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APG3016C > Module 3 - Differential Data Processing Techniques > Flashcards

Module 3 - Differential Data Processing Techniques Flashcards

1
Q

The pseudo-range equation for GPS measurements is as follows:

(i) Identify and describe the main observable.

A

Δ𝑑τ€―œ = measured time offset between the received signal from the satellite and the signal
generated in the receiver.

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2
Q

What does PDOP stand for?

A

Position Dilution of Precision

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3
Q

True or False: GPS is a range-rate system

A

False

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4
Q

True or False: In GPS pseudo-ranging, the term β€œpseudo” is because time is measured rather than range?

A

False

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5
Q

Calculate the expected effect of an orbit bias of 3m over a baseline length of 30 km

A

30000*3/20000000 = 0.0045m = 0.005m

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6
Q

Name one other satellite dependent bias

A

Satellite clock bias

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7
Q

Systematic errors – reduced by modelling

A

dion, dtrop

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8
Q

Measured values

A

delta t (Δ𝑑τ€―œ)

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9
Q

Unknown quantities

A

dxp, dyp, dzp, Ξ΄t

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10
Q

Initial estimates – provisional values

A

xp0, yp0, zp0

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11
Q

Correction supplied in the navigation message (not modelled)

A

cdt

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12
Q

Known values – constants, or determined from information contained in the navigation
message

A

c, xi, yi, zi

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13
Q

True or False: Refraction of GPS signals causes systematic errors which can be modelled in order to reduce
their effects on the measurements

A

True

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14
Q

True or False: Horizontal positions are more accurately determined than vertical positions using GPS due to
the refraction effects on the signal

A

True

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15
Q

What does GDOP stand for?

A

Geometric dilution of precision

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16
Q

Describe four (4) attributes of the GPS orbits.

A 
55 degree inclination
20200 km above the earth’s surface
6 orbital planes
4 per orbital plane (24 constellation)
Period of 11h58 min
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17
Q

Why is this process called pseudo-ranging and not simply ranging?

A

Ξ”t; pseudo ranging is uncorrected for clock offset

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18
Q

Where does PDOP come from and what does it mean?”

A

The DOP stands for Dilution of Precision and PDOP stands for Position Dilution of Precision. PDOP
is a value derived from the square root trace of the Qxx matrix, or N-1 or (AtPA)-1. With PDOP only
the first three elements are used as these are the position elements. This value indicates the
relative geometry of the satellites and the receiver. It ranges from about 2 to 6 – the lower the
PDOP the better the geometry.

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19
Q

What is the most likely reason for

inaccurate position fix?

A

Multipath

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20
Q

Tropospheric and ionospheric refraction are modelled
What is/are the common feature(s) of these models?
Explain this/these features.
Explain how a GNSS user (with an old code-correlating receiver using L1 only) can adopt a setting on
the controller/receiver to avoid these residual errors after modelling from becoming huge

A

The common term/feature is the relationship between the error and the secant of the zenith angle.
The errors increase to β€œinfinity” near the horizon and the modelling thereof becomes unstable
near the horizon. So after modelling, there would still be huge residual errors. A simple method is
for the user to set the β€œsatellite cut-off angle” to 10 to 15 degrees above the horizon. All satellite
signals received below this angle will be ignored.

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21
Q

Give at least three errors/biases that are not explicitly contained within the GPS pseudorange model above, and state, for each one, whether a DGPS system such as OmniStar would reduce their effect on the solution?

A

Multipath – not reduced through differencing;
Orbit Errors – DGPS will reduce
Receiver Resolution – no, DGPS will not help
Antenna Phase centre – not an issue for code phase measurements – accuracy level is not sensitive to this
Satellite constellation geometry – DGPS wont help much as PDOP at the base station and the rover is likely to be similar, unless there are obstructions at the rover, in which case DGPS will help.

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22
Q

True or False: Ionospheric refraction is a function of the frequency of the GPS carrier signals. This allows the determination of the effect of refraction on the signals if more than one frequency signal is transmitted by the satellite and observed by the receiver.

A

True

23
Q

True or False: In code phase GPS, averaging over about 20 minutes will substantially reduce the effects of multipath

A

True

24
Q

True or False: Increased baseline length reduces correlation of the errors that affect GPS measured ranges, and therefore decreases the benefits of code phase differential corrections and carrier phase differencing

A

True

25
Q

True or False: Receiver resolution affects carrier beat phase measurements and code phase measurements equally

A

False

26
Q

Which systems are Ground-Based

System?

A

Trignet, LAAS, Nasa Global DGPS,

27
Q

Which systems are Satellite-Based

System?

A

Quasi-zenith satellite system, WAAS

28
Q

Which systems Use geostationary
satellites or near geostationary
satellites for
communications?

A

Quasi-zenith satellite system, Omnistar, WAAS

29
Q

Which systems Uses
internet/GPRS/UHF/
VHF for
communications?

A

Trignet, LAAS, Nasa Global DGPS

30
Q

What is the GPS antenna phase centre error?

A

It is the electronic centre at which the electromagnetic radiation(signal) from the satellite appears to be received – it is the point to which the measurement refers, or the β€œzero”

31
Q

Is this error frequency dependent?

A

Yes it is – the antenna phase centre for L1, L2(C), L5 will be different

32
Q

What size could this error be for carrier phase observations?

A

1cm

33
Q

What simple field technique can be employed to mitigate against this error?

A

Use the same make and model of antenna and orient these in the same direction (a compass is helpful) so that any offset affects both terminals of the baseline vector equally

34
Q

Motivate the benefit of using DGPS rather than stand-alone GPS

A

DGPS will reduce unmodelled biases common to the satellites and due to refraction. With DGPS corrections, the student will be able to achieve more accurate position fixes for the plants enabling them to be relocated more easily and plotted more accurately. Without DGPS you would get 2-6m 95% of the time. With DGPS this can be reduced to 1-2 m or better.

35
Q

Explain what OmniSTAR is and the main reason why this is not the best solution

A

OmniSTAR is a commercial SBAS DGPS provider – pay for the DGPS corrections

36
Q

an error source that is not reduced or eliminated in DGPS and what he/she should do about reducing this source of error in the field.

A

Multipath could affect the results. The student should be careful to avoid reflective surfaces such as bill boards and cars near the surveyed points. If this is not possible, the student could stand between the potentially reflective object and the receiver and use his/her body as a multipath shield.

37
Q

What is RINEX and when would you use it in GPS positioning?

A

RINEX is a standard format for GPS data storage and exchange – you would mostly use it if you were going to do GPS post processing as all your raw data would be stored in this format. It has file formats for the observations files, navigation files etc.

38
Q

What does the carrier phase equation for GPS equation model/represent?

A

The equation represents the 3D range vector between the satellite and the receiver using the
measured quantity of the carrier phase shift.

39
Q

Explain the main purpose of a receiver performing β€œbetween epoch differencing” in the solution of a
GPS position.

A

The purpose is to form a differenced equation in which the integer ambiguity unknown has been
eliminated by differencing – this can be combined in a triple difference which is then used to calculate an
initial solution of the receiver coordinates and removal of β€œcycle slips” which occur when there is loss of
lock.

40
Q

Which of the following is used to obtain the final position fix in carrier beat phase positioning:
between satellite single differencing, between receiver single differencing, between receiver-satellite
double differencing, or between epoch differencing?

A

between receiver-satellite double differencing

41
Q

True or False: Multipath effects on GPS positions can be reduced if a choke-ring antenna is used, if the site is occupied for about 30 minutes, and if an antenna with a good gain pattern is used. However, multipath is not reduced in differential GPS or double differencing.

A

True

42
Q

True or False: Orbit bias is a satellite-dependent error and its effect on positioning is thus eliminated from the computations when using double-differencing.

A

False

43
Q

True or False: Satellite clock bias is a satellite-dependent error and its effect on the positioning is thus eliminated from the computations when using double-differencing.

A

True

44
Q

True or False: Receiver resolution is the ability of the receiver to resolve the integer ambiguities

A

False

45
Q

True or False: Receiver clock bias is so small that one does not need to consider this in carrier phase GPS positioning.

A

False

46
Q

True or False: The effect of antenna phase centre differences on GPS positioning can be reduced by using the same type of antennas and orienting them in the same direction at each set-up using a compass

A

True

47
Q

True or False: Satellite constellation - receiver geometry does not affect GPS positioning to any significant level as there are enough satellites available now and they are so far away compared to the scale of the survey.

A

False

48
Q

Draw a sketch to illustrate real-time DGPS

A

Refer to notes

49
Q

Give brief explanations of the UHF and GPRS media for data transmission for DGPS purposes.

A

UHF (radio) is ultra-high frequency – needs line of sight and uses little power – useful for small scale
surveys.
GPRS is General Packet Radio Service and is used to broadcast radio/music over internet – small volume
data and so high speed.

50
Q

What is RTCM SC 104?

A

RTCM SC 104 is a serial protocol used for broadcasting pseudo range corrections from differential-
GPS reference stations. It is an attempt to introduce a standard for the transmission of range
correction data, and has gained fairly wide acceptance

51
Q

Explain the main difference between SBAS and GBAS and what essential feature of these systems is
similar?

A

SBAS: Satellite based augmentation systems use space based communication methods to disseminate GPS
corrections for DGPS applications. In other words, communications satellites are involved in broadcasting
DGPS corrections.
GBAS: Ground based augmentation systems use land based communication of differential corrections.
Both systems rely on a network of ground base stations to compute the DGPS corrections.

52
Q

What is a β€œbaseline” in carrier phase differential GPS?

A

A baseline is the 3D space vector between the antenna at the base station and the antenna at the rover – or between any two GPS antennas operating at the same time and which are combined in a double difference solution.

53
Q

Why are two receivers required as a minimum for a solution using carrier phase GPS?

A

There are too many unknowns in the range equation using the carrier phase measurement. Each time you add another satellite, more unknowns are added, largely due to the integer cycle ambiguities and also the satellite and receiver clock biases. So adding more observations does not allow for the number of equations to be equal or greater than the number of unknowns. The only way to solve the equation is to difference the observation equations in such a way as to remove some of the unknowns. Then adding more satellites (and hence observation equations) allows the number of equations to become larger than the number of unknowns.

54
Q

complete observation equation for pseudo-range positioning: 𝑐.Ξ”π‘‡π‘—βˆ’π‘.π‘‘π‘‘π‘—βˆ’π‘‘π‘–π‘œπ‘›βˆ’π‘‘π‘‘π‘Ÿπ‘œπ‘=πœŒπ‘–π‘—βˆ’π‘.𝛿𝑑𝑗

A
  • 𝑑𝑑𝑗 (satellite clock offset and drift correction obtained through the navigation message)
  • π‘‘π‘–π‘œπ‘› (ionospheric refraction correction)
  • π‘‘π‘‘π‘Ÿπ‘œπ‘ (tropospheric refraction correction)
  • 𝑐 (Speed of light in a vacuum)
  • πœŒπ‘–π‘— (true range)

APG3016C (5 decks)

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