IEEE - Aerospace and Electronic Systems - January 2023 - 9

Braasch
measure of the change-of-range over time. The benefit of
these measurements is that they are two orders ofmagnitude
more precise (lower noise) than measurements made with
the codes. The disadvantage is that they do not provide absolute
range, only range change. Interferometric techniques
were applied to these measurements such that highly precise
baselines between two receivers could be determined. For
the surveying application, one receiver would be placed
over a geodetic monument (marker) and the second receiver
was operated at the location to be surveyed. Although coverage
(i.e., at least four satellites in view) was limited to just a
few hours per day in the early 1980s, the time savings to the
surveyors (versus traditional techniques) was enormous. It
was well worth their time to schedule their surveys during
the periods oflimited coverage.
The first civilian handheld GPS receiver was the
Figure 4.
GPS constellation. Six orbital planes, inclined at 55 degrees with
respect to the equator, with four satellites in each. [Image courtesy
U.S. National Coordination Office for Space-Based Positioning,
Navigation, and Timing.]
50 YEARS OF RECEIVER DEVELOPMENT
During the initial phase of GPS development, prototype
receivers (called " user equipment " or UE) cost the program
over US$200,000 each. One early receiver occupied
two entire full-height 19-inch racks (it even had an
operator console with two chairs!) [5]. The JPO originally
had two goals: drop five bombs in the same hole
and provide receivers at a cost of US$10,000 each [6].
The bombing accuracy was demonstrated early in the
program and the receiver cost goal was achieved by the
1990s. Early receivers came in two varieties: sequencing
receivers consisted of a single channel that was timemultiplexed
among the four satellites being tracked.
Multichannel receivers had a higher SWAP-C penalty
but provided better accuracy and higher data rates. In the
present day, all receivers are so-called " all-in-view "
receivers with sufficient channels to track all satellites
above the local horizon. Over the decades GPS receivers
have become virtually indispensable to all branches of
the military.
The earliest civilian GPS receivers were so-called
" time transfer " receivers. Since accurate time is embedded
in the broadcast signal, a receiver at a known location
needed only track one satellite to " transfer " accurate time
from the satellite to the receiver [13]. Early generation
time transfer receivers were still bulky (see Figure 5) but
provided time accuracy well under 100 nanoseconds (in
the era before SA). Similar units were used in the earliest
tests ofGPS in civil aviation [14].
Another " early adopter " was the surveying community.
Since the GPS broadcast signals are continuous wave, Doppler
measurements (equivalent to line-of-sight velocity measurements)
can be continuously integrated to provide a
JANUARY 2023
Magellan Nav 1000 (a second-generation unit is shown in
Figure 6). It was introduced in 1989 and retailed for
US$3000 [15]. Both the size and cost of handheld units
dropped dramatically over the next decade. By 1999,
Magellan released the Blazer (a slightly larger than palmsized
unit) and the cost was sufficiently low that this
author received his unit as a gift given to each attendee of
a navigation conference in 2000. [Note: The author has no
financial interest in Magellan. The receivers are just convenient
examples.]
Figure 5.
Stanford Telecommunications Inc. 5026 Time Transfer Receivers
(ca. early 1980s). Two units are shown in the photo. One was a
conventional time transfer receiver and the second was modified
for carrier-phase tracking [16]. [Photo by the author.]
IEEE A&E SYSTEMS MAGAZINE
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IEEE - Aerospace and Electronic Systems - January 2023

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