How to Calculate Satellite Ground Station Contact Probability
Contact planning underpins satellite mission success, especially for Earth observation constellations that rely on frequent downlinks. Rather than modelling every pass individually, mission teams often need a quick probability that at least one downlink will succeed each day given their pass plan and expected reliability. This walkthrough provides that probability using a simple binomial model adjusted for weather and maintenance.
Use the calculation alongside link-budget checks from the satellite downlink budget margin guide and laser interlink availability insights from the laser interlink availability analysis. Together they provide a coherent readiness view for ground and space segments.
Definition
Daily contact probability expresses the likelihood that at least one downlink succeeds within a day across all scheduled passes. It folds in baseline pass reliability—capturing pointing, modulation, coding, and interference margins—and derates that probability by weather or maintenance effects.
The metric is intentionally conservative: it assumes independent passes, so correlated failures such as ephemeris errors or systemic ground-station faults may warrant additional contingency planning.
Variables and units
Capture variables on a per-day basis. Use percentages for probabilities to align with operational reporting.
- N – Expected passes per day (count; fractional allowed when averaging).
- p – Baseline per-pass success probability (%).
- w – Weather downtime fraction (%).
- m – Maintenance downtime fraction (%).
- Pdaily – Probability of at least one successful contact in the day (%).
Per-pass success probabilities should already incorporate link-budget margins, ground-segment diversity, and modulation/coding schemes. If you rely on multiple ground stations, include their diversity gains in p rather than counting them as separate passes unless time slots truly overlap.
Formula
We derate the baseline per-pass success rate by multiplicative availability factors for weather and maintenance, then compute the complement of the probability that all passes fail.
Availability = (1 − w) × (1 − m)
peff = p × Availability
Pdaily = 100 × [1 − (1 − peff)N]
Because probabilities are capped at 100%, ensure peff stays between 0 and 1. When weather or maintenance downtime exceeds 50%, re-examine your pass plan or add ground-station diversity.
Computation steps
1. Forecast passes
Use orbit propagators and ground-station visibility tools to estimate daily pass counts. Average over several days if pass opportunities vary with beta angle or pointing constraints.
2. Establish baseline success
Derive per-pass success from historical contacts or link simulations. Include modulation and coding scheme margins, antenna pointing error, and interference assumptions. For optical links, substitute cloud-free line-of-sight probability.
3. Apply derates
Set weather and maintenance downtime percentages based on site climatology and maintenance schedules. If multiple sites operate independently, compute availability for each and combine via weighted averages.
4. Compute Pdaily
Plug values into the formula. Interpret low probabilities as cues to add passes, introduce diversity, or tighten pointing budgets.
5. Communicate risk
Translate the probability into mission assurance terms. For example, a 90% daily probability means roughly three failures per month. Combine this with latency requirements from the SAR tasking priority score methodology if downlink timeliness affects imaging requests.
Validation
Validate the model by replaying a month of contact logs: compute actual daily success rates and compare to model predictions. Investigate deviations—persistent underperformance may signal unmodelled pointing issues or scheduling conflicts.
Perform sensitivity testing by varying N and p by ±10%. Missions with narrow pass windows or high latency requirements should target probabilities above 95% by adding ground stations or improving link margins.
Limits
The independence assumption breaks if ephemeris errors, software bugs, or regulatory outages affect all passes simultaneously. Consider layering deterministic safeguards such as data recorder capacity checks and alternative relay paths. Optical links warrant separate cloud correlation models when sites share weather systems.
Also remember that achieving one contact may still miss latency objectives. If payload data must be delivered within specific timeframes, compute probability within that window rather than across the entire day.
Embed: Ground-station contact probability calculator
Use the embedded calculator to quantify daily downlink assurance and justify changes to pass planning or station maintenance windows.