satcomms cgp l7 2011

8/12/2019 Satcomms Cgp L7 2011

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Satellite Communications

ELEM026

Professor Clive Parini

Lecture 5 Communication Satellites

2010


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NAVSTAR GPS Principle is the accurate measurement of distance from the receiver of

each of a number (4) of satellites which transmit accurately timedsignals as well as other coded data giving the satellites position

A 3D ranging system based on the knowledge of the precise positionof the satellites in space.

The distance between the user and the satellite is calculated byknowing the time of transmission of the signal from the satellite andthe time of reception at the receiver and the fact that the signalpropagates at the speed of light.

The whole GPS system can be divided into 3 main segments:- SPACE CONTROL USER


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Principle of time based navigation - 2DRequires precision clocks(oscillators) at each station

No active participation by userLow cost listen only receiverTo solve for the 2 unknownlatitude and longitude 2

independent measurements of

range are required (2 equations

with 2 unknowns)

Transmitters transmit uniquesignal with time of transmission

encoded into it

Users receiver contains accurateclock synchronisedwith those of

transmitters so transmissiondelay!T can be determined

Range determined from radiowave propagation speed C


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Principle of time based navigation - 3D Extend to 3D by adding

additional transmitter in thethird dimension

Achieved on a global scaleusing satellite based transmittersin a global constellation

27 (24 +3 spares) satellites in12hr circular orbits at an altitudeof 20,183km

Orbits inclined at 550 to equatorin 6 orbital planes

At least 4 satellitescan be seenat any time above 150elevationangle of view for most places onthe earths surface


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Satellite trajectories as viewed from earth

Ground track for 2 orbits (24 hours) is shown. One satellite orbit shown in bluePattern repeats every day although given satellite in give place is seen 4minutes earlier each day


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View of constellation from a user

At Poles At 45 degree latitude

(London 52 degrees) Equator

Red dots represent satellite position at any one time, blue line is track for one satellite

Centre of sky chart is zenith, outer circle is the horizon


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How it works Although it is technically possible to keep the clocks

on all the satellites synchronised (to of order 1nsec)via pair of caesium and rubidium atomic clocks , theuser cannot have such a clock.

Cost about 100,000 and not very portable! So user has cheap crystal based oscillator.

The measured time for the signal from one satellite toreach the receiver is thus the transmit time plus the usersclock offset from GPS time

Range measurement is thusR

1

=C("t1

+ "T)

C = speed of light; R1 = pseudo range

"t1 = transmit signal time; "T= user clock offset


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Additional unknown is user clock offset so we now

have 4 unknowns (x,y,z, user clock offset)


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Satellite keeping of GPS time

The whole system works of a time standard calledGPS timewhich is maintained by the master

control station. It is possible for a satellite clock to vary slightly

from this time but these errors are determined by

the system and transmitted to the user along with

the time of transmission and other useful data


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System with known satellite clock offsets

My clock

offset is


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4 equations 4 unknownsfor satellite 1

R1 = C("t1 +"T-#1)

let corrected range due to satellite clock error be R'1

R'1 = C("t1 +"T)

true distance is = C"t1 =R'1 $c"T

=R'1 $CB

In spherical coordinates user position is

ux,uy ,uz

(x1 -ux )2 +(y1 -uy )

2 +(z1 -uz )2 = (R'1 $CB )

2

where x1,y1,z1are the known satellite positions

Remaining 3 equations are : -

(x2 -ux )2

+(y2 -uy )2

+(z2 -uz )2

= (R'2 $CB )2

(x3 -ux )2 +(y3 -uy )

2 +(z3 -uz )2 = (R'3 $CB )

2

(x4 -ux )2 +(y4 -uy )2 +(z4 -uz )2 = (R'4 $CB )2

unknowns are : ux uy uz CB


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Velocity measurement

The users velocity can also be determined by measuringthe Doppler Shift of the received carrier frequency of the

signal from each of the 4 satellites

As in the case of time, an error due to the offset of thereceiver oscillator frequency with GPS Time can beremoved using a 4 satellite measurement

Set of equations with the 3 velocity components plus thisoffset again gives 4 equations with 4 unknowns

Unknowns are Vx, Vy, Vz, user oscillator offset


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NAVSTAR GPS SPACE SEGMENT

Satellite operates in earth pointing 3-axis stabilised modePowered by solar arrays and rechargable batteries (for eclipse operation)Down link is at several frequencies in L-band L1=1575.42 andL2=1227.6MHz operating on right hand circular polarisation

Control Uplink and downlink is at S-band (2 -4)GHzDown link antenna is 12 element helical array producing a shaped beam


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Fig 7 Ground Control segment shown monitoring one satellite in the constellation

Master

control

Monitoring stations

S-band

Uplink=1783MHz

Downlink=2227MHz

L1 and L2 downlinks

L1 and L2 downlinks


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GROUND CONTROL SEGMENT-2

Monitoring stations around the globe at accurately knownlocations receive the satellite user down link signals andforward this raw data to the master control station. Theyhave accurate atomic clocks locked to GPS time.

There data is analysed and deviations of the satellites clockfrom GPS time is determined as well as corrections to thesatellites predicted position in space (ephemeris). {i.e. itsdeviation from modified Keplers laws}

These are then relayed up to the satellite by the GroundAntenna using the S-band link


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User communications down link GPS was originally a US military system and is still today

administered by the US DOD.

There are 2 levels of accuracy Standard Positioning Service (SPS) - unrestricted use using the

L1=1575.42 MHz

Precision Positioning Service (PPS)-DOD authorised users onlyusing both L1=1575.42 MHz and L2=1227.6MHz

Both use CDMAas the multiplexing process and BPSK as themodulation method

For SPS the spreading code is 1023 bits long and are called the C/A(course acquisition) codes and the chip rate is 1.023Mbits/sec. Thespectrum of this modulated code is 1.023MHz either side of the L1 carrier.

For PPS an additional code P(Y) is transmitted on L1 and a second on L2.In both cases the chip rate is 10.23Mbits/sec with a bandwidth of

20.46MHz. The P(Y) code is pseudo random and 37 weeks long! Inantispoofing mode (Y-code) its further encrypted so never repeats.


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SPS using the C/A code

Each satellite has a unique 1023 bit spreading code and this code iscontinually transmitted every 1msec. Hence chip rate of 1.023Mbits

The code is modulated by a 50bit/sec navigation message so each bitof data spans 20 spreading code transmissions

1500 bit message sub-divided into 5 sub frames of

300 bits each

The HOW bit gives theaccurate transmission time

Last frame is multiplexedtaking 25 frames of the C/A

code to transmit the complete

message


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PPS The availability of signal propagation timings using 2

frequencies gives the ability to predict the effect of theionosphere on the propagation speed of light, soenhancing accuracy to a few metres

The level of dithering of the clock signal (not now used) isknown so it can be removed

The very long (37 week or infinite in case of anti-spoofmode) spreading code means that the code is truly randomso that any integration length (or averaging as in ourexample) will yield the desired signal from the noise, thelonger the integration length the more the recovered signalraises above the background noise.

So long as you can generate the identical random code atthe receiver, and this is done by having a known key


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User terminal

Each system has same basic design of antennaplus microwave front end receiver. Received CDMA signal split into a number of

parallel channels enabling navigation messages

from individual satellites to be received in parallelthus achieving fastest lock time. 12 channel

receivers are common today


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block diagram of receiver

Filter and

Pre-amp

RF/IF

downconverter

A/D conversion

Channel 1

Channel 2

Channel n

Frequency

synthesiser

Reference

oscillator

DSP

CLOCK

Navigation

receiver

processor

CPU, user

control and

Display

antenna


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Fig 10 (a) code phase timing, (b) carrier phase timing

Carrier phase

The raw propagation time is

determined for a given

satellite by loading into the

CDMA receiver correlator

the code for the desiredsatellite and delaying it in

time until correlation is

achieved with the incoming

signal

This form of timing is often calledCode Phase timing as it attempts to

match the phase of the incoming

code with the receivers own

generated code, as illustrated in fig

10a. Since the chip rate is about

one microsecond and the accuracy

to which the code phase can belocked is about 1% of its period

then the timing accuracy is about

10nsec, corresponding to a

position error of 3metres


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Carrier Phase

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Timing can be improved if Carrier Phase is used to give afiner timing for the received edge of the incoming pseudo-random code, as shown in fig 10b.

The receiver can measure the carrier phase to about 1%accuracy by keeping a running count of the Doppler

frequency shift of the carrier since the satellite acquisition

the overall phase measurement contains an unknownnumber of carrier cycles, N, between the satellite and the

user.

If this Carrier Cycle Integer Ambiguitycan be determinedaccuracies of order 1mm could be achieved. The

techniques employed by differential GPS (DGPS),described later aim to determine N.


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High Sensitivity GPS receivers

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High Sensitivity GPS receivers use large banks of correlators anddigital signal processing to search for GPS signals very quickly.

This results in very fast times to first fix when the signals are at

their normal levels, for example outdoors.

When GPS signals are weak, for example indoors, the extraprocessing power can be used to integrate weak signals to the point

where they can be used to provide a position or timing solution.

GPS signals are already very weak when they arrive at the Earthssurface. The GPS satellites have transmitters that only deliver 27

W from a distance of 20,200 km in orbit above the Earth. By the

time the signals arrive at the user's receiver, they are typically as

weak as "160 dBW, equivalent to one tenth of a millionth billionth

of a watt. This is well below the thermal noise level in itsbandwidth.

Outdoors, GPS signals are typically around the "155 dBW level.


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High Sensitivity GPS receivers -cont

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Conventional GPS receivers integrate the received GPS signals forthe same amount of time as the duration of a complete C/A codecycle which is 1 ms. This results in the ability to acquire and track

signals down to around the "160 dBW level.

High Sensitivity GPS receivers are able to integrate the incomingsignals for up to 1,000 times longer than this and therefore acquire

signals up to 1,000 times weaker. A good High Sensitivity GPSreceiver can acquire signals down to "185 dBW, and tracking can be

continued down to levels approaching "190 dBW.

High Sensitivity GPS can provide positioning in many but not allindoor locations. Signals are either heavily attenuated by the building

materials or reflected as in multipath.Given that High Sensitivity GPS receivers may be up to 30 dB moresensitive, this is sufficient to track through 3 layers of dry bricks, or

up to 20 cm of steel reinforced concrete for example.


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Selective Availability (SA) The role of SA was to deny accurate positioning to non-

authorised users

This was achieved by dithering the satellite clocks in apseudo random fashion to corrupt the the rangemeasurements

Authorised users had a key to that allows them to removedithering before processing

SPS accuracy was thus limited to about 100m To circumvent SA the user needs to know the amount of

dithering and can be simply done by monitoring the GPSsatellite with a receiver at a known location.

The variation in position with time was largely due to thedithering process so if this can be transmitted by radio toother local users they can remove the effects of dithering.Hence removed since yr 2000.


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Commercial use of 2 frequencies Besides redundancy and increased resistance to jamming, the

benefit of having two frequencies transmitted from one satellite isthe ability to measure directly, and therefore remove, the

ionospheric delay error for that satellite.

As the ionosphere is a highly dynamic charged media itspermittivity is also dynamic and so the speed of light fluctuates by a

small frequency dependant amount, hence leading to positionalerrors. Ionospheric delay is one of the largest remaining sources of

error in the GPS signal for a static receiver.

Without such a two-frequency measurement, a GPS receiver mustuse a generic model or receive ionospheric corrections from another

source. As part of a general development of NAVSTAR GPS theintroduction of a second civilian signal channel L2C was begun in

2006 (with the IIR-M) satellites, which by about 2016 will provide a

24-satellite constellation with this capability. 27


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Local Area Differential GPS (LADGPS)

The concept of Local Area Differential GPS (LADGPS) is to place a GPS referencereceiver at a surveyed (known) location, compute the differences in latitude, longitude

and geodetic height between the GPS measured position and the known surveyed

location. The GPS reference receiver is a survey-grade GPS that performs GPS carrier

tracking and can work out its own position to a few millimetres.

For real-time LADGPS these differences are immediately transmitted to the localreceivers by a low frequency radio link (VHF or UHF) and they employ this data to

correct their own GPS position solutions. This requires that all the receivers make

pseudorange measurements to the same set of satellites to ensure that errors are common. Where these is no need for real-time measurement, such as terrain mapping, the local

receiver needs to record all of its measured positions and the exact time and satellite data

etc., then post processing of the data along with that from the reference receiver yields

the required accurate locations.

In both cases the basic measurement errors (or biases) related to each satellitemeasurement such as ionospheric and tropospheric delay errors, receiver noise and clock

offset, orbital errors etc. can be determined and corrected for.

Table 1 [2] gives the estimates of the pseudorange error components from varioussources in SPS mode. The total rms range error is estimated at 33m, and with LADGPS

the error drops by a factor of ten.28


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Table 1 GPS C/A code pseudorange error budget (after [2])

Segment source Error source GPS

1 sigma error

(metres)

LADGPS

1 sigma error

(metres)

Space

Satellite clock stability

Satellite perturbations

Selective availability

Other (thermal radiation etc.)

3.0

1.0

32.3

.5

0

0

0

0

Control Ephemeris prediction error

Other (Thruster performances

etc)

4.2

0.9

0

0

User

Ionospheric delay

Tropospheric delay

Receiver noise and resolutions

Multipath

Other (interchannel bias, etc.)

5.0

1.5

1.5

2.5

0.5

0

0

2.1

2.5

0.5

SYSTEM

Total (rms)

33.3

3.3


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GPS sources of ERROR Satellite clock error:- 1nsec = 0.3m on ground (1.5m)

Errors tracked and transmitted to user Atmospheric Delays :-propagation of radio through ionosphere &

troposphere not exactly at speed of light (2.5m)

Receiving information on more than one frequency (authorised users) Knowledge of receiver to satellite elevation angle plus estimate of C Atmospheric correction data from satellite supplied by monitoring stations

Orbital errors (1.5m) Ephemeris data supplied in navigation message

Multipath :- signal received from more than one path upsetting timing(worst inside buildings , cities etc) (2.5m)

Careful choice of location of the 4 satellites used for a fix can helpaccuracy


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LADGPS -cont Protocols have been defined for communicating between reference

station and user and one such is that from the Radio Technical

Commission for Maritime Services Commission (RTCM-104). The

data rate is low (200 Baud) so can be transmitted to the remote

receiver in a number of ways including a GPRS mobile phone

connection.

The error in the estimated corrections will be a direct function of thedistance between the reference and remote receivers, it is possible touse a number of reference receivers providing a perimeter to the roving

remote receiver [3].

As mentioned in the previously the receiver can measure the carrierphase to about 1% accuracy by keeping a running count of the Doppler

frequency shift of the carrier since the satellite acquisition by the

receiver, but the overall phase measurement contains an unknownnumber of carrier cycles, N, between the satellite and the user (fig 11).

31


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Fig 11 Carrier phase as a function of time for a given satellite link

Earth surface

user

Satellite orbital track

at times t0etc.

N

t0

t1

t2 t3

"+#1

"+#2"+#3


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DGPS in surveying

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DGPS in agriculture

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Transportable DGPS reference

station Baseline HD by CLAAS

for use in satellite-assisted

steering systems in modern

agriculture


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LADGPS -cont The recording of this data over time can be done at both the

reference and roving receiver for the SAME set of satellites at the

SAME time. Combining this data in a form of interferometry

leads to a set of equations over time that can be solved to

determine the values of N (Carrier Cycle Integer Ambiguity) for

each satellite received by the reference and roving receivers. The

corresponding Code Phase measured data can be used to limit the

size of the integer ambiguity to about 10$to aid the solution.

A brute force solution to determining N could then be applied bycalculating the least squares solution for each time iteration and

finding the minimum residual, but this is a large computational

task (of order 300,000 residuals for each time point for a 10$

ambiguity [3]).

A better approach uses advanced processing techniques to choosesuitable trial values for N [3] leading to 20cm accuracy in near

real-time and 1mm accuracy with post-processing.35


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Performance of Differential GPS

Blue and green

are 2 differentlocations

SPS mode

Strongly effected by SA

Differential GPS

with base-station

near the 2 sites


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Differential GPS with Carrier phase measurements

giving more accurate time of arrival measurement

Blue with base-station at 10Km distance

Green with base-station at 10m distance


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Dilution of precision due to poor choice

of the four satellite locations


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Navstar upgrades

First batch of satellites launched in 1978 (called Block 1) Improvements over the years with Blocks II, IIA and IIR

and next generation IIF

Block IIF Transmit civilian code on L2 removing atmospheric effects

offering 10m accuracy

Third frequency to be added to system for all users would improveaccuracy by about an order of magnitude

Increased transmit power


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GPS Applications

Monitoring car usage for insurance purposes -amile driven at night is 10 times more dangerousthan one driven at 8am - US insurance company

Installed in mobile phone for emergency calllocation

Building & surveying via differential GPS As a universal time standard for CDMA 3G

mobile system

Air traffic control and automatic landing systems


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Future systems: ESAs GALILEOBy offering dual frequencies asstandard,, Galileo will deliver real-time

positioning accuracy down to the metrerange,Part (18 satellites) OperationalCapability in 2014.The fully deployed Galileo systemconsists of 30 satellites (27 operational

+ 3 active spares), positioned in threecircular Medium Earth Orbit (MEO)

planes in 23616 km altitudeWith an accuracy of better than onebillionth second in one hour, the clocks

on the Galileo satellites will allow you toresolve your position anywhere on theEarth's surface to within 45 cmtwo clocks on board, one based on theRubidium atomic frequency standard

and the other using a passive Hydrogenmaser

ESA - European Space Agency

satcomms cgp l7 2011

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