Why you need FarEarth during in-orbit commissioning

Sep 8

3 min read

Once your satellite is in orbit, the commissioning phase starts. Although the satellite operator is usually responsible for commissioning, most people do not realize how valuable a platform like FarEarth is during this phase. This blog explains how FarEarth helps you commission your visible-near-infrared (VNIR) camera.

The purpose of satellite commissioning is to ensure that the satellite is operating normally. This means two things: ensure the satellite and its sensors are reporting the correct data, and the satellite is “flying” correctly.

Raw to Level 0

The first step is to decode the data from your satellite. We decode and format your raw data into a standard Level 0 format. This is the main input used for processing in FarEarth. See FarEarth’s levels here: Image products | Knowledge Hub

FarEarth creates a custom decoder for your specific satellite based on data documentation provided by the sensor manufacturers and bus integrator. It is important to understand that FarEarth not only uses pixel data but also the NavAtt and ephemeris from the bus to produce a scientifically traceable product.

Data validation

During the first downlinks, it may happen that there are corrupted and/or missing packets in the datasets. FarEarth analyses the datasets to ensure all required data is present and in good order. Any problems are flagged and reported in FarEarth. This information helps the satellite operator narrow down problems quickly to get your satellite operational as soon as possible. These problems might be due to radio frequency (RF) downlinking problems or there might be a problem with the satellite’s software and/or configuration.

Let’s look at the data received from your satellite which is included in the FarEarth Level 0 product. For a VNIR imager, at a very minimum, the data below needs to be present:

Pixel data (for all bands)
FarEarth will flag any dropped data
Timing information
Each pixel needs to be timestamped. In the case of a pushbroom camera, it may be that each scanline is recorded at its own time. In the case of a frame camera, it may be that the entire frame has the same timestamp.
Navigation and Attitude (NavAtt) data
All NavAtt measurements need to be timestamped
Ephemeris data, which contains the satellite’s position and velocity (usually provided as two 3D vectors)
Attitude data, which contains the satellite’s orientation (usually provided in quaternion form)
Other interesting measurements
General state-of-health of the satellite, such as battery voltages, current consumption during imaging, power generated by the solar panels, and so forth

Understanding your satellite

Once we are confident that the correct data is downlinked, no packets are missing and the values are validated, we can move on to the next level of processing.

Level 0 is just a clean, structured version of raw data. We still need to interpret what this data means, and for this we need to understand your satellite’s physical layout, sensor specification, and operating modes.

We ask questions like... where exactly is the attitude determination and control system (ADCS) located and how it is orientated on the satellite? What is the nominal flight path with respect to the ADCS? What is the orientation of the imager with respect to the bus?

Why is this important? It may be that the satellite’s ADCS and science instrument are not aligned. For example, the satellite might need to yaw by 90 degrees during imaging.

FarEarth produces geolocated accurate radiometric readings of the Earth. For this the platform takes the following into account for each pixel of every image it processes:

when it was recorded
what sensor configuration (TDI, binning, etc) was used
what the temperature of the sensor was
where the satellite was
how the sensor and satellite were orientated

Some of the imager information FarEarth uses during processing is:

the number of bands
imaging mode (pushbroom or pushframe)
band binning settings
band TDI settings
band integration times
band line-rates or frame-rates
band spectral responses
detector instantaneous-field-of-view (IFOV)
sensor swath width
detector ground-sampling-distance
expected band modulation-transfer-function (MTF)
expected band signal-to-noise ratios (SNR)
temperature sensors on the instrument and more

All this information is used to determine whether your satellite is operating nominally. Only after nominal operation is achieved can your satellite be considered commissioned.

A glimpse of a FarEarth report

Let’s look at some snippets from a real-life example. The image below shows the flight path of the satellite (red) and the view path of the imager (green). Imaging started at the small dot and ended at the large dot. The thumbnail overlay shows the raw recorded pixel values.

The image below shows the roll of the satellite before, during (red) and after imaging. The satellite was stable, but with a large roll of around 18 degrees.