Tutorial HRPT AHRPT

Tutorial: HRPT and AHRPT

What it is. HRPT/AHRPT is the high-resolution direct broadcast of polar-orbiting weather satellites. It carries the full AVHRR radiometer data, giving about a 1 km picture, far sharper than the 137 MHz APT/LRPT services. Active satellites are MetOp-B and MetOp-C (AHRPT on 1701.3 MHz) and the FY-3 series. NOAA-15, 18 and 19 are retired; their HRPT is gone from the air and the NOAA entries are for old recordings. Because the satellite moves fast, you need a dish that follows it.

What you need

• Radio. A fast radio: the mode asks for 6 Msps, the lowest rate that works is 4 Msps. HackRF, Airspy, SDRplay, PlutoSDR are named in the panel; an RTL-SDR is too slow. See Supported-radios.
• Antenna. A dish of about 1 m or more that tracks the satellite, with a low-noise amplifier and a filter at the feed. A rotator can drive the dish: see Radio-Hamlib-rigs for rig and rotator control. Antenna power via bias-tee or injector.
• Frequencies from the repository's frequency database. Metop AHRPT 1701.300 MHz (mode default); FengYun-3 AHRPT 1704.500 MHz (approximate; also X-band at 7.8 GHz, not covered); NOAA HRPT 1698.000 MHz (also 1702.5 and 1707.0 MHz, retired). Tuning range 1690 to 1710 MHz.

Before you start

• Plan: the pass planner (View > Passes) gives AOS, LOS and maximum elevation; high passes work best. Expect Doppler of up to +-40 kHz; the receiver follows it.
• Gain: moderate; the 1.7 GHz band has terrestrial signals, so a filter is needed.
• Tracking accuracy matters: the dish beam is narrow.

Step by step

1. Download TLEs and set your position in View > Passes.
2. Mode list, group Weather & science, entry Weather pictures, 1.7 GHz.
3. Choose the Variant: automatic tries MetOp, then FY-3, and the retired NOAA HRPT only now and then, until frames decode; or choose a variant by hand.
4. Start the receiver while MetOp-B or MetOp-C is overhead, and keep the dish on it.
5. Watch the picture grow.

What you should see

The AVHRR picture grows line by line. A Packets table counts frames. The signal is a QPSK carrier about 2.33 Msym/s wide (4.5 MHz channel).

Reading the screen

• Picture with views of colour or one of five channels. Colour: near infrared (2) as red, visible (1) as green and the inverted thermal channel 4 as blue. A channel alone is grey. Channel 3B (APID 104) is a separate stream.
• Save PNG saves the picture at full size into the hrpt folder. New picture starts again. Follow scrolls with the newest line.
• Packets table: counts per packet type. State, Frequency and Variant.
• FY-3 AHRPT: frames are counted only, not turned into pictures.

Options and settings

Variant (automatic or fixed), channel view, follow.

Troubleshooting

• No frames. Dish not tracking, or wrong variant.
• Frames then gaps. Pointing drift; improve tracking.
• Rate too low. Under 4 Msps nothing decodes.
• Overload. Add a 1.7 GHz filter.
• Only NOAA tried. The NOAA satellites are retired; choose MetOp.

Tips

Record the IQ of the pass (Auto record IQ) and decode it afterwards if you want to try different settings.

Legal and privacy notes

Direct broadcast services are open. Receive only. See Legal.

Under the hood

MetOp AHRPT: QPSK 2.33 Msym/s, K = 7 Viterbi, Reed-Solomon (255,223) x 4, CCSDS packets, AVHRR lines. Retired NOAA HRPT: BPSK, Manchester, 665.4 kbit/s.

Related

Mode-HRPT-AHRPT, Tutorial-Meteor-M-LRPT, Tutorial-GOES-HRIT, Tutorial-Weather-satellite-images, Radio-Hamlib-rigs