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CONS 127 > GNSS > Flashcards

GNSS Flashcards

1
Q

Important

Global Navigation Satellite Systems(GNSS)

A

Gps was the first GNSS system

gps is the gnss american system

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2
Q

History of GPS

A
  • was developed by the US military from 1973
  • Become operational in 1995 with 24 satelites
  • many people starting using it from 1996 and it became a civilian asset from 20000
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3
Q

How do GNSS systems work?

A
  • three system segments:
    1. Space segment
    2. Control segiment
    3. User segment
  • Satilites are communicating with control stations(two way signals)
  • Satelites send one way signal to reciver
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4
Q

Space Segment (GPS-NAVSTAR)

A
  • Minimum of 24 satellites on 6 orbits
  • from any point on earth,at least 4 satellites are visible for reciver

the space segments are always trying to improve

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5
Q

User Segment

A

thats you

ex:your phone

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6
Q

Control Segment

A
  • monitor station
  • master control station
  • ground station
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7
Q

How does satelites determine a location of an object?

trielation=uses distance calculation not angle (triangulation)

A

Example - which city is:
o 175 km from Amsterdam
o 320 km from London
o 185 km from Luxemburg

circles - using 4 satelites:

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8
Q

Satellite Navigation

A
  • Trilaterationrather than triangulation.
    -In the case of GNSS, uses the speed of light to calculate distances
  • Ranging (distance) measurements are calculated by determining differences in time(how far is the statelite from the reciver we can know what our position is)
  • Combining the distances and satellitelocations, the receiver can find its latitude, longitude, and height.
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9
Q

Calculating Location

A
  • General rule: the more
    signals received (from
    different satellites), the
    more accurate the
    calculated position

so to find your location u need 3 but to accurately and make sure we know your position 4 satelite

GPS uses 4

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10
Q

the setelites determine your position by :

A
  • Download almanac (information on satellites, their status, health
    etc.)
  • Download ephemeris (predicted location of each satellite)
  • Download GPS date and time
  • Measure ∆T to at least 4 satellites
  • Determine range
  • Calculate X, Y, Z
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11
Q

Ephemeris:

A

gives the detailed information of where the sitelite is

  • Detailed information on the day and time, satellite accuracy and health, age of data, satellite clock correction coefficients, and orbital parameters.
  • It is valid for 2 hours and is different for each satellite.
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12
Q

Almanac

A

not 100% necessarie

  • Contains less accurate orbital information than ephemerides
  • is valid for 90 days
  • It can speed up the time to the receiver takes to find the first few satellites.
  • It is the same system wide.

Therefore a receiver can work without the almanac but ephemerides data is always needed.

less acurate

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13
Q

How is ∆T (how far away are the setelites from the reciver) measured?

A
  • Comparing the time that the transmitted PRN code is received with the known time the PRN code was generated provides the time difference
  • The time difference (t) is multiplied by the speed of light (c) to estimate the
    distance to the satellite (P)
  • P = c(t) = 299,792,458(0.03) = 8,994 kilometers
  • These calculations happen instantaneously by the GPS receiver
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14
Q

Measuring Travel Time

A

the atonic clocks need to be increbly precise because if the satellite clocks were off by just 1 millionth of a second, our
position would be off by 3000 meters!

Signals travel a meter in about 3 billionths of a second (i.e., speed of
light)

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15
Q

recivers

A

you pay more more accurate it gets

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16
Q

How accurate is your phone as a reciver?

A
  • 3-15m 95% of the time (clear
    view of the sky)
  • Accuracy is influenced by:
    o Number and position of satellites
    o Atmospheric effects
    o Obstructions (trees, buildings etc.)
    o Receiver quality
    o Corrections / post-processing
17
Q

Differential Gps accuracy?

is the most accurate reciver today also the most expensive

A

maximum Horizontal accuracy :10mm and maxium vertical accuracy:20mm

18
Q

Sources of GPS Error

A
  • Radio waves cannot pass through some objects: buildings, some trees, mountains.
  • Radio waves can bounce or be reflected off of certain objects: water, large metal objects, buildings. Signal will not be accurate.
  • These can result in an inaccurate measurements due to multi path errors
19
Q

To Get the Best Measurement

A
  • Remain in the open
  • Avoid buildings and tall trees
  • Take several measurements and average them
  • Be patient

Remember the satellites are orbiting, and a satellite that was not visible
earlier may come into view of the receiver and give you a more accurate
measurement

20
Q

Dilution of Precision (DOP)

A

A measure of the geometry of the visible GPS constellation

a good DOP/ Low Delusion of Precison is when the setelites are all spread out in the sky,in different location and agles = good accuracy

a poor DOP/ High Delusion of Precison is when the setiles are all bunched togetherc = bad accuracy

21
Q

Dilution of Precision

A

PDOP = Position Dilution Of Precision (Most Commonly Used)

  • VDOP = Vertical Dilution Of Precision
  • HDOP = Horizontal Dilution Of Precision
  • TDOP = Time Dilution Of Precision

Very Good 1-3
Good 4-5
Fair 6
Suspect >6

Mission Planning Is Critical to Obtain
Good DOP

22
Q

Mission Planning

A

For given date and location provides information on satellite visibility
and expected accuracies (PDOP)

23
Q

Costs of Errors

A

GPS Receiver 1-2
Clock Errors 1
Ephemeris Errors 1
Tropospheric Delays 1
Ionospheric Delays(ions interacts with the radio waves wchich delays its conection to the satelites) 10
Multipath Errors(radiu signals beounce off diferent objects) 0.5
Total 10 - 20

24
Q

Differential GNSS

A

Differential correction is used to increase the accuracy of GPS location. A base station with known location can compute corrections for remote receivers. Corrections can be made in real time or by post-processing

25
Q

Real-Time-Kinematic GPS (RTK)

A
  • Positional Accuracy: +/- 2cm or so
  • Same satellite constellation
    o Base station and rover(s)
  • Carrier Phase
    o Track 5 satellites minimum
  • Radio link
    o More information
    o Fast transmission
    o Real-time result
26
Q

What’s the difference between DGPS(differential GPS) and RTK GPD?

A

DGPS
* Satellites
* Base station with known location
(Often a permanent station)
* **Less correctional information
transmitted **than RTK
* Transmitted slower
* Could be in real time
* Post processing

vs

RTK GPS
* Satellites
* Base station with known location
(Base station is mobile)
( Bring your base station to your site)
(More information transmitted than DGPS)
( Newer algorithm)
* Transmitted faster
* Real time correction

27
Q

A-GPS

A
  • A-GPS = Assisted GPS
  • When a receiver is turned on, it needs to find orbit and clock
    data of relevant satellites. This can take up to several minutes in
    some cases.
  • A-GPS improves start-up performance of a GNSS receiver by
    using the cellular network towers that pass information about
    location and relevant satellites to the receiver.

reduce errors

28
Q

Dead Reckoning

reduce errors

A
  • Calculating position by using previously determined position,
    speed, and course
    (same as navigation dead reckoning)
  • Can be used to overcome some limitation of GNSS technology (e.g.
    in tunnels, blocked lines of sight to the satellites)

ex:tunnels

29
Q

Going Further – Positioning in Closed
Environments

A
  • IPS = Indoor positioning systems
  • Using Wi-Fi
  • Bluetooth
30
Q

Applications of GPS

A
  • Transportation (navigation)
  • Mapping and surveying
  • Agriculture
  • Aviation
  • Environment
  • Public safety & disaster relief
  • Recreation
  • Wildlife
31
Q

what method/principle is used by a reciver to find your position

A

Trilateration

32
Q

what are the steps the recvier use to find your location

A
  • Download almanac (information on satellites, their status, health
    etc.)
  • Download ephemeris (predicted location of each satellite)
  • Download GPS date and time
  • Measure ∆T to at least 4 satellites
  • Determine range
  • Calculate X, Y, Z
33
Q
  • What is the accuracy of GNSS positioning?
A

**Depends on the receiver **
for you phone it would be 3 to 15 m 95% of the time

but for a expensive Trimble GS receiver with a antenna attached using differential GSS it might be a horizontal accuracy of 10 mm and a vertical accuracy of 20 mm

34
Q

What does DOP stand for and what does it measure?

A

stand for dilution of precision geometry of the satellites if we have a good dilution of precision, all the satellites are spread out nicely if we have a portion of precision

35
Q

What is RTK GPS?

A

This is a newer updated version of the differential GPS that usually includes a base station that is mobile that you’ll take out to the field with you and set up and it has its own known location and it will transmit correctional information to your receiver Real time so that very quickly and it has a newer updated algorithm relative to differential GPS transmits more correctional information than differential GPS

CONS 127 (19 decks)

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