Geostationary Arc Census

The geostationary arc is a ring of satellites parked at 35,786 km altitude, orbiting at the same rate the Earth rotates. From any fixed location on the ground, these satellites appear motionless — arranged in a band across the southern sky (in the Northern Hemisphere) at a specific elevation that depends on your latitude. The Carryout G2 can sweep the full azimuth range, measure RSSI and DVB lock at each position, and produce a complete census of every Ku-band satellite visible from your site.

What you’ll measure

A full-arc census produces three data products:

• RSSI profile — a signal strength curve across 360 degrees of azimuth, showing peaks at each satellite’s orbital slot
• Lock detections — positions where the BCM4515 achieves DVB carrier lock, confirming active DVB-S/S2 transmission
• SNR readings — signal quality at each locked position, useful for distinguishing strong regional satellites from weak ones at the edge of your coverage

From the Americas, expect to detect 10-30 satellites depending on your longitude, local horizon obstructions, and atmospheric conditions. The DISH Network fleet (110W, 119W, 129W), DirecTV (99W, 101W, 103W), and Bell TV (82W, 91W) satellites produce the strongest signals in North America.

Prerequisites

• Motors homed and calibrated (see Calibration & Homing)
• TV search disabled (see Disabling TV Search)
• LNA enabled (dvb > lnbdc odu)
• Serial output logged to a file for post-processing (e.g., picocom --logfile census.log)

You also need the GEO arc elevation for your latitude. The arc passes through a range of elevations depending on the satellite’s longitude relative to yours, but the midpoint elevation (due south) is a good starting position:

Latitude	GEO arc elevation (due south)
25 N	~55 deg
30 N	~50 deg
35 N	~46 deg
40 N	~41 deg
45 N	~36 deg
50 N	~31 deg

The actual arc traces a shallow curve across the sky — highest due south, lower toward the east and west horizons. For a first census, a single EL value near the midpoint captures most satellites. A multi-elevation sweep (described below) catches the ones at extreme orbital slots.

Elevation limits

The Carryout G2’s firmware enforces an elevation range of 18-65 degrees. If the GEO arc at your latitude exceeds 65 degrees due south (below ~22 N latitude), you won’t be able to reach the highest part of the arc. This experiment works best between 25 N and 55 N latitude.

Procedure

1. Home the motors and set elevation.

   TRK> mot
   MOT> h 0
   MOT> h 1
   MOT> a 1 41

   Replace 41 with the GEO arc elevation for your latitude from the table above.

2. Enable the LNA.

   MOT> q
   TRK> dvb
   DVB> lnbdc odu
   DVB> q
   TRK> mot

3. Verify RSSI baseline. Point away from any known satellite and check the noise floor.

   MOT> q
   TRK> dvb
   DVB> rssi 10
   Reads:10  RSSI[avg: 498  cur: 502]
   DVB> q
   TRK> mot

   Readings near 489-502 with LNA active confirm the receiver is working. If you see ~233-238, the LNA is not powered.

4. Run the H-pol sweep. The boot default is 18V (H-pol). Sweep the full azimuth range at 1-degree resolution, cycling 32 transponders at each position.

   MOT> azscanwxp 0 360 100 32

   This takes approximately 20-40 minutes depending on dwell time per transponder. Each measurement outputs:

   Motor:0 Angle:<cdeg> RSSI:<adc> Lock:<0/1> SNR:<dB> Scan Delta:<step>

   Let it run to completion. Do not send any serial input during the sweep.

5. Run the V-pol sweep. Switch to 13V (V-pol) and repeat.

   MOT> q
   TRK> dvb
   DVB> lnbdc odu
   DVB> q
   TRK> mot
   MOT> azscanwxp 0 360 100 32

   Many satellites transmit on both polarizations using different transponders. Running both sweeps doubles your detection rate.

6. Optional: multi-elevation sweep. The GEO arc curves downward at the east and west edges. Run additional sweeps at elevations 3-5 degrees above and below your midpoint to catch satellites at extreme orbital slots.

   MOT> a 1 38
   MOT> azscanwxp 0 360 100 32
   MOT> a 1 44
   MOT> azscanwxp 0 360 100 32

Interpreting results

Parse the serial log to extract the Angle, RSSI, Lock, and SNR fields. Plot RSSI versus azimuth angle — satellites appear as distinct peaks rising above the ~500 noise floor.

What the peaks mean:

Signal characteristic	Interpretation
RSSI > 1000, Lock=1, SNR > 5 dB	Strong satellite with active DVB carriers
RSSI 600-1000, Lock=0	Weak satellite or non-DVB carrier (C-band spillover, beacon-only)
RSSI > 1000, Lock=1, multiple adjacent angles	Wide beam — dish is resolving the satellite’s angular extent (rare for GEO at Ku-band)
Narrow RSSI peak, no lock	Possible non-DVB Ku-band emitter or adjacent satellite sidelobe

Identifying satellites by position

To identify satellites, convert each peak’s azimuth to a true bearing (accounting for your dish’s North alignment) and compare against the known GEO orbital slot for that bearing from your location. Online tools like SatBeams or the UCS Satellite Database provide orbital longitudes for all registered GEO satellites.

Azimuth to orbital longitude

The relationship between azimuth bearing and GEO orbital longitude depends on your ground station’s latitude and longitude. For a quick check: due south corresponds to the orbital longitude equal to your own longitude. East of south maps to satellites east of your longitude, and west of south maps to satellites west. The exact mapping is non-linear — use a satellite look-angle calculator for precise conversion.

Post-processing the scan data

The azscanwxp output is line-oriented and straightforward to parse. Each line contains fixed-format fields separated by spaces. A minimal Python script to extract the data:

import re

pattern = re.compile(
    r"Motor:(\d+)\s+Angle:(-?\d+)\s+RSSI:(\d+)\s+"
    r"Lock:(\d)\s+SNR:(-?\d+)\s+Scan Delta:(\d+)"
)

with open("census.log") as f:
    for line in f:
        m = pattern.search(line)
        if m:
            az_cdeg = int(m.group(2))
            rssi = int(m.group(3))
            lock = int(m.group(4))
            snr = int(m.group(5))
            print(f"{az_cdeg/100:.1f},{rssi},{lock},{snr}")

Plot the resulting CSV as RSSI versus azimuth. Peaks above the noise floor are satellite detections. Lock=1 markers on the plot confirm DVB carrier presence. Satellites with high RSSI but no lock are transmitting non-DVB signals (telemetry beacons, C-band spillover into the Ku-band feed, or non-standard modulation).

Dual-polarization comparison

Overlay the H-pol and V-pol sweeps on the same plot. Satellites that appear in both are transmitting on both polarizations (most do). Satellites that appear in only one polarization are either single-pol or too weak on the other polarization to detect. The RSSI difference between H-pol and V-pol at the same azimuth reflects the satellite’s polarization plan and the dish’s cross-polarization isolation.

Going further

• Repeated surveys over weeks detect new satellite launches, orbital relocations, and end-of-life drift. A satellite that wasn’t there last month is either newly launched or repositioned from another orbital slot.
• Seasonal variation — atmospheric attenuation changes with weather. Run the census on clear days for the most consistent baseline, then compare against rainy-day runs to see which satellites drop below the detection threshold first (the weakest ones).
• Compare with published catalogs — the UCS Satellite Database lists all known GEO objects. Your census will find a subset (Ku-band DVB transmitters only), but any detection not in the database is worth investigating.
• Beam calibration — once you’ve identified a strong satellite, use it as a reference source for the antenna pattern measurement experiment.
• Feed the census into DVB SIGINT — satellites you detect here become targets for the DVB Signal Intelligence experiment, where you characterize each satellite’s transponder plan in detail.
• Resolution refinement — if two satellites are close together (2-3 degrees), decrease the scan resolution from 100 centidegrees (1 degree) to 25 or 50 centidegrees to resolve them. The Carryout G2’s beam width is approximately 3-4 degrees at Ku-band, so satellites closer than that will merge into a single peak.

Radio Telescope Mode Detailed guide to the azscanwxp command, output format, and sky mapping workflow.

Console Command Reference Full firmware command inventory for the DVB, MOT, and PEAK submenus.