Vsat(GPS)

Contents IntroductionApplicationsImplementationAccess ControlAccess MethodsInterference, Modulation and CodingEarth Stations*

IntroductionVSAT = Very Small Aperture TerminalEarly Earth Stations in commercial systems were very large and expensive (30 m).Need to make system more affordable to end user:Increased transmit power from satellite.Higher frequenciesResult: Smaller ES antenna size required.

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Large Antenna SystemsBreakpoint between large and small antennas is at about 100 wavelengths.Above breakpoint, back-fed configurations such as Cassegrain or Gregorian are economically and technically viable (subreflectors need to be at least 10 wavelengths).Below breakpoint, terminals called Small Aperture Terminals.Smaller Antennas Tighter Link Budgets

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Typical Antenna SizesAt C-band: below 5 meters (100 wavelength at 6 GHz).Extrapolation of terminology:USAT = Ultra Small Aperture Terminal.Standard VSAT antennas (Intelsat tables)Smaller antennas are also included in the concept of VSAT or USAT (DTH, MSS, etc). These systems will be studied separately in this course.

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*Intelsat Standard for VSAT antennas

Satellite Communication System

APPLICATIONS*

VSAT SYSTEMSUnderlying objective of VSAT Systems: bring the service directly to the end-userMajor reasons for doing thisReduce hierarchical distribution network (make more efficient and faster - e.g. POS credit)Reduce distribution costsLeapfrog technology in developing countries (e.g. VSAT/WLL) *Point of Service

*VSAT/WLL - 1Telecommunications and roads are the two major economic growth requirements for developing countriesMajor telecommunications infrastructure does not exist in many developing countriesSOLUTIONDistribute links to communities by satellite/VSATUse Wireless Local Loop from the VSAT

VSAT/WLL - 2The geostationary satellite is used to link a large number of VSATs with the main switching center in a large city. Each VSAT acts as the link to the local switching center in the village or rural community, with the final mile of the telephony link being carried over a Wireless Local Loop.

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*VSAT/WLL - 3


VSAT/WLL 4User density dependencyEconomic advantages of VSAT/WLL solution depends primarily on user density.Physical distances, major transportation routes, and geographic barriers, as well as the individual countrys demographics and political influences, can alter the breakpoints.*

Motivation to use VSAT/WLL*The last mile problemHard to reach areasReliability Time to deploy (4-6 months vs. 4-6 weeks)FlexibilityCostVS


VSAT/WLL 5User density dependency*Approximate economic break-points in the implementation choices for serving new regions with different population densities.

*POS/VSATHandles small traffic streams.Intermittent traffic stream: Demand Assigned Multiple Access (DAMA)Message sent to main hub (usually a request for credit authorization), short message received in response. Transaction transparent to the user.

IMPLEMENTATIONS*

*VSAT IMPLEMENTATION - 1There are several ways VSAT services might be implementedOne-Way (e.g. TV Broadcasting satellites)Split-Two-Way (Split IP) Implementation (return link from user is not via the satellite; e.g. DirecTV)Two-Way Implementation (up- and down-link)We will be looking at Two-Way Implementation only

*VSAT IMPLEMENTATION - 2There are basically two ways to implement a VSAT Architecture

STAR VSATs are linked via a HUBMESH VSATs are linked together without going through a large hub

*VSAT IMPLEMENTATION - 3Higher Propagation delayUsed by TDMA VSATsHigh central hub investmentSmaller VSAT antenna sizes(1.8 m typically)Lower VSAT costsIdeally suited for interactive dataapplicationsLarge organizations, like banks, with centralized data processing requirements Lower Propagation delay (250 ms)Used by PAMA/DAMA VSATsLower central hub investmentlarger VSAT antenna sizes(3.8 m typically)Higher VSAT costsSuited for high data trafficTelephony applications and point-to-point high-speed links

VSAT STAR ARCHITECTURE - 2In this network architecture, all of the traffic is routed via the master control station, or Hub.If a VSAT wishes to communicate with another VSAT, they have to go via the hub, thus necessitating a double hop link via the satellite.Since all of the traffic radiates at one time or another from the Hub, this architecture is referred to as a STAR network.*

*VSAT STAR ARCHITECTURE - 2


All communications to and from each VSAT is via the Master Control Station or Hub

VSAT

Community

Master Control Station

(The Hub)

*VSAT STAR ARCHITECTURE - 3Topology of a STAR VSAT network viewed from the satellites perspectiveNote how the VSAT communications links are routed via the satellite to the Hub in all cases.


VSAT MESH ARCHITECTURE - 1 In this network architecture, each of the VSATs has the ability to communicate directly with any of the other VSATs. Since the traffic can go to or from any VSAT, this architecture is referred to as a MESH network. It will still be necessary to have network control and the duties of the hub can either be handled by one of the VSATs or the master control station functions can be shared amongst the VSATs.*

VSAT MESH ARCHITECTURE - 2*


*VSAT MESH ARCHITECTURE - 3Topology of a MESH VSAT network from the satellites perspective

Note how all of the VSATs communicate directly to each other via the satellite without passing through a larger master control station (Hub).


*ADVANTAGES OF STARSmall uplink EIRP of VSAT (which can be a hand-held telephone unit) compensated for by large G/T of the Hub earth stationSmall downlink G/T of user terminal compensated for by large EIRP of Hub earth stationCan be very efficient when user occupancy is low on a per-unit-time basis

*DISADVANTAGES OF STARVSAT terminals cannot communicate directly with each other; they have to go through the hubVSAT-to-VSAT communications are necessarily double-hopGEO STAR networks requiring double-hops may not meet user requirements from a delay perspective

*ADVANTAGES OF MESHUsers can communicate directly with each other without being routed via a Hub earth stationVSAT-to-VSAT communications are single-hopGEO MESH networks can be made to meet user requirements from a delay perspective

*DISADVANTAGES OF MESHLow EIRP and G/T of user terminals causes relatively low transponder occupancyWith many potential user-to-user connections required, the switching requirements in the transponder will almost certainly require On-Board Processing (OBP) to be employedOBP is expensive in terms of payload mass and power requirements

ACCESS CONTROL*

Access Control ProtocolsInternational Standards Organization has specified the Open Systems Interconnection ISO/OSI.ISO-OSI considers a seven layer stack for interconnecting data terminals. Conceptual model.Satellite Link occupies the physical layer (bits transport)VSAT Network must have terminal controllers at each end of the link (network and link layers).Network control center is responsible for the remaining layers.*

ACCESS CONTROL PROTOCOLS*


USER TWO

USER ONE

APPLICATION

PRESENTATION

SESSION

TRANSPORT

NETWORK

LINK

PHYSICAL

APPLICATION

PRESENTATION

SESSION

TRANSPORT

NETWORK

LINK

PHYSICAL

Access Control ProtocolsIn this example, User One and User Two are conducting a two-way communications session with each other. Each user interacts with their local device (e.g. a computer keyboard/visual display unit) at the Application Layer of the ISO-OSI stack. Their transaction is then routed via the various layers, with suitable conversions, etc., until the content is ready to be transmitted via the physical layer (where the satellite link is).*

Delay ConsiderationsSatellite Scenario:Typical slant path range for GEO satellite: 39,000 kmOne way transmission: ESSatelliteES: 2 x RangeOne way delay: 2 x (range/velocity) = 260 msFiber Optic Transcontinental Link: 4000 km or about 13 ms delayAdditionally to either case: Processing delay.Several tens to over a hundred ms.

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*DELAY CONSIDERATIONS - 110 ms one-way delayTypical on terrestrial links


Rolling Time Window of 60 ms

A1

Signal transmission continues in an uninterrupted stream between User 1 and User 2 since User 1 receives the acknowledgement signals from User 2 within the required time of 60 ms.

B1

A2

0 ms

B2

60 ms

120 ms

Time Line of User No. 2 (the receiver)

Time Line of User No.1 (the sender)

DELAY CONSIDERATIONS - 2Previous Slide: Illustration of a communications link with a 10 ms one-way delay and a 60 ms windowIn this example, a packet or frame is sent at instant A1 from User 1 to User 2. User 2 receives the transmission without error and sends an acknowledgement back, which is received at instant A2, 20 ms after the initial transmission from User 1. This is well within the time window of 60 ms. The time window rolls forward after each successful acknowledgement. Thus the transmission from User 1 at instant B1 is received by User 2, and the acknowledgement received by User 2 at instant B2, within the new rolling time window of 60 ms. Each packet or frame is successfully received in this example.

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*DELAY CONSIDERATIONS - 3260 ms one-way delay


Rolling Time Window of 60 ms

A1

B1

C1

0 ms

D1

120 ms

There are no signal transmissions from User 1 to User 2 in these two intervals because the rolling 60 ms window has timed out in the protocol used by User 1 since no acknowledgement signals have been received from User 2 in the required interval of 60 ms.

A2

240 ms

C2

B2

D2

360 ms

480 ms

Time Line of User No. 2 (the receiver)

Time Line of User No.1 (the sender)

DELAY CONSIDERATIONS - 4Previous Slide: Illustration of a communications link with a 260 ms one-way delay and a 60 ms windowIn this example, a packet or frame is sent at instant A1 from User 1 to User 2. User 2 receives the transmission without error and sends an acknowledgement back, which is received at instant A2, 260 ms after the initial transmission from User 1. Unfortunately, instant A2 is well after the rolling window time out of 60 ms. Transmissions from User 1 are automatically shut down by the protocol when the time out of 60 ms is exceeded. Ignoring processing delays in this example, User 1 is only transmitting for 60 ms in every 260 ms, thus drastically lowering the throughput. Again, no propagation errors are assumed to occur.

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Protocol Changes - 1 VSAT protocol acts as processing buffer to separate the satellite network form the terrestrial network (spoofing). VSAT networks are normally maintained as independent, private networks, with the packetization handled at the user interface units of the VSAT terminals. The satellite access protocol (with a larger time-out window) is handled in the VSAT/Hub Network kernel, which also handles packet addressing, congestion control, packet routing and switching, and network management functions. Protocol conversion and, if necessary, emulation is handled by the Gateway equipment.

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*PROTOCOL CHANGES


Fig. 2.2.1 of VSAT Systems and Earth Stations, Supplement No. 3 to the Handbook on Satellite Communications, International Telecommunications Union, Geneva, 1994 (for updates on this handbook, please refer to http://www.itu.int)

Design Considerations Using basic concepts introduced in TCOM507: Link Budget, Multiple access, Modulation Schemes. Frequency Allocation: Considered a Fixed Satellite Service (FSS), allocation frequencies at :C band (4/6 GHz)Ku band (14/11 GHz) increasingly common todayKa band (30/20 GHz) considered for future applications Small antennas Small sensitivity (small G/T). Restrictions in transmitted power flux density from satellite to satisfy regulatory restrictions due to frequency sharing with terrestrial systems (C band). A common solution is to use spread-spectrum techniques.

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ACCESS METHODS*

Multiple Access Possibilities Choice aiming to maximize the use of common satellite and other resources amongst all VSAT sites.Methods considered:Pre-Assigned Multiple Access (PAMA)Demand Assigned Multiple Access (DAMA) FDMA = Frequency Division Multiple AccessTDMA = Time Division Multiple AccessFixed Assigned TDMAALOHA & Slotted ALOHADynamic ReservationCDMA = Code Division Multiple Access

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FDMA Frequency Division Multiple Access Here all VSATs share the satellite resource on the frequency domain only. Allows smaller receiver bandwidth (less noise power) Smaller maximum transmit power requirements. Operates both in star and mesh topologies.Example:QPSK (M=2), 64 kbps (Ri), FEC (k/n= ), roll-off 0.5 (a)Rb = Ri/r = 128 kbpsRs = Rb/M = 64 kbaudsTransmit bandwith = Bt = (1 + a) * Rs = 96 kHz(Allow guard band for frequency drift : 120 kHz)Receive bandwidth = Br = Rs = 64 kHz

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*Schematic of a 64 kbit/s equivalent voice channel accessing a satellite using FDMAInbound link:VSATs Satellite Hub StationExample: Star - Inbound Link - FDMA


Star Inbound FDMA Example (cont.)The 64 kbit/s information rate is contained in a bandwidth of 96 kHz when transmitted to the satellite. The bandwidth of the satellite transponder (from frequency f1 to frequency f2) is divided up, or channelized, into increments of 96 kHz so that a large number of VSATs can access the transponder at the same time.Each of the 96 kHz channels requires a certain amount of spectrum on either side to guard against drift in frequency, poor VSAT filtering, etc. The 96 kHz channels plus the guard bands on either side add up to a channel allocation of about 120 kHz per VSAT.*

Star Inbound FDMA Example (cont.)From a spectrum allocation viewpoint, therefore, a typical 36 MHz satellite transponder would permit the simultaneous access of 300 VSATs, each of which is transmitting the equivalent of a 64 kbit/s voice channel. Because each VSAT uses a single channel continuously on the uplink, it is often referred to as SCPC - Single Channel Per Carrier - FDMA.

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FDMA Implementation OptionsPAMA (Pre Assigned Multiple Access) - implies that the VSATs are pre-allocated a designated frequency. Equivalent of the terrestrial leased line solutions, PAMA solutions use the satellite resources constantly. Consequently there is no call setup delay which makes them most suited for interactive data applications or high traffic volumes . As such PAMA is used typically to connect high data traffic sites within an organization. SCPC (Single Channel Per Carrier) refers to the usage of a single satellite carrier for carrying a single channel of user traffic. The frequency is allocated on a pre-assigned basis in case of SCPC VSAT's. The term SCPC VSAT is often used interchangeably with PAMA VSAT.

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FDMA Implementation OptionsDAMA (Demand Assigned Multiple Access) - network uses a pool of satellite channels, which are available for use by any station in that network. On demand a pair of available channels are assigned so that a call can be established. Once the call is completed, the channels are returned to the pool for an assignment to another call. Since the satellite resource is used only in proportion to the active circuits and their holding times, this is ideally suited for voice traffic and data traffic in batch mode. DAMA offers point to point voice, fax, and data requirements and supports video conferencing.

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Outbound Link - TDM Return link: HubSatelliteVSATs Star Topology: typically a single, wide-band stream in Time Division Multiplexing (TDM) format*Note: What is the difference between TDM and TDMA???(usually used interchangeably, but not exactly the same)

Satellite Communication System**Answer: In TDM, all multiplexed data channels come from the same transmitter, which means that clock and carrier frequencies do not change. In TDMA, each frame contains a number of independent transmissions (time slots contain information from different data sources usually transmitted from different locations).


*Example: Outbound Link - TDMSchematic of the TDM downlink outbound channel from the hub, via the satellite, to the individual VSAT terminals


Example: Outbound Link TDM (cont.) The 300 individual, narrow-band, inbound channels received at the hub from the VSATs are sent back to the VSATs in a single, wide-band, outbound TDM stream at a combined transmission rate 20 Mbit/s. Each VSAT receives the downlink TDM stream and then demodulates and decodes it (i.e. changes the modulated bandpass signal into a baseband line code and removes the FEC). *

Example: Outbound Link TDM (cont.) The line code is then passed through a demultiplexer which is used to extract the required part of the stream that contains the equivalent 64 kbit/s voice channel destined for that VSAT terminal. Carrier recovery and bit recovery circuits are used in the receiver in order to be able to identify the exact position of the required VSAT channel in time. The bandwidth of the satellite transponder (from frequency f1 to frequency f2) is fully occupied in this example.

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*Transponder Sharing:TDM-Outbound, FDMA-Inbound36 MHz satellite transponderInbound narrow-band VSAT channelsOutbound wide-band TDM streamIn the example here, 18 MHz of spectrum is allocated for each side of the system connection. On the uplink to the satellite, the collection of FDMA narrow-band channels transmitted by the VSATs co-exists in the same transponder with the wide-band TDM stream transmitted up by the hub. On the downlink from the satellite, the hub receives the collection of individual narrow-band channels while the wide-band TDM downlink stream is received by each VSAT. The precise frequency assignment can vary to suit the capacity of the VSAT network.


Another option for Inbound LinkMulti-Frequency TDMA (MF-TDMA)If we used TDMA instead of FDMA, in the example, each VSAT would have to be able to transmit (at discontinuous intervals) at a power much higher than that need by one single channel (larger bandwidth).Solution Hybrid TDMA-FDMA approachEach VSAT transmits a burst rate at 5 times the bandwidth of a normal single VSAT single-channel rate.Equivalent to say that each frequency is shared in 5 time-slots, one for each VSAT. Saves power at VSAT transmitter compared to pure TDMA.

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*Example: Inbound MF-TDMA


Example: Inbound MF-TDMA (cont.) In this particular case, each group of five VSAT terminals (A, B, C, D, and E) share the same frequency assignment, that is they all transmit at the same frequency. However, they each have a unique time slot in the TDMA frame when they transmit, so that they do not interfere with each other. The bursts from each VSAT are timed to arrive at the satellite in the correct sequence for onward transmission to the hub. Other frequencies (not shown in the picture) shared among other groups of five VSATs.

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CDMA OptionAdds spectral efficiency in interference-limited environments (facilitates frequency reuse).Allows reception below noise floor due to signal spreading in larger bandwidth (spread-spectrum).Initially employed for encryption and military purposes.Off-axis emission is closely specified by the ITU-R and is a key element in Up-Link Power Control design. When LEO constellations are sharing the same frequency bands as GEO systems, the use of CDMA may confer some advantages for coordination purposes at the expense of system capacity.

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*How a VSAT can cause interference to other satellite systems2o2oGeostationary orbit arc:satellites at 2o spacingVSATWSATUSAT(1)USAT(2)Beamwidth ofVSAT In this example, the VSAT is transmitting to a wanted satellite (WSAT) but, because the antenna of the VSAT is small, its beam will illuminate two other adjacent, unwanted satellites (USATs) that are 2o away in the geostationary arc. In a like manner, signals from USAT (1) and USAT(2) can be received by the VSAT, thus causing the potential for interference if the frequencies and polarizations used are the same.


INTERFERENCE, MODULATION AND CODING*

*Interference Scenario - 1Main lobe and first sidelobes of VSAT antennaGain, Gw (dB), in the direction of the wanted satelliteGain, Gu (dB), in the direction of the interfered-with satellitePath to the satellite which will have a fixed path loss and a variable loss due to propagation impairmentsWSATUSATVSAT with an HPA power of P (dBW)Gain of the antenna of the interfered-with satellite, Gs (dB), towards the VSAT


Interference Scenario - 2 Previous slide shows the interference geometry between a VSAT and a satellite of another system. The EIRP of the VSAT towards the interfered-with satellite [P(dBW) + Gu(dB)] is the interference power from the VSAT into the interfered-with satellite. To develop the interference link budget, the Gain of the interfered-with satellite in the direction of the VSAT, Gs(dB), would be used, plus any additional effects along the path (such as site shielding, if used, expected rain effects for given time percentages, etc.)

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Coding and ModulationModulation Scheme: High index modulation schemes use bandwidth more effectively. High index modulation schemes also require more link margin, more amplifier linearity. They are also more susceptible to interference and harder to implement. Typically systems work with BPSK or QPSK.Coding Scheme: Inner code. Outer interleaving code (Reed-Solomon) to protect against burstiness.

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EARTH STATIONS*

VSAT Earth Station - 1 *Outdoor Unit (ODU)Inter-facility link (IFL)Indoor Unit (IDU)


VSAT Earth Station - 2The VSAT Outdoor Unit (ODU) is located where it will have a clear line of sight to the satellite and is free from casual blockage by people and/or equipment moving in front of it. It includes the Radio Frequency Trasceiver (RFT).The Inter Facility Link (IFL) carries the electronic signal between the ODU and the Indoor Unit (IDU) as well as power cables for the ODU and control signals from the IDU.The IDU is normally housed in a desktop computer at the Users workstation and consists of the baseband processor units and interface equipment (e.g. computer screen and keyboard). The IDU will also house the modem and multiplexer/demultilexer (mux/demux) units if these are not already housed in the ODU.*

*VSAT Earth Station - Block DiagramAntennaFeedLNCHPCDEMMODToData Terminal EquipmentIFLOutdoor Unit (ODU)Indoor Unit (IDU)RFTBase Band Processor(BBP)RFTIFLIDU


VSAT Earth Station Blocks DescriptionThe Low Noise Converter (LNC) takes the received RF signal and, after amplification, mixes it down to IF for passing over the inter facility link (IFL) to the IDU. In the IDU, the demodulator extracts the information signal from the carrier and passes it at base band to the Base Band Processor. *

VSAT Earth Station Blocks DescriptionThe data terminal equipment then provides the application layer for the user to interact with the information input. On the transmit operation, the user inputs data via the terminal equipment to the baseband processor and from there to the modulator.The modulator places the information on a carrier at IF and this is sent via the inter facility link to the High Power Converter (HPC) for upconversion to RF, amplification, and transmission via the antenna to the satellite.*

*Hub Station - 1


Hub Station - 2The line interface equipment handles the terrestrial ports to the host computer.The control bus via the hub control interface allows all of the transmit, receive, and switching functions to be carried out.The transmit Processing and Control Equipment (PCE) prepares the TDM stream for the outbound link to the VSATs.This stream passes through the IF interface (the equivalent of the interfacility link of the VSAT) to the Up-Converter (UC) that mixes the IF to RF.*

Hub Station - 2The High Power Amplifier (HPA) amplifies the TDM stream and the antenna transmits the signal.On the receive side, the antenna passes the individual inbound MF-TDMA signals to the Low Noise Amplifier (LNA) for amplification prior to Down Conversion (DC), demodulation, and so on to the user.

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Satellite Communication System**


MCSMCS

Global Positioning System (GPS)*


GPS CreationThe U.S. Department of Defense decided that the military had to have a very precise form of worldwide positioning. And fortunately they had the kind of money ($12 Billion!) it took to build it.

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What is GPSWorldwide radio-navigation system formed from a constellation of 24 satellites and their ground stations. Uses satellites as reference points to calculate positions accurate to a matter of meters (advanced forms of GPS can achieve centimeter accuracy).GPS receivers miniaturized and becoming very economical and accessible to the end users.Applications in cars, boats, planes, construction equipment, movie making gear, farm machinery, etc.*

GPS SatellitesName: NAVSTAR Manufacturer: Rockwell International Altitude: 10,900 nautical miles Weight:1900 lbs (in orbit) Size:17 ft with solar panels extended Orbital Period: 12 hours Orbital Plane: 55 degrees to equitorial plane Planned Lifespan: 7.5 years Current constellation: 24 Block II production satellites Future satellites: 21 Block IIrs developed by Martin Marietta. *

Ground Control StationsAlso known as the "Control Segment.Monitor the GPS satellites, checking both their operational health and their exact position in space. The master ground station transmits corrections for the satellite's ephemeris constants and clock offsets back to the satellites themselves. The satellites can then incorporate these updates in the signals they send to GPS receivers. There are five monitor stations: Hawaii, Ascension Island, Diego Garcia, Kwajalein, and Colorado Springs.

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How GPS worksThe basis of GPS is "triangulation" from satellites (formally speaking, trilateration). To "triangulate," a GPS receiver measures distance using the travel time of radio signals. To measure travel time, GPS needs very accurate timing which it achieves with some specific techniques. Along with distance, the receiver needs to know exactly where the satellites are in space. High orbits and careful monitoring contribute to this accuracy.Finally the receiver must correct for any delays the signal experiences as it travels through the atmosphere. *We will see each step next

1 - Triangulation from Satellites Use satellites in space as reference points for location on earth.How does the knowledge of distance from three (or more) satellites allow the position determination? *

Triangulation - BasicsPosition is calculated from distance measurements (ranges) to satellites. Mathematically we need four satellite ranges to determine exact position. Three ranges are enough if we reject ridiculous answers or use other auxiliary. Another range is required for technical reasons to be discussed later.*

Distance to one satelliteSuppose we measure our distance from a satellite and find it to be 11,000 miles. (How we measure that distance is the subject of further discussion)Knowing that we're 11,000 miles from a particular satellite narrows down all the possible locations we could be in the whole universe to the surface of a sphere that is centered on this satellite and has a radius of 11,000 miles. *

Distance to two satellitesNext, suppose we measure our distance to a second satellite and find out that it's 12,000 miles away. That tells us that we're not only on the first sphere but we're also on a sphere that's 12,000 miles from the second satellite. Or in other words, we're somewhere on the circle where these two spheres intersect. *11,000 miles sphere12,000 miles sphere

Distance to three satellitesIf we then make a measurement from a third satellite and find that we're 13,000 miles from that one, that narrows our position down even farther, to the two points where the 13,000 mile sphere cuts through the circle that's the intersection of the first two spheres. *11,000 miles sphere12,000 miles sphere13,000 miles sphereThree measurements put us at one of these two points

Triangulation - SummaryBy ranging from three satellites we can narrow our position to just two points in space. To decide which one is our true location we could make a fourth measurement. But usually one of the two points is a ridiculous answer (either too far from Earth or an impossible velocity) and can be rejected without a measurement. A fourth measurement does come in very handy for another reason however, but we will see that later.Next we'll see how the system measures distances to satellites.

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2 - Measuring distance from a satelliteFrom last section: position is calculated from distance measurements to at least three satellites. But how to measure the distance?Solution: By timing how long it takes for a signal sent from the satellite to arrive at the receiver.

* Speed of light: c = 300,000 km/sec Distance to satellite is d = c x TdThe problem is measuring the travel time.

Measuring Travel TimeA Pseudo Random Code (PRC) is transmitted from each satellite. Physically it's a pseudo-random sequence of "on" and "off" pulses.Receiver knows the time of transmission of the satellite sequence.By synchronizing the received sequence with a locally generated sequence, the receiver can identify the relative delay between the satellite and its location.*Transmission from satelliteReception at GPS receiverTd = Time elapsed between satellite and receiver

Reasons for using pseudo random sequencesAvoid accidental synchronism with other interfering signal. The patterns are so complex that it's highly unlikely that a stray signal will have exactly the same shape. Since each satellite has its own unique Pseudo-Random Code they allow satellite identification. So all the satellites can use the same frequency. Pseudo-random sequences also make it more difficult for a hostile force to jam the system. In fact the Pseudo Random Code gives the DoD a way to control access to the system. Most importantly, the spread-spectrum effect gives spreading gain, which allows the receiver to amplify the signal at de-spreading. This enhances the link budget and allows economical GPS receiver (portable units with low gain antennas).*

GPS SignalsThe GPS satellites transmit signals on two carrier frequencies. The L1 carrier is 1575.42 MHz and carries both the status message and a pseudo-random code for timing. The L2 carrier is 1227.60 MHz and is used for the more precise military pseudo-random code.

Navigation Message: low frequency signal added to the L1 codes that gives information about the satellite's orbits, their clock corrections and other system status. *

Pseudo-Random CodesThere are two types of pseudo-random code.The first pseudo-random code is called the C/A (Coarse Acquisition) code. It modulates the L1 carrier. It repeats every 1023 bits and modulates at a 1MHz rate. Each satellite has a unique pseudo-random code. The C/A code is the basis for civilian GPS use. CA code is at 1.024 Mbps.The second pseudo-random code is called the P (Precise) code. It repeats on a seven day cycle and modulates both the L1 and L2 carriers at a 10MHz rate. This code is intended for military users and can be encrypted. When it's encrypted it's called "Y" code. Since P code is more complicated than C/A it's more difficult for receivers to acquire. That's why many military receivers start by acquiring the C/A code first and then move on to P code. P code is at 10.24 Mbps.*

Summary Measuring DistancesDistance to a satellite is determined by measuring how long a radio signal takes to reach the user from that satellite. To make the measurement we assume that both the satellite and the users receiver are generating the same pseudo-random codes at exactly the same time. By comparing how late the satellite's pseudo-random code appears compared to the receiver's code, the receiver determines how long the signal took to reach it. Multiply that travel time by the speed of light and you've got distance. *

Summary Measuring DistancesDistance to a satellite is determined by measuring how long a radio signal takes to reach the user from that satellite. To make the measurement we assume that both the satellite and the users receiver are generating the same pseudo-random codes at exactly the same time. By comparing how late the satellite's pseudo-random code appears compared to the receiver's code, the receiver determines how long the signal took to reach it. Multiply that travel time by the speed of light and you've got distance. *But to measure the time a perfect synchronism would be required!!

3 - TimingTiming is critical: 1ms means a 200 mile error!Remember that both the satellite and the receiver need to be able to precisely synchronize their pseudo-random codes to make the system work. On the satellite side, timing is almost perfect because they have incredibly precise atomic clocks on board. But what about receivers on the ground? *

Position error due to wrong timing*

Timing at receiversIf our receivers needed atomic clocks (which cost upwards of $50K to $100K) GPS would be non-economical.Solution to this problem is to make an extra satellite measurement. This is one of the key elements of GPS and as an added side benefit it means that every GPS receiver is essentially an atomic-accuracy clock. In other words: if three perfect measurements can locate a point in 3-dimensional space, then four imperfect measurements can do the same thing. *

How timing works at receiversIf timing was perfect (i.e. if receiver's clocks were perfect) then all satellite ranges would intersect at a single point (which is the receivers position). But with imperfect clocks, a fourth measurement, done as a cross-check, will NOT intersect with the first three. So the receiver's computer can detect the discrepancy in time measurements and recognize that it is out of synchronism with universal time. Since any offset from universal time will affect all of receiver measurements, the receiver looks for a single correction factor that it can subtract from all its timing measurements that would cause them all to intersect at a single point. That correction brings the receiver's clock back into sync with universal time, providing atomic accuracy time to it.*

How timing works at receivers (cont.)Once receiver has the timing correction it applies to all the rest of its measurements and allows precise positioning. One consequence of this principle is that any GPS receiver will need to have at least four channels so that it can make the four measurements simultaneously.*But for the triangulation to work we not only need to know distance, we also need to know exactly where the satellites are. In the next section we'll see how we accomplish that.

Summary - TimingAccurate timing is the key to measuring distance to satellites. Satellites are accurate because they have atomic clocks on board. Receiver clocks don't have to be too accurate because an extra satellite range measurement can remove errors. *But for the triangulation to work we need not only to know distance, we also need to know exactly where the satellites are. NEXT SECTION

4 - Satellite Position in SpaceOn the ground all GPS receivers have an almanac programmed into their computers that tells them where in the sky each satellite is, moment by moment. *

Monitoring Satellite PositionOrbits constantly monitored by the Department of Defense. They use very precise radar to check each satellite's exact altitude, position and speed. Errors in position caused by gravitational pulls from the moon and sun and by the pressure of solar radiation on the satellites. The errors are usually very slight because of high orbit (MEO), but for accuracy they must be taken into account. *

Monitoring Satellite Position (cont.)Once the DoD has measured a satellite's exact position, they relay that information back up to the satellite itself. The satellite then includes this new corrected position information in the timing signals it's broadcasting. That is why a GPS signal is more than just pseudo-random code for timing purposes. It also contains a navigation message with ephemeris information as well.*

Summary Satellite PositionTo use the satellites as references for range measurements we need to know exactly where they are. GPS satellites are being at high orbits (MEO), are very predictable. Minor variations in their orbits are measured by the Department of Defense. The error information is sent to the satellites, to be transmitted along with the timing signals. *

5 Additional Errors Assumption distance to a satellite can be calculated by multiplying a signal's travel time by the speed of light was simplified so far: speed of light is only constant in a vacuum. As a GPS signal passes through the charged particles of the ionosphere and then through the water vapor in the troposphere it gets slowed down, and this creates the same kind of error as bad clocks. *

Correcting delay errorsTo minimize the errors described, one can predict what a typical delay might be on a typical day. This is called modeling and provides considerable improvement but with limitations because atmospheric conditions are rarely typical.

Another technique to minimize on these atmosphere-induced errors is to compare the relative speeds of two different signals. This "dual frequency" measurement is very sophisticated and is only possible with advanced receivers:Physics says that as light moves through a given medium, low-frequency signals get "refracted" or slowed more than high-frequency signals. By comparing the delays of the two different carrier frequencies of the GPS signal, L1 and L2, we can deduce what the medium (i.e. atmosphere) is, and we can correct for it. Unfortunately this requires a very sophisticated receiver since only the military has access to the signals on the L2 carrier.

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Other sources of errorMultipath error: The signal may bounce off various local obstructions before it gets to our receiver. Atomic clocks imperfections (small not null). Position detection errors.Geometric Dilution of Precision.Intentional errors (removed in 2000) by the DoD. The policy was called "Selective Availability" or "SA" and the idea behind it was to introduce inaccuracies to make sure that no hostile force or terrorist group could use GPS to make accurate weapons.

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Geometric Dilution of PrecisionBasic geometry itself can magnify these other errors with a principle called "Geometric Dilution of Precision" or GDOP. It sounds complicated but the principle is quite simple. There are usually more satellites available than a receiver needs to fix a position, so the receiver picks a few and ignores the rest. If it picks satellites that are close together in the sky the intersecting circles that define a position will cross at very shallow angles. That increases the gray area or error margin around a position. If it picks satellites that are widely separated the circles intersect at almost right angles and that minimizes the error region. Good receivers determine which satellites will give the lowest GDOP.

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Geometric Dilution of Precision (cont.)*

Summary - Correcting ErrorsThe earth's ionosphere and atmosphere cause delays in the GPS signal that translate into position errors. Some errors can be factored out using mathematics and modeling. The configuration of the satellites in the sky can magnify other errors. Differential GPS can eliminate almost all error. *

GPS Flavors"Differential GPS," involves the use of two receivers. One monitors variations in the GPS signal and communicates those variations to the other receiver. The second receiver can then correct its calculations for better accuracy."Carrier-phase GPS" takes advantage of the GPS signal's carrier signal to improve accuracy. The carrier frequency is much higher than the GPS signal which means it can be used for more precise timing measurements. "Augmented GPS" (aviation industry) involves the use of a geostationary satellite as a relay station for the transmission of differential corrections and GPS satellite status information. These corrections are necessary if GPS is to be used for instrument landings. The geostationary satellite would provide corrections across an entire continent. *

Differential GPSError in position location is bias plus random error. Bias is same over a wide area caused by delay in atmosphere, ephemeris error, etc.Fixed receiver at a known location can measure bias error.Radio communication link to user allows removal of bias error. Extra receiver and data links increases cost considerably.Used to be more essential for civil applications before removal of Selective Availability (2000).*

GPS AccuracyC/A (civil): About 10 meters

P (military): Can get down to centimeter with the use of differential GPS techniques.

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GPS ApplicationsCivil Location - determining a basic position Tracking - monitoring the movement of people and things. Timing - providing atomic clock precision.Military: primary targeting and navigation system for US armed forces.Surveying: Mapping and locating land areas.Vehicular Navigation: on-car navigation systems.Ship navigation: Especially in coastal and inland waters.Aircraft navigations and landing: with development of Augmented GPS by FAA.*

GPS LimitationsReceiver must have line of sight to four or more satellites.Cannot work indoors of if sky is blocked (by buildings or other solid obstructions).Accuracy in vertical dimension is lower than in horizontal.CA code may be vulnerable to interference and jamming.*

Other options of navigation systemsLandmarks: Only work in local area. Subject to movement or destruction by environmental factors.Dead Reckoning:Very complicated. Accuracy depends on measurement tools which are usually relatively crude. Errors accumulate quickly.Celestial:Complicated. Only works at night in good weather. Limited precision.OMEGA:Based on relatively few radio direction beacons. Accuracy limited and subject to radio interference.LORAN:Limited coverage (mostly coastal). Accuracy variable, affected by geographic situation. Easy to jam or disturb.SatNav:Based on low-frequency doppler measurements so it's sensitive to small movements at receiver. Few satellites so updates are infrequent.*

GMU - TCOM 508 - Spring 2001GMU - TCOM 508 - Spring 2001Class: April-12-2001(C) Leila Ribeiro, 2001*(C) Leila Ribeiro, 2001GMU - TCOM 508 - Spring 2001GMU - TCOM 508 - Spring 2001Class: April-12-2001(C) Leila Ribeiro, 2001*(C) Leila Ribeiro, 2001

Vsat(GPS)

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