An attempt was made to determine if future satellite altimeters, operating at K-band, should use a single transmitter or an array of pulsed amplifiers. The two systems were compared as a function of reliability, power, service life, system requirements, and weight. Preliminary results indicate the modular system to be superior in service life and reliability although it requires more power and is substantially heavier than the single system. No concrete conclusions were reached as to which system should be used

Greene, A. H.

Hudson, E. F.

English

NASA Technical Reports Server

iii
SATELLITE ALTIMETERS AFTER SKYLAB AND
GEOS-C --- SHOULD THEY UTILIZE A SINGLE
TRANSMITTER OR AN ARRAY OF PULSED AMPLIFIERS?
19
A_H. Greene and E_ F. Hudson
Raytheon Company
Equipment Divis ion
Wayland, Massachusetts 01778
Introduction
The first spacecraft radar altimeters to operate over the ocean
were those which flew on the Saturn I launch vehicles in 1964 and 1965
{reference 1). These radars, which were used to measure the altitude
of the vehicles during their ascent, operated at 1.6 GHz (L-Band) and
transmitted 5-kw pulses of 1-#sec duration. A brief summary of the
results is given in Table 1.
The first altimeter to fly in Earth orbit will be the one in the
Skylab S-193 Microwave Experiment in 1973. This radar will operate
at 13.9 GHz {K -Band) and transmit Z-kw pulses of 10-ns, 100-ns,
-U
and 130-ns compressed to 10-ns. The second altimeter planned for
Earth orbital operation is the one for the GEOS-C satellite scheduled
for late 1973 or early 1974. This altimeter will also operate at 13.9
GHz and will transmit pulses of about the same length and peak power
_s the Skylab altimeter.
ig-i
https://ntrs.nasa.gov/search.jsp?R=19730006661 2020-03-17T07:40:44+00:00Z
.Table I
Saturn Radar Altimeter F1i_hts
{Source: Reference I }
Flight Humber SA-4 SA-5
Date of Flight 3128/64 1129164
Peak Altitude to
which altimeter 6Z km Z64 km
was operated
Amount of Data 120 sec 157 sec
Quality of Data ÷20m bias Noisy
SA-6 SA-? SA-9
5/28/64 9118/64 2116165
195 km 210 km 499 km
6Z9 sec 59Z sec184 sec
÷90mbias ÷100mbias +100mbias
One of the things all these altimeters have in common is their
reliance on a single transmitter having a single microwave power tube.
The Saturn altimeter utilized a L-Band Triode. The Skylab altimeter
utilizes a K -Band Traveling Wave Tube (TWT). The GEOS-C altimeter
U
is expected to utilize a K -Band Magnetron, or a K -Band TWT, or
U U
possibly a combination of both in order to have two modes of operation,
a global mode at low power for extended periods and an intensive data
mode at high power for short periods.
The question addressed by this paper is: should the Earth ob-
servation satellite altimeters that come after Skylab and GEOS-C con-
tinue to use the single transmitter and microwave tube approach, or
should they be designed as an array of solid-state K -Band transmit/
u
receive modules ?
19-2
,j
If the choice had to be made today, it would have to be the single
.nsmitter designed around a TWT or Magnetron. Solid-state module
technology at K -Band for the output power levels required for these
U
radars is not yet available. However, the choice does" not have to be
! today. The design of the altimeters for the dedicated satellites
i
should await the return of the design data from the Skylab and GEOS-C
"ments and that data will not be available for another two to three
,ears. By that time, the state-of-the-art in K -Band solid-state
u
;chnology hopefully will have advanced to the point where the radar
ner will have a choice. •
System Requirements
The radar designer will be faced with the task of designing a
ed radar that, like its predecessors, will operate as a pulsewidth
imited system. For this condition, the signal-to-noise ratio per
S/N, is given by:
_,Z _rO c T Zy-tGZ_ w r
_J e
ili
S/N : 3 3 (I)
h N F L Sk T a (4w) (re÷h)
Table Z defines each term and gives the values used in subse-
uent calculations.
19-3
I
,<
J
4
4
J
'<
i
,7
Term
Y
P,
G
G°
r
e
k
i
T
a
Table 2
Pulse Limited Radar S/N Equation Terms
Value D_finition
36.4 dB
2.16 cm
+3.0 dB
Pulse compression ratio
Transmitted pulse peak power
Antenna gain. Assumes constant
illumination over a circular aperture
60-cm in diameter having a 3-dB
beamwidth of approximately 2° and
antenna efficiency of approximately O. 6.
,=
R-f wavelength equivalent to a
frequency of 13.9 GHz
Ocean backscatter coefficient at
vertical incidence for sea-state 5.
3x108 m/see Velocity of light
- Compressed pulsewidth
6370 km
1000 km
1.38x10"Z3joules/°K
290°K
I 5.5dB
4.1dB
Radius of the Earth
,, , , =
Height of the satellite above the ocean
Boltzman constant
Absolute reference temperature of
the receiver
Noise figure rc_erred to T a
Sum of all system losses including those
projected for the radar equipment and
attenuation due to propagation through
the troposphere.
19-4
4|
Accuracy studies have shown that rms altitude errors reduce as
T is narrowed and as S/N is increased, up to a point. T cannot use-
fully be narrowed below the wave height, and higher S/N ratios yield
little improvement if increased above 12 dB. Since the radar should
provide the desired accuracy of 10-cm over most ocean conditions,
a wave height corresponding to sea-state 5 is taken to limit the min-
imum value of T which sets T at 20-ns.
"For a three gate tracker, the 20-ns pulsewidth results in an
rms error of about 0. Z-cm and a bias uncertainty of about ÷ 5.5-cm.
The radar is assumed to have a prf of 1000 Hz, and the tracker time
constant is set at 0.5 seconds. Raising the prf will not be effective
because of pulse-to-pulse correlation which will occur, and the in-
tegration time must be kept low enough to provide sufficient resolu-
tion of surface topographical features, given a satellite horizontal
velocity in the neighborhood of 7 km/sec.
Substituting the S/N of 12 dB and the compressed pulsewidth of
20-ns into equation (1) gives a value for YPt equal to Z00 kw.
The radar designer may now consider what pulse compression
ratio to employ in order to reduce the pulse peak power requirements.
For the purposes of this paper, a compression ratio of 500:1 will be
used and then two systems will be examined, first a single transmitter
system utilizing a'TWT, and second a system based on an array of
solid-state T/R modules. Both systems will transmit 400-watt
10-/_sec long pulses compressed to 20-ns. The prf will be 1000 Hz.
19-5
:!i
i
Single Transmitter System
A block diagram for a single transmitter system using a TWT
power amplifier, a high v'oltage power supply and modulator, and
pulse compression is shown in Figure 1. Upon receipt of a trigger
from the logic and control circuits, the impulse generator will provide
a signal to the pulse compression network. The output of the network
will be switched, filtered, gated, frequency converted, and finally
amplified in a high powered TWT stage. The transmitted signal is
then fed through a duplexer to the antenna.
The received signals will be directed by the duplexer to a
tunnel diode amplifier, converted in frequency, amplified, and then
compressed in a pulse compression network and processed to produce
range, pulse shape, and AGC data.
The reliability and life of such a system will depend to a large
extent on the Traveling Wave Tube and its associated high voltage
power supply. The HVPS will operate at several kilovolts, and in the
space environment this can present problems with electrical break-
down, corona, arcing, and electromagnetic interference. With a
single transmitter in the system, it is estimated that system life will
be on the order of 10 3 hours of assured operation. With an extra
transmitter which can be switched in if the primary unit fails, the
system life expectancy would be almost doubled.
ii
I TDA !'
Ante . Duplexer
Pulse
Comprcssion
Network
&
Detector I Processor
Local
Oscillator
I
High
Voltage
Power
Supply
&
Modulator
SSB
Converter
Logic
& Commands & Clock Signals
Control 'I' ,
_er _ .STch M Cr,stal!
Arn_ti_ider [ J Gate ] I°scil,at°r[
Figure l Bl_ock Diagram of Single Transmitter System
19-7
Range
Pulse
Shape
AGC
It is estimated the system would require about 70 watts of dc
power, exclusive of the power required to reposition the antenna in
the event the radar senses it is more than 0.5 ° off the vertical. It
may also be desirable for the antenna to scan to either side every so
often so that the sea-state can be measured over a swath instead of just
along the ground track. System weight, including the second trans-
mitter, is estimated to be approximately 60 lbs.
Array of Solid-State T/R Modules
A block diagram of an altimeter that would use a modular array,
with a large number of solid-state low power amplifiers in place of the
high power TWT, is shown in Figure 2. Each of the transmit/receive
modules would contain several of the elements shown in Figure 1,
such as a power amplifier, duplexer or circulator, low noise ampli-
fier, and switching circuits. The output of theimpuise gcnerator
would be fed through the pulse compression network and the wide
pulse would be switched, filtered, gated, and frequency converted.
A feed system would distribute the signal to the T/R modules. The
received signals from the modules would be combined in a feed system
and the single output converted to the I-f, compressed and processed.
In order to estimate the number of T/R modules, it will be
assumed that each radiating element in the antenna is fed directly
by a module.
19-8
ITIR .Modules
_-cZP
Feed
Local
Oscillator
HConverter
I l C°mpressi°n
l-f _ Ne&orkAmplifier
I AGCI Detector
Logic& !Control
_p
Filter
&
Gated
Amplifier
Processor
Commands & Clock Signals
I PulseCompression
Network
t-- 3 J,
Gate
1
H-iGenerator
l
H-rOsciUator
Figure 2 - Block Diagram of Solid-State Module System
I
°
i9-9
Range
Pulse
Sl_ape
AGC
iIn an equilateral-triangular arrangement of radiating elements,
the maximum area, A , allowed per element if grating lobes are to
e
be avoided is:
2
A = 2X
where Om is the maximum scan angle away from boresight
(reference 2).
Therefore, the minimum number of elements is:
n=A
a
A
e
where A
a
is the antenna aperture area.
(3)
Since A = _D 2
a
4
z x ..(4)
A 2 ° beamwidth is desirable to minimize satellite pitch and roll
control requirements, and since X = 2. 16-cm, D is selected as 60-cm,
as indicated in Table 2. 0 is selected as 10 ° so it will be possible to
m
19-10
i',7
3
51
i
Z
scan the beam at times out to 10 ° off the vertical in order to measure
sea-state to either side of the satellite ground track. Substituting these
values for _, D, and _m in equation (4) gives n = 700 elements. Since
Pt is 400-watts, the output required of each module becomes approx-
imately 570-milliwatts per module.
An example of where the K -Band state-of-the-art is today is the
u
module recently built and tested by Sylvania under an Air Force contract
(reference 3). This module provides 100-milliwatts using a two-stage
avalanche diode as the power amplifier in the module.
The life of the solid-state T/R modules is expected to be on the
order of 105 hours per module, and the system life in this case should
be about 5 x 104 hours of assured operation. The high voltages of the
single transmitter system have been eliminated, and the highest voltages
will be well under 100 volts.
However, system power requirements will be significantly greater
' than those for the single transmitter system. It "is estimated that system
power, exclusive of beam steering, will be approximately 120 watts.
System Weight will also be higher. It is estimated the array system
will weigh approximately 100 lbs.
Summary and Conclusions
A summary of the reliability, power, and weight estimates for
the two system approaches is given in Table 3.
.°
19-11
If progress is made in avalanche diodes and microwave hybrid
integrated circuit fabrication techniques at Ku-Band, the use of the
modular array approach should be considered carefully. Although it
will require more power and be substantially heavier, its advantages
in life and reliability may more than outweigh these disadvantages.
When sufficient progress has been made, this question should be
rigorously examined in detail, rather than the cursory way ithas been
examined here, so that a sound decision may be made on the system
approach that should be adopted for the altimeters that come after
Skylab and GEOS- C.
Table 3
Summa ry
Single
Transmitter
System
Array
Solid-State
T/R Modules
_ystem Life (Estimatedl _ x 103 hours 5 x 104 hours
_ystem Power (Estimated: 70 watts 120 watts
_ystem Weight (Estimated: 60 Ibs. 100 lbs.
iReferences
1. Performance of Saturn Radar Altimeter M.
MSFC, NASA TM X-53277, I0 June 1965
Coleman, NASA
2. Array Antennas T. Cheston and J. Frank, APL, Chapter 11
in Radar Handbook M. Skolnik, NRL, McGraw-Hill 1970
e Integrated K-Band Transceiver Report No. AleAL-TR-71-26
prepared under Air Force Contract F33615-67-C-1896,
A. Solomon, Sylvania, January 1971
19-13


Satellite altimeters after Skylab and GEOS-C - should they utilize a single transmitter or an array of pulsed amplifiers?

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