Indian Institute of Hams' Official Website

Introduction

Satellite Space

Chuck Towns (K6LFH) in his garage with OSCAR II

Amateur Satellites
 
Project OSCAR (Orbiting Satellite Carrying Amateur Radio), a west coast USA-based group which built and launched the very first amateur radio satellites began in 1961. OSCAR I was launched on December 12th, 1961 barely four years after the launch of Russia's first sputnik. OSCAR I was followed by OSCAR II six months later. These satellites were built, quite literally in people's basements and garages. Originally these were "bleep sats" but now they carry sophisticated repeaters or transponders.
 
Even today's satellites give out bleeps (beacons) on a particular frequency so that you know the satellite is passing by. The time the satellite is visible (in range) to an observer is called a satellite "pass". During the pass, you are in the "footprint".

How is a satellite designed?
 
In simple terms a communication satellite is usually nothing but an orbiting repeater in the sky. The satellite picks up your signals on a particular frequency (uplink) and re-transmits the same on a different frequency (downlink). Your original signals are hence enhanced and made available to a huge area depending upon the coverage (footprint) of the satellite.

The transmitter-receiver combination on a satellite is called a transponder. Satellites do not have the physical space to separate receive and transmit antennae, so they use different bands ie. most satellites have their uplink frequencies in the V.H.F band and their downlink frequencies in the U.H.F band. You may not be able to hear a satellite on its exact downlink frequency right here on earth. The frequency on which you may get the signal from the satellite may vary due to doppler shifts. A doppler shift is the variation in frequency caused by the motion of the satellite or the earth

FM Repeater vs Linear Transponder

Satellite Space

The above self explanatory diagram shows the difference between a Narrow Band FM Repeater and a Linear Transponder (Courtesy: Emily Clarke WØEEC)

 

Satellite Space

Design of a typical communication satellite.  (Courtesy: Emily Clarke WØEEC)

Satellite Orbit Types and Attitudes

Communications satellites are put into orbit by means of rocket-powered launch vehicles, or boosters. Most are placed into geostationary orbit, in which the satellite follows a circular path around the Earth in the plane of the Equator, at an altitude of 35,900 km (22,300 miles). At this height the satellite's period of revolution around the Earth is the same as the Earth's period of rotation, so that the satellite maintains a continuous position above the same spot on the globe.

 
A satellite can be put into any of the following orbit types:

Low Earth Orbit (LEO) < 800Km
Geo-Stationary Orbit (GSO)
Geo-synchronous Trasfer Orbit (GTO) ~ 32000Km
Polar Orbit ~ 800-1000Km
Polar Sun - synchronous Orbit
Equatorial orbit 

Throughout its service life, a satellite must be maintained in the correct attitude (i.e., pointing in the right direction). Attitude is controlled by one of three methods: "spin-stabilizing" the entire satellite, including the antennas; spinning the body of the satellite while the antenna platform is "despun," or counter-rotated, in order to point stably at some point on Earth; and maintaining "three-axis" stability of the entire satellite by means of an internal gyroscope and small thruster jets.

 

Modes of Operation

 

Mode Uplink (to the satellite) Downlink (from the satellite)
A 145 MHz 29 MHz
B 435 MHz 145 MHz (also known as UVMode)
J 145 MHz  435  Mhz  -- here JA is analog and JD is Digital
JL 1.2 GHz / 145 MHz  29 MHz
K 21 MHz  29 MHz
KA 21 MHz / 145 MHz  29 MHz
KT 21 MHz  29 MHz / 145 MHz
L 1.2 GHz  435 MHz
S 1.2 GHz  2.4 GHz
T 21 GHz  145 MHz
UV 435 MHz  145 MHz - same as Mode 'B'
V 29 MHz  145 MHz - inverse of Mode 'A'

 

HAMSAT - India's First Amateur Satellite
HAMSAT HAMSAT
Mr. Nagesh Upadhyay (VU2NUD) and Mr.William Leijenaar (PE1RAH) [Photo Courtesy: PE1RAH]

Launched on 5th May 2005 10:15 AM from Sri Hari Kota Launch Range (SHAR), hamsat was a co-passenger along with another of ISRO's satellite known as Cartosat I on a Polar Satellite Launch Vehicle (PSLV). HAMSAT or VUSAT as hams call it, is Indian Space Research Organisation's (ISRO) theme-based micro satellite. The HAMSAT micro satellite is India's contribution to the international community of Amateur Radio operators.  This effort is also meant to bring ISRO's satellite services within the reach of the common man and popularize space technology among the masses.
       
        It is a cuboidal structure of 630mm X 630mm X 550mm with a mass of about 42.5 Kg.  It has aluminium honey-comb structure with passive thermal control. Hamsat will be flown in a near-circular polar Low Earth Orbit(LEO). Body mounted solar panels and COTS (Commercial Off The Shelf) Lithium-Ion batteries are the main source of power. The satellite spins at a rate of about 4rpm with the spin axis orientation being ± 3degs. Spin stabilization is provided with on-board autonomy.

 

HAMSAT
HAMSAT

Tri-axial magnetometer and Twin Slit Sun Sensor with magnetic torquers act as actuators providing the required inputs to the Bus Electronics.  Bus Electronics consists of  MAR31750 processor.  This system also provides for attitude control, telemetry and telecommand for the satellite.

Communication for the satellite is on VHF (137MHz for TM and 149MHz for TC). There are two payload transponders - Indian and Dutch.   Dutch transponder was delivered by William Leijenaar, PE1RAH. Both these operate in Mode-B with 435.25MHz uplink and 145.90MHz downlink. Indian transponder is identified by an un-modulated carrier on 145.936MHz and Dutch transponder has a CW signal on 145.860MHz. The output power is about 30dBm. Bandwidth is 60KHz for the Indian transponder and 50KHz for the Dutch transponder. Only one transponder is `on' at a time. Antennae are turnstile are common for TM/TC and payload transponders.

Apart from MAR31750 processor-based Bus Electronics, COTS Li-Ion batteries and Ga-As solar panels, other new technologies incorporated are micro strip VHF 180deg. hybrid coupler, micro strip 4-way power divider and negative-B dot law for attitude control. Programmed tracking of Hamsat is being carried out by ISRO's two stations at Bangalore and Lucknow.

 

Glossary

Alpha Testing     
The first stage of product testing after the prototype 
development. The sequence is: conception, design, prototype, alpha testing, beta testing, and production.
 
AMSAT
The AMateur SATellite Corporation. A United States-
based non-profit organization committed to the development and encouragement of amateur satellite activity.
 
Analog
A signal that varies in a continuous manner (e.g., voice,
music, and voltage and currents that vary in a continuous manner). (See digital.)
 
Apogee
When the satellite is at it's highest altitude.
 
ARRL
Acronym for American Radio Relay League.
 
Asynchronous
Also called Start-Stop transmission. Digital signals
which are sent as groups of a specified length with start and stop bit indicators at the beginning and end of each group. Usually used when time intervals between transmitted groups may be uneven. (See synchronous.)
 
AX.25
Amateur packet radio protocol version of the X.25
protocol. Usually used in reference to the data link layer protocol in use by most amateur packet stations.
 
BBS
Also called CBBS, PBBS, Mailbox. Acronyrn for Bulletin
Board System. An automated computer system which can be controlled from a remote location. Usually capable of sending and receiving messages and files.
 
Beta testing
The fifth and usually final stage of product development
before general production and release. The sequence is: conception, design, prototype, alpha testing, beta testing, and production.
 
Bipolar Keying
A technique in which a binary "1" is represented by a
positive pulse and a binary "0" is represented by a negative pulse. Bipolar keying is the system used by NRZI on amateur packet radio.
 
Digital
A discrete or discontinuous signal whose various states are
identified with specified values. (See analog)
 
Doppler
A shift in frequency caused by satellite motion.
 
Downlink
The frequency used to recieve a satellite.
 
Footprint
A circular area where the satellite is line of sight.
 
FSK
Acronym for Frequency Shift Keying. A method of
frequency modulation in which the frequency varies.
 
GEO
A satellite in a Geosynchronous Orbit (35,680km)
 
Half Duplex
A circuit designed for transmission in either direction on
two separate channels but not both directions simultaneously.
 
HEO
A satellite in a High Earth Orbit (> 20,000km)
 
Inclination
The angle of the satellite where equator = ZERO.
 
JAMSAT
The Japanese AMSAT affiliate.
 
JAS-1
A packet radio satellite designed by JAMSAT which
features a flying mailbox and digipeater capabilities. (See RUDAK, Pacsat.)
 
LEO
A satellite in a Low Earth Orbit (400 - 2000km)
 
Mailbox
(See BBS.)
 
MSK
Acronym for Minimum Shift Keying. A modulation
method similar to FSK in which the shift in hertz is equal to half the signaling rate in BPS.
 
Orbit
The path a satellite travels around the earth.
 
OSCAR
Acronym for Orbital Satellite Carrying Amateur Radio.
 
Packet
A group of bits including data and control elements which
is transmitted as a whole. Technically, a packet is not formed until the network layer; however, many refer to frame transmissions on the data link layer as packets. (See frame.)
 
PACSAT
Contraction of PACket SATellite. A packet satellite
designed by AMSAT which will feature a flying mailbox similar to terrestrial BBSs. (See JAS-l, RUDAK.)
 
Perigee
When the satellite is at it's lowest altitude.
 
PSK
Acronym for Phase Shift Keying. A method of transmitting
digital information in which the phase of the carrier is varied in accordance with the digital signal.
 
Simplex
Operation over a single channel in one direction at a time.
 
TNC
Acronym for Terminal Node Controller. A device which
assembles and disassembles frames. Usually includes some form of a user interface and command set. May be implemented in hardware or software. Used in conjunction with a radio, modem, and terminal for packet radio applications. (See node, PAD, FAD, packet controller.)
 
Uplink
The frequency used to transmit to a satellite.