A Ka-band (35 GHz) altimeter would be much less affected by the ionosphere than one operating at Ku-band, and would have enhanced performance in terms of vertical resolution, time decorrelation of echoes, spatial resolution and range noise. With the design of an adapted tracker algorithm, near-continuous altimetric tracking above all kinds of surface could be performed, which is especially important when approaching or leaving coasts. The main drawback is that Ka-band electromagnetic waves are sensitive to rain. However, this does not prevent them from acquiring a fairly high percentage of measurements, except for strong rain rates.
Use of the Ka-band for an altimeter would provide:
- Low ionospheric attenuation
—-this can be considered as negligible, except for some exceptional ionospheric situations– and would therefore eliminate the need for a dual-frequency altimeter. The DORIS system can provide data for ionospheric corrections whenever there are significant perturbations.
- Higher pulse repetition frequency (4 kHz).
The decorrelation time of sea echoes at Ka-band is shorter than at Ku-band. This makes it possible to increase significantly the number of independent echoes per second compared with Ku-band altimeters.
- Larger bandwidth (up to 500 MHz).
The 500 MHz bandwidth that can be used at Ka-band provides a high vertical resolution (0.3 m) which is improved with respect to other altimeters (including Jason-1 and Envisat).
- Better description of sea surface roughness than at Ku-band
The eight-millimetre wavelength is better suited to describing the slopes of small facets on the sea surface (capillary waves, etc.) and enables more accurate measurement of the backscatter coefficient over calm or moderate seas, thus leading to a noise reduction of a factor of two compared to Poseidon, for wave heights greater than 1 m.
- ” Lower radar penetration of snow and ice (penetration of snowpack is less than one centimetre at Ka-band, compared with five metres at Ku-band) The altimetric observation and height restitution thus correspond to a thin subsurface layer. This should improve measurements of snowpack with respect to ice aging in the surface layers of the polar ice caps. Moreover, ice grain size would also be measurable. Combined with better spatial resolution, Ka-band would therefore allow closer monitoring of sea and continental ice.
The one major drawback of Ka-band is that attenuation due to water or water vapour in the troposphere is high. Rain cells –which are often dense and frequent in the Tropics– will remain a constraining factor, since the radar wave can be attenuated by 2 dB in heavy rain. Typically, if the rain rate is higher than 1.5 mm/h, the radar echoes will be unusable (whereas at Ku-band, echoes are hardly affected at rain rates less than 3 mm/h). However, impact studies carried out on the basis of seven years of TMR data from Topex/Poseidon show that rain rates of over 1.5 mm/h only occur globally 10 per cent of the time. A Ka-band altimeter would therefore still be able to acquire measurements 90% of the time. If the satellite is on a sun-synchronous orbit, rain frequency will also have to be factored in (it rains most often in the Tropics between 6:00 and 12:00 a.m. and 6:00 and 12:00 p.m.). Conversely, this 1.5-mm/h threshold will also be likely to lead to more accurate mapping of rain cells over the ocean –one of the major remaining unknown factors in the global water budget– and yield more reliable climatology data.
– Phalippou L., E. Caubet and E. Thouvenot, A Ka-band altimeter for future altimetry missions, IGARSS’2000, 2000.
– Rémy F., B. Legresy and P.Vincent, New scientific opportunities from Ka-band altimetry, IGARSS’99, Hamburg, Germany, 1999.
– Jacques Verron and The AltiKa Mission Group, AltiKa: a Ka-band altimetry system for operational altimetry during the GMES period, 15 years of progress in radar altimetry symposium, Venice, 2006.
– Verron J., P. Bahurel and P. Vincent, AltiKa: Etude de la circulation océanique mésoéchelle par altimétrie en bande Ka sur microsatellite, Research proposal to CNES, 2001.
– Vincent, P., N. Steunou, E. Caubet, L. Phalippou, L. Rey, E. Thouvenot and J. Verron: AltiKa: a Ka-band Altimetry Payload and System for Operational Altimetry during the GMES Period, Sensors, Special Issue: Satellite Altimetry: New Sensors and New Applications, 2006.