1. The signal that had to be answered

Carl Sagan’s Contact is about a message from somewhere else and the human machinery built to receive it. Most of the novel is not about the aliens; it is about antennas, protocols, committees, and the logistical problem of paying attention to a faint signal for long enough to understand it. The desktop’s communications are less dramatic, but the same truth applies: a spacecraft is only useful if someone on the ground can hear what it says and tell it what to do next.

Entry 265 opened the operations arc. This entry sizes the ground segment and decides how often the desktop must talk to Earth.

2. What the ground segment must do

The ground segment has four jobs:

  • Command uplink: send commands and schedules to the desktop.
  • Telemetry downlink: receive housekeeping data and event logs.
  • Payload data downlink: receive images, processed data, and files.
  • Tracking: measure range and range-rate to refine the orbit.

These jobs do not all need the same link. Commands and telemetry can use a low-rate link. Payload data may need a higher-rate link or a different provider.

For a LEO desktop at roughly 500 km, the practical options are:

  • UHF: simple, cheap, and well supported by amateur and small-sat ground stations. Data rates are low, typically a few kbps to a few tens of kbps. Good for commands and telemetry, marginal for images.
  • S-band: the small-sat workhorse. Data rates from hundreds of kbps to a few Mbps. Good for telemetry and moderate payload data.
  • X-band: higher rate, typically several Mbps to tens of Mbps. Good for high-volume payload data but more expensive in space and on the ground.
  • Commercial relay: use another satellite network as a relay, such as Starlink, Iridium, or a dedicated small-sat relay service. Provides near-continuous connectivity at a price.

For the desktop, the baseline is S-band for command, telemetry, and routine payload data, with UHF as a backup command link if S-band fails. Commercial relay is an option later if customers need faster turnaround.

4. How often contact is needed

The required contact cadence depends on autonomy. A highly autonomous spacecraft needs only occasional check-ins. A spacecraft with low autonomy needs frequent contact.

For the desktop’s target autonomy level — moderate — a reasonable cadence is:

  • Two to four S-band contacts per day for command upload, telemetry downlink, and orbit determination.
  • Additional contacts as needed for high-priority payload data or time-critical manoeuvres.
  • Continuous UHF receive capability is not required; UHF is a backup for emergency command.

Each contact lasts roughly 5–15 minutes, depending on the ground station elevation mask and the data volume. At 500 km, a single ground station can see the spacecraft for about 10 minutes per pass. Four stations roughly evenly spaced around the globe can give four contacts per orbit, or about 60 contacts per day. A small operation with two to three stations gets a few contacts per day, which is enough for moderate autonomy.

A link budget converts transmitter power, antenna gain, path loss, and receiver sensitivity into a maximum data rate. For the desktop:

Parameter S-band telemetry S-band payload data
Frequency 2.2 GHz 2.2 GHz
Spacecraft transmit power 2 W 2 W
Spacecraft antenna gain 2 dBi 6 dBi
Ground antenna gain 20 dBi 20 dBi
Path loss at 500 km ~158 dB ~158 dB
Margin 6 dB 6 dB
Supported data rate ~100 kbps ~500 kbps

These are approximate. A higher-gain spacecraft antenna or a larger ground antenna increases the rate. A more distant spacecraft or a lower elevation decreases it.

6. Data volume per contact

At 100 kbps for housekeeping and telemetry, a 10-minute pass gives about 6 MB of telemetry. That is plenty for a healthy spacecraft.

At 500 kbps for payload data, a 10-minute pass gives about 30 MB. If the imager produces 1 MB images, that is roughly 30 images per pass. For routine operations, this is adequate. For a large imaging campaign, either the link rate must increase or the data must be stored and downlinked over multiple passes.

7. Ground station ownership

There are three ownership models:

  • Own stations: build or lease a few antennas. Higher capital cost, lower marginal cost per pass. Good if the fleet grows.
  • Commercial network: pay per pass or per minute. Lower capital cost, higher marginal cost. Good for the first spacecraft.
  • Customer-provided: some customers have their own ground infrastructure. Useful for government or institutional payloads.

For the first desktop, a commercial ground network is the lowest-risk choice. It avoids capital expenditure and lets the operator learn the actual data demand before investing in antennas. As the fleet grows, owning a few strategic stations may become cheaper.

8. Tracking and orbit determination

Every ground contact provides ranging data. With GPS on board, the desktop already knows its orbit well, but ground-based ranging adds redundancy and helps refine manoeuvre execution. Orbit determination is important for collision avoidance, for predicting future contacts, and for pointing the ground antenna.

For the desktop, GPS is the primary orbit source. Ground ranging is secondary, used for calibration and anomaly investigation.

What this changes

  • The desktop’s baseline link is S-band for command, telemetry, and routine payload data.
  • UHF is a backup command path.
  • Two to four ground contacts per day are sufficient for moderate autonomy.
  • A 10-minute S-band pass can deliver roughly 30 MB of payload data at 500 kbps.
  • A commercial ground network is the lowest-risk choice for the first desktop.
  • GPS is the primary orbit source; ground ranging is secondary.
  • The next entry will define the onboard flight software and autonomy budget.