Lecture 1, General Principles (http://www.satcom.co.uk/article.asp?article=3)

Introduction

Welcome to the first lecture in the RPC Satcom Tutorial series.

At any point in the lecture, you can skip to a latter section, or back to earlier sections using the guide on the left.

If you want to check out the other Lectures in this series, you can find Lecture 2 (Space Segment) and Lecture 3 (Earth Segment).

The Pioneers of Satellite Communications

-Konstantin Tsiolkovsky (1857 - 1935)
Russian visionary of space flight First described the multi-stage rocket as means of achieving orbit.

Link: The life of Konstantin Eduardovitch Tsiolkovsky
 

-Hermann Noordung (1892 - 1929)
Postulated the geostationary orbit.

Link: The Problem of Space Travel: The Rocket Motor

-Arthur C. Clarke (1917 - )
Postulated the entire concept of international satellite telecommunications from geostationary satellite orbit including   coverage, power, services, solar eclipse.

Link: "Wireless World" (1945)

Historical Perspective

1968
  • INTELSAT III - to give 3 Ocean Region Coverage

1971 

  • ITU-WARC for Space Telecommunications
  • INTELSAT IV Launched
  • INTERSPUTNIK - Soviet Union equivalent of INTELSAT formed

1976 

  • MARISAT - First civil maritime communications satellite service started

1977 

  • EUTELSAT - European regional satellite
  • ITU-WARC for Space Telecommunications in the Satellite Service

1980 

  • INTELSAT V launched - 3 axis stabilised satellite built by Ford Aerospace

1983 

  • ECS (EUTELSAT 1) launched - built by European consortium supervised by ESA

1984 

  • UK's UNISAT TV DBS satellite project abandoned

1989 

  • INTELSAT VI - one of the last big "spinners" built by Hughes

1990 

  • IRIDIUM, TRITIUM, ODYSSEY and GLOBALSTAR S-PCN projects proposed - CDMA designs more popular
  • EUTELSAT II

1992 

  • OLYMPUS finally launched - large European development satellite with Ka-band, DBTV and Ku-band SS/TDMA payloads - fails within 3 years

1993 

  • INMARSAT II - 39 dBW EIRP global beam mobile satellite - built by Hughes/British Aerospace

1994 

  • INTELSAT VIII launched - first INTELSAT satellite built to a contractor's design
  • Hughes describe SPACEWAY design

1996 

  • INMARSAT III launched - first of the multibeam mobile satellites (built by GE/Marconi)

1997 

  • IRIDIUM launches first test satellites
  • ITU-WRC'97

1999 

  • AceS launch first of the L-band MSS Super-GSOs - built by Lockheed Martin
  • Iridium Bankruptcy - the first major failure?

2000 

  • Thuraya launch L-band MSS Super-GSO

2004 

  • Teledesic network planned to start operation

INTELSAT

INTELSAT is the original "Inter-governmental Satellite organisation".

It once owned and operated most of the World's satellites used for international communications, and still maintains a substantial fleet of satellites.

INTELSAT is moving towards "privatisation", with increasing competition from commercial operators (e.g. PanAmSat, Loral Skynet, etc.).

INTELSAT Timeline:

  • Interim organisation formed in 1964 by 11 countries
  • Permanent structure formed in 1973
  • Commercial "spin-off", New Skies Satellites in 1998
  • Full "privatisation" by April 2001

INTELSAT has 143 members and signatories listed here.

INTELSAT Structure:

EUTELSAT
-Permanent General Secretariat opened September 1978

-Intergovernmental Conference adopted definitive statutes with 26 members on 14 May 1982

-Definitive organisation entered into force on 1 September 1985

  • General Secretariat -> Executive Organ
  • ESC Council -> EUTELSAT Board of Signatories
  • Secretary General -> Director General
  • Current DG is Giuliano Berretta

-Currently almost 50 members

-Moving towards "privatisation"

  • Limited company owning and controlling all assets and activities
  • Also a "residual" intergovernmental organisation which will ensure that basic principles of pan-European coverage, universal service, non-discrimination and fair competition are observed by the company

EUTELSAT Structure:

Advantages & Disadvantages

Advantages:
  • Flexible (if transparent transponders)
  • Easy to install new circuits
  • Circuit costs independent of distance
  • Broadcast possibilities
  • Temporary applications (restoration)
  • Niche applications
  • Mobile applications (especially "fill-in")
  • Terrestrial network "by-pass"
  • Provision of service to remote or underdeveloped areas
  • User has control over own network
  • 1-for-N multipoint standby possibilities

Disadvantages

  • Large up front capital costs (space segment and launch)
  • Terrestrial break even distance expanding (now approx. size of Europe)
  • Interference and propagation
  • Congestion of frequencies and orbit
When, and when not, to use satellites
When to use satellites:
  • When the unique features of satellite communications make it attractive
  • When the costs are lower than terrestrial routing
  • When it is the only solution

Examples:

  • Communications to ships and aircraft (especially safety communications)
  • TV services - contribution links, direct to cable head, direct to home, SNG
  • Data services - private networks
  • Overload traffic
  • Delaying terrestrial investments
  • 1-for-N diversity
  • Special events

When to use terrestrial:

  • PSTN - satellite is becoming increasingly uneconomic for most trunk telephony routes
  • but, there are still good reasons to use satellites for telephony such as: thin routes, diversity, very long distance traffic and remote locations.
  • Land mobile/personal communications - in urban areas of developed countries new terrestrial infrastructure is likely to dominate (e.g. GSM, FLMPTS)
  • but, satellite can provide fill-in as terrestrial networks are implemented, also provide similar services in rural areas and underdeveloped countries
Frequency Bands Allocated to the FSS
Frequency bands are allocated to different services at World Radio-communication Conferences (WRCs). Allocations are set out in Article S5 of the ITU Radio Regulations.

It is important to note that (with a few exceptions) bands are generally allocated to more than one radio service

  • Sharing
  • Interference
  • -> CONSTRAINTS

Bands have traditionally been divided into "commercial" and "government/military" bands, although this is not reflected in the Radio Regulations and is becoming less clear-cut as "commercial" operators move to utilise "government" bands.

The Satellite Link
Radio Links:

Antenna Gain: 

g = (Power density on-axis at given distance) / (power density from isotropic source at same distance)

g = 4π.a/λ2

Antenna Aperture:

a = η.π.r2

Power density from transmit antenna at receive aperture is:

pden = (pt.gt)/(4π.d2)

Power collected by receive antenna:

pr = ((pt.gt)/(4π.d2)).ar.α = pt.gt.gr(λ/(4π.d))2

Path transmission loss ("free space loss"):

lp = ((4π.d)/λ)2

Thermal noise at output of receive antenna:

n = k.T.B

Carrier-to-noise ratio:

pr/nr = (pt.gt.gr.α)/(lp.k.T.B)

Hence "Link Budget Equation" expressed in dB:

C/N = Pt+Gt-Lp-La+(G/T)|r-k-10.log(B)

Note: |r means evaluated at r.

When calculating the link budget, we must take account of additional factors:

  • Interferences from other radio systems
  • Intermodulation noise
  • Additional loss, interference and noise due to propagation events (e.g. rain, rain-scatter, noise degradation, ducting, etc.)

Figure of merit (G/T) is a useful way of categorising the performance of a receiving system. 

In a constant RF field, different receiving systems (with the same receive bandwidth) will yield a C/N proportional to the system G/T.

EIRP is the usual way of combining transmit power and gain:

EIRP = Pt+Gt

So, the link budget equation is normally expressed as:

C/N = EIRP - Lp - La - Lm + G/T - 10log(B) + 228.6

The satellite link:

Uplink and downlink are analysed seperately as individual radio links:

Overall Link Quality:

C/N = C/(Nup+Ndown+Iup+Idown+Nimod)

(c/ntot)-1=(c/nup)-1+(c/ndown)-1+(c/iup)-1+...

Copyright 2002 Satcom Online (http://www.satcom.co.uk)
11/12/2017  11:13:44