Self-Assessment Quiz: Communications, Navigation, and Data Handling

Twenty questions to check your grasp of link budgets, antennas, the Deep Space Network, bands and coding, navigation, and radiation-hardened computers. Answer each before opening the key. Aim for 16 or more. Use $c = 2.998\times10^8\ \text{m/s}$ and, where needed, $-228.6\ \text{dBW/K/Hz}$ for Boltzmann's constant.

Question 1

In a link budget worked in decibels, you combine the terms by:

A) multiplying gains and dividing by losses B) adding gains and subtracting losses C) adding everything D) taking the logarithm of the sum

Question 2

Received signal power in free space falls with distance $d$ as:

A) $1/d$ B) $1/d^2$ C) $1/d^3$ D) $e^{-d}$

Question 3

Free-space path loss is written $20\log_{10}(4\pi d/\lambda)$ rather than $10\log_{10}$ because:

A) it is measured in different units B) the quantity inside is squared, and $10\log_{10}(x^2) = 20\log_{10}(x)$ C) radio waves travel at twice the speed of light D) it accounts for two antennas

Question 4

The effective isotropic radiated power (EIRP) of a transmitter is:

A) $P_t - G_t$ B) $P_t \times G_r$ C) $P_t + G_t$ (in dB) D) the received power

Question 5

The gain of a parabolic dish antenna scales as:

A) $D/\lambda$ B) $(D/\lambda)^2$ C) $\lambda/D$ D) $D^2\lambda$

Question 6

A very high-gain antenna necessarily has:

A) a very wide beam B) a very narrow beam C) no beam at all D) low efficiency

Question 7

The Deep Space Network uses three complexes spaced ~120° apart in longitude so that:

A) each can use a different frequency B) a target stays in view of at least one as Earth rotates C) they can point at three planets at once D) they share the same weather

Question 8

"Arraying" antennas means:

A) painting them for camouflage B) electronically combining several dishes to act as one larger antenna C) pointing them at different spacecraft D) stacking them vertically

Question 9

For a link with fixed physical dish sizes at both ends, moving to a higher frequency:

A) always weakens the received signal B) increases received power, roughly as $f^2$ C) has no effect on received power D) only helps if the distance also increases

Question 10

The purpose of forward error correction (channel coding) on a deep-space link is to:

A) increase the transmitter power B) let the receiver correct errors without a retransmission, lowering the required $E_b/N_0$ C) point the antenna more accurately D) reduce the free-space path loss

Question 11

Ranging determines a spacecraft's __ by ____:

A) velocity; measuring frequency shift B) distance; timing a signal's round trip C) sky position; interferometry with a quasar D) temperature; measuring noise

Question 12

Doppler tracking measures a spacecraft's:

A) distance B) line-of-sight (radial) velocity C) plane-of-sky angle D) spin rate

Question 13

Delta-DOR provides the coordinate that ranging and Doppler cannot — the spacecraft's:

A) distance B) radial velocity C) angular position across the sky D) mass

Question 14

Radiation-hardened processors are much slower than consumer chips mainly because:

A) they run at lower voltage to save power B) they use larger feature sizes and heavy redundancy to resist upsets C) space is cold D) they are older only by accident

Question 15

A "single-event upset" (SEU) is:

A) a permanent destruction of the chip B) a bit flipped by a single energetic particle (a soft error) C) a software bug D) a loss of the radio link

Question 16 (True/False, justify)

"The best way to raise a deep-space probe's data rate is to add transmitter power." True or false? Justify in one sentence.

Question 17 (True/False, justify)

"Doubling the distance to a probe halves the received signal power." True or false? Explain.

Question 18 (True/False, justify)

"Delta-DOR needs two widely separated ground stations and an observation of a quasar." True or false? Say why the quasar is there.

Question 19 (Short answer)

A probe at $10\ \text{AU}$ ($1.496\times10^{12}\ \text{m}$) downlinks at $8.4\ \text{GHz}$ ($\lambda = 0.0357\ \text{m}$). Compute the free-space path loss in dB.

Question 20 (Short answer)

New Horizons returned data at about 1 kbit/s from Pluto but far faster from Jupiter, using the same hardware. Explain why, in one or two sentences, and estimate the ratio of the two rates. (Pluto ≈ 32 AU, Jupiter ≈ 5 AU.)

Answer Key

Q Ans Note
1 B In dB, multiplication becomes addition; gains add, losses subtract.
2 B Inverse-square spreading: $P_r \propto 1/d^2$.
3 B The loss factor is $(4\pi d/\lambda)^2$; the square becomes a factor of 20.
4 C EIRP $= P_t + G_t$ (dB) — the isotropic power that gives the same on-axis flux.
5 B $G = \eta(\pi D/\lambda)^2$: gain grows as diameter-in-wavelengths squared.
6 B Gain and beamwidth are reciprocal: $G \approx 29{,}000/\theta_{3\text{dB}}^2$.
7 B Earth's rotation sweeps antennas through longitude; three sites keep continuous coverage.
8 B Combining collecting areas raises $G/T$ by a few dB.
9 B Two gains rise as $f^2$ each, path loss as $f^2$: net $P_r \propto f^2$ for fixed apertures.
10 B Coding gain (~7 dB for concatenated codes) lowers the required $E_b/N_0$.
11 B $d = c\tau/2$ from the round-trip light time.
12 B $\Delta f/f = -v/c$ gives radial velocity.
13 C Interferometry between two stations fixes a plane-of-sky angle.
14 B Bigger nodes hold more charge (harder to flip); TMR/EDAC add redundancy — both cost speed.
15 B A soft, non-destructive bit flip from one particle.
16 False Power enters the budget only linearly (dB for dB), and deep-space power is scarce; antenna gain appears squared at both ends, so bigger dishes/arraying and colder receivers help far more.
17 False Received power falls as $1/d^2$, so doubling the distance cuts it to a quarter, not a half.
18 True Two stations form the interferometer baseline; the quasar (known sky position) is observed to cancel common clock/plasma/atmosphere errors — the "delta."
19 — $L_{\text{fs}} = 20\log_{10}(4\pi(1.496\times10^{12})/0.0357) = 20\log_{10}(5.27\times10^{14}) \approx 294\ \text{dB}$.
20 — Received power and data rate fall as $1/d^2$; rate ratio $\approx (32/5)^2 \approx 41$, so Jupiter runs roughly forty times faster than Pluto on identical hardware.

Topics to review by question

Questions Topic Section
1, 2, 3, 4, 17, 19 The link budget and free-space path loss §26.1
5, 6 Antenna gain and beamwidth §26.2
7, 8, 16 The Deep Space Network and arraying §26.3
9, 10, 20 Bands, coding, and data rate §26.4
11, 12, 13, 18 Navigation: ranging, Doppler, delta-DOR §26.5
14, 15 Radiation-hardened computers §26.6