Journal of Global Positioning Systems (2005)
Vol. 4, No. 1-2: 139-143
Using GPS to enhance digital radio telemetry
K J Parkinson
School of Surveying and Spatial Information Systems, University of New South Wales, Sydney NSW 2052 Australia
Tel: [phone] Email:
Received: 27 November 2004 / Accepted: 12 July 2005
Abstract. The precise time available from the atomic
clocks orbiting the earth in GPS satellites is used in many
systems where time synchronization is important. The
satellite clocks are monitored and adjusted by ground
based control telemetry to within one microsecond of
Universal Time. A number of commercial GPS receivers
have the ability to provide a time synchronised output,
typically one pulse per second, that is locked to this
precise time base. This easily accessible timing source is
often the justification for including a GPS receiver as an
integral component of a complex system. There are
additional benefits to be gained from integrating a GPS
receiver as an embedded component of a mobile radio
telemetry system, where GPS information can also be
used to enhance the overall performance. This paper
examines some research into combining some
transmission techniques with time synchronisation from
GPS receivers located in the mobile and in the base
equipment to improve a digital radio channel. Using this
combined approach, a reverse data channel can be
eliminated where a single direction data stream is the
predominant requirement.
Key words: 1PPS, FEC, SFH, TDMA, FDMA,
Synchronisation.
1 Introduction
Radio communicatio ns systems for digital telemetry have
undergone many enhancements over the past few
decades. The ability to reliably transmit high speed digital
data streams is a common requirement. As an example,
the IEEE 802.11 Radio Frequency (RF) Local Area
Network (LAN) has been widely used, not only in the
office environment, but for many applications because of
its relatively low cost. As with any RF data channel,
some real challenges arise when the remote stations
become mobile, and data rates are pushed higher. They
suffer from well known propagation d ifficulties includ ing
multi-path and deep signal fade (Lee, 1993), all of which
introduce data errors. While there are a number of
methods used to correct these errors on the fly, the impact
is often unacceptable transmission delays. Particularly
damaging to throughput are the methods of successively
reducing the data speed and transmission retries. For
many static applications such as PC networks, error
correction and data integrity are more important than
variable delays. The delays experienced during error
recovery must be tolerated by the user. For real time
systems, this situation is unacceptable because it
lengthens the transmission delay making the system
response time unpredictable.
This project investigated the application of GPS within a
digital telemetry system to bring advantages of
robustness, simplification and to help so lve some of these
problems. This allowed the combining of some
conventional methods in a system architecture that may
not otherwi se be c ons i dere d vi abl e .
2 802.11 RF LAN Te sts
2.1 Initial equipment setup
The aim of this first step was to build and verify the
equipment at each end of the radio link before
introducing new radio channel equipment using GPS.
Commercial 802.11b RF LAN equipment was used as the
initial “proof of concept” transport medium connected as
shown in Figure 1. 802.11 systems typically use Direct
Sequence Spread Spectrum (DSSS) radio system in the
licence-free Industrial Scientific and Medical (ISM)
2.4GHz band for the physical layer with a TCP/IP
interface to the host equipment.

140 Journal of Global Positioning Systems
During development, an opportunity was taken to
measure the real time performance of the RF LAN
equipment under mobile conditions for later comparison.
The accurate time information from a variable number of
SuperStar II GPS receivers was transmitted back to a
central computer. An embedded microprocessor in an
FPGA was used to format a 150 byte message packet and
to synchronise the transmission start time to the 1PPS
signal from the GPS receiver. The message transmission
was started from each remote station simultaneously at
100ms intervals. Because the transmission start time was
known, this was able to be compared with the arrival time
as measured at the central GPS receiver, in order to reveal
the transmission delay.
Figure 1. Test system data flow
2.2 Test results and analysis
Although the link was reliable under all Line Of Sight
(LOS) conditions, the transmission delay was found to be
quite unacceptable. The delay increased with the number
of remote stations added, even under ideal conditions, as
shown in Figure 2. The small drop in delay using 5
stations suggests that the data packet being sent was close
to the optimum size for the internal RF protocol block.
Delays of this type were expected because the
conventional approach when un-correctable errors are
detected in an RF LAN is to use an Automatic Repeat
reQuest (ARQ) technique. Naturally, this reduces
throughput as it takes extra time to re-send the data.
Different message lengths and repetition rates were not
tested but it is suspected that the data was broken into
smaller blocks to help Forward Error Correction (FEC)
and reduce the number of ARQ requests.
Changing the minimal tuning parameters for message
length and wait time produced almost no improvement.
This was not intended to be an exhaustive test but it did
show that the equipment is not ideal for real time
deterministic transmission between more than 3 to 5
remote mobile stations sharing one base station.
0
100
200
300
400
500
600
700
123456789101112131415
Stations
mS
Figure 2. 802.11 ave rage transmission delay
3 A GPS Synchronous solution
A new system was needed to deliver continuous bursts of
real time digital data over a variable LOS distance of up
to 5km from up to 30 remote mobile transmitters, at
100ms intervals. The remote transmitter needed to be
light-weight, battery-operated and small enough to be
man- portable. Con s istent transmission delay for real time
performance and data integrity were important.
The time base from the GPS network was employed as an
integral part of the system. The One Pulse Per Second
(1PPS), available from many GPS receivers (Mumford,
2003), was provided in this case by Novatel Superstar II
receivers as a synchronous time base for both transmit
and receive. Testing confirmed that the 1PPS signal
between two Superstar II receivers was on average no
more than 150ns apart.
This allowed the system to be fully synchronised at both
ends using a combination of Slow Frequency Hopping
(SFH), Time Division Multiple Access (TDMA) and
Frequency Division Multiple Access (FDMA) to provide
robust performance.
The ISM band transmitter output was set at a maximum
of 400mW which allowed a run time of approximately 2
hours at full power using a small 950mAH cell phone
size Lithium Ion battery. Receiver sensitivity was -
100dB.
The data speed selected was 288Kbps giving a bit time of
3.47µs. This was fast enough to do the job but low
enough to reduce excessive exposure to propagation-
induced data errors.
The modulation scheme used was Gaussian Minimum
Shift Keying (GMSK) for RF power amplifier efficiency
and lower battery drain (Eberspacher and Vogel, 1999).
An FPGA was used in both the transmitter and receiver to
perform signal processing, error recovery, frequency hop
and synthesiser control functions.
GPS
RCVR1
GPS
RCVRn
802.11b
TRANSCEIVER1
802.11b
TRANSCEIVERn
802.11b
TRANSCEIVER GPS RECEIVER
COMPUTER
FPGA1
FPGAn

Parkinson: Using GPS to enhance digital radio telemetry 141
Figure 3. Remote transmitter architecture
3.1 Frequency Hopping Spread Spectrum (FHSS)
A lack of useable spectrum in the ISM band means that
there is a risk of interference from other users. Slow
Frequency Hopping (SFH) using 75 channels, spaced at
5MHz intervals between 2.405GHz and 2.470GHz, was
chosen for this system. Although it is recognised that this
approach is vulnerable to partial-band interference, to
overcome this, the technique of Dual Frequency Diversity
(DFD) (Proakis and Saheli, 2000) was used. The same
information is transmitted on two successive frequency
hops within the 100ms epoch and the received data from
each hop is combined to improve interference rejection
and anti-jam capability
Because 1PPS and GPS time were used to synchronise
the system, the start time and duration of each hop is
known at each transmitter-receiver pair. This simplified
the design considerably because the usual adaptive timing
recovery circuits and tracking procedures were not
required.
3.2 Time Divi sion Multipl e A ccess (T DMA)
TDMA is a common technique used to increase capacity
in a communication channel. A successful radio example
of this is the Global System for Mobile communications
(GSM) mobile telephone network. In order to maintain
synchronisation of mobiles, the GSM base station
transmits signals on a dedicated channel (Eberspacher
and Vogel, 1999). The mobiles must use these signals to
synchronise both time and operating frequency.
When using TDMA in a GPS synchronous system, the
need for complicated time slot synchronising is
eliminated. Each time slot is determined relative to the
1PPS signal. In this case the time slots chosen were
8.5ms long which allowed 5 different transmissions of
2448 bits at 288Kbps in each half (50ms) of the 100ms
epoch, as shown in Figure 4.
3.3 Combining SFH, TDMA and FDMA
Using some custom-designed logic in an FPGA at each
end of the radio link it was possible to combine both
techniques described above with FDMA using the 1PPS
signal.
With a pre-allocated orthogonal frequency hopping plan
that was known to all transmitters and receivers, it was
possible to dedicate a transmitter-receiver pair to a given
channel in a given time slot. This effectively added
FDMA to the system. Furthermore, it was possible to
have a group of transmitter-receiver pairs operate in
parallel, knowing that the fr equency in use was exclusive
to each member of the group. By distributing the channel
occupancy of the receivers across the 100ms epoch, it
was possible to use less receivers than remote
transmitters. The receivers operated in every time slot
while the transmitters only op erated in two time slots per
epoch to achieve DFD.
In this case there were 6 parallel operating receivers using
5 time slots per half epoch (50ms) receiving data from 30
remote transmitters.
Figure 4. Time slots for one receiver
The maximised use of the spectrum and the radio
equipment in this way requires that the hopping table
entries are random to satisfy the channel occupancy time
and avoid adjacent channel interference. The hopping
table contained 300 entries which allowed sequential
channel use over a 30 second period before repeating the
sequence.
3.4 Error handling approaches
Error handling in transmission often employs a
combination of procedures to detect and correct errors
after receiving.
Because the data stream contained GPS measurements,
there was an opportunity to interpolate some missing
samples in downstream processing. Any samples that
were completely unrecoverable due to failure to correct

142 Journal of Global Positioning Systems
errors were marked as unusable. Based on this it was
decided to eliminate the ARQ function from the
architecture to reduce complexity and power
consumption.
This is somewhat unique to the application, as it removes
the need for a guaranteed delivery mechanism. In
addition to using DFD, this did mean that stronger
embedded data stream FEC measures were required to
recover errors where they could have been recovered by
ARQ.
A combination of approaches was used, as explained
below.
3.4.1 Forward Error Correction (FEC)
The FEC technique used relies on the transmission of
enough redundant data so that multiple bit errors can be
corrected. In this case convolutional encoding was
performed in the transmitter while the Viterbi algorithm
(Viterbi and Omura 1979) was used for decoding in the
receiver.
The encoder processed the message bits with k=1 and
v=2 doubling the number of bits to give 100% data
redundancy. The constraint length (K) was set to 9 to gain
increased robustness. This was built into the transmitter
FPGA using an additive shift register of K-1 stages to
encode the data using polynomials (1) for the first bit and
(2) for the second:
g0(x) = 1+x+x2+x3+x5+x7+x8 (1)
g1(x) = 1+x+x2+x3+x4+x8 (2)
As the K value increases to strengthen the error
correction capability, the downside is the increased
processing load in the Viterbi decoder. Although the
selection of K=9 was large, this posed no problem at the
receiver end because an FPGA is ideally suited to this
task.
3.4.2 Interleaving
Interleaving is used to reduce the susceptibility to fading
by spreading the data so that all adjacent bits are
separated. The level of protection against fade duration is
impossible to set for all likely conditions of the radio
channel because of the dynamically changing
environment. Signal fading characteristics have been
modelled (Lee, 1993) to show that fade rate increases as
speed increases while fade depth is inversely proportional
to speed, and can fluctuate over a large dynamic range
from 10 dB to 50 dB.
In this case the message was broken into four blocks of
612 bits giving 612/288000 = 2.125ms of fade duration
protection. The buffer was arranged as a rectangular
matrix so that data was written by columns and ex tracted
by rows, as shown in Figure 6. This is done inside a
RAM buffer in the FPGA before transmit, and
reconstructed using the reverse procedure in the receiver.
Figure 6. Transmit interleaver
3.4.3 Error checking
The ability to check for errors after receiving is essential
because not all errors can be corrected. In this case two
measures were used to confirm the reliability of the data.
The first was a 32 bit Cyclic Redundancy Check (CRC)
generated from the buffer in the transmitter FPGA and
appended to the message before transmission. This gave
the ability to detect all error bursts of 32 bits or less at the
receiver. Bursts greater than this were also detected but
with only slightly less reliability.
The second integrity check was to verify that the 64 bit
predicted GPS time value from the Superstar II receiver
Measurement Record 23 was actually incrementing in
100ms steps. Any variation of this in the data from the
telemetry receiver gave an indication of an error in the
data.
4. Performance
The transmission delay was as expected from a
synchronous system, and is s hown overlayed w ith the RF
LAN results in Figure 7. The transmission delay was
110ms, consistently measured regardless of the number
of transmitters operating.

Parkinson: Using GPS to enhance digital radio telemetry 143
0
100
200
300
400
500
600
700
123456789101112131415
Stations
mS
RF LAN
New System
Figure 7. Transmission delay comparison
Table 1 gives a comparison of Bit Error Rates (BER)
between different communication paths, including the
new system described here.
Table 1. BER comparison
Communi cat i on path Nominal BER
RF, No error correction 10-1 to 10-3
RF LAN 10-5
GSM 10-5 to 10-6
New telemetry system 10-7
5. Concluding remarks
GPS provides advantages when designed into a radio
telemetry system because of the ability to make use of
synchronisation. While this system has the benefit of
multiple receivers, effectively providing a dedicated
channel during each frequency hop, there is the added
option to use TDMA. The concept allows design
flexibility and scalability to meet a number of
requirements limited only by the selected frequencies and
the processing speed of the electronics.
One of the key system features is that when the number
of remote transmitters is scaled up, there is no penalty in
transmission delay. The limit may only be a regulatory
issue with channel occupancy and dwell time.
Acknowledgements:
The guidance of Prof. Chris Rizos, Dr. P. Wakeman and
Dr. Joel Barnes are gratefully acknowledged in
supporting this work.
References
Eberspacher J.; Vogel H J. (1999): GSM Switching, Services
and Protocols, J. Wiley & Sons, Chichister, England
Lee W C Y. (1993): Mobile Communication Design
Fundamentals, 2nd ed, J. Wiley & Sons, New York
Mumford P J. (2003): Relative timing characteristics of the
one pulse per second (1PPS) output pulse of three GPS
receivers, Satnav 2003, Melbourne, Australia
Proakis J G.; Salehi M. (2000): Contemporary communication
systems using MATLAB, Brookes/Cole, California, 416-
421
Viterbi A.; Omura J K. (1979): Principles of Digital
Communication and Coding, McGraw-Hill, New York