How to calculate satellite contact times from an orbit
Congratulations! After months of analysis and system design, your team has concluded their satellite mission trade studies. They know the optimal orbital plane parameters, the number of satellites per plane, a selection of ground antenna providers with sites for contacting your satellite, and popular areas of interest (AOIs) that your mission will cover on behalf of prospective customers. Now your team needs to calculate satellite visibilities (also known as access times) and contact schedules between your spacecraft and ground targets around the globe every single day for duration of your mission.
What are satellite visibilities?
A satellite visibility is the period of time when a line of sight exists from a satellite to a ground target. This is also referred to as satellite access time. While a minimum requirement, this is just a piece of the puzzle to successfully connect to a satellite. Antenna restrictions, such as minimum pointing elevation in reference to the horizon, play a role in determining the actual ground contact times.
What are satellite ground contacts?
A ground contact is a period of time in which a terrestrial antenna can reliably communicate with a satellite. For Low Earth Orbit (LEO) missions, this usually lasts only 6 - 12 minutes at link speeds slower than 3G.
For most missions, ground contacts serve as the only moments mission operations will have communication with the satellite. During these ground contacts, spacecraft commands & schedules must be uploaded and satellite state-of-health data must be downloaded. Payload data (e.g. image data or infrared sensor data) will also need to be downloaded but the satellite design will dictate if you need separate ground contacts for payload and health data due to the antenna orientation on the spacecraft.
Reserve every ground contact you can get! Not so fast my friend. There are two issues with this thought – 1) commercial ground station providers charge per contact and 2) some satellites cannot perform ground contacts and operate their payload (i.e. camera) simultaneously. Effectively, scheduling ground contacts can be a balance between cost and a loss of revenue for your company.
So that settles it; never reserve a ground contact! No.
The operations team should answer the question, “What is the fewest number of ground contacts required to downlink all satellite state-of-health data and all payload data?” Drivers will include onboard data storage limitations, ground station locations, and service availability requirements.
There are several scenarios that might complicate your plan:
- A satellite enters an anomaly mode and cannot operate its payload. It now needs to be returned to service ASAP to support the customer and business.
- A customer wants an image in the next 2 hours and only certain satellites have access to the target. A satellite needs to be tasked prior to the collection opportunity and the image downloaded within the 2 hour timeframe.
- A ground antenna fails or your ground station provider bumps your contact for a higher priority customer.
- There are more satellites than ground stations. There must be a balance between state-of-health monitoring across all satellites.
A mission operations team must balance all of these variables. Managing nominal and anomaly operations requires a detailed understanding of all ground contact access times to get the most out of your fleet.
Why are satellite visibilities so important?
Your satellite needs to operate its payload to generate revenue:
- 📈 Gather imagery and thermal sensing data to monitor weather and forest fires, study agriculture emissions, or track national security interests.
- 🎯 Provide secure position, navigation and timing data for first responders, financial institutions, and aerial vehicles.
- 🛰 Relay data from one side of the planet to the other to provide telecomm and broadband services to the 4 billion disconnected users.
To operate effectively, we need to know the satellite’s planned flight path relative to the areas of interest (AOI) on the ground. Often, mission planning utilizes the satellite ground track (a satellite's position, past and future, projected onto the surface of the earth) to illustrate this.
NASA AQUA Satellite Ground Track as shown by Quindar
Here are a few real-world examples:
- NASA needs to determine when its Suomi NPP Satellite will be over Yosemite National Forest in California to get infrared measurements to calculate the spread of wildfires
- An imaging satellite needs to determine when its satellites are over Ukrainian airfields to help with battle damage assessment (BDA) in the Ukraine-Russia conflict
- Satellites providing global internet need to know when they will be over high demand areas to slew their fleet and increase the available bandwidth
For each of these scenarios, the mission operations team needs to understand the details of each satellite visibility to best coordinate real time ground events, spacecraft activities, and ground station contacts.
How to calculate satellite ground visibilities
For the sake of this blog, it’s assumed that you know how to calculate where a satellite is (orbit determination) and where it is going (propagation), but a future blog will help explain this analysis in more detail.
Satellite position is typically stored in the Earth Centered Inertial (ECI) reference frame. Ground locations are typically stored using latitude, longitude, and altitude - although using an Earth Centered Earth Fixed (ECEF) frame would also suffice. The graphic below shows the ECEF frame, which stays fixed with Earth's rotation, verse the ECI frame that is stationary relative to Earth's rotation.
ECI vs ECEF Frame
Converting all satellite ECI positional data to latitude, longitude, and altitude can build a ground track visualization. The next biggest factor is defining the field of view (FOV) for what constitutes a feasible visibility and thus a ground contact.
When calculating visibilities for a ground contact, the field of view of the satellite antenna is important. For example, a satellite may have an omni-antenna (signals equally in all directions) or a directional antenna that concentrates signals in a certain direction. The FOV affects the quantity and duration of each contact within a ground track.
Then there are additional masks that must be applied both on the ground and on the spacecraft. Certain ground stations have obstructions that limit communications at certain azimuth and elevation angles - such as mountains and buildings. Synthetic aperture radar (SAR) imagery can only occur between certain ground elevation angles. Whatever your mission may be, there will be constraints that need to be considered.
Below is an output of ground access times from NASA's AQUA and TERRA satellites against various ground targets around the world using Quindar’s Ground Access Calculator service. Our service allows for quick ground access calculations of all your ground targets, from each of your satellites, in seconds. Just plug in any valid Two Line Element (TLE) with a set of ground targets and our tool will calculate ground access times up to a user selected period of time. TLEs can also be pulled automatically from Space-Track.
Ground access times for AQUA and Terra from Quindar's web app
What can you do with this information?
- Analyze the contact quality with metrics like contact duration and range.
- Test new orbital planes against ground station locations to optimize access times.
- Select new ground antenna locations to increase access times for your constellation.
- Plan out your next collection opportunities for an imaging satellite.
- Coordinate tests with ground terminals for your service provider satellites, like Position, Navigation, and Timing (PNT) or communications.
- View overlapping access times with satellites and ground targets to maximize operations.
- Provide a mission plan of all ground contacts for TT&C and payload contacts over the next 7 days.
Everything in the ground access calculator is also available via API, allowing users to create custom analysis and feed downstream design and operations. No installation required. The ground access calculator also feeds into the Quindar ground contact reservation system, automatically maintaining the optimal schedule to increase payload availability and drive business.
- An optimized ground contact schedule will limit your costs and maximize your revenue.
- Your operations team will need to know all possible ground contact times in case of contingency operations.
- Mission planning strategies for an earth observation satellite are coupled very tightly with the satellite’s ground track.
- Calculating ground access times takes in several inputs including spacecraft orbit or position, ground target location, instrument field of views, and ground station field of views.
- The Quindar ground access calculator can analyze and reserve optimal contact times via web app and API.