network of baselines
3. Shorter baselines resolved in 15-20 minutes with dual frquency rapid or fast static 1cm + 1ppm
93
Classical static GNSS applications
1. Survey control
2. Large scale geodetic network
3. National and global network
4. Monitoring tectonic movement
5. Deformation networks
94
The vector between the reference and rover receivers is known as a
Baseline
95
The rovers position is determined by
Computing the baseline vecotr components and adding to the base station position
96
If the base station position is approximate (e.g you have accepted the SPP solution) then there will be
Systematic error in absolute position
97
What is required to deteemine correct coordinates for baselines
A local transformation
98
Used to remove most of the errors from baseline vector components
Differencing techniques
99
Three differencing techniques
1. Single difference
2. Double difference
3. Triple difference
100
A single difference is when
Measurements between one satellite and two receivers are subtracted
101
In a single difference, since both receivers are observing the same satellite at the same time, the satellite clock error is
Completely removed
102
In a single difference, if receivers are 10-30km from each other, observations will be
Subject to almost the same irbital errors, and similar atmospheric errors that (mostly) cancel
103
A double difference is when
The difference between 2 single differences removes/reduces all the errors removed/reduced in single differencing, but also removes the receiver clock errors
104
The differencing technique used by most GNSS software
Double difference
105
A triple difference is formed by
Differencing two double differences taken at two different times, keeps same satellites, same receivers, but different times/epochs and removes constant carrier phase integer ambiguity (if no loss of lock / cycle slips)
106
Triple differences can be used to
Repair the number (integer) carrier phase wavelengths if a cycle slip (or loss of lock) occurs
107
The base and rover both collect data and corrected positions are
Computed later by post-processing on a computer
108
RTK gives position on the
Receiver display, hence called "real time" kinematic
109
Characteristics of post-process kinematic (PPK)
1. Marginally more accurate than real time kinematic (RTK)
2. No concern about data transmission delay (latency)
3. Setting out (or "stake out") is not possible
4. Need to be sure ambiguities are resolved before moving
110
GNSS positioning can be in real time if there is
A radio data-link to transmit the GNSS data continuously from the base to rover
111
Radio data link can be alternatively through
A geo-stationary satellite, internet or cell phone network
112
A radio, mobile phone link or internet connection is need to
Transmit the GNSS data from a base station to a rover (or rovers)
113
The power of private radio transmission may be limited by
Law
114
Transmission delay of the data to the rover will deteriorate
Positioning performance (compared to post-processing) (known as latency)
115
For a post-processed kinematic survey (PPK) setting out is
Not possible
116
RTK uses little memory meaning ____ is stored but not ____
Points with coordinates are stored, but not the transmitted GNSS data
117
For post processing, does GNSS data need to be stored
Yes
118
Repeater (parrots) transmitters may be used to
Extend the range of a base station
119
Mobile internet is the most common usage of RTK GNSS data transmission except for when
Areas of limited or no mobile coverage, or obstructed environments, in which where repeaters are preferred
120
Five advantages of RTK
1. Coordinates may be displayed in real time, thus setting out is possible (huge advantage)
2. Also possible to see if ambiguity tracking has been lost and they need to be re-initialised / re-fixed
3. Acceptable tolerances can be chacked before leaving the field
4. Smaller memory requirements to store coordinates and not the raw GNSS data (however memory is cheap these days)
5. Provides cm-level accuracy provided that the baseline length is not too long and/or sufficient time has passed (typically less than 20km)
121
Examples of where coordinates being displayed in real time is an advantage
1.A critical road infrastructure being built, where the road workers will work on the fly waiting for each point to be staked out
2. Also able to navigate to exiting marks e.g finding cadastral marks
122
Precise Point Positioning (PPP) characteristics
1. Post-processed and real time
2. precise satellite orbits and satellite clocks, e.g international GNSS service (IGS), system provided, commercial etc
3. Code and carrier phase measurements
- Ionosphere: dual frequency ionosphere combination
-Troposphere: modelled or setimated
- Estimated parameters: 3 Receiver coordinates, 1 receiver clock, 1 troposphere (at zenith), and 1 carrier phase ambiguity for each satellite
- (integer) ambiguity resolution is possible
123
Precise point positioning (PPP) benefits
1. Single GPS receiver
2. No local base station (or RTK network) required
3. No spatial operational range limits
4. No requirement for simultaneous observations (base + rover)
5. Position in terms of a global reference system e.g ITRF2008, ITRF2014
124
Precise point positioning (PPP) limitations
1. Initialisation time (typically > 20 minutes)
- initialisation time limits real-time applications ...but becoming shorter
2. Needs timely access to precise orbit and clock products for real time
3. Current position in global frame
- Transform to local geodetic datum e.g NZGD2000
- Does not account for regional deformation
