Reprinted from Radiokit Elettronica April and

By Guido Emiliani, I4GU Marciano Righini, I4MY Giampaolo Rossini, IW4CSG

Say you've been receiving the NOAA weather satellite APT signals on 137.5 and 137.62 MHz for years, and you're proud of the fine pictures that you can display on your PC monitor. Analog APT has been a very successful system of weather image dissemination—it's simple to receive and convert into pictures.

But APT transmits only one third of the lines generated by the AVHRR, the instrument that scans the Earth surface and produces the images. All the NOAA instrument data are included in the HRPT digital transmission, but you thought that displaying all the lines would go beyond your technical or economical capabilities. This was probably true a few years ago, but now—well, let's see if it's still true.

Via Colombo Lolli 8 48100 Ravenna Italy

We'll examine the difficulties of homebrewing an HRPT receiving station, but we'll also discover how easy it is if you have a good signal at your disposal. In fact, the greatest difficulty is acquiring a strong and steady signal from the satellite. If you can do this, the rest of the system is within the capacity of a homebrewer, and the software necessary to complete the system is available free to everybody.

Radio Frequency Section

These are the components of our RF section:

a) A 1.4-meter parabolic dish with an fid ratio of 0.38.

b) A cylindrical horn, 17cmlongand 12 cm in diameter, at the focus of the dish. Inside the horn, two quarter-wave monopoles (probes), each 4 cm long, are placed at 4.5 cm from the horn's back end, 90° apart. To obtain circular polarization, one monopole must be fed 90° out of phase with respect to the other. The delay is obtained with two lengths of RG-59: one 11.65 cm long (X), and the other 14.55 -cm long (A.+A./4). The proper polarization is left-handed (the dish inverts the right-hand polarization from the satellite); thus, circling counterclockwise, the first probe you meet should be connected to the longer cable. The ends of the two cables are connected together and go to c) A low-noise amplifier (LNA) for 1700 MHz, to be placed at the rear of the horn.

The system also includes:

d) Yaesu G-5400B dish azimuth and elevation rotators.

e) The Kansas City Tracker (which is optional, because manual tracking is easy with this system).

f) A downconverter that converts the HRPT signal to a "transportable" frequency. (If your receiver can tune the S band and is close to the dish, you may not need a converter.)

g) An ICOMIC-R7000 receiver, with which we receive the converter's output frequencies or (if we don't use a converter) the NOAA satellite's frequencies (1698 and 1707 MHz). The R7000's rear panel includes a jack that outputs a 10.7-MHz 2nd-IF signal. We use this signal to drive our HRPT decoder, (Be careful: this jack also sources 9 V dc, so you may need to use a blocking capacitor.)

There are many more possible arrangements, but we agree on one point: If you cannot install the "external" part of the system (items a, b, c, d), this project is not for you. Again, the secret for success i s a very good RF signal from the satellite.

Fig 1 block-diagrams the "internal" part of the system—the hardware described in this article.

10.7-MHz Amplifier and Band-Pass Filter

See Fig 2. Because the signal level from the 10.7-MHz output of the R7000 is very weak, we need amplification to drive the following stage. A white dot marks the input of the MAR-6 MMIC, and the output is the pin on the opposite side. The other two pins must be grounded.

The filter, consisting of two FM2/3 IF transformers, should have a bandwidth of about 1.5 MHz at -3 dB. This is a critical section of the system; if you do not have the proper test equipment (as is our case), you will have to adjust it on the satellite signal for a noise-free picture.

Phase Demodulator and Split-Phase-Low to Nonreturn-to-Zero Decoder

See Fig 3. This section performs the following functions:

a) It demodulates the 10.7-MHz RF signal (in IC1, a Plessey SL1451) and supplies a split-phase-low (Manchester) coded signal.

Table 1—HRPT Parameters

Transmit frequencies

Polarization Carrier modulation Lines per frame Line rate Number of words Word rate Number of bits Bit rate

Words per image Spectral channels

1698 MHz for the NOAAs transmitting APT on 137.5 MHz 1707 MHz for the NOAAs transmitting APT on 137.62 MHz Right hand circular Digital split phase, phase modulated 1

6 per second 11090 per line 66540 per second

10 per word; bit 1 = MSB (transmitted first) 665400 per second 2048 per line 5:

Channel 1 Channel 2 Channel 3 Channel 4 Channel 5

0.58 - 0.68 0.725 - 1.1 3.55 - 3.93 10.3 - 11.3 11.5 - 12.5

Digital Split Phase

Fig 1—Block diagram of the HRPT system.

Mmic Drive Uhf Amp
Fig 2—The 10.7-MHz amplifier and band-pass filter.

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Phase Sensitive Demodulation
dimSLr

Fig 3—The phase demodulator and split-phase-low to nonreturn-to-zero decoder.

b) It decodes the SPL data into nonreturn-to-zero (in IC3, a Harris HD-6409).

IC1 is a Plessey PLL FM detector-intended for satellite TV reception,1 Inside the chip is a transistor (pin 3 emitter, pin 4 base, pin 5 collector) that must be connected to an external coil (in our case, a small IF transformer) to act as an oscillator at 10.7 MHz. The HRPT signal from the filter of Fig 2 is fed into pin 11. The video output (pin 14) drives a dual tuning diode (Dl, a BB204) via isolating network Ll-Rl. It also drives the SL1451's loop feedback 1 input (pin 1) via the filter consisting of CI and R2. The gain of the SL1451's internal input RF amplifier is programmable by varying the voltage on pin 10; the gain is maximum with pin 10 connected to Vcc via a 330-kfl resistor.

The 665.4 kbit/s split-phase-low signal passes through a post-detection low-pass filter (R3-C2), which is designed to cut off above twice the bit rate (1330.8 kHz), and on to IC2, which outputs it at a 5 V level.

The decoding process is based on an HD-6409 Manchester encoderdecoder (Harris Corporation), a chip that converts Manchester code into NRZ code and provides clock recovery. The SPL data is fed into the decoder's Unipolar Data Input (UDI) pin. The HD-4609's SDO (serial data out) provides decoded serial NRZ data synchronous with the decoder clock (DCLK). The decoder requires an oscillator with a frequency 16x or 32x the bit rate. The HD-6409's speed selector (SS) pin (17) sets this, with SS low producing a 16x clock and SS high a 32x clock. We use the 32x mode, so the frequency of the free-running oscillator should therefore be 21292.8 kHz for clock synchronization with the incoming data (665.4 kHz x 32 = 21292.8 kHz). A frequency counter connected to the clock output (DCLK) via a l-k£2 resistor is very useful in showing when the internal clock is locked to the incoming data rate (665.4 kHz) (Doppler shift causes the clock to shift over 30 Hz between AOS and LOS.)

IC3's reset (RST) pin provides control of the decoder outputs. When RST is low, SDO and DCLK are forced low RST is connected to the error detector in order to resolve the ambiguity

1GEC Plessey Semiconductors, "Designing with the SL-1451 Phase Locked Loop," Consumer IC Handbook, Sep 1991, pp 8-23 to 8-25.

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Fig 4—The serial-to-parallel converter, frame sync and error detector. 6 QEX

problem. If frame sync is detected by IC10-12, the clock error generated is fed to rst so that the phase of the clock is changed by 180°.

IC3's rst input is normally kept high by IC12. If you test this circuit subsection without connecting it to the following section (Fig 4), temporarily tie rst high.

Serial-to-Parallel Converter, Frame Sync and Error Detector

See Fig 4. This section:

aj Performs the serial-to-parallel conversion.

b) Generates the frame sync (line start) at the last bit of the 60-bit sequence transmitted at the beginning of each line.

c) Generates the frame sync (error) at the last bit of the 60-bit sequence if it detects a phase error (that is, if the value of the bits is inverted).

d) Feeds the parallel data into the DMA computer interface (Fig 6) subdivided into bytes corresponding to the data words.

The serial-to-parallel conversion is performed by passing the serial data through a 24-bit shift register (IC4-6). Our system makes use of the eight most significant bits of the 10-bit word. The eight data lines are derived from the last six outputs of the first register (IC4, a 74164) and the first two of the second shift register (IC5, another 74164). The bus is fed into a D-type register (IC13, a 74374) that is clocked by the word strobe (a high at the end of each word). The eight parallel lines are connected to the DMA computer interface.

Only the last 24 bits of the 60-bit frame sync sequence transmitted at the beginning of each frame (line) are detected. When the 24th bit enters the first register (IC4), a low (frame sync flag) is generated at pin 19 of IC9 (a 74688); this is the line start flag.

This section also detects the inverted sequence in order to resolve the ambiguity problem. The effect of a phase error is that the data appears inverted (0 instead of 1 and vice versa). This situation is detected by checking the serial data for frame sync as well as frame sync and correcting the clock if frame sync is found. This signal, generated at pin 19 ofIC 12 (another 74688), is fed into the r.st input of the HD-6409 decoder (IC3).

Word Strobe Detector

See Fig 5. This section performs these functions:

Msb 74151 Pin Configuration

Fig 5—The word strobe detector.

a) It divides the flow of clock pulses into blocks of 10 bits that correspond to the bytes or words of the data, and provides a pulse (word strobe) at the last (tenth) bit of each word.

b) It divides the word strobe by 5 (in IC18, a 4017) so that you can choose one of the five channels transmitted by the satellite. IC18's five output lines carry the data of five multiplexed channels, each of which represents a view of the same area of the Earth as viewed by five bands of the light spectrum. A front-panel-mounted BCD switch selects the channel outputted by IC19, a 74151. (It's often practical to choose one channel and ignore the other four. This allows you to display a real-time image and avoids filling up your hard disk with data you won't use.)

c) It supplies the computer with the data of all the five channels if the divide by 5 (IC18) is bypassed. This option is implemented by the BCD switch. Storing five channels implies the creation of a 50-Mbyte file for a 15-minute pass, which is not always feasible. Moreover, after the pass it is necessary to explode the fi ve ch annels, an operation which requires another 50 Mbytes of hard-disk space.

d) It blocks the data not belonging to the images that are present on the line, ie, the first 750 bytes and the last 100 bytes of the frame.

e) It signals with D2, a front-panel-mounted LED, that the system recognizes the frame sync (line start); that is, the repetitive sequence situated at the beginning of each frame (line).

When the frame sync pulse (line start) is fed into the IC15A's CK input, Q goes high and therefore the clock pulses pass through IC16B and reach IC17, which divides the frequency by 10 thus generating the word strobe (66.54 kHz)—a pulse at the end of each 10-bit word. The word strobe:

1) Drives IC13 of Fig 4 to signal that the 8 bits present at its output are an image word.

2) Drives the DMA computer interface (via IC19 and IC16C) if you want

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05

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S

02

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A7

5

A6

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Fig 6—The DMA interface. 8 QEX

to acquire all 5 channels. If you want to receive only one channel, IC18 divides the word strobe by 5, generating the sample strobe—a pulse that occurs every 50 bits, at the end of all the words of one of the five multiplexed channels. The desired channel can be selected by means of the BCD switch.

3) Drives counters IC20-23, the outputs of which are connected to the inputs of both the start comparators (IC24-25), and the stop comparators (IC26-27). The start pulse (a low at pin

19 of 1C25) is generated at word 745 of the line (the first word of earth data is 751 minus 6 frame sync words); the stop pulse (a low at pin 19 of IC27) is generated at word 10984 (the last word of earth data is 10990 minus 6 frame sync words).

When the start signal is applied to IC15B's clock input, Q goes high, IC16C allows the strobe to pass, and LED D2 blinks 6 times per second, signaling that the system works. At the same time, Q goes low, thus enabling

Table 2—HRPT Line Format

Function

Frame sync

Telemetry, TIP, etc Earth data

Auxiliary sync 100

Telemetry, TIP, etc Earth data

Auxiliary sync 100

Number

Word

Word Code

of Words

Position

<& Meaning

MSB LSB

6

1

1010000100

2

0101101111

3

1101011100

4

0110011101

5

1000001111

6

0010010101

744

7 to 750

10240

751

Ch 1 - Sample 1

752

Ch 2 - Sample 1

753

Ch 3 - Sample 1

754

Ch 4 - Sample 1

755

Ch 5 - Sample 1

756

Ch 1 - Sample 2

757

Ch 2 - Sample 2

758

Ch 3 - Sample 2

759

Ch 4 - Sample 2

760

Ch 5 - Sample 2

761

Ch 1 - Sample 3

10985

Ch 5 - Sample 2047

10986

Ch 1 - Sample 2048

10987

Ch 2 - Sample 2048

10988

Ch 3 - Sample 2048

10989

Ch 4 - Sample 2048

10990

Ch 5 - Sample 2048

10991 to 11090

NOAA Satellite Status

NOAA 9 Downlink: APT 137.62 MHz; HRPT 1707 MHz

Passes: morning (descending), early evening (ascending) NOAA 10 Downlink: APT 137.5 MHz; HRPT 1698 MHz

Passes: early morning (descending), afternoon (ascending) NOAA 12 Downlink: APT 137.5 MHz; HRPT 1698 MHz

Passes: morning (descending), afternoon (ascending) NOAA 14 Downlink: APT 137.62 MHz; HRPT 1707 MHz

Passes: early afternoon (ascending), night (descending)

divide-by-5 IC18. IC18 is reset after the fifth count. The stop signal resets both flip-flops IC15A and B, and, via outputs Q and Q of IC15A, the counters and the dividers.

Frame sync and strobe are sent to the direct memory access (DMA) computer interface by means of separate shielded cables.

DMA Computer Interface

See Fig 6. This board, which installs in an expansion slot on the PC system board, performs these functions:

a) It activates the data input when requested by the receive software; then it resets the DMA request.

b) It recognizes the line start pulse generated by IC9.

c) It makes a direct memory access request for the PC whenever a data item is acquired.

The three buffers IC32, IC33 and IC36 isolate the PC bus from the rest of the board. IC32 feeds the data into the PC bus. IC35 acts as memory address decode logic (310le). IC34 is a software-driven switch.

When the software keeps output Q of IC34 (pin 2) high, the line-start signal is applied to IC30A which, in turn, activates IC29B. Under this condition, the strobe signal reaches IC30B, whose output Q (pin 5) generates a Dma REQuest (DREQ). Then the computer responds by activating the Dma ACKnowledge (DACK) line, which resets IC30B and enables IC32 to feed data (D0-D7) into the data bus.

Our computer is a 486DX, 33-MHz machine with 4 Mbyte of RAM. The hard disk is a 340-Mbytes Western Digital with a VESA controller; it has 15-ms access time. The video adapter card is a Cirrus 5428 with 1 Mbyte of memory. Slower machines may not be able to perform all the options of the program (eg, storing five channels, displaying and storing at the same time), but they should be able to do all the basic functions.

Software

We used Turbo Pascal as our programming tool. The program, produced by Giampaolo, IW4CSG, covers HRPT as well as APT/WEFAX and HR (digital) Meteosat, but this article does not deal with the hardware interfaces necessary for the latter. The pictures can be dispayed in various formats, stored on disk and processed (zoomed, enhanced and cleaned if single pixels y.

Photo 1—NOAA-IO, 27 January 1991, 17:50-18:05 UTC, Ch 4 IR. The sky over central Europe is overcast. Only the snow-capped Alps emerge from the fog. A huge vortex spins over the Mediterranean.
Radio Electronics August 1992
Photo 2—NOAA-11, 29 August 1992, 12:53-13:08 UTC, Ch 2 VIS. Two great rivers, the Dnieper and the Danube, flow into the Black Sea. The picture also shows the Marmara Sea between the Straits of Bosphorus and Dardanelles.

Photo 3—NOAA-12, 1 June 1994, 07:37-07:52 UTC, Ch 2 VIS. The picture shows southeastern England, the Strait of Dover, Holland, Belgium and northern France. London is barely visible because of a veil of mist; Paris is a large dark spot. Many more smaller spots mark other cities and towns.

Bering Strait
Photo 4—NOAA-11, 8 August 1994, 14:46-15:01 UTC, Ch 2 VIS. The Balkan Peninsula crossed by the Danube and the Aegean Sea between Greece and Turkey.

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Glossary

AOS—Acquisition Of Signal.

APT— Automatic Picture Transmission. Analog system for the transmission of environmental images used by the NOAA weather satellites. The APT signal derived from the AVHRR (see below) consists of a multiplexed output of two selected channels of this instrument. The APT signal modulates a 2400-Hz subcarrier, which is then frequency-modulated on a VHF RF carrier and transmitted to ground stations.

AVHRR—Advanced Very High Resolution Radiometer. The instrument, which scans the Earth's surface by means of a rotating mirror, is sensitive in five spectral regions. The scan of the AVHRR is converted to a digital format, which is then phase-modulated on an S-band carrier and transmitted to ground stations. Frame—Major frame corresponds to three successive lines during which the same TIP (TIROS Information Processor) data is transmitted. Minor frame means single line. In this article, the word frame is used with the meaning of scan line. The HRPT frame rate is 6 per second. In a frame there are 11090 10-bit words. Frame sync—A sequence of 60 bits (six words) at the beginning of each frame. In this article, frame sync means also the line start pulse generated at the 60th bit of the sequence.

HRPT— High Resolution Picture Transmission. Digital system for the transmission of environmental images generated by the AVHRR. The HRPT is provided in a split-phase format to the S-band transmitter. LOS— Loss Of Signal.

NRZ— Nonreturn-to-Zero. A code representing the binary values (logic 0 and logic 1) with a static level maintained throughout the data cell. Consequently, there is no transition between two successive bits of the same binary value. Sample strobe— Word strobe divided by 5; therefore a pulse at the end of all the words of one of the 5 multiplexed images.

Split-Phase-Low— Also known as Biphase-Low or Biphase Manchester code, represents data with a level transition in the middle of the data cells. The direction of the transition indicates the binary value of data. A logic 0 is defined as a low-to-high transition in the middle of the data cell, and a logic 1 as a high-to-low mid-bit transition. In Manchester, the serial data stream contains both the clock and the data, with the position of the mid-bit transition representing the clock and the direction of the transition representing data. Therefore, there is no phase variation between the clock and the data. Word strobe—A pulse generated at the last bit of each word.

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Hrpt Noaa Dish

Photo 7—Southern Italy and Sicily on the monitor of our weather satellite receiving station.

Hrpt Noaa Dish
Photo 8—The entire HRPT receiving/control setup.
Photo 9—The NOAA HRPT dish antenna.

or lines have gotten lost because of noise).

The software is free, is not copy-protected, and can be downloaded as METEO.ZIP from the ARRL BBS at 860594-0306 and meteo.zip from the QEX FTP site (ftp:// arrl.org/pub/qex/).

The Pictures

All the pictures have been received and printed by the authors. Our fax machine is a vintage Muirhead-Jarvis Picture Receiver, type D-356D, It produces 8 x 10-inch images on photographic paper. The software for printing the pictures was developedby Cludio Pagnani; this article does not describe the related hardware. no

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