Solar Radio Monitoring

The Sun is one of the strongest natural radio sources in the sky. At Ku-band frequencies (10—12 GHz), the quiet Sun produces a noise temperature of roughly 10,000 K, rising to 50,000 K or more during solar flares and coronal mass ejections. That is well above the ~300 K thermal noise of the atmosphere, and the Carryout G2’s BCM4515 tuner can detect it without any hardware modifications.

This is one of the oldest experiments in radio astronomy — Karl Jansky and Grote Reber both detected solar radio emission in the 1930s and 1940s. You are repeating their work with better equipment and a motorized mount.

What you’ll measure

An RSSI increase of approximately 50—200 ADC counts above the clear-sky noise floor when the dish beam passes through the Sun. The exact magnitude depends on solar activity, atmospheric conditions, and how well centered the Sun is in the beam.

The dish’s beam width at 12 GHz is roughly 2—3 degrees. The Sun’s angular diameter is about 0.5 degrees, so it fits comfortably within a single beam and acts as a point-like source for this experiment.

Why Ku-band solar detection works

The Sun’s radio emission at microwave frequencies comes from the chromosphere and corona, not the photosphere. The brightness temperature at 12 GHz is much higher than the ~5,800 K optical surface temperature because the corona is a hot, optically thin plasma. During active solar periods (large sunspot groups, flares), gyro-synchrotron emission from energetic electrons in coronal magnetic loops can raise the effective temperature by an order of magnitude at Ku-band.

Prerequisites

• Motors homed and calibrated (see Calibration & Homing)
• TV search disabled (see Disabling TV Search)
• LNA enabled (dvb then lnbdc odu)
• Solar AZ/EL for your location and time — use Stellarium, NOAA Solar Calculator, or any solar position calculator
• Serial logging to a file (e.g., tee or a terminal recorder)
• Clear sky preferred for first attempt (clouds attenuate Ku-band and may mask the solar signal)

Do not stare at the Sun through the dish

This experiment measures radio-frequency energy, not visible light. There is no optical hazard from the dish itself. However, if you are outdoors adjusting the feed or checking alignment, standard solar safety applies — the dish reflector can concentrate sunlight at the focal point. Do not place your hand, face, or any optic at the feed position while the dish is pointed near the Sun.

Method 1: Direct pointing

Point the dish at the Sun’s computed position and record RSSI. This is the simplest approach and gives you an immediate measurement.

Elevation constraint

The Carryout G2’s EL range is 18—65 degrees. The Sun is only detectable when its elevation falls within this range, which depends on your latitude and the time of year. At mid-latitudes (35—45N), the Sun exceeds 18 degrees EL for most of the day between March and October. In winter, the window is shorter. Check your solar calculator for the specific times when the Sun is between 18 and 65 degrees elevation.

1. Get the Sun’s current AZ/EL. Use your solar calculator. Example: AZ = 185.3, EL = 42.7.

2. Enter the motor submenu and position the dish.

   TRK> mot
   MOT> a 0 185.3
   MOT> a 1 42.7

   Wait for both moves to complete, then verify with a:

   MOT> a
   Angle[0] = 185.30
   Angle[1] = 42.70

3. Return to the DVB submenu and take an RSSI reading.

   MOT> q
   TRK> dvb
   DVB> rssi 50
   Reads:50  RSSI[avg: 612  cur: 608]

   Compare this to a clear-sky reading at the same elevation but offset 10—15 degrees in azimuth (away from the Sun). The difference is the solar contribution.

4. Record the off-Sun baseline.

   DVB> q
   TRK> mot
   MOT> a 0 200.0
   MOT> q
   TRK> dvb
   DVB> rssi 50
   Reads:50  RSSI[avg: 498  cur: 501]

   In this example, the Sun adds approximately 114 counts above the baseline.

Method 2: Drift scan

Hold the dish at a fixed elevation matching the Sun’s declination-projected altitude and let Earth’s rotation sweep the Sun through the beam. This produces a clean time-series with minimal mechanical variables.

1. Calculate the Sun’s peak elevation for the day. The Sun crosses your local meridian at solar noon. At that moment, its elevation is 90 - |latitude - declination|. The elevation changes slowly near transit — within about +/- 1 degree over 20 minutes.

2. Point the dish south (or north, in the southern hemisphere) at the solar noon elevation.

   TRK> mot
   MOT> a 0 180.0
   MOT> a 1 48.5

3. Start the streaming RSSI monitor about 30 minutes before solar noon.

   MOT> q
   TRK> adc
   ADC> m

   The adc m command streams RSSI values continuously, one per line with carriage-return overwrite. Log the serial output to a file. You will see a gradual RSSI rise as the Sun enters the beam, a peak near solar noon, and a symmetric decline as it exits.

4. Let it run for at least one hour centered on solar noon. The Sun moves at 15 degrees per hour in azimuth, so the full beam transit (2—3 degree beam width) takes roughly 8—12 minutes depending on elevation.

5. Stop the monitor by sending q or pressing any key.

6. Record an off-Sun baseline at the same elevation. After the Sun has passed through the beam, leave the dish pointing south and take another set of RSSI readings. The clear-sky values before and after transit should be nearly identical — any difference indicates atmospheric changes or receiver drift.

Drift scan bonus: beam width measurement

A drift scan across the Sun gives you a free measurement of the dish’s azimuth beam width at the Sun’s elevation. The RSSI curve’s full-width at half-maximum (FWHM), converted from time to degrees using the 15 deg/hour solar rate (adjusted for elevation: actual rate = 15 * cos(elevation) degrees/hour in the AZ plane), is the beam width projected onto the sky. See the Antenna Pattern Measurement experiment for a more rigorous approach.

Interpreting results

Observation	Meaning
RSSI on-Sun 50—200 counts above off-Sun baseline	Normal quiet-Sun detection
RSSI on-Sun > 300 counts above baseline	Elevated solar activity (large sunspot group or flare)
No detectable difference	Check LNA is enabled, verify solar position calculation, try again near solar noon when EL is highest
Asymmetric drift scan peak	Dish EL slightly off from Sun’s EL — re-center and repeat
Gradual baseline drift over hours	Receiver temperature drift — take off-Sun reference readings periodically

The absolute RSSI value depends on the transponder frequency, LNA gain, and atmospheric conditions. The relative difference (on-Sun minus off-Sun) is what matters. Consistent measurements across days track solar activity trends.

Sanity checks

If you see no solar signal at all, work through this checklist:

1. Verify LNA is enabled: dvb rssi 10 should read ~490—510 with LNA active, ~233—238 without it
2. Confirm your solar position calculation against a second source (Stellarium vs. web calculator)
3. Try the measurement at solar noon, when the Sun is at maximum elevation and atmospheric path length is shortest
4. Scan +/- 3 degrees in both AZ and EL around the computed position — your alignment or solar position may be off by a degree or two

Correlation with external data

The Dominion Radio Astrophysical Observatory (DRAO) publishes daily 10.7 cm (2.8 GHz) solar flux measurements — the standard index of solar radio activity. While your Ku-band measurements are at a different frequency, the two correlate during active solar periods.

• DRAO Solar Radio Flux (NRCan)
• NOAA Space Weather Prediction Center

Compare your daily peak RSSI deltas with the published 10.7 cm flux. Days with high solar flux (> 150 SFU) should produce visibly stronger Ku-band detections.

The 10.7 cm flux and Ku-band flux are not directly proportional — they originate from different layers of the solar atmosphere and respond differently to various types of solar activity. Gradual Rise and Fall (GRF) events from large sunspot groups show up at both frequencies. Impulsive flare bursts are often stronger at Ku-band, where gyro-synchrotron emission peaks, than at 10.7 cm.

Going further

Solar flare monitoring. Set up the drift scan as a continuous monitor running unattended. A solar flare produces a sudden RSSI spike — sometimes hundreds of counts in seconds — correlated with NOAA/GOES X-ray alerts. Log timestamped RSSI data and overlay it with GOES X-ray flux plots for a direct comparison. The GOES satellite X-ray data is freely available from NOAA SWPC in near-real-time. An M-class or X-class flare should produce a clear, time-correlated spike in your Ku-band RSSI. The radio burst typically precedes the X-ray peak by a few minutes (the Neupert effect).

Multi-polarization solar measurement. Use peak rssits instead of dvb rssi to measure both H-pol and V-pol simultaneously. Solar radio emission at Ku-band can be partially polarized during flares, particularly during the impulsive phase when non-thermal electrons spiral in coronal magnetic fields. Comparing Even_sig (H-pol, 18V) and Odd_sig (V-pol, 13V) during active periods may reveal polarization structure not visible in total-intensity measurements.

Elevation scanning across the solar disk. Move in fine EL steps (0.1 degree) across the Sun while recording RSSI at each position. Combined with AZ stepping, this produces a 2D brightness map of the Sun — a radio image showing active regions if the dish has sufficient angular resolution. At 2—3 degrees beam width versus the Sun’s 0.5 degree diameter, you will not resolve individual sunspots, but you can detect asymmetry in the brightness distribution during high-activity periods.

Seasonal baseline tracking. Record the on-Sun RSSI delta at the same time each day over weeks or months. The quiet-Sun Ku-band flux varies with the 11-year solar cycle. During solar maximum (next expected ~2025—2026), daily detections should be consistently stronger than during solar minimum.

Active region tracking. Large sunspot groups rotate with the Sun (period ~27 days). If a large active region produces a measurably higher RSSI, you may detect its return ~27 days later as it rotates back to face Earth. This requires consistent daily measurements over at least one solar rotation.

Antenna Pattern Measurement The beam width you measure in a solar drift scan can be validated against a full antenna pattern characterization.

Radio Telescope Guide The azscanwxp foundation for automated sky sweeps -- the same workflow extends to solar scanning at specific elevations.

Console Command Reference Full details on dvb rssi, adc m, peak rssits, and all other firmware commands used in this experiment.