Rain Fade Radiometry

Rain fade is the attenuation of microwave signals by precipitation — the same effect that kills satellite TV reception during thunderstorms. At Ku-band (10—12 GHz), raindrops are a significant fraction of a wavelength and scatter incoming RF energy. The heavier the rain, the more signal you lose.

By locking the dish onto a geostationary satellite (a constant, known signal source) and logging RSSI over hours or days, you turn the Carryout G2 into a calibrated atmospheric radiometer. Your measurements can be directly compared against the ITU rain attenuation models used by satellite link engineers worldwide.

What you’ll measure

Signal attenuation in dB caused by precipitation along the dish’s line of sight. Typical values at Ku-band:

Rain rate	Expected attenuation
Light rain (2 mm/hr)	1—3 dB
Moderate rain (10 mm/hr)	5—10 dB
Heavy rain (25 mm/hr)	10—20 dB
Downpour (50+ mm/hr)	20—40 dB

These numbers are path-integrated — they depend on your elevation angle (longer slant path through rain at low EL) and the vertical extent of the rain cell. A satellite at 40 degrees elevation has a slant path through a 3 km rain cell of about 4.7 km. At 20 degrees elevation, the same rain cell produces a slant path of about 8.8 km — nearly double the attenuation for the same rain rate on the ground.

Prerequisites

• Motors homed and calibrated (see Calibration & Homing)
• TV search disabled (see Disabling TV Search)
• LNA enabled (dvb then lnbdc odu)
• Known geostationary satellite position for your location — DISH Network 110W, 119W, or 129W are strong targets in the continental US
• A local weather data source: personal weather station, Weather Underground, or NOAA ASOS station for rainfall rate
• Serial logging to a file with timestamps
• Patience — you need actual rain, which is not on-demand

Geostationary look angles

A geostationary satellite sits at a fixed AZ/EL from your location. Use a satellite look angle calculator (e.g., dishpointer.com or Stellarium with satellites plugin) to find the AZ/EL for your specific latitude/longitude. For a station at 40N latitude, DISH 119W is roughly AZ 220, EL 40 — but calculate your own.

Locking onto the beacon

Before you can measure rain fade, you need a stable reference signal. A locked geostationary transponder gives you a steady RSSI baseline.

1. Position the dish at the satellite’s computed AZ/EL.

   TRK> mot
   MOT> a 0 220.5
   MOT> a 1 40.2

2. Enable the LNA and check for signal.

   MOT> q
   TRK> dvb
   DVB> lnbdc odu
   DVB> rssi 50
   Reads:50  RSSI[avg: 1247  cur: 1251]

   An RSSI well above 1000 indicates a strong signal. If you are near the noise floor (~500), the dish is not pointed at the satellite — adjust AZ/EL in 0.5 degree increments.

3. Verify carrier lock.

   DVB> qls

   The quick lock status should show Lock = 1 with a positive SNR value. If Lock = 0, you are either off-target or the transponder configuration does not match. Try selecting different transponders with t <n> and checking each.

4. Fine-tune pointing for maximum RSSI. Nudge AZ by +/- 0.5 degrees while watching RSSI. Then do the same for EL. Park at the peak.

   DVB> q
   TRK> mot
   MOT> a 0 220.3
   MOT> q
   TRK> dvb
   DVB> rssi 50
   Reads:50  RSSI[avg: 1285  cur: 1290]

5. Record your clear-sky baseline RSSI and the exact AZ/EL. This is your 0 dB reference point. All attenuation measurements are relative to this value.

Logging RSSI through a weather event

The dish stays locked at the satellite’s position. The atmosphere changes; the dish does not move.

1. Start periodic RSSI logging. Use a script or manual samples at regular intervals (every 1—5 minutes). For each sample:

   DVB> rssi 100
   Reads:100  RSSI[avg: 1283  cur: 1279]

   The 100-sample average smooths out short-term fluctuations. Record the timestamp and the avg value.

   Alternatively, use adc m for continuous streaming if you want sub-second resolution during a fast-moving storm cell.

2. Log weather data simultaneously. Record rainfall rate (mm/hr) from your weather station at the same timestamps. Temperature and humidity are useful secondary data.

3. Continue through the weather event. A passing thunderstorm might last 30—90 minutes. A frontal system can produce measurable attenuation for hours. Let the logger run.

4. Record the clear-sky recovery. After the rain stops, RSSI should return to within a few counts of your original baseline. If it does not, check for water on the dish surface (which also attenuates) or a temperature drift in the receiver.

Automating the data collection

For unattended logging, write a script that opens the serial port, sends dvb rssi 100 at a fixed interval, and parses the response. Birdcage’s rotctld server exposes RSSI over TCP (R 10 command on port 4533), which may be easier to script against than raw serial. See the Radio Telescope Guide for the rotctld RSSI extensions.

Converting RSSI to attenuation

RSSI values from the BCM4515 are raw ADC counts, not calibrated power in dBm. To get attenuation in dB, you work in relative terms:

attenuation_dB = 10 * log10(RSSI_clear / RSSI_rain)

Where RSSI_clear is your baseline and RSSI_rain is the value during precipitation. This assumes the ADC response is roughly linear in power over the range you are measuring, which is a reasonable approximation for the moderate dynamic range involved.

RSSI_clear	RSSI_rain	Attenuation
1280	1200	0.28 dB
1280	800	2.04 dB
1280	400	5.05 dB
1280	100	11.07 dB
1280	50	14.08 dB

If RSSI drops to the noise floor (~500 with LNA), you have lost lock and the attenuation exceeds your measurement range. Record it as ”> X dB” where X is computed from RSSI_clear / noise_floor.

Interpreting results

Plot your RSSI time series against rainfall rate. You should see:

• Clear inverse correlation. RSSI drops when rain rate increases, with heavier rain producing deeper fades. The correlation is strongest with instantaneous rainfall rate, not accumulated total — a brief downpour at 50 mm/hr produces a deeper (but shorter) fade than steady drizzle at 2 mm/hr that deposits more total water.
• Path length dependence. Lower elevation angles (longer slant path through the atmosphere) produce more attenuation for the same rain rate. If you can lock onto satellites at different elevations simultaneously (with a second dish or by switching targets), the ratio of attenuations reveals the rain cell’s vertical extent.
• Scintillation. Even on clear days, you may notice small RSSI fluctuations of 0.5—1 dB on timescales of seconds to minutes. This is tropospheric scintillation from turbulent mixing of air masses at different temperatures and humidity. It sets the noise floor for your attenuation measurements and is itself an interesting atmospheric parameter.
• Hysteresis during freezing precipitation. Wet snow and melting-layer effects can produce more attenuation than the equivalent liquid rain rate would suggest. The “bright band” at the melting layer (~2 km altitude in winter) is a known phenomenon in radar meteorology, and you may see it as excess fade that does not match surface rainfall.
• Dish wetting effect. Water films on the dish surface add attenuation even after rain stops. A wet dish surface can add 1—3 dB at Ku-band. You will see this as a slow recovery after the rain ends, with RSSI gradually returning to baseline as the dish dries.

Polarization effects

Raindrops are not spherical. Large drops are oblate (flattened by aerodynamic drag), which means horizontally polarized signals experience more attenuation than vertically polarized ones. The effect increases with rain rate.

Use peak rssits to measure both polarizations simultaneously:

PEAK> rssits
Even_sig = 823, Odd_sig = 891

Even_sig is H-pol (18V), Odd_sig is V-pol (13V). During rain, you should see H-pol fade more than V-pol. The ratio between them is called the differential attenuation and is directly related to raindrop size distribution.

ITU-R reference models

ITU-R Recommendation P.838 defines the specific attenuation coefficient (dB/km) as a function of rain rate and frequency. P.618 extends this to slant-path attenuation for earth-space links. Your measurements test these models directly. At 12 GHz with horizontal polarization, P.838 predicts roughly 0.5 dB/km at 10 mm/hr and 2.5 dB/km at 50 mm/hr.

Going further

Long-term dataset. Run the logger through an entire rain season. Accumulate hundreds of rain events and plot attenuation vs. rainfall rate on a log-log scale. Fit the ITU-R P.838 power-law model A = k * R^alpha to your data and compare your fitted coefficients against the published values for your frequency and polarization.

Dual-polarization differential attenuation. Systematically compare H-pol and V-pol fade during each event. The differential attenuation is a proxy for median raindrop diameter — larger drops are more oblate and produce more differential fade. This is the same principle used by dual-pol weather radars (ZDR measurement).

Cloud attenuation. On overcast but rain-free days, you may detect a small attenuation (0.1—0.5 dB) from liquid water content in clouds. This requires a very stable baseline and careful temperature-drift correction, but it is within the measurement range.

Multi-satellite comparison. Lock onto two or more geostationary satellites at different elevation angles and log RSSI simultaneously (or alternate between them every few minutes). Different slant paths through the same rain cell let you triangulate the rain cell height and horizontal extent.

Snow and ice detection. Dry snow produces very little attenuation at Ku-band (ice crystals are weak scatterers). But the melting layer — the altitude where snow transitions to rain — produces pronounced attenuation (the “bright band” effect). By logging RSSI during winter precipitation events where the surface temperature is near freezing, you can detect the onset and cessation of the melting layer as temperature changes. This has direct applications in weather radar calibration and hydrometeorological research.

Antenna Pattern Measurement Characterize your dish's beam pattern so you can distinguish rain fade from pointing error.

Radio Telescope Guide RSSI interpretation, LNA setup, and the azscanwxp workflow that underlies the DVB experiments.

Console Command Reference Full details on dvb rssi, adc m, peak rssits, and qls commands.