The Altika instrument is made of a mono-frequency Ka-band radar altimeter and a bi-frequency radiometer used to correct altimetry measurements from wet troposphere crossing effects.
Why use Ka-band?
As Ka-band is new for spatial altimetry applications, it was necessary to assess the problem of electromagnetic wave interaction with surrounding environment at this frequency band. Therefore, the following paragraphs present a summary of the studies conducted about respectively ionosphere and troposphere.
From this point of view, Ka band is highly privileged with respect to Ku and C bands, because of ionospheric effects proportionality to the inverse of the squared frequency. The following results have been shown:
- ionospheric attenuation: negligible
- Faraday rotation: negligible
- chirp distortion: negligible (B/f ~1.4 % versus 2.3% in Ku band and 6% in C band)
- distance bias: the typical time-frame will be 0.02 ns (i.e. 3 mm in distance), with maximum values (very rare) of 0.25 ns (i.e. 4 cm in distance).
To conclude, the ionosphere can be neglected most of the time, but external information (ionosphere maps deduced from other space observations) can be used in extreme cases.
Effects are globally more important than in Ku band. They consist in time and attenuation impacts.
The tropospheric delay can reach up to 15 to 20 ns roundtrip; this delay has to be corrected.
Two solutions are possible:
- the use of model data or other external information;
- a dedicated on-board wet troposphere measurement system.
This measurement system consists in a microwave radiometer. Considering the altimeter instrument quality and mission objectives, it is necessary (and sufficient) to implement a bi-frequency radiometer (24/37 GHz). The same choice had been done for previous altimetry missions like Geosat Follow-on, ERS-2, ENVISAT, etc.
The attenuation impacting the Ka-band altimeter is part of the instrument sizing through the link budget impact. Potential sources of attenuation are the air, clouds or fog (water particles), ice, as well as rain precipitations (or snow or hail).
The attenuation budget to take into account includes the following:
- Air: Ka band (at 35.75 GHz) lies within a water propagation window. Despite this, the total attenuation (related to the O2 + H2O content) depends highly on humidity, with values lying between 0.4 and 2.1 dB when humidity varies between 7.5 and 50 g/m³.
- Clouds: typical attenuations are between 0.05 and 0.2 dB for altocumulus and altostratus, between 0.2 and 0.4 dB for stratus, nimbostratus and stratocumulus, and between 0.2 and 0.7 dB for cumulus and cumulonimbus. Some higher attenuation values are possible (even reaching 2 to 8 dB) but are too exceptional to be taken into account in our case.
- Fog: thanks to a generally very small thickness, attenuation to be taken into account does not exceed 0.1 dB.
- Ice: as the specific attenuation in Ka band is about a few 10-3 dB/km/(g/m³), total attenuation is negligible.
- Hail: attenuation models in Ka band are complex (due to hailstone size and possible presence of a liquid surface layer). Fortunately, this phenomenon is exceptional enough to be neglected.
- Rain and snow: specific attenuations (in dB/km) depend on the fall rate. The total attenuation to be taken into account in rainy conditions does not exceed 1.8 dB (in 95% of cases), or 1.2 dB (in 90% of cases). There are three possibilities from AltiKa's standpoint:
- if it does not rain at all, data will be 'nominal';
- if the rainfall rate exceeds (typically) 1.5 mm/h, data will be missing or 'damaged';
- if the rainfall rate is weak, lower than this value (i.e. about 5% of time), the waveform will be affected and its inversion will be possible by using a specific dedicated algorithm, which will provide by the way some additional information on rain characteristics.
General budget: when summing-up (despite some correlations between terms) all previously detailed contributions, the attenuation to be taken into account for the instrument sizing has been fixed in a range from 2.5 to 3 dB. The main contributor is not always rainfall for considered availability levels, water vapour being a generally bigger contributor.
The Altika instrument has the main following characteristics:
|Altimeter band||35.75 GHz ± 250 MHz|
|Pulse bandwidth||500 MHz|
|Pulse duration||110 µs|
|Altimeter pulse repetition frequency||~3.8 kHz (adjustable along the orbit)|
|Echo averaging (altimeter)||~25 ms|
|Spectrum analyzer (altimeter)||128 points|
|Altimeter link budget||11 dB (sigma naught = 6.5 dB)|
|Antenna diameter||1000 mm|
|Focal length||700 mm|
|Radiometer band||23.8 GHz ± 200 MHz|
37 GHz ± 500 MHz
|Radiometric resolution||< 0.4 K|
|Radiometric accuracy||< 3 K|
|Radiometric averaging||200 ms|
|Data rate||38 kbits/s|
|Mass (altimeter+radiometer)||< 42 kg|
|Power consumption (altimeter+radiometer)||< 100 W|