CLASSIFICATION OF GPS RECEIVERS
GPS receivers can be divided into
various groups according to different criteria. In the early stages two basic
technologies were used as the classification criteria viz. Code correlation
receiver technology and sequencing receiver technology, which were equivalent
to code dependent receivers and code free receivers. However, this kind of
division is no longer justifiable since both techniques are implemented in
present receivers.
Another
classification of GPS receivers is based on acquisition of data types
e.g.
-
C/A code receiver
-
C/A code + L1 Carrier phase
-
C/A code + L1 Carrier phase + L2 Carrier phase
-
C/A code + p_code + L1, L2 Carrier phase
-
L1 Carrier phase (not very common)
-
L1, L2 Carrier phase (rarely used)
Based on technical realization of
channel, the GPS receivers can be classified
as:
-
Multi-channel receiver
-
Sequential receiver
-
Multiplexing receiver
GPS
receivers are even classified on the purpose as:
-
Military receiver
-
Civilian receiver
-
Navigation receiver
-
Timing receiver
-
Geodetic receiver
For geodetic application it is essential to use the carrier
phase data as observable. Use of L1 and L2 frequency is also essential along
with P-code.
Examples
of GPS Receiver
GPS receiver market is developing
and expanding at a very high speed. Receivers are becoming powerful, cheap and
smaller in size. It is not possible to give details of every make but
description of some typical receivers given may be regarded as a basis for the
evaluation of future search and study of GPS receivers.
Classical Receivers
Detailed description of code
dependent T1 4100 GPS Navigator and code free Macrometer V1000 is given here:
T1 4100 GPS Navigator was
manufactured by Texas Instrument in 1984. It was the first GPS receiver to
provide C/A and P code and L1 and L2 carrier phase observations. It is a dual
frequency multiplexing receiver and suitable for geodesist, surveyor and
navigators. The observables through it are:
-
P-Code pseudo ranges on L1 and L2
-
C/A-Code pseudo ranges on L1
-
Carrier phase on L1 and L2
The data are recorded by an
external tape recorder on digital cassettes or are downloaded directly to an
external microprocessor. A hand held control display unit (CDU) is used for
communication between observer and the receiver. For navigational purposes the
built in microprocessor provides position and velocity in real time every three
seconds. T1 4100 is a bulky instrument weighing about 33 kg and can be packed
in two transportation cases. It consumes 90 watts energy in operating mode of
22V - 32V. Generator use is recommended. The observation noise in P-Code is
between 0.6 to 1 m, in C/ A code it ranges between 6 to 10 m and for carrier
phase it is between 2 to 3 m.
T1 4100 has been widely used in numerous
scientific and applied GPS projects and is still in use. The main disadvantages
of the T1 4100 compared to more modern GPS equipment's are
-
Bulky size of the equipment
-
High power consumption
-
Difficult operation procedure
-
Limitation of tracking four satellites
simultaneously
-
High noise level in phase measurements
Sensitivity of its antenna for
multipath and phase centre variation if two receivers are connected to one
antenna and tracking of seven satellites simultaneously is possible. For long
distances and in scientific projects, T1 4100 is still regarded useful.
However, due to imposition of restriction on P- code for civilian, T1 4100
during Anti Spoofing (AS) activation can only be used as a single frequency C/A
code receiver.
The
MACROMETER V 1000, a code free GPS receiver was introduced in 1982 and was the
first receiver for geodetic applications. Precise results obtained through it
has demonstrated the potential of highly accurate GPS phase observations. It is
a single frequency receiver and tracks 6 satellites on 6 parallel channels. The
complete system consists of three units viz.
-
Receiver and recorder with power supply
-
Antenna with large ground plane
-
P 1000 processor
The processor is essential for
providing the almanac data because the Macrometer V 1000 cannot decode the
satellite messages and process the data. At pre determined epoches the phase
differences between the received carrier signal and a reference signal from
receiver oscillator is measured. A typical baseline accuracy reported for upto
100 km distance is about 1 to 2 ppm (Parts per million).
Macrometer II, a dual frequency
version was introduced in 1985. Though it is comparable to Macrometer V 1000,
its power consumption and weight are much less. Both systems require external
ephemeredes. Hence specialized operators of few companies are capable of using
it and it is required to synchronize the clock of all the instruments proposed
to be used for a particular observation session. To overcome above
disadvantages, the dual frequency Macrometer II was further miniaturized and
combined with a single frequency C/A code receiver with a brand name MINIMAC in
1986, thus becoming a code dependent receiver.
Examples
of present Geodetic GPS Receivers
Few of the currently available
GPS receivers that are used in geodesy surveying and precise navigation are
described. Nearly all models started as single frequency C/A-Code receivers
with four channels. Later L2 carrier phase was added and tracking capability
was increased. Now a days all leading manufacturers have gone for code-less,
non- sequencing L2 technique. WILD/ LEITZ (Heerbrugg, Switzerland) and MAGNAVOX
(Torrance, California) have jointly developed WM 101 geodetic receiver in 1986.
It is a four channel L1 C/A code receiver. Three of the channels sequentially
track upto six satellites and the fourth channel, a house keeping channels,
collects the satellite message and periodically calibrates the inter channel
biases. C/A-code and reconstructed L1 carrier phase data are observed once per
second.
The dual frequency WM 102 was
marketed in 1988 with following key features:
-
L1 reception with seven C/A code channel tracking
upto six satellites simultaneously.
- L2 reception of up to six
satellites with one sequencing P- code channel
Modified
sequencing technique for receiving L2 when P-code signals are encrypted.
The observations can be recorded
on built in data cassettes or can be transferred on line to an external data
logger in RS 232 or RS 422 interface. Communication between operator and receiver
is established by alpha numerical control panel and display WM 101/102 has a
large variety of receiver resident menu driven options and it is accompanied by
comprehensive post processing software.
In 1991, WILD GPS system 200 was
introduced. Its hardware comprises the Magnavox SR 299 dual frequency GPS
sensor, the hand held CR 233 GPS controller and a Nicd battery. Plug in memory
cards provide the recording medium. It can track 9 satellites simultaneously on
L1 and L2. Reconstruction of carrier phase on L1 is through C/A code and on L2
through P-code. The receiver automatically switches to codeless L2 when P-code
is encrypted. It consumes 8.5 watt through 12-volt power supply.
TRIMBLE NAVIGATION (Sunny vale,
California) has been producing TRIMBLE 4000 series since 1985. The first
generation receiver was a L1 C/ A code receiver with five parallel channels
providing tracking of 5 satellites simultaneously. Further upgradation included
increasing the number of channels upto tweleve, L2 sequencing capability and
P-code capability. TRIMBLE Geodatic Surveyor 4000 SSE is the most advanced
model. When P-Code is available, it can perform following types of
observations, viz.,
-
Full cycle L1 and L2 phase measurements
-
L1 and L2, P-Code measurements when AS is on and
P-code is encrypted
-
Full cycle L1 and L2 phase measurement
-
Low noise L1, C/A code
-
Cross-correlated Y-Code data
Observation noise of the carrier
phase measurement when P-code is available is about ñ 0-2mm and of the P-code
pseudoranges as low as ñ 2cm. Therefore, it is very suitable for fast ambiguity
solution techniques with code/ carrier combinations.
ASHTECH (Sunnyvale, California)
developed a GPS receiver with 12 parallel channels and pioneered current
multi-channel technology. ASHTECH XII GPS receiver was introduced in 1988. It
is capable of measuring pseudoranges, carrier phase and integrated dopler of up
to 12 satellites on L1. The pseudoranges measurement are smoothed with
integrated Doppler. Postion velociy, time and navigation informations are
displayed on a keyboard with a 40-characters display. L2 option adds 12
physical L2 squaring type channels.
ASHTECH XII GPS receiver is a
most advanced system, easy to handle and does not require initialization
procedures. Measurements of all satellites in view are carried out
automatically. Data can be stored in the internal solid plate memory of 5
Mbytes capacity. The minimum sampling interval is 0.5 seconds. Like many other
receivers it has following additional options viz.
-
1 ppm timing signal output
-
Photogrammetric camera input
-
Way point navigation
-
Real time differential navigation and provision of
port processing and vision planning software
In 1991, ASHTECH P-12 GPS receiver was marketed. It has 12
dedicated channels of L1, P-code and carrier and 12 dedicated channels of L2,
P-code and carrier. It also has 12 L1, C/A code and carrier channels and 12
code less squaring L2 channels. Thus the receiver contains 48 channels and
provides all possibilities of observations to all visible satellites. The
signal to noise level for phase measurement on L2 is only slightly less than on
L1 and significantly better than with code-less techniques. In cases of
activated P-code encryption, the code less L2 option can be used.
TURBO ROGUE SNR-8000 is a
portable receiver weighing around 4 kg, consumes 15-watt energy and is suitable
for field use. It has 8 parallel channels on L1 and L2. It provides code and
phase data on both frequencies and has a codeless option. Full P-code tracking
provides highest precision phase and pseudo rages measurements, codeless
tracking is automatic 'full back' mode.
The code less mode uses the fact that each carrier has identical modulation of
P-code/Y-code and hence the L1 signal can be cross-correlated with the L2
signal. Results are the differential phase measurement (L1-L2) and the group
delay measurement (P1-P2)
Accuracy specifications are :
P-Code pseudo range 1cm (5 minutes integration) Codeless pseudo
range 10cm (5 minutes
integration) Carrier phase
0.2
- 0.3 mm
Codeless phase 0.2 - 0.7 mm
One of the important features is
that less than 1 cycle slip is expected for 100 satellite hours.
Navigation Receivers
Navigation receivers are rapidly
picking up the market. In most cases a single C/A code sequencing or
multiplexing channel is used. However, modules with four or five parallel
channels are becoming increasingly popular. Position and velocity are derived from
C/A code pseudoranges measurement and are displayed or downloaded to a personal
computer. Usually neither raw data nor carrier phase information is available.
Differential navigation is possible with some advanced models.
MAGELLAN NAV 1000 is a handheld
GPS receiver and weighs only 850 grams. It was introduced in 1989 and later in
1990, NAV 1000 PRO model was launched. It is a single channel receiver and
tracks 3 to 4 satellites with a
2.5 seconds update rate and has a RS 232 data port.
The follow up model in 1991 was
NAV 5000 PRO. It is a 5-channel receiver tracking all visible satellites with a
1-second update rate. Differential navigation is possible. Carrier phase data
can be used with an optional carrier phase module. The quadrifilar antenna is integrated
to the receiver. Post processing of data is also possible using surveying
receiver like ASHTECH XII located at a reference station. Relative accuracy is
about 3 to 5 metres. This is in many cases sufficient for thematic purposes.
Many hand held navigation
receivers are available with added features. The latest market situation can be
obtained through journals like GPS world etc.
For most navigation purpose a
single frequency C/A code receiver is sufficient. For accuracy requirements
better than 50 to 100 meters, a differential option is essential. For
requirement below 5 meters, the inclusion of carrier phase data is necessary.
In high precision navigation the use of a pair of receivers with full geodetic
capability is advisable. The main characteristics of multipurpose geodetic
receiver are summarized in Table 4.
Table 4. Overview of geodetic
dual-frequency GPS satellite receiver (1992)
Some of the important features for selecting a geodetic
receiver are :
-
Tracking of all satellites
-
Both frequencies
-
Full wavelength on L2
-
Low phase noise-low code noise
-
High sampling rate for L1 and L2
-
High memory capacity
-
Low power consumption
-
Full operational capability under anti spoofing
condition
Further, it is recommended to use
dual frequency receiver to minimize ion-spherical influences and take
advantages in ambiguity solution.
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