Digital satellite communication systems require careful management of frequency stability. Historically, frequency stability has been accomplished by continuously powered, high cost, high performance reference oscillators. Today's low cost mobile satellite communication equipment must operate under wide ranging environmental conditions, stabilize quickly after application of power, and provide adequate performance margin to overcome RF link impairments unique to the land mobile environment. Methods for frequency stabilization in land mobile applications must meet these objectives without incurring excessive performance degradation. A frequency stabilization scheme utilizing the LORAN (Long Range Navigation) system is presented

Ernst, Gregory J.

Kee, Steven M.

Marquart, Robert C.

English

NASA Technical Reports Server

N92-24195
Frequency Stabilization for Mobile
Satellite Terminals via LORAN
Gregory J. Ernst, Steven M. Kee,
and Robert C. Marquart
Hughes Network Systems, Inc.
Gcrmantown, Maryland 20875
ABSTRACT
Digital satellite communication systems re-
quire careful management of frequency
stability. Historically, frequency stability has
been accomplished by continuously powered,
high cost, high performance reference oscil-
lators. Today's low cost mobile satellite com-
munication equipment must operate under wide
ranging environmental conditions, stabilize
quickly after application of power, and provide
adequate performance margin to overcome RF
link impairments unique to the land mobile en-
vironment. Methods for frequency stabilization
in land mobile applications must meet these ob-
jectives without incurring excessive perfor-
mance degradation. A frequency stabilization
scheme utilizing the Loran L(___ng range
_navigation) system is presented.
INTRODUCTION
Position determination and reporting are re-
quirements in many mobile satellite terminal ap-
plications. Presently, the Loran-C system is a
widely available and a low cost means of
providing position determination. Many mobile
satellite terminals include an integrated Loran-C
Receiver for this purpose. A high stability fre-
quency reference can be derived from the
received Loran-C signal. Application of this
technique to transmit-only mobile satellite ter-
minals is particularly useful.
As shown in Figure 1, the Loran-C Receiver
determines the frequency offset between a volt-
age controlled oscillator reference and the
received Loran-C signal. Periodically, the
mobile satellite terminal baseband processor
system interrogates the Loran-C Receiver to
retrieve this frequency offset. After data
processing, the baseband processor corrects the
frequency offset by tuning the voltage con-
trolled oscillator via a digital to analog con-
verter. This negative feedback control loop
removes the frequency offset effects due to both
temperature and time.
The implementation of this control loop in-
corporates algorithms to ensure reliable fre-
quency stabilization. The Loran-C Receiver
computes the frequency offset and derives a
received signal quality indicator. The baseband
processor ignores frequency offsets accom-
panied with a poor signal quality indication.
Otherwise, the frequency offset is processed to
establish an updated frequency correction. The
most recent frequency correction is always
stored in nonvolatile memory so that upon
power application, a stabilized frequency can be
obtained promptly. During periods of extended
Loran-C outages, the stability is maintained be-
cause the voltage controlled oscillator is tem-
perature compensated.
REQUIREMENTS
The mobile satellite terminal must provide
long-term and short-term frequency stability un-
der the following conditions:
• Extreme operating temperatures
• Rapid recovery after power cycling
• Impaired Loran-C reception
• Aging effects over time and tempera-
ture
International Mobile Satellite Conference, Ottawa, t990
A9
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• Automatic operation without user inter-
vention
In addition, the stabilized oscillator reference
provides the critical clock timing and phase
noise performance for the mobile satellite ter-
minal.
CONCEPTUAL MODEL
Figure 2 shows a conceptual diagram of the
mobile satellite terminal frequency stabilization
scheme using the Loran system as the reference
source. A quartz crystal voltage controlled os-
cillator (VCXO) is used as the frequency refer-
ence source within the mobile satellite terminal.
This VCXO has stability characteristics as
described in Figure 3. As shown in the Time
Domain Characteristics of the VCXO, the
quartz crystal oscillator by itself is sufficient to
provide the short-term stability. However, the
long-term frequency drift can become large
when uncompensated. Utilizing the Loran sys-
tem for frequency stabilization eliminates the
effects of long-term frequency drift.
The Loran-C Receiver utilizes the 100 kHz
Loran signal as the reference input, r(t), to the
frequency stabilization control loop. Embedded
within the position determination process is a
frequency comparison algorithm which cal-
culates the frequency offset between the VCXO
controlled output, c(t), and the reference input.
The frequency offset is represented by e(t).
The digital controller provides the overall
operational control for the frequency stabiliza-
tion process. The input to the digital controller
is the output of a sampled analog to digital con-
version process, e(kT). The sampling period T
is defined by the baseband processor interroga-
tion interval of the Loran-C Receiver. The
sampled frequency offset data is processed by
the digital controller to produce a frequency cor-
rection, m(kT). The digital to analog converter
develops an analog tuning voltage to the VCXO
based upon the frequency correction.
The VCXO is temperature compensated and
not ovenized; therefore, it can quickly stabilize
in frequency when power is applied. The fre-
quency characterization of the VCXO shown in
Figure 3 forms the basis to determine the con-
trol loop parameters. In addition, the VCXO
frequency tuning range must be sufficient to
tune to the desired center frequency for tem-
perature variations and design life of the mobile
satellite terminal. When the Loran system is not
available, the oscillator must be frequency com-
pensated for temperature variations so that
mobile satellite terminal operation is not dis-
rupted. The long-term aging effects can be
easily maintained to less than 0.1 x 10-6 with
only monthly Loran availability. Temperature
stability without Loran availability is deter-
mined by the design of the VCXO but can be
less than 0.1 x 10 -6 with a state-of-the-art im-
plementation.
IMPLEMENTATION
The implementation of the frequency
stabilization control loop has been developed
using a 16 MHz VCXO. The output of the 16
MHz VCXO provides the reference frequency
for critical timing including position determina-
tion and carrier generation. The VCXO tuning
voltage is the output of an 8-bit digital to analog
converter (DAC). The initial value for the DAC
is stored in nonvolatile memory and is applied
as the frequency correction upon application of
power and initialization of the mobile satellite
terminal. The DAC value is updated as required
to compensate for the frequency drift of the
VCXO.
The frequency offset from the Loran-C
Receiver is derived from the Loran system
timing as well as the average of all received
Loran signals. This averaging minimizes the ef-
fects of Doppler induced errors due to the
geographic separation of Loran system transmit-
ting stations. The frequency offset provides a
resolution of 62.4 x 10 -_ Hz per bit.
After initialization of the mobile satellite ter-
minal, the baseband processor waits for a
received signal quality indication from the
Loran-C Receiver. During periods of poor sig-
nal quality, the frequency correction is not up-
dated. For extended periods of poor signal
quality as a result of Loran-C impairments (e.g.,
interference, shadowing, areas of inadequate
coverage), the frequency stability of the mobile
satellite terminal is provided by the VCXO.
AIO
International Mobile Satellite Conference, Ottawa, t990
The frequency stabilization loop operates con-
tinuously once there is good received signal
quality. The algorithms within the baseband
processor correct the VCXO frequency at a rate
that is sufficient to track out the effects of tem-
perature and aging.
The baseband processor examines the fre-
quency offset, and increments or decrements the
frequency correction as required. The resolu-
tion of the frequency correction through the
DAC is nominally 0.05 x 10 -6 per bit. The fre-
quency correction is updated whenever the fre-
quency offset is equal to or greater than
0.1 x 10 -6. When the loop has been adjusted to
a frequency offset of less than 0.1 x 10 -6, the
baseband processor compares the current fre-
quency correction to the value stored in non-
volatile memory. The nonvolatile memory is
updated if the difference between these two
values is greater than 0.5 x 10-6.
TEST RESULTS
Figure 4 shows the measured response of the
frequency stabilization loop to a +1.00 x 10 -6
change of the VCXO frequency. Initially, the
VCXO frequency was +0.17 x 10 -6 from the
nominal 16.00 MHz. A step change to
+1.17 x 10 -6 from the nominal 16.00 MHz was
applied to the VCXO. The frequency offset
response attained 63% of its final value in 50
seconds. In 180 seconds, the frequency offset
had attained 95% of its final value. This test
was performed for -1.00 x 10 -6 change to the
VCXO frequency and the results were com-
parable to the positive change in VCXO fre-
quency. Both of these tests were repeated
through successive trials, temperature cycling,
and power cycling. Again, the results were con-
sistent for a 1.00 x 10 -6 change in VCXO fre-
quency.
Incremental changes of the VCXO frequency
were tested up to the + 3.00 x 10 -6 acquisition
limit of the Loran-C Receiver. The results
showed that for a +3.00 x 10 -6 step, 95% of the
frequency offset was attained in 44 minutes on
the average. A -3.00 x 10 -6 step resulted in an
average of 42 minutes to attain 95% of the fre-
quency offset.
Testing of the frequency stabilization control
loop also included parametrics, acquisition and
tracking, firmware integration, and error han-
dling. Parametrics and acqusition and tracking
tests were also conducted over temperature and
time.
Extensive field tests of the Hughes Network
Systems SkyRider TM Mobile Satellite Terminal
have demonstrated frequency stability perfor-
mance of better than 0.03 x 10 -6 over tempera-
ture and time with continuous Loran coverage.
CONCLUSION
An implementation scheme for obtaining a
stabilized frequency reference for land mobile
applications has been presented. A combination
of long-term frequency drift compensation via
Loran system and short-term frequency stability
characteristics of a crystal oscillator meets the
requirements for an accurate and stable mobile
satellite terminal frequency source. This fre-
quency stabilization technique is particularly ap-
plicable to transmit only or random access type
mobile satellite terminals.
International Mobile Satellite Conference, Ottawa, 1990
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_ LORAN-C
100 KI-Iz
NONVOLATP I::
MEMORY
TUNING
VOLTAGE
40O41
05/08/90
Fig. 1. Functional Diagram
_ J (PERIOD = T)
.IDIGITAL Im(kT)_n '] m(t)_I I
"ICONTROLLERf-'---_L_J'_ VCXO I
c(t)
4OO42
05/08/90
T = SAMPLING PERIOD
r(t) = REFERENCE INPUT
c(t) = CONTROLLED OUTPUT
e(t) = r(t) - c(t), ACTUATING SIGNAL
re(t) = MANIPULATED SIGNAL
Fig. 2. Conceptual Model
A12
International Mobile Satellite Conference, Ottawa, t990
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International Mobile Satellite Conference, Ottawa, 1990


Frequency stabilization for mobile satellite terminals via LORAN

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