This is only a preview of the May 1992 issue of Silicon Chip. You can view 47 of the 96 pages in the full issue, including the advertisments. For full access, purchase the issue for $10.00 or subscribe for access to the latest issues. Articles in this series:
Items relevant to "Build A Telephone Intercom":
Articles in this series:
Articles in this series:
Articles in this series:
|
AMATEUR RADIO
BY GARRY CRATT, VK2YBX
Receiving weather satellite signals; Pt.2
Last month, we looked at the polar orbiting
weather satellites and discussed the equipment
necessary to receive and decode images
transmitted by them. This month, we look at the
SHF transmissions which originate from the
Japanese GMS-4 satellite.
The Japanese GMS-4 satellite is located in geostationary orbit at 140°
east. This satellite transmits enhanced
images on a frequency of 1691MHz,
requiring the use of microwave techniques for good quality reception.
When one considers the minute signal levels reaching the Earth's surface
(typically .05µV), it becomes apparent that considerable care must be
taken to receive and display weather
images from this satellite. The GMS
satellite uses a deviation of ±126kHz
and so the receiver must have a minimum IF bandwidth of 260kHz. This
increased bandwidth means that the
received noise is about 10 times larger
in amplitude than from a VHF polar
orbiting satellite having a deviation
of ±18kHz.
In order to recover useable signals ,
the antenna must be capable of pro-
This image from the Japanese GMS-4 satellite clearly shows the cyclone that
eventually crossed the Queensland coast during March 1992.
86
SILICON CI-IIP
viding sufficient gain between the incoming signal level of -134dBm and
the typical receiver sensitivity of
-1 lOdBm. This equates to a gain of
26dB, requiring a dish having a diameter of at least 1. 5 metres. Such dishes
are often available from satellite TV
dealers, who often have damaged units
of no use at 12GHz but still quite
useable at the frequency of the GMS
satellite. Fig. l shows the gain that can
be expected from dishes of various
_diameters.
The path loss between spacecraft
and Earth at 1691MHz is approximately 188dB and as the output signal from the satellite is 5W (+37dBm),
and the gain of the spacecraft antenna
is around 17dBi, the calculated signal
at the ground is -134dBm.
Receiver requirements
To obtain a good signal, the receiver
should also have a noise figure of
around 1.5-ZdB. A popular configuration is to use a microwave mixer and
local oscillator chain feeding a VHF
receiver. Particular care must be taken
to ensure that the local oscillator is
kept as stable as possible: it will be
oscillating at about 1500MHz, to produce an IF of 137MHz (for example),
and any drift in the oscillator will be
multiplied by 15 or so (assuming the
oscillator runs a 20MHz crystal). Thus,
a frequency offset of lkHz will become a shift of 75kHz at 1500MHz.
The most important parameter of
the GMS receiving system is the IF
bandwidth of the receiver. Some
weather satellite enthusiasts have attempted to use scanning receivers for
the purpose, as they cover the SHF
frequency range. However, these receivers have a wideband FM bandwidth of 150-lB0kHz, causing poor
signal-to-noise ratio and severe limiting of greyscale resolution. A correctly
ing this distance for maximum signal.
Fortunately, for those without the
resources or time to construct a GMS
system, specially designed and prebuilt components .are available. PH
Communications - phone (07) 264
1575 - produces a built up GMS receiver, downconverter and dipole feed
system. They also produce a suitable
1.7GHz LNA (low noise amplifier) for
those who think they need it. S.CISAT
Products are finalising a complete
GMS receiver/downconverter. QUORUM Communications (address details in last month's issue) produce a
suitable downconverter, model SDC16918.
Polar orbiters
Taken from one of the NOAA polar orbiting satellites, this infrared image of the
United Kingdom shows quite a lot of detail, including many small towns.
designed receiver must have an IF
bandwidth of 260-280kHz.
Unfortunately, most receiver designs featured in overseas magazines
are suitable only for METEORSAT or
GOES satellites, which have an IF
bandwidth of 40kHz. With this kind
of signal improvement over GMS
transmissions (+8.25dB), signals can
be heard on four phased Yagi antennas. While GMS transmissions can be
heard using such equipment, pictures
cannot be produced, due to the reduced signal levels.
As 1691MHz is a frequency used
exclusively for weather satellite transmissions, commercial feedhorns for
this frequency are not readily available. Because the efficiency of the
dish is related to the type of feedhorn
arrangement used, the design of this
component is also very important. The
first step towards constructing a suitable feedhorn is to determine the focal point of the dish, so that the
feedhorn can be correctly positioned.
This can be calculated using the formula F = D2 /2c, where F is the focal
point, D is the diameter and c is the
depth of the dish.
Designs for "coffee can" feedhorns
can be found in the ARRL Antenna
Book, and Jessop's VHF UHF Manual.
A table in the latter book shows both
the 3dB and lOdB beam width required
for various values of F/D. This table
enables constructors to select a suitable feedhorn design, once the F/D of
the dish is known, ensuring that the
dish is fully illuminated and operating at peak efficiency.
As an example, a feedhorn suitable
for a dish having an F/D ratio of 0.56
can be constructed using a 12cm long,
18cm wide can, containing a quarter
wavelength monopole (3cm), mounted
3cm from the rear of the tube. The
focal point is measured from the inside centre of the dish to the inside
edge of the feedhorn.
As is the case with all microwave
receiving systems, the feed must be
rotated to the correct satellite polarity, corresponding to maximum signal level, and the focal point should
also be fine tuned by carefully adjust-
Fig.1: Dish Diameter vs. Gain
Diameter
Gain (dBi)
0.6
18
1.2
24
1.5
26
1.8
27.5
2.0
29
2.4
30
3.0
32
There is yet another mode of transmission used by the polar orbiting
weather satellites described in our first
article. These polar orbiters produce
extremely high resolution pictures and
the data is transmitted on frequencies
similar to GMS.
In the case of NOAA 9 & 11, this
frequency is 1707MHz. For NOAA 10
it is 1698MHz. Data is collected from
spacecraft instruments such as the
Advanced Very High Resolution Radiometer (AVHRR), the Operation Vertical Sounder (TVOS), the Space Environment Monitor (SEM), the Data
Collection System (DCS) and the
spacecraft telemetry system.
The subject of HRPT signal reception is quite complex and extremely
interesting. Dedicated newsletters for
enthusiasts are available, such as the
Journal of the Environmental Satellite Amateur Users Group. This is
published by the Dallas Remote
Imaging Group, 4209 Meadowdale
Drive, Carrollton, TX 75010 USA. Several copies perused in our office indicate that this newsletter contains a
high level of quality information.
Further reading
(1) "High Resolution Weather Satellite Pictures," M. L. Christieson, Wireless World December 1981 and January 1982.
(2) "Tracking Low Earth Orbit Satellites At LIS Band", "Break In" NZART
publication, March 1989.
Acknowledgment
I would like to thank Mr Brian Buckingham and Mr Fred Lehner for their
time and assistance in supplying background information.
SC
MAY 1992
87
|