3.1. Instruments overview
  3.1.1. General
  3.1.2. AMI Image mode
  3.1.3. AMI Wave mode
  3.1.4. AMI Wind Mode
  3.1.5. Radar Altimeter
  3.1.6. Along Track Scanning Radiometer (ATSR)
  3.1.7. Precise Range and Range Rate Equipment (PRARE)
  3.1.8. Laser Retroreflector

3.2. Laser Retroreflector
  3.2.1. High Rate Data (LINK 1)
  3.2.2. Low Rate Data (LINK 2)
    3.2.2.1. Low Rate Data Contents and Formatting
    3.2.2.2. Low Rate Real-Time and Playback Data
Higher level

Data processed in the Greund Stations are generated by the following Instrumentation carried by the ERS-1 satellite:

• The C-Band Active Microwave Instrumentation (AMI) comprising side-looking radar systems with three in-flight selectable functional modes for high r‚solution imaging, and measurements relating to wave spectra and wind scatterometry over ocean surface.

• The Ku-Band Radar Altimeter (RA), comprising a nadir looking instrument for measurements over ocean and ice surface.

For completeness two more devices on board the satellite are mentioned for which no ground processing at the Ground Stations is required:

• The Along-Track Scanning Radiometer and Microwave Sounder (ATSR) ) is a passive instrument consisting.of an infrared radiometer (ATSR) and a microwave sounder (MWS). Both instruments can be operated at the same time. ATSR data only are extracted at the LR Data Transcription Facility located at Fucino and sent to Rutherford Appleton Laboratories (RAL) and to the PAF's for off-line processing.
• The Precision Range and Range Rate equipment (PRARE) and
• The Laser Retro Reflector.

The function of the AMI Image Mode is to produce -high-resolution radar-images of the Earth's surface. In this mode, the instrument operates as a Synthetic Aperture Radar (SAR).

A swath of approximately 100 km is illuminated to the side of the satellite (see Figure 3-1).

The high-resolution image requiring both high RF power and a high data rate precludes prolongd measurement and on-board storage. Image mode operation is therefore performed only during line-of- sight communication with an ERS-1 ground station.

3.1.3. AMI Wave mode

The function of the AMI Wave Mode is to measure the radar reflectivity of the sea surface influenced by ocean waves.

For wave mode, the input signals and the target area are reduced from that of the image mode in both the along-track and across-track directions, having the effect of reducing the output data rate to enable on-board storage of the wave data (see Figure 3-1)

The data acquired is first processed as in the SAR image mode, except that there is no Doppler Ambiguity Estimator. In a second step, the wave image is converted to a two-dimensional spectrum of the SAR image.

In the AMI Wind Mode (scatterometer) the sea surface is sequentially illuminated by RF pulses from different angles and the backscattered signal (sigma naught) is measured to determine the average sea surface radar reflectivity. From this, the wind characteristics are determined by using a model which relates wind speed and wind direction relative to the radar beam to sea surface radar reflectivity. Unambiguous wind measurements require that each ocean patch is illuminated under at least three different directions. This is achieved by using three spaceborne antennas, one looking sideward (midbeam antenna), one looking 45° forward (forebeam antenna), and one looking 45° aftward (aftbeam antenna) with respect to the spacecraft flight direction (Figure 3-2). All three antennas form a fan beam with a narrow azimuth pattern and a relatively wide elevation pattern in order to cover a wide swath (measurement area approximately 500 km) parallel to the subsatellite track (Figure 3-3).

Inside the swath a regular grid of points, called (measurement-) nodes, is defined, the spacing of which is 25 km across track and approximately 25 km along track.

Wind speed and direction can be calculated from either two or three beams. The number of beams used is indicated for each cell.

The conversion from sigma naught to wind speed and direction gives an ambiguous result. In a second step, an attempt is made to remove the ambiguity. If that is not possible, it is indicated in the output product and the most likely solution is given.

The RA of the ERS-1 is a nadir-looking active microwave instrument. Over ocean it is used to determine the significant wave height, the wind speed and the mesoscale topography. Over ice it is used to determine the ice surface topography and ice type. Processing of RA data over land will be experimentally attempted in off-line mode by the PAF's

The microwave measurements comprise the time delay between transmission and reception of a pulse, the slope of the leading edge of the return pulse, the amplitude of the return pulse, and the echo waveforms.

These measurements are used as follows:

• The altitude is determined from the measured delay time after correction of propagation delays caused by ionosphere and troposphere.
• The significant ocean wave height (SWH) is calculated from the slope of the leading edge of the return echo.
• The wind speed over sea surfaces is estimated from the power level of the backscatter signal.

The Along-Track Scanning Radiometer and Microwave Sounder (ATSR) is a passive instrument consisting of an advanced four-channel infrared radiometer and a two-channel nadir viewing microwave sounder. Unlike the other ERS-1 instruments, the ATSR is an experimental package resulting from an ESA Announcement of Opportunity for a scientific add-on payload package.

The raw ATSR data will be sent to the Rutherford Appleton Laboratory (RAL), UK, and to the PAF's for further processing.

The product from RAL will consist of sea surface temperature images and cloud top temperatures. The IR measurements will be corrected for atmoospheric influence using the microwave measurements and using the two-atmospheric-path method - the 45 forward view and the nadir view.

The Precise Range and Range Rate Equipment (PRARE), also an instrument from the Announcement of Opportunity, is a highly accurate instrument which will be used for orbit determination at decimeter level of accuracy as well as for various geodetic applications. In this two-way microwave ranging system, the on-board equipment performs the measurements in X-band with some additional fonctions in S-band for ionospheric error correcting purposes.

The ground stations are dedicated X-band regenerative transponders. They are small, mobile units of modest cost. Both units, the ground equipment and the spaceborne one, are in many parts identical. One major difference is the S-band unit, which is a transmitter in the spacecraft and a receiver in the ground equipment. The S-band down-link is needed for the determination of the ionospheric effects. The PRARE memorizes the global measurements

with ground stations and dumps the data with the S-band down-link to its principal ground station at DLR in Oberpfaffenhofen, where the further processing will be performed.

The Laser Retroreflector (LRR) permits accurate range measurements using laser ranging stations on the. ground. The data are acquired by these laser ground stations and sent to the German PAF in Munich for further processing.

The instrument data transmitted by the satellite in the X-Band down-link consists of two independent types: High Rate (HR) in Link 1 and Low Rate (LR) in Link 2. These are described below.

High Rate data consists of formatted AMI image mode SAR data plus Instrument Data Handling and Transmission (IDHT) General Header. The data is put into frames of 256 bytes each, of which the first 6 bytes are allocated to a frame header generated by the IDHT formatter. One complete SAR data set, corresponding to one pulse return (range line), is contained in a format of 29 frames.

The IDHT format generation rate is somewhat greater than the SAR data set generation rate, to avoid synchronisation problems and to accommodate the variation in SAR pulse generation rate. The IDHT will generate a format with Pseudorandom Noise (PRN)-fill data whenever a data set is not available from the AMI.

Low Rate Data is generated by five different sources:

• AMI operating in wave-mode
• AMI operating in wind-mode
• ATSR
• Radar Altimeter
• IDHT General Header

This data is transmitted in 412-byte transfer frames. The first 10 bytes of each frame are used for the frame header, and the last two for frame checksum, with the remaining 400 bytes available for LR data.

The LR data sources are multiplexed on a frame-by-frame basis by the IDHT, using a round-robin polling sequence. The IDHT transmission rate is intentionally greater than the net data generation rate of the LR data sources, so the IDHT will generate frames of zero-fill data when none of the LR sources has data available during a polling sequence.

Each of the LR data sources generates data organized as source packets, which contain packet header, auxiliary data, and measurement data. Source packets typically are larger than the transfer frame data field, so that scveral transfer framcs are generally required to transmit a source packet The size and organisation of the source packets are different for each of the data sources.

Two separate streams of LR data are transmitted to the Ground Station: real-time data and playback data.

• Real-time data is that which is acquired by the satellite instruments and transmitted directly to the ground station during the period when the satellite is within view of an ESA ground station.
• Playback data is that which is acquired during the majority of each orbit, when the satellite is not within view of an ESA ground station. This data is recorded on board the satellite in the downlink format and transmitted to the ESA ground station at a higher rate during the time in which the satellite is within view of the station (once per orbit).

Keywords: ESA European Space Agency - Agence spatiale europeenne, observation de la terre, earth observation, satellite remote sensing, teledetection, geophysique, altimetrie, radar, chimique atmospherique, geophysics, altimetry, radar, atmospheric chemistry