Sending Signals Back

Sending signals back is a complex process, and depends upon the satellite and the amount of detail involved. A radio link of some kind is required, but the more detail that is needed, the higher the frequency that must be used in order not to unduly slow down the rate of transfer. On the polar orbiters, with their 1km square resolution, the amount of information to be transmitted is such that only a microwave link will suffice. This is because the bandwidth of the transmission increases in proportion to the amount of information carried.
 Compare the quality of a medium wave radio broadcast with the same data on broadcast FM and you will appreciate why this is so - the medium wave transmission is at a frequency of around 1 MHz or less and to accomodate a reasonable number of stations packed into a fairly narrow band, recalling that each needs about 4.5 kHz to send a usable signal, it isn't possible to send a wider range of frequencies with that carrier frequency. Going up to VHF (around 90 - 100 MHz) allows much wider spacing of signals and consequently more bandwidth and better quality of reception. Visual images contain very much more information than audio signals, so as a result, to avoid loss of data, much higher frequencies still have to be used.

The NOAA polar orbiters generate images from five (or six) onboard sensors, each of which independently provides an image. As the Earth surface beneath is scanned, the output of each sensor is first digitised and stored, a line at a time. The downward link uses a frequency around 1690 - 1700 MHz and the digital data is used to phase modulate the radio frequency signal which is sent down as a binary encoded bit-stream. On reception the bit-stream is recovered and stored, and with the lines from each sensor being sent sequentially, the individual images are built up. This process is referred to as High Resolution Picture Transmission or HRPT for short. An analogous process is also used in the Chinese Fengyun series of polar orbiting satellites. In this case there are ten data channels making it possible to obtain very highly detailed colour images. Clearly all of this is a complex process and rather specialised (not to mention expensive) equipment is required at the receiving station to take full advantage of it. An 'off the shelf' receiving system might start at around 2000 and whilst this may not be out of the reach of those with the expertise, it is primarily used by professional observers involved in weather prediction services generally. That is not to say that there aren't individuals who have set up their own receiving stations. More details can be found on the links and data reception pages.

If this is rather off-putting for those interested in obtaining their own images, then take heart and read on !   It doesn't have to be nearly so complex or expensive...

The earliest satellites in the TIROS series used a data transmission system of much more modest specification which required relatively simple receiving equipment. With the contemporary NOAA satellites, the system is still in use in tandem with HRPT and is usually referred to as Automatic Picture Transmission - APT for short. The important practical differences from the user's point of view are that only two data channels are transmitted [one visible and one infra-red], the image resolution is significantly lower [about 4km] and a significantly lower radio frequency [137 - 138 MHz] is used for the downlink. These are not serious limitations for many weather observation purposes and are available to everyone.

The process begins with selection of the data; generally channels 2 and 4 are used, certainly during daylight when adequate illumination is present. At night time, the visible channel would have little or no data to display, so channel 3 is substituted in its place. To reduce the data overhead, from the data streams, every fourth line is selected (hence the approximately 4km resolution) and the two channels are transmitted consecutively at a rate of 120 lines per minute i.e. two per second. The transmission process comprises first the amplitude modulation of a 2400 Hz carrier by the data; this 2400 Hz carrier is then used to frequency modulate the 137 Mhz RF carrier which is transmitted down to Earth - the transmitter power is of the order of 5 to 10 watts, varying somewhat from one satellite to another.

A somewhat directional antenna is used to ensure an adequate signal strength over the Earth's surface beneath roughly corresponding to the area over which the satellite can be observed. This area is often referred to as the footprint of the satellite. The antenna is designed so that right-handed circular polarisation is produced, the point being that were linear polarisation to be used, this could give rise to some signal loss. It was found that the plane of polarisation of the transmission can be rotated by a variable amount on propagation through the Earth's atmosphere. Whilst requiring slightly more complex antenna systems at the receiving end, this helps ensure that variations in the received signal strength are minimised. In order to indicate which channel is being transmitted, each is preceeded by a brief toneburst, 832 Hz for the visual data and 1040 Hz for the infra-red, and in addition to all of this other data is added to display stepped grey scales (for image intensity calibration) and temperature read-out from the infra-red images. All in all, there is a great deal of information available, and certainly sufficient to help make useful weather forecasting possible.

A quite similar system is used with the Russian Meteor and Resurs polar orbiting spacecraft, although only a single image is transmitted in this case (the latter does carry other types of imager as well), and the same receiving gear can be used to receive it with appropriate adjustments. Their images also contain calibration data, but the sensor sensitivity usually has been chosen to maximise visibility of cloud arrays and underlying land masses often are hard to see except under favourable lighting conditions. Unfortunately, there is no APT system on board the Fengyun series of satellites so images from here can only be obtained the hard way !

When it comes to the geostationary satellites, diferent provisions apply again. With the European Meteosat system for instance, the data from the various sensors is stored line by line as described earlier and is then multiplexed in a downward raw data stream. This data is referred to as the primary data and is received by the satellite control centre at Darmstadt in Germany. Here it undergoes considerable processing in which the image is divided up into segments and various other data including political boundaries and latitude/longitude markers are overlaid.  It is the transmitted back to the satellite from which it is re-broadcast to data users. The primary transmission is referred to as PDUS (Primary Data User System) and the re-transmitted data as SDUS (Secondary User Data System). Whilst there are some organisations and individuals who receive PDUS, most users make use of SDUS. This transmission also is sent at around 1700 MHz as with HRPT, but an encoding system not unlike APT is used. Provided a suitable down-converter is available (from 1700 to 137 MHz) this data can then be received on equipment used for APT with suitable parameter adjustments. In SDUS, the different segments extracted from the primary data are sent in a prearranged time sequence, so that a given desired area can be received as and when required. It should be noted that for commercial and to an extent political reasons, most of the METEOSAT data is encrypted, so that additional hardware is required to view all images. However, some unencrypted images are sent at regular intervals which can be received without further complication. Happily, it has not been the policy of US, Russian, Chinese or Japanese governments so far to encrypt data from their satellites, but it cannot be taken for granted that this will always be the case, unfortunately.

Whilst the simple APT system described above gives a relatively straightforward means of receiving satellite pictures, unfortunately, its days are numbered. Technology has moved on apace as satellites have come and gone. Already the next generation of weather satellites is moving from drawing board to implementation, and these will use a different data transmission system, referred to as LRPT (Low Rate Picture Transmission) Whilst not all the details of how it will work are yet fully in the public domain, it will probably use a variation of HRPT with a digital rather than an analogue modulation scheme. The upside is that it will give resolution comparable to present day HRPT; the downside that the receiving gear will undoubtedly be more complex and less amenable to home construction. BUT, that's still some way off - there are still some analogue satellites yet scheduled to be launched and APT should be around in some form till around 2010 or so. A lot could happen in that time, so there's everything to be said for going ahead now, and facing the changes as and when they arise !

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© Stuart Hill [Updated November 2002]