GPS L1 Band Civilian Simulator
MOHANCHUR SARKAR, BHANU PANJWANI
Navigation Simulation Division
Indian Space Research Organization (ISRO)
Space Applications Centre, Ahmedabad, Gujarat
INDIA
Abstract:- This paper presents the design and development of the GPS L1 Band Civilian Simulator, developed at
Space Applications Centre (ISRO) for the realization of GPS Receivers and for the proliferation of GPS based
applications. This paper describes the need of a GPS Simulator along with its major capabilities and supported
features. The simulator generates the actual RF signals in the L1 Band for any receiver location and time. This paper
also presents the signal structure of GPS L1 Band Civilian Signal. The GNSS Navigation Simulators in general and
GPS Simulator in particular with RF signal generation capability is a highly expensive product, which may not be
economically viable for small industries and academic institutions. The development of this GPS Simulator is a
classic exhibition of how simulation and modelling can help realize systems, which can solve real world problems
and create scenarios, which ultimately leads to the success of critical missions in which a GPS receiver is used.
Different models have been extensively used in the realization of this simulator, starting from the modelling of the
satellite motion, satellite clock degradation, and effects of Ionosphere and troposphere on transmitted signals along
with high dynamic receiver motion under different scenarios. The development of this GPS Simulator will help
various Indian Academic Institutions and Industries to start their independent GPS Receiver development
activity, aid in numerous GPS based application development, testing and also assist researchers to work in the
Satellite based Navigation Technology.
Key-Words: - Satellite, Navigation, GPS, Simulator, Modulation, Elevation, Ionosphere, Receiver
Received: April 14, 2024. Revised: October 9, 2024. Accepted: November 8, 2024. Published: December 27, 2024.
1. Introduction
The Global Positioning System (GPS) remains the
most predominant satellite based navigation system,
the use of which is so ubiquitous and pervasive that its
limit is restricted to the evolutionary thinking of
mankind. Being the foremost satellite based
navigation system, with a global coverage, there
seems to be no degradation in its popularity even with
the advent of GNSS from other countries like Galileo
[21], Glonass [22], Beidou [23], [24] and other
regional constellations like NavIC [2], [4], [5] of India
and QZSS of Japan.
With the increasing use of GPS, the need for the
development of GPS Receivers is increasing and
companies are still coming up with interest in GPS
Receiver development. Along with the design and
development of GPS Receivers, there is an increasing
challenge in the qualification of GPS Receivers, and
to ascertain the performance of the receivers in
mission critical applications and even scenarios where
GPS Receiver usage is involved in safety of life. The
use of GPS in automatic driving has opened a new
horizon and challenge in maintaining the receiver
performance and response to the signal integrity under
various unforeseen scenarios. The use of GPS in
aviation has already been well established. All these
applications of GPS especially for use in high speed
trains, aviation, automobiles necessitates the
development of new innovative test cases to ascertain
the realistic situation [14].
For the design, development, testing and
qualification of GPS receivers under various
application scenarios, the need of a GPS Simulator
becomes inevitable. Navigation Simulators in general
and GPS Simulators in particular are systems, which
mimic the GPS constellation and generate the same
RF signals as will be received from a live GPS
constellation. Starting from the conceptualization to
the actual deployment in a live scenario, the GPS
Simulators are used to nurture the Receiver
development process, fine-tune the various internal
algorithms by providing the receivers with the
stimulus it is supposed to receive in reality. As
satellite based navigation technology progresses with
new applications constantly coming up in the market,
the need of GPS Simulators will go on increasing to
help realize different types of GPS Receivers needed
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to fulfill various applications dependent on satellite
based positioning techniques [14]. With the rising
demand for Navigation Simulators, the market size of
Simulators is expected to be 430 million USD by
2030 as per market survey [30].
This paper discusses the design and development of
GPS Simulator, which has been realized at Space
Applications Centre, Ahmedabad a branch of the
Indian Space Research Organization (ISRO). The
remainder of the paper is organized as follows.
Section 2 gives a brief overview of the GPS
Constellation and the system. Section 3 describes the
GPS Signal Structure. In Section 4 the need of GPS
Simulator is explained in detail. Section 5 describes
the GPS Simulator specification, architecture and the
features supported. Section 6 discusses all the test
cases and validation of the simulator performance.
Section 7 concludes the paper.
2 GPS Overview
The Global Positioning System (GPS) one of the
greatest gift to mankind by the United States, which
has been operational from 1993 with a huge number
of satellites being launched in phases called blocks,
each being an evolution of its predecessor. Starting
from Block I the present GPS satellites belong to IIR-
M, IIF, IIIA and future IIIF to be launched. With
seven satellites of Block IIR, seven of Block IIR-M,
twelve of Block II F and six from BLOCK IIIA,
thirty-two satellites are operational in GPS
constellation at present.
The satellites are organized into six equally spaced
orbital planes, with each plane having slot for 4
satellites. The 6 planes are having an inclination of 55
degrees and separated by a 60 degree right ascension
of the ascending node [19]. This 24 satellite
constellation is guaranteed, though the number of
operating satellites are more than 24 to ensure better
performance. In June 2011, the 3 slots have been
expanded to increase the number of satellites to 27.
The specialty of BLOCK IIR-M satellites are the
addition of 2nd civil signal on L2 (L2C) [28] along
with the new military M Code signal [26], which was
an evolution of the earlier P(Y) code signals [27].
BLOCK IIF satellites started the third civil signals in
L5 frequency specially for aviation use [28]. The
latest GPS III/IIIF satellites has introduced the fourth
civilian signal in the L1 Band (L1C) [10], [11], [12] as
part of the GPS modernization [28]. The life span of
the satellites have increased from 7.5 to 15 years for
the latest GPS III-F satellites [29].
3 GPS Signal Structure
The L1 Band GPS Signal uses a family of codes
called Gold Codes, which is a Pseudorandom Noise
(PRN) Code repeating itself every 1ms. The code is
generated using a Linear Feedback Shift Register,
having a length of 1023 bits transmitted at a chipping
rate of 1.023 Mcps. It basically involves the modulo 2
addition of two subsequences G1 and G2, where the
G2 sequence being delayed by pre negotiated number
of chips to generate different sets of C/A codes [17].
The Navigation Data of the GPS L1 signal is
organized as 5 subframes, each of duration 6 secs.
Each subframe is further subdivided into 10 words.
The collection of 5 subframes is termed as a page. The
contents of the subframe changes with the page
number. Each page of GPS Navigation Data is 1500
bits long and takes 30 secs to transmit as the Nav Data
is transmitted at 50 bps. Each subframe is 300 bits
long and takes 6 secs to be transmitted. Each subframe
is further subdivided into 10 words of 30 bits each.
Each subframe starts with a Telemetry Word followed
by a Handover word (HOW).
All the information about the GPS constellation is
covered in 25 pages. The general structure of the first
three subframes remains fixed across all the pages.
The structure of the remaining two subframes 4 and 5
varies with the page numbers. The contents of the
subframes used in the GPS Navigation Data are
described in the following sub-sections.
3.1. TLM Word
Each subframe carries the telemetry word, which has a
fixed pattern to signify the start of a subframe. The
value of the fixed pattern is 0x8b, which acts as the
start of the frame delimiter of the subframe, after
which there are 16 reserved bits, followed by the 6
bits of parity. Each word ends with 6 parity bits and
effectively contains 24 information bits. The TLM is
used by the receiver to ascertain the start of a
subframe and is used to ensure the receiver is locked
to the actual bit sequence and to avoid the possibility
of a false lock.
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3.2. Handover Word
Every subframe after the TLM word contains the
Handover Word (HOW). It is used to convey the
satellite time to the receiver. The 17 bits of HOW
often referred to as the Z-count contains the start time
of the next subframe, communicated in terms of the
time of week count (TOWC). The GPS TOWC gets
initialized to zero at the Saturday & Sunday Midnight
crossing. The value of HOW gets incremented every
subframe which is of 6 secs duration. Hence, the value
of HOW increments from 0 to 131071 corresponding
to 604800 seconds in a week.
In this context, it needs to be emphasized that bit 18 of
the HOW carries the ALERT flag, which when set to
1 indicates that the SV (Satellite Vehicle) URA (User
Range Accuracy) is even degraded than what is being
transmitted in Subframe1, and the use of the particular
SV for position calculation is at the discretion of the
receiver [20].
HOW word in bit 19 carries the (A-S) flag called the
Anti-Spoofing flag. When the bit becomes high it
indicates that anti spoofing mode is enabled in that
particular GPS satellite and the Y code is transmitted
in place of P Code [27]. The HOW word in bits 20, 21
and 22 also contains the subframe ID for the
transmitting subframe for which it is part of, and
varies from 1 to 5 depending on the subframe
transmitted.
The following 2 bits of HOW, bit 23 and 24 are
calculated by solving the parity calculation equations,
with zeros in bit 29 and 30, the details of which can be
found in the parity calculation algorithm [20]. In
HOW, the last 2 parity bits are always 0. If pi denotes
the six parity bits of any word, with i ranging from 0
to 5, then p4 = 0, p5 = 0.
If di with i ranging from 0 to 23 denote the source bits
of the HOW word, the data bit 23 and 24 of the HOW
word d22 and d23 are calculated as in equation (1) and
(2).
d23 = last_D30 ^ d0 ^ d2 ^ d4 ^ d5 ^ d6 ^ d8 ^ d9 ^ d13 ^
d14 ^ d15 ^ d16 ^ d17 ^ d20 ^ d21 ^ p4
(1)
d22 = last_D29 ^ d2 ^ d4 ^ d5 ^ d7 ^ d8 ^ d9 ^ d10 ^ d12 ^
d14 ^ d18 ^ d21 ^ p5 ^ d23
(2)
where, last_D30 and last_D29 are the data bits for the
previous word of the subframe.
3.3 Subframe 1
The subframe1 starts with the Telemetry word and the
HOW which is same for all the subframes. The main
information conveyed in Subframe1 of GPS L1 C/A
code is the week number. GPS satellites have a unique
way of conveying the satellite time to the receivers.
The GPS week started from 6 January 1980 at
00:00:00 hours. It starts with the value of zero and
gets incremented every week till 1023, after which it
is again reset to 0. This event, which happens after
1024 weeks or 19.69 years approximately is called a
“week rollover”. The value of 1023 comes as the
week number, which is represented in 10 bits.
When the receiver gets the week number it can
actually calculate the absolute week starting from 6th
Jan, 1980 after taking care of the rollover events. Then
the Z-count, which is obtained from the HOW as
explained earlier for every subframe is multiplied by
six to get the time in terms of TOWC. Thereafter, the
receiver counts the number of navigation bits passed
each of 20 ms duration and the number of chips within
the code, with each chip of 977.517 ns duration to
calculate the absolute time at which the signal is
transmitted from the satellite [17].
The subframe 1 carries information which are all
related to the satellite time, which includes the
satellite clock bias (af0), the rate of degradation of the
satellite clock (af1) or the first order variation of the
satellite clock and the second order degradation of the
satellite clock (af2) are transmitted. The other
parameters of Subframe 1 are the toc or the time of
clock, which signifies the time in terms of TOWC in
which the above mentioned clock parameters (af0, af1
and af2) are calculated by the ground segment and
uploaded to the satellite. The other information sent in
subframe1 is the Tgd or the total group delay [20].
The first thing done by the receiver while doing the
position determination is the time correction. After a
successful acquisition, tracking, demodulation of the
navigation data bits, the time is corrected [19] using
the following equation.
t = af0 + af1 * (t –toc) + af2 * (t – toc)2 + Tr
(3)
where, Tr is the relativistic correction [19].
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The other information in Subframe1 are the User
Range Accuracy (URA) index of 4 bits, a value which
indicates to the receiver the amount of confidence in
the transmitted satellite data. In case of degradation of
the satellite clock or accuracy of the parameters
transmitted, the URA index is increased so that the
receiver can take the decision of including that
satellite in position calculation. In addition to this, the
subframe1 carries the Satellite Health parameters in 6
bits.
3.4. Subframe 2 and 3
Subframe 2 and 3 contain the ephemeris parameters of
the transmitting satellite. The ephemeris parameters
are needed by the receiver to calculate the exact
position of the satellite in the ECEF (Earth Centered
Earth Fixed) reference frame [16] at every point of
time. The main orbital parameters include the
eccentricity of the orbit, semi major axis, the
inclination of the orbit with respect to the equatorial
plane. The longitude of the ascending node, which
signifies the longitude in the equatorial plane, which
the satellite orbit intersects as the satellite moves from
south to north [19] is also transmitted along with the
argument of perigee, which represents the orientation
of the orbital plane in space. The mean anomaly
depicts an angle which is used to calculate the true
anomaly or an angle with respect to the perigee point
where the satellite is at any point of time is also part
of Subframe 2.
In addition to this, other parameters include the toe or
the time in terms of TOWC in which the ephemeris
parameters are calculated. Along with these, there are
six perturbation parameters signifying the
perturbations the satellite orbit undergoes. Apart from
this information, the rate of the right ascension of
ascending node and the rate of change of orbital
inclination is also transmitted in these two subframes.
The navigation message or the parameters in the
subframe changes every two hours and is signified by
the issue of data ephemeris (IODE) and Issue of Data
Clock (IODC) parameters.
Almanac represents the orbital and clock parameters
of a satellite with reduced precision. It consists of all
the parameters generally carried as part of the
ephemeris parameters in Subframe 2 and 3 and clock
parameters in Subframe 1, but with a reduced
precision so as to accommodate more number of
parameters in the subframes. The perturbation
parameters are generally not part of Almanac.
3.5. Subframe 4
The contents of Subframe 4 and 5 changes with each
page. The Almanac of all the satellites in the
constellation are transmitted by each satellite.
Subframe 4 contains the almanac parameters for
Satellite number 25 to 32. Along with this, the
parameters for ionospheric correction (8 coefficients)
using the Klobuchar Model [15] are also transmitted
in Page 18 of Subframe 4.
The GPS constellation also serves as a universal
source of time used widely for time synchronization
applications. As the GPS signals are available all the
time globally, these signals can be used to derive time
and are essential for geographically distributed
terminals or systems like MF-TDMA (Multi
Frequency Time Division Multiple Access) based
VSAT terminals and other applications in general
which require time synchronization.
The globally accepted and widely used time standard
is the UTC (Universal Time Coordinated), which is
adjusted with the introduction of leap seconds to keep
pace with the slowing down of earth rotation with
time as and when needed. As the GPS system time
does not use any concept of leap seconds, the GPS
time is ahead of UTC. Presently, the GPS time is
ahead of UTC by 18 seconds. This information is
conveyed to the receiver in page 18 of Subframe 4.
3.6 Subframe 5
Subframe 5 contains the almanac parameters for
satellite 1 to 24 in each page from page 1 to page 24.
In page 25, the Subfarme 5 contains the time of
almanac, week number and the satellite health
parameters in 6 bits. The MSB of the 6 bits is a
summary of the satellite health with a zero indicating
all OK and non-zero values indicates issues in the
satellite health.
In this context, it needs to be understood, the need for
a satellite to transmit the almanac parameters for all
other satellites in the GPS constellation. Once one
satellite channel is locked by the receiver, the almanac
parameters can be used to find the other satellites
visible to the receiver. Once the visible satellites are
known, the receiver acquisition engine can search for
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only those satellites, thereby reducing the search time
instead for searching for all satellites.
Moreover, in the acquisition process of a GPS
satellite, the receiver needs to know the code offset
and the doppler frequency of the satellite concerned so
that the appropriate frequency bin can be directly
searched. From the almanac parameters, the position
of all the satellites along with the approximate
Doppler frequency of the satellite can be calculated
which aids the acquisition process [18].
With a combination of 25 pages, which takes 12.5
minutes for the satellite to transmit, the receiver is
made aware of the complete prevailing state of the
GPS constellation by the navigation data transmitted
by the GPS satellites. Every subframe of GPS has 10
words of 30 bits each. The last 6 bits of each word is
the parity bits. The 24 bits of each prevailing word
and the last 29th and 30th bit of the previous word are
used to calculate the 6 bit parity the details of which
are available in [20].
4 Need of GPS Simulator
The GPS Constellation is operational for years,
however still for the GPS Receiver development cycle
the GPS Simulator is an indispensable system. The
GPS signals are already available from the open sky,
but still the live sky signal is not the best choice for
GPS Receiver development. The operational check of
the GPS Receivers can certainly be done using the live
sky signals, but the design, development and
qualification of receiver algorithms need a simulator
mainly because of the reasons described in the
following sub-sections.
As we know, the main error sources in GPS receiver
position computation are the errors due to satellite
ephemeris and satellite clock performance prediction.
Errors are also introduced due to inaccurate estimation
of the Ionospheric & Tropospheric condition as the
signal passes through these layers of the Atmosphere.
The ionospheric error remains the most significant
source of error as it is very difficult to predict the
ionospheric Total Electron Content (TEC) density
[17], especially in equatorial anomaly [6] region. The
multipath phenomenon in dense urban environment
also contributes to the GPS error budget along with
errors arising from receiver noise [17].
The live GPS constellation signal has all the errors
discussed before in a combined form. During the GPS
receiver development cycle, the effects of different
error sources like the satellite ephemeris errors,
satellite clock errors, ionospheric errors, tropospheric
errors, multipath errors on the receiver performance
need to be separately addressed. If a live GPS signal is
used, is becomes very difficult and non-deterministic
to exactly quantify the contribution of each error
source. On the contrary, using a GPS Simulator, each
and every error contributing sources can be separately
made active. The verification of the concerned
receiver algorithms can be accomplished and
necessary fine tuning generally needed during the
development phase can be achieved.
The amount of error present for each error
contributing source cannot be ascertained in a live
GPS signal. For example, the ionospheric errors
present in a live GPS signal cannot be quantified with
very high accuracy. However, using a GPS Simulator,
an exact amount of Ionospheric error may be
introduced in the signal. There is also a possibility in
the Simulator of no ionospheric errors being injected
in the signal at all, which is never possible using a live
GPS Signal. This criterion is true for all the error
contributing sources mentioned. The capability of
quantifiable error injection actually helps in the design
and development of new algorithms and performance
verification of existing methodologies implemented in
the receiver.
Whenever, any new algorithm is designed and
developed, the need for repeatedly testing the
algorithm with the same stimulus becomes very
important. The performance of the algorithm can be
evaluated only if the test environment remains the
same. With a live GPS signal, it is difficult to recreate
repeatable scenarios for testing. At every instance of
time, the GPS satellites in the constellation will
change their position, the predicted satellite ephemeris
and the satellite clock errors will be different. Along
with this, the effect of ionosphere and troposphere
cannot remain the same and the multipath error may
also vary. The overall geometry created by the visible
GPS satellites and the receiver, which in turn governs
the Geometric Dilution of Precision (GDOP) [19],
will differ leading to a different error value in the
position computation. The signals from the GPS
satellites will arrive at the receiver with different time
offsets and Doppler values, which would be different
in every test iteration. This becomes a major
bottleneck in testing using a live signal.
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Fig. 1. GPS L1 Band Simulator Architecture
On the contrary, using GPS Simulator, the state of the
constellation, the exact amount of errors from the
different error sources, the geometry of the
constellation, all can be repeated ensuring signals
generated being similar even upto the code and carrier
phase values. This helps in the design and
development of various receiver algorithms and also
in comparing different algorithm efficacy or
comparing the performance of receiver from different
make.
The most important utility of the GPS Simulator
comes when the newly designed GPS Receiver needs
to undergo a dynamic scenario testing. Using the live
GPS signals, the receiver needs to be mounted in high
speed automobiles, trains or aircrafts for high
dynamics testing, which is quite inconvenient, time
consuming and not economically viable especially
when the receiver development is in its infancy. Using
a GPS Simulator, the designed receiver can be
subjected to these dynamic environments, in a highly
controllable and repeatable manner, which will highly
aid in the high dynamic receiver performance
evaluation. The performance of receiver in high speed
trains, unmanned landing of an aircraft, guidance of a
satellite launch vehicle can all be ascertained in a
controlled laboratory environment. The performance
of the algorithms can be fine tuned after analysis of
the dynamic test results. This approach is economical,
reduces the development time and gives confidence in
the receiver capability before the actual deployment in
the field.
All these above mentioned reasons make the GPS
Simulator an indispensable system in the GPS
Receiver development process.
5 GPS Simulator Architecture
A 12 channel GPS L1 Band Simulator has been
designed and developed at Space Application Centre
(ISRO), which has the capability to simulate the 32
satellites of the GPS constellation civilian signals in
the L1 frequency Band. The GPS Simulator mimics
the entire GPS Constellation and generates GPS RF
signal in the L1 Band as will be received by a GPS
receiver from the actual GPS Constellation.
The GPS Simulator consists of a Simulation Software,
FPGA Signal Generation Unit and RF Up-Conversion
Unit. The scenario simulation along with the
generation of the navigation messages is performed in
the software. These simulation parameters are utilized
by the FPGA based unit for the complete signal
synthesis as per GPS ICD [19].
The realization of the GPS Simulator primarily
involves the synthesis of 32 GPS satellite signals in
real time. This entails implementation of the codes
pertaining to the individual satellites, the navigation
data and the signal modulation. The combined satellite
signals for all the satellites in L1 Band is available
from the RF output of the Simulator as shown in Fig
1.
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GPS being a global constellation, using MEO orbiting
satellites, not all satellites are visible to a receiver at
any point of time. The aim of the GPS Simulator is to
present the RF signals of only those satellites that are
visible to the receiver. This creates a challenge in the
Simulator realization, as at some given point of time,
some satellites move below the horizon and new
satellites come up the horizon. The signals only from
the satellites above the horizon need to be
incorporated in the generated RF signal from the
Simulator.
The first task of the GPS Simulator is to determine the
number of visible satellites from the receiver location.
The GPS Simulator has 12 hardware RF channels
corresponding to 12 satellites visible from any point
on the earth. The set of visible satellites is
dynamically controlled to accurately simulate the
rising and setting of satellites. Once the simulation
scenario starts, it is ascertained that a satellite is
included in the visible set only when it comes above a
certain elevation angle threshold. Then the RF signal
generation process is configured in such a way that
only signals from the visible satellites are presented to
the Receiver.
There are some software based GPS IF Signal
Simulation techniques available in [3][7][8][9][13],
which are primarily targeted for development and
evaluation of Software based GPS Receivers [18].
The Simulator design presented in this paper uses a
combination of hardware and software design as
shown in Fig 1. Authors have realized a NavIC Tri-Band
(L1, L5, S) SPS Simulator using a similar approach for the
design and development of NavIC Receivers [1]. The
broad specification of the Simulator is as shown in
Table I.
TABLE I
GPS SIMULATOR SPECIFICATIONS
Hardware Channels
Number of Channels
12 channels
PRN code selectivity
Any PRN number among
1 to 32
Output Frequency
GPS L1
1575.42 ± 20.46 MHz
Reference Clock Specification
Reference clock source
10MHz (OCXO)
Frequency Stability –
Long term
≤ ±10-9
External reference input
10 MHz, 0dBm ± 3 dB
Phase noise of
Reference source
(10MHz)
≤-135 dBc/Hz @ 1KHz
≤-143 dBc/Hz @ 10KHz
≤-144 dBc/Hz
@100KHz
≤-146 dBc/Hz @ 1000
KHz
Signal quality
In band spurious
≤-40dBc
2nd Harmonic spurious
≤-40dBc
Spurious free dynamic
range (SFDR)
≥48dBc
Carrier Suppression
≥ 30dB
Amplitude Imbalance
≤ ± 0.5 dB
Phase Imbalance
≤ ± 2 deg.
Error Vector Magnitude
< 10% RMS
RF Output power level
RF Signal Power Level
-130dBm (Typical)
Range
±20 dB
Resolution
0.1dB
In the GPS Simulator, all the signal structures
described in Section III have been implemented with
the generation of Pseudo Random Codes, navigation
data and the signal modulation as per the data
structure and generation of the actual RF signals in the
L1 Band. The inputs required to setup the GPS
Simulation Scenario includes parameters like receiver
position, the simulation time and enabling / disabling
of different error sources. The Simulator generates
the modulated RF signal as would have been received
from the actual GPS Constellation at the specified
receiver location and simulation time.
The receiver performance needs to be evaluated at
different locations. This is important to ascertain the
receiver position accuracy in different parts of the
world as the Geometric Dilution of Precision
(GDOP)[19] of the GPS Constellation varies with
location and time. The simulation time can also be
changed to any date and time in past or future to check
the performance of the receiver.
As the GPS signals traverse through the ionosphere of
the atmosphere, it is subjected to a phenomenon called
code-carrier divergence [17], where the code is
delayed and the carrier is advanced. This effect has
been implemented in the GPS Simulator where the
code and carrier is subjected to the effects as it
happens in reality. The GPS signals are also delayed
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by the troposphere of the atmosphere [16]. The
Simulator has the capability to simulate the
tropospheric effects. In addition to this, errors
occurring due to satellite ephemeris, satellite clock
and satellite perturbation can be simulated in the
Simulator. The Simulator has the provision to
simulate fixed power levels for individual satellites
along with the precise variation of power arising from
the motion of the satellite and receiver, even taking
into account the receiver antenna gain pattern.
Moreover, the high dynamic scenarios can also be
tested using the Simulator without leaving the
laboratory environment.
The performance of the GPS Simulator has been
tested using various test cases as described in the
following section. The ultimate RF output signal of L1
Band is available at -130 dBm from the RF output
port of the Simulator. The Simulator also has the
provision to generate 1PPS and 10 MHz signals
generally needed for receiver development and
testing.
6 Test and Evaluation
Several tests have been conducted to characterize
the performance of the Simulator related to signal
fidelity, SFDR, and signal harmonics as shown in
Table II. The GPS Simulator has also been tested
with M/S Accord [25] developed GPS Receivers as
shown in Fig 5 and the receiver GUI shown in Fig 6.
The Receiver is provided with the input RF Signal
generated by the Simulator. All the visible GPS
satellite signals for the L1 Band are acquired by the
M/S Accord Receiver for the L1 Band as shown in Fig
6 along with the position solution being achieved.
TABLE II
SIGNAL FIDELITY TEST RESULTS
Measured
Values
-56dBc
-66 dBc
55 dBc
61dB
0.2 dB
6 %
The static test, where the receiver position remains
fixed with time, has been conducted with all the
atmospheric impairments like ionosphere and
troposphere [16]. At the start of the configured
simulation scenario, a total of 10 GPS satellites were
visible to the receiver which includes PRN 2, 5, 6, 11,
12, 19, 20, 24, 25 and 29. The ionospheric errors in
meters as imparted by the simulator for all visible
satellites is shown in Fig2. Similarly, the tropospheric
errors generated by the simulator for the set of visible
satellites is shown in Fig3. It can be seen that the
tropospheric error is maximum, when the elevation
angle is low [17], as is the case for satellite number 29
in the simulated scenario. The variation of the
elevation angle as calculated and logged by the
Simulator is shown in Fig4. The simulator logged
receiver position values were compared with the
receiver obtained values and their difference
calculated every second. The resultant 3D RMS error
is approximately 17 cm.
Fig. 2. Ionospheric Errors imparted by Simulator
International Journal of Electrical Engineering and Computer Science
DOI: 10.37394/232027.2024.6.31
Mohanchur Sarkar, Bhanu Panjwani
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Volume 6, 2024
Fig. 3. Tropospheric Errors imparted by Simulator
Fig. 4. Elevation Angle as logged by Simulator
Fig. 5. GPS Simulator Test Setup
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Volume 6, 2024
Fig. 6. GPS Simulator with M/S Accord GPS Receiver [25]
The capability of the Simulator to generate dynamic
simulation scenarios is verified by creating a scenario
where the Receiver moves at a constant velocity of
515 m/s (1854 km/hr) in a circular track. This is the
highest velocity permissible for civilian Receivers. All
the effects arising because of the high receiver
dynamics have been successfully synthesized by the
Simulator, like the very high Doppler, which will be
experienced due to the high variation in the satellite
and receiver relative velocity. The Doppler values
imparted by the Simulator for the respective satellites
is shown in Fig7. If the Simulator is perfectly creating
the dynamic scenario in the lab environment, where
actually the receiver is remaining static, but the
received signal is artificially being subjected to the
effects as if it is moving at a very high velocity by the
simulator, the computed receiver velocity will be very
high. The Receiver acquires the signal even in the
presence of the very high Doppler values and the
velocity obtained by the Receiver matches with the
simulator imparted velocity of 515 m/s as is shown in
Fig8. In Fig8, considering the dynamic scenario, the
Receiver GUI shows the PRN locked, the C/No
achieved, the pseudoranges of the visible satellites,
azimuth & elevation which was as expected from a
live GPS constellation signal. This shows the efficacy
of the GPS Simulator in recreating the dynamic
scenario as the receiver is transparently processing the
signal as if the signals are coming from a live GPS
constellation. This is the main objective of the
developed GPS Simulator, to artificially generate a
test environment, which is close to the reality and that
has been achieved.
Fig. 7. Doppler frequency imparted by Simulator
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DOI: 10.37394/232027.2024.6.31
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Volume 6, 2024
Fig. 8. GPS Receiver GUI with dynamic scenario
7 Conclusion
The design and development of the GPS Simulator
has been undertaken at Space Applications Centre,
ISRO, which generates the GPS constellation civilian
signals in the L1 Band. The full constellation of 32
GPS satellites has been realized in the Simulator. The
simulator synthesizes the propagation effects with all
the error contributing sources as the signal traverses
from the satellite to the receiver. The signal structure
of GPS as per the ICD [19] has been realized in the
simulator. As satellite based navigation proliferates,
throughout the world in general and India in
particular, there will be a huge demand of GPS
receivers and increased industry participation. Many
Indian small scale industries and academic institutions
will engage in research on satellite based navigation
and associated applications. Considering the
importance and huge cost implication of simulators in
the complete receiver development eco-system, this
developed simulator will be beneficial to academic
institutions and industries. This simulator will help
drive the GPS receiver development from
conceptualization, testing to ultimate product delivery
and aid in the envisaged future utilization of GPS
based applications.
Acknowledgement:
Authors are grateful to Shri. Deval Mehta, Group
Director, Navigation Technology and Applications
Group (SAC-ISRO), Dr. Khagindra Kumar Sood,
Deputy Director, SatCom and SatNav Application
Area (SAC-ISRO) and Dr. Nilesh.M. Desai, Director
of Space Applications Centre (SAC), ISRO for their
encouragement and support towards this work.
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DOI: 10.37394/232027.2024.6.31
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E-ISSN: 2769-2507
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Volume 6, 2024
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Contribution of Individual Authors to the
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Policy)
The authors equally contributed in the present
research, at all stages from the formulation of the
problem to the final findings and solution.
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Scientific Article or Scientific Article Itself
No funding was received for conducting this study.
Conflict of Interest
The authors have no conflicts of interest to declare
that are relevant to the content of this article.
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DOI: 10.37394/232027.2024.6.31
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