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:
- Type -- traveling wave tube
- Peak power (RF) -- 20 Watts (minimum)
- Power consumption -- 70 Watts
Receiver:
- Type -- Dual conversion (500Mhz, 0h)
- Automatic gain control -- 0 to 63 decibels (1-dec steps)
Antenna:
- Type -- 1-m parabolic
- Gain -- >37.6 decibels
- Beamwidth -- 2.0 degrees
Weight:
- Signal processor -- 47 pounds
- RF Section -- 144 pounds
Envelope (in):
- RF section -- 41.25 (diameter) X 11.5 (height) = 15,369 cubic inches (-antenna)
- Antenna -- 41.25 (diameter) X 19.125 (height) = 25,559 cubic inches (incl. feed)
- Signal processor -- 20 (length) X 13.5 (width) X 10 (height) = 2700 cubic inches
*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.
For waveform sample gain correction, the onboard tracker operated on a set of 60 waveform samples in the power spectrum outputs of a digital filter bank. Effects, such as in-band ripple and band-edge rolloff of anti-aliasing low-pass filters in the altimeter receiver were removed by individual waveform sample gain correction factors.
The correction processes of the attitude determination (and related corrections) started with computation of a voltage proportional to attitude (VATT) based on the amplitude of the last eight waveform samples.
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
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