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Radar Altimeter

Summary

A radar altimeter, which is carried aboard satellites, collects elevation signals from land and ocean surfaces. These data are used to determine topographical features.

Table of Contents

1. Sensor/Instrument Overview
2. Sensor/Instrument Layout, Design, and Measurement Geometry
3. Manufacturer of Sensor/Instrument
4. Calibration
5. References
6. Document Information

1.Sensor/Instrument Overview

Sensor/Instrument Name

Radar altimeter

Sensor/Instrument Introduction

The radar altimeter is an instrument on a satellite that measures the distance from the satellite to the Earth's surface using a radar signal. The instrument transmits an electronic pulse in the microwave frequency to the Earth's surface. The microwave pulse reflects off the surface and returns to the sensor. Altitude is determined from the pulse travel time (from transmit to receive) and from the waveform of the returned pulse.

Sensor/Instrument Mission Objectives

The radar altimeter measures the altitude of the Earth's surface to determine ice sheet topography and small-scale ocean roughness.

Key Variables

Satellite orbit, tropospheric/ionospheric signals, and geoid model.

Scanning or Data Collection Concept/Principles of Operation

The radar altimeter sends electromagnetic pulses at a microwave frequency to a land or ocean surface, then detects the reflected signals. The waveform data are then transmitted to a ground receiving station.

2.Sensor/Instrument Layout, Design, and Measurement Geometry

Sensor Description

The two major subsystems of the radar altimeter are: A peak power (RF) section and a signal processor.

*GEOSAT radar altimeter

Transmitter:

Receiver:

Antenna:

Weight:

Envelope (in):

*The GEOSAT and SEASAT radar altimeters share mechanical, thermal, and electrical interface characteristics.

Radar Altimeter Characteristics for Various Satellites

Satellite Frequency (Ghz) Bandwidth Wavelength (m) Range Resolution (m) Pulse Compression Wave Height
ERS-1 13.50 400.00 0.02 0.10 8000.00
ERS-2 13.50 0.02 0.1 58000.00 .13 m
Geos-3 13.90 80.00 0.02 0.50 30.00 ±25% (4-10 m)
GEOSAT 13.50 320.00 0.02 0.10 30000.00 ±10% (1-20 m)
GEOSAT Follow-On 13.50 0.02 0.018 58000.00 .035 m
SEASAT 13.50 320.00 0.02 0.10 1000.00 ± 10% (1-20 m)
Skylab 13.90 100.00 0.02 1.00 13.00 1-2 m
TOPEX/Poseidon 5.3&13.6 320.00 .0566*amp;.02205 0.03 58000.00 .13 m

3.Manufacturer of Sensor/Instrument

This section is not applicable.

4.Calibration

Specifications

The GEOSAT radar altimeter contained an onboard calibration mode that was invoked twice daily to track waveform sample gain and attitude, wave height, automatic gain control, and height.

5.References

MacArthur, John L., Paul C. Marth, Jr., and Joseph G. Wall. 1987. The Geosat radar altimeter. Johns Hopkins APL Technical Digest 8(2): 176-81.

Moody, Thomas B. 1996. Radar altimetry, Internet home page, University of Texas at Austin, Texas. http://www.ae.utexas.edu/courses/ase389/sensors/alt/alt.html.

6.Document Information

Document Revision Date

February 3, 1998

Document Review Date

February 2, 1998

Document Curator

NSIDC Writers

Document URL

http://nsidc.org/data/docs/daac/altimeter_instrument.gd.html