ASF Logo
North American Space Agency (NASA) Logo

Alaska Satellite Facility - Distributed Active Archive Center

Seasat – Glossary

Term Definition
Azimuth / Along-track The direction the satellite is moving. SAR can be referred to in (Range, Azimuth) coordinates using either time or distance (since time and distance are interchangeable in a SAR system).
Azimuth Reference Function A frequency-modulated chirp whose parameters depend on the velocity of the spacecraft, the pulse repetition frequency (PRF), and the absolute range. The chirp is Fourier transformed into Doppler space and multiplied by each column of range-migrated data in order to focus the data in azimuth, accounting for the phase shift of the target as it moves through the aperture.
BER Bit Error Rate.
Bit One binary digit.
Byte Eight binary digits.
Caltones Calibration tones.
Chirp A waveform created by sweeping the frequency from low to high.
Cleaned File Decoded signal file that has all currently addressed data errors corrected.
Datatake One pass of the satellite over a ground station. Since the satellite was constantly imaging and sending telemetry packets, a datatake will contain a contiguous swath of radar echoes from the ground.
Decode Decode (of Telemetry) is the process of unpacking telemetry packets into usable data. Generally, a telemetry packet will have three parts:
  1. Sync Code — Packets start with a synchronization code (sync code), this is a recognizable pattern that signals the start of a data packet. It is vital in determining the start of packets, and thus, how to interpret the rest of the information in a packet.
  2. Metadata and Status — Next follows some metadata, generally containing timing information and satellite status. These are typically packed into the smallest number of bits possible. Thus, they have to be interpreted and expanded in order to gain information about the data packet that was received.
  3. Payload — Finally, each telemetry packet will contain some amount of payload, i.e. actual data samples. Again, these will be packed into as few bits as possible and thus must be decoded into usable data sizes (typically, byte values). Additionally, it is common for a single payload to be so small as to not be able to contain an entire "line" of an image. Thus, in most cases, multiple payloads have to be combined to create a single imaged line.
Delay to digitization Parameterizes the time between when a satellite emits a pulse and when return echoes are recorded. Field in Seasat metadata.
ESA Standard Nodes Node locations defined by ESA to tag images at a given location. For node 0 to 1800 the center latitude is (node * 0.05) degrees, while for nodes 1801 to 3600 the center latitude is 180.0 – (node * 0.05) degrees.
Fill flag Fill Flag should be 1 when no valid SAR data is sent, 0 if payload is valid.
Fixed File Decoded signal file that has some number of data errors corrected.
Focusing The transformation of raw signal data into a spatial image. In its most abstract form, this is the process of performing a frequency domain correlation of the received signal with a 2-D system transfer function. In practice, this process is performed in several 1-D steps, including range compression, range migration, and azimuth compression.
Frame
  1. Creation of an imagery product of a specific size with a known geolocation, e.g. framing of data.
  2. Seasat telemetry packet ordering system, labeling minor frames in sequence 0...60, e.g. minor frame number.
Georeferenced An image in which some number of locations have map coordinates associated with them. This may be as little as the coordinates for the corners. Or, it could be as many as coordinates for the entire image.
Header Seasat metadata. Also, Seasat metadata files.
I&Q Complex samples of radar echoes. The in-phase and quadrature components of samples.
Metadata Data that describes data. For Seasat this includes timing and platform status information.
Minor Frame One telemetry packet. For Seasat, these are 1180 bits of data.
Nadir Point The point directly underneath the satellite on the earth’s surface. The satellite height above the earth plus the earth radius at the nadir point equals the magnitude of the satellite position vector.
Offset Video Real samples of radar echoes. The total bandwidth is in the "video" range (i.e. MHz), while the center of the bands are “offset” from 0 frequency.
PRF Pulse Repetition Frequency.
Range / Across-track The direction the satellite is looking. For SAR, this is nearly 90 degrees from the along-track direction. SAR can be referred to in (Range, Azimuth) coordinates using either time or distance (since time and distance are interchangeable in a SAR system).
Range Line The data samples from a single radar return collected in the across-track direction.
Range Migration The migration of range compressed pixels to compensate for the hyperbolic shaped reflection of a target as it moves through the synthetic aperture. The target will migrate in the azimuth direction as a linear trend plus a hyperbola. The shape of this migration path is calculated from the precise orbital information and then removed from each range line during the range migration portion of SAR correlation.
Range Reference Function The range reference function is a replica of the transmitted radar pulse that is used as a matched filter to be correlated with each row of raw SAR data.
SAR Correlator Software that performs SAR focusing. There are several different algorithms that can be implemented, but they all have the goal of transforming raw signal data into spatial imagery.
SAR Processing Algorithm ROI uses the Range Doppler algorithm which has three basic steps:
  1. Range Compression — convolution of the received radar echoes with the range reference function,
  2. Range migration — migration of range compressed pixels to compensate for the hyperbolic shaped reflection of a target as it moves through the synthetic aperture, and
  3. Azimuth compression — convolution of the compressed range migrated data with the azimuth reference function.
Sentinels Marker used to indicate the beginning or end of a particular block of information.
Side-band Frequency bands on either side of a signal carrier — A signal has a symmetric frequency spectrum: the positive frequency band is symmetric to the negative frequency band about 0 frequency. To create a radio frequency signal, a chirp is mixed with a pure sinusoid at the desired radar frequency (L-band for Seasat), which then shifts the entire spectrum up to be centered around the L-band frequency with a side-band of frequencies on one side of the carrier and another side-band on the other side of the carrier. These are all positive frequencies now, but there are two side-bands created from the positive and negative portions of the original chirp spectrum.
Slant Range to First Pixel The direct line of sight distance from the satellite to the first place imaged on the ground. This is calculated using two-way round-trip time of the radar pulse from the satellite to the ground.
SLC Single Look Complex.
Swath One pass of the satellite over a ground station. Since the satellite was constantly imaging and sending telemetry packets, a datatake will contain a contiguous swath of radar echoes from the ground.
SyncPrep SyncPrep 6.6.10 (SKY © 2013 Vexcel Corporation). Software used to byte-align raw data read from tapes.
Telemetry The highly automated communications process by which measurements are made and other data collected at remote or inaccessible points and transmitted to receiving equipment for monitoring. Telemetry is used by manned or unmanned spacecraft for data transmission. In practice, satellite telemetry data comes in discreet data packets. For Seasat, these are referred to as "minor frames," each of which comprises 1180 bits of data.
TLE Two Line Element giving the Keplerian elements necessary to calculate the orbital path of a satellite.