Full text: Systems for data processing, anaylsis and representation

as small as 5cm x 5 cm. These receivers are 
relatively new on the market and are ideal for 
system integration applications where GPS is 
merely a component in the overall system. Most of 
these receivers have 5 to 8 channels which means 
that all the satellites which are in view may not 
be tracked. Receivers use algorithms based on the 
PDOP or the satellite elevation in order to select 
satellites. Tests have shown that the algorithm 
may affect the receiver's performance under 
shading conditions (McLellan et al., 1994). 
Another distinguishing factor regarding GPS 
engines is the availability of raw data to the user. 
Some receivers do not output the measurements, but 
only the internally computed positions, whereas 
other receivers output both the pseudorange and 
carrier phase data. This feature is important for 
system developers since raw data is usually 
required. The cost of the C/A code engine class of 
receivers is generally between $500 - $8,000 (US), 
depending on the performance. These receivers 
will play a major role in the GIS market since they 
can add a georeferencing component to a GIS system 
at relatively low cost. 
The second class of the receivers is the C/A code 
handheld type which are effectively GPS engines 
housed in a small, lightweight data collector. 
Many of these receivers are targetted at the GIS 
market such that the receiver has software to log 
attribute information in addition to positions. In 
the past, one of the limitations of these receivers 
is the achievable positioning accuracy of typically 
a few metres. Many manufacturers are currently 
developing handheld systems which can deliver 
sub-metre or even cm-level positions. Position and 
attribute information can be downloaded to a GIS 
system in an automated process. These receivers 
generally range in price from $800-$12,000 (US). 
For surveying applications, where cm-level 
accuracies are needed a full C/A code receiver is 
generally used. These receivers have a 
sophisticated user interface which allows more 
flexibility in the operation of the unit and the 
number of channels range from 8 to 12 so that all 
satellites in view may be tracked. Raw data is 
generally available to the user which may be 
processed by manufacturer-supplied software or 
alternatively by third party programs. The 
quality of the receiver oscillator as well as the 
antenna are generally higher than either the 
engine or handheld receivers, so that errors (i.e. 
measurement noise and antenna phase centre 
stability) will be minimized. These receivers 
range in cost from $12,000 to $20,000 (US) and may 
be used for high-end GIS applications where the 
achievable accuracy is important. Alternatively, 
these receivers are often used for differential base 
stations in GIS applications since they have the 
all-in-view capability. 
The final class of receiver is the so-called P 
codeless unit which gives dual frequency carrier 
phase (and possibly pseudorange) data without 
access to the encrypted P code (i.e. the Y code). 
These receivers are the current state-of-the-art 
and are generally priced in the 35,000-45,000 (US) 
range. The main applications of these receivers are 
for high accuracy rapid static surveying, on-the- 
fly kinematic surveying (OTF) as well as high 
precision static surveying over long baseline 
lengths (say >50 km). 
Table 1: GPS Receiver Classes and Features 
Class General Features 
- low cost 
- 5-10 channels 
C/Acode - system integration required 
engine — - raw data may not be available 
- flexible architecture 
- manufacturer post-processing 
software generally not available 
- low-mid cost 
- 5 or 6 channels 
Handheld - may have manufacturer supplied 
C/Acode software for m-level accuracies 
receiver - may interface to GIS package 
- mid-range cost 
- 8-12 channels 
C/Acode - sophisticated user interface 
receiver - manufacturer supplied software for 
cm-level accuracies 
- premium cost 
- 9-12 channels 
- sophisticated user interface 
P codeless - complete system with manufacturer- 
receiver supplied post-processing software 
- dual frequency data available after 
GPS becomes operational 
- quality of L2 data may vary B 
Receiver hardware costs are reducing at a fast rate 
which is mainly driven by the increasing use of the 
GPS system. An interesting phenomenon that is 
occurring is that the spread in the cost, i.e. from 
the lowest to the most expensive, is growing wider. 
This is due to the emergence of GPS engines which 
are 'bare-bones' receivers as compared to the full 
turn-key systems that include software and 
extensive customer support. Figure 2 shows the 
trend in GPS hardware over the past several 
years, after McDonald (1989). 
GPS Receiver Cost $US (000's) 
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