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AMATEUR RADIO
BY GARRY CRATT, VK2YBX
Receiving weather satellite
signals - the hardware required
Interested in receiving off-air pictures from
the weather satellites? Here's a rundown on
the equipment that you'll need & where you
can obtain it.
Ever since the advent of weather
satellites , amateur radio enthusiasts
have been involved in the reception
of images from spacecraft. Apart from
the meteorological aspects of weather
satellite reception, there are a number
of technical challenges to be overcome if successful reception is to be
accomplished. Areas such as satellite
antennas , receivers , data processing
and storage techniques all add to the
challenge of satellite reception.
There are several levels of weather
satellite transmissions that can be received and each level provides higher
· resolution and more varied formats all at an increasing level of complication, as far as the receiving equipment
is concerned.
Some satellites produce high resolution imagery which is sent at high
speed on microwave frequencies.
Such signals require significant effort
and investment to be received. Other
satellites, such as the polar orbiting
"birds", use a much slower rate of
transmission, in the VHF bands, and
can easily be decoded using relatively
simple equipment.
These satellites use APT (automatic
picture transmission), a system devised to allow users in remote locations direct access to meteorological
data at low cost.
APT resolution is typically 2-3km,
allowing definite identification of cities, rivers, lakes and other major land
features and their proximity to cloud
masses. APT transmissions are FM,
with an audio subcarrier that has been
amplitude modulated. The amplitude
peaks on the subcarrier correspond to
white areas, while the lowest amplitude corresponds to black. Pictures
are transmitted at either 120 or 240
lines per minute.
What's needed
This German-made satellite receiver (model MR-137) & its companion decoder
unit (model FX-666) are one way of receiving weather satellite signals. Other
hardware options are discussed in the text.
42
SILICON CHIP
In order to receive APT signals from
a polar orbiting spacecraft, we need a
suitable antenna, a receiver, a decoder
and a display unit. These days, with
the cost of PC-compatible computers
reaching their lowest levels ever, most
operators will choose this kind of
hardware to handle the display and
decoding functions. However, this still
leaves the antenna and receiver, which
must both have particular performance characteristics for good reception. ·
The antenna should be horizontally
polarised and have a high angle of
radiation to allow reception for the
maximum amount of time as the satellite becomes "visible", then passes
overhead and disappears over the horizon. We discussed the construction
of a suitable turnstile antenna in the
tained. Other frequencies sometimes.
used are 137.06, 137.12, 137.15,
137.33, 137.45 and 138.8MHz.
VHF APT transmissions from these
polar orbiting spacecraft use wideband
FM with no pre-emphasis. Because
the maximum deviation (of NOAA
spacecraft) is ±18kHz, the maximum
modulating frequency is 4kHz and
the maximum Doppler shift is ±3kHz,
we need a receiver with an IF bandwidth of 50kHz. This will present a
problem to anyone proposing to use a
standard scanning receiver, as the typical NBFM bandwidth is 15kHz and
the wideband FM bandwidth is
180kHz.
Alternative receivers
A J-pole antenna is ideal for weather
satellite reception. You can either use
a commercial unit, such as the unit
pictured here, or build the design to
be published in a forthcoming issue of
SILICON CHIP.
November 1991 issue of the magazine.
The next requirement is for a good
VHF FM receiver, capable of reception in the 136-138MHz band. Many
enthusiasts use a suitable scanning
receiver which, despite not having
ideal IF bandwidth characteristics for
weather satellite reception, can be
used to produce fair results. Dedicated kit receivers are also available
at moderate prices.
Some older crystal locked 2-metre
FM receivers could also be pressed
into service, although the cost of crystals will often make the purchase of a
scanning receiver a more attractive
proposition.
The accompanying table shows a
list of current VHF channels and the
spacecraft that use them. As can be
seen, some satellites share the same
frequency (one will be turned off by
ground station command if there is
any chance of interference), so that
maximum frequency utilisation is ob-
One solution, suggested by Paul
Hayden VK4ZBV, is to use one of the
old 60kHz crystal filters from an early
commercial 2-way radio transceiver.
He also suggests an alternative approach, which is to use a crystallocked converter in conjunction with
a surplus ex-army transceiver, such
as the C45/46 or PRC9/10. Both these
transceivers have a receiver IF bandwidth of 50kHz and can be converted
to solid state operation. It is also possible to purchase a custom made
10. 7MHz filter from Hy-Q Crystals in
Melbourne.
Considering the highest altitude a
polar orbiting weather satellite is
likely to attain is 3000km, and that
the resultant path attenuation at
137MHz is likely to be in the order of
-145dB or so, the receiver must be
capable of producing 20dB of quieting for an input signal of0.5µV (based
on the spacecraft transmit power of 5
watts). This level of performance is
reasonably easy to obtain, particularly
if using a FET preamplifier at the antenna. Users should be aware of the
close proximity of TV channel 5A
and the adverse affect such signals
will have on VHF weather satellite
reception.
Software
Because we have elected to use a
computer and video monitor as the
decoder and display (and memory)
devices, the choice of a suitable software program is the next project to
tackle. There is an enormous amount
of weather satellite software available,
ranging from barely adequate to brilliant. Computer programs to suit
Amiga, IBM, Commodore 64 and
STOCK
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2 x AA Battery Holder ....... ..... $0.28
12-Way terminal block 10A ... $1.60
SEMICONDUCTORS
1N4004 Diodes ..................... $0.07
1N4007 Diodes ................. .... $0.10
1N4148 Diodes ... .... .. ... ...... ... $0.04
MJ15003 Transistor ..... ... ...... $6.50
MJ 15004 Transistor .......... .... $6.50
MJ15024 Transistor ........... .. . $8.60
MJ 15025 Transistor ... .. ..... ... . $8.60
MJ13007 Transistor ......... ..... $4.87
MFR571 Transistor .. .. ....... .. .. $4.27
NE555 IC .. ... ... .. ............. ....... $0.30
TL494 IC .... .. .. ...... .... ... .. ..... ... $2.83
MC14514 IC .......................... $3.00
MC34063 IC ............ .............. $2.98
ALL ITEMS 10 PLUS 10% OFF
HYCAL ELECTRONICS
Unit 4, 62 Gt. Western Hwy,
Parramatta, NSW 2150.
Phone (02) 633 5477
Fax (02) 891 5640
Credit Cards Welcome
APRIL 1992
43
terns, Wichambrook Newmarket CB8
8QA, England. Phone (0440) 82 0040
of fax (0440) 82 0281. Their US distributor is Spectrum International Inc,
PO Box 1084, Concord, MA 01742,
USA. Phone (508) 263 2145 or fax
(508) 263 7008.
A comprehensive range of receivers, decoders, antennas, and other
hardware for both APT, GMS and
HRPT reception is available from:
Quorum Communications Inc, 1020
Main Street Suite A, Grapevine, TX
76051, USA. Phone (817) 488 4861 or
fax 817 481 8983.
For those sufficiently affluent,
Mitsubishi Electric can supply a suitable Video Copy Processor, allowing
black and white prints of any video
screen to be made for posterity. Model
P75E can produce high resolution
prints at 640 X 580 dots resolution in
handy 200 X 154mm print size - ideal
for the family album!
Next month, we will discuss hardware requirements for the reception
of GMS and HRPT satellite signals.
A turnstile antenna can also be used for weather satellite reception although the
results are generally not as good as from a J-pole. This is a commercial unit but
you could also use the build-it-yourself design published in the November 1991
issue of SILICON CHIP.
Tandy COCO computers are available,
and may also be for other types of
computer.
Perhaps the best way to evaluate
available software is to contact a local
user group or scan some of the available bulletin boards for information
on such programs.
Hopkins, 4 Handsworth Street,
Capalaba, Qld 4157. The software is
available from: Mr M. Delahunty, 42
Villiers Street, New Farm, Qld 4004.
A complete weather satellite receiving system and all discrete components including software can be obtained from Timestep Weather Sys-
Hardware guide
A suitable weather satellite receiver
kit is available from Stewart Electronic
Components, phone (03) 543 3733 or
fax (03) 543 7238. The kit was described in the February 1988 issue of
"Australian Electronics Monthly"
(which has now ceased publication).
A complete standalone receiver and
decoder is available from Mr Volker
Wraase, Kronsberg 10 D-2300
Altenholz/Kiel, West Germany. The
receiver model is "MR-137", while
the decoder is the" FX-666".
A suitable pre-assembled and tested
137MHz preamplifier is available
from: (1) SCISAT Products, PO Box
307, Kenmore, Qld 4069. (2) Vanguard
Electronic Labs, 196-23 Jamaica Avenue, Hollis NY 11423, USA.
An IBM PC/XT/ AT plug-in decoder
board is available from Mr David G.
44
SILICON CHIP
VHF WEATHER SATELLITES
Country
Satellite
Freq. (MHz)
USA
NOAA9
136.620
USA
NOAA 10
137.500
USA
NOAA 11
137.620
USA
NOAA12
137.500
USSR
Meteor 2-17
137.300
USSR
Meteor 2-18
137.300
USSR
Meteor 2-19
137.850
USSR
Meteor 2-20
137.850
USSR
Meteor 3-2
137.850
USSR
Meteor 3-3
137.300
USSR
Meteor 3-4
137.850
Further reading
(1) Communications Satellites - A
Monitors Guide. 3rd edition by Larry
V. A. Horn. Published by Grove Enterprises, PO Box 98, Brasstown, N.C.
28902, USA.
(2) US Department of Commerce,
NOAA Technical Report NESDIS 44.
NOAA Office of Constituent Affairs,
Herbert C Hoover Building, Room
6815 A, 14th & Constitution Avenue,
Washington DC 20230, USA.
(3) "TIROS-N Series Direct Readout
Services Users Guide" and "The
WEFAX User's Guide"; both available
from The Co-ordinator, Direct Readout
Services, United States Department
of Commerce, National Oceanic and
Atmospheric Administration, National Earth Satellite Service, Washington DC 20233, USA.
In addition, NOAA has a large range
of technical publications (NESS61 to
NESS115) relating to weather satellite reception. These can be obtained
by 'Yriting to the National Technical
Information Service, US Department
of Commerce, Sills Building, 5285 Port
Royal Road, Springfield, VA 22161,
USA.
Information on related subjects can
be obtained from the Environmental
Data Service, D822, 6009 Executive
Boulevard, Rockville, MD 20852,
USA.
SC
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