Week 7: GNSS I Flashcards
(110 cards)
1
Q
When did Russia launch Sputnik, the worlds first satellite
A
October 4th, 1957
2
Q
After analyzing Sputniks orbit using the doppler shift, what was suggested if the satellites position and orbit was known and predictable
A
It was suggested the doppler shift could locate a receiver on earth
3
Q
The US launched the transit 1A prototype in ____ (a failure), and transit 1B in ____
A
1A in Sept 1959, and 1B in April 1960
4
Q
Successful satellite positioning tests began when
A
1960
5
Q
Satellite positioning became operational in the navy when
A
1964
6
Q
Satellite positioning was primarily used for
A
Military use, e.g submarines to surface and reset inertial guidance systems
7
Q
What was required in satellite receivers
A
Passive receivers
8
Q
Purpose of passive receivers in satellite positioning
A
- Data is stored on receivers and positions not displayed in real-time
- Enemy could not intercept ground transmissions
- Unlimited users at one time
9
Q
A single satellite pass has a positioning accuracy of
A
200m
10
Q
The Doppler Transit system was superseded by
A
NAVSTAR
11
Q
What does NAVSTAR stand for
A
Officially NAVigation Satellite Timing And Ranging, known as global positioning system
12
Q
NAVSTAR was developed from ____, first satellite launched by US in ____ and fully operational by ____
A
Developed from 1973, first satellite launched in 1978, fully operational by 1993
13
Q
Because the development design of GPS is more than 50 years old, how does this explain complexity for survey
A
- Focus is on 1970s military needs
- Civilian surveying, or navigation better than 100m, was never intended
14
Q
GPS is now jointly operated by
A
The US military (DoD) and Department of Transport
15
Q
Number of useable GNSS satellites in early 2025 is around
A
125
(GPS 31, GLONASS 26, Galileo 27, Beidou 35, QZSS 4, IRNSS 2)
16
Q
The four segments of GNSS
A
- Space segment
- Control Segment
- User Segment
- Ground segment
17
Q
The space segment involves
A
The satellites or SVs
18
Q
GPS has a minimum of ____ satellites in orbits 20,200km above the earth,
A
24
19
Q
GPS has an orbital period of
A
Half a sidereal day (some GNSS use different altitudes than GPS and therefore different orbital peiods
20
Q
The control segment involves
A
Stations positioned around the earth to control and monitor the satellites, calculate orbits, and upload orbit/ephemeris data
21
Q
The user segment involves
A
The users of GNSS
22
Q
The ground segment involves
A
International GNSS service, which provides GNSS data, precise ephermeris, satellite clocks and ionosphere models for civilian users
23
Q
Our clocks are aligned with universal coordinated time (UTC) which is based on
A
Average solar time (24 hours between successive transits of the sun over an observers median
24
Q
A siderial day is based on
A
The interval between successive transits of a star over an observers meridian
25
Is a solar or sidereal day shorter
Sidereal day, by about 3m 56s
26
After 1 orbit of a GPS satellite in half a sidereal day, the Earth has completed
Half a revolution
27
After 2 orbots (1 sidereal day) the Earth has completed almost
A full revolution and the satellites have completed 2 full orbits
28
Satellites return to the same apparent position in the sky ____ each day as measured by solar time
3m 56s earlier
29
A sidereal day is defined as
The interval between successive transits of the sun over an observers meridian
30
A solar day is defined as
The interval of time between successive transits of the sun over an observers meridian
31
Does it take longer for the sun or a star to transit an observers meridian
The sun (earth must turn a little further for the sun to be over the meridian)
32
What computes the position of all GPS satellites
Control segment ground stations
33
What accurately predicts the positions where the satellite will be
Broadcast ephemeris
34
Broadcast ephemeris are sent to satellites by the control segment every
Two hours, and then broadcast from satellites to users
35
Current broadcast ephemeris accuracy
1m
36
Precise ephemeris
Post-computed positions where the satellite was and clock offsets
37
Precise ephemeris is online from international GNSS service (IGS) (not broadcast) with a latency of
12-19 days
38
Current precise ephemeris accuracy
0.025m
39
Precise ephemeris is preferred for
Extremely accurate work and long baselines (e.g a national control network)
40
IGS also provide an ultra rapid predicted orbit accurate to about
5cm for real time use
41
Each satellite has ____ very accurate clocks which run on ____
3 - 4 very accurate clocks which run on GPS time
42
GPS time runs at the same rate as
UTC (average solar time)
43
GPS time has not had leap seconds applied (18 seconds) since
6 Jan 1980
44
GPS time is exactly ____ seconds ahead of UTC
18 seconds
45
Leap seconds complicate calculating the
Orbit of a satellite moving at 4km/sec
46
The control segment also monitors
Satellite clocks and calculates clock corrections
47
Each satellite continuously broadcasts two electromagnetic signals in the "L" band known as
L1 and L2
48
The latest GPS satellites have a third frequency known as
L5
49
L1 and L2 (and L5) are what type of waves
Carrier waves
50
Precise atomic clocks in satellites have a frequency of
10.23 HZ
51
The P(Y) code is for military use and is on both L1 and L2 for what reason, and what is this called
- To protect against malicious imitation of the P code it is encrypted with a W code to generate the encrypted P(Y) code
- Known as anti spoofing (AS)
52
AS is able to be switched on and off, and is currently
On
53
Anti spoofing also prevents civilian access to
L2 and to the more accurate 29.3m wavelength (chip length) of the P(Y) code
54
The code for civilian use that exists only on L1 is ____ and has a less accurate wavelength of
C/A code with a wavelength of 293m
55
The navigation message (NavData) is modulated onto both L1 and L2, and includes what six components
1. A satellite clock correction determined by the control segment
2. Satellite (broadcast) ephemeris predicted by the control segment
3. An almanac
4. An atmospheric correction for the ionosphere
5. Satellite health information
6. Plus other stuff like control segment data we don't need
56
The satellite clock correction is used to
Correct the satellite clock and hence the measured satellite-to-receiver range
57
Satellite (broadcast) ephemereis is used for
1. Storing SV orbital information (one set per SV)
2. Used to compute the position of the satellite broadcasting the ephemeris
58
An almanac is used for
Approximate SV orbital information: just enough for a receiver to find and track all satellites
59
An atmospheric correction for the ionosphere is used to
Correct the signal propogation time through the atmosphere
60
Satellite health information provides
Info on SV malfunctions, bad / do-not-use SV etc
61
In GNSS, there are two observables that can be used to determine the satellite to receiver distance:
Pseudoranges and carrier phase
62
Using the observables, pseudoranges (PR) and carrier phase (CP), there are two methods of determining postition using GNSS which are
Code ranging and carrier-phase based positioning
63
Code ranging (consumer grade) usage
1. AKA, point positioning or single point positioning (SPP)
2. 3D position: (X Y Z) i.e point
3. Using pseudo range observations
64
Carrier phase based positioning (survey grade) usage
1. Relative positioning between two (or more) receivers
2. 3D baseline (X Y Z) i.e vector
65
An advanage of code ranging is that positioning is possible with
A single receiver (both handheld and geodetic receivers)
66
Point positions are fast to compute, but with
Low accuracy (+- 1-5m)
67
What occurs in code ranging to determine the distance to each satellite
A receiver measures the time taken (transit time) for the C/A code to travel from SV to receiver
68
Code ranging equation
Distance (or range) = time for the code to travel to the receiver (signal transmission time) x signal velocity (i.e speed of light in vacuum)
69
How many clocks are need in the EDM to measure signal transit time both ways
One clock
70
In EDMs, with one clock, the signal transit time both ways x the speed of light gives us
The two way distance
71
The returning signal in EDMs is timed using the same clock that created the outgoing signal, meaning there is
No time synchronisation bias in this measurement
72
EDM system is limited to how many users
One
73
In GNSS, how many clocks are needed to measure signal transit
Two, one in the satellite and one in the receiver
74
What is required by the two clocks in GNSS for accurate readings
They keep the same time
75
When the two clocks in GNSS readings lack synchronisation, users must
Estimate the clock bias
76
GNSS system has a ____ user system
Multi user
77
Ideally, both SV and the receiver would have both a very precise clock set to
GPS time (or at least a known offset to GPS time)
78
How many SVs are needed to determine a 3D position mathematically assuming no receiver clock bias i.e X,Y,Z
Three SVs
79
Why do receivers have less accurate clocks
Size and cost
80
Due to clock errors in the receivers, we therefore always need how many satellites for a 3D position point position in X Y Z
Four
81
The four satellites needed for point positioning determine what
The range error due to a lack of clock synchronisation, as receiver clock error will be the same for all satellites
82
All SVs have on board atomic clocks which generate individual
C/A codes
83
C/A codes are modulated onto what carrier wave
L1 Carrier wave
84
When a receiver locks onto an SV, the receiver generates
A replica signal identical with that of the SV
85
The replica code is compared with the received satellite code, and the receiver code is time-shifted until it is
In phase with the satellite code
86
The amount the receiver code is shifted to is equal to
The observed transmission time between the satellite and receiver
87
The downside to the receiver code being time-shifted is
It includes any biases, including receiver clock error.
88
The amount the receiver code is time-shifted to multiplied by the speed of light gives us the
Distance Pseudo-range
89
The accuracy of three dimensional positioning accuracy is
1-5m at 95% confidence level (CL)
90
There is a ____% chance of readings being less accurate then 5m
5%
91
What receivers are inadequate for high precision surveying
Receivers that have Code ranging only
92
Millimetre positioning precision is possible when
1. Measurements made not on the modulated C/A and P codes, but on the carrier phase waves themselves (with a short wavelength of about 0.19m)
2. There are equally precise phase measurements
93
For carrier phase observations, the distance between the satellite and receiver is
Distance = Integer carrier phase ambiguity (units of cycles) x Wavelength + Phase measurement consisting a faractional wavelength a plus a count b of the number of whole cycles since the receiver locked on x Wavelength
94
Challenges in finding the carrier phase ambiguity (or integer ambiguity)
1. Long distances involved (20,000 km)
2. The fact that the satellites are always moving ( 4.5km/second)
3. The pseudoranges accuracy is many cycles so can't resolve ambiguity
95
The starting point is for the receiver to use code-ranging to determine an approximate position, also referred to as
Single point position (SPP)
96
How many receivers are needed to solve for N so that most other biases can be removed or greatly reduced
Two
97
Estimating ambiguities is called
Floating them
98
Floating ambiguities involves
Allowing them to be estimated with non integer values
99
In order to resolve ambiguities, we need observations for up to
Several minutes to hours
100
Once the carrier phase ambiguity (N) is solved, the carrier phase observations can be used to estimate
The receiver positions at the mm-cm level
101
When the carrier phase ambiguities are correctly estimated, the baseline is
Recomputed holding the number of complete wavelengths fixed at an integer number (Fixed solution)
102
What comes as a result of a fixed solution
Baseline (relative) accuracy improves from decimeter-level in a single-epoch "float solution" to 10-50mm in the fixed solution
103
Carrier phase ambiguity cycles (integer) steps
1. Baseline float solution - estimate carrier phase ambiguity (N) as a real number
2. Determine integer carrier phase ambiguity - different approaches are used for RTK and static GNSS
3. Baseline fixed solution - recompute baseline using integer N - THe N term becomes a known term or correction
104
Once the ambiguity has been resolved, and if the receiver remains locked onto a satellite, it can generally
Keep track of the change in the whole (carrier phase) wavelengths, i.e count the wavelengths
105
Once you have gained lock or initialised, and have a fixed solution, tracking can be maintained unless what occurs
Cycle slip
106
Examples of cycle slips
1. The line of sight to a satellite is obstructed
2. Adverse atmospheric conditions are encountered
3. Multipathing occurs
4. The receiver will revert to a float solution until lock is regained
107
The receiver can always move when tracking SVs as long as it doesn't
Lose lock (kinematic GNSS)
108
The basis of kinematic GNSS and real time kinematic (RTK) GNSS
Gaining a fixed solution before starting to move
109
Carrier phase ambiguity known =
Fixed solution
110
